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MANUFACTURING TECHNOLOGY RESEARCH

TEXTILES ADVANCES IN RESEARCH AND APPLICATIONS

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MANUFACTURING TECHNOLOGY RESEARCH

TEXTILES ADVANCES IN RESEARCH AND APPLICATIONS

BORIS MAHLTIG EDITOR

Copyright © 2018 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. We have partnered with Copyright Clearance Center to make it easy for you to obtain permissions to reuse content from this publication. Simply navigate to this publication’s page on Nova’s website and locate the “Get Permission” button below the title description. This button is linked directly to the title’s permission page on copyright.com. Alternatively, you can visit copyright.com and search by title, ISBN, or ISSN. For further questions about using the service on copyright.com, please contact: Copyright Clearance Center Phone: +1-(978) 750-8400 Fax: +1-(978) 750-4470 E-mail: [email protected]. NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book.

Library of Congress Cataloging-in-Publication Data ISBN: H%RRN

Published by Nova Science Publishers, Inc. † New York

CONTENTS Preface

vii

Chapter 1

Advances in Research and Applications of Smart Textiles Ali Akbar Merati

Chapter 2

Microwave Assisted Preparation for the Realization of Functional and Colored Textiles Haoqian Miao, Elena Schüll, Kerstin Günther and Boris Mahltig

29

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features to Cellulosic Fabrics: An Overview Giulio Malucelli

61

Wastewater Treatment from Textile Industry Using Advanced Oxidation Processes C. López-López, J. Martín-Pascual and J. M. Poyatos

87

Chapter 3

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Green Electrospinning of Nanofiber Mats from Biopolymers for Medical and Biotechnological Applications Timo Grothe, Nils Grimmelsmann, Sarah Vanessa Homburg and Andrea Ehrmann Achieving Electrical Conductivity in Textiles: An Overview of Current Techniques Thomas Grethe, Karoline Günther, Thomas Weide and Anne Schwarz-Pfeiffer Metal Coatings Effect Pigments for Textile Functionalization to Realize UV and IR Protective Applications Jieyang Zhang and Boris Mahltig

1

115

133

151

vi Chapter 8

Chapter 9

Contents Luminous Textiles for UV-Protection and Light Effect Applications Egon Dalponte, Boris Mahltig and Christof Breckenfelder

167

Double Functional Finishing for Textile Fabrics: A Combination of UV-Protective and Antimicrobial Properties Ghulam Shabbir, Hajo Haase and Boris Mahltig

183

Chapter 10

3D-Printing on Textile Fabrics Christoph Döpke, Yasmin Martens, Nils Grimmelsmann and Andrea Ehrmann

201

Chapter 11

Simulation Driven Design of Braided Products Yordan Kyosev

217

Chapter 12

Stab Resistance of Textiles Priscilla Reiners

233

Chapter 13

Sustainable Knitted Noise Insulation: The Development of Woolen Interior Elements with Acoustic Absorption Properties Maureen Mann, Sonja Vater, Frank Heimlich and Ellen Bendt

269

An Analysis of Impact Behavior of Warp Knit Spacer Fabric for the Evaluation of Suitability in Protective Applications N. Gokarneshan

285

Advances in Research and the Application of Ligno-Cellulosic Fibres Emphasising Sustainability Sanjoy Debnath

319

Chapter 14

Chapter 15

Chapter 16

Textile Applications for German Heath Wool Janina Krolzik, Roland Werner and Ellen Bendt

337

Chapter 17

Plasma Functionalisation Maike Rabe

345

Chapter 18

Photochromic Textiles E. Perrin Akçakoca Kumbasar and Seniha Morsunbul

359

Chapter 19

The Vintage Supply Chain: The Role of Networks Ruth Marciniak and Julie McColl

381

Index

399

PREFACE Textile materials or materials made from textiles are probably the most important materials. Textile materials are present everywhere in daily life in modern times but also since thousands of years. In this moment you are reading this book and you are also in tough with textile materials, which are your clothes. In your room you may see now as well several home textiles as carpet, curtains or as part of your furniture. If you drive your car or you are passenger in an air plane, you are placed in between many different types of textiles. Beside these obvious textile applications, there are also many more hidden applications, which could be e.g., textile filters, textiles in productions processes or as part of a composite material. Altogether, it can be summarized, modern life without textiles is not possible. This book is dedicated to advances in textile research and application. Keep in mind that the final textile in the final application is the product of many different production steps, altogether named also as the “textile chain”. Very many different disciplines are part of this textile chain, as chemistry and machine engineering but also something like agricultural science, if you think on cotton or wool fiber production. The life of a textile material starts with the fiber production, this can be a natural process for plant fibers, animal fibers or regenerated fibers from natural resources. In contrast for chemical fibers, their origin is the chemical synthesis. After fiber production, the yarn production follows, which is in the field of spinning technology. From the yarn the textile materials as fabrics or non-wovens are produced using processes as weaving, knitting, braiding etc. This textile products are further modified by dyeing and printing processes. Often also finishing is used to realize special functional properties on a textile product. Finally, the textile product is realized as cloth, home textile or technical application. Beside technical aspects, also design and marketing play an important role – especially to realize a successful product, accepted and bought. After following this summary, it is obvious that this current book cannot cover every simple aspect of textile and textile products. Therefore, this book headlines with every

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chapter a topic in area of textile research and application, which is a topic in actual scientific research but also in industrial development. According to this statement, this book is dedicated to readers from universities and industry. For industry, to promote ideas for new innovative processes and products. For universities, especially for graduated students, who wish to broaden their expertise in very different fields of the textile world. Also established scientists could gain a benefit, if they use this book as a tool to generate new ideas for further research. Besides all these thoughts on possible benefits, please do not forget one important benefit for reading or viewing this book: to enjoy it.

In: Textiles: Advances in Research and Applications ISBN: 978-1-53612-855-0 Editor: Boris Mahltig © 2018 Nova Science Publishers, Inc.

Chapter 1

ADVANCES IN RESEARCH AND APPLICATIONS OF SMART TEXTILES Ali Akbar Merati* Advanced Textile Materials and Technology Research Institute and Textile Engineering Department, Amirkabir University of Technology, Tehran, Iran

ABSTRACT Smart textiles are a relatively new area of research and convergence of different science such as materials, physics, chemistry, electrical engineering, wireless & mobile telecommunication and nanotechnologies. They have many potential applications in the fields of medical, protection, security communication and textile electronics. Smart textiles can be defined as textiles that are able to sense and respond to changes in their environment. The objective of this chapter is to present the latest research results together with basic concepts related to the main fields of applications of smart textiles. Future trends in this area of research are presented and issues regarding technology development and its uptake are highlighted.

Keywords: smart textile, intelligent apparel, chromic materials, conductive materials, electronic textiles, phase change materials, shape memory materials

INTRODUCTION Fine and elastic fibers are usually used in order to make comfortable fabric and clothing. The fibers, yarns and fabrics have to meet special requirements concerning *

Corresponding Author Email: [email protected].

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processability and wearability. They should be able to withstand handling that is typical for textiles, for example weaving, washing and wrinkling, without damaging functionality. Therefore, fabrics need to have a low mechanical resistance to bending and shearing so that they can be wearable and comfortable. The terms “wearable textiles ,” “electronic textiles,” “intelligent textiles” or “smart textiles,” are associated to those clothing which integrate electronic components, or which are made of smart materials. Smart textiles are the results of a disciplinary approach that creates an intersection and overlapping of researches in different fields such as textile, physics, chemistry, medicine, electronics, polymers, biotechnology, telecommunications, information technology, microelectronics, wearable computers, nanotechnology and micro-electromechanical machines. Shape memory materials (SMMs), phase change materials (PCMs), chromic materials, optic fibers, conductive materials, mechanical responsive materials, intelligent coating/membranes, micro and nanomaterials and piezoelectric materials are applied in smart textiles. Smart textiles are ideal vehicle for carrying active elements that permanently monitor our body and the environment, providing adequate reaction should something happens. The objective of this chapter is to present the latest research results together with basic concepts related to the most advanced researches and applications of smart textile. This chapter highlights all the main fields of applications of smart textiles such as healthcare, health monitoring, medicine, personal protective equipment, personal communication, textile antennas, garments for motion capture and sensors. The scientific issues and proposed solutions regarding various results, prototypes and achievements obtained in the best academic and industrial laboratories worldwide are presented in a rigorous scientific way. At the same time, practical solutions and their realization, believed to be of the interest to industrial partners, are presented and explained.

SMART TEXTILES The smart textiles are a new generation of fibers, yarns, fabrics and garments that are able to sense stimuli and changes in their environments, such as mechanical, thermal, chemical, electrical, magnetic and optical changes, and then respond to these changes in predetermined ways (Ibrahim 2014; Tao 2001; Culshaw 1996; Seinivasan & Mcfarland 2001). The objective of smart textile is to absorb a series of active components essentially without changing its characteristics of flexibility and comfort. Comfort is very important in textiles. Therefore, adequate actuators are needed that can heat, cool, insulate, ventilate and regulate moisture to keep the comfort of textiles and prevent the stresses and fatigue. In order to make a smart textile, firstly, conventional components such as sensors, devices and wires are being reshaped in order to fit in the textile, ultimately the research activities trend to manufacture active elements made of fibers, yarns and fabrics

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structures. The use of the smart system should not require any additional effort and their weight should not reduce operation time of the rescue worker. They are multifunctional textile systems that can be classified into three categories of passive smart textiles, active smart textiles and very smart textiles (Schwarz et al. 2010). The smart textiles have some of the capabilities such as biological and chemical sensing and responding, power and data transmission from wearable computers and polymeric batteries, transmitting and receiving radio frequency signals and automatic voice warning systems as to ‘dangers ahead’. These capabilities make the smart textiles technologies appropriate in military, mountain climbing, sports, healthcare and medicine, police, and firefighting applications. Smart textiles as a carrier of sensor systems can measure heart rate, temperature, respiration, gesture and many other body parameters that can provide useful information on the health status of a person. The smart textiles can support the rehabilitation process and react adequately on hazardous conditions that may have been detected. The reaction can consist of warning, prevention or active protection. After an event has happened, the smart textile is able to analyze the situation and to provide first aid. There are many research areas in integrated processes and products of smart textiles such as wearable electronics and photonics, adaptive and responsive structures, biomimetic, bioprocessing, tissue engineering and chemical /drug releasing. There are some areas that the research activities have reached the industrial application. Optical fibers, shape memory polymers, conductive polymers, textile fabrics and composites integrated with optical fiber have been already used in smart textiles. Shape memory polymers have been applied to textiles in fiber, film and foam forms, resulting in a range of high performance fabrics and garments, especially sea-going garments. Fiber sensors, which are capable of measuring temperature, strain/stress, gas, biological species and smell, are typical smart fibers that can be directly applied to textiles. Conductive polymer -based actuators have achieved very high levels of energy density. Clothing with its own senses and brain, like shoes and snow coats which are integrated with Global Positioning System (GPS) and mobile phone technology, can tell the position of the wearer and give him/her directions. Biological tissues and organs, like ears and noses, can be grown from textile scaffolds made from biodegradable fibers.

SMART TEXTILES CONTAINING PHASE CHANGE MATERIALS Phase change materials (PCMs) are thermal storage materials that are used to regulate temperature fluctuations. The thermal energy transfer occurs when a material changes from a solid to a liquid or from a liquid to a solid. This is called a change in

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state, or phase. Incorporating microcapsules of PCMs into textile structures improves the thermal performance of the textiles. PCMs store energy when they change from solid to liquid and dissipate it when they change back from liquid to solid. It would be most ideal, if the excess heat a person produces could be stored intermediately somewhere in the clothing system and then, according to the requirement, activated again when it starts to get chilly. The most widespread PCMs in textiles are paraffin-waxes with various phase change temperatures (melting and crystallization) depending on their carbon numbers. PCMs can be applied to fibers in a wet-spinning process, incorporated into foam or embedded into a binder and applied to fabric topically, or contained in a cell structure made of a textile reinforced synthetic material. In manufacturing the fiber, the selected PCM microcapsules are added to the liquid polymer or polymer solution, and the fiber is then expanded according to the conventional methods such as dry or wet spinning of polymer solutions and extrusion of polymer melts. Fabrics can be formed from the fibers containing PCMs by conventional weaving, knitting or nonwoven methods, and these fabrics can be applied to numerous clothing applications. In this method, the PCMs are permanently locked within the fibers, the fiber is processed with no need for variations in yarn spinning, fabric knitting or dyeing and properties of fabrics (drape, softness, tenacity, etc.) are not altered in comparison with fabrics made from conventional fibers. The microcapsules incorporated into the fibers in this method have an upper loading limit of 5–10% because the physical properties of the fibers begin to suffer above that limit, and the finest fiber available is about 2.2 dtex. Due to the small content of microcapsules within the fibers, their thermal capacity is rather modest, about 8–12 J/g. Usually PCM microcapsules are coated on the textile surface. Microcapsules are embedded in a coating compound such as acrylic, polyurethane and rubber latex, and applied to a fabric or foam. In lamination of foam containing PCMs onto a fabric, the selected PCMs microcapsules can be mixed into a polyurethane foam matrix, from which moisture is removed, and then the foam is laminated on a fabric (Pause 2001). Typical concentrations of PCMs range from 20% to 60% by weight. Microcapsules should be added to the liquid polymer or elastomer prior to hardening. After foaming (fabricated from polyurethane) microcapsules will be embedded within the base material matrix. The application of the foam pad is particularly recommended because a greater amount of microcapsules can be introduced into the smart textile. In spite of this, different PCMs can be used, giving a broader range of regulation temperatures. Additionally, microcapsules may be anisotropically distributed in the layer of foam. The foam pad with PCMs may be used as a lining in a variety of clothing such as gloves, shoes, hats and outerwear. Before incorporation into clothing or footwear the foam pad is usually attached to the fabric, knitted or woven, by any conventional means such as glue, fusion or lamination.

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The addition of PCMs to fabric-backed foam significantly increases the weight, thickness, stiffness, flammability, insulation value, and evaporative resistance value of the material. It is more effective to have one layer of PCM on the outside of a tightfitting, two layer ensemble than to have it as the inside layer. This may be because the PCMs closest to the body did not change phase. PCM protective garments should improve the comfort of workers as they go through these environmental step changes (e.g., warm to cold; cold to warm, etc.). For these applications, the PCM transition temperature should be set so that the PCMs are in the liquid phase when worn in the warm environment and in the solid phase in the cold environment (Mattila 2006). The effect of PCMs in clothing on the physiological and subjective thermal responses of people would probably be maximized if the wearer was repeatedly going through temperature transients (i.e., going back and forth between a warm and cold environment) or intermittently touching hot or cold objects with PCM gloves (Mattila 2006). The PCM microcapsules are also applied to a fibrous substrate using a binder (e.g., acrylic resin). All common coating processes such as knife over roll, knife over air, screen-printing, gravure printing, dip coating may be adapted to apply the PCM microcapsules dispersed throughout a polymer binder to fabric. The conventional pad– mangle systems are also suitable for applying PCM microcapsules to fabrics. The formulation containing PCMs can be applied to the fabric by the direct nozzle spray technique. There are many thermal benefits of treating textile structures with PCM microcapsules such as cooling, insulation and thermo regulating effect. Without PCMs, the thermal insulation capacity of clothing depends on the thickness and the density of the fabric (passive insulation). The application of PCMs to a garment provides an active thermal insulation effect acting in addition to the passive thermal insulation effect of the garment system. The active thermal insulation of the PCM controls the heat flux through the garment layers and adjusts the heat flux to the thermal circumstances. The active thermal insulation effect of the PCM results in a substantial improvement of the garment’s thermo-physiological wearing comfort (Pause 2000). Intensity and duration of the PCM’s active thermal insulation effect depend mainly on the heat-storage capacity of the PCM microcapsules and their applied quantity. In order to ensure a suitable and durable effect of the PCM, it is necessary to apply proper PCM in sufficient quantity into the appropriate fibrous substrates of proper design. The PCM quantity applied to the active wear garment should be matched with the level of activity and the duration of the garment use. Furthermore, the garment construction needs to be designed in a way which assists the desired thermo-regulating effect (Mattila 2006). Thinner textiles with higher densities readily support the cooling process. In contrast, the use of thicker and less dense textile structures leads to a delayed and therefore more efficient heat release of the PCM. Further requirements on the textile substrate in a garment application include sufficient breathability, high flexibility, and mechanical stability.

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In order to determine a sufficient PCM quantity, the heat generated by the human body has to be taken into account carrying out strenuous activities under which the active wear garments are worn. The heat generated by the body needs to be entirely released through the garment layers into the environment. The necessary PCM quantity is determined according to the amount of heat which should be absorbed by the PCM to keep the heat balance equalized (Pause 2000). It is mostly not necessary to put PCM in all parts of the garment. Applying PCM microcapsules to the areas that provide problems from a thermal standpoint and thermo-regulating the heat flux through these areas is often enough. It is also advisable to use different PCM microcapsules in different quantities in distinct garment locations. There are many applications including apparel, home textiles and technical textiles for smart textiles containing PCMs (Keyan et al. 2012). PCMs are used in winter and summer clothing not only in high-quality outerwear and footwear, but also in the underwear, socks, gloves, helmets and bedding of world-wide brand leaders. Seat covers in cars and chairs in offices can consist of PCMs. In outdoor apparels, PCMs are being used in a variety of items such as smart jackets, vests, men’s and women’s hats and rainwear, outdoor active-wear jackets and jacket lining, golf shoes, trekking shoes, ski and snowboard gloves, ski boots and earmuffs. In protective garments, PCMs are being used in a variety of items such as fire fighters protective clothing, bullet proof fabrics, space suits, sailor suits and so on. The specified roles of PCMs in outdoor and protective smart textiles are the absorption of body heat surplus, insulation effect caused by heat emission of the PCM into the fibrous structure and thermo-regulating effect, which maintains the microclimate temperature nearly constant. A new generation of military fabrics feature PCMs which are able to absorb, store and release excess body heat when the body needs it resulting in less sweating and freezing, while the microclimate of the skin is influenced in a positive way and efficiency and performance are enhanced. In the medical textiles field, a blanket with PCM can be useful for gently and controllably reheating hypothermia patients. Also, using PCMs in bed covers regulates the micro climate of the patient. In domestic textiles, blinds and curtains with PCMs can be used for reduction of the heat flux through windows. In the summer months large amounts of heat penetrate the buildings through windows during the day. At night in the winter months the windows are the main source of thermal loss. Results of the test carried out by Pause (2000) on curtains containing PCM have indicated a 30% reduction of the heat flux in comparison to curtains without PCM. One example of practical application of PCM smart textile is cooling vest (TST Sweden Ab) (http: //www.tst-sweden.com/en). This is a comfort garment developed to prevent elevated body temperatures in people who work in hot environments or use extreme physical exertion. The cooling effect is obtained from the vest’s 21 PCM elements containing Glauber’s salt which start absorbing heat at a particular temperature

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(28ºC). Heat absorption from the body or from an external source continues until the elements have melted. After use the cooling vest has to be charged at room temperature (24ºC) or lower. When all the PCMs are solidified the cooling vest is ready for further use.

SMART TEXTILES CONTAINING SHAPE MEMORY MATERIALS Shape memory materials (SMMs) are able to ‘remember’ a shape, and return to it when stimulated, e.g., with temperature, magnetic field, electric field, pH -value and UV light (Lendlein & Kelch 2002; Okano & Kikuchi 1996; Koshizaki et al. 1992; Srinivasan & McFarland 2001; Otsuka & Wayman 1998). An example of natural shape memory textile material is cotton, which expands when exposed to humidity and shrinks back when dried. Such behavior has not been used for aesthetic effects because the changes, though physical, are in general not noticeable to the naked eye. The most common types of such SMMs are shape memory alloys and polymers, but ceramics and gels have also been developed. Commercialized shape memory products have been based mainly on metallic shape memory alloys (SMAs), either taking advantage of the shape change due the shape memory effect or the super-elasticity of the material, the two main phenomena of SMAs. Shape memory polymers (SMPs) and shape memory gels are developed at a quick rate. There are many potential applications of SMPs in industrial components like automotive parts, building and construction products, intelligent packing, implantable medical devices, sensors and actuators, etc. SMPs are used in toys, handgrips of spoons, toothbrushes, razors and kitchen knives, also as an automatic choking device in smallsize engines (Wang et al. 1998). One of the most well-known examples of SMP is a clothing application, a membrane called Diaplex. The membrane is based on polyurethane based SMPs developed by Mitsubishi Heavy Industries. Polyurethane is an example of SMPs which is based on the formation of a physical cross-linked network as a result of entanglements of the high molecular weight linear chains, and on the transition from the glassy state to the rubber -elastic state (Mather et al. 2000). Shape memory polyurethane (SMPU) is a class of polyurethane that is different from conventional polyurethane in that these have a segmented structure and a wide range of glass transition temperature (Tg). The long polymer chain s entangle each other and a three-dimensional network is formed. The polymer network keeps the original shape even above Tg in the absence of stress. Under stress, the shape is deformed and the deformed shape is fixed when cooled below Tg. Above the glass transition temperature polymers show rubberlike behavior. The material softens abruptly above the glass transition temperature Tg. If the chains are stretched quickly in this state and the material is rapidly cooled down again

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below the glass transition temperature the polynorbornene chains can neither slip over each other rapidly enough nor become disentangled. It is possible to freeze the induced elastic stress within the material by rapid cooling. The shape can be changed at will. In the glassy state the strain is frozen and the deformed shape is fixed. The decrease in the mobility of polymer chains in the glassy state maintains the transient shape in polynorbornene. The recovery of the material’s original shape can be observed by heating again to a temperature above Tg. This occurs because of the thermally induced shapememory effect (Mather et al. 2000). The disadvantage of this polymer is the difficulty of processing because of its high molecular weight (Otsuka & Wayman 1998). Some of the other SMPs such as polynorbornene, Polyethylene/nylon-6 graft copolymer, styrene -1, 4-butadiene block copolymer, ethylene oxide -ethylene terephthalate block copolymer, polymethylene-1 and 3-cyclopentane) polyethylene block copolymers are suitable for textiles applications. SMPs can be laminated, coated, foamed, and even straight converted to fibers. There are many possible end uses of these smart textiles. The smart fiber made from the shape memory polymer can be applied as stents, and screws for holding bones together. Shape memory polymer coated or laminated materials can improve the thermophysiological comfort of surgical protective garments, bedding and incontinence products because of their temperature adaptive moisture management features. Films of SMPs can be incorporated in multilayer garments, such as those that are often used in the protective clothing or leisurewear industry. The SMPs reverts within wide range temperatures. This offers great promise for making clothing with adaptable features. Using a composite film of SMPs as an inter-liner in multilayer garments, outdoor clothing could have adaptable thermal insulation and be used as protective clothing. A SMP membrane and insulation materials keep the wearer warm. Molecular pores open and close in response to air or water temperature to increase or minimize heat loss. Apparel could be made with shape memory fiber. Forming the shape at a high temperature provides creases and pleats in such apparel as slacks and skirts. Other applications include fishing yarn, shirt neck bands, cap edges, casual clothing and sportswear. Also, using a composite film of SMPs as an interlining provides apparel systems with variable tog values to protect against a variety of weather conditions.

SMART TEXTILES CONTAINING CHROMIC MATERIALS Chromic materials are the general term referring to materials, which their color changes by the external stimulus. Due to color changing properties, chromic materials are also called chameleon materials. This color changing phenomenon is caused by the external stimulus and chromic materials can be classified depending on the external

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stimulus of induction. Photochromic, thermochromic, electrochromic, piezochromic, solvatechromic and carsolchromic are chromic materials that change their color by the external stimulus of heat, electricity, pressure, liquid and an electron beam, respectively (Mattila 2006). Most photochromic materials are based on organic materials or silver particles. Thermochromic materials change color reversibly with changes in temperature. The liquid crystal type and the molecular rearrangement type are thermochromic systems in textiles. The thermochromic materials can be made as semi-conductor compounds, from liquid crystals or metal compounds. The change in color occurs at a pre-determined temperature, which can be varied. Electrochromic materials are capable of changing their optical properties (transmittance and/or reflectance) under applied electric potentials. The variation of the optical properties is caused by insertion /extraction of cations in the electrochromic film. Piezochromism is the phenomenon where crystals undergo a major change of color due to mechanical grinding. The induced color reverts to the original color when the fractured crystals are kept in the dark or dissolved in an organic solvent (Mattila 2006). Solvatechromism is the phenomenon, where color changes when it makes contact with a solvent or liquid. Materials that respond to water by changing color are also called hydrochromic and smart textiles containing this kind of materials can be used, e.g., for swimsuits. The majority of applications for chromic materials in the textile sector today are in the fashion and design area, in leisure and sports garments. In workwear and the furnishing sector a variety of studies and investigations are in the process by industrial companies, universities and research centers. Chromic materials are one of the challenging material groups when thinking about the future textiles. Color changing textiles are interesting, not only in fashion, where color changing phenomena will exploit for fun all the rainbow colors, but also in useful and significant applications in soldier and weapons camouflage, workwear and in technical and medical textiles. The combination of SMMs and thermochromic coating is an interesting area which produces shape and color changes of the textile material at the same time Mattila 2006).

SMART TEXTILES CONTAINING OPTICAL FIBERS Optical fibers are currently being used in textile structures for several different applications. Optic sensors are attracting considerable interest for a number of sensing applications (Kersey et al. 1993; Xu et al. 1993). There is great interest in the multiplexed sensing of smart structures and materials, particularly for the real-time evaluation of physical measurements (e.g., temperature, strain) at critical monitoring points. One of the applications of the optical fiber s in textile structures is to create flexible textile-based displays based on fabrics made of optical fibers and classic yarns (Deflin & Koncar 2002). The screen matrix is created during weaving or knitting, using the texture of the

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fabric. Integrated into the system is a small electronics interface that controls the LEDs that light groups of fibers. Each group provides light to one given area of the matrix. Specific control of the LEDs then enables various patterns to be displayed in a static or dynamic manner. This flexible textile-based displays are very thin size and ultralightweight. This leads one to believe that such a device could quickly enable innovative solutions for numerous applications. Bending in optical fibers is a major concern since this causes signal attenuation at bending points. Integrating optical fibers into a woven perform requires bending because of the crimping that occurs as a result of weave interlacing. However, standard plastic optical fiber (POF) materials like polymethylmethacrylate, polycarbonate and polystyrene are rather stiff compared to standard textile fibers and therefore their integration into textiles usually leads to stiffen of the woven fabric and the textile touch is getting lost (Markus et al. 2008). Alternative fibers with appropriate flexibility and transparency are not commercially available yet.

CONDUCTIVE MATERIALS IN SMART TEXTILES Several conductive materials are in use in smart textiles. Conductive textiles include electrically conductive fibers, yarns, fabrics and articles made from them (Marchini 1991). Flexible electrically conducting and semi-conducting materials, such as conductive polymers, conductive fibers, threads, yarns, coatings and ink are playing an important role in realizing lightweight, wireless and wearable interactive electronic textiles. Generally, conductive fibers can be divided into two categories such as naturally conductive fibers and treated conductive fibers. Naturally conductive fibers can be produced purely from inherently conductive materials, such as metals, metal alloys, carbon sources, and conjugated polymers (ICPs). Highly conductive flexible textiles can be prepared by weaving or knitting thin wires of various metals. These textiles have been developed for higher degrees of conductivity. Metal conductive fibers are very thin filaments with diameters ranging from 1 to 80 μm produced from conductive metals such as ferrous alloys, nickel, stainless steel, titanium, aluminum and copper. Since they are different from polymeric fibers, they may be hard to process and have problems of long term stability. These highly conductive fibers are expensive, brittle, heavier and lower processability than most textile fibers. Treated conductive fibers can be produced by the combination of two or more materials, such as non-conductive and conductive materials. The conductive textiles can be produced in various ways, such as by impregnating textile substrates with conductive carbon or metal powders, patterned printing, and so forth. Conducting polymers, such as polyacetylene (PA), polypyrrole (PPy), polythiophene (PTh) and polyaniline (PAn), offer

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an interesting alternative. Among them, PPy has been widely investigated owing to its easy preparation, good electrical conductivity, good environmental stability in ambient conditions and because it poses few toxicological problems (Omastova et al. 1997; Thiéblemont et al. 1995). PPy is formed by the oxidation of pyrrole or substituted pyrrole monomers. Electrical conductivity in PPy involves the movement of positively charged carriers or electrons along polymer chain s and the hopping of these carriers between chains. The conductivity of PPy can reach the range 102 S cm-1, which is next only to PA and PAn. With inherently versatile molecular structure s, PPys are capable of undergoing many interactions. The brittleness of PPy has limited the practical applications of it. The processability and mechanical properties of PPy can be improved by incorporating some polymers into PPy (Ruckenstein & Chen 1991; Truong et al. 1998). However, the incorporation of a sufficient amount of filler generally causes a significant deterioration in the mechanical properties of the conducting polymer, in order to exceed the percolation threshold of conductivity (Chen et al. 1991). Another route to overcoming this deficiency is by coating the conducting polymer on flexible textile substrates to obtain a smooth and uniform electrically conductive coating that is relatively stable and can be easily handled (Gregory et al. 1991; Heisey et al. 1993). The conductive fibers obtained through special treatments such as mixing, blending, or coating are also known as conductive polymer composites (CPCs), can have a combination of the electrical and mechanical properties of the treated materials (Markus et al. 2008). Polymer fibers may be coated with a conductive layer such as PPy, copper or gold (Tariq 2013). PPy-based composites may overcome the deficiency in the mechanical properties of PPy, without adversely affecting the excellent physical properties of the substrate material, such as its mechanical strength and flexibility. The resulting products combine the usefulness of a textile substrate with electrical properties that are similar to metals or semi-conductors. Fibers containing metal, metal oxides and metal salts are a proper alternative for metal fibers. The conductivity will be maintained as long as the layer is intact and adhering to the fiber. Chemical plating and dispersing metallic particles at a high concentration in a resin are two general methods of coating fibers with conductive metals. Fibers containing conductive carbon are produced with several methods such as loading the whole fibers with a high concentration of carbon, incorporating the carbon into the core of a sheath–core bicomponent fiber, incorporating the carbon into one component of a side–side or modified side–side bicomponent fiber, suffusing the carbon into the surface of a fiber. Nanoparticles such as carbon nanotubes can be added to the matrix for achieving conductivity. Conductive fibers can also be produced by coating fibers with metal salts such as copper sulfide and copper iodide. Metallic coatings produce highly conductive fibers; however adhesion and corrosion resistance can present problems. It is also possible to coat and impregnate conventional fibers with conductive

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polymers, or to produce fibers from conductive polymers alone or in blends with other polymers. Conductive fibers /yarns can be produced in filament or staple lengths and can be spun with traditional non-conductive fibers to create yarns that possess varying wearable electronics and photonics degrees of conductivity. Also, conductive yarns can be created by wrapping a nonconductive yarn with metallic copper, silver or gold foil and be used to produce electrically conductive textiles. Conductive threads can be sewn to develop smart electronic textiles. Through processes such as electrodeless plating, evaporative deposition, sputtering, coating with a conductive polymer, filling or loading fibers and carbonizing, a conductive coating can be applied to the surface of fibers, yarns or fabrics. Electrodeless plating produces a uniform conductive coating, but is expensive. Evaporative deposition can produce a wide range of thicknesses of coating for varying levels of conductivity. Sputtering can achieve a uniform coating with good adhesion. Textiles coated with a conductive polymer, such as PAn and PPy, are more conductive than metal and have good adhesion, but are difficult to process using conventional methods. Adding metals to traditional printing inks creates conductive inks that can be printed onto various substrates to create electrically active patterns. The printed circuits on flexible textiles result in improvements in durability, reliability and circuit speeds and in a reduction in the size of the circuits. The printed conductive textiles exhibit good electrical properties after printing and abrading. The inks withstand bending without losing conductivity. However, after twenty washing cycles, the conductivity decreases considerably. Therefore, in order to improve washability, a protective polyurethane layer is put on top of the printed samples, which resulted in the good conductivity of the fabrics, even after washing (Kazani et al. 2012). Currently, digital printing technologies promote the application of conductive inks on textiles. Electrically conductive textiles make it possible to produce interactive electronic textiles. There are many applications for conductive textiles. They can be used for communication and antennas, entertainment, health care, safety, homeland security, computation, thermal purposes, protective clothing, wearable electronics and fashion. The application of conductive smart textile in combination with electronic devices is very widespread. In location and positioning, they can be used for child monitoring, geriatric monitoring, integrated GPS (global positioning system) monitoring, livestock monitoring, asset tracking, etc. In infotainment, they can be used for integrated compact disc players, MP3 players, cell phones and pagers, electronic game panels, digital cameras, and video devices, etc. In health and biophysical monitoring, they can be used for cardiovascular monitoring, monitoring the vital signs of infants, monitoring clinical trials, health and fitness, home healthcare, hospitals, medical centers, assisted-living units, etc. They can be used for soldiers and personal support of them in the battlefield, space programs, protective textiles and public safety (firefighting, law enforcement ), automotive,

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exposure -indicating textiles, etc. They can be also used to show the environmental response such as color change, density change, heating change, etc. Fashion, gaming, residential interior design, commercial interior design and retail sites are other application of conductive smart textiles.

SMART TEXTILES CONTAINING ELECTRONIC DEVICES AND CONDUCTIVE MATERIALS The components of an electronic smart textile that provide several functions are sensors unit, network unit, processing unit, actuator unit and power unit. On the smart textile, several of these functions are combined to form services. Providing information, communication or assistance are possible services. Because mobility is now a fundamental aspect of many services and devices, these smart textiles can be used for health applications such as monitoring of vital signs of high-risk patients and elderly people, therapy and rehabilitee, knowledge applications such as instructions and navigation and entertainment applications such as audio and video devices (Tao 2005). In order to form flexible circuit boards, printing of circuit patterns is carried out on polymeric substrates such as films. Fabric based circuits potentially offer additional benefits of higher flexibility in bending and shear, higher tear resistance, as well as better fatigue resistance in case of repeated deformation. Different processes that have been described in literature for the fabrication of fabric based circuits include embroidery of conductive threads on fabric substrates, weaving and knitting of conductive threads along with nonconductive threads, printing or deposition and chemical patterning of conductive elements on textile substrates. The insulating fabric could be woven, non-woven, or knitted. The conductive threads can be embroidered in any shape on the insulating fabric irrespective of the constituent yarn path in a fabric. One of the primary disadvantages of embroidery as a means of circuit formation is that it does not allow formation of multi-layered circuits involving conductive threads traversing through different layers as is possible in the case of woven circuits. Conductive threads can be either woven or knitted into a fabric structure along with nonconductive threads to form an electrical circuit. One of the limitations of using weaving for making electrical circuits is that the conductive threads have to be placed at predetermined locations in the warp direction while forming the warp beam or from a creel during set up of the machine. Different kinds of conductive threads can be supplied in the weft or filling direction and inserted using the weft selectors provided on a

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weaving machine. Some modifications to the yarn supply system of the machine may be needed in order to process the conductive threads that are more rigid. In most conventional weft knitting machines, like a flatbed machine, the conductive threads can be knitted in the fabric only in one direction, i.e., the course (or cross) direction. In order to keep the conductive element in a knit structure straight, one can insert a conductive thread in the course direction such that the conductive thread is embedded into the fabric between two courses formed from non-conductive threads. Processes that have been employed to form a patterned conductive path on fabric surfaces include deposition of polymeric or nonpolymeric conducting materials and subsequent etching, reducing, or physical removal of the conductive materials from certain regions. Thus, the conductive material that is not removed forms a patterned electrical circuit or a region of higher conductivity. The biggest problem associated with patterning of circuits from thin conductive films (polymeric or metallic) deposited on fabric substrates is that use of an etching agent for forming a circuit pattern leads to nonuniform etching, as some of the etching liquid is absorbed by the threads of the underlying substrate fabric (Locher et al. 2002; DeAngelis et al.1995; Kuhn & Kimbrell 1987; Stoppa & Chiolerio 2014; Spoerry & Co AG, Swiss Shield® 2017; Scheibner et al. 2003; Ohmatex, Smart Textile Technology 2017; Hertleer 2004; Gough 2004; Eleksen Ltd. 2017; Pressure Profile Systems, Inc. 2017; Swallow and Thompson 2017; Viry et al. 2014). Another problem with deposition of conductive films on fabric substrates is that bending the fabric may lead to discontinuities in conductivity at certain points. There are different device attachment methods like raised wire connectors, solders, snap connectors, and ribbon cable connectors in electronic smart textiles. Soldering produces reliable electrical connections to conductive threads of an electronic textile fabric but has the disadvantage of not being compatible with several conductive threads or materials like stainless steel. Moreover, soldering of electronic devices to threads that are insulated is a more complex process involving an initial step of removal of insulation from the conductive threads in the regions where the device attachments are desired and insulation of the soldered region after completion of the soldering process. The main advantage of employing snap connectors is the ease of attachment or removal of electronic devices from these connectors, whereas the main disadvantages are the large size of the device and the weak physical connection formed between the snap connectors and the devices. Ribbon cable connectors employ insulation displacement in order to form an interconnection with insulated conductor elements integrated into the textiles. A v-shaped contact cuts through the insulation to form a connection to the conductor. Firstly, the ribbon cable connector is attached to the conductive threads in an e-textile fabric and subsequent electronic devices and printed circuit boards are attached to the ribbon cable connector. One of the advantages of employing ribbon cable connectors for device attachment is the ease of attachment and removal of the electronic devices to form the electronic textiles.

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SOME OTHER RESEARCHES AND APPLICATIONS OF SMART TEXTILES Each technique of smart textiles manufacturing process shows advantages and disadvantages and our aim in this section is to highlight some typical commercialized smart textile products (Stoppa & Chiolerio 2014). Swiss Shield® yarns that are turned into fabrics ensure an efficient shielding from electromagnet fields. Their degree of shielding, which can be modulated according to the type of application, ranges to over 80 dB. Swiss Shield® yarn is manufactured in various forms for home textiles, garments, industrial and military applications (Spoerry & Co AG, Swiss Shield® 2017). Numerous materials can be employed as the base for Swiss Shield® yarns, from cotton to artificial fibers such as polyester, polyamide, aramids and the like. The monofilament can be made from copper, bronze, silver, gold, aluminum, inox or other conductive materials. Swiss Shield® curtains prevents electro-smog from penetrating into interiors and act as an invisible protective shield that will effectively and safely protect the room from unwelcome external electromagnetic radiation (Spoerry & Co AG, Swiss Shield® 2017). The Textile Research Institute of Thuringia-Vogtland (TITV, Greiz, Germany) has produced conductive threads by coating a conventional Nylon 66 yarn with sliver layers, called ELITEX® with a specific conductivity of about 1.2 × 103 S·cm−1 (Scheibner et al. 2003). The Danish company of Ohmatex developed physiological and environmental sensors for inclusion in medical devices, sportswear, and protective clothing (Ohmatex, Smart Textile Technology 2017). They produce textile cables, connectors and sensors using conductive yarns. A Flemish consortium of universities and companies, among them the textile department of Ghent University, developed a prototype suit called Intellitex. It is a biomedical suit meant for the long term monitoring of heart rate and respiration of children at the hospital. To measure the ECG, a three-electrode configuration is used. Two measurement electrodes are placed on a horizontal line on the thorax, a third one, acting as a reference (Right Drive Leg, RDL), is placed on the lower part of the abdomen (Hertleer et al. 2004). Philips Research Laboratory (Redhill, UK), developed a stretch sensor integrated into a garment. The stretch sensor, which is produced out of conductive and elastic yarns knitted together, is based on the fact that the electrical resistance changes when stretching the sensing material. Thus, it can be used to control the volume of music or changing the track (Gough 2004). The British company Eleksen Limited, formerly Electrotextiles (Tunstall, UK), commercializes a soft and flexible textile based sensory fabric under the tradename ElekTex® Smart Fabric Interfaces. ElekTex is not composed of wires -instead it is composed of conductive fibers with other traditional textile fibers. This combination results into a durable, reasonably priced, washable and even wearable 3D structure (Eleksen Ltd. 2017). Many other researchers in research institutes and companies have developed various kinds of sensing systems for smart

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textiles (Pressure Profile Systems, Inc. 2017; Swallow, S.; Peta Thompson 2017; Viry et al. 2014; Benito-Lopez et al. 2009). Goose Design and product design agency PDD revealed a concept for a new cycling jacket that combined safety and fashion. The ILLUM jacket was based on emerging technology including printed electroluminescent ink and printed photovoltaic technology (solar power), 360 degrees around the body and cut into several ergonomic panels with red light at the back, white at the front and the photovoltaic source at the shoulders and top of the back, designed to be discreet once switched off (Consultancy Goose Design, “PDD Illum Project” 2017). Thermotron of UNITIKA (Osaka, Japan) is a particular fabric able to converts sun light into thermal energy while storing heat without wasting it. Inside the Thermotron there are micro particles of zirconium carbide which allow the fabric to absorb and filter sunlight. The inner layer of the fabric withholds the heat generated and prevents it from becoming lost, thus providing a salutary effect on the human body (Unitika LTD. 2017). This company has developed KOKAGE MAX advanced heat-shielding material that efficiently shields the wearer against heat, by blocking most visible and ultraviolet rays, radiation -shielding waterproof sheet and Saracool that efficiently shields the wearer against the heat-carrying (infrared) rays of the sun, keeping the wearer cool. It has excellent UV protection and see-through prevention properties. Researchers at Katholieke Universiteit Leuven and University Malaysia Perlis were the first to develop a fully textile waveguide antenna using a material inspired unit cell that is also used in composite right/left-handed transmission lines. The antenna is compact, robust and can be used for 2.45 and 5.4 GHz dual-band WLAN applications (50. Yan & Soh 2014). Patria (Halli, Finland) is a company with expertise in textile antenna design. It develops textile antennas composed by conventional or industrial fabrics, and typically conductive antenna parts are made out of modern conductive fibers (Patria. 2017). In this sector TexTrace AG (Frick, Switzerland) provides the manufacturing line as well as the components for industrial in-house production of woven RFID labels. Integrate RFID and the label will provide added value from garment manufacturing through logistics to sales and after-sales management (TEXTRACE 2017).

SMART TEXTILES IN HEALTH The continuous monitoring of vital signs of some patients and elderly people is an emerging concept of health care to provide assistance to patients as soon as possible either online or offline. A wearable smart textile can provide continuous remote monitoring of the health status of the patient. Wearable sensing systems will allow the user to perform everyday activities without discomfort. The simultaneous recording of

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vital signs would allow parameter extrapolation and inter-signal elaboration, contributing to the generation of alert messages and synoptic patient tables. In spite of this, a smart fabric is capable of recording body kinematic maps with no discomfort for several fields of application such as rehabilitation and sports (Pacelli et al. 2001). The working principles of these smart textiles and some solutions for integration of electronic components are summarized to show the current evolution toward ‘embedded everywhere’. This evolution will only be successful if the devices are unobtrusive and reliable for the user and for the diagnostician. The advances in electrodes for monitoring, textile sensors and textile-based actuators are key points in the development of highefficiency smart textiles for healthcare, health monitoring and medicine. The embedding electronics and conductive wires into fabric is a basic requirement to make smart textiles that can serve as platforms for a wide variety of applications in the field of healthcare. Wearable body sensor network based on smart textiles for healthcare applications is a great solution to enable the ubiquitous noninvasive health monitoring in people's daily life. However, developing smart textile-based body sensor networks poses significant technical challenges for the sensor and sensor network design such as miniaturization of sensors, imbedding sensors in noninvasive wearable structures, the integration of radio integrated circuits and modules with body-worn antennas, energy consumption minimization, and data security. The challenges of designing electronic textiles-based body sensor networks, strategies and solutions to overcome the problems of body sensor network have been the concern of many research activities.

SMART TEXTILES IN PERSONAL PROTECTIVE EQUIPMENT In the field of the technical textile industry, the personal protective equipment is a critical and certain requirement that has been an important field of research and innovation as well as business and technological development for many years. During the last decade, researches have focused on developing new personal equipment to protect workers, soldiers, firefighters and other professionals often exposed to hazardous situations. Getting real-time information about a person's health, his environment, or even the state of his protective equipment can be essential for both the worker and his team in order to make relevant strategic decisions. Smart textiles as a new and progressive technology have many potential applications in personal protective equipment including fire fighters protective clothing, bullet proof fabrics, space suits, sailor suits and so on. Smart textiles can be functionalized for their membrane and responsive permeability properties using SMPs, polymer gels, superabsorbent polymers, grafted polymer brushes, and polymeric ionic liquids. The smart textiles can be also developed to improve the thermal comfort using phase change materials that can provide additional heat or coolness as the need arises. Several types of textile monitoring sensors, including

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electrocardiographic, respiratory, temperature and humidity sensors are used in the personal protective equipment that can be developed using the smart materials and technologies. Currently, phase change materials are being used in a variety of outdoor apparel items such as smart jackets, vests, men’s and women’s hats and rainwear, outdoor active-wear jackets and jacket lining, golf shoes, trekking shoes, ski and snowboard gloves, boots, earmuffs and protective garments. In protective garments, the absorption of body heat surplus, insulation effect caused by heat emission of the PCM into the fibrous structure and thermo-regulating effect, which maintains the microclimate temperature nearly constant, are the specified functions of PCM contained smart textile. Phase change materials are used both in winter and summer clothing. PCM is used not only in highquality outerwear and footwear, but also in the underwear, socks, gloves, helmets and bedding of world-wide brand leaders. Seat covers in cars and chairs in offices can consist of phase change materials. The addition of PCMs to fabric-backed foam significantly increases the weight, thickness, stiffness, flammability, insulation value, and evaporative resistance value of the material. It is more effective to have one layer of PCM on the outside of a tightfitting, two layer ensemble than to have it as the inside layer. This may be because the PCMs closest to the body did not change phase. PCM protective garments should improve the comfort of workers as they go through these environmental step changes (e.g., warm to cold; cold to warm, etc.). For these applications, the PCM transition temperature should be set so that the PCMs are in the liquid phase when worn in the warm environment and in the solid phase in the cold environment (Mattila 2006). The effect of phase change materials in clothing on the physiological and subjective thermal responses of people would probably be maximized if the wearer was repeatedly going through temperature transients (i.e., going back and forth between a warm and cold environment) or intermittently touching hot or cold objects with PCM gloves (Mattila 2006). One example of practical application of PCM smart textile is cooling vest (TST Sweden Ab) (http: //www.tst-sweden.com/en 2017). This is a comfort garment developed to prevent elevated body temperatures in people who work in hot environments or use extreme physical exertion. The cooling effect is obtained from the vest’s 21 PCM elements containing Glauber’s salt which start absorbing heat at a particular temperature (28ºC). Heat absorption from the body or from an external source continues until the elements have melted. After use the cooling vest has to be charged at room temperature (24ºC) or lower. When all the PCMs are solidified the cooling vest is ready for further use (http: //www.tst-sweden.com/en 2017). A new generation of military fabrics feature PCMs which are able to absorb, store and release excess body heat when the body needs it resulting in less sweating and freezing, while the microclimate of the skin is influenced in a positive way and efficiency and performance are enhanced. In the medical textiles field, a blanket with PCM can be

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useful for gently and controllably reheating hypothermia patients. Also, using PCMs in bed covers regulates the micro climate of the patient. In domestic textiles, blinds and curtains with PCMs can be used for reduction of the heat flux through windows. In the summer months large amounts of heat penetrate the buildings through windows during the day. At night in the winter months the windows are the main source of thermal loss. Results of the test carried out by Pause (2001) on curtains containing PCM have indicated a 30% reduction of the heat flux in comparison to curtains without PCM.

SMART TEXTILES IN PERSONAL COMMUNICATION AND TEXTILE ANTENNAS A wearable smart textile system basically comprises of sensors to detect body or environmental parameters, a data processing unit to collect and process the obtained data, an actuator that can give a signal to the wearer, an energy supply that enables working of the entire system, interconnections that connect the different components and a communication device that establishes a wireless communication link with a nearby base station components that provide several functions (Ajmera et al. 2013). In recent years, communication, as a part of smart textile functionalities, has attracted the attentions of many researchers (Hertleer et al. 2008; Roh et al. 2010a; Roh et al. 2010b; Salvado et al. 2012; Sanjari et al. 2013; Sanjari et al. 2014; Sanjari et al. 2015a; Sanjari et al. 2015b). In practice, communication refers to the transfer of information. Generally, the majority of electronic components can be placed on the inner clothing layer. These components include various sensors, a central processing unit (CPU) and communication equipment. Analogous to ordinary clothing, additional heating to warming up a person in cold weather conditions is also associated with this layer. Thus, the inner layer is the most suitable for batteries and power regulating equipment, which are also sources of heat. The outer clothing layer contains sensors needed for environment measurements, positioning equipment that may need information from the surrounding environment and numerous other accessories. The physical surroundings of smart clothing components measure the environment and the virtual environment accessed by communication technologies. Soldier and weapons camouflage is possible by using chromic materials in outer layer of smart textiles. The data transfer requirements in the smart textiles can be divided into internal and external. The internal transfer services are divided into local health and security related measurements. Many of the services require or result in external communications between the smart textile and its environment. For communication between the different components of smart textile applications, both wired and wireless technologies are applicable. An applied solution for data transferring is often a compromise based on

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application requirements, operational environment, available and known technologies, and costs. Wired data transfer is in many cases a practical and straightforward solution. Thin wires routed through fabric are an inexpensive and high capacity medium for information and power transfer. The embedded wires inside clothing do not affect its appearance. However, wires form inflexible parts of clothing and the detaching and reconnecting of wires decrease user comfort and the usability of clothes (Rantanen et al. 2002). The cold winter environment especially stiffens the plastic shielding of wires. In hard usage and in cold weather conditions, cracking of wires also becomes a problem (Rantanen et al. 2002). The connections between the electrical components placed on different pieces of clothing are another challenge when using wires. During dressing and undressing, connectors should be attached or detached, decreasing the usability of clothing. Connectors should be easily fastened, resulting in the need for new connector technologies. A potential alternative to plastic shielded wires is to replace them with electrically conductive fibers. Conductive yarns twisted from fibers form a soft cable that naturally integrates in the clothing’s structure keeping the system as clothing-like as possible. Fiber yarns provide durable, flexible and washable solutions. Also lightweight optical fiber s are used in wearable applications, but their function has been closer to a sensor than a communication medium (Lind et al. 1997; Lee & Kwon 2001). The problem of conductive fibers is due to the reliable connections of them. Ordinary wires can be soldered directly to printed circuit boards, but the structure of the fiber yarn is more sensitive to breakage near the solder connections. Protection materials that prevent the movement of the fiber yarn at the interface of the hard solder and the soft yarn must be used. Optical fibers are commonly used for health monitoring applications and also for lighting purposes (http: //www.lumitex.com/medical -devices 2013). In practice, the transfer of information in the smart textiles can be divided into two categories: short-range and long-range. There are different ways to accomplish the first goal. Using electrical wires in cloths, infrared or Bluetooth technology and personal area network (PAN) are some of these techniques. From practical point of view, embedded wiring is very tiresome to the wearer. Low-power wireless connections provide increased flexibility and also enable external data transfer within the personal space. The infrared rays to work effectively require a direct connection between sender and receiver. But this would be difficult or impractical for devices located inside cloths. In view of the fact that this area needs further development and despite some disadvantages, it seems that Bluetooth technology and personal area network are more practical ways to connect two wearable devices on the user (Tao 2005). Also, existing and emerging WLAN and WPAN types of technologies are general purpose solutions for the external communications, providing both high speed transfer and low costs. For wider area

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communications and full mobility, cellular data networks are currently the only practical possibility. The antennas are used for long-range and wireless communication. Antennas are designed and produced in various types and geometries. Wire antennas (dipole antennas, loop antennas, helix antennas), aperture antennas, microstrip antennas, reflector antennas, lens antennas and array antennas are the most common types (Huang & Boyle 2008). It seems, just planar antennas are suitable for wearable applications. In this case, there is a possibility to hide the antenna beside of the fact that the comfort of wearer is not affected. Lamination, embroidery, weaving, knitting and printing are some of the methods which have been used to produce wearable antennas (Roh et al. 2010; Salonen et al. 2004; Scarpello et al. 2012; Yao & Qiu 2009). Roh et al. (2010) designed and produced wearable textile antenna with multiple resonance frequencies for the reception of FM signals using conductive embroidery of metal composite embroidery yarn on a polyester woven substrate. They attached the antenna to a jacket and evaluated the effect of body conditions on the antenna performance. Salonen et al. (2004) prepared a wearable WLAN antenna that made out of knitted copper and aracon fabric. They concluded that to avoid the adverse effects of discontinuities, the fabrics should be densely knitted. In addition, they acknowledged that the printed circuit board antennas can be replaced by textile antennas in wearable systems. Wearable antenna technology has evolved from simple rectangular patches to more complex topologies and uses a range of textile materials. Because of their planar structure, microstrip antennas, also called patch antennas, are more appropriate for embedding into garments. A microstrip antenna in its simplest configuration consists of a radiating patch on one side of a dielectric substrate (Electrical permittivity ≤ 10), which has a ground plane on the other side. The patch can assume virtually any shape, but conventional geometrical shapes are generally used to simplify analysis and performance prediction. These antennas have several advantages compared to conventional microwave antennas and therefore many applications over the broad frequency ranging from 100 MHz to 50 GHz are expected (Bahl & Bhartia 1980). Unlike rigid antennas, wearable antennas endure a lot of stresses during using or washing. These stresses can deform the shape of the antenna and consequently change its characteristics. Due to these challenges, some attempts have been made to provide an understanding of such situations (Sanjari et al. 2013; Sanjari et al. 2014; Sanjari et al. 2015a; Sanjari et al. 2015b; Bai & Langley 2012; Kaija et al. 2010). Compressive stresses are one of the most probable stresses which would be applied to the cloths. In compression, because the thickness of substrate is much greater than the thickness of patch and ground, changing to its dimensions is more effective on the shift of resonance frequency. Sanjari et al. (2013; 2014; 2015a; 2015b) simulated the deformation caused in antenna under uniaxial compressive loading and compared the results of simulation with those of experimental measurements. They showed a good agreement between analytical predictions, full wave simulations and experimental measurements.

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SMART TEXTILES FUTURE TRENDS Smart textiles and wearable electronic devices are becoming increasingly popular, as they offer significant enhancements to human comfort, health and well-being. The development of high value-added products such as smart fabrics, wearable consumer and medical devices and protective textiles has increased rapidly in the last decade. Recent advances in stimuli-responsive surfaces and interfaces, sensors and actuators, flexible electronics, nanocoatings and conductive nanomaterials has led to the development of a new generation of smart and adaptive electronic fibers, yarns and fabrics for application in E-textiles. However, improvements in sensors, flexible & printable electronics and energy devices are necessary for wider implementation and nanomaterials and/or their hybrids are enabling the next phase convergence of textiles, electronics and informatics. They are opening the way for the integration of electronic components and sensors (e.g., heat and humidity) in high strength, flexible and electrically conductive textiles with energy storage and harvesting capabilities, biological functions, antimicrobial properties, and many other new functionalities. Nanomaterials such as carbon nanotubes, silver nanowires graphene and other 2D materials are viewed as key materials for the development of wearable electronics for implementation in healthcare and fitness monitoring, electronic devices incorporated into clothing and smart skin’applications (printed graphene-based sensors integrated with other 2D materials for physiological monitoring). These materials are naturally more suitable for integration with flexible, soft or glass substrates owing to their two dimensional nature and can potentially offer the electronic performance needed for lowpower systems. Unlike today’s ‘wearables’ tomorrow’s devices will be fully integrated into the garment through the use of conductive fibers, multilayer 3D printed structures and two dimensional materials such as graphene. Products utilizing two dimensional materials such as graphene inks will be integral to the growth of wearables. The market for wearables using smart textiles is forecast to grow rapidly in next decades. Smart textiles in the military and protection sector accounted for the largest share in comparison with other segments such as sports & fitness, medical & healthcare, home & lifestyle, industrial and fashion. Major drivers identified for the growth of the smart textiles market are growing trends in the wearable electronics market, increasing popularity of sophisticated gadgets with advanced functions, miniaturization of electronic components, and rapid growth of low-cost smart wireless sensor networks. There are also some factors such as high costs, lack of exhaustive standards and regulations which are restraining the growth of the market.

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REFERENCES Ajmera, N., Dash, S. P. and Meena, C. R., (2013). Smart Textile, www.fibre2fashion.com. Bahl, I. J. and Bhartia, P. (1980). Microstrip Antennas. United Kingdom: Artech House. Bai, Q. and Langley, R. (2012). Crumpling of PIFA Textile Antenna. IEEE Transactions on Antennas and Propagation, 60(1), 63-70. Benito-Lopez, F., Coyle, S., Byrne, R., Smeaton, A., O'Connor, N. and Diamond, D., (2009). Pump Less Wearable Microfluidic Device for Real Time pH Sweat Monitoring. Proceeding of the Eurosensors XXIII Conference, Lucerne, Switzerland, 6–9 September 2009. Chen, Y. P., Qian, R. Y., Li, G. and Li, Y., (1991). Morphological and mechanical behaviour of an in situ polymerized polypyrrole/Nylon 66 composite film, Polym. Commun. 32(6), 189–192. Consultancy Goose Design, “PDD Illum Project.” Available online: http: //www.goose. london/ (accessed on 26 April 2017). Culshaw, B., (1996). Smart Structures and Materials, Artech House, USA. DeAngelis, A. R., Child, A. D. and D. E. Green, (1995). Patterned conductive textiles, US Patent 5624736. Deflin, E. and Koncar, V., (2002). For communicating clothing: The flexible display of glass fiber fabrics is reality, Second International Avantex Symposium, Frankfurt, Germany. Eleksen Ltd. Available online: http: //www.eleksen.com (accessed on 26 April 2017). Gough, P., (2004). Electronics and Clothes: Watt to Wear? Proceeding of Wearable Electronic and Smart Textiles, Leeds, UK, 11 June 2004. Gregory, R. V., Kimbrell, W. C. and Huhn, H. H., (1991). Electrically conductive nonmetallic textile coatings, J. Coated Fabrics, 20(1), 167–175. Heisey, C. L., Wightman, J. P., Pittman, E. H. and Kuhn, H. H., (1993). Surface and adhesion properties of polypyrrole-coated textiles, Textile Res. J., 63(5), 247–256. Henri Bouas-Laurent and Heinz Dürr, (2001). Organic Photochromism, IUPAC Technical Report, Pure Appl. Chem., Vol 73, no. 4, pp 639–665. Hertleer, C., Grabowska, M., Van Langenhove, L., Catrysse, M., Hermans, B., Puers, R., Kalmar, A., Van Egmond, H. and Matthys, D., (2004). Toward a Smart Suit. Proceeding of Wearable Electronic and Smart Textiles, Leeds, UK, 11 June 2004. Hertleer, C., Tronquo, A., Rogier, H., and Langenhove, L. (2008). The Use of Textile Materials to Design Wearable Microstrip Patch Antennas. Textile Research Journal, 78(8), 651-658. http: //www.lumitex.com/medical -devices (accessed on 22 June 2013). http: //www.tst-sweden.com/en (accessed on 26 April 2017).

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Huang, Y. and Boyle, K. (2008). Antennas: From Theory to Practice. USA: John Wiley & Sons. Ibrahim H. Mondal, (2014). Textiles: History, Properties and Performance and Applications, Nova Science Publishers, INc. New York, ISBN 9/3/85339/496 (eBook). Kaija, T., Lilja, J. and Salonen, P. (2010). Exposing Textile Antennas for Harsh Environment. Paper presented at the the 2010 Military Communications Conference Unclassified Program - Waveforms and Signal Processing Track, San Jose, California, USA. Kazani, I., Hertleer, C., De Mey, G., Schwarz, A., Guxho, G. and Van Langenhove, L., (2012). Electrical Conductive Textiles Obtained by Screen Printing. Fibres and Textiles in Eastern Europe, 20, 1(90) 57-63. Kersey, A. D., Davis, M. A. and Morey, W. W., (1993). Quasi-distributed Bragg-grating fiber -laser sensor, Proceedings OFS ’9, Florence Italy, postdeadline paper PD-5. Keyan K., Ramachandran, T., Shumugasundaram, O. L., Balasubramaniam, M. and Ragavendra, T., (2012). Microencapsulation of PCMs in Textiles: a review, Journal of Textile and Apparel, Technology and Management, Volume 7, Issue 3, 1-10. Koshizaki, N., Yasumoto, K., Yano, S. and Yoshida, H., (1992). Intelligent Functionalities of Composite Materials, Bulletin of Industrial Research Institute, No 127, 99–128. Kuhn, H. H. and Kimbrell Jr., W. C., (1987). Electrically conductive textile materials and method for making same, US Patent 4803096. Lee, K. and Kwon, D., (2001). Wearable master device using optical fiber curvature sensors for the disabled, International Conference on Robotics & Automation, Seoul, Korea, 892–896. Lendlein, A. and Kelch, S., (2002). Shape-memory polymers, Angew. Chem. Int., 41, 2034–2057. Lind, E. J., Jayaraman, S., Eisler, R. and McKee, T., (1997). A sensate liner for personnel monitoring applications, 1st International Symposium on Wearable Computers (ISWC), Cambridge, MA, USA, 98–105. Locher, I., Kirstein, T. and Tröster, G., (2002). Electronic Textiles, Proc. of ICEWES Conference, Cottbus, Germany, 1-14. Marchini, F., (1991). Advanced applications of metalized fibers for electrostatic discharge and radiation shielding, J. Coated Fabrics, 20, 153–166. Mather, P. T., Jeon, H. G. and Haddad, T. S., (2000). Strain recovery in POSS hybrid thermoplastic, Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem., 41, 528–529. Mattila, H., (2006). Intelligent textiles and Clothing, Woodhead Publishing Limited, England, ISBN 1 84569 005 2. Ohmatex, Smart Textile Technology. Available online: http: //www.ohmatex.dk (accessed on 26 April 2017).

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Okano, T. and Kikuchi, A., (1996). Intelligent biointerface: remote control for hydrophilic hydrophobic property of the material surface s by temperature, Proceeding of the third international conference on intelligent materials, Third European conference on smart structures and materials, Lyon, France, edited by Gobin P F and Tatibouët J, 34–41. Omastova, M., Pavlinec, J., Pionteck, J. and Simon, F., (1997). Synthesis, electrical properties and stability of polypyrrole-containing conducting polymer composites, Polym. Int., 43(2), 109–116. Otsuka, K. and Wayman, C. M., (1998). Shape memory materials, Cambridge University Press, 203–219, ISBN: 052144487. Pacelli, M., Paradiso, R., Anerdi, G., Ceccarini, S., Ghignoli, M., Lorussi, F., Scilingo, E. P., De Rossi, D., Gemignani, A. and Ghelarducci, B., (2001). Sensing threads and fabrics for monitoring body kinematic and vital signs, Proceedings of Fibers and Textiles for the Future Conference, Tampere, Finland, 55–63. Patria. Available online: http: //www.patria.fi/ (accessed on 26 April 2017). Pause, B., (2000). Tailored to the purpose: Computer-optimized development of thermoregulated active wear. Lecture No. 333. International Avantex-symposium, Frankfurt, Germany, 8 p. Pause, B., (2001). New possibilities in medicine: Textiles treated with PCM microcapsules. Lecture No. 627, 10th International Symposium for Technical Textiles, Nonwovens and Textile Reinforced Materials, 7 p. Pause, B., (2001). Possibilities for air-conditioning buildings with Phase Change Material, Technical Tex. Int., 44 (1), 38–40. Pressure Profile Systems, Inc. Available online: http: //www.pressureprofile.com/ presStrip.php (accessed on 26 April 2017). Rantanen, J., Impiö, J., Karinsalo, T., Malmivaara, M., Reho, A., Tasanen, M., and Vanhala, J., (2002). Smart clothing prototype for the arctic environment, Personal and Ubiquitous Computing, 6(1), 3–16. Roh, J. S., Chi, Y. S. and Kang, T. J. (2010a). Wearable textile antennas. International Journal of Fashion Design, Technology and Education, 3(3), 135-153. Roh, J. S., Chi, Y. S., Lee, J. H., Tak, Y., Nam, S. and Kang, T. J. (2010b). Embroidered wearable multiresonant folded dipole antenna for FM reception. IEEE Antennas and Wireless Propagation Letters, 9, 803-806. Rothmaier, M., Luong, M. P. and Clemens, F., (2008). Textile Pressure Sensor Made of Flexible Plastic Optical Fibers, Sensors, 8, 4318-4329. Ruckenstein, E. and Chen, J. H., (1991). Polypyrrole conductive composites prepared by coprecipitation, Polymer, 32(7), 1230–1235. Salonen, P., Rahmat-Samii, Y., Hurme, H. and Kivikoski, M. (2004). Effect of conductive material on wearable antenna performance: a case study of WLAN antennas. Paper

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In: Textiles: Advances in Research and Applications ISBN: 978-1-53612-855-0 Editor: Boris Mahltig © 2018 Nova Science Publishers, Inc.

Chapter 2

MICROWAVE ASSISTED PREPARATION FOR THE REALIZATION OF FUNCTIONAL AND COLORED TEXTILES Haoqian Miao1, Elena Schüll1, Kerstin Günther2 and Boris Mahltig1,* 1

Niederrhein University of Applied Sciences, Faculty of Textile and Clothing Technology, Mönchengladbach, Germany 2 Gesellschaft zur Förderung von Medizin-, Bio- und Umwelttechnologien, GMBU e.V., Jena, Germany

ABSTRACT The use and the benefit of microwave radiation in textile functionalization processes are presented. First, a general introduction into microwave radiation and microwave assisted processes is given. Following, examples from textile dyeing and UV-protective textiles are given. In the end, the realization of photoactive textiles by microwave assistant application of titania is presented. Altogether, it is shown, that microwave assisted procedures can be a powerful tool in textile functionalization methods.

Keywords: photoactivity, textile finishing, microwave synthesis, titania, dyeing, UV protection, fluorescence, polyester fiber

*

Corresponding Author Email: [email protected].

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INTRODUCTION Microwave heating is a process which directly introduces the thermal energy into the heated components and the reactive compounds. For this, microwave heating is in many cases more effective compared to conventional heating processes (Nimtz 1980). Especially in high pressure processes the microwave assisted heating can be used to reach short heating durations and altogether short process duration s, which can be 10 times faster compared to the analogous processes driven by conventional heating (Gedye et al. 1986; Strauss & Trainor 1995). In chemical synthesis, especially for pharma production, microwave assisted processes are well established (Larhed & Hallberg 2001; Wathey et al. 2002). However in the textile world, microwave processes are seldom used for synthesis purposes. Mainly reported is the use of microwave radiation for textile drying and fixation after dyeing or finishing is performed (Burkinshaw & Marshall 1986; Dittrich et al. 1992; Steiner 1982). For this, the aim of the actually presented chapter is to give the reader a short introduction into microwave technique and introduce him to some textile functionalization processes realized under microwave assistance. At first presented is a dyeing process adapted from a conventional HT-process to a microwave process. Here the microwave is used as heating source in the hydrothermal process. Following this procedure, UV protective textiles and fluorescence textiles are realized. Second, the preparation of photoactive textiles by microwave assistance is presented. Textiles are suitable substrates to carry photocatalytic materials as titania, which can be introduced for different applications as waste water cleaning or cleaning of air pollution (Böttcher et al. 2010; Dastjerdi & Montazer 2010). Two approaches are actually presented to produce photoactive titania onto textile surfaces. One approach aims on the microwave treatment of a titania containing finishing agent, which is a sol-gel based system. Photoactive properties can be introduced by this microwave treatment. The other microwave approach uses the microwave application as part of a hydrothermal process to promote the condensation of the titaniumorganic precursor to photoactive titania directly on the textile surface (Mahltig & Miao 2017). The water and acid necessary for the condensation are transported via the gas phase to the textile. The photoactivity is determined by different methods especially by decomposition of organic dye (AcidOrange7) under illumination with UV-light. The determined photocatalytic effect of the realized titania materials strongly depends on the applied process temperature but also on process duration. Significant photoactivity is reached even after short processes of only 5 minutes and moderate process temperatures around 120°C. Altogether, microwave assisted processes can be as well used for textile functionalization, especially if short process duration s and mild thermal conditions are requested.

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MICROWAVE TECHNOLOGY Microwaves belong to electromagnetic radiation. In the electromagnetic spectrum microwave radiation is placed between infrared light and radiowaves. Microwaves contain frequencies between 300MHz to 300 GHz – related to a wavelength in the range of 100 cm to 0.1 cm (Pozar 2009, Gerthsen & Vogel 1993). Microwave spectroscopy investigates the rotation of molecules with electromagnetic radiation of frequency in the range of 1 to 1000 GHz, which is related to wavelengths from submillimeter to centimeter range (Göpel & Ziegler 1994). Microwaves have a lot of utilizations in human society. In the communication field, microwave technology is extensively used for telecommunications. In the navigation field, Global Navigation Satellite Systems (GNSS) including the Chinese Beidou, the American Global Positioning System (GPS) and the Russian GLONASS broadcast navigational signals in various bands between about 1.2 GHz and 1.6 GHz. Further, radar uses microwave radiation to detect the range, speed, and other characteristics of moving objects (Bartlett 2013). Another important application common in daily life is the heating by microwave radiation (Chandrasekaran et al. 2013). Microwave heating is not only used in domestic but also industry, e.g., for drying purposes, in analytics, in production processes and in environmental applications (Sengutta 2009; Krämer 1999; Menendez et al. 2010; Jones et al. 2002). Microwave radiation is used during textile dyeing processes for drying and fixation purposes after application of the dye bath to the textile fabric. The process duration can be significantly decreased by use of microwave technique (Breuer 2011). The company Mageba (Bernkastel-Kues, Germany) offers microwave devices especially for the drying of ropes and narrow fabrics. The advantages of this drying method – especially in comparison to infrared drying – are the higher penetration depth of the microwave radiation into the rope and the selected heating of the water compared to the surrounding fiber material. By this, the drying is possible at lower temperatures, which is especially advantageous if the fiber material is thermal sensitive. Mainly the application for pre-drying processes is mentioned in patent literature (Stang 2004; Stang 2011). The laboratorial and industrial used microwave technology can be applied for acid digestion, ashing, food testing, drying, curing, compositional analysis, solvent extraction, and synthesis etc. (Camel 2000; Du et al. 2011; Kappe 2004; Kappe et al. 2012; Nakashima et al. 1988). Microwave synthesis can be as well used for fast preparation of silver nanoparticles (Mahltig et al. 2009). When the microwave is applying to heat up material, it is often named differently, e.g., dielectric heating, electronic heating, RF heating, high-frequency heating. These names all describe the same phenomenon that a high-frequency alternating electric field

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or microwave electromagnetic radiation heats a dielectric material (Mingos & Baghurst 1991; Osepchuk 1984). Molecular rotation and vibration occur in materials containing polar molecules having an electrical dipole moment, with the consequence that they will align themselves with an electromagnetic field (Fowler & Raynes 1981; Herman & Wallis 2016). If the field is oscillating, as it is in an electromagnetic wave or in a rapidly oscillating electric field, these molecules rotate continuously aligning with it (Bishop 1990). As the field alternates, the molecules reverse direction. Rotating molecules push, pull, and collide with other molecules (through electrical forces), distributing the energy to adjacent molecules and atoms in the material. Once distributed, this energy appears as heat (Lidström et al. 2001). Temperature is related to the average kinetic energy of the atoms or molecules in a material, so stimulating the molecules in this way increases the temperature of the material. Thus, dipole rotation is a mechanism by which energy in the form of electromagnetic radiation can raise the temperature of an object (Zlotorzynski 1995; Göpel & Ziegler 1994). Microwave chemistry is the science of applying microwave radiation to chemical reactions (Kappe et al. 2012). Microwaves act as high-frequency electric field s and will generally heat any material containing mobile electric charge s, such as polar molecules in a solvent or conducting ions. Semiconducting and conducting samples heat when ions or electrons within them form an electric current and energy is lost due to the electrical resistance of the material (Nimtz 1980).

Glass vessel (transparent to microwave energy)

Metal vessel (conducts heat)

Samplesolvent mixt ure

Samplesolvent mixt ure Microwave

Temperature gradient

Conventional heating process

Microwave heating process

Figure 1. Conventional heating process (left) versus microwave heating process (right). The temperature gradient in the conventional heating process leads to unevenness of the temperature of heated sample. The microwave can easily penetrates the whole sample, which heats the sample in different parts of the vessel evenly.

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Angle Clamp IntelliVent Pressure Control System (10ml vessel)

Pressure Control System for 80ml vessel Hot Keys

Number Keys Screen

Figure 2. Laboratory microwave system used for the actual presented investigations and experiments The Discover System (Model series: 908010, manufacturer: CEM co.).

Conventional heating usually involves the use of a furnace or bath, which heats the walls of the reactor by convection or conduction. The core of the sample takes much longer to achieve the required temperature (Kappe 2004). One of the advantages of microwave heating is the elimination of wall effects. Microwave heating is able to heat the target compounds without heating the entire furnace or bath, which save time and energy. Figure 1 shows a comparison between conventional heating process and microwave heating process. It is also able to heat sufficiently this objects throughout their volume. Microwave frequencies penetrate conductive materials, including semi-solid substances like meat and living tissue, to a distance defined by the skin effect. The penetration essentially stops where all the penetrating microwave energy has been converted to heat in the substances. In most laboratory situations, the penetration is considered to be thorough (Kappe 2004). In literature also an intensive and controversial discussion is presented on the fact that microwave radiation can influence chemical reactions with other effects additional to heating (Kuhnert 2002). These other effects are also named “non-thermal effect” and the background of them could be a direct interaction of the microwave radiation with the reacting molecule. This image is similar to photoreactions started and driven by illumination with UV-light (Wöhrle et al. 2012). However, after intensive investigations

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Kuhnert stated that there are obviously no so-called non-thermal effect in microwaveassisted reactions in organic chemistry (Kuhnert 2002). Different compounds covert microwave radiation to heat by different amounts. This selectivity allows some parts of the object being heated to heat more quickly or more slowly than others. A heterogeneous system, which comprising different substances or different phases may be heated anisotropic. As a result, it can be expected that the microwave field energy will be converted to heat by different amounts in different parts of the system (Schepps & Foster 1980). This inhomogeneous energy dissipation means that molecular scaled hot spots appear. At those hot spots, the microwave field in the heating device is especially strong, so the thermal effect at those spots is high (Coleman 1991; Kriegsmann 1997). To avoid so-called hot-spot effects, often sample vessels are rotated or moved in other way in the microwave field. Also the stirring of liquid components and solutions can help to avoid hot-spot effects. Another issue or better limitation of microwave heating is related to the volume of the heating subject. If the subject is too thick, the microwave radiation is not able to heat it up regularly (Metaxas & Meredith 1983). For the current investigations presented in the next sub-sections of this chapter a socalled focused microwave system available from CEM was used (see Figure 2). This device is presented in further literature more in detail (Sengutta & Meier 2002; Theis & Ritter 2011). The aim of this focused microwave device is to gain the same microwave intensity and by this the same heating rate in the whole volume of the reaction vessel. By this application the formation of hot-spots is avoided. The used reaction vessel has a total volume of 80 mL and can be filled up safely with 50 mL of liquid. This vessel can be also closed and be used as a kind of autoclave system with high pressure of several bar as process pressure for the microwave assisted synthesis. Such focused microwave heating system is also reported for the synthesis of azo disperse dyes, which can be afterwards used for dyeing of polyester (El-Apasery 2008).

OPTICAL MODIFICATION OF TEXTILES BY MICROWAVE PROCESSES In year 2010, Ahmed & El-Shishtawy describe in a review several new technologies for dyeing of textile materials (Ahmed & El-Shistawy 2010). One technology they present is the use of microwave processes. They give examples for natural fibers like cotton or flax dyed with reactive dyes but also for polyester fabrics dyed with dispersed dyes. Reactive dyes are applied on cotton fabrics in a conventional dyeing process and a dyeing process used microwave irradiation for heating the dye bath. The process temperature was set to 60°C and the duration of microwave heating is in between 5 to 20 minutes. Compared to the conventional dyeing process the reported microwave driven

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process saves 75% of dyeing time. The fastness according to washing, light, abrasion and perspiration is tested as well (Haggag et al. 2014). The same research group reports also on the dyeing of polyester with disperse dyes under microwave heating. The experiments are done with a household microwave and the maximum process temperature for water based systems is 100°C, because of the boiling point of water. To gain under this experimental condition s adequate dyeing results on polyester fabric, a carrier system is added to the dye bath (Haggag et al. 1995). Instead of using a carrier system, also the addition of organic solvents is reported to enable polyester dyeing with microwave heating at process temperatures of 100°C or less (Kim et al. 2003). For application of disperse dyes onto polyester fabrics, the use of microwave is also reported as a tool for the fixation (Chiao-Cheng & Reagan 1983). In this case the dispersed dyes are applied by a padding procedure and afterwards analogously to a thermosol process the microwave method is applied. Important for this application process is the presence of boiled water (Chiao-Cheng & Reagan 1983). The microwave for aftertreatment of pad-dyed fabrics is as well demonstrated for natural fabrics. The fixation of reactive dyes on wool with short fixation times of 30 to 60 seconds by microwave application is possible (Delaney & Seltzer 1972). For such processes an adequate heating chamber with steam is constructed (Kawaguchi 1979). Compared to these previous experiments and investigations, the actual study is related to polyester fabrics functionalized in a HT-process driven by a focused microwave-system -CEM Focused MicrowaveTM Synthesis System (Model Discover). This microwave device can work as closed system under high pressure and with process temperatures above 100°C. No addition of carriers or organic solvent is needed to gain adequate results of dyeing. The dyeing is performed with two conventional red dyes (Dianix Deep Red SF and Dianix Red AC-E01 from DyStar) and one fluorescent dye (Bemacron Luminous Red SEL-4B from CHT – Tübingen, Germany) (Bone et al. 2007; Avinc et al. 2009; Ferus-Comelo 2009). Further a functionalization is done by application of a disperse UV-absorber (Tanuval UVL from Tanatex, Netherlands ). The application of the microwave process is compared with a conventional dyeing process with same dyes and functional agents. This conventional dyeing is performed in a dyeing machine – Mathis Polycolor HT. The process temperature is set to 130°C and the process duration (including heating up and cooling down) is set to 1 h 50min. For the microwave process a discover system from CEM is used. This microwave apparatus is equipped with a dyeing vessel of maximal 80mL capacity, which is filled with 50mL of dyeing bath. The polyester fabric is placed together with a stirrer into this vessel as depicted in Figure 3. For the microwave process, temperatures from 80°C to 140°C are set with a process duration of 5 or 15 minutes. For heating and cooling 10 minutes or less are set. The microwave operates at the maximum power of 250 Watts. The process pressure is determined by the process temperature in the autoclave system. An example for relation

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Haoqian Miao, Elena Schüll, Kerstin Günther et al.

of process temperature and pressure as function of process duration is presented in Figure 4. In this example the process temperature is set to 140°C.

Figure 3. Schematic drawing of placement of textile sample in the reaction vessel containing the dye bath.

After all performed dyeing procedures, the treated polyester fabric is cleaned by a reductive process. This reductive cleaning is done to remove the not fixed dye, which is present on the fiber surface after treatment with the dye bath. For alkaline cleaning, the chemical Cyclanon Clear ARC in a concentration of 2g/L is applied at 70°C for 10 minutes. A final rinsing is performed with acetic acid. To determine the color properties of dyed fabrics, a photospectrometer from Shimadzu is used. By this photospectrometer, the diffuse reflection of light with wavelength in the range of 220nm to 800 is measured. This device also enables the calculation of the Kubelka-Munk function (KM-function) from the measured refection spectra. 160

5

temperature

hold time

140 4 120 3

100 80

2

pressure

60

process pressure [bar]

process temperature [C°]

run time

1 40 20

0 0

3

6

9

12

15

18

process duration [minutes]

Figure 4. Process temperature and pressure for a dyeing process at 140°C driven in the microwave system. The hold time is set to 15 minutes.

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COLOR APPLICATION For color application under microwave assistance the dyeing of polyester fabrics with two disperse dyes - Dianix Deep Red SF and Dianix Red AC-E01 – is used. These dyes are applied in microwave process with different process temperatures and two different durations 5 minutes or 15 minutes. For comparison, both dyes are applied with a conventional process driven at 130°C. The reflection spectra of resulted dyed polyester fabrics with Dianix Deep Red SF are presented in Figure 5. It is clearly seen, that the color intensity increased with increasing process temperature and process duration of the microwave process. A color intensity similar to the one reached with the conventional process is gained for microwave processes driven at 130°C or 140°C. These temperatures are obviously needed to open up the polyester structure, so the disperse dye can diffuse into the polymer matrix of the polyester fiber. By a first view on the reflection spectra, it is clear, with the microwave assisted HT-process the dyeing procedure can be speed up to few minutes, while the conventional procedure takes more than one hour. Nevertheless, the minimum at the reflection spectra are of high intensity and closed together. For this, it is quite difficult to distinguish the color intensity of these strong colored samples. To enable here a further data evaluation, the minimum of reflection spectra and the maximum of the related KM-function are recorded as function of process temperatures (Figures 6 and 7). 60

60

microwave process time 5min

50

40

reflection [%]

reflection [%]

50

microwave process time 15 min

30

80°C 20

40

30

80°C 20

100°C

100°C

10

130°C

10

conventional

conventional

140°C

0 300

400

500

600

wavelength [nm]

700

800

130°C; 140°C

0 300

400

500

600

700

800

wavelength [nm]

Figure 5. Spectra of diffuse reflection of polyester fabrics after dyeing with Dianix Deep Red SF. The dyeing is performed under microwave assistance at different process temperatures of 80°C, 100°C, 130°C or 140°C. The process duration is 5 or 15 minutes. For comparison a reflection spectrum of a conventional dyed polyester fabric is shown.

It is clearly seen in Figure 6, the reflection minimum which is an indicator for the color intensity is decreasing as function of the used process temperature of the microwave process. With the highest microwave process temperatures applied for 15

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minutes similar or even stronger color intensities compared to the conventional process can be reached. With the dye Dianix Red AC-E01 even with 100°C process temperature in the microwave same color intensities as with the conventional dyeing are reached. For this it should be stated, that not only the process parameter but also the type of dye has a certain influence on the dyeing result. Probably, there are dyes with higher affinity for microwave processes. dye1; 5min dye1; 15min dye2; 5min dye2; 15min

18

reflection at minimum [%]

16 14 12 10 8 6

dye 2; conventional

4 dye 1; conventional

2 0 80

90

100

110

120

130

140

process temperature [°C]

Figure 6. The reflection at minimum for dyed polyester fabrics is given as function of process temperature of microwave process. The dyeing is performed with Dianix Deep Red SF (dye1) and Dianix Red AC-E01 (dye2). The duration of microwave process is set to 5 or 15minutes. The reflection values gained after analogous conventional dyeing are shown as reference lines. 24

KM-function [a.u.]

20

16 dye 1; conventional

12

dye 2; conventional

8

dye1; 5min dye1; 15min dye2; 5min dye2; 15min

4

0 80

90

100

110

120

130

140

process temperature [°C]

Figure 7. The maximum of KM-function for dyed polyester fabrics is given as function of process temperature of microwave process. The dyeing is performed with Dianix Deep Red SF (dye1) and Dianix Red AC-E01 (dye2). The duration of microwave process is set to 5 or 15 minutes. The KMfunction values gained after analogous conventional dyeing are shown as reference lines.

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The evaluation of the KM-function leads to similar results (Figure 7). Here the color intensity is directly correlated to the KM-function which increases with the process temperature of the microwave process. By evaluation of the KM-values, the differences between the both process duration s of 5 minutes or 15 minutes are more obviously. It is clearly seen, that with the longer duration of 15 minutes the reached coloration is of significant higher intensity. However, even this longer microwave process duration is significant shorter (less than 20%) compared to the analogous conventional dyeing process.

FLUORESCENCE EFFECTS AND UV-PROTECTION For application of fluorescent effects on polyester, the red fluorescent dye Bemacron Red SEL-4B is applied in the focused microwave device. The process temperature for the microwave process is set to 80°C to 140°C and the process duration is 5 or 15 minutes. For comparison an analogous HT-dyeing process at 130°C is performed. The dyeing results are investigated by measurements of diffuse reflection as depicted in Figure 8. It is clearly seen, that the microwave process driven at the same process temperature as the conventional process – 130°C – leads to a deeper coloration of the treated polyester fabric. By using the microwave process more intensive coloration with significantly shorter process duration can be realized. However, lower process temperatures with 100°C or even 80°C are not suitable for microwave processes to compete with the conventional dyeing process. A systematic evaluation of the dyeing results is given in Figure 9. In Figure 9 the reflection value at the minimum is depicted as function of microwave process temperature and shown for both process duration s. In this dyeing process, the process temperature is the most important parameter influencing the intensity of resulted coloration. Compared to the temperature, the influence of process duration is low. Especially for the lower investigated temperatures of 80°C and 100°C, there is no significant difference, if the microwave process is driven at 5 or 15 minutes. Probably a certain process temperature is needed to support a certain penetration of the dye into the fiber by diffusion process es. If this temperature is not reached, even a longer process duration cannot increase the dye up-take significantly. With the process temperatures of 130°C or 140°C, a longer process duration leads to a stronger coloration of the dyed polyester. In these cases of higher temperature, obviously an in principal possible diffusion is further promoted, so more dye molecules penetrate the polyester fiber during the microwave dyeing process.

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Figure 8. Spectra of diffuse reflection of polyester fabrics after dyeing with the fluorescent dye Bemacron Luminous Red SEL-4B. The dyeing is performed under microwave assistance at different process temperatures of 80°C, 100°C, 130°C or 140°C. The process duration is 15 minutes. For comparison a reflection spectrum of a conventional dyed polyester fabric is shown.

The amount of up-taken fluorescent dye is directly correlated with the strength of fluorescent effect of the treated polyester fabric. Altogether it can be concluded, that by microwave assisted process the polyester fabric can reach stronger fluorescent effect after significantly shorter process duration s. 45

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Figure 9. The reflection at minimum for dyed polyester fabrics is given as function of process temperature of microwave process. The dyeing is performed with fluorescent dye Bemacron Luminous Red SEL-4B. The duration of microwave process is set to 5 or 15minutes. The reflection value gained after analogous conventional dyeing is shown as reference line.

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The effectivity for UV-protection of a textile can be effectively determined by the spectrum of diffusive transmission for UV-light with a wavelength of 250 nm to 400 nm. A lower transmission value indicates obviously a better UV-protection (Mahltig et al. 2005). By application of an UV absorber, this transmission can be decreased to small values. However, some UV-absorbers are also optimized, that they do not absorb visible light (Rauch et al. 2004). In fact a complete transparence for visible light and a zero transmission for UV light is an ideal demand, which cannot be fulfilled by real compounds. For this, uncolored UV-absorbers do not lead to a perfect UV-protection for UV-light just below 400 nm. In microwave assisted processes, UV-absorbers can be applied on polyester fabrics to realize an UV-protective textile. These textile are evaluated by transmission spectroscopy, as shown in Figure 10. 35

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Figure 10. Spectra of diffusive transmission of polyester fabrics treated with the UV-absorber Tanuval under different process conditions in the microwave system. The microwave process is driven at 80°C, 100°C or 140°C for a duration of 5 or 15 minutes. As reference, a spectrum of untreated polyester fabric and a spectrum of a polyester fabric treated with Tanuval in a conventional process is given.

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It is clearly seen, that the application of the UV-absorber significantly decreases the transmission of the polyester fabric in the range of 300 to 400 nm. Below 300 nm, the transmission is even for the untreated polyester near 0%, probable because the aromatic structure of the polyester fabric. Also the addition of titaniumdioxide as white pigment could be a reason for a certain decrease in transmission for the untreated polyester reference (Bajaj et al. 2000). The application of the UV-absorber in a conventional HTprocess is done at 130°C process temperature and improve significantly the UVprotective properties of the polyester fabric (Figure 10). In Figure 10 this conventional functionalized fabric is compared to microwave-assisted functionalization at different process temperatures 80°C, 100°C and 140°C. The composition of the dye bath is the same for all applications. The microwave process driven at 80°C leads to a significant decrease in transmission for UV-light but the results are not that good as reached after the conventional application. Competitive or even better compared to the conventional application is the microwave application at 100°C and 140°C. The better results are realized for a microwave treatment of longer duration of 15 minutes. Probable even under microwave assisted application a certain amount of time is needed to force the UV absorber molecules into the matrix of the polyester fiber. Nevertheless, the microwave process with 100°C process temperature is competitive to the conventional process at 130°C. This is a significant lower process temperature, applied for only 25% of the conventional needed process duration. For further development it is here especially important, that with a process temperature of 100°C there is no need for using an autoclave system. For this, it can be a first starting point for a microwave driven non HTfunctionalization of polyester textiles.

PHOTOCATALYTIC MODIFICATION To give an appropriate introduction, following first a short general introduction into photoactivity is given. After this, in a further sub-section different microwave assisted preparations are presented, which enable the preparation of photoactive titania onto textile substrates. Beside preparation also some rough analytic characterizations of these titania materials are described.

Short Introduction to Photoactivity In general, photocatalysis can be defined as the acceleration of a photoreaction in the presence of a catalyst. This so-called photocatalyst is only working in the presence of

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light and absorbs light during the process. It can be included in the category of advanced oxidation process, if the photocatalyst is working for an oxidation process (Andreozzi et al. 1999). Often mentioned is the heterogeneous photocatalysis. In heterogeneous catalysis, the catalyst is placed in a different phase from the reactants. Mostly the catalyst is a solid material while the reactant is present in a liquid or in the gas phase, e.g., a dye stuff in waste water or a toxic gas in air. Heterogeneous photocatalysis is a discipline which includes a large variety of reactions, as e.g., mild or total oxidations, dehydrogenation, hydrogen transfer, water detoxification and gaseous pollution removal. Most common heterogeneous photocatalysts are semiconductors from transition metal oxides. Unlike the metals which have a continuum of electronic states, semiconductors possess a void energy region where no energy levels are available to promote recombination of an electron and hole produced by photoactivation in the solid. The void region, which extends from the top of the filled valence band to the bottom of the vacant conduction band, is as well called band gap (Linsebigler et al. 1995). This band gap is also named as energy gap. It is an energy range in a solid where no electron state s can exist (Brus 1986). The term "band gap" refers to the energy difference between the top of the valence band and the bottom of the conduction band (Hybertsen & Louie 1986). Electrons are able to jump from one band to another. However, in order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different semiconductor materials. Electrons can gain enough energy to jump to the conduction band by absorbing either a phonon (heat) or a photon (light ). A semiconductor will not absorb photons of energy less than the band gap. The energy of the formed electron-hole pair produced by a photon is equal to the bandgap energy (Weller 1967). One of the most prominent photocatalysts is titaniumdioxide TiO2 in the anatase crystalline type. This titania photocatalyst is activated by UV-light of 390 nm or lower wavelength according to the band gap of anatase (Mahltig et al. 2007). Altogether, mainly three different crystalline forms of the TiO2 exist which are rutile, anatase and brookite (Willmes 1993). Additionally titania can also occur in amorphous state. The band gap of the titania modifications are different, for anatase it is 3.20eV related to a wavelength of light 385nm, while for rutile it is 3.05eV related to a wavelength of 420nm (Veronovski et al. 2010). The photooxidation process with oxygen is well catalyzed by anatase but not with pure rutile, brookite or amorphous titania. For this, the preparation of photoactive titania materials is usually aimed on anatase and its modifications. One prominent commercially available photoactive titania product is P25 supplied by Degussa /Evonik. P25 is a combination of anatase and rutile (Ohno et al. 2001). The modification of photoactive properties of TiO2 by doping with noble metals,

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copper, iron, silica or manganese is as well possible and intensively reported (Sakthivel et al. 2004; Xin et al. 2005; Zaleska 2008; Mahltig & Textor 2008; Mahltig et al. 2011; Mahltig et al. 2013; Schmidt et al. 2012). In the environmental protection area, TiO2 has been a popular subject since years. Researchers mainly focus on the purification of air and water (Böttcher et al. 2010). Tan et al. reported the conversion of carbon dioxide into gaseous hydrocarbons using titanium dioxide in the presence of water (Tan et al. 2006). A form of solar water disinfection, which is supported titaniumdioxide photocatalyst was described by McCullagh (McCullagh et al. 2007). According to the supposed mechanism of photocatalysis by titanium dioxide, during the process, free radicals are generated. Because of the generation of the highly reactive free radicals, titaniumdioxide is used by many researchers as an effective antibacterial material (Brunet et al. 2009; Nonami et al. 2004). In the textile field, photoactive titania is used to realize self-cleaning and antimicrobial textiles (Bozzi et al. 2005; Meilert et al. 2005; Mahltig & Textor 2008; Mahltig & Haufe 2010)

Modification of Titania Sol-Gel Systems The idea of this approach is to prepare a sol-gel coating agent for textile functionalisation based on photoactive titania materials. Most titania sol-gel coating agents are containing amorphous titania nanoparticles forming after application onto textile substrate an as well amorphous titania coating. Such amorphous titania materials usually contain no or only small photoactive properties (Mahltig et al. 2007). If these coatings are thermal treated at temperatures of >400°C, a transfer to photoactive anatase type is possible. However, such temperatures would destruct the textile substrates. For this, several years ago a solvothermal treatment for titania sol-gel systems was developed which leads to formation of anatase already in the coating solution. By this, anatase coatings on textile materials without thermal aftertreatment are possible (Mahltig et al. 2007). According to this approach following two sol-gel systems are treated in a microwaveassisted solvothermal process. The sol system A is prepared as following: 56ml TTIP (TiO2 precursor -titaniumtetraisopropoxide) is added into 26ml ethanol. This solution is stirred for 30min. 13ml ethanol, 3ml distilled water and 1.2ml nitric acid are mixed under strong stirring and are added dropwise to the first TTIP containing solution. The dropping process has an approximately duration of two hours. This sol-gel system A is treated in the microwave at process temperatures between 100°C to 150°C, while the process pressures are between 11 bar to 18 bar (see Figure 11). The prepared samples exhibit a strong yellow coloration, with increasing color intensity as function of process temperature in the microwave process (Figure 12).

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Figure 11. Development of process temperature and process pressure at a microwave-assisted reaction with set for control temperature. The shown curves are recorded for a microwave process with a duration of 5 minutes driven with the titania sol-system A.

Figure 12. Microwave radiated titania sol system A. The sols which are treated under different temperature control points are distinguished by the boxes.

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Figure 13. The titania sol-gel system B is more cloudy when the temperature control point of microwave radiation was higher, samples prepared with process temperatures of 120°C and 140°C are totally opaque.

The second investigated titania sol-system B is developed according to the synthesis route of photoactive TiO2 from Qi et al. in Hong Kong Polytechnic University (Qi et al. 2007). The preparation of sol-system B is described as following. 50 ml distilled water and 2.14ml HNO3 are added into an Erlenmeyer flask, respectively. Afterwards 20ml acetic acid are added into the same container. Dilution of the as-prepared solution is performed with distilled water to 200ml. The resulting solution is further named as Solution A. Afterwards, a dropping funnel is employed to add TTIP into Solution A. When the first few drops go into the acidic solution, some white precipitate occurres immediately. After 24h stirring, all white precipitate is gone. The final solution is transparent with a hint of light yellow. These solutions are treated in the microwave device at process temperatures of 100°C to 140°C for 5 minutes. After this process the solutions turn into a white nontransparent appearance. This whiteness is increasing as function of process temperature (see Figure 13).

Vapor Phase Deposition of Photoactive Titania As a different preparation method for photoactive titania onto textile surfaces a kind of vapor phase deposition process is developed (Mahltig & Miao 2017). Roughly spoken, the textile substrate is impregnated by the TTIP precursor for the titania formation. The water and acid necessary for the hydrolysis and condensation of TTIP to titania are applied from vapor phase by using a microwave driven evaporation process. The developed process is described following in detail.

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Figure 14. Preparation of the textile substrate with the TTIP-precursor for titania synthesis in the microwave-assisted process. Two microscope slides are used to squeeze the extra TTIP precursor out of textile sample.

The scoured cotton fabric is cut into small size. The width of the fabric sample is equal to the width of microscope slide used as sample holder, but the length of fabric sample is one-third shorter than that (Figure 14). During the microwave radiation process, the TTIP treated fabric sample is laying on the slide, and the slide is standing inside an 80ml thick-wall vessel. The vessel will contain a small amount of solution (acid and water) in the bottom. In order to prevent the TTIP coated fabric sample from touching the surface of the solution, the samples need to be shorter than the slide (the bottom of the slide is immersing in the solution). The specific dimensions of the fabric samples are 55mm × 25mm. The preparation route of the used acidic solution is according to a report of Qi et al.. In their reported synthesis route, the TTIP is added dropwise into the acidic solution (Qi et al. 2007). However, the actual method is slightly different, because actually the precursor come in contact to the acidic solution by evaporation of the acidic medium. An acidic solution is prepared in the following route. Firstly, 30ml distilled water are added into a 150ml Erlenmeyer flask. Secondly, 10ml acetic acid and 1.07ml nitric acid (65%) are added into the same Erlenmeyer flask. Finally, the Erlenmeyer flask is filled until the volume of the acidic solution is 100ml. The microwave instrument which is employed here is a Discover System from CEM Corporation. The model 908010 (shown in Figure 2) is designed to enhance the ability to perform chemical reactions under controlled conditions on a laboratory scale. The impregnation of the textile fabric is done with diluted TTIP, in ratio of 1: 1 and 1: 3 with ethanol. For this impregnation, the fabric sample is laid on the microscope slide (supporting slide) evenly and 1ml precursor (dilute TTIP) is dropped slowly onto the fabric. Another microscope slide (covering slide) is laid on the top of the fabric and a 200g weight is set on the covering slide to press extra precursor out (Figure 14). The

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squeezing process takes only 5 seconds. A forceps is used to transfer this impregnated fabric with the supporting slide into the 80ml thick wall vessel which containing 3ml asprepared acidic solution (Figure 15). After placing the vessel into the cavity of the microwave device and closing the pressure detector, the microwave radiation process is ready to run. During the microwave radiation process, the fabric sample stays on the slide by the adhesive force of wet precursor. The fabric sample has no contact to the acidic solution which is on the bottom of the vessel. However, when the temperature of the system increased, the water molecules and acid molecules gain the thermal energy and escape from the liquid phase and enter the gas phase (evaporation ). When the acidic solution gasifies, the gas molecules have the opportunity to react with the precursor on the surface of fabric and microscope slide. The hydrolysis of TTIP and condensation polymerization to TiO2 occur consequently. The proper temperature can transform the amorphous TiO2 into crystalline states. Different temperatures lead to TiO2 of different crystalline forms (Chen et al. 1995). In most studies, it needs 400°C to transform the TiO2 to anatase phase, and 500°C to transform to rutile phase (Wang & Ying 1999). When the microwave system is cooled down, the fabric sample is transferred to an oven for drying with the supporting slide. The drying process upholds 5min under the temperature of 80°C. The process temperatures in the used microwave process are varied in the range between 60°C to 140°C with a 20°C interval. The hold time (process duration) is pre-set with 5min and 15min.

Figure 15. Transfer of textile sample with TTIP precursor into the 80ml reaction vessel of the microwave device.

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Figure 16. SEM-image of titania coating on fabric. The preparation is done with 1: 3 diluted TTIP and microwave process temperature of 120°C applied for 15 minutes.

Figure 17. SEM-image of titania coating on fabric. The preparation is done with 1: 1 diluted TTIP and microwave process temperature of 120°C applied for 15 minutes.

After the complete process is finished, the formed TiO2 can be clearly detected by scanning electron microscope SEM onto the textile substrates (see Figures 16 and 17). In case of the more diluted TTIP application (ratio 1: 3), only nanoscaled TiO2 particles are detected. These nanoparticles appear as single particles and as aggregates (Figure 16). In

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case of the higher concentrated TTIP application (ratio 1: 1), beside the nanoparticles also larger structures are deposited. These structures contain also cracks, what is a hint to a high coating thickness (Mahltig & Textor 2008; Krüger et al. 2006). The formed structures are almost similar independent from process temperature and duration of the microwave synthesis. Probably with the application of the more concentrated precursor to much TiO2 is applied leading to the formation of larger broken structures onto the textile substrate. The photoactivity of the prepared titania coated fabrics is determined by photodecomposition of the organic dye Acid Orange 7, as described in detail previously (Mahltig et al. 2007; Böttcher et al. 2010). By photooxidation, the organic dye AO7 is decomposed under illumination with UV-light and the comparison to the dye concentration under dark conditions indicates the photoactivity of the present titania material. The concentration of AO7 after the testing is determined in the dye solution (Figures 18 and 19) and on the textile fabrics (Figures 20 and 21). In this test the decomposition of the dye AO7 is the parameter for the intensity of the photocatalytic effect. This dye is solved into the testing solution with a starting concentration of 0.1 mmol/L. After contact with the titania coated textile samples with and without illumination with UV-light, the dye concentration in the solution is determined again by using a UV/Vis spectrometer. A stronger decrease in dye concentration is probable related to a stronger photocatalytic effect. However, it has to be kept in mind that beside the photodecomposition also other processes can lead to a decrease of the dye concentration. One main additional process is the absorption of the dye molecules by the textile fabric and by the titania coating on it. For this, it is absolutely necessary to determine the remaining dye concentration with similar test condition but without UV light (under dark conditions). The reference measurements are absolutely necessary, to rank the intensity of the photocatalytic effect in an adequate way. The Figures 18 and 19, summarize both measurements under UV light exposition and the reference measurements under dark conditions. The remaining dye concentration is here given as function of temperature in the microwave process. It is obvious, with increasing microwave process temperature the concentration of the dye is decreased. Under exposition to UV-light this decrease is stronger, so the photocatalytic activity is clearly determined and this activity is stronger with higher process temperatures. Nevertheless, it is also determined that the dye concentration is significantly decreased even if the reference testing is performed without UV-light. For this, a strong up-take of the dye AO7 by the textile and the titania material can be stated. Interesting is that this absorption increases with increased microwave process temperatures. Probably under these conditions titania material is formed which has a higher absorptive capacity. Such a higher absorption capacity is also an important aspect for the photocatalytic effectivity, because for the photocatalytic effect a certain contact of titania material to the

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dye molecules is necessary and an absorbed dye has of course an intensive contact to the absorbing titania matrix. 1:3 ratio application

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Figure 18. Testing on photoactivity with the dye acid orange 7 AO7. Presented is the remaining dye concentration after contact with the titania coated fabric under different conditions; dark test – 19 hours placement of the fabric in the dye solution without light, UV test – 6 hours placement under a UVlamp. The results are depicted as function of microwave process temperature. The sample preparation is done with 1: 3 diluted TTIP and microwave applied for 5 minutes or 15 minutes. 1:1 ratio application

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Figure 19. Testing on photoactivity with the dye acid orange 7 AO7. Presented is the remaining dye concentration after contact with the titania coated fabric under different conditions; dark test – 19 hours placement of the fabric in the dye solution without light, UV test – 6 hours placement under a UVlamp. The results are depicted as function of microwave process temperature. The sample preparation is done with 1: 1 diluted TTIP and microwave applied for 5 minutes or 15 minutes.

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Figure 20. Testing of photoactivity with the dye acid orange 7 AO7. Presented are the reflection spectra of textile samples after placement in the dye solution under different conditions; dark test – 19 hours placement of the fabric in the dye solution without light, UV test – 6 hours placement under a UVlamp. The results are depicted for microwave process temperature 60°C and 120°C. The sample preparation is done with 1: 3 diluted TTIP and microwave applied for 5 minutes.

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Figure 21. Texting of photoactivity with the dye acid orange 7 AO7. Presented is the difference of reflection of textile samples at wavelength of 480nm between textiles tested with UV test and placement under dark conditions. The results are depicted as function of microwave process temperature. The sample preparation is done with 1: 1 or 1: 3 diluted TTIP and microwave applied for 5 minutes or 15 minutes.

Beside the color intensity of the aqueous dye solution, also the color properties of the textile samples are determined after the testing procedure with the dye AO7. This color properties are determined by UV/Vis spectroscopy in arrangement of diffusive reflection. Related spectra are presented in Figure 20. The absorption maxima of the dye AO7 is at

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486nm, this is related to the minimum determined in the reflection spectra. A deeper reflection minimum is correlated to a stronger dye up-take by the textile. The spectra of samples tested in the UV-test and under dark conditions are compared and the difference in the reflection minima after both tests is directly related to the photoactivity of the prepared samples. A stronger difference indicates that the once absorbed dye is decomposed afterwards by a photocatalytic process driven under UV-light exposure. To generate here an overview, for all prepared samples this difference in reflection at the minimum is correlated to the microwave process temperature used for sample preparation (Figure 21). This difference in reflection is clearly increasing as function of the process temperature in microwave process. Until a process temperature of 100°C, the reflection difference increases moderately to values around 20%. The change of process temperature to 120°C changes the situation drastically to values of 50% or more. For this, it could be concluded that for preparation of strong photocatalytic titania materials by the currently presented process a minimum process temperature of 120°C is necessary. For samples prepared with temperatures till 120°C, the other parameters as concentration of the precursor or the process duration are of less importance. However, with the highest investigated process temperature of 140°C, the other parameters show their influence. Highest activity – with values of 80% - is reached with a process temperature of 140°C, the lower process duration of 5 minutes and the smaller precursor concentration. In fact, this is a surprising result. Of course, a higher process temperature can be easily correlated to a stronger formation of photocatalytic species. However, a lower concentration of precursor should be also related to less photoactive titania on the textile surface. An explanation can be done with the structure of the formed titania on the fiber surface, as shown in Figures 16 and 17. With smaller precursor concentration, the formed titania occurs mainly in nanoparticular form. These nanoparticles are supposed to have a high specific surface area and by this a larger area to get in contact with the dye in the solution. A higher contact area can cause higher reaction rate s for the photocatalytic process. An analogous argument can be used to explain the difference resulting from the process duration. Probable with longer process duration the formed titania nanoparticles can sinter together and by this the photoactive surface area is decreased. By this argumentation the higher photoactivity gained after the shorter process time of 5 minutes can be explained. Altogether it can be concluded that the microwave-assisted process can be a powerful tool to realize photoactive functionalized textiles. The most important process parameter is the temperature but also moderate temperatures of 120°C or 140°C are sufficient to reach significant photoactivity. These temperatures are in the range of temperatures used in conventional textile processes. Other parameters as concentration of functional component or process duration influences the gained photoactivity probable mainly by surface effects. Such surface effects are difficult to predict, so for improvement of the microwave process usually intensive experimental optimizations are required.

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SUMMARY AND CONCLUSION Microwave assisted processes are versatile tools for dyeing, finishing and functionalization of textile materials. Conventional functionalization can be realized significantly faster and at lower process temperature. This is demonstrated in the current overview on the examples of dyeing, fluorescent textiles and UV protective textiles. Beside the improvement of conventional procedures, also new materials can be realized by using microwave assistance in textile functionalization, this is shown actually by the preparation of different photocatalytic materials on textile substrates. However it should be mentioned additionally, that the use of microwave applications for different textile treatments is known since decades but only few applications have found the way towards commercially products and procedures. One probable reason for this are technical challenges in case of an up-scale in microwave devices. The actual shown experiments are performed with a focused microwave system giving nearly the same intensity of the microwave field in the whole reaction vessel. In case of up-scaling the shown procedures, a certain experimental effort has to be done especially to gain homogenous distributed properties on the whole treated textile fabric. Nevertheless, the chances of microwave assisted textile functionalization are obviously clear and a future development of new techniques and materials in this field can be expected.

ACKNOWLEDGMENTS The authors would like to thank Thomas Heistermann and Thomas Grethe for many helpful discussions and experimental help. For funding of the electronmicroscopic equipment the authors acknowledge very gratefully the program FH-Basis of the German federal country North-Rhine-Westphalia NRW. Experimental work presented is part of the Master thesis performed by Haoqian Miao (2016, Hochschule Niederrhein) and the Bachelor thesis performed by Elena Schüll (2017, Hochschule Niederrhein). All product and company names mentioned in this chapter may be trademarks of their respective owners, also without labeling.

REFERENCES Ahmed, N.S.E., El-Shishtawy, R.M. (2010). The use of new technologies in coloration of textile fibers. J. Mater. Sci., 45, 1143-1153.

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Dastjerdi, R., Montazer, M. (2010). A review on the application of inorganic nanostructured materials in the modification of textiles: Focus on anti-microbial properties, Colloid and Surfaces B: Biointerfaces, 79, 5-18. Delaney, M.J., Seltzer, I. (1972). Microwave Heating for Fixation of Pad-dyeings on Wool. Journal of the Society of Dyers and Colourists, 88, 55-59. Dittrich, J.-H., Töpert, G., Kreitz, J., Böhnke, B., Naefe, P. (1992). Procedural investigations to prevent the yellowing in the continuous HF drying of yarn packages and Wollkammzügen. Melliand textile reports, 73, 529-532 and 589-596. Du, Z., Li, Y., Wang, X., Wan, Y., Chen, Q., Wang, C., Lin, X., Liu, Y., Chen, P., Ruan, R. (2011). Microwave-assisted pyrolysis of microalgae for biofuel production. Bio resource Technology, 102, 4890-4896. El-Apasery, M.A. (2008). Solvent-Free One-Pot Synthesis of Some Azo Disperse Dyes under Microwave Irradiation: Dyeing of Polyester Fabrics. J. Appl. Polym. Sci., 109, 695-699. Ferus-Comelo, M. (2009). Dye demand/supply ratio as a predictor for the unlevelness of disperse-dyed polyester. Coloration Technology, 125, 352-356. Fowler, W., Raynes, W.T. (1981). The effects of rotation, vibration and isotopic substitution on the electric dipole moment, the magnetizability and the nuclear magnetic shielding of the water molecule. Molecular Physics, 43, 65-82. Gedye, R., Smith, F., Westaway, K., Ali, H., Baldisera, L., Laberge, L., Rousell, J. (1986). The use of microwave ovens for rapid organic synthesis. Tetrahedron Letters, 27, 279-282. Gerthsen, C., Vogel, C. (1993). Physik, Springer-Verlag, Berlin, 17th edition. Göpel, W., Ziegler, C. (1994). Struktur der Materie: Grundlagen, Mikroskopie und Spektroskopie, B.G. Teubner Verlagsgesellschaft, Stuttgart. Haggag, K., Hanna, H.L., Youssef, B.M., El-Shimy, N.S. (1995). Dyeing Polyester With Microwave Heating Using Disperse Dyestuffs, American Dyestuff Reporter, 22-35. Haggag, K., El-Molla, M.M., Mahmoued, Z.M. (2014). Dyeing of cotton fabrics using reactive dyes by microwave irradiation technique. Indian Journal of Fibre & Textile Research, 39, 406-410. Herman, R., Wallis, R.F. (2016). Influence of Vibration‐Rotation Interaction on Line Intensities in Vibration‐Rotation Bands of Diatomic Molecules. J. Chem. Phys., 23, 637. Hybertsen, M.S., Louie, S.G. (1986). Electron correlation in semiconductors and insulators: Band gaps and quasiparticle energies. Physical Review B, 34, 5390-5413. Jones, D.A., Lelyveld, T.P., Mavrofidis, S.D., Kingman, S.W., Miles, N.J. (2002). Microwave heating applications in environmental engineering – a review. Resources, Conservation and Recycling, 34, 75-90. Kappe, C.O. (2004). Controlled microwave heating in modern organic synthesis. Angewandte Chemie International Edition, 43, 6250-6284.

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In: Textiles: Advances in Research and Applications ISBN: 978-1-53612-855-0 Editor: Boris Mahltig © 2018 Nova Science Publishers, Inc.

Chapter 3

SOL-GEL AND LAYER BY LAYER METHODS FOR CONFERRING MULTIFUNCTIONAL FEATURES TO CELLULOSIC FABRICS: AN OVERVIEW Giulio Malucelli* Politecnico di Torino, Dept. Applied Science and Technology, Alessandria, Italy

ABSTRACT This chapter reviews the recent advances referring to the use of sol-gel processes and layer by layer (LbL) treatments for conferring multifunctional features to cellulosic fabrics (namely cotton and its blends with polyester). In fact, these two approaches, which have been successfully applied to both natural and synthetic fabrics, can be very effective in providing multifunctional features to the treated fabrics, through the formation of a ceramic - or polymer /ceramic-based coating (sol-gel ), or by depositing a nanostructured organic, inorganic or hybrid assembly (LbL) on the underlying textile. Sol-gel is based on a bottom-up strategy, starting from different precursors, from which it is possible to tailor the final surface properties of the treated substrates. Conversely, layer by layer exploits a top-down method, suitable for the design of very thin and, at the same time, performing coatings. Among the different possible multifunctional properties the two approaches can provide to cellulosic substrates, fire retardancy, hydrophobicity, wear resistance and anti-bacterial activity seem to fulfill the up-to-date needs coming from either the academic or industrial world. This work thoroughly reviews the development and the recent advances achieved by the two strategies, showing their potentialities and current limit ations.

*

Corresponding Author Email: [email protected].

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Keywords: cotton fabrics, sol-gel processes, layer by layer deposition, surface engineering, multifunctional surfaces, fire retardancy, wear resistance, hydrophobic surfaces, anti-bacterial coatings

INTRODUCTION The recent technological innovations and the need for new functions are engendering an increasing demand for novel materials and structures, which could remarkably widen their application fields and markets. In this context, the possibility of conferring multifunctional features to different substrates through the exploitation of surface engineering strategies seems to open new pathways towards the development of smart tailored materials, for which the surface structure and composition represent the key point affecting their overall behavior. It is worthy to note that this approach, irrespective of the achieved performances, is very often easy to perform on different types of materials (plastics, metals, ceramics, composites …), does not imply changes of the bulk properties and is cost -effective (Martin 2011). Fabric substrates have emerged from the materials that deserve surface treatment s, as they can be quite easily modified exploiting the standard finishing treatments already employed in the industrial processes; furthermore, the surface engineering methods selected for modification of the fabrics can provide interesting multifunctional features, making these substrates suitable for wider uses. In this context, sol-gel and the layer by layer (LbL) techniques are the main strategies that received great attention over the last 10 to 15 years. The former represents an example of soft-chemistry bottom-up strategy based on hydrolysis and condensation reactions taking place in a liquid medium that contains selected alkoxy precursors. This technique, schematized in Figure 1, allows synthesizing new multifunctional materials with a high degree of homogeneity at molecular level and with exceptional physicochemical features. As clearly reported in the open scientific literature, several parameters contribute to the development of the sol-gel processes, hence affecting the structure /morphology of the resulting oxidic networks: temperature and pH, type of (semi)metal atom and alkyl/alkoxide groups, water /alkoxide ratio, reaction time and presence of cosolvents (Alongi & Malucelli 2012). Unlike sol-gel, LbL is a typical top-down approach, suitable for the deposition of very thin assemblies (ranging from hundreds of nanometers to microns) on several types of substrates, comprising plastics, metals and also fabrics. Although the first application of this approach dated in 1966 (Iler 1966), this method was forgotten until the early 1990s, when Decher’s group developed a suitable strategy for its practical use (Decher & Hong 1991).

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 63 OR

OR H+

RO

M

OR

+ H 2O

RO

OR

OH + ROH

STEP 1: Hydrolysis

OR

OR M

M

OR OH + HO

OR

M

OR OR

H+

RO

OR

M

OR O

OR

M

OR + H2O

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STEP 2a: Condensation with water formation

OR M OR

OR OH + RO

M OR

OR OR

OR

H+ RO

M OR

O

M

OR + ROH

OR

STEP 2b: Condensation with alcohol formation

Figure 1. Scheme of sol-gel reactions occurring in acidic conditions (M=(semi)metal, such as Si, Ti).

From an overall point of view, the LbL assembly mainly consists in an alternate adsorption of electrically oppositely charged chemical species (namely, polyelectrolytes, nanoparticles, nano-objects) on a specific substrate. Most of its applications, as clearly depicted in the open scientific literature, exploit a step-by-step coating build-up based on electrostatic forces (Malucelli 2016a). However, LbL can also utilize different types of interactions (e.g., covalent bond s (Ichinose 1998, Fang 1997), hydrogen bonds (Stockton & Rubner 1997, Shimazaki 1998), stereo-complex formation (Serizawa 2000, Serizawa 2001), donor-acceptor interactions (Wang 2007), etc.). A general scheme of the most used LbL assembly based on electrostatic interactions is shown in Figure 2: it essentially exploits the alternate immersion of the substrate into oppositely charged polyelectrolyte solutions (or nanoparticle dispersions). The concentration of the solutions/dispersions is very low (usually in between 0.5 and 1 wt.%); besides, water is usually the medium utilized for their preparation, hence avoiding the use of organic solvents. As a result, an assembly of positively and negatively charged layers piled up on the underlying substrate is achieved, giving rise to a total surface charge reversal after each immersion step. In addition, the suitable components involved in the LbL assembly can be selected from a wide assortment of products: in fact, cationic or anionic polyelectrolytes, metallic or oxidic colloids, layered silicates, even biomacromolecules have been successfully used.

Giulio Malucelli

1 cycle = 1 layer

Fabric

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cationic polyelectrolyte solution/ nanoparticle suspension

washing

washing

anionic polyelectrolyte solution/ nanoparticle suspension

Figure 2. Scheme of the layer by layer method based on electrostatic interactions.

Unlike some standard and well-known thin film deposition methods (such as plasma treatments and impregnation /exhaustion processes, which are very common as finishing treatments in the textile industry), the layer by layer deposition takes place in “mild” and somehow “environmentally-friendly conditions.” In fact, it is usually carried out at atmospheric pressure and room temperature, almost always using very diluted water -based solutions or suspensions. In addition, it allows tailoring the final properties of the obtained assembly by easily incorporating selected functional species in the deposited layers. Therefore, from an overall point of view, sol-gel and LbL methods can be successfully exploited for providing fibers or fabrics with multifunctional features, depending on the structure of the sol-gel precursors or of the LbL constituents, hence

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 65 finely tuning the desired specific properties to provide the textile with. In particular, it is possible to coat the underlying textile substrate with a fully inorganic or hybrid organicinorganic (referring to both sol-gel and LbL) or with a fully organic coating (as far as the only LbL strategy is considered). This chapter reviews the current state of the art related to the use of these surface engineered strategies for conferring multifunctional features (among them, fire retardancy, hydrophobicity, wear resistance and anti-bacterial activity), to cellulosic substrates, discussing both their potential advances and current limit ations.

MULTIFUNCTIONAL FEATURES PROVIDED BY SOL-GEL COATINGS The sol–gel method has already demonstrated its outstanding potential for the design of tailored materials with a high degree of homogeneity at the molecular scale and with outstanding physico-chemical, thermo-mechanical and barrier properties. As already shown in Figure 1, this technique is based on two-step hydrolysis /condensation reactions of (semi)metal alkoxides, among which, tetramethoxysilane, tetraethoxysilane, aluminium isopropoxide, titanium tetraisopropoxide are very often utilized. The aforementioned reactions favor the formation of fully inorganic or hybrid organic– inorganic coatings at or near room temperature, on the basis of the chemical structure and reactivity of the employed precursors. Referring to textiles, sol-gel processes have been successfully exploited for providing them with multifunctional features; more specifically, great attention was directed towards the design of “smart” textiles. For this purpose, different kinds of properties have been considered, hence leading to the obtainment of flame retardant, super-hydrophobic, antimicrobial or UV radiation protective fabrics. Besides, it was also possible to improve the dye fastness of the treated fabrics (Mahltig & Textor 2006, Cireli & Onar 2008), to immobilize biomolecules (Li 2007), to perform anti-wrinkle finishing (Huang 2006), and to provide the treated fabrics with sensor characteristics (Caldara 2012, Van der Schueren 2012) or photocatalytic features (Moafi 2011, Colleoni 2012).

Sol-Gel Flame Retardant Coatings The sol-gel method has been used from the 1950s, but only quite recently for providing the textiles with flame retardant properties. In particular, it was found that the ceramic phases (in form of either coatings or particles) formed on the sol-gel treated fabrics could behave as a thermal shield during the exposure of the fabrics to a flame or to a heat source. This physical barrier that limits oxygen and heat transfer impedes the

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formation of combustible volatile species that promote the further textile degradation, favoring, at the same time, the formation of an aromatic stable carbonaceous residue, called char (Alongi & Malucelli 2012, Alongi & Malucelli 2015). On the other hand, as the fabrics have a limited thickness, the deposited sol-gel coatings can protect them only partially. As a consequence, the flame retardant properties provided by these coatings are really effective only when they work in synergistic or joint effects with other active components: this situation can be fulfilled by combining the solgel ceramic phases with other flame retardant active species, such as P- and/or Ncontaining molecules. In addition, a quite recent step-forward of the sol-gel technique has allowed designing dual-cure processes, which can be exploited for preparing hybrid organic-inorganic protective coating s (Malucelli 2016b) through the combination of a photo-induced polymerization process together with a thermal treatment that promotes the buildup of sol-gel ceramic phases (Alongi 2011a). The first researches on the sol-gel method applied to textiles were focused on the development of fully inorganic coatings on the fabrics. Hribernik et al. applied sol-gel derived silica coatings (thickness about 350 nm), aiming at reducing the flammability of regenerated cellulose fibers (i.e., viscose) (Hribernik 2007). In particular, tetraethylorthosilicate (TEOS) was utilized as precursor of the silica coating: the treated fibers showed enhanced thermal stability (with 20°C temperature increase of the first degradation step) and flame resistance with respect to the untreated counterparts; furthermore, the glowing combustion of the residue showed an important 40°C temperature increase. The obtained sol-gel coatings, when applied to cotton, polyester and cotton/polyesterrich blends, were able to delay the degradation reactions in nitrogen and air, as revealed by thermogravimetric analyses. In addition, the silica coatings were capable to significantly lower the peak of the heat release rate (pkHRR) and to increase the time to ignition (TTI) during cone calorimetry tests (Alongi 2011b). Pursuing this research, different precursors for silica, even in combination with other preformed oxidic phases have been applied to cellulosic fabrics, thoroughly assessing the effect of different adopted process parameters on the morphology of the sol-gel derived coatings. Alongi and co-workers investigated the effect of the experimental condition s adopted for the sol-gel treatments (namely, temperature and time of the thermal treatment and the precursor: water molar ratio) on the achieved flame retardancy of cotton (Alongi 2011c). More specifically, cone calorimetry tests showed 56% increase of time to ignition (TTI) and 15% reduction of peak of heat release d rate (pkHRR) when the sol-gel process was performed at 80°C for 15 h, keeping a 1: 1 water: tetramethylortosilicate (TMOS) molar ratio. These findings demonstrated that the sol-gel coating acts as a thermal insulator, shifting the degradation temperature towards higher values and favoring the

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 67 formation of a stable char, as derived from cellulose dehydration (Alongi &Malucelli 2015). Then, the role of the chemical structure of the silica precursor (in terms of type and number of hydrolysable alkoxy functionalities) was thoroughly investigated, comparing the flame retardant features of cotton treated with TMOS (bearing 4 methoxy groups) with the fabric treated with TEOS (bearing 4 ethoxy groups) and tetrabuthylorthosilicate (TBOS, bearing 4 buthoxy groups) (Alongi 2012a). Vertical flame spread tests revealed that the burning rate lowers in the presence of even a very limited amount of silica coating (from 2.50 to 1.45 mm/s for untreated cotton and TMOS-treated counterpart, respectively), hence increasing the total burning time. Furthermore, the final residue was remarkably increased (from 10, untreated fabric, to 35 and 48 wt.%, for TEOS- and TMOS-treated counterparts, respectively). Therefore, it was concluded that a shorter precursor chain length lowers the fabric flammability. In addition, the combustion behavior, as assessed by cone calorimetry tests, was thoroughly investigated: to this aim, the combustion behavior of TEOS was compared with that of other alkoxysilanes bearing a different number of hydrolysable groups, namely diethoxy(methyl)phenylsilane (DEMPhS), 3-aminopropyl trimethoxysilane (APTES), triethoxy(ethyl)silane (TEES), 1,4-bis(triethoxysilyl)benzene (bTESB) and 1,2-bis(triethoxysilyl)ethane (bTESE). As revealed by the cone calorimetry results, it was found that: 

 

the alkoxysilanes bearing a limited number of hydrolysable groups (i.e., 2 or 3) behave like TEOS, though they promote an increase of smoke release and give rise to very thin and incoherent residues a compact and thicker residue is obtained when an amino group (APTES) is replaced with an alkyl chain (TEES); bTESB and bTESE, which bear a high number of alkoxy groups, are able to change cotton flammability; more specifically, the fabrics treated by bTESB do not burn even after repeated flame applications.

The same group also studied the effect of the precursor type on the overall fire behavior of cotton fabrics: for this purpose, these latter were treated with alumina, zirconia or titania sol-gel derived coatings, starting from aluminium isopropylate, tetraethylortho-zirconate and tetraethylortho-titanate precursors (Alongi 2012b). The best flame retardancy performances were obtained with the silica coating, for which 56% TTI increase and 20% pkHRR decrease were achieved in cone calorimetry tests. Conversely, alumina and titania coatings ensured the best enhancements of abrasion resistance, assessed by Martindale tests. Pursuing this research, alumina micro- or nano-particles were embedded in sol-gel silica coatings, aiming at designing new coatings with enhanced fire and tribological features. In particular, irrespective of the size of the used alumina particles, the sol-gel

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systems coated on cotton or cotton-linen fabrics were found to enhance the abrasion resistance, to decrease the total burning rate by 45% and to increase the final residue (+46%) after horizontal flame spread tests (Alongi 2013). As previously mentioned, the fire behavior of sol-gel derived coatings can be further ameliorated by exploiting their synergistic or joint effects with other flame retardants, mostly containing P or P and N elements. This goal can be fulfilled either by using alkoxysilane precursors that bear both silane and phosphate groups or by grafting a phosphoric acid source to an alkoxysilane precursor. A further method exploits the incorporation of an alkoxysilane precursor bearing both silane and phosphate groups into P- and N-containing compounds. Irrespective of the type of chosen strategy, the presence of phosphate groups promotes the dehydration of the cellulosic substrate, instead of the formation of flammable volatile organic products, hence giving rise to the creation of a stable char (Alongi & Malucelli 2015). The concurrent presence of P and Si elements in the same precursor has been exploited for obtaining hybrid organic-inorganic coatings, which act as char developers, due to the presence of the phosphoric acid source, but also as thermal protective coating s (owing to the inorganic ceramer). In this context, hybrid phosphorus -silicon organic-inorganic coatings were synthesized on cotton, starting from diethylphosphatoethyltriethoxysilane (DPTES) as a precursor and using a multistep sol-gel process (from 1 to 6 deposited layers) (Alongi 2012c). In cone calorimetry tests, the sol-gel treated fabrics showed a decreased burning time, as revealed by the flame out values (66, 62 and 80 vs. 116 s for 1, 3 and 6 layers and untreated cotton, respectively). Besides, the obtained hybrid coatings were capable of limiting the development of volatile species, as indicated by total smoke release (TSR) values (20, 15 and 6 vs. 26 m2/m2 for 1, 3 and 6 layers and untreated cotton, respectively). Finally, a further pre-hydrolysis step of the precursor was found to improve the washing fastness of the treated fabrics (up to 5 washing cycles, according to the ISO 6330 standard) (Alongi 2013). Pursuing this research, possible synergistic effect s between sol-gel derived silica coatings, bearing both silane and phosphate groups, with P- and N-containing compounds, were assessed. For this purpose, DPTES was mixed with APTES or APTES and a melamine-based resin (M) and applied to cotton (Brancatelli 2011). An increase of char yields, as revealed by thermogravimetric analyses in air (42 and 38 wt.% for APTES/DPTES- and APTES/DPTES/M-treated samples, respectively) was found. The further replacement of the N-source with N,N,N,’N,’N,”N”-hexakis-methoxymethyl[1,3,5]triazine-2,4,6-triamine (MF) and the optimization of DPTES/MF relative ratios were found to enhance the char-forming character of the obtained coatings that approached 70 wt.% of residue after thermogravimetric analyses in air (Alongi 2012d). The occurrence of synergistic effect s, estimated through Lewin’s synergistic effectivity (Lewin 2001) was assessed for sol-gel derived coatings deposited on cotton

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 69 fabrics and doped with different phosphoric acid sources (i.e., aluminium phosphinate, or a mixture of aluminium phosphinate, melamine poly(phosphate) and zinc and boron oxide, or ZrP nano-platelets (Alongi 2011d)). More specifically, time to ignition (TTI, from 14, untreated cotton, to 40 s) and the limiting oxygen index (LOI, from 19%, untreated cotton, to 30%) were found to remarkably increase by simply mixing the solgel precursor with 5 wt. % of the above-mentioned phosphorus -based compounds. Very recently, a new combination of a functional sol-gel coating based on (3trimethoxysilylpropyl) diethylenetriamine and phenylphosphonic acid as a phosphorous rich component was applied to cotton, polyester and a 65: 35 cotton: polyester blend (Kappes 2016). The treated fabrics were able to self-extinguish in small-scale flame tests with either surface or edge ignition (according to EN ISO 15025). In a very recent work, a two-step process for obtaining flame retardant and superhydrophobic cotton fabrics was designed by Przybylak and co-workers (Przybylak 2016). In particular, a sol-gel coating based on aminopropyltriethoxysilane was exploited for immobilizing ammonium dihydrogen phosphate and/or guanidine carbonate: this way, it was possible to provide cotton with flame retardant features. Subsequently, the flame retarded cotton was further combined with a fluorofunctional silane or polysiloxane, hence gaining high hydrophobicity. The occurrence of synergistic effect s between P- and N-containing compounds and organosilicon species was demonstrated and allowed reaching high flame retardant properties: in particular, 71.6% limiting oxygen index and 90% decrease in Qmax (i.e., the maximum specific heat release rate assessed in Pyrolysis-Combustion Flow Calorimetry tests) with respect to the untreated fabric were documented. Very recently, a hybrid organic–inorganic coating containing P, Si and N was synthesized starting from APTES and phenylphosphonic dichloride and applied to cotton fabrics by using a sol-gel method (Liu 2016). The treated fabrics showed an improved thermal stability and fire behavior: in particular, the deposited hybrid coating promoted the self-extinction of the fabrics immediately after the removal of the ignition source. The obtained results were interpreted on the basis of the formation of both a ceramic layer on the fiber surface upon heating and of a thermally stable aromatic char.

Sol-Gel Antimicrobial Coatings Several sol–gel systems with antimicrobial features have been developed up to now, in order to prevent possible biocontamination phenomena. In particular, the sol-gel antimicrobial coating formulations for fabrics can be classified into three different systems:

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



Photoactive TiO2–based coatings with permanent antimicrobial features, which work upon UV exposure due to photomineralization processes occurring on their surface (Ollis 2000); Polycationic coatings, obtained from hydrolysis products of long-chained tetraalkylammoniumsilane precursors, which show permanent antimicrobial properties (Isquith 1972); Temporary antimicrobial sol–gel coatings with embedded biocides and controlled release of biocides (Bottcher 1992).

The third strategy seems to be the most performing in the textile world, as it makes possible to embed a wide selection of inorganic biocides (such as Ag, Cu or boric acid) and any organic biocides. Mahltig and co-workers exploited sol-gel silica coatings containing silver, silver salts or biocidal quaternary ammonium salts (i.e., cetyltrimethylammoniumbromide and octenidine) for conferring antimicrobial features to cotton /polyester blend fabrics (Mahltig 2004). In particular, the antimicrobial efficacy, wash-out and long-term behavior were thoroughly investigated on the basis of the type of embedded biocides. All the prepared sol-gel coatings were found to inhibit the growth of fungi and bacteria, especially when embedding octenidine or silver. Xing and co-workers treated cotton fabrics with silica sols from water glass and then with silver nitrate solution (Xing 2007). Using E. coli as a model for Gram-negative bacteria assessed the antimicrobial activity. The results showed that the treated cotton fabrics possess an excellent antimicrobial effect and washing fastness (up to 50 washing cycles ). Furthermore, XPS analyses revealed the presence of silver in two different states, i.e., Ag+ and Ag2+, on the surface of the treated fabrics: both the states turned out to contribute to the observed antimicrobial activity. Tomsic and co-workers exploited a sol-gel method for preparing water and oil repellent and antimicrobial coatings for cotton fibres (Tomsic 2008). To this aim, a commercially available fluoroalkyl functional water-borne siloxane (FAS ), nanosized silver (Ag) and a reactive organic–inorganic binder (RB) were used and applied according to two different procedures: cotton fabrics were impregnated with a sol mixture containing all three components or first impregnated with the Ag–RB sol and then with the FAS sol. Apart from the good hydrophobic/oleophobic features shown by the sol-gel treated fabrics even after 10 washing cycles, their antibacterial activity against E. coli and S. aureus was found to depend on the application procedure of the sol-gel coatings. In particular, the separate deposition of antibacterial Ag nanoparticles followed by the application of the oleophobic and antibacterially non-active fluoroalkyl functional waterborne siloxane turned out to be preferable to the application of the sol mixture in one step only.

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 71 Pursuing this research, the same group treated cotton fabrics with a diureapropyltriethoxysilane [bis(aminopropyl)-terminated polydimethylsiloxane (1000)] (PDMSU) sol−gel hybrid coating, also in the presence of 1H,1H,2H,2Hperfluorooctyltriethoxysilane (PFOTES) (Vilčnik 2009). The obtained mixed PFOTES−PDMSU finishes applied on cotton fabrics turned out to enhance both hydrophobicity and oleophobicity of the treated substrates. The antibacterial activity of sol-gel treated fabrics was assessed with the transfer method (EN ISO 20743: 2007): the reduction of E. coli bacteria on unwashed cotton fabrics was nearly 100% and was still high after 10 washing cycles (60.6%). Tarimala et al. exploited a sol–gel method for synthesizing a dodecanethiol-capped silver nanoparticle–doped silica coating (Tarimala 2006). The antibacterial activity tests performed on woven cotton fabrics treated with the prepared doped sol revealed an excellent performance of the silica coating against E. coli, hence suggesting the possibility of using the so-treated fabrics as disposable bandages.

Sol-Gel UV-Protective Coatings The emission of UV radiation in the spectral region from 280 to 400 nm may be responsible of such health issues as sunburn, allergies or even, in very limited cases, skin cancer (Lapidot 2003). Thus, the development of effective systems able to ensure protection from UV radiation is an up-to-date issue. Apart from the products that are directly put on the skin, the design of UV-protective fabrics may represent a solution in order to face this issue. In this context, different parameters directly related to the fabrics, such as their chemical structure and composition, grammage, porosity, color, play an important role. Xin et al. developed a UV-blocking sol-gel derived coating for cotton fabrics: to this aim, a nanosol, prepared at room temperature by mixing titanium tetraisopropoxide with absolute ethanol, was directly applied to the cellulosic substrate (Xin 2004). The presence of titania domains on the surface of the fabrics provided an excellent UV protection; furthermore, the deposited coating showed a good adhesion to the underlying fibers and a high washing fastness (the UPF – ultraviolet protection factor – was equal to 50+ even after 55 home launderings). Finally, the sol-gel coating did not show any detrimental effect on the mechanical strength of the treated substrates, as confirmed by bursting strength tests. Abidi and co-workers exploited a sol-gel method and applied nanosols containing tetraethyl orthotitanate and tetraethyl orthosilicate to lightweight 100% cotton fabric (Abidi 2007). After drying and curing, the modified fabrics show a high UV protection, which was strictly correlated to the titania content in the nanosol: this finding was ascribed to the increase of the refractive index of the sol-gel coatings deposited on the

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fabric surface. Furthermore, the obtained UV protective systems showed a high washing fastness, due to the formation of covalent bond s between the hydroxyl groups of cellulose and those of the titania and titania–silica network. Pursuing this research, the same group modified cotton fabrics by titania nanosols derived from a sol-gel method, using tetrabutyl orthotitanate as the active species (Abidi 2009). The treated substrates showed a good UV protection, which was not affected by repeated home launderings (for up to 18 cycles according to AATCC TM 124); furthermore, it was found that coffee and red wine stains were successfully decomposed by exposing of the stained fabric to the UV radiation. Paul et al. compared the UV protection effect provided by preformed ZnO nanoparticles deposited on cotton fabrics, with that of titania nanoparticles obtained by sol-gel reactions starting from titanium isopropoxide (Paul 2010). As compared to ZnO nanoparticles, the sol-gel derived titania counterparts were found to be more performing in the UV protection of the underlying fabrics and more resistant to washing treatments (i.e., to 10 cycles of domestic washings). Wang and co-workers investigated the effect of dumbbell-shaped ZnO crystallites obtained from a sol-gel method on the UV-protection of cotton fabrics (Wang 2005). In particular, in the UV-range, complete blocking was achieved using the obtained ZnO crystallites. Onar and co-workers treated cotton fabrics by a pad–dry–cure method with different amounts of Ag-doped, Ti-based transparent solutions prepared with a sol–gel technique (Onar 2007). The fabrics modified with a 30 atom % Ag doped, Ti-based solution, exhibited a good UPF rating of 15+, which was 200% higher with respect to the untreated fabric. Furthermore, the proposed treatments also showed a good washing fastness after 10 washing cycles according to BS EN ISO 105-C06-A1S Standard. Sundaresan et al. designed a multifunctional treatment based on the application of titania nanoparticles, obtained through a sol-gel method, on cotton fabrics. In particular, two different average TiO2 nanoparticle sizes (i.e., 7 and 12 nm), were obtained and applied to the cellulosic substrate (Sundaresan 2012). The proposed treatment provided the cellulosic fabrics with UV-protection, antimicrobial and self-cleaning activity. In particular, as far as UVprotection is considered, 12 nm TiO2 nanoparticles showed higher UPF values as compared to the smaller nanoparticles; besides, the UV-protection was retained until 10 washing cycles.

MULTIFUNCTIONAL FEATURES PROVIDED BY LAYER BY LAYER COATINGS In the first research works appeared in the scientific literature, the layer by layer technique was addressed to the build-up of coatings with outstanding oxygen barrier

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 73 properties (Jang 2008, Grunlan & Jang 2008, Priolo 2010); then, it was also utilized for providing different substrates with anti-reflection (Podsiadlo 2007), electrical conductivity (Rivadulla 2010) or antibacterial properties (Dvoracek 2009, Gentile 2015). In general, the LbL coatings have been intentionally designed for modifying the surface of substrates having a high surface to bulk ratio, such as plastic films and fabrics: in fact, these substrates possess a high surface accessible for the LbL assembly with respect to the bulk. In the following, the state of the art about the multifunctional features that can be conferred to cellulosic fabrics by designing specific LbL assemblies is thoroughly reported.

Multifunctional Layer by Layer Coatings During the last 10 years, several types of LbL assemblies have been designed for expressly providing flame retardant features (i.e., to enhance thermal stability and fire retardancy of the cellulosic substrate) to different fabrics; in particular, fully inorganic, hybrid organic/inorganic and fully organic coatings have been proposed. As far as fully inorganic LbL systems are considered, one of the pioneering works was done by Laufer and co-workers, who assembled positive alumina-coated silica (10 nm) and negative silica nanoparticles (10 or 40 nm) on cotton fabrics, exploiting a dipping method (Laufer 2011). In particular, the deposition of up to 20 silica/silica bilayers was able to provide self-extinction to the treated fabrics during vertical flame spread tests and to increase the time to ignition in cone calorimetry experiments. The same silica /silica architectures were then applied to cotton fabrics by using a spray-assisted layer by layer deposition; the obtained results were compared with those achieved by dipping. More specifically, the horizontal spray method was found to homogeneously cover the fabric surface, conferring, at the same time, the best thermal protection to the underlying substrate during combustion tests (Alongi 2013, Carosio 2013b). Among the other types of inorganic nanoparticles utilized for assembling fully inorganic LbL coatings, Polyhedral Oligomeric Silsesquioxane (POSS®) salts, positively or negatively charged, have been considered. The first example refers to the use of water -soluble OctaAmmonium POSS ((+) POSS) and OctaTetramethylammonium POSS ((-)POSS), respectively as cationic and anionic components, for the design of 20 bilayers thin coatings (Li 2011a). Aminopropyl silsesquioxane oligomer was also considered as possible substitute cationic species. Micro cone calorimetry tests performed on the LbL treated cotton fabrics showed the formation of more than 12 wt.% char; furthermore, the afterglow time was found to significantly decrease during vertical flame spread tests and

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the LbL treatment allowed preserving the fabric texture and the shape of the individual fibers. The outcomes from the fully inorganic LbL assemblies pushed the scientific community towards the design of intumescent LbL coatings: in fact, thanks to their structure and composition, the intumescent LbL coatings can offer both a barrier effect (due to the presence of the inorganic layers) and a charring effect (due to the reactive intumescent layers). The first example of intumescent LbL coating on cotton combined a poly(allylamine) (which acted as a carbon source and blowing agent) with sodium phosphates (as acid source) (Li 2011b). The intumescent LbL-treated fabrics self-extinguished in vertical flame spread tests; besides, it was not possible to ignite the treated fabrics under the cone, using 35 kW/m2 heat flux. Chitosan and ammonium polyphosphate, i.e., a carbon source and a acid source/blowing agent, respectively, were combined in a layer by layer coating suitable for conferring flame retardant properties to cotton -polyester blends (Carosio 2012). In particular, during horizontal flame spread tests, the fabrics treated by 20 bilayers exhibited decreased combustion kinetics and did not show afterglow phenomena; furthermore, the obtained residues were coherent and thick. Instead of using intumescent LbL coatings based on bilayers, quad-layers made of poly (diallydimethylammonium chloride) / poly (acrylic acid) / poly (diallydimethyl ammonium chloride) / ammonium polyphosphate, were applied to cotton fabrics. The proposed LbL assemblies allowed the fabrics achieving self-extinction, decreasing, at the same time, the heat-related parameters (Alongi 2012e, Carosio 2013a). An oligoallylamine and its phosphonated counterpart were synthesized (Guirao 2013) and exploited in intumescent LbL assemblies on cotton fabrics, exploring the effect of the adopted experimental parameters (namely, molecular weight s of the synthesized products and pH conditions for the building up of the LbL coatings (Carosio 2015). First, a high thermal and thermo-oxidative stability was assessed through thermogravimetric analyses performed in nitrogen and air, along with the formation of very high residues at 800°C (37 and 31%, in nitrogen and air, respectively). The thickness of the LbL structures was influenced by the pH chosen during the deposition process. Afterglow phenomena were not detected on the LbL-treated fabrics, irrespective of the molecular weight of the components of the assembly; furthermore, horizontal flame spread tests clearly indicated that high molecular weight layers are more effective in lowering the total burning time and increasing of the final residues at the end of the tests. Huang et al. applied an intumescent flame retardant-polyacrylamide (derived from the copolymerization of N1-(5,5-dimethyl-1,3,2-dioxaphosphinyl-2-yl)-acrylamide with acrylamide) in combination with graphene oxide nanosheets on cotton fabrics (Huang 2012). The thermal stability of cotton was enhanced by the deposition of the LbL assembly, as assessed by thermogravimetric analyses. Cone calorimetry tests revealed a

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 75 decrease of pkHRR, as well as an increase of TTI in the presence of the deposited coating; in addition, the assembly was responsible for the formation of a continuous and coherent char after the combustion tests. The possibility of designing multifunctional flame -retardant, self-healing and superhydrophobic LbL assemblies on cotton fabrics was assessed and demonstrated by Chen et al., who exploited a solution -dipping method, with the sequential deposition of a trilayer of branched poly(ethylenimine), ammonium polyphosphate and fluorinated-decyl polyhedral oligomeric silsesquioxane (Chen 2015). In vertical flame spread tests, the LbL-treated fabrics showed an intumescent behavior, with the formation of a porous char layer that allowed achieving self-extinction. Besides, the fluorinated layer provided superhydrophobic features to the underlying cellulosic substrate, along with a selfhealing behavior. In fact, a simple water rinsing process allowed the LbL assemblies restoring the superhydrophobicity when this latter was lost. A significant part of the research activity on LbL coatings carried out during the last 5 to 10 years has been addressed to the setup of low environmental impact renewable intumescent coatings that, apart from flame retardant purposes, may provide the fabrics with multifunctional features (as an example, antibacterial properties). In particular, it was possible to exploit different biomacromolecules for the buildup of “green” treatments on different textile substrates. One of pioneering works was done by Laufer and co-workers, who combined chitosan (carbon source, positively charged) with phytic acid (acid source, negatively charged) in a 30 bi-layered assembly on cotton fabrics (Laufer 2012). The pH of the aqueous LbL solutions was found to strongly affect the composition of the obtained LbL nanocoatings: in particular, those obtained at pH 6 were thicker than the analogous prepared at pH 4 and with a different phytic acid content (48 vs 66 wt.% for pH 6 and 4, respectively). Vertical flame spread tests proved self-extinction for the fabrics coated with high phytic acid content assemblies, while untreated cotton was completely consumed. In addition, the fabrics treated with pH 4 aqueous solutions exhibited a significant decrease of pkHRR and of total heat release (-60 and -76%, respectively) with respect to the untreated fabric, during microcombustion calorimetry tests. Fang and co-workers applied an LbL intumescent flame retardant coating, comprising chitosan (positively charged) and ammonium polyphosphate (negatively charged) on cotton fabrics (Fang 2015a). The deposition of 20 bilayers remarkably reduced the burning time and efficiently limited fire propagation, with only 27 mm length in horizontal direction damaged. After vertical flame spread tests, the char formed on the LbL-treated fabrics was able to preserve the textile texture that showed a considerable mechanical strength. The occurrence of an intumescent effect was confirmed by the formation of several bubbles on the surface of the burnt fibers treated with 20 bilayers. Lastly, both pkHRR and total heat release decreased after the deposition of the LbL coating, as assessed by microcone calorimetry tests.

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The same group further designed a polyhexamethylene guanidine phosphate ammonium polyphosphate LbL coating exhibiting flame retardant and antimicrobial features (Fang 2015b). In particular, vertical flame spread tests carried out on the LbLtreated fabrics displayed a reduced burning time, no afterglow, and an increased residue, maintaining the original textile texture. Lastly, antimicrobial tests performed on the treated fabrics showed an antimicrobial action on E. coli and S. aureus. Very recently, ammonium polyphosphate was replaced with potassium alginate, extracted from seaweed: in doing so, it was possible to combine the intumescent effect of antimicrobial polyhexamethylene guanidine phosphate with the low impact character of potassium alginate (Fang 2016). The LbL-treated cotton fabrics turned out to show a significant decrease of peak of heat release rate and total heat release in microcombustion tests. At the same time, the antimicrobial effect on E. coli and S. aureus exerted by the LbL assemblies was assessed: more specifically, the inactivation of 100% E. coli and S. aureus was achieved in 5 and 30 min, respectively. The possibility of using DNA (deoxyribonucleic acid) in the design of intumescent LbL coatings has been quite recently assessed. In fact, the structure and chemical composition of this biomacromolecule mimics that of an all-in-one intumescent system (Alongi 2013b). In particular, its phosphate groups can behave as acid source, the deoxyribose units provide carbon and the nitrogen -containing bases may release blowing agent s (i.e., ammonia and nitrogen) (Alongi 2013c). Carosio et al. designed LbL anionic DNA /cationic chitosan assemblies on cotton, depositing up to 20 bilayers (Carosio 2013c). The DNA layers embedded in the assembly enhanced the char forming character of chitosan, hence leading the fabric to selfextinction in horizontal flame spread tests. In addition, 40% reduction of heat release rate, as well as a significant increase of the final residue was assessed in cone calorimetry tests performed on the fabrics treated with 20 bilayers. Referring to hybrid organic-inorganic LbL systems, the combination of nanoparticle layers together with the intumescent properties provided by organic layers has been repeatedly reported in the open scientific literature as a feasible method for achieving high performing treatments on different fabric substrates. Alongi et al. designed one of the first LbL hybrid coating on cotton -polyester blend fabrics, coupling chitosan, ammonium polyphosphate and silica nanoparticles (bearing positive or negative charges) (Alongi 2012f). The resulting assemblies provided the treated substrates with an increased flame retardant behavior, as assessed by horizontal flame spread and cone calorimetry tests. In particular, this latter was strictly correlated with both the morphology and the physical stability of the obtained LbL assemblies i.e., with the growth of a homogeneous and coherent LbL coating. Huang and co-workers designed LbL hybrid coatings for cotton, comprising poly(acrylic acid) and amino -functionalized montmorillonite nanoplatelets layers (Huang 2012). The treated fabrics showed decreased combustion kinetics. In addition, the

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 77 formation of a blown charred structure including montmorillonite nanoplatelets was assessed on the residues after combustion tests through SEM microscopy. Recently, montmorillonite nanoplatelets, in combination with dimethyl diallyl ammonium chloride-allyl glycidyl ether, were also utilized in LbL hybrid assemblies deposited on cotton fabrics (Gao 2015). In particular, an LbL modified technique, involving a dipping step and a subsequent padding, drying and curing, was exploited. The deposited coatings increased the char residue formed at high temperature, as shown by thermogravimetric analyses; an increase of the total burning time and of the final residue was obtained after vertical flame spread tests, as well as a slight increase of LOI values. Very recently, Chen et al. designed a LbL multifunctional coating with electrical conductivity, flame retardant and antimicrobial features (Chen 2016). For this purpose, up to 20 bilayers consisting of polyhexamethylene guanidine phosphate and potassium alginate-carbon nanotubes were deposited on cotton. The formation CNTs networks on the fabric surface was responsible for an excellent electrical conductivity. At the same time, the assembled LbL coating significantly enhanced the thermal stability and fire behavior of the underlying fabric, notwithstanding an increased inhibition effect of the growth of E. coli as a function of the number of deposited bilayers. One of the main drawbacks on the use of LbL coatings on fabrics refers to the water soluble character of many active components of the assemblies: as a consequence, the deposited coatings cannot tolerate repeated washing cycles, which significantly limits their potential applications. Therefore, several attempts have been carried out in order to solve this durability issue, although it is still an up-to-date challenge. In addition, it is also greatly needed to fabricate LbL assemblies in a more efficient way, i.e., by largely diminishing the number of layer deposition cycles required for conferring the envisaged final properties to the underlying fabric. For this purpose, quite recently, Carosio and Alongi combined the LbL process with the UV-curing technique (Pappas 1992): an anionic UV-curable aliphatic acrylic polyurethane latex doped with ammonium polyphosphate and in the presence of a suitable photoinitiator was coupled with chitosan and LbL deposited on cotton fabrics by dipping (Carosio & Alongi 2015). Subsequently, the obtained assembled coating was UV-cured: the thin coating formed on cotton fibers showed an enhanced flame retardant behavior; at the same time, the assembly provided the substrate with the self-extinction even after a washing cycle performed in water at 65°C for 1 h, hence showing an enhanced durability. Very recently, Tian and co-workers exploited the LbL method also for conferring a robust UV protection to cotton fabrics; to this aim, graphene oxide and chitosan layers were combined in the LbL assembly (Tian 2015, Tian 2016). The treated fabrics exhibited more than 40-fold increase of UPF value than control cotton (UPF rating at 9.37). In addition, the LbL-treated cotton showed an excellent washing fastness, even after 10 times water laundering.

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Another effort was very recently made by Casale et al. who obtained cotton fabrics with enhanced water resistance, self-extinction in flammability tests and good antimicrobial activity, by combining DNA and chitosan in a LbL assembly, exploiting the ability of chitosan to undergo UV-curing when exposed to UV radiation in the presence of a suitable photoinitiator (Casale 2016). One of the last challenging issues of the LbL method refers to the prospect of designing a continuous process, reasonably exploitable for commercial and industrial purposes. Quite recently, Chang and co-workers proposed a continuous layer by layer system suitable for depositing polymer −clay nanoassemblies on cotton fabrics and for enhancing their fire behavior (Chang 2014). In particular, layers of branched polyethylenimine with urea and diammonium phosphate (positively charged) and clay nanoparticles (negatively charged) were continuously applied to the fabric substrate up to 50 bilayers, using a single “roll-to-roll” process and avoiding the rinsing step. The proposed continuous LbL technique was found to confer an increased thermal stability to the fabrics and to enhance the combustion parameters (i.e., to increase TTI and to decrease the heat release rate) as proved by thermogravimetric analyses and cone calorimetry tests, respectively.

CONCLUSION This chapter has clearly demonstrated the importance, practicability and consistency of sol-gel processes and layer by layer methods as robust surface engineering approaches for providing cellulosic fabrics (and their blends with polyester) with multifunctional features (i.e., fire retardancy, anti-bacterial activity, UV-protection, wear resistance ). This goal can be successfully achieved by simply “playing” with the chemical structure of the sol-gel precursors or of the LbL components, hence tuning the obtained coatings towards the final application. Notwithstanding the wide attention, the two strategies have merited until today, especially from the scientific/academic world, they still represent a challenging issue for several reasons here summarized. In particular, referring to sol-gel approaches, their current main limitation may be ascribed to the “chemistry behind the process” that could limit the scale-up of the technique to fit the already existing finishing treatments at an industrial scale. A further limitation refers to the possible detrimental effect of the treatment on the hand/comfort of the fabrics, even, in some cases, when hybrid organic-inorganic sol-gel treatments are specifically designed on the cellulosic substrates. Conversely, at present, the Layer by layer approach shows some limitations in view to a possible scale-up at an industrial level that should ensure the commercial valorization /exploitation of this surface engineering method. Besides, the up-to-now weak durability

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 79 of the proposed multifunctional assemblies seems to limit and to exert a negative influence on a large-scale exploitation of this strategy. However, the valuable research work that has been performed in the last years within the scientific community is intended to demonstrate that a further development and optimization of these strategies may overcome their current limit ations.

REFERENCES Abidi, N., Hequet, E., Tarimala, S., Dai, L. L. (2007). Cotton Fabric Surface Modification for Improved UV Radiation Protection Using Sol–Gel Process. J. Appl. Polym. Sci. 104, 111-117. Abidi, N., Cabrales, L., Hequet, E. (2009). Functionalization of a Cotton Fabric Surface with Titania Nanosols. ACS Appl. Mater. Interfaces 1, 2141-2146. Alongi, J., Carletto, R. A., Di Blasio, A., Carosio, F., Bosco, F., Malucelli G. (2013). Intrinsic Intumescent-Like Flame Retardant Properties of DNA -Treated Cotton Fabrics. Carbohydr. Polym. 96, 296-304. Alongi, J., Carletto R. A., Di Blasio, A., Carosio, F., Bosco, F., Malucelli G. (2013b). DNA: a Novel, Green, Natural Flame Retardant and Suppressant for Cotton. J. Mater. Chem. A 1, 4779-4785. Alongi, J., Carosio, F., Frache, A., Malucelli, G. (2013c). Layer by Layer Coatings Assembled Through Dipping, Vertical or Horizontal Spray for Cotton Flame Retardancy. Carbohydr. Polym. 92, 114-119. Alongi, J., Carosio, F., Malucelli, G. (2012e). Influence of Ammonium Polyphosphate/poly(acrylic acid )-based Layer by Layer Architectures on the Char Formation in Cotton, Polyester and their Blends. Polym. Degrad. Stab. 97, 1644-1653. Alongi, J., Carosio, F., Malucelli, G. (2012f). Layer by Layer Complex Architectures Based on Ammonium Polyphosphate, Chitosan and Silica on Polyester-Cotton Blends: Flammability and Combustion Behavior. Cellulose 19, 1041-1050. Alongi, J., Ciobanu, M., Malucelli, G. (2011a). Cotton Fabrics Treated with Hybrid Organic-inorganic Coatings Obtained Through Dual-cure Processes. Cellulose 18, 1335-1348. Alongi, J., Ciobanu, M., Carosio, F., Tata, J., Malucelli, G. (2011b). Thermal Stability and Flame Retardancy of Polyester, Cotton, and Relative Blend Textile Fabrics Subjected to Sol-Gel Treatments. J. Appl. Polym. Sci. 119, 1961-1969. Alongi, J., Ciobanu, M., Malucelli, G. (2011c). Sol-gel Treatments for Enhancing Flame Retardancy and Thermal Stability of Cotton Fabrics: Optimisation of the Process and Evaluation of the Durability. Cellulose 18, 167-177.

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Alongi, J., Ciobanu, M., Malucelli, G. (2011). Novel Flame Retardant Finishing Systems for Cotton Fabrics Based on Phosphorus-Containing Compounds and Silica Derived from Sol-Gel Processes. Carbohydr. Polym. 85, 599-608. Alongi, J., Ciobanu, M., Malucelli, G. (2012a). Sol-Gel Treatments on Cotton Fabrics for Improving Thermal and Flame Stability: Effect of the Structure of the Alkoxysilane Precursor. Carbohydr. Polym. 87, 627-635. Alongi, J., Ciobanu, M., Malucelli, G. (2012b). Thermal Stability, Flame Retardancy and Mechanical Properties of Cotton Fabrics Treated with Inorganic Coatings Synthesized through Sol-Gel Processes. Carbohydr. Polym. 87, 2093-2099. Alongi, J., Colleoni, C., Malucelli, G., Rosace, G. (2012c). Hybrid Phosphorus-Doped Silica Architectures Derived from a Multistep Sol-Gel Process for Improving Thermal Stability and Flame Retardancy of Cotton Fabrics. Polym. Degrad. Stab. 97, 1334-1344. Alongi, J., Colleoni, C., Rosace, G., Malucelli, G. (2012d). Thermal Stability, Flame Retardancy and Mechanical Properties of Cotton Fabrics Treated with Inorganic Coatings Synthesized Through Sol-Gel Processes. J. Therm. Anal. Calorim. 110, 1207-2016. Alongi, J., Colleoni, C., Rosace, G., Malucelli, G. (2013a). The Role of Pre-Hydrolysis on Multistep Sol-Gel Processes for Enhancing the Flame Retardancy of Cotton. Cellulose 20, 525-535. Alongi, J., Malucelli, G. (2012). State of the Art and Perspectives on Sol–Gel Derived Hybrid Architectures for Flame Retardancy of Textiles. J. Mater. Chem. 22, 2180521809. Alongi, J., Malucelli, G. (2013). Thermal Stability, Flame Retardancy and Abrasion Resistance of Cotton and Cotton-Linen Blends Treated by Sol-Gel Silica Coatings Containing Alumina Micro- or Nano-Particles. Polym. Degrad. Stab. 98, 1428-1438. Alongi, J., Malucelli, G. (2015). Thermal Degradation of Cellulose and Cellulosic Substrates. In: Reactions and Mechanisms in Thermal Analysis of Advanced Materials. Scrivener Publishing LLC, Beverly. Bottcher, H. (1992). Specific Optical and Electrical Properties of Vacuum-Deposited Thin Films of Dyes. J. Prakt. Chem. 334, 14-24. Brancatelli, G., Colleoni, C., Massafra, M. R., Rosace, G. (2011). Effect of Hybrid Phosphorus-Doped Silica Thin Films Produced by Sol-Gel Method on the Thermal Behavior of Cotton Fabrics. Polym. Degrad. Stab. 96, 483-490. Caldara, M., Colleoni, C., Guido, E., Re, V., Rosace, G. (2012). Development of a Textile-optoelectronic pH Meter Based on Hybrid Xerogel Doped with Methyl Red. Sens. Actuators B 171-172, 1013-1021. Carosio, F., Alongi, J. (2015). Few Durable Layers Suppress Cotton Combustion due to the Joint Combination of Layer by Layer Assembly and UV-Curing. RSC Adv. 5, 71482-71490.

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 81 Carosio, F., Alongi, J., Malucelli, G. (2012). Layer Ammonium Polyphosphate-based Coatings for Flame Retardancy of Polyester-Cotton Blends. Carbohydr. Polym. 88, 1460-1470. Carosio, F., Alongi, J., Malucelli, G. (2013a). Flammability and Combustion Properties of Ammonium Polyphosphate-/poly(acrylic acid )-based Layer by Layer Architectures Deposited on Cotton, Polyester and their Blends. Polym. Degrad. Stab. 98, 1626-1630. Carosio, F., Di Blasio A., Cuttica, F., Alongi, J., Frache, A., Malucelli G. (2013b). Flame Retardancy of Polyester Fabrics Treated by Spray-Assisted Layer-by-Layer Silica Architectures. Ind. Eng. Chem. Res. 52, 9544-9550. Carosio, F., Di Blasio, A., Alongi, J., Malucelli, G. (2013c). Green DNA -Based Flame Retardant Coatings Assembled through Layer by Layer Polymer 54, 5148-5153. Carosio, F., Negrell-Guirao, C., Di Blasio, A., Alongi, J., David, G., Camino, G. (2015). Tunable Thermal and Flame Response of Phosphonated Oligoallylamines Layer by Layer Assemblies on Cotton. Carbohydr. Polym. 115, 752-759. Casale, A., Bosco, F., Malucelli, G., Mollea, C., Periolatto, M. (2016). DNA -chitosan Cross-linking and Photografting to Cotton Fabrics to Improve Washing Fastness of the Fire-resistant Finishing. Cellulose 23, 3963-3984. Chang, S., Slopek, R. P., Condon, B., Grunlan, J. C. (2014). Surface Coating for FlameRetardant Behavior of Cotton Fabric Using a Continuous Layer-by-Layer Process. Ind. Eng. Chem. Res. 53, 3805-3812. Chen, X., Fang, F., Zhang, X., Ding, X., Wang, Y., Chen, L., Tian, X. (2016). FlameRetardant, Electrically Conductive and Antimicrobial Multifunctional Coating on Cotton Fabric via Layer-by-Layer Assembly Technique. RSC Adv. 6, 27669-27676. Chen, S., Li, X., Li, Y., Sun, J. (2015). Intumescent Flame-Retardant and Self-Healing Superhydrophobic Coatings on Cotton Fabric. ACSNano 9, 4070-4076. Cireli, A., Onar, N. (2008). Leaching and Fastness Behavior of Cotton Fabrics Dyed with Different Type of Dyes Using Sol-gel Process. J. Appl. Polym. Sci. 109, 97-105. Colleoni, C., Massafra, M. R., Rosace, G. (2012). Photocatalytic Properties and Optical Characterization of Cotton Fabric Coated via Sol–gel with Non-crystalline TiO2 Modified with Poly(ethylene glycol ). Surf. Coat. Technol. 207, 79-88. Decher, G., Hong, J. D. (1991). Buildup of Ultrathin Multilayer Films by a Self-assembly Process. 1. Consecutive Adsorprtion of Anionic and Cationic Bipolar Amphiphiles on Charged Surfaces. Makromol. Chem. Macromol. Symp. 1991, 46, 321-327. Dvoracek, C. M., Sukhonosova, G., Benedik, M. J., Grunlan J. C. (2009). Antimicrobial Behavior of Polyelectrolyte–Surfactant Thin Film Assemblies. Langmuir, 25, 1032210328. Fang, F., Chen, X., Zhang, X., Cheng, C.; Xiao, D., Meng, Y., Ding, X., Zhang, H., Tian, X. (2016). Environmentally Friendly Assembly Multilayer Coating for Flame Retardant and Antimicrobial Cotton Fabric. Prog. Org. Coatings 90, 258-266.

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Fang, M. M., Kaschak, D. M., Sutorik, A. C., Mallouk, T. E. (1997). A Mix and Match Ionic-covalent Strategy for Self-assembly of Inorganic Multilayer Films. J. Am. Chem. Soc. 119, 12184-12191. Fang, F., Zhang, X., Meng, Y., Gu, Z., Bao, C., Ding, X., Li, X., Chen, X., Tian, X. (2015a). Intumescent Flame Retardant Coatings on Cotton Fabric of Chitosan and Ammonium Polyphosphate via Layer-by-Layer Assembly. Surf. Coatings Technol. 262, 9-14. Fang, F., Xiao, D., Zhang, X., Meng, Y., Cheng, C., Bao, C., Ding, X., Cao, H., Tian, X. (2015b). Construction of Intumescent Flame Retardant and Antimicrobial Coating on Cotton Fabric via Layer-by-Layer Assembly Technology. Surf. Coatings Technol. 276, 726-734. Gao, D., Li, R., Lv, B., Ma, J., Tian, F., Zhang, J. (2015). Flammability, Thermal and Physical-Mechanical Properties of Cationic Polymer/Montmorillonite Composite on Cotton Fabric. Composites Part B 77, 329-337. Gentile, P., Frongia, M. E., Cardellach, M., Miller, C. A., Stafford G. P., Leggett, G. J., Hatton P. V. (2015). Functionalised Nanoscale Coatings Using Layer-By-Layer Assembly for Imparting Antibacterial Properties to Polylactide-co-Glycolide Surfaces. Acta Biomater. 21, 35-43. Grunlan, J. C., Jang W.-S. (2008). Layer-By-Layer Assembly of Nano Brick Walls: Tailoring Film Growth and Gas Permeability. ACS Polym. Preprints, 49, 342. Guirao, C., Carosio, F., Boutevin, B., Cottet, H., Loubat, C. (2013). Phosphonated Oligoallylamine: Synthesis, Characterization in Water, and Development of Layer by Layer Assembly. J. Polym. Sci. Part B: Polym. Phys. 51, 1244-1250. Hribernik, S., Smole, M. S., Kleinschek, K. S., Bele, M., Jamink, J., Gaberscek, M. (2007). Flame Retardant Activity of SiO2 -coated Regenerated Cellulose Fibers. Polym. Degrad. Stab. 92, 1957-1965. Huang, G., Liang, H., Wang, X., Gao, J. (2012). Poly(Acrylic Acid)/Clay Thin Films Assembled by Layer-by-Layer Deposition for Improving the Flame Retardancy Properties of Cotton. Ind. Eng. Chem. Res. 51, 12299-12309. Huang, K. S., Nien, Y. H., Hsiao, K. C., Chang, Y. S. (2006). Application of DMEU/SiO2 Gel Solution in the Antiwrinkle Finishing of Cotton Fabrics. J. Appl. Polym. Sci. 102, 4136-4143. Huang, G., Yang, J., Gao, J., Wang, X. (2012). Thin Films of Intumescent Flame Retardant-Polyacrylamide and Exfoliated Graphene Oxide Fabricated via Layer-ByLayer Assembly for Improving Flame Retardant Properties of Cotton Fabric. Ind. Eng. Chem. Res. 51, 12355-12366. Ichinose, I., Kawakami, T., Kunitake, T. (1998). Alternate Molecular Layers of Metal Oxides and Hydroxyl Polymers Prepared by the Surface Sol-Gel Process. Adv. Mater. 10, 535-539.

Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 83 Isquith, A. J., Abbott, E. A., Walters, P. A. (1972). Surface Bonded Antimicrobial Activity of an Organosilicon Quaternary Ammonium Chloride. Appl. Microbiol. 24, 859-863. Jang, W. S., Rawson, I., Grunlan J. C. (2008). Layer-by-layer Assembly of Thin Film Oxygen Barrier. Thin Solid Films 516, 4819-4825. Kappes, R. S., Urbainczyk, T., Artz, U., Textor, T., Gutmann, J. S. (2016). Flame Retardants Based on Amino Silanes and Phenylphosphonic Acid. Polym. Degrad. Stab. 129, 168-179. Lapidot, N., Gans, O., Biagini F., Sosonkin, L., Rottman, C. (2003). Advanced Sunscreens: UV Absorbers Encapsulated in Sol-Gel Glass Microcapsules. J. Sol–Gel Sci. Technol. 26, 67-72. Laufer, G., Carosio, F., Martinez, R., Camino, G., Grunlan, J. C. (2011). Growth and Fire Resistance of Colloidal Silica-Polyelectrolyte Thin Film Assemblies. J. Colloid Interface Sci. 356, 35669-35677. Laufer, G., Kirkland, C., Morgan, A., Grunlan, J. C. (2012). Intumescent Multilayer Nanocoating, Made with Renewable Polyelectrolytes, for Flame-Retardant Cotton. Biomacromolecules 13, 2843-2848. Lewin, M. (2001). Synergism and Catalysis in Flame Retardancy of Polymers. Polym. Adv. Technol. 12, 215-222. Li, F. Y., Xing, Y. J., Ding, X., Zu, Y. (2007). Immobilization of Papain on Cotton Fabric by Sol–gel Method. Enzyme Microb. Technol. 40, 1692-1697. Li, Y. C., Mannen, S., Schulz, J., Grunlan, J. C. (2011a). Growth and Fire Protection Behavior of POSS-based Multilayer Thin Films. J. Mater. Chem. 21, 3060-3069. Li, Y. C., Mannen, S., Morgan, A. B., Chang, S. C., Yang, Y. H., Condon, B., Grunlan, J. C. (2011b). Intumescent All-Multilayer Nanocoating Capable of Extinguishing Flame on Fabric. Adv. Mater. 23, 3926-3930. Liu, Y., Pan, Y. T., Wang, X., Acuña, P., Zhu, P., Wagenknecht, U., Heinrich, G., Zhang, X. Q., Wang, R., Wang D.-Y. (2016). Effect of Phosphorus-Containing Inorganic– Organic Hybrid Coating on the Flammability of Cotton Fabrics: Synthesis, Characterization and Flammability. Chem. Eng. J. 294, 167-175. Mahltig, B., Fiedler, D., Böttcher, H. (2004). Antimicrobial Sol-Gel Coatings. J. Sol-Gel Sci. Technol. 32, 219-222. Mahltig, B., Textor, T. (2006). Combination of Silica Sol and Dyes on Textiles. J. SolGel Sci. Technol. 39, 111-118. Malucelli, G. (2016a). Layer-By-Layer Nanostructured Assemblies for the Fire Protection of Fabrics. Mater. Lett. 166, 339-342. Malucelli, G. (2016b). Surface-Engineered Fire Protective Coatings for Fabrics through Sol-Gel and Layer-by-Layer Methods: An Overview. Coatings 6, 1-23.

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Sol-Gel and Layer by Layer Methods for Conferring Multifunctional Features … 85 Stockton, W. B., Rubner, M. F. (1997). Molecular-Level Processing of Conjugated Polymers. 4. Layer-by-Layer Manipulation of Polyaniline via Hydrogen-Bonding Interactions. Macromolecules 30, 2717-2725. Sundaresan, K., Sivakumar, A., Vigneswaran, C., Ramachandran, T. (2012). Influence of Nano Titanium Dioxide Finish, Prepared by Sol-Gel Technique, on the Ultraviolet Protection, Antimicrobial, and Self-Cleaning Characteristics of Cotton Fabrics. J. Ind. Text. 41, 259-277. Tarimala, S., Kothari, N., Abidi, N., Hequet, E., Fralick, J., Dai, L. L. (2006). New Approach to Antibacterial Treatment of Cotton Fabric with Silver Nanoparticle– Doped Silica Using Sol–Gel Process. J. Appl. Polym. Sci. 101, 2938-2943. Tian, M., Hu, X., Qu, L., Du, M., Zhu, S., Sun, Y., Han, G. (2016). Ultraviolet Protection Cotton Fabric Achieved via Layer-by-Layer Self-Assembly of Graphene Oxide and Chitosan. Appl. Surf. Sci. 377, 141-148. Tian, M., Tang, X., Qu, L., Zhu, S., Guo, X., Han, G. (2015). Robust Ultraviolet Blocking Cotton Fabric Modified with Chitosan /Graphene Nanocomposites. Mater. Lett. 145, 340-343. Tomsic, B., Simoncic, B., Orel, B., Cerne, L., Forte Tavcer, P., Zorko, M., Jerman, I., Vilcnik, A., Kovac, J. (2008). Sol–gel Coating of Cellulose Fibres with Antimicrobial and Repellent Properties. J Sol-Gel Sci Technol. 47, 44-57. Van der Schueren, L., De Clerck, K., Brancatelli, G., Rosace, G., Van Damme E., De Vos W. (2012). Novel Cellulose and Polyamide Halochromic Textile Sensors Based on the Encapsulation of Methyl Red into a Sol–gel Matrix. Sens. Actuators B 162, 27-34. Vilčnik, A., Jerman, I., Šurca Vuk A., Koželj, M., Orel, B., Tomšič, B., Simončič, B., Kovač, J. (2009). Structural Properties and Antibacterial Effects of Hydrophobic and Oleophobic Sol-Gel Coatings for Cotton Fabrics. Langmuir, 25, 5869-5880. Wang, X., Naka, K., Wang, C., Itoh, H., Uemura, T., Chujo, Y. (2007). Layer-by-layer Films Based on Charge Transfer Interaction of ϕ-conjugated Poly(dithiafulvene) and Incorporation of Gold Nanoparticles into the Films. J. Appl. Polym. Sci. 103, 16081615. Wang, R. H., Xin, J. H., Tao, X. M. (2005). UV-Blocking Property of Dumbbell-Shaped ZnO Crystallites on Cotton Fabrics. Inorg. Chem. 44, 3926-3930. Xin, J. H., Daoud, W. A., Kong, Y. Y. (2004). A New Approach to UV – Blocking Treatment for Cotton Fabrics. Text. Res. J. 74, 97-100. Xing, Y. J., Yang, X. J., Dai, J. J. (2007). Antimicrobial Finishing of Cotton Textile Based on Water Glass by Sol–Gel Method. J. Sol-Gel Sci. Technol. 43, 187-192.

In: Textiles: Advances in Research and Applications ISBN: 978-1-53612-855-0 Editor: Boris Mahltig © 2018 Nova Science Publishers, Inc.

Chapter 4

WASTEWATER TREATMENT FROM TEXTILE INDUSTRY USING ADVANCED OXIDATION PROCESSES C. López-López1,2, J. Martín-Pascual1,2 and J. M. Poyatos1,2,* 1

Department of Civil Engineering, University of Granada, Granada, Spain 2 Institute for Water Research, University of Granada, Granada, Spain

ABSTRACT In recent years, the use of natural resources in industrial sectors has increased, specifically in the textile industry. One of the main problems generated by the textile industry is related to the wastewater generated. The textile industry is considered one of the main generators of complex dangerous water pollutants with high contents of organic and inorganic matter, suspended solids, turbidity and colour, among others. The main points of generation of these pollutants are focused on the emission of air, solid and liquid wastes, energy consumption and generation of wastewater with high contents of slowly biodegradable compounds. Different types of synthetic dyes and pigments are used in this industry; however, the most used are the azo dyes, followed by phthalocyanines, both of which contain highly stable molecules. The removal of this pollution is difficult using conventional biological treatments, which are insufficient to remove the high pollution load and have a great impact on the environment. In this chapter we present physical-chemical treatments to treat wastewater. These more intensive technologies are capable of removing these recalcitrant and toxic substances characterised by low biodegradability. The technology based on Advanced Oxidation Processes (AOPs) involves the transformation of organic contaminants in less complex species and even full mineralisetion. The studies done with H2O2/UV, photoFenton (Fe2+/H2O2/UV) and heterogeneous photocatalysis (TiO2/H2O2/UV) processes have shown very high efficiency of colour and organic matter removal. *

Corresponding Author Email: [email protected].

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C. López-López, J. Martín-Pascual and J. M. Poyatos The high turbidity in the water caused by the colouration can produce a shield that reduces the penetration of UV light and causes a decrease in the efficiency of AOP. Therefore, pre-treatment must be considered for these technologies. CoagulationFlocculation (CF) as pre-treatment can be a reliable combination to remove excess turbidity in the wastewater and increase the efficiency of AOPs.

1. PROBLEMS OF WASTEWATER IN THE TEXTILE INDUSTRY The industrial development and the improvement of analytical methods have led to the concern about complex bioresistant pollutants in the wastewater effluents, which has become an important aspect to consider in the textile industry. This industry is one of the largest groups of industries causing intense water pollution, because it is one of the most important consumers of resources (energy and water). This high consumption is especially great during the cleaning of the raw material, which involves numerous steps of washing throughout the production manufacturing process (Lin and Chen, 1997; Rodríguez et al., 2002; Al-Kdasi et al., 2004; Scharnk et al., 2007; Blanco et al., 2014). According to Rajeshwar et al. (2008), each year around 0.7 million tons of organic synthetic dyes are produced, mainly for use in the textile sector, leather goods, industrial paint, food, plastics, cosmetics and consumer electronics. Wastewater from the textile industry is complex and consists of a mixture of many polluting substances, generally synthetic organic compounds with high complex molecular structure s and large molecular weight s (McMullan et al., 2001; Fu et al., 2002). This wastewater contains a wide variety of dyes and chemicals with a relatively low biological oxygen demand (BOD) and high chemical oxygen demand (COD), suspended solids (SS) (Correia et al., 1994; Al-Kdasi et al., 2004) and matter in dissolved and/or colloidal form. Moreover, the dyes also have intense colours and high contents of other soluble substances (Correia et al., 1994; Dae-Hee et al., 1999). The effects of the direct discharge of this wastewater into water bodies are water pollution and flora and fauna disruption (Kumar-Verma et al., 2012), which leads to a considerable environmental problem (Savin and Butnaru, 2008). The presence of colour with the high level of organic and reactive compounds could transform in toxic and/or mutagenic products in contact with a natural water mass (Weisburger, 2002, Blanco et al., 2012). Due to the complexity of the compounds contained in textile industrial wastewater, its biodegradability is a particularly complex parameter. Effluents from the textile industry contain toxic compounds, most of them non-biodegradable (Ledakowicz and Gonera, 1999). The high levels of coloured products and toxic chemicals introduced into the environment are not easily eliminated by conventional biological treatments (Garcia et al., 2009; Kumar-Verma et al., 2012).

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About 20% of toxic compounds discharged into the water are caused by the textile industry; the most of them come from products manufactured outside of the European Union, especially from Asian countries. The European Commission is concerned with the use of CMR (Carcinogenic, Mutagenic and Toxic for Reproduction) products in the textile industry. Some of the pollutants introduced by the textile industries are called emerging pollutants that are currently unknown or unrecognised pollutants. The presence of these compounds in the environment is not necessarily new but at the moment concern regarding the environment and the public health is high. In recent years, the concern about emerging compounds has increased due to their continued and excessive use and their release into the environment (Al-Khazrajy and Boxall, 2016). In the twentieth century alone, more than 100,000 chemicals that are used in daily life, either in households, industries or agriculture, have been introduced into the environment (Primel et al., 2012). The detection of these pollutants is possible thanks to new technologies and emerging pollutants are detected in wastewater, water surfaces, soils and sediments around the world in very small concentrations (from ng/L to μg/L) (He et al., 2016). The trace concentrations of these compounds have adverse effects, most of which are currently unknown. In addition, there are numerous emerging contaminants, from pharmaceutical products (Martín et al., 2012; Al-Khazrajy and Boxall, 2016; He et al., 2016), personal care and hygiene products (Camacho-Muñoz et al., 2009; PeñaÁlvarez y Castillo-Alanís, 2015; He et al., 2016), perfluorinated compounds (Arvaniti y Stasinakis, 2015; Wilkinson et al., 2016; Zahn et al., 2016), hormones (Robles-Molina et al., 2014), abuse drugs (Martínez-Bueno et al., 2011; Robles-Molina et al., 2014; Wilkinson et al., 2016) and organic compounds (Pivnenko et al., 2015; Wilkinson et al., 2016; Zahn et al., 2016). In spite of the variability of the textile wastewater composition, according to Crespi (1994), the following general characteristics can be surmised: 1. 2. 3. 4. 5. 6. 7.

Great variability of flow and load, depending on the industry. High content of colloidal and suspended particles. Organic load higher than that of an urban wastewater. High pollution in soluble form, therefore the wastewater is generally coloured. Generally do not contain toxic products. Deficient in nutrients. Low concentration of pathogenic microorganisms.

The textile industry uses numerous types of dyes and synthetic pigments depending on the nature of the industry. These compounds include azo, anthraquinones, triarylmethanes and phthalocyanine groups. Azo dyes account for 70% of over one million tons of dyes produced per year, making them the largest group of dyes released into the environment (dos Santos et al., 2007). Phthalocyanines used to obtain blue and

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green dyes are the second dye type used (Forgacs et al., 2004; Shu and Chang, 2005). Initially, the phthalocyanines were used as pigments in the textile industry but were subsequently used in various applications, such as biology, medicine and industry; this highlights the importance of these complex molecules (Jancûla and Maršalek, 2011). Phthalocyanines are present in industrial wastewater, as shown in numerous studies carried out on textile wastewater (Alinsafi et al., 2007, Fu et al., 2002; Sena and Deminrer, 2003; Shu and Chang, 2005; Kalsoom et al., 2012; Ghasemi et al., 2013). These dyes and pigments are generally non-biodegradable and are composed of chemically stable molecules. In addition, dyes and pigments are designed to resist degradation by different external agents, which make them difficult to neutralise using conventional systems, such as biological processes (Khalid et al., 2011). These characteristics make chemical synthetic dyes highly recalcitrant and capable of inducing toxicity in aquatic organisms and, in some cases, in humans (Bandala et al., 2008). This has forced the search for new technologies capable of eliminating these contaminants using a combination of several physical, chemical and biological methods (Velegraki et al., 2006; Fatta-Kassinos et al., 2011). The aim of the present chapter is to summarise the use of Advanced Oxidation Processes (AOPs) to treat textile wastewater and to show the most relevant results obtained using three oxidation processes: H2O2/UV, photo-Fenton and heterogeneous photocatalysis (TiO2/H2O2/UV).

2. TECHNOLOGIES FOR TEXTILE WASTEWATER TREATMENT As a consequence of the problems caused by this wastewater type, the use of advanced technologies is necessary for its treatment. Given the recalcitrant and poor biodegradability of these effluents, biological processes are not efficient. Physicalchemical treatments, such as coagulation-flocculation, membrane technology, such as reverse osmosis, ultrafiltration and nanofiltration, adsorption and AOPs, are used reliably (Salas Colotta, 2003; Holkar et al., 2016). Coagulation-flocculation is based on the addition of polyelectrolytes that form flocs with the dye molecules, thereby facilitating their removal by settling. The removal efficiencies of this process are high, but in the process a polluted concentrated sludge is generated (GiPavas et al., 2017). The membrane technology based on physical separation, including reverse osmosis, ultrafiltration and nanofiltration, effectively separate the dye molecules and other compounds that are larger than the pore size of the selected membrane. Membrane technologies are widely recognised as an adequate option for wastewater treatment and reclamation because they can selectively remove contaminants from wastewater. (Choo et al., 2007; Alventosa-deLara et al., 2014; Thamaraiselvan and Noel, 2015; Avdicevic et al., 2017). The adsorption process is based

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on the physical retention of the dye molecules on the surface of the adsorbent used. The efficiency of this process is influenced by the interaction between the dye and adsorbent, the specific surface area of the adsorbent, the size of the dye molecule, temperature, pH and contact time. Activated carbon is an effective adsorbent for a wide range of dyes. Carbon-based materials have varied applications due their remarkable properties that enable tailoring to a particular process, however decolourisation has some limitation, especially when dealing with bulkier and complex species (Holkar et al., 2016; Khamparia and Jaspal, 2017). Other alternatives for the elimination of recalcitrant pollutants from textile industry wastewater are AOPs (Agustina et al., 2005; Sillanpää et al., 1998; Choi et al., 2013; Del Moro et al., 2013), on which the present chapter is focused. These are a good option when conventional processes are insufficient to remove persistent pollutants (Moreira et al., 2005).

2.1. Advance Oxidation Processes AOPs are physical chemical processes based on the addition of different compounds. They have been used successfully in the removal of organic matter and colour. AOPs are a complex and synergistic treatment of textile wastewater, involving the conversion of organic pollutants to short species and even their complete mineralisetion through the generation of highly reactive free radical oxidants (Rodrigues et al., 2008). These processes can eliminate toxic substances and increase the biodegradability of organic pollutants (Zayas Perez et al., 2007). AOPs have been extensively studied and are considered as an improved technological alternative to conventional wastewater treatment (Yonar et al., 2006). The AOPs based on the use of UV radiation and oxidants such as hydrogen peroxide (H2O2) have been used successfully for removing many different substances. The aim of these processes is to eliminate non-biodegradable soluble compounds and/or high chemical stability in wastewater (Poyatos et al., 2010). Due to the high reactivity of the OH• radicals it is possible to remove both organic and inorganic compounds, often to the point of extensive mineralisetion of the target pollutant (Pignatello et al., 1999), thus achieving a reduction of COD, Total Organic Carbon (TOC) and toxicity in the treated wastewater (Del Moro et al., 2013). AOPs are technologies based on physical-chemical processes that lead to the chemical oxidation of a large range of pollutants, under low pressure and temperature conditions. This oxidation causes the formation of highly reactive oxygen species, specifically OH• radicals, with an oxidation potential equal to 2.8 Volts (Papoutsakis et al., 2015). These oxidising species can be generated by photochemical means (including sunlight) or other forms of energy, thereby achieving a high efficiency in the oxidation of

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organic matter. OH• radicals are often combined with metal or semiconductor catalysts and UV radiation (Homem and Santos, 2011). AOPs can be used as the main treatment or as part of the purification process to increase their efficiency. AOPs are especially useful as a pre-treatment prior to a biological treatment to remove contaminants resistant to biodegradation (Pulgarin et al., 1999; Kajitvichyanukul and Suntronvipart, 2006; Tantak and Chaudhari, 2006; Lucas et al., 2007) or as a post-treatment process (Scott and Ollis, 1995) to improve the characteristics of the water before discharge to water bodies. In addition, AOPs are considered as a potential future treatment for water due to their high efficiency in disinfecting most viruses, bacteria and protozoa (Guo et al., 2009; Rubio et al., 2013), with Escherichia coli being the most common method used to microbiologically control water quality (Pitkänen et al., 2007). ADVANCED OXIDATION PROCESSES

HOMOGENEOUS PROCESSES

WITHOUT ENERGY

WITH ENERGY

ULTRAVIOLET RADIATION

ULTRASOUND ENERGY

HETEROGENEOUS PROCESSES

ELECTRICAL ENERGY

MICROWAVE

O3 in alkaline medium

Photocatalytic ozonation

Catalytic ozonation

O3/UV

O3/US

Electrochemical oxidation

MW/H2O2

O3/H2O2

H2O2/UV

H2O2/US

Anodic oxidation

MW/catalyst

H2O2/catalyst

O3/H2O2/UV

Heterogeneous photocatalysis

Electro-Fenton

Photo-Fenton Fe2+/H2O2/UV

UV vacuum

Figure 1. Classification of Advanced Oxidation Processes. Abbreviations used: O3 ozonation, H2O2 hydrogen peroxide, UV ultraviolet, US energy with ultrasound, Fe2+ ferrous ion. (Modified from Poyatos et al., 2010).

Figure 1 shows the numerous existent AOPs, which can be classified as heterogeneous or homogenous. In the more commonly used homogeneous process, the

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oxidant and the catalyst exist in single phase, whilst in the heterogeneous process the catalyst has a different phase. Homogeneous processes can be further subdivided into processes that use energy and processes that do not use energy (Poyatos et al., 2010). AOPs include a combination of oxidants (e.g., H2O2 or ozone ), catalysts (e.g., Fe2 + or TiO2) and energy (UV radiation or ultrasounds). The AOPs most investigated in wastewater treatment are heterogeneous and homogeneous photocatalytic methods based on UV radiation or solar irradiation, ozonisetion, Fenton process, ultrasonic, electrochemical and wet-air oxidation (Rizzo, 2011). A chemical wastewater treatment using AOPs can produce complete mineralisetion of the pollutants in CO2, water and inorganic compounds, or these compounds can be transformed into innocuous products (Poyatos et al., 2010; Del Moro et al., 2013). In addition, partial decomposition of nonbiodegradable organic pollutants can lead to biodegradable intermediates (Sarria et al., 2002). Of these different processes, the present chapter is focused on the UV/H2O2, photoFenton and photocatalysis heterogeneous (TiO2/H2O2/UV).

2.1.1. H2O2/UV This process, which involves the combination of UV radiation with hydrogen peroxide (UV/H2O2), is one of the most appropriate and promising technologies for the removal of toxic organic compounds from wastewater (Alnaizy and Akgerman, 2000; Aleboyeh et al., 2005). This AOP involves the formation of potent and non-selective hydroxyl radicals (OH•) generated by the photolysis of H2O2 when UV radiation (hv) is applied, as shown in reaction R(1) (Alaton et al., 2002; Poyatos et al., 2010): H2O2+hν→ν2OH•

(1)

This photolysis rate does not depend on pH and increases under more alkaline conditions, so the total photolysis of the system could be affected by pH. The following propagation reactions are given in R(2), R(3) and R(4) (Glaze et al., 1987): H2O2+OH•→OH2•+H2O

(2)

H2O2+OH2•→OH• +O2+H2O

(3)

2OH2•→H2O2+O2

(4)

The advantages of the H2O2/UV process include the wide range of applications for the degradation of contaminants such as dyes, phenols and pesticides, with an accelerated rate of oxidation, potential disinfection and meticulous mineralisation of contaminants in the wastewater (Cao and Mehrvar, 2011).

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This system has been applied for the degradation of several azo dyes in industrial wastewater, suggesting that it is an effective method for the treatment of wastewater contaminated with dye (Zuorro et al., 2013). Some studies report the efficiency of the process on particular dyes and the effect of the variables in the process. López-López et al. (2013) reported COD and colour removal efficiencies of 86.38 and 68.95%, respectively, by operating the process under a pH of 7 for a treatment time of 120 minutes. However the efficiency of the process is severely affected by the colouring type: Lopez-Lopez et al. (2015), using the same process on commercial dyes, attained higher efficiencies for organic matter and colour removal (91.42 and 99.60%, respectively), operating at pH 3 for a treatment time of 120 minutes. Schrank (2007) obtained only 52% for Vat Green 01 dye removal with the H2O2/UV process and 0.5 g/L H2O2. One of the most important parameters to consider in the design of an AOP in general, and the H2O2/UV process in particular, is the concentration of oxidant. For example, López-López et al. (2013) reported averages of 86.38 ± 2.9%, 63.03 ± 3.7%, 45.73 ± 3.9%, 35.49 ± 4.5% and 22.94 ± 8.9% using H2O2 concentrations of 5, 2, 1, 0.5 and 0.25 g/L, respectively. Similarly, the colour removal obtained at pH 7 was 68.95, 55.68, 26.44, 10.96, and 2.75% using H2O2 concentrations of 5, 2, 1, 0.5 and 0.25 g/L, respectively. Obviously, the higher oxidant concentration is, the higher efficiency became. López-López et al. (2013) studied the effect of pH on the process by using a pigment that changed the pH from 3 to 11 and operating at concentrations of H2O2 between 0.25 and 5 g/L. The results obtained in that study showed that under neutral pH the H2O2/UV process removed more organic matter and colour, independent of the concentration of H2O2 Another fundamental aspect studied by Lopez-López et al. (2014) was the influence of the exposure time in the process. For example, the rate of colour removal in the H2O2/UV process was relatively constant at 90 minutes for 0.25 g/L (47.4%) and 75 minutes with 0.5 g/L (82.2%). However, for TOC removal, indicated values increased with time for all concentrations of H2O2, while removal rates increased with H2O2 concentration independent of time. According to the results attained by these authors, the H2O2/UV process is an effective technology for removing many contaminants in wastewater. However, the selection of the conditions and the influences between them affect the organic matter and colour removal. Figure 2 shows a schematic diagram of the laboratory-scale photoreactor used by López-López et al., (2013). The reactions are carried out in a batch or continues reactor, which includes a lamp for UV generation. To remove the heat produced from the lamp, the temperature must be controlled with a refrigeration system to maintain it at a constant temperature. Normally, the photoreactor is covered with an opaque material to avoid

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interference with other external radiation and is placed on a magnetic stirrer in order to keep the sample homogeneous.

Figure 2. Photoreactor diagram used for the Advanced Oxidation Process.

2.1.2. Photo-Fenton Henry J. Fenton first described the photo-Fenton oxidation reagent (H2O2 in the presence of Fe2+) and demonstrated that H2O2 could be activated in the presence of Fe2+ ions to oxidise tartaric acid (Fenton, 1894). As such, the photo-Fenton process has proved to be a good alternative for treating textile wastewater containing high levels of dyes, recalcitrant organic compounds and suspended solids by improving the biodegradability of the effluent (Vedrenne et al., 2012; Yu et al., 2013). This process appears to have the ability to completely decolourise and mineralise the dyes from the textile industry in a short reaction time (Lucas and Peres, 2006). Fenton’s oxidation appears to be the most promising method, in terms of cost effectiveness and simplicity of operation (Tekin et al., 2006). Fenton is a homogeneous oxidation process and is considered to be a metal-catalysed oxidation reaction, in which iron is the catalyst (Tekin et al., 2006; Saritha et al., 2007) and includes reactions of peroxides (usually H2O2) with iron ions to form active oxygen species (OH•) that oxidise organic or inorganic compounds present in the wastewater (Pignatello et al., 2006). The mixture of ferrous sulphate and H2O2 generates OH• radicals that are capable of oxidising organic pollutants in wastewater (Sivakumar et al., 2013) and the additional formation of hydroxyl radicals could increase with UV light. Thus, by increasing the Fe2+ concentration the overall reaction is accelerated (Modirshahla et al., 2007). As the concentration of dye increases, it is necessary to increase the concentration of both Fe 2+ and H2O2 to ensure the presence of OH• radicals to degrade organic matter (ArslanAlaton et al., 2009, Prato-Garcia and Buitrón, 2013). The reactions for the production of OH• radicals in the photo-Fenton process can be represented by the reactions given below (Sun and Pignatello, 1993; Poyatos et al., 2010; Maezono et al., 2011):

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C. López-López, J. Martín-Pascual and J. M. Poyatos Fe2++H2O2→Fe3++OH- +OH•

(5)

H2O2+hv→2OH•

(6)

Fe3++H2O+hv→Fe2++OH•+H+

(7)

The photo-Fenton process has been used successfully in the removal of organic compounds and can completely decolourise water. Employing the same conditions as in the H2O2/UV process, López-López et al. (2015) studied the removal of organic matter and colour using the photo-Fenton process. The rates of colour removal for the photo-Fenton process were 87.2, 91.0 and 93.3% for 1, 2 and 5 g/L peroxide, respectively, after 120 minutes. These rates were lower than those reported by Elmorsi et al. (2010) for photo-Fenton treatment of the Mordant red 73 dye, where 99% decolourisation was seen within 15 min. In relation to the organic matter removal, the photo-Fenton process was achieved more than 94% removal after 120 min with 5 g/L of H2O2. When Fe2+ was used as the catalyst, after 15 minutes of reaction, for all concentrations of H2O2, the colour yields are very high, with the lowest concentration of H2O2 achieving a colour removal above 58%.

2.1.3. Heterogeneous Photocatalytic (TiO2/H2O2/UV) The term photocatalysis is applied to the acceleration of a photoreaction by the action of a catalyst. The use of TiO2 in these processes is of great interest due to its high photocatalytic activity, lack of toxicity, biological and photochemical stability in aqueous solutions and its chemically inert nature (Pekakis et al., 2006; Sakkas et al., 2009). Processes consisting of a heterogeneous photocatalysis (TiO2/H2O2/UV), which degrade pollutants by the generation of hydroxyl radicals using a catalyst (TiO2), an oxidant (H2O2) and UV radiation, are advantageous due to the lack of limitations of mass transfer, operation under environmental conditions and the possible use of solar irradiation (Pekakis et al., 2006). In addition, the catalyst itself is inexpensive and readily available (Velegraki et al., 2006). In these systems, the hydroxyl radicals (OH•) are more likely to form and a larger number of oxidant species may appear in the electron (e) and proton (h+) generation processes (García et al., 2007): TiO2+hv→TiO2 (e-+h+)

(8)

Wastewater Treatment from Textile Industry … TiO2h++OH-ad→TiO2+OH• H2O2+e-→OH•+OH-

97 (9)

(10)

The heterogeneous photocatalysis process (TiO2/H2O2/UV) is a technology that can also easily decolourise and greatly reduce the organic load of dyes and related effluents (Velegraki et al., 2006; Chen et al., 2009). The TiO2/H2O2/UV process was found to be the most effective method for colour and organic matter removal by López-López et al. (2015). At lower H2O2 concentrations, colour removal rates were higher (77.6 and 85% for 0.25 and 0.5 g/L of H2O2, respectively) and organic matter removal rates were 46.81 and 73.85%, respectively, at the same H2O2 concentrations after 120 minutes. At higher H2O2 concentrations the colour and organic matter removal rates were 99.9% and 94.55%, respectively, at 120 minutes.

3. COAGULATION-FLOCULATION PROCESS AS A PRE-TREATMENT Considering the principles of AOPs based on UV, the presence of turbidity is a limiting aspect for the efficiency of the treatment (Islam et al., 2011). Textile wastewater contains particles of a wide variety of shapes, sizes and densities (Aguilar et al., 2003; Sörensen and Larssson, 1992) and the removal of these particles from the wastewater is of great interest because they can absorb the chemical and microbiological contaminants present in the water (Lawler, 1997). Additionally, high concentrations of particles in water promote greater heat absorption from solar radiation and, consequently, the dissolved oxygen in the sample decreases. Furthermore, due to the turbidity present in the water, the penetration of UV into the water sources is reduced; therefore, the efficiency of the process can be affected (Prado and Espulgas, 1999). As a consequence of an excessive level of turbidity, the efficiency of the photomineralisation and photodisinfection of the contaminants present in the water is reduced due to the effect of shielding, which attenuates the light penetration (Chin et al., 2004, Rincon and Pulgarin, 2005). Turbidity leads to weakened penetration of direct light and decreased absorption of light photons (Seien and Soleymani, 2012). For this reason, Yonar et al. (2006) recommended a pre-treatment to eliminate turbidity to ensure the success of the oxidation process. One of the alternative solutions for this turbidity problem in the influent of AOP could be Coagulation-Flocculation (CF). CF process is a common treatment used in water effluents (Rodrigues et al., 2008), as it is a versatile method that has shown high efficiency of removal in the COD and SS

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(Guida et al., 2007). The flocculation is a process based on two stages, which cannot be applied independently. Coagulation is the phase in which colloidal particles are destabilised by the neutralisation of their electrical charges through the addition of chemical compounds. The amount of coagulant that it is added to the water should be taken into account in this process, as the turbidity of the water can be increased when the concentration is higher than necessary. Flocculation is the stage that follows the coagulation, in which all the electrically discharged particles are put into contact with one another so that the destabilised particles enter into an equilibrium around inert particles (flocculants), thereby resulting in the formation of larger flocs that can be decanted or separated by membranes. The CF process usually consists of the rapid dispersion of a coagulant into the wastewater, followed by an intense agitation, commonly defined as a rapid mixing (Rossini et al., 1999). This can achieve a greater removal of particles in the effluent and provide improved filtered water compared to conventional treatment processes (Rahman et al., 2010). This process also removes colour and turbidity in industrial wastewater (Meriç et al., 2005; Oller et al., 2011), and is a favoured option for this process (Oller et al., 2011). The coagulants most commonly used for treating contaminated water are aluminium and iron salts; although a great variety of them, both of inorganic and organic nature, can be found. In general, the hydrolysis reactions of trivalent metals can be presented as R(11) (Ching et al., 1994a; Stephenson and Duff, 1996): xMe3++yH2O2 → Mex(OH)y(3x-y)++yH+

(11)

The hydrolysis reactions that take place with the hydrated ferric ion are R(12) and R(13) (Fair et al., 1971): [Fe(H2O)6]3++H2O → [Fe(H2O)5(OH)]2++H3O+

(12)

[Fe(H2O)5(OH)]2++H2O → [Fe(H2O)4(OH)2]++H3O+

(13)

Hydroxo ferric complexes have a great tendency to polymerise, thereby producing the simplest reaction R(14) or R(15): 2[Fe(H2O)5(OH)]2+ → [Fe2(H2O)8(OH)2]4++2H2O

(14)

[Fe2(H2O)8(OH)2]4++H2O → [Fe2(H2O)7(OH)3]3++H3O+

(15)

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Al3+ behaves very similarly to Fe3+ but its hydrolysis is apparently more complicated. Aluminium salts are easily hydrolysed and the acidity of the aqueous aluminium ion is lower than that of the ferric ion. The species formed during the hydrolytic reactions for aluminium are shown in reaction (16) (Fair et al., 1971; Hutchison and Healy, 1990): [Al(H2O)6]3+→[Al(H2O)5OH]2+→[Al6(OH)15]3+→[Al8(OH)20]4+→Al(OH)3(s) → Al(OH)4](16) pH is one of the most important factors because it determines, for each coagulant, the nature of the species present in the water and its solubility. There is a pH zone for each coagulant where a good flocculation occurs in the short term using a given dose of coagulant (Aguilar et al., 2002). It is important to keep in mind that the colloidal particles tend to remain in suspension when they are stable. As such the stability must be reduced so that it is easier to join larger nuclei and increase their density to make facilitate removal by sedimentation. Therefore, the chemical reagents in the process are added, which eliminate the electrical charge that keeps them separated and can be agglomerated more easily. This charge can be positive or negative, although most of the colloids of wastewater developed a negative charge (Fair et al., 1971; Narkis and Rebhun, 1983; Van Benschoten and Edzwald, 1990a; Nemerow and Dasgupta 1991; Aguilar et al., 2002). Some novel pre-hydrolysed coagulants, such as Polyaluminium chloride (PACl), Polyaluminium ferric chloride (PAFCl), Polyferrous sulphate (PFS) and Polyferric chloride (PFCl) were found to be more effective and suggested for the decolourisation of textile wastewater (Kumar Verma et al., 2012). López-López et al. (2016) tested five coagulants of this type (SICOAG C-21, FLOCUSOL-PA/18, FLOCUSOL-CM/1, SIFLOC C 40 L PLUS and SIFLOC C-3) in order to analyse the effect of CF as pretreatment to colour, turbidity and total organic carbon (TOC) removal from effluent by AOPs. After the CF process the colour removal was higher than 84.48% in all cases; however, the coagulant used presented differences in the turbidity and TOC removal. The TOC removal presented lower efficiencies than colour, which changed between 52.90 and 82.19% with SICOAG C-21 and FLOCUSOL-CM/1, respectively. In relation to the turbidity removal, a high range of values was obtained for the efficiencies, ranging from 9.52 to 98.21% (López-López et al., 2016). Table 1 shows the results of total colour, turbidity and TOC removal after CF and AOP. It can be seen that the colour removal in all cases is practically 100%. In the case of turbidity removal, there are differences depending on the coagulantflocculants used. The coagulants-flocculants SICOAG C-21 and FLOCUSOL-PA/18 produce a turbidity removal higher than 92 and 98%, respectively. However, in the case

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of the other three coagulants-flocculants, turbidity removal is very low. The different turbidity removal obtained can allow us to analyse the effect of the turbidity in the behaviour of the AOP, due to the fact that turbidity removal plays a crucial role as the penetration of UV light through the wastewater is important for the success of the process in the long run (Yonar et al., 2006). The TOC removal, for all cases, is higher than 79%. Table 1. Total removal (%) of colour, turbidity and TOC from the raw effluent after CF and AOP (López-López, 2016) Colour

SICOAG C-21 (5 g/L)

FLOCUSOL-PA/18 (5 g/L)

FLOCUSOL-CM/1 (0.1 g/L)

SIFLOC C 40 L PLUS (0.1 g/L)

SIFLOC C-30 (1 g/L)

H2O2/UV photo-Fenton TiO2/H2O2/UV H2O2/UV photo-Fenton TiO2/H2O2/UV H2O2/UV photo-Fenton TiO2/H2O2/UV H2O2/UV photo-Fenton TiO2/H2O2/UV H2O2/UV photo-Fenton TiO2/H2O2/UV

99.94 100.00 100.00 99.66 99.77 99.94 99.69 99.62 99.93 99.33 99.12 99.73 99.28 99.09 99.84

Turbidity % total removal 92.86 92.86 92.86 98.21 98.21 98.21 10.58 11.11 11.64 19.79 19.79 20.32 30.77 30.77 31.79

TOC 97.10 98.39 99.29 95.12 98.20 98.20 79.51 86.90 94.43 89.95 93.25 94.91 91.11 94.23 97.01

The use of CF as a pre-treatment of an AOP that uses catalyst improves the efficiency in colour and organic matter removal with respect to the process without pre-treatment.

KINETIC MODELLING OF AOPS A significant advantage of AOPs is that they can be mathematically modelled at several different levels to determine the optimum conditions of H2O2 and exposure time, based on known kinetic pathways and constant reaction rate s that can provide the most information best fit between model and laboratory data (Crittenden et al., 1999). Table 2 shows the kinetic equations used by López-López et al. (2015).

Ct/Co

Wastewater Treatment from Textile Industry … a)

1 0,8 0,6 0,4 0,2 0

Ct/Co

0

15

Ct/Co

45 60 75 Time (minutes)

90

105 120

15

30

45 60 75 Time (minutes)

90

105 120

15

30

45 60 75 Time (minutes)

90

105 120

30

45 60 75 Time (minutes)

90

105 120

30

45 60 75 Time (minutes)

90

105 120

c)

1 0,8 0,6 0,4 0,2 0 0

Ct/Co

30

b)

1 0,8 0,6 0,4 0,2 0 0

d)

1 0,8 0,6 0,4 0,2 0 0

Ct/Co

101

15

e)

1 0,8 0,6 0,4 0,2 0 0

15

Figure 3. Ct/C0 ratio for AOP TiO2/H2O2/UV for each concentration of H2O2: 0.25 (a), 0.5 (b), 1 (c), 2 (d) and 5 (e) g/L for each type of water used in the different investigations: synthetic water with commercial dye, real water with commercial dye and pre-treated water with CF pre-treatment prior to the AOP: experiment 1 (SICOAG C-21), experiment 2 (FLOCUSOL-PA/18), experiment 3 (FLOCUSOL-CM/1), experiment 4 (SIFLOC C 40 L PLUS) and experiment 5 (SIFLOC C-30) (LópezLópez, 2016).

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C. López-López, J. Martín-Pascual and J. M. Poyatos Table 2. Kinetic equations (modified of López-López et al., 2015)

Model Pseudo-first Order

Pseudo-second Order

Equation

Parameter

𝐶𝑡 = 𝐶𝑜 − (𝐶𝑒 ∗ (1 − 𝑒 −𝑘1′∗𝑡 ))

𝐶𝑡 = Co −

t ∗ k2′ ∗ Ce2 1 + k2′ ∗ Ce

Zero Order

𝐶𝑡 = 𝐶𝑜 + 𝑘𝑜 ∗ 𝑡

First Order

𝐶𝑡 = 𝐶𝑜 ∗ 𝑒 𝑘1∗𝑡

C0: initial TOC concentration (mg/L)

Ct: TOC concentration at any time, t, (mg/L)

Ce: the removal capacity at equilibrium (mg/L)

t: time of process (min)

Second Order

Third Order

Elovich type equation

𝐶𝑡 =

𝐶𝑜 1 + 𝑘2 ∗ 𝐶𝑜 ∗ 𝑡

𝐶𝑜 2 √ 1 + 𝑘3 ∗ 𝐶𝑜 2 ∗ 𝑡 1 1 𝐶𝑡 = 𝐶𝑜 + ln(𝑎𝑏) + ln(𝑡) 𝑏 𝑏

k1’,2’,0,1,2,3: Velocity constant

a: initial TOC concentration rate (mg/L min)

b: removal constant (mg/L)

The pseudo-first-order model for the three processes studied by Ho and McKay (1998), Ho et al. (2000), Calero et al. (2011) and López-López et al. (2013, 2016) indicated the highest correlation between empirical and modelled data with an average of 0.965, suggesting that this model could be used to design AOPs. The rate constants increased with H2O2 concentration and indicated that heterogeneous photocatalysis (Figure 3) was the most rapid way to remove organic matter at the same H2O2 concentration. With heterogeneous photocatalysis, increasing the concentration of H2O2 increased the degradability of water, thereby confirming that by using TiO2 as a catalyst, high rates of organic matter removal are attained; in addition, less processing time is required.

GENERAL DISCUSSION Considering the results obtained by the numerous authors who have studied the application of AOPs in the textile industry, some relevant aspect can be summarised. The

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efficiency of colour and organic matter removal increases with the oxidant concentrations, up to a limit whereby no more oxidant is consumed. An excess of H2O2 could be hazardous for the environment and so it is important to choose the minimum concentration to attain the efficiency required. Moreover, an increase in the exposure time also improved the efficiency to the oxidant; once H2O2 is used up the process finished. The greater the exposure time, the greater the energetic consumption; thus, it is especially important for the design of these processes to study the characteristics of the influent. In general terms, the addition of a catalyst to a conventional AOP improves the efficiency of the process; however, the cost of the process increases too. Among the different process studied, generally the most reliable is TiO2/H2O2/UV, although with certain textiles wastewater the photo-Fenton can attain similar or even better removal rates. Considering the negative effect of the turbidity in the efficiency of the AOP, the use of a pre-treatment must be considered, especially in wastewater with high concentrations of colloidal particles.

REFERENCES Aguilar, M.I., Sáez, J., Lloréns, M., Soler, A. and Ortuño, J.F. (2002). Tratamiento físicoquímico de aguas residuales. Coagulación-Floculación. Universidad de Murcia. [Physical-chemical treatment of wastewater. Coagulation-Flocculation. University of Murcia] Aguilar, M.I., Sáez, J., Lloréns, M., Soler, A. and Ortuño, J.F. (2003). Microscopic observation of particle reduction in slaughterhouse wastewater by coagulation– flocculation using ferric sulphate as coagulant and different coagulant aids. Water Research, 37: 2233–2241. Agustina, T.E., Ang, H.M. and Vareek, V.K. (2005). A review of synergistic effect of photocatalysis and ozonation on wastewater treatment. Journal of Photochemistry and Photobiology C: Photochemistry Reviews, 6: 264–273. Alaton, I.A., alcioglu, I.A. and Bahnemann, D.W. (2002). Advanced oxidation of a reactive dyebath effluent: comparison of O3, H2O2/UV-C and TiO2/UV-A processes. Water Research, 36: 1143–1154. Aleboyeh, A., Moussa, Y. and Aleboyeh, H. (2005). The effect of operational parameters on UV/H2O2 decolourisation of Acid Blue 74. Dyes and Pigments, 66: 129-134. Alinsafi, A., Evenou, F., Abdulkarim, E.M., Pons, M.N., Zahraa, O., Benhammou, A., Yaacoubi, A. and Nejmeddine, A. (2007). Treatment of textile industry wastewater by supported photocatalysis. Dyes and Pigments, 74: 439-445.

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In: Textiles: Advances in Research and Applications ISBN: 978-1-53612-855-0 Editor: Boris Mahltig © 2018 Nova Science Publishers, Inc.

Chapter 5

GREEN ELECTROSPINNING OF NANOFIBER MATS FROM BIOPOLYMERS FOR MEDICAL AND BIOTECHNOLOGICAL APPLICATIONS Timo Grothe, Nils Grimmelsmann, Sarah Vanessa Homburg and Andrea Ehrmann* Bielefeld University of Applied Sciences, Faculty of Engineering and Mathematics, Bielefeld, Germany

ABSTRACT Electrospinning can be used to create mats from nanofibers consisting of polymers, possibly blended with other organic or inorganic components. Compared with other textile fabrics, such as common nonwovens, these nanofiber mats possess large surfacevolume ratios, allowing for enhanced interactions with their environment. This property makes them suitable for applications in wound dressing, drug delivery, biotechnological filter technology, etc., especially if prepared from biopolymers with intrinsically antimicrobial or other qualities. In a recent project, the possibilities to create such nanofiber mats from diverse (bio-)polymers by “green” electrospinning, i.e., electrospinning from aqueous solutions or other nontoxic solvents, are investigated. The article depicts the latest results from needleless electrospinning pure polymers and polymer blends as well as typical physical and chemical properties of the created nanofiber mats.

Keywords: electrospinning, nanospinning, nanofiber, biopolymer, polymer blend, green electrospinning

*

Corresponding Author Email: [email protected].

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INTRODUCTION Electrospinning belongs to the primary spinning methods, allowing for producing fine fibers or fiber mats from polymer solutions or melts. The diameters of electrospun fibers usually range from some ten nanometers to a few micrometers. Electrospinning can be performed using a needle through which the polymer solution or melt is pressed or by coating a rotating drum or a fine wire (Koombhongse et al. 2001; Bognitzki et al. 2000; Theron et al. 2001), resulting in different positioning of the nanofibers on a substrate. The process of needleless electrospinning using a wire system is described in detail in the section “Methods.” The principle technology of electrospinning has been known for decades and was patented in 1934 (Formhals 1934). Nevertheless, the electrospinning technology experienced a revival only in the 1990s when it was shown that a broad variety of different polymers could be spun from solutions (Reneker and Chun 1996) or melts (Larrondo and Manley 1981). Besides polymers which are typically used in the textile industry, such as polyester, polyamides, aramide, poly(acrylonitrile ), etc., biopolymers are of large interest, such as poly(ethylene glycol ), polypeptides, proteins, etc. Due to their large inner surface, nanofiber mats are especially used in applications which necessitate large contact areas between the fiber material and the environment, e.g., as filters in medical or biotechnological areas, for tissue engineering, wound dressing, drug delivery, etc. Additional to the fiber diameter, the morphology of the nanofibers can be modified by carefully tailoring the spinning and solution /melt parameters. Besides round cross-shapes, flat ribbons were produced (Koombhongse et al. 2001, Frenot and Chronakis 2003) or combinations of fibers and membranes (Grimmelsmann et al. 2017b). With too low viscosities, nano droplets are formed instead of fibers. Too low electrode-substrate distances or too high electrical fields may cause formation of undesired beads, a phenomenon which occurs more or less frequently depending on the spinning polymer. It was attributed to capillary break-ups of the polymer jets, thus being dependent on the charge density on the polymer jets, their surface tension and the viscoelastic properties of the solutions (Deitzel et al. 2001; Fong et al. 1999). Fiber diameters, on the other hand, depend mainly on the polymer concentration in the solution (Deitzel et al. 2001). While most polymers are spun from acids or toxic solvents, the idea of “green” electrospinning arose around 2010, avoiding harmful organic solvents by spinning from polymeric solutions (Sun et al. 2001), emulsions or suspensions (Agarwal and Greiner 2011). In the latter case, spinning agents, such as high-molecular weight poly(ethylene glycol) (PEG), can be used to process a dispersion to water -resistant nanofiber mats (Bubel et al. 2013). Spinning from aqueous solutions often necessitates an additional crosslinking step after the actual spinning process, e.g., by UV irradiation (Giebel et al. 2013a; Giebel et al.

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2013b) or heat treatment (Shi and Yang 2015; Jiang et al. 2016). The crosslinking efficiency, on the other hand, often depends on the nanofiber mat properties and thus on the spinning parameters (Giebel et al. 2013a). Functionalization of such stabilized nanofiber mats can be achieved by coating them, e.g., with dyes, antibacterial materials, metal nanoparticles, or the like (Giebel et al. 2013b; Wang et al. 2015; Bunyatova et al. 2016; Castilla-Casadiego et al. 2016). On the other hand, such nanofiber mats can be used for controlled drug delivery (Salehi et al. 2014; Chen et al. 2015) or protein release (Wang et al. 2015). The biopolymers used in this examination and their potential applications are described more in detail in the “Results” section.

METHODS Electrospinning The process of needleless electrospinning is depicted in Figure 1. Firstly, a carriage is filled with the polymer solution or melt. This carriage travels along the high voltage electrode, formed by a thin stainless steel wire. A fine spinning nozzle surrounds the wire and allows the polymer solution to coat it. This coating is forced to the ground electrode due to the high voltage field between both electrodes before the solvent evaporates. The ground electrode is shielded by a movable substrate, here a polypropylene mat. During flying, the polymer drops are drawn in the electric field, resulting in formation of long, thin fibers which form so-called Taylor cones between the electrodes, i.e., cone-shaped jets of fibers. After impinging on the substrate, the fibers build a mat of nanofibers which are in most cases rigidly fixed on each other. It should be mentioned that with needleless electrospinning it is not possible to create infinitely long fibers which could be carefully drawn away from the substrate afterwards, as it can be made possible in needle-electrospinning. Several techniques can be used to implement the basic idea of electrospinning. While several research groups prefer working with self-built equipment, mostly used for needleelectrospinning, our group work s with a commercially available machine, the “Nanospider Lab” produced by Elmarco (Czech Republic ), as depicted in Figure 2. This equipment enables production of relatively large-scale nanofiber mats in the order of magnitude of 1 m²/h, with significant dependencies of this value on the spinning parameters. For industrial applications, the process can be upscaled to larger machines. The spinning parameters can be modified in large ranges. Typical parameters are, e.g., high voltages between 25 kV and 80 kV; currents between 0.05 mA and 0.3 mA; carriage speeds between 100 and 200 mm/s; nozzle diameters between 0.6 mm and 1.5 mm; electrode-substrate distances between 120 mm and 240 mm; and relative humidity

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in the spinning chamber between 30% and 40%. Higher relative humidities may result in severe problems, e.g., with cotton -candy-like fiber connections between high voltage electrode and substrate, flashovers, etc.

Figure 1. Principle of the electrospinning process.

Figure 2. Electrospinning apparatus “Nanospider Lab.”

Biopolymers and Other Ingredients For the spinning solutions, a broad variety of different materials was used: aloe vera (alexmo cosmetics ); chitosan; commercial agar -agar 15% in aqueous solution; commercial gelatin powder (as used for baking) 15% in aqueous solution; commercial gelatin leaves 15% in aqueous solution; and pure gelatin powder (Abtei, Germany) 55% in aqueous solution. In some experiments, the latter was blended with 19% wax (KahlWax 6592) to increase water resistance. As a spinning agent as well as for basic tests with the pure material, PEG with a molecular weight of 600,000 daltons (concentration 8%) purchased from S3 Chemicals

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was used in some experiments. The concentrations of the spinning solutions were chosen due to their viscosity which must not be too high for needleless electrospinning. Different salts (NaCl, KCl and KBr, purity > 99%, Carl Roth) as well as PEDOT: PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) were used to modify the conductivity of the spinning solutions. Diverse additional materials were integrated into the polymer solutions, e.g., forest fruit tea (MAYFAIR), a ferrofluid (AstroMedia), and chlorophyll. Optical evaluation of the nanomats was done using a confocal laser scanning microscope (CLSM) VK 9000 by Keyence. All images shown here have a nominal magnification of 2000 x.

RESULTS Poly(ethylene glycol ) Poly(ethylene glycol) (PEG), also called Poly(ethylene oxide) (PEO), is a water soluble biopolymer with very good spinnability. Solely, it can be used in applications from skin creams or toothpaste to pharmaceutics to medicine. PEG can be used as a coating for fibers from other materials (Liu et al. 2003; Wang et al. 2000). In electrospinning, it is often used as a spinning agent, e.g., in combination with collagen, PVDF or cellulose (Wongchitphimon et al. 2011; Zeugolis et al. 2008; Sungkaew et al. 2010), while only few reports about spinning pure PEG nanofibers can be found in the scientific literature (Deitzel et al. 2001; Chen et al. 2007; Fortunato et al. 2014). In an earlier project, we have investigated the morphology of PEG nanofiber mats created by needleless electrospinning and identified the PEG concentration in the aqueous solution as the most crucial parameter. Other spinning parameters, such as the voltage, the electrode-substrate distance, the carriaged speed etc. influenced the fiber mat density, but a significant influence on the fiber mat morphology was not observed (Grothe et al. 2017a). On the other hand, the molecular weight of the PEG under examination had to be chosen carefully to enable fiber formation (Grothe et al. 2017b). It was found that at too low molecular weights, strong Taylor cones were visible, but no fibers were formed on the substrate. Too high molecular weights, however, impeded reaching the necessary concentrations for electrospinning since the viscosity of the spinning solution became too high. Generally, molecular weights between 300 kD and 600 kD in combination with concentrations between 6% and 12% PEG in aqueous solution were found to be suitable for needleless electrospinning. It should be mentioned that these values differ from those found for needle-electrospinning (Grothe et al. 2017a; Jaeger et al. 1998).

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Figure 3 depicts PEG samples produced under different conditions which are given in Table 1 for an overview of possible morphologies, depending on the spinning solution and process parameters. By changing these parameters, fiber densities, thicknesses, and straightness are modified, until transformation from fibers to membrane-like structures start (cf. sample D).

Figure 3. PEG electrospun nanofiber mats created by different spinning and solution parameters, cf. Table 1.

Table 1. Parameters of the nanofiber mats depicted in Figure 3 Sample A B C D

Mol. weight 600 kD 600 kD 600 kD 600 kD

Concentration 6% 6% 8% 8%

U 35 kV 25 kV 70 kV 75 kV

Carriage speed 100 mm/s 200 mm/s 250 mm/s 250 mm/s

Electr. dist. 140 mm 140 mm 190 mm 140 mm

Aloe Vera Aloe vera belongs to the materials typically used for medical applications. It contains more than 70 active components, e.g., minerals, amino acid s, vitamines, or enzymes (Surjushe et al. 2008). Due to its antiseptic (Surjushe et al. 2008) and anti-inflammatory properties (Hutter et al. 1996), its ability to increase wound healing (Fulton 1990; Chithra

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et al. 1998) and collagen cross-linking (Heggers et al. 1996) and to support the immune system (Ro et al. 2000), it can be used as a part of bandages etc. In electrospinning, aloe vera cannot be used solely, but is usually co-spun with PEG or another spinning agent (Grothe et al. 2017c). Figure 4 depicts a nanofiber mat created from aloe vera 2.5% in aqueous solution and PEG 8% in aqueous solution. With this relation, relatively thick and homogenous mats are produced with nanofiber diameters of approximately 500 nm.

Figure 4. Electrospun aloe vera / PEG nanofiber mat.

Casein Casein can absorb water to a large extend, making it smooth to the skin and comfortable in contact, especially for people with skin diseases such as neurodermatitis. It is known to have a natural antibacterial effect, to promote biomineralization (SzykWarzynska et al. 2014) and to work as a size-selective molecular device which attracts macromolecules like proteins or polysaccharides (Peixoto et al. 2015). In electrospinning, casein is usually combined with other materials (Grothe et al. 2017; Tomasula et al. 2016) or in graft copolymers (Dong and Gu 2002). For the experiments shown here, 25% casein in aqueous solution and 8% PEG in aqueous solution were mixed in different ratios. Figure 5 depicts the result of mixing 5 ml casein solution with 1.5 ml PEG solution. Apparently, this mixing ratio is not perfect, leading to an inhomogeneous mat with several thicker and denser parts as well as visible diameter modifications on the fibers, apparently resulting from droplet formation during the spinning process. Combining 5 ml casein solution with only 1 ml PEG solution results in denser and more regular nanofiber mats (Grothe et al. 2017c), while adding water leads to very fine fibers with droplets. Spinning casein solely seems to be impossible.

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Figure 5. Electrospun casein / PEG nanofiber mat.

Chitosan Chitosan belongs to the typical filter materials, amongst others due to its antifouling (Lim 2016) and antibacterial properties (Cooper et al. 2013; Wang et al. 2016). It is used as a coating on filters for disperse dyes in water (Zhao et al. 2015) or on cotton fabrics for filtering Gram-positive and Gram-negative bacteria (Ferrero et al. 2014; Periolatto et al. 2014). Additionally, chitosan can be applied for medical or biotechnological scaffolds, e.g., for growing human mesenchymal stem cells (Muzzarelli et al. 2015; Aljawish et al. 2016).

Figure 6. Electrospun chitosan /PEG nanofiber mat.

In electrospinning, chitosan is usually blended with PEG, resulting in morphologies which strongly depend on the chitosan: PEG ratio (Wei et al. 2015; Lemma et al. 2016; Grimmelsmann et al. 2017a; Grimmelsmann et al. 2017b). The nanofiber mat depicted in

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Figure 6 was produced using a chitosan: PEG ratio of 3: 2, while higher amounts of chitosan resulted in strong bead formation.

Agar Agar is a typical cell growth medium for biotechnological applications. It is reported to be not spinnable solely from aqueous solutions (Sousa et al. 2015a); however, cospinning with different polymers from other solution was successfully performed (Sousa et al. 2015b).

Figure 7. Electrospun agar /PEG coating on PP substrate fibers.

In our experiments, the temperature of the spinning equipment, especially cartridge and nozzle, was shown to strongly influence the possible spinning duration since at low temperature the viscosity of the agar solution became too high to be spinnable any longer. Combining agar and PEG in a weight-ratio of 2: 1 and electrospinning from pre-heated equipment resulted in dense nanofiber mats which turned into coatings for ratios around 20: 1. Figure 7 depicts such a coating, gained from spinning agar-agar 15% in aqueous solution in combination with 10% of PEG 8% in aqueous solution.

Gelatin Gelatin is also a typical material for tissue engineering (Elamparithi et al. 2017) which can be blended with other materials, such as poly(glycerol sebacate), to increase the biocompatibility of the resulting nanofiber mat (Hu et al. 2017). Here, different approaches to electrospin gelatin are depicted.

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Figure 8 (upper left panel) shows the results of electrospinning gelatin leaves. This attempt led to coating the PP fibers of the substrate instead of producing nanofibers. From pure gelatin (pharmaceutical grade), thick flat fibers were spun. Adding PEG to the pure gelatin solution interestingly resulted in electrospraying instead of electrospinning, opposite to previous experiments in which PEG has proven to work well as a spinning agent. Finally, adding wax to the pure gelatin spinning solution in order to increase the water resistance resulted in short, wax-coated fibers. Apparently, gelatin belongs to the materials which should ideally be spun solely or in combination with other polymers than PEG.

Figure 8. Electrospun gelatin nanofibers from different aqueous solutions.

PEG with Different Dopants While the former sub-chapters concentrated on the possibility to spin non-solely spinnable polymers by adding PEG, here different modifications of PEG spinning solutions and additions are depicted which can be used to add different functions or to modify the spinning process. Figure 9, e.g., shows blends of PEG with different concentrations of NaCl. Salts can be used to modify viscosity (Frenot and Chronakis 2003) and conductivity (Murugendrappa et al. 2000; Blomberg et al. 2017) of the spinning solutions and thus to change the physical properties of the resulting nanofiber mats. Here, it is clearly visible that high concentrations of NaCl destroy fiber formation, most probably due to the

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extremely increased conductivity of the solution which was approx. one order of magnitude higher than suggested for needleless electrospinning.

Figure 9. Electrospun PEG nanofibers with different amounts of NaCl.

Another possibility to increase the conductivity of an electrospun nanofiber mat is the addition of the conductive polymer PEDOT: PSS to a PEG solution, as depicted in Figure 10 (left panel). However, similar problems arise as with the addition of salts – the desired conductivities cannot be reached with relatively low PEDOT: PSS concentrations (here: 22%), while higher amounts of the conductive polymer result in the typical problems with too conductive spinning solutions (Grothe et al. 2016). Another physical property of nanofiber mats which can be increased by blending PEG with another material is the magnetism of the fiber mat. Figure 10 (right panel) depicts a nanofiber mat produced from PEG co-spun with a ferrofluid. The different brightnesses in the nanofiber mat show that not all areas have the same magnetic properties; however, on larger scales, these results may be sufficient. A problem in electrospinning this solution is the missing compatibility of water -based PEG solutions and oil -based ferrofluids. Further research should try to implement magnetic nanoparticles directly in the aqueous PEG solution.

Figure 10. Electrospun PEG nanofibers with PEDOT: PSS and ferrofluid.

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Figure 11. Electrospun PEG nanofibers with chlorophyll and forest fruit tea.

Finally, Figure 11 shows PEG nanofiber mats blended with chlorophyll (coated area in left panel) and forest fruit tea (dark areas in right panel). In both cases, the color of the nanofiber mats is changed on macroscopic scales, and their smell allows identification of the respective additional indegredient. In a similar way, PEG nanofiber mats could also be used for drug delivery.

CONCLUSION Nanofiber mats were electrospun from PEG and diverse other biopolymers as well as additional ingredients, used for modifying the physical properties of the mats. In most cases, by carefully adjusting the solution and spinning parameters, it was possible to gain nanofiber mats with different morphologies and other properties.

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mediator release in guinea pig lung mast cells activated with specific antigen antibody reaction. J. Pharmacol. Exp. Ther., 292, 114-121. Salehi, R., Irani, M., Eskandani, M., Nowruzi, K., Davaran, S., Haririan, I. (2014). Interaction, Controlled Release, and Antitumor Activity of Doxorubicin Hydrochloride From pH -Sensitive P(NIPAAm-MAA-VP) Nanofibrous Scaffolds Prepared by Green Electrospinning. International Journal of Polymeric Materials and Polymeric Biomaterials, 63, 609-619. Shi, J. J., Yang, E. L. (2015). Green electrospinning and crosslinking of polyvinyl alcohol /citric acid. Journal of Nano Research, 32, 32-42. Sousa, A. M. M., Souza, H. K. S., Uknalis, J., Liu, S. C., Goncalves, M. P., Liu, L. S. (2015a). Electrospinning of agar /PVA aqueous solutions and its relation with rheological properties. Carbohydrate Polymers, 115, 348-355. Sousa, A. M. M., Souza, H. K. S., Uknalis, J., Liu, S. C., Goncalves, M. P., Liu, L. S. (2015b). Improving agar electrospinnability with choline -based deep eutectic solvents. Int. J. Biological Macromolecules, 80, 139-148. Sun, J. Y., Bubel, K., Chen, F., Kissel, T., Agarwal, S., Greiner, A. (2010). Nanofibers by Green Electrospinning of Aqueous Suspensions of Biodegradable Block Copolyesters for Applications in Medicine, Pharmacy and Agriculture. Macromolecular Rapid Communications, 31, 2077-2083. Sungkaew, S., Thammakhet, C., Thavarungkul, P., Kanatharana, P. (2010). A New Polyethylene Glycol Fiber Prepared by Coating Porous Zinc Electrodeposited onto Silver for Solid-phase Microextraction of Styrene. Analytica Chimica Acta, 664, 4955. Surjushe, A., Vasani, R., Saple, D. G. (2008). Aloe vera: A short review. Indian Journal of Dermatology, 53, 163-166. Szyk-Warzynska, L., Kilan, K., Socha, R. P. (2014). Characterization of casein and polyL-arginine multilayer films. J. Colloid Interface Science, 423, 76-84. Theron, A., Zussman, E., Yarin, A. L. (2001). Electrostatic fields-assisted alignment of electrospun nanofibres. Nanotechnology, 12, 384-90. Tomasula, P. M., Sousa, A. M. M., Liou, S.-C., Li, R., Bonnaillie, L. M., Liu, L. S. (2016). Short communication: Electrospinning of casein /pullulan blends for food grade application. Journal of Dairy Science, 99, 1837-1845. Wang, R. Z., Wang, Z., Lin, S., Deng, C., Li, F., Chen, Z. J., He, H. (2015). Green fabrication of antibacterial polymer /silver nanoparticle nanohybrids by dualspinneret electrospinning. RSC Advances, 5, 40141-40147. Wang, L., Zhang, C., Gao, F., Pan, G. (2016). Needleless electrospinning for scaled-up production of ultrafine chitosan hybrid nanofibers used for air filtration. RSC Advances, 6, 105988-95.

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Wang, X. Y., Yuan, Y. H., Huang, X. C., Yue, T. L. (2015). Controlled release of protein from core-shell nanofibers prepared by emulsion electrospinning based on green chemical. Journal of Applied Polymer Science, 132, 41811. Wang, Z., Xiao, C., Wu, C., Han, H. (2000). High-performance Polyethylene Glycolcoated Solid-phase Microextraction Fibers Using Sol-gel Technology. Journal of Chromatography A, 893, 157-168. Wei, H. Q., Zhang, F. H., Zhang, D. W., Liu, Y. J., Leng, J. S. (2015). Shape-memory behaviors of electrospun chitosan /poly(ethylene oxide) composite nanofibrous membranes. J Appl. Polymer Sci., 132, 42532. Wongchitphimon, S., Wang, R., Jiraratananon, R., Shi, L., Loh, C. H. (2011). Effect of Polyethylene Glycol (PEG) as an Additive on the Fabrication of Polyvinylidene Fluoride-co-hexafluropropylene (PVDF-HFP) Asymmetric Microporous Hollow Fiber Membranes. Journal of Membrane Science, 369, 329-338. Zeugolis, D. I., Paul, R. G., Attenburrow, G. (2008). Extruded Collagen-Polyethylene Glycol Fibers for Tissue Engineering Applications. Journal of Biomedical Materials Research, 85B, 343-352. Zhao, X., Liu, Y., Wang, C. and Liu, Q. S. (2015). Structure and filtration performance of fibrous composite membranes containing environmentally friendly materials for water purification. Fibers and Polymers, 16, 2586-2592.

In: Textiles: Advances in Research and Applications ISBN: 978-1-53612-855-0 Editor: Boris Mahltig © 2018 Nova Science Publishers, Inc.

Chapter 6

ACHIEVING ELECTRICAL CONDUCTIVITY IN TEXTILES: AN OVERVIEW OF CURRENT TECHNIQUES Thomas Grethe*, Karoline Günther, Thomas Weide and Anne Schwarz-Pfeiffer Hochschule Niederrhein – University of Applied Sciences, Moenchengladbach (Germany )

ABSTRACT Electronic textiles are subjected to an increasing attention since the combination of sensory or actuary functions with textile materials is quite promising. Initially the integration of electronic functions into textiles for recreational purposes was reported, for example integrating control elements for a music player in a jacket, or light application in a dress (Post et al. 2000). In this early stage conventional electronic components were integrated in the textile structures. The ongoing development process leads to higher integration levels by empowering textile structures themselves to act as electronic devices. An ambitious example is the fabrication of fiber based transistors, which was reported in 2005 by Lee et al. (Lee et al. 2005). Less demanding approaches like realizing resistors and capacitors with textile materials is also often referred to (Castano et al., 2014). These simple structures can act as sensors to detect for instance force (Meyer et al., 2010; Shu et al., 2010), elongation (Cochrane et al., 2010; Mattmann et al., 2011), moisture (Devaux et al., 2011; Weremczuk, 2012), and temperature (Sibinski et al., 2010; Schwarz-Pfeiffer, 2015). These types of textiles are already commercialized. This development shifts possible fields of application from consumer goods to technical appliances. Therefore, electronic functions can be found with increasing extent in technical textiles. The field of application spans from geo textiles *

Corresponding Author Email: [email protected].

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Thomas Grethe, Karoline Günther, Thomas Weide et al. (Heyse et al., 2015), building materials (Davis et al., 2013) to medical textiles (Hoffmann et al., 2011). In all the addressed sectors the availability of textile integrated reliable conductive tracks is crucial as well as the connection technology.

INTRODUCTION Therefore, common approaches like copper printed circuits or even cables cannot be considered to achieve the goal of a high integration level. Hence, other opportunities have to be evaluated. Before looking into further details, it is helpful to understand the different requirements of an electrical conductor: In the first place, obviously, a low electric resistance is necessary, but on a second look additional functions are crucial, depending on the field of application. Namely, the isolation against other conductors but also against surrounding media needs to be addressed. Faulty insulations can lead to injuries if high energies are transmitted, but also low current applications like sensors or data lines can suffer from malfunctions due to faulty insulations. In a textile environment the requirements of insulation materials can also include long-term high flexibility, washing resistance, and mechanical strain resistance. In the case of signal transmission lines also the issue of electromagnetic shielding is of high importance. Since in common electronic applications coaxial cables are widely used, this technique cannot easily be transferred to textile applications. It is usually desirable to achieve a low capacitance of a coaxial cable, which is realized by a distance between the signal transmitting wire and the outer shielding, which gives such cables a diameter of a few millimeters, which is not well compatible with textile integration for the most parts. Different techniques can be applied to ensure a conduction of electric signals and power along the textile material. In general terms, the options can either be fiber or fabric based. In the first case conductive yarns are applied by different techniques like weaving, embroidering or braiding dependent on the used yarn and the aimed application. Yarn materials can be separated in two systems: conductive coated yarns and bulk metal components as yarn or yarn component. Such approaches show specific challenges, for example the mechanical properties of these yarns render common textile processing techniques difficult due to their susceptibility to abrasion or breakage. On the other hand fabric based concepts often apply coating of conductive compounds like pastes or inks onto the textile substrate. Here other requirements such as good conductivity, elasticity of the film, adhesion to the fabric have to be met.

YARN BASED CONDUCTORS This approach might represent the highest possible integration level for all yarn or filament based textile materials. A lot of different approaches exist to obtain an electric

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conductivity in yarns. First, conductive coatings of nonconductive mono- or multifilament yarns with conductive materials are often realized. The coating materials are varying widely and are usually applied by any kind of paste coating, like dip coating, immersing, etc. by chemical deposition or physical vapor deposition. Examples for the first approach are dip coating of cotton threads with a CNT containing liquid resulting in resistances of a few ten ohms per cm (Shim et al. 2008), coating of polyethylene yarns with polyaniline (a conductive polymer) obtaining resistances in the same order of magnitude (Devaux et al. 2007), and also other intrinsically conductive polymers (ICPs) are used like polypyrrole (Xue et al. 2004; Maity et al., 2014) which can be in situ polymerized on the fiber. The conductivity of ICPs can be explained by the combination of two concepts: First the molecular orbital s need to be extended along the complete molecular chain. This can usually be obtained by an extended -System. A primitive example of such a molecule is polyacetylene (Figure 1). The conjugated carbon -carbon double bonds lead to a widely delocalized -System allowing electrons to reside anywhere along the molecular structure. For textile applications, the most common ICPs are polyethylene dioxythiophene (PEDOT), polypyrrole (PPy) and polyaniline (PANI). These offer better properties regarding degradation and processability. The conjugated bond structure of those polymers creating a delocalized -system is depicted in Figure 2. Beside these structural requirements for conductivity also a reasonable charge carrier mobility is necessary. A common way to introduce charge carriers is doping, which is also known for inorganic semiconductors like silicon. Contrary to the inorganic systems, where doping is usually accomplished by introducing impurities in the crystal lattice, ICPs are usually doped due to oxidation of the polymer. The mechanism can be illustrated by looking on the molecular orbital s of hexatriene (Figure 3). In the neutral state all electrons reside in the highest occupied molecular orbital (HOMO). It is therefore not possible for an electron to be moved along the molecular axis, since there are no allowed states for it to reside along its movement. By introducing an electron (reduction) it will fill in the lowest unoccupied molecular orbital (LUMO) of the molecule with plenty possible states to fill in while moving along the molecular axis, therefore conductivity can be achieved. The same mechanism is present in the case of oxidizing the molecule, where an electron is removed, resulting in an unoccupied state in the HOMO, which allows electrons to settle in. This will lead to an apparent movement of the free state, which is commonly referred to as hole conduction.

Figure 1. Structure of polyacetylene, the first synthesized and investigated conductive polymer, but unfortunately, this substance is sensitive to oxidation (Berets et al. 1968) and therefore gained no commercial impact.

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Figure 2. Structures of different ICPs: a) polyethylene dioxythiophene, b) polyaniline, c) polypyrrole.

In an inorganic semiconductor like silicon, doping is accomplished for example by aluminum or phosphorous which also leads to freely moveable charge carriers. These impurities a rather immobilized in the crystal lattice and therefore most inorganic semiconductors are not really sensitive to oxidation 1. ICPs are mostly amorphous and due to their electron configuration quite susceptible to oxidation. Besides unintended doping or dedoping the material can also be permanently degraded by oxidation.

Figure 3. MO structure of hexatriene. 1

In fact, it is possible to adsorb oxygen on an inorganic semiconductor surface, which leads to a change of the bandgap of the material, but this effect only takes place at the crystal surface. In a bulk material this is usually not of a concern, but by using a semiconductor with a high surface to volume ratio, this effect can be used to sense gases.

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However, doping of ICPs by oxidation obviously introduces a charge on the polymeric backbone. Therefore, a compensation by additional ions is necessary. For example, polyethylene dioxythiophene can be doped with polystyrene sulfonate (PEDOT: PSS) and polypyrrole (ppy) is usually in situ polymerized using an iron catalyst which also acts as a dopant. In the case of PEDOT: PSS the charges are located on the sulfonate ion (negative) and the polymeric backbone (positive) forming polarons. Charge transport is than accomplished by moving of these polarons along the electric field gradient. PEDOT: PSS also offers another interesting property: It is mostly transparent to the visible part of the spectrum, which allows fabricating translucent electrodes. The case of polyaniline (PANI) is a bit more complicated: The conductive variant of the molecule consists of an intramolecular blend of oxidized and reduced parts called the emeraldine base (Figure 4).

NH

N

NH m

N

n

Figure 4. Emeraldine base, reduced part left, oxidized part right.

Figure 5. Movement of the oxidized section along the molecular axis by protonation/deprotonation and valence resonance, according to Focke et al. (Focke et al. 1987).

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The proposed conduction mechanism involves both species after a protonation of the base forming the emeraldine salt (Figure 5, second structure from top). Due to valence resonance the oxidized section emerges further down the molecular axis, which results in a net charge movement (Focke et al. 1987). The counter ion can be conceived as the corresponding anion of the used Brönsted acid. PANI offers also the ability to transport charge carriers between different polymer molecule s by a proton exchange. The further above mentioned in situ polymerization of polypyrrole is an example of a chemical deposition of conductive materials on fiber surfaces from a liquid phase, a technique also often utilized to deposit metals on fiber surfaces. This is mostly accomplished by reducing a noble metal salt in situ leading the reaction to a particulate precipitation of the metal. For example this can be used to deposit copper (Kim et al., 2004; Jiang et al., 2011; Schwarz et al., 2012), gold (Schwarz et al. 2010), nickel (Pinto et al., 2004) and silver (Lu et al. 2010; Feng et al., 2011). Another option is physical vapor deposition of metals on fibrous surfaces (Shahidi et al, 2014; Amberg et al., 2008). In this case a low pressure plasma discharge removes neutral atoms from a metal target which than are deposited on the fiber surface. These technique requires a more complex machinery than liquid based coating methods. The yarn has either to be placed in the recipient or a set of differentially evacuated vacuum chambers have to be applied allowing to transport the yarn material through a small orifice in the outermost chamber. The latter setup can be used for a continuous coating. It has to be noted, that physical vapor deposition yields only good results on monofilaments, since the rough surface topology of spun multifilaments exhibits interruptions in the very thin coating.

Figure 6. SCHEME of a differentially pumped vacuum chamber: a) high volume pump e.g., roots pump, b) vacuum pump e.g., rotary pump, c) main treatment chamber, d) prechamber, the yarn material is ducted from left to right across the chambers.

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However, coating of yarns comes with a set of drawbacks: Abrasion resistance of the coating is usually not sufficient to employ these yarns successfully in textile production processes like weaving, embroidering or knitting. Especially yarns from PVD are susceptible to abrasion. Furthermore, most yarn based substrates offer an elasticity which is necessary for the textile processing, but most coatings will alter their conductivity by elongation mostly due to lateral departing of the conductive elements in the coating (molecules in intrinsic conductive polymers, metal nano crystallites, or carbon particles, etc.). This might be interesting for the use in textile sensor systems but is disadvantageous for current or data lines. Another issue emerges due to the quite low share of conductive material in the net mass of the yarn leading to a low cross section of the conductive material itself. The resistance of a metal coated yarn can easily be calculated using the resistivity of the coated metal. The resistivity can be conceived as a proportionality factor between the resistance and the ratio of length and cross sectional area of a conductor. 𝑅=𝜌

𝑙 𝐴

(1)

An exemplary yarn with a diameter of 500 µm and a silver coating (resistivity of silver: 1.586*10-8 Ohm/m (Matula et al. 1979)) obtained by PVD of about 100 nm exhibits according to Eq. 1 a resistance of approx. 100 Ohm/m. Also most of the coated yarns are not isolated, which renders them mostly unsuitable for commercial applications in the clothing sector and also in the sector of technical textiles. Intrinsic conductive polymers as coating agents show in most cases a susceptibility to foreign substances and degradation by UV light and/or oxygen impairing their conductivity. This is mostly provoked by an unintended doping of the polymers by these substances changing the band gap of the polymer. However, these effects can be very interesting to realize sensors based on such polymers. An approach to gain higher conductivity is to integrate metal filaments in yarns. A common approach is to entwine a metal wire around a textile yarn. This ensures a higher elongation of the yarn under strain without rupturing the conductive element, since the metal is usually less elastic than the yarn. On the other hand, the textile haptics of the yarn is altered (Schwarz et al., 2011; Guo et al., 2012). Already commercialized is a blend of stainless steel and polyester staple fibers spun to a conductive yarn, additionally a variant consisting of pure stainless steel staple fibers exist. These materials offer electric conductivity combined with a good textile haptics and a good processing ability. However, due to the use of staple fibers the contact resistances in the yarn dominate the conductivity leading to a higher resistance than expected by the metal content. Unfortunately, an insulation of these yarns is not offered and may possibly be hard to achieve.

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Figure 7. Example of a wire entwined around a staple fiber yarn.

Figure 8. Micrograph of a commercially available steel staple fiber yarn.

In our research group the concept of integrating metal wires in textile staple fiber yarns was taken further while maintaining the textile haptics and realizing an insulation: Enameled metal wires, e.g., from copper, are widely used in electronic components like transformers and other inductive components. In first steps, these wires were integrated in 100% cotton material by conventional ring spinning, which was successfully accomplished. Due to the circular motion of spun yarn in the process, the metal component exhibits a helical structure, which lead to an only partially positioning of the wire in the middle of the yarn cross section (Figure 9). However, it was obviously desirable to keep the wire in the inner core of the yarn to achieve a complete textile characteristic of the resulting thread.

Figure 9. Left: micrograph of a ring spun yarn with metal core, the highlighted area shows the metal core penetrating the yarn surface; right: cross sectional view.

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An alternate spinning process called friction spinning was also applied to incorporate such wires into a staple fiber yarn. This technology is an open-end technology which means the inserted fiber bundle is opened into single fibers to piece them at the end of an already spun yarn end by friction and twisting. For this process, one or multiple draw frame slivers can be used. The first friction spinning apparatus was invented by Ernst Fehrer, Helmut Fuchs, and Franz Koenig in 1976 (Fehrer et al. 1976). The commercial use of their friction spinning technology is known under the name DREF-3000 and is mainly used for core-shell yarn structures for technical applications as demonstrated here. Their spinning technique employs two drums feeding the staple fibers into the process, while a core yarn is lead axially along the drums. The drums are slightly tapered, equipped with a mesh surface and connected to a vacuum pump. This leads to an adhesion of the fibers to the rotating drums, and friction occurs at the gap between the drums forming the resulting yarn (Figure 10). The tapered geometry increases the friction in axial direction in order to densify the produced yarn.

Figure 10. Basic working principle of a friction spinning machine creating a core-shell-yarn; a) staple fibers from a draw frame sliver, b) spinning drums, c) connection to vacuum pump, d) core yarn, e) core-shell-yarn.

Figure 11. Left: micrograph of a friction spun yarn with metal core; right: cross sectional view.

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An enameled copper wire was then introduced as core component and wool staple fibers were used to create a core-shell-yarn. It is clearly visible that in this case the metal core is well integrated into the yarn without any surface contact or break through (Figure 11). However, it has to be considered that the coverage ratio is also influenced by the produced yarn count, the used fiber type and fiber diameter. As mentioned earlier, these metal core hybrid yarns lack elasticity and tenacity and are therefore prone to breakage during textile processing. In particular, sewing and embroidering showed to be impossible. However, other processing techniques were successfully carried out. In the first place, using a special embroidery head enables one to use these yarns as cover thread which is fixated by an additional yarn. This allows realizing nearly unlimited designs of these conductive structures. In a second approach the integration of these yarns in a narrow woven fabric was realized. In this case the yarn material was introduced as weft component. Due to the nature of a narrow weaving machine the weft yarn is not cut at the selvedge but lead back. This resulted in a mesh like conductive surface consisting of a single, non-crossing conductive strand.

Figure 12. Embroidered patch with metal core yarn.

Figure 13. Narrow fabric with incorporated metal core yarn as weft yarn (white).

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APPLICATIONS OF CONDUCTIVE YARNS Commercially available with a significant market share are only metal based yarns, being metal coated, incorporating metal filaments or consisting of metal filaments. Polymer coated yarns are not yet commercialized most probably for the reasons mentioned above. Since most of these metallic yarns lacking an insulation, the fields of applications are mostly: Heating, medical electrodes (contact to the skin ), and antistatic applications. They are also used as power supply lines for smart textiles, however, the lack of insulation renders these approaches more experimental. It has to be mentioned, that some yarn suppliers offer silver coated yarns with a polyurethane isolation, rendering these materials promising for the field of smart textiles, but removing the isolation without damaging the underlying coating may be difficult.

FABRIC BASED CONDUCTORS Another approach to introduce an electric conductivity into textile materials can be accomplished by coating the textile substrate with conductive materials. The integration level is lower than in yarn based approaches, but some requirements can be met much better by areal coating of a textile. These requirements can either be structural or being dictated by the field of application. Structural advantage of areal coating is the ability to quickly change design patterns of conductive tracks or areas or the ability to use individual patterns. This can be done either by screen printing or digital printing of conductive pastes. The field of application may lead to an advantageous coating rather using conductive yarns if wide areas need to be conductive. This can be the case for textile electrodes or capacitance based textile sensors. Different materials exist to realize conductive coatings on textiles. First, carbon containing coating pastes are already commercially available, which can be applied by a simple knife coating technique. Usually these pastes contain carbonaceous materials (mostly sooth) at a sufficient amount to achieve percolation embedded in a polymeric coating paste. This approach enables easy to process and cost efficient coatings. Also intrinsically conductive polymers (ICPs) are utilized for conductive coatings. PEDOT: PSS solution for coating applications are known since quite a long time, but were based on organic solvents. For textile applications it is highly desirable to replace these solvents by water. Therefore, water based PEDOT: PSS dispersions are commercially available today which incorporate water soluble polymers of higher molecular weight. These provide some assistance in geometrical aligning the ICP molecules during the drying process which usually leads to higher conductivity than without these auxiliaries.

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Also Isolation is an issue for these technique as it is for the conductive yarns. But in the case of coating this can be addressed much easier by applying a non-conductive top coating on the conductive layer. On the other hand contacting such coatings is more challenging than for conductive yarns. The latter can on some cases even be soldered, but also contacting by embroidery is possible. Conductive areal coatings, especially multilayer assemblies, cannot be sewn or otherwise penetrated. It is also not possible to solder on carbonaceous materials or ICPs. Therefore the most applied technique is using conductive adhesives or coating on a partially metalized substrate. Also clamping and crimping can be utilized. Some care hast to be taken by contacting ICP coatings with metals (wires, connectors, etc.), since an unintended schottky contact could be made. For example schottky contacts for Al/PEDOT: PSS and Al/PPy are reported (Abthagir et al. 2001). Choosing metals with a higher work function should avoid this issue.

APPLICATIONS OF CONDUCTIVE AREAL COATINGS Fabric based coatings are less widespread than conductive yarns. Some applications are known for heating textiles and electroluminescent textiles in which luminescent particles are incorporated between to coated electrodes, one being made of PEDOT: PSS. By applying an alternating electric field, these particles can be excited to emit light (Graßmann et al. 2017). However, none of these applications gained a market relevance.

Printing Techniques A sub technique of areal coating is printing of conductors onto textiles. Here either screen printing (Kazani et al., 2012; Yang et al., 2013; Virkki et al., 2015) or inkjet printing (Stempien et al., 2016) can be conceived. The latter is of growing attention due to its high flexibility and resource efficiency in the textile sector especially for nonconductive applications. While printing inks need to show specific rheological behavior and sometimes also distinct dielectric properties, printing conductive tracks on textiles by this technique needs additional requirements to be met. In most cases printing inks for conductive applications can be divided into two groups: filler based and intrinsically conductive. Intrinsically conductive inks usually consist of a share amount of conductive polymers, like PEDOT: PSS in a suitable dispersion for inkjet printing. Challenges for these are achieving a sufficient low viscosity to be printable by simultaneously maintaining a high conductivity. As for knife coated PEDOT: PSS layers, degradation and oxidation have to be minimized. Textile applications of inkjet printed ICPs are quite

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rare but some approaches are known e.g., for the fabrication of textile EL-devices (Hu et al. 2012) or textile strain sensors (Calvert et al. 2007). Filler based inks contain metallic or carbonaceous particles and can therefore achieve higher conductivities than intrinsically conductive inks. However, the particle size of such fillers is highly crucial as well as the agglomeration behavior of the fillers. Both effects can lead to clogging of the printing nozzle or, worse, to clogging of the supplying tubes. To circumvent those issues, nano sized particles are employed in printing inks, but they need to be sintered after printing to connect the conductive particles together and allow percolation (Perelaer et al. 2006, Park et al. 2007). Another approach features the in situ reduction of silver ions on the substrates surface. The authors claim, that due to their chosen reduction agent, metallic silver forms after passing the nozzle and no annealing is necessary afterwards (Walker et al. 2012). Textile applications are reported for example as antennas (Whittow et al. 2014) or elastic conductors (Matsuhisa et al. 2015). Due to this aforementioned challenges, screen printing of conductive materials on textile fabrics is much more prevalent. Depending on the mesh size, different particle diameters and also nano wires can be printed. Furthermore, viscosity is not that crucial, since it influences the maximal achievable resolution but does not infringe the process at all. So on textile substrates transmission lines are reported (Locher et al. 2007), all printed waterproof conductive tracks (Yang et al. 2013), and also electrodes for medical applications are screen printed on fabrics (Paul et al. 2014).

ACKNOWLEDGMENTS The authors wish to thank the following members of the Niederrhein University of applied sciences: Mr. Roland Werner for conducting the spinning experiments, Mr. Frank Heimlich and Ms. Jiaming Gu for producing the woven structures, and Ms. Marina Normann for embroidering the conductive yarns.

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Jiang, S. X. & Guo, R. H. (2011). Electromagnetic shielding and corrosion resistance of electroless Ni–P/Cu–Ni multilayer plated polyester fabric. Surface and Coatings Technology, 205(17), 4274-4279. Kazani, I., Hertleer, C., De Mey, G., Schwarz, A., Guxho, G. & Van Langenhove, L. (2012). Electrical conductive textiles obtained by screen printing. Fibres & Textiles in Eastern Europe, 20(1), 57-63. Kim, S. H., Oh, K. W. & Bahk, J. H. (2004). Electrochemically synthesized polypyrrole and Cu-plated nylon/spandex for electrotherapeutic pad electrode. Journal of Applied Polymer Science, 91(6), 4064-4071. Lee, J. B. & Subramanian, V. (2005). Weave Patterned Organic Transistors on Fiber for E-Textiles,” IEEE Transactions on Electron Devices, 52, 269-275. Locher, I. & Tröster, G. (2007). Screen-printed Textile Transmission Lines, Textile Research Journal, 7(11), 837-842. Lu, X., Jiang, S. & Huang, Y. (2010). Ultrasonic-assisted electroless deposition of Ag on PET fabric with low silver content for EMI shielding, Surface & Coatings Technology, 204, 2829–2833. Maity, S. & Chatterjee, A. (2014). Polypyrrole based electro-conductive cotton yarn. Journal of Textile Science and Engineering, 4(6), 171. Matsuhisa, N., Kaltenbrunner, M., Yokota, T., Jinno, H., Kuribara, K., Sekitani, T. & Someya, T. (2015). Printable elastic conductors with a high conductivity for electronic textile applications, Nature Communications, 6, 7461. Mattmann, C., Clemens, F. & Tröster, G. (2008). Sensor for measuring strain in textile. Sensors, 8(6), 3719-3732. Matula, R. A. (1979). Electrical resistivity of copper, gold, palladium, and silver, Journal of Physical and Chemical Reference Data, 8, 1147, 1260. Meyer, J., Arnrich, B., Schumm, J. & Troster, G. (2010). Design and modeling of a textile pressure sensor for sitting posture classification. IEEE Sensors Journal, 10(8), 1391-1398. Park, B. K., Kim, D., Jeong, S., Moon, J. & Kim, J. S. (2007). Direct writing of copper conductive patterns by ink-jet printing, Thin Solid Films, 515(19), 7706-7711. Paul, G., Torah, R., Beeby, S. & Tudor, J. (2014). The development of screen printed conductive networks on textiles for biopotential monitoring applications, Sensors and Actuators A, 206 35– 41. Perelaer, J., de Gans, B. J. & Schubert, U. S. (2006). Ink-jet Printing and Microwave Sintering of Conductive Silver Tracks, Adv. Mater., 18, 2101–2104. Pinto, N. J., Carrion, P. & Quinones, J. X. (2004). Electroless deposition of nickel on electrospun fibers of 2-acrylamido-2-methyl-1-propanesulfonic acid doped polyaniline. Materials Science and Engineering: A, 366(1), 1-5. Post, E. R., Orth, M, Russo, P. R. & Gershenfeld, N. (2000). E-broidery: Design and fabrication of textile-based computing. IBM Syst. J., 39, 840–860.

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Schwarz, A., Hakuzimana, J., Kaczynska, A., Banaszczyk, J., Westbroek, P., McAdams, E., Moody, G., Chronis, Y., Priniotakis, G., DeMey. G., Tseles, D. & VanLangenhove, L. (2010). Gold coated para-aramid yarns through electroless deposition, Surface & Coatings Technology, 204, 1412–1418. Schwarz, A., Hakuzimana, J., Westbroek, P., Mey, G. D., Priniotakis, G., Nyokong, T. & Langenhove, L. V. (2012). A study on the morphology of thin copper films on paraaramid yarns and their influence on the yarn ’s electro-conductive and mechanical properties. Textile Research Journal, 82(15), 1587-1596. Schwarz, A., Kazani, I., Cuny, L., Hertleer, C., Ghekiere, F., De Clercq, G. & Van Langenhove, L. (2011). Electro-conductive and elastic hybrid yarns–The effects of stretching, cyclic straining and washing on their electro-conductive properties. Materials & Design, 32(8), 4247-4256. Schwarz-Pfeiffer, A., Hoerr, M. & Mecnika, V. (2015). Textiles with integrated sleepmonitoring sensors. Advances in Smart Medical Textiles: Treatments and Health Monitoring, 197. Shahidi, S., Moazzenchi, B. & Ghoranneviss, M. (2015). A review-application of physical vapor deposition (PVD) and related methods in the textile industry. The European Physical Journal Applied Physics, 71(3), 31302. Shim, B. S., Chen, W., Doty, C., Xu, C. & Kotov, N. A. (2008). Smart Electronic Yarns and Wearable Fabrics for Human Biomonitoring made by Carbon Nanotube Coating with Polyelectrolytes, Nano Lett., 8 (12), 4151–4157. Shu, L., Hua, T., Wang, Y., Li, Q., Feng, D. D. & Tao, X. (2010). In-shoe plantar pressure measurement and analysis system based on fabric pressure sensing array. IEEE Transactions on Information Technology in Biomedicine, 14(3), 767-775. Sibinski, M., Jakubowska, M. & Sloma, M. (2010). Flexible temperature sensors on fibers. Sensors, 10(9), 7934-7946. Stempien, Z., Rybicki, E., Rybicki, T. & Lesnikowski, J. (2016). Inkjet-printing deposition of silver electro-conductive layers on textile substrates at low sintering temperature by using an aqueous silver ions -containing ink for textronic applications. Sensors and Actuators B: Chemical, 224, 714-725. Virkki, J., Björninen, T., Merilampi, S., Sydänheimo, L. & Ukkonen, L. (2015). The effects of recurrent stretching on the performance of electro-textile and screenprinted ultra-high-frequency radio -frequency identification tags. Textile Research Journal, 85(3), 294-301. Walker, S. B. & Lewis, J. A. (2012). Reactive Silver Inks for Patterning HighConductivity Features at Mild Temperatures, J. Am. Chem. Soc., 134 (3), 1419–1421 Weremczuk, J., Tarapata, G. & Jachowicz, R. (2012). Humidity sensor printed on textile with use of ink-jet technology. Procedia Engineering, 47, 1366-1369.

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Whittow, W. G., Chauraya, A., Vardaxoglou, J. C., Li, Y., Torah, R., Yang, K., Beeby, S. & Tudor, J. (2014). Inkjet-Printed Microstrip Patch Antennas Realized on Textile for Wearable Applications, IEEE Antennas and Wireless Propagation Letters, 13, 71-74. Xue, P., Tao, X. M., Kwok, K. W. Y., Leung, M. Y. & Yu, T. X. Electromechanical Behavior of Fibers Coated with an Electrically Conductive Polymer, Textile Research Journal, 74(10), 929 – 936. Yang, K., Torah. R., Wei, Y., Beeby, S. & Tudor, J. (2013). Waterproof and durable screen printed silver conductive tracks on textiles, Textile Research Journal, 83(19), 2023-2031.

In: Textiles: Advances in Research and Applications ISBN: 978-1-53612-855-0 Editor: Boris Mahltig © 2018 Nova Science Publishers, Inc.

Chapter 7

METAL COATINGS EFFECT PIGMENTS FOR TEXTILE FUNCTIONALIZATION TO REALIZE UV AND IR PROTECTIVE APPLICATIONS Jieyang Zhang and Boris Mahltig Hochschule Niederrhein – University of Applied Science, Faculty of Textile and Clothing Technology, Mönchengladbach, Germany

ABSTRACT The aim of the actual development is the realization of a coated textile material with protective properties against ultraviolet light (UV) as well as against infrared light (IR). To fulfill these demands a double coating is developed for textile substrates. The double coating contains mainly two different coatings: A first base coating – to equalize the textile topography and to support UV-protection and a second top coating – containing metal effect pigments to introduce IR-reflection and coloration effects. The optical transmission of prepared samples are determined by UV/Vis/IR-spectroscopy in diffusive measurement arrangement for wavelength from 220 nm to 1400 nm. Surface properties of coated samples are recorded by using scanning electron microscopy SEM and light microscopy. The wash fastness as well as the abrasion stability of the coatings are tested. Altogether the developed coating approach can be the starting point for the final development for a textile based light protective and light management device.

Keywords: effect pigments, optical spectroscopy, UV-protection, IR-reflection



Corresponding Author Email: [email protected].

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1. INTRODUCTION In daily life, people are exposed mainly to three different types of light: UV light, visible light and IR light. Simplest these types of light can be distinguished by their wavelength ; UV nmVis 750nm > > 400nm; IR> 750nm (Hesse et al. 2008). The radiation of the sun reaching the earth surface contains usually UV light from 290 nm up to IR light with 4000 nm wavelength (Hölzle & Hönigsmann 2005). The light intensity and composition is of course dependent on the geographical position and the weather (Krizek et al. 2005). Also artificial light from man-made light sources (lamps) can contain significant amounts of UV and IR light (Maslowsky 2013; Mahltig et al. 2013). For incandescent lamps (lightbulb), the emission of infrared light is significantly higher compared to the amount of emitted visible light (MacIssac et al. 1999). For human eye only visible light of wavelength between 400 to 750 nm is visible, so a decrease of UV- and IR-light exposure cannot be detected by human eye and will not influence the lighting situation, e.g., in a building (Hesse et al. 2008). UV light can cause sun burns, blindness or even skin cancer, so the protection against UV-light is quite reasonable (Brasch et al. 1991). UV protection by textile materials means that the transmission of UV-light (0.1% and from the 3.period or higher are recorded) Pigment paste Shinedecor 5000 Shinedecor 9350 Shinedecor 9355

content Aluminum flake; 24-28% Gold bronze; 38-42% Gold bronze; 33-37%

pH Color value 6.5-7.5 Silver white

Elemental composition [wt-%] Cu Ag Al Zn --0.5 +/- 55.6 +/- --0.1 3.0

Si ---

P 0.3 +/0.1

7.3-8.3 Copper 52.5 +/red 1.8

---

---

---

1.4 +/0.1

---

7.3-8.3 Gold yellow

---

0.2 +/0.1

14.4 +/1.1

1.5 +/0.1

---

32.7 +/1.2

Table 2. Paste recipe for coating agent containing different metallic effect pigments. All amounts are given in wt-%. The weight content of the metallic effect pigment in the final coating recipe is 10% Recipe components

Lutexal Thickener HIT Water Edolan CA Edolan XCI Shinedecor paste

Amounts of components used, for recipes with different Shinedecor effect pigment pastes Shinedecor 5000 Shinedecor 9350 Shinedecor 9355 0.5 2.5 3 28 41 36.9 30 30 30 1.5 1.5 1.5 40 25 28.6

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In Table 2 the recipe composition of the most used 10% recipe is given. The application of the coating recipes on the polyester fabric is done by knife on air coating. Afterwards drying is performed in a Mathis Lab dryer at 80°C for 2 minutes. In the end of the process a curing is performed at 160°C for a duration of 3 minutes.

2.2. Analytical Methods Light microscopy is performed with a light microscope VHX-600 from Keyence. The actual zoom lens is a VH – Z250R with a possible magnification in the range of 250 to 2500. A profile measurement unit VHX – S15 is used to gain profile pictures. Scanning electron microscopy SEM is performed with a TM 3000 Tabletop microscope from Hitachi. For SEM measurements the acceleration voltage is set to 15kV. This used SEM device contains additionally an EDS (energy dispersive spectroscopy) unit Quantax 80 from Bruker, which enables the determination of the elemental composition on sample surfaces. The optical properties are determined in the spectral range of 220 nm to 1400 nm and give for this information on the coloration of samples but also on their ability to support protection against UV and IR radiation. These measurements are performed in arrangement of diffuse transmission. In the measurement arrangement of diffuse transmission all light which is transmitted through the sample is collected and summarized. In comparison to common transmission measurements, in case of the actually used arrangement beside the direct transmitted light also the light scattered during the transmission is detected. A SHIMADZU UV-2600 UV-VIS Spectrometer is used as testing devise. This spectrometer is equipped with two detectors. One detector for detection of UV light and visible light and the other detector for infrared light. These detectors are switched at a wavelength of 700 nm, leading in the recorded spectra to a small step in the signal. For determining the washing fastness of coated fabric samples, fabric samples are washed several times at 40℃ for 75 minutes in the washing device SIEMENS SIWAMAT IQ 714. The samples are cut into 38 mm diameter round shape by using James Heal sample cutter. The washing detergent is TANDIL gentle washing powder containing no brightener. The usage of the detergent is 20 g per 2.5 kg washing material. The fabric material is added into the washing cylinder to keep the weight of treated textiles at 2.5 kg. The coated fabric samples are weighed before and after washing. The optical properties of samples are also evaluated after washing. According to EN ISO 12947-1, the abrasion testing is operated by using a Martindale from James Heal. The fabrics samples are cut in 38 mm diameter round shape by using James Heal sample cutter. According to the abrading fabric requirement in ISO 12947-1, a flat woven wool fabric from James Heal against the test specimen is abraded. After several abrasion cycles, the weight and the light transmission of coated fabric samples are evaluated.

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3. RESULTS AND DISCUSSION To support an appropriate structure to the reader, this section is split into three subchapters. At first the properties of the pure pigments and surface properties of the coated textile substrates are described. Following the optical properties and the protective functionalization gained by the coatings are presented. In the end, important properties related to the practical application, as wash fastness and abrasion fastness, are presented and discussed.

3.1. Materials and Surface Properties Altogether three different metal effect pigments are used to prepare the effect coating. These three metal effect pigments can be sorted into two different categories – an aluminum pigment and gold bronze pigments (Table 1). The aluminum pigment Shinedecor5000 is of silver white coloration and contain beside aluminum as main component also small amounts of the elements silver and phosphorous. These both elements are probable present to enhance the stability of the aluminum pigments in aqueous binder systems (Wissling 1999). Without stabilization of pigment surface, aluminum pigments react with water based solutions under the formation of hydrogen. This reaction is especially enhanced, if the binder system used for coating preparation contains an alkaline pH. The pigments contain nearly a round and regular shape with diameters up to 80 m (Figure 1). This shape of an effect pigment is often also named as “silver dollar” (Wißling et al. 2006). In contrast, the both gold bronze pigments Shinedecor9350 and Shinedecor9355 are significantly colored. They are mainly copper based (Table 1). For Shinedecor9355 also a content of zinc and aluminum is observed, probably responsible for the yellow coloration of this pigment. Both gold bronze pigments contain also an amount of around 1.5 wt-% silicon. This silicon is probable a part of a protective layer on the pigment surface, preventing the corrosion of these copper containing pigments. The size of the gold bronze pigments is smaller compared to the aluminum pigments. Also their shape is less regular (Figure 1). Such a shape is often also named as “corn-flake” structure (Wißling et al. 2006).

3.2. Optical Properties The optical properties are determined by transmission spectroscopy and recorded for the samples in transmission spectra for each type of prepared samples (Figures 3 to 5). Figure 2 shows as references the transmission spectra of the uncoated textiles of polyester

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and cotton used as substrates for further coating experiments. The polyester fabric contains in the range from 400 to 1400 nm transmission values of 50% to 60%. For UV light with a wavelength below 400 nm, the transmission is decreased and lowest values with a transmission of only 5% are reached for wavelengths below 300 nm. This low transmission of UV light can be explained by light absorption caused by the aromatic structure of the polyester polymer (PET, polyethyleneterephthalate ). Nevertheless the main UV radiation from sunlight is in the range from 300 nm to 400 nm and in this area the fabric exhibits a significant transparency. For this, this untreated polyester fabric is not suitable for UV protection applications and still a further protective finishing or coating is necessary to introduce UV protective properties. The reference cotton with a higher weight per area and a denser structure contains a lower transmission from 400 to 1400 nm with transmission values around 35%. However, the transmission against UV light is higher compared to the values of the polyester reference, so this cotton is as single material as well not a suitable material for UV protective application. In a first sample series the effect of a coating containing 10% of the effect pigments is investigated (Figure 3). By application of the effect pigment coating, the transparency is significantly decreased on the whole investigated spectral range. For all coatings, below a wavelength of 300 nm the transmission is near zero, so for that range the UV protection is given. This decrease can be explained by the absorption properties of the metal pigments in the UV range and the polyurethane component in the binder system. However in the range from 300 to 400 nm still significant transmission values are determined, so a full UV protection cannot be reached by these effect coatings. The transmission of visible light and infrared light is strongly determined by the type of used effect pigment. Lowest transmission values can be reached by use of the aluminum pigment (Shinedecor5000), which is supposed to contain a higher light reflection property compared to the other both investigated pigments. The gold bronze pigments contain in the area of visible light a shoulder in the spectra, which is related to their color. Although a significant decrease in the light transmission is reached by this first series of coatings, the transmission values are still too high for a light protective application. For this, following further coating series are performed to optimize the protective properties. For this improvement, the effect coating is applied as second coating on a previous applied base coating containing only the pure binder. The idea for using the base coating is to even the rough textile surface structure and to optimize by this the arrangement of the flat anisotropic pigments in the second layer. A certain decrease in transmission can be obtained by this double coating method (Figure 4). This decrease is stronger for the more rough cotton and mainly related to the infrared light. In contrast, by this arrangement a full UV protection cannot be reached. To improve the UV-protection, the addition of another effective UVabsorber material is required. This improvement is done by addition of titanium dioxide pigments to the base coating. The transmission spectra of the double coating system containing a base coating with titania pigments are presented in Figure 5. By use of this

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titania additive in the base coating, the transmission in the full UV range up to 400 nm can be decreased to values below 5% for both types of textile substrates (polyester and cotton) (Figure 5). For this, a full UV protection can be reached by this double coating system. Beside the strong influence of the TiO2 on the optical properties in the UV range, also the TiO2 application decreases significantly the transmission in the range for visible and infrared light. Titania is an UV absorber but also a white pigment with strong reflective properties for visible and IR light. This additional reflection is probable the reason for the further decrease of transmission of visible and IR light, if titania is applied in the base coating. Lowest transmission values are gained by combination with the used aluminum pigment Shinedecor5000. So for the current investigation, the combination of a titania containing base coating and an aluminum pigment top coating is most advantageous for radiation protection purposes.

5000

9355

Figure 1. Light microscopic images of polyester fabrics coated with recipes containing 10% of metallic effect pigments Shinedecor 5000 or Shinedecor 9355. The images are recorded in magnification of 1000X. 70 60

transmission [%]

polyester 50 40

cotton 30 20 10 0 400

600

800

1000

1200

1400

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Figure 2. Transmission spectra of the both textile substrates polyester and cotton. Reference spectra of the uncoated substrates.

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reference 5000 9350 9355

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0 400

on cotton fabric

reference 5000 9350 9355

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0

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wavelength [nm]

Figure 3. Transmission spectra of the fabrics with effect pigment coatings. Applied are four different effect pigments (Shinedecor types) with a concentration of 10% in the coating recipe. 70 70

reference 5000 9350 9355

on cotton fabric

on polyester fabric

60

transmission [%]

transmission [%]

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reference 5000 9350 9355

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50 40 30 20 10 0

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wavelength [nm]

Figure 4. Transmission spectra of the fabrics with effect pigment coatings. Applied are double coating, with a first base coating without any pigment and a second top coating containing four different effect pigments (Shinedecor types) with a concentration of 10% in the coating recipe. 70

70

on cotton fabric

60

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reference 5000 9350 9355

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40 30 20 10 0

0 400

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1000

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1200

1400

400

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1200

1400

wavelength [nm]

Figure 5. Transmission spectra of the fabrics with effect pigment coatings. Applied are double coating, with a first base coating containing titania white pigments and a second top coating containing four different effect pigments (Shinedecor types) with a concentration of 10% in the coating recipe.

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3.3. Fastness Properties The fastness properties are investigated with the most advantageous coating system containing titania in the base coating and the aluminum effect pigment in the second effect coating.

3.3.1. Washing Stability To gain information on the washing fastness the transmission spectra of the samples are recorded before and after 5 or 10 washing cycles (Figure 6). Also the transmission value at = 1000 nm in the infrared range is recorded as function of pigment concentration in the coating recipe and the number of washing cycles (Figure 7). Further the weight of coated samples before and after washing is depicted (Figure 8). The transmission spectra of samples in the washing tests are depicted in Figure 6 together with the reference spectrum of the samples containing only the titania base coating. The change of transmission as function of number of washing cycles is clearly seen but strongly depends on the type of coated textile and the wavelength of light. On polyester fabrics, the increase in transmission after washing is stronger compared to coated cotton. This statement is especially valid for visible and infrared light. Even on polyester after 10 washing cycles, the transmission values are lower compared to the reference spectrum of the titania containing base coating. For this it could be estimated, that by the washing process the effect coating is not completely removed and can act still as a protection component decreasing the transmission of light. Probably the adhesion of the binder layer onto the polyester fabric is weaker compared to the situation on the cotton fabric. The change of transmission for UV-light (