Advances in the Dyeing and Finishing of Technical Textiles || The use of nanotechnology in the finishing of technical textiles

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<ul><li><p> Woodhead Publishing Limited, 2013</p><p>280</p><p>12The use of nanotechnology in the fi nishing of </p><p>technical textiles</p><p>M. L. GU LR A JA N I, Indian Institute of Technology Delhi, India</p><p>DOI: 10.1533/9780857097613.2.280</p><p>Abstract: The use of nanotechnology in the fi nishing of technical textiles has resulted in imparting new and more complex functions on textile substrates as well as improvements in existing functions such as durability without losing the fabrics feel and texture, with the minimum use of chemicals. The evolution, advancement, theory and technology of application of some of the commercially successful nano-fi nishes that impart hydrophobic, super-hydrophobic, self-cleaning and antimicrobial properties are discussed in this chapter.</p><p>Key words: Lotus Effect, NanoSphere, oleophobic fi nish, silanes, fl uorocarbon, photocatalytic, nanosilver, titania, hydrophobic, super-hydrophobic, self-cleaning, antimicrobial.</p><p>12.1 Introduction</p><p>Finishing of technical textiles is generally carried out to give them a par-ticular function. Nanotechnology allows the development of new and more complex functions as well as improvements in existing functions such as durability without losing the fabrics feel and texture. Application of various types of nanotechnologies for the fi nishing of textiles has been recently reviewed (Gulrajani, 2006; Holme, 2007; Siegfried, 2007; Sawhney et al., 2008; Dastjerdi and Montazer, 2010; Gowri et al., 2010).</p><p>The most signifi cant early development of nano-fi nishes for textiles came through the research of Dr David Soane. After almost 20 years at the Uni-versity of California, Berkeley, Dr Soane left academe and, using his garage as a lab, began devising ways of using nanotechnology to add unusual prop-erties to natural and synthetic textiles, without changing a fabrics look or feel. He fl oated the fi rst nanotechnology-based company, Nano-Tex, in 1998, specifi cally catering to the textile industry. The main attribute of the tech-nology was to directly bind functional chemicals to the fi bres, instead of binding them as a side chain of a polymeric compound, thereby improving their durability with minimal effect on the basic properties of the substrate.</p><p>At about the same time the pioneering work of Professor W. Barthlott of the University of Bonn, Germany, led to an understanding of the </p></li><li><p> The use of nanotechnology in the fi nishing of technical textiles 281</p><p> Woodhead Publishing Limited, 2013</p><p>mechanism by which the leaves of the lotus and other plants utilize super-hydrophobicity as the basis of self-cleaning. Barthlott now owns a patent and the Lotus Effect trademark, which has formed the basis of the NanoSphere-based stain protection and oil- and water-repellent textile fi nishes of Schoeller Textil A G of Switzerland.</p><p>Further impetus for the development of nano-fi nishes for textiles came from the work of Dr Walid Daoud and Dr John Xin of Hong Kong Poly-technic University. These scientists invented an effi cient way to coat cotton cloth with tiny particles of titanium dioxide. These nanoparticles act as catalysts that help break down carbon-based molecules, requiring only sun-light to trigger the reaction. The inventors surmised that these fabrics could be used to make self-cleaning clothes, able to tackle dirt, environmental pollutants and harmful microorganisms.</p><p>Today we have a plethora of textile fi nishes, all based on the basic research and development carried out by the above-mentioned pioneers. Applica-tion of inorganic nanoparticles such as titanium dioxide, silver, zinc oxide, copper, gallium, gold nanoparticles, carbon nanotubes, nano-layered clay, and their nanocomposites, to textile substrates to impart antimicrobial properties, is another area where the application of nanotechnology to the fi nishing of textiles is being explored extensively. New methods of applica-tion of nanoparticles by nano-coating, electro-spraying, layer-by-layer deposition, chemical vapour deposition, sol-gel deposition and polymer fi lm roughening are being researched and commercially exploited to impart super-hydrophobicity to textile substrates (Xue et al., 2010a). A large number of functionalities can be imparted to textiles by the application of nanoparticles and nanostructured materials. These have been discussed by Siegfried (2007) and are summarized in Table 12.1. The evolution and pro-gression of nano-fi nishes and their application to textiles will be discussed in this chapter.</p><p>12.2 Hydrophobic nano-fi nishes</p><p>Most commercial hydrophobic fi nishes are based on fl uorocarbon copoly-mers. These are dispersions of fl uorinated acrylates having comonomers that have reactive methalol or epoxy groups that may react to form a cross-linked network and become covalently bonded to the surface of the fi bres. These fi nishes form low-energy semi-permeable fi lms that protect the fi bres in treated fabrics and considerably reduce surface tension. The critical surface tension (C) of CF3 is 6 mN m1. In fl uorocarbon fi nishes the critical surface tension (C) depends on the chain length of the fl uorinated side chain and requires a minimum chain length of n = 9 (Holme, 2003).</p><p>To develop a more durable hydrophobic and oleophobic fi nish that does not block the pores of the fabric by formation of polymer fi lm thereby </p></li><li><p>282 Advances in the dyeing and fi nishing of technical textiles</p><p> Woodhead Publishing Limited, 2013</p><p>Table 12.1 Technical textiles and nanoparticles</p><p>Functional textile Nanoparticle/nanostructure</p><p>Product description</p><p>Self-cleaning textiles/stain resistant</p><p>TiO2, fl uoroacrylates, SiO2*, CNT</p><p>Stain-repellent furniture textiles, umbrellas, easy to clean luggage, self-cleaning pants, ties, coats</p><p>Antibacterial Ag, chitosan, SiO2*, TiO2, ZnO</p><p>Anti-odour underwear, socks, insoles, helmets and other sports gear, furniture textiles and bed sheets, kitchen sponges, towels; biocidal facial masks, blankets, patient dresses, surgical gloves</p><p>Conductive/antistatic textiles</p><p>Cu, polypyrrole, polyaniline</p><p>Smart clothes with sensing function, isolating carpets and fl oor coverings, suits with electromagnetic functions, spark-preventing fi lters</p><p>UV-blocking textiles</p><p>TiO2, ZnO UV-blocking sports clothing with integrated sun protection, shirt fabric, coating agents, umbrellas</p><p>Flame-retardant textiles</p><p>Sb3O2, CNT, boroxosiloxane, montmorillonite</p><p>Flame-resistant suits, gloves, carpets, curtains, furniture textiles, seat cushions, linings</p><p>Reinforced textiles CNT Bulletproof jackets and vestsControlled release </p><p>of active agents, drugs or fragrances</p><p>Montmorillonite, SiO2*</p><p>Insect-repelling jackets, tents; fragrance-emitting furniture textiles, carpets, curtains; drug-releasing wound dressings</p><p>Luminescent textiles</p><p>Stimuli-sensitive colorants</p><p>Textiles with colour changing effects</p><p>Thermal insulating textiles</p><p>Nanoporous Si structure</p><p>Thermally insulating mountain jackets for low temperatures, shoe insoles</p><p>* SiO2-nanosol-coating as matrix for embedded active species (biocides, dyes, fragrances).</p><p>making it more breathable Soane and co-workers (Soane and Offord, 2002; Linford et al., 2005) patented a large number of multifunctional (nano) molecules that were capable of being covalently and non-covalently attached to cellulosic and proteinous fi bres. Some of these multifunctional molecules were block copolymers or graft copolymers, having plural functional groups such as binding groups, hydrophobic groups, hydrophilic groups and oleo-phobic groups. These groups may be present in the form of hydrophobic and hydrophilic regions. In these multifunctional molecules the hydrophilic groups, such as the carboxyl groups, act as reactive groups. These may be present in the form of polycarboxylic acid or as polyanhydrides, such as poly(maleic anhydride) polymer.</p></li><li><p> The use of nanotechnology in the fi nishing of technical textiles 283</p><p> Woodhead Publishing Limited, 2013</p><p>One such multifunctional molecule may be represented as in Fig. 12.1. A reaction scheme of a multifunctional molecule with cotton is shown in Fig. 12.2. Where a hydrophilic reactive molecule of poly(maleic anhydride) fi rst reacts with the hydro- or fl uoroalkyls, having preferably C8 or C9 (for </p><p>R</p><p>A</p><p>X</p><p>( )m</p><p>( )o</p><p>(O)n</p><p>12.1 Multifunctional molecule of Dr Shone, where m, n = 0 or 1, o = 0 or 2. R is a linear, branched, or cyclic hydrocarbon or fl uorocarbon having C1 to C30 hydrocarbon or fl uorocarbon groups. A is SO2, CONH, CH2 or CF2. X is a nucleophilic group capable of reacting with a hydroxyl, amine or thiol group.</p><p>Poly(maleic anhydride)</p><p>OO O</p><p>OO Om</p><p>( )</p><p>+R</p><p>R = hydro-and fluoroalkyls</p><p>HO RR = hydro- and fluoroalkyls Copolymerize</p><p>O O</p><p>oO</p><p>OO</p><p>OO</p><p>R</p><p>( )( )n</p><p>OO O R</p><p>( ) ( )m n</p><p>Poly(maleic anhydride-co-fluoroalkene-co-hydroalkene)</p><p>Cotton clothingH2O, NaH2PO2, heat</p><p>H2O, NaH2PO2,heat, cotton clothing</p><p>Cotton</p><p>OH OH OH OHHOOO O</p><p>O</p><p>Cotton</p><p>OH OH OH OHOHOO</p><p>OOO</p><p>O</p><p>RO</p><p>O( )</p><p>( )no</p><p>R</p><p>nm( )( )</p><p>12.2 Reaction schemes of a multifunctional molecule formation and attachment with cotton to form whiskers on the surface that are fl oating in air away from the fabric surface.</p></li><li><p>284 Advances in the dyeing and fi nishing of technical textiles</p><p> Woodhead Publishing Limited, 2013</p><p>maximum hydro- and oleophobicity as discussed above), it forms a multi-functional molecule, having hydrophobic, oleophilic and hydrophilic groups or regions. Subsequently this multifunctional molecule reacts with the hydroxyl groups of cotton or other cellulosic fi bres and amino groups of wool to form hydrophobic whiskers on the surface of the fabric, without blocking its pores.</p><p>It is claimed that the attached multifunctional molecules modify the surface properties of the treated fabric and impart water repellency, grease repellency, soil resistance, detergent-free washing, and increased speed of drying, in addition to improved strength and abrasion resistance without affecting the materials air permeability or breathability. Due to the multi-plicity of bonds and the ability of the molecule to easily diffuse into the fi bre because of its small molecular size (nano size) the durability of the fi nish is much better than that of the conventional fl uorocarbon acrylate polymer-based fi nish.</p><p>This original research formed the basis of the fi rst commercially success-ful nano-fi nish, originally named Nano-CareTM (now AquapelTM) and mar-keted by Nano-Tex. Dr Soane demonstrated that 10100 nanometre whiskers attached to cotton fi bres modify the surface tension so much that almost nothing can soak into and stain the treated fabric including red wine, soy sauce and chocolate syrup. To obviate the environmental concerns of fl uorochemicals, long-chain non-fl uoro copolymers containing hydropho-bic alkyl acrylates and maleic anhydride have been prepared and applied to cotton fabrics to get hydrophobic fabrics similar to those mentioned in Dr Soanes patents, with limited success (Prusty et al., 2010).</p><p>12.3 Super-hydrophobic nano-fi nishes</p><p>Hydrophobic fl uorocarbon fi nishes as discussed above lower the surface energy and can give a maximum water contact angle of roughly 120. To achieve self-cleaning ability with a super-hydrophobic fi nish, a contact angle of above 150 is required. This type of fi nish is obtained by increasing the surface roughness, which provides a larger geometric area for a relatively small projected area. The roughened surface generally takes the form of a substrate with a multiplicity of microscale to nanoscale projections or cavities.</p><p>Cassie and Baxter (1944) were the fi rst to observe that rough surfaces repelled water due to the air enclosed between the gaps in the surface. This enlarges the water/air interface while the solid/water interface is minimized. In this situation, spreading does not occur; the water forms a spherical droplet. The self-cleaning propensity of plant leaves with a rough surface was investigated and reported by Barthlott and Neinhuis in 1997. These investigators analysed the surface characteristics by high-resolution SEM </p></li><li><p> The use of nanotechnology in the fi nishing of technical textiles 285</p><p> Woodhead Publishing Limited, 2013</p><p>and measured the contact angle (CA) of leaves from 340 plant species cultivated at the Botanical Garden in Bonn. The majority of the wettable leaves (CA &lt; 110) investigated were more or less smooth, without any prominent surface sculpturing. In particular, epicuticular wax crystals were absent. In contrast, water-repellent leaves exhibited various surface sculp-tures mainly epicuticular wax crystals in combination with papillose epi-dermal cells. Their CAs always exceeded 150. They observed that on water-repellent surfaces, water contracted to form spherical droplets. It ran off the leaf very quickly, even at slight angles of inclination (</p></li><li><p>286 Advances in the dyeing and fi nishing of technical textiles</p><p> Woodhead Publishing Limited, 2013</p><p>is applied, resulting in the reduction of contact angle hysteresis and adhe-sive force (Jung and Bhushan, 2009).</p><p>The relation between the roughness of hydrophobic surfaces and the contact angle was established many years ago by Wenzel (1936) and Cassie and Baxter (1944) (see Fig. 12.4). The Wenzel equation relates to the homo-geneous wetting regime and yields the Wenzel apparent contact angle, W, in terms of the Young contact angle, Y, and the roughness ratio, r:</p><p>cos W = r cos Y</p><p>The roughness ratio is defi ned as the ratio of the true area of the solid surface to its nominal area. This equation shows that when the surface is hydrophobic (Y &gt; /2), roughness increases the contact angle.</p><p>The Cassie and Baxter equation describes the heterogeneous wetting regime and gives CB, the CB apparent contact angle, as</p><p>cos CB = rf f cos Y + f 1</p><p>In this equation, f is the fraction of the projected area of the solid surface that is wetted by the liquid, and rf is the roughness ratio of the wet area. When f = 1, rf = r, and the CB equation turns into the Wenzel equation.</p><p>It has been shown (Marmur, 2004) that the heterogeneous wetting regime is practically preferred by nature as the super-hydrophobic state on lotus </p><p>Liquid</p><p>Solid</p><p>Saturatedvapour </p><p>SV</p><p>LV</p><p>SL</p><p>q</p><p>LV, SV, and SL are the surface energies of theliquid-liquid, solid-vapour and solid-liquid.</p><p>Youngs equation: LV, SVSL = cos q</p><p>(a)</p><p>(b) (c)</p><p>cos qW = r cos qYWenzel Equation</p><p>cos qCB = rf f cos qY + f-1Cassie and Baxter Equation </p><p>qY is Youngs q</p><p>12.4 (a) Youngs wetting equation; (b) homogeneous wetting on a hydrophobic, rough surface; (c) heterogeneous wetting on a hydro-phobic, rough surface.</p></li><li><p> The use of nanotechnology in the fi nishing of technical textiles 287</p><p> Woodhead Publishing Limited, 2013</p><p>leaves. Moreover, the structures that trap air give low sliding angles required for self-cleaning. The relationship between sliding angles and contact angles on super-hydrophobic surfaces with roughness has been established.</p><p>Miwa et al. (2000) also prepared a transparent super-hydrophobic fi lm where the sliding angle was approximately 1 for a 7 mg water droplet. On this fi lm there was almost no resistance to the sliding of water d...</p></li></ul>

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