8.1 Introduction

The concept of textiles coating dates back to antiquity era. The simplest coating process was born when our primary ancestors realized that they could get waterproofing features on their outfits by simply smearing the animal fats on clothes. Since then, humans have always been trying to introduce novel materials to textiles by coating [1]. The main objective of the finishing process in the textile industry is adding or improving the functions of current textile-based products or introducing new properties to the substrate. The application of numerous materials, chemicals, and methods to coat the fabrics has led to introduction of many new products in the market with novel features. Finishing, coating, and laminating processes cover various fields, aspects, and contexts in the textile industry, each of which warrants its own technical prerequisites, knowledge, expertise, and equipment. The coated textiles have several layers, and the overall properties of each layer determine the final characteristics of the product. In general, the coating process is defined as a process in which a thick polymer solution, paste, or other substances are applied to a textile substrate to form a continuous, durable, and uniform layer of coating formulation on the substrate. In the laminating process, a thin pre-prepared polymer film is affixed to the surface of a fabric using heat, mechanical bonding, pressure, binders, and adhesives [2]. There are two common types of coating in the textile industry including fluid coating and dry compound coating [3]. The final application, nature of substrate, polymer type, and viscosity of coating paste and solution define the methods and equipment to be employed in coating [3]. For each of these coating methods, different types of polymers, solvents, and instruments should be used. The main steps of a textile coating process in industry are applying the coating formulation to the fabric surface, stabilizing the coating layer in the curing step, cooling, and winding up the coated products to rolls [3]. The first step of the coating process is spreading a viscous paste or solution on textiles and allowing the solvent to evaporate, leaving a polymer network on textiles [4]. In a coated product, the textile substrate provides strength and mechanical properties and the coating layer introduces novel functions to the surface [4]. Some main parameters such as the type of polymer used in the formulation, the substrate type, and the method of coating all have a direct impact on the ultimate features of the coated products. The coating is not limited to woven or knitted fabrics, and it can be applied to any form of fibrous materials such as fibers, yarns, and nonwovens [1]. The most commonly used polymers in the textile coating industry are polyvinyl chloride (PVC), polyvinyl acetate (PVA), acrylics, polyurethanes (PUs), and polyvinylidene chloride (PVDC), among others [4].

Nowadays, textiles with high performance are required in different aspects of life, and therefore, researchers continue to seek new approaches to integrate nanoscience into textile productions. The functionalized textiles should provide some advantages such as easy care, comfort, health, and hygiene for the end users; however, this should not be at the expense of sacrificing the intrinsic features of common textiles [5]. Most of the research carried out in this field focuses on growing and introducing a thin layer of nanomaterials and nanocrystals on textiles as one of the most feasible methods of surface coating [6]. In general, a smart textile is categorized as a new generation of products that can actively detect and sense various external stimuli including environmental, chemical, mechanical, thermal, pH, and electrical changes and then react to them according to the defined functionalities. The main hurdle in producing such smart textiles is the lack of acceptable washing fastness of coating layers. Different categories of materials are used to fabricate smart textiles such as phase change materials (PCMs), shape memory materials (SMMs), conductive materials, hollow glass microspheres, photocatalytic structures, chromic materials, mechanical responsive materials, polymers, nanowires, and noble metals, to name but a few. Depending on the type of nanomaterials used in the coating process, different functionalities can be introduced to textiles such as self-cleaning, antimicrobial property, UV protection, wrinkle resistance, oil and water repellence, and flame retardancy. Also, different coating methods such as sol–gel, spraying, immersion, plasma, chemical and physical vapor evaporation, and sono-processing have widely been used by researchers to coat the textiles [2].

This chapter provides a general overview on some of the novel aspects of nanocoatings that are currently used to produce smart textiles. To this end, some fundamental approaches to prepare nanocoatings will be assessed, and the obtained novel functionalities on textiles along with their associated mechanisms will be discussed.

8.2 Fabrication Methods of Nanocoatings

8.2.1 Sol–Gel

The main idea behind using the sol–gel method is creating an inorganic or organic network from a colloidal solution synthesized from precursors [7]. The sol in general is defined as a stable suspension of metal oxide particles in a liquid medium [7]. In a colloidal sol, the sizes of dispersed particles are too small in a way that the gravity force on them is negligible and the van der Waals forces and surface chemistry are influential parameters [8]. The gel is an interconnected porous 3D rigid network of suspended matters in the sol that are developed throughout the liquid medium. Depending on the type of initial sol, the formed gel is named as polymeric or colloidal gel. In the gel structure, there is a thermodynamic equilibrium between the solid gel networks and the liquid that exists in their structure. Depending on the nature of liquid media in the system, the gel is categorized as either aquagel (hydrogel) or alcogel. In an aquagel, water is the main liquid medium, while in an alcogel, the dominant liquid medium is alcohol. The existence of liquid in the structure of gels leads to a soft structure of gels. After drying the gels by removing the liquid from the network, a xerogel will be obtained [7]. The most commonly used precursors for synthesizing the metal oxide colloidal sols are metal alkoxides. The alkoxides are classified as a group of materials in which organic ligands have linked to metal or metalloid atoms [8]. The synthesized process of sols starts with a hydrolysis of precursors in the presence of water molecules. The amount of water and catalyst in the system determines the progress rate of hydrolysis. The next step is polycondesation process, which occurs by the reaction of hydrolyzed precursors with each other generating either water or alcohol by-products [8]. The influential factors in these reactions are pH, time, temperature, and concentrations of precursors and catalysts. The metal alkoxides react instantly with water due to their reactive alkoxide groups (–OR), which exist in their structure. During the hydrolysis process, the alkoxide groups are replaced with hydroxyl groups, which, in turn, results in formation of metaloxane links (M–O–M) [9]. Figure 8.1 illustrates the hydrolysis and condensation reactions of metal alkoxides in a sol–gel method.

The sol–gel process has been used in coating different substrates including textiles [10]. Application of sols to a substrate and then drying it results in formation of a thin and transparent layer of nanomaterials on fibrous matters. The common deposition methods of sols to substrates are dip coating, spray coating, and spin coating [11]. The hydrolysis and drying conditions are effective on some parameters such as density, porosity, cracking, and the mechanical properties of the coating layers [11].

Sol–gel method has been considered as one of the most feasible methods for surface functionalization of textiles. There are some advantages in this method including (i) obtaining a thin and transparent film; (ii) protecting the substrate against dirt, heat, and microorganisms; (iii) bolstering the mechanical characteristics of substrates; (iv) developing multifunctional coatings; and (v) feasibility [12]. After applying the sols to the textile surface, a padding step should be introduced to remove the excess uptake of sols and create a thin layer of three-dimensional network of nanoparticles on the surface. After deposition of nanoparticles on the textile surface, a curing process is recommended to improve the washing fastness and mechanical stability of the coating layer (Figure 8.2) [12, 13]. During the curing step, the M–OH groups remained from the precursor can establish hydrogen bonds with the hydroxyl groups of substrate as well as covalent bonds with the functional groups of fiber [9]. Figures 8.3 and 8.4 show the mechanisms of cross-linking processes, which take place during the curing step.

The final features of synthesized sol–gel coating on textiles mainly depend on some parameters such as rate of hydrolysis and condensation reactions of precursors, pH, temperature, time of reaction, precursor concentration, catalyst type, ageing temperature, aging duration, and drying conditions [9]. Textiles can be coated with sols simply through a dip-coating process. This technique is based on soaking the substrate into a sol and then removing it based on a predefined speed under certain pressure and temperature. The applied layer of nanoparticles is transformed to gel after drying and stabilized during the curing process. The thickness of the coating on substrates can be controlled by the number of coating layers and also the viscosity of sols and the concentration of nanoparticles [14].

8.3 Sol–Gel Coatings on Textiles

8.3.1 Self-Cleaning Coatings

8.3.1.1 Photocatalytic Self-Cleaning Nanocoatings

One of the new functionalities that can be produced through the sol–gel method on textiles is the self-cleaning property. There are two types of self-cleaning surfaces including photocatalytic and superhydrophobic, each of which has its own mechanisms to clean the substrate surface. For the former one, the photocatalytic activity of TiO₂ nanoparticles plays the main role in eliminating the pollutions from the textile surface. Titanium dioxide is a semiconductor that is widely used in different fields and products such as paints, cosmetic materials, and waste water treatment systems [15]. TiO₂ nanoparticles show photocatalytic activity under the illumination of UV ray with wavelengths greater than TiO₂‘s band gap energy, which is 388 nm. Under UV light, the electrons of the valence band of TiO₂ absorb the energy of UV light and promote to the conduction band producing negative electrons and positive holes. These negative and positive species, in turn, react with water and oxygen molecules producing superoxide anions and hydroxyl radicals. These two products are considered as active species that can decompose dirt and pollution adsorbed on the surface of textiles. Figure 8.5 illustrates the photocatalytic mechanism of TiO₂ nanoparticles under UV light.

Daoud and Xin [16, 17] in a series of publications investigated different aspects of cellulosic substrates coated with TiO₂ nanosols. They synthesized TiO₂ sol from titanium tetra isopropoxide (TTIP) in the media of ethanol and water mixture in the presence of acetic acid and nitric acid. The sol was synthesized after refluxing the ingredients together at 60°C for 16 h. The prepared sol was applied to cotton fabric surfaces through a conventional dip–pad–dry–cure process. Some features of fabrics such as ultraviolet protection factor (UPF) and antimicrobial activity against different pathogen microorganisms such as Klebsiella pneumoniae (a gram-negative bacterium) and Staphylococcus aureus (a gram-positive bacteria) were investigated [16, 18]. Qi et al. [19] synthesized TiO₂ sol at different temperatures and applied it to the surface of cotton fabrics. It was demonstrated that coated cotton fabrics possessed photocatalytic self-cleaning property due to the presence of anatase TiO₂ nanoparticles. The coated fabrics successfully decomposed the coffee and red wine stains under simulated sunlight (Figures 8.6 and 8.7).

Daoud et al. [20] applied TiO₂ sol on keratin fibers and investigated its self-cleaning property. The main challenge in applying nanosols to the proteinous fibers was the lack of adequate functional groups in the wool fiber structure to bond with TiO₂ nanoparticles. Therefore, they modified the surface of wool fibers through an acylation process using succinic anhydride and DMF to introduce new functional groups to wool [20]. Tung et al. [21–24] investigated the self-cleaning property of wool fabrics coated with TiO₂ nanoparticles, optimizing the TiO₂ sol preparation procedure. They assessed and optimized the influential parameters in TiO₂ sol preparation such as temperature, stirring time, TTIP concentration, and acid catalyst. They synthesized two types of sols using hydrochloric acid (H-sol) and nitric acid (N-sol) and compared the crystallinity, particle size, and photocatalytic property of nanoparticles coated on textiles (Figures 8.8 and 8.9).

Although the applications of TiO₂ sols to the textiles were promising in terms of decomposing stains, it was observed that photocatalytical removal of stains needed a long period of UV irradiation. Also, it had a very low efficiency under visible light. Therefore, researchers tried to boost the photocatalytic activity of TiO₂ nanoparticles through modifying sol synthesis through doping with noble metals and dyes and combining with other compounds such as SiO₂. In a research conducted by Pakdel et al. [25, 26], the self-cleaning property of nanocoatings was improved through integrating different concentrations of silica in the synthesis of sols. They prepared the TiO₂/SiO₂ sols with the compositions of 30/70 (1:2.33), 50/50 (1:1), and 70/30 (1:0.43) and coated on wool and cotton fabrics through a dip–pad–dry–cure method. It was reported that the presence of SiO₂ in the coating system rendered the surface of wool and cotton fabrics super-hydrophilic [25, 26]. The prepared TiO₂/SiO₂ coatings particularly TiO₂/SiO₂ 30/70 showed higher efficiency compared with pure TiO₂ on both wool and cotton fabrics (Figures 8.10 and 8.11). The synergistic role of silica was linked to its role in increasing the surface acidity of nanocoatings, increasing the surface area in the surrounding of the photocatalyst, and lowering the overall refractive index of nanocomposites [26].

Further research in this field was carried out to increase photoefficiency of the coating on textiles. One of the main research objectives was expanding the activation territory of nanoparticles applied to textiles towards the visible light region. Incorporating noble metals was considered as an effective remedy to improve the photoactivity of TiO₂ nanoparticles under UV and shift the photocatalytic activity threshold of TiO₂ nanoparticles to the visible region [27]. There are some associated mechanisms regarding the role of noble metals in increasing the photocatalytic activity of TiO₂ nanoparticles. It was confirmed that the presence of metals on TiO₂ surface can reduce the recombination rate of generated electrons and holes. This, in turn, would increase the production of active species of superoxide anions and hydroxyl radicals, hence a higher photocatalytic activity. At the same time, TiO₂/metal systems can show a higher photocatalytic activity under visible light due to the synergistic role of metals in producing a surface plasmon resonance and narrowing the semiconductor’s band gap [27, 28]. Wang et al. [29] synthesized the ternary nanocomposite system of Au/TiO₂/SiO₂ through the sol–gel method and applied it to cotton fabrics. It was observed that the Au/TiO₂/SiO₂ nanocomposites showed higher photoefficiency compared with bare TiO₂ on cotton surface. The efficiency of coated samples was compared based on the degradation rate of red wine stains under visible light [29]. Pakdel et al. [30, 31] synthesized TiO₂/metal/SiO₂ through the sol–gel method and investigated the role of metal type and concentration on the photocatalytic activity of coated wool and cotton fabrics. The effect of metal type (Pt, Au, and Ag) and their concentration on photocatalytic and antibacterial activities were investigated. They assessed the photoinduced efficiency of nanocoatings based on the degradation of red wine stains on wool and decomposition of methylene blue solution under visible light sources [30]. Figure 8.12 shows the self-cleaning property of wool samples coated with TiO₂/Au/SiO₂ nanocomposites under filtered simulated sunlight.

8.3.1.2 Self-Cleaning Surface Based on Superhydrophobic Coatings

Another type of self-cleaning surface relies on superhydrophobic properties of coated surfaces. A surface is defined with superhydrophobic property when contact angle is 90°<θ≤180° and contact angle hysteresis is very low (typically <10°) [32]. The superhydrophobic property is also recognized as lotus effect because this behavior was first observed in nature on the leaves of a lotus plant, a symbol of purity [33]. It has been proved that the geometric structure and chemical composition of surface are two main factors that contribute to superhydrophobicity. In the main, there are two major approaches to confer a superhydrophobic property to a surface including introducing micro/nanostructure roughness to the surface or using chemicals with low surface free energy to modify the nanostructured surfaces [34, 35]. The electron microscopy investigation of lotus leaves demonstrated that the surface of leaves has been covered with a lot of nubs sticking out of the surface, which were covered with smaller rough scale epicuticular wax crystalloids [36]. Due to the waxy and rough structure, water droplets will not be able to spread on the surface and therefore will be rolling off on top of the hierarchical structures on coated surface and remove any contaminating particles [37]. Understanding the reason of lotus effect has spawned many new research investigations to expand this property to different surfaces such as textiles, building materials, and car paints. Utilizing chemicals that lower the free surface energy along with making microlevel and nanolevel roughness structures on the surface can even result in higher contact angles on coated samples [38]. Some methods such as sol–gel, electrochemical deposition, chemical vapor deposition, template synthesis, and hydrothermal approach, among others, can be used to construct the rough coating surface [35]. In order to reduce the free surface energy, some compounds such as alkyl- and fluoro-alkyl-substituted silanes and fluorine in the form of fluorocarbon polymers can be added to the coating system [34]. In the sol–gel method, the process of preparing sols can be modified to incorporate the hydrophobic compounds in the coating systems. To this end, some compounds of silane monomers with long alkyl chains such as hexadecyltriethoxysilane and polymer additives such polysiloxanes can be used [39]. Also, fluorinated additives such as fluoroalkylsilane monomers and polymers with perfluorinated alkyl side chains can be used to produce oleophobic properties on the textiles [39]. However, the interest in using fluorinated compounds in coating systems is diminishing due to its potential risks to human health and environment.

Mahltig and Böttcher [40] used the sol–gel method to produce pure and modified silica sols and assessed their water repellency after modifying with different additives on polyamide and polyester/cotton fabrics. Silica sols were modified with alkyltrialkoxysilanes, polysiloxane derivatives, and a fluorine-containing silane. It was observed that fabrics coated with sols containing hexadecyltrimethoxysilane and triethoxytridecafluorooctylsilane showed suitable water repellency. Shorter alkyl-chain length did not result in acceptable hydrophobicity [40]. Wang et al. [41] demonstrated that through applying the fluoro-containing silica nanoparticles to the surface of textiles such as wool, cotton, and polyester, a superhydrophobic property can be obtained. A co-hydrolysis of tetra-ethylorthosilicate together with fluorinated alkyl silane in NH₃·H₂O–ethanol media was carried out to produce the solution. The contact angle of water droplets on the fabrics treated by this method was larger than 170°. The size of nanoparticles on the surface of polyester ranged within the region of 50–150 nm [41]. Daoud et al. [42] coated cotton fabrics with a superhydrophobic layer of silica synthesized by low-temperature sol–gel method. A mixture of hexadecyltrimethoxysilane (HDTMS), tetraethylorthosilicate (TEOS), and 3-glycidyloxypropyltrimethoxysilane (GPTMS) was used to prepare the finishing sol. The coated sample showed a contact angle of around 141°. A slight reduction of water contact angle from 141° to 105° after 30 cycles of washing was observed highlighting the suitable stability of the coating layer on the cotton surface. It was claimed that the suitable stability of the hydrophobic layer is due in large measure to the linkages created between GPTMS and cotton fabric. Moreover, it was observed that the bursting strength and air permeability were improved; conversely, the tensile strength declined by 5% [42]. Gao et al. [43] produced hydrophobic cotton and polyester fabrics through incorporating hexadecyltrimethoxysilane (HDTMS) in the coating process. The samples were dip-coated in silica sol and then treated with hydrolyzed HDTMS for 1 h. It was observed that applying silica layer to the surface of fabrics before coating with hydrolyzed HDTMS led to higher water contact angle due to the increased surface roughness. They achieved water contact angles of 155.1° and 143.8° on cotton and polyester fabrics, respectively [43]. Xue et al., [44] produced UV-protective-superhydrophobic cotton fabrics through coating TiO₂ sol to cotton. Then, hydrophobization process was carried out through immersing the coated samples into a 1% solution of 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane (PFTDS). Afzal et al. [45] reported a superhydrophobic cotton fabric with photocatalytic activity that was fabricated through the low-temperature sol–gel method. In the first step, they synthesized the TiO₂ sol through the sol–gel method, applied to the fabrics surface, and then modified the coating with TTCP (meso-tetra(4-carboxyphenyl)porphyrin) to shift the photocatalytic activity threshold to the visible region. The coated samples were further functionalized with trimethoxy(octadecyl)silane (OTMS) solution to get the superhydrophobic property. The samples coated with OTMS/TTCP/TiO₂ showed a water contact angle of 156° (Figure 8.13) [45].

Some of the main drawbacks of liquid-repellent coatings on textiles are poor washing durability, low resistance to physical abrasion, and in some cases high stiffness of the coated samples. These issues have made these types of functionalized products inappropriate options for daily use [46]. Therefore, some methods such as covalent bonding with the substrate, crosslinking the coating layers, and using an elastomeric nanocomposite as the coating material, or introducing a self-healing function, have been recommended to tackle these drawbacks [46]. In order to improve the durability of coatings, Zhou et al. [47] filled polydimethylsiloxane (PDMS) with fluorinated alkyl silane (FAS) functionalized silica nanoparticles to fabricate a superhydrophobic coating on polyester fabric. The hydrophobic silica particles were synthesized through co-hydrolysis and co-condensation of tetra ethyl orthosilicate under an alkaline condition in the presence of a fluorinated alkyl silane [47]. They reported a water contact angle of 171° and a sliding angle of 2° for coated samples, which can be considered as a very high superhydrophobicity on samples. The coated samples underwent 500 cycles of wash, and it was observed that the water contact angle and sliding angle variations were less than 5° indicating the excellent washing fastness. They also tested the resistance of coated samples against boiling water condition as well as abrasion, and it was found out that the superhydrophobicity was still stable. In another research, Zeng et al. [32] produced hydrophobic silica particles and then applied to cotton fabrics along with SU-8, which is an epoxy-based photoresist compound (Figure 8.14). The coating was stabilized on the fabric by UV irradiation, followed by heat treatment. They obtained a coating layer that had a contact angle of 163° and was durable against organic solvents, acid and base solutions, as well as multiple washing cycles.

8.3.2 Antimicrobial Sol–Gel Nanocoatings

In recent years, the population growth and the diversity of microorganisms in living environments have motivated researchers to introduce advanced antimicrobial features to fibrous materials. Textiles can provide suitable breeding grounds for microorganisms such as warmth, moisture, and required nutrients, facilitating their proliferation. The growth of bacteria on textiles can negatively impact fabric’s physical and aesthetic features such as strength, color, and smell, among others [31, 48]. There is a great demand in the market for some antimicrobial textile products such as sportswear, shoe linings, socks, and underwear. Moreover, these functional textiles can be introduced to different types of textiles such as upholstery covers, outdoor textiles, air filters, automotive textiles, carpets, and medical textiles [49]. The antimicrobial property on textiles can be achieved by different types of organic and inorganic antimicrobial agents. The application of inorganic nanoparticles such as TiO₂, Ag, ZnO, Au, and CNT and their nanocomposites to coat textiles have been widely used by researchers [48]. The coated antimicrobial textiles should be efficient enough to eliminate different types of microorganisms while posing no toxicity to the consumers. At the same time, applied coatings need to be durable enough to withstand numerous cycles of launderings and hot pressing processes [49]. Also, the antimicrobial coating should not hamper the intrinsic features of the textile substrate. Due to the continuous contact between the textiles and human skin, it is important that the finished products do not impose any change to the bacterial ecology of human skin. This can interfere with the normal functionality of bacteria species, which usually inhabit on human skin, and their existence is necessary for balanced human health [49].

Among different coating methods to fabricate antimicrobial textiles, the sol–gel technique is considered as a versatile and feasible approach. Through the sol–gel method, the photoactive nanomaterials such as TiO₂ can be synthesized and applied to textiles [12]. Also, it is possible to incorporate some bioactive compounds into the inorganic matrices through this method [12]. Nanocoatings containing metal oxides or metals show antimicrobial properties based on different mechanisms. The antimicrobial efficiency of metal oxides such as TiO₂ nanoparticles results from their photocatalytic activity. The active species of super oxide anions and hydroxyl radicals generated from TiO₂ particles can react with the cell wall of bacteria; as a result, they would be able to destroy the cell wall and then the cell membrane, endotoxin, of E. coli [50]. The destruction of cell structure causes leakage of macromolecular compounds, such as proteins, minerals, and genetic materials causing the death of cells [50–52]. Further AFM analysis of antimicrobial activity of TiO₂ nanoparticles demonstrated that following the decomposition of endotoxin, a lipopolysaccharide, the intracellular material of cells leaks out, causing the death of bacteria [53]. However, heavy metals such as silver follow different strategy to deactivate the bacteria. The antimicrobial property of silver results from the silver ions generated from the silver particles in the presence of moisture. The antimicrobial efficiency of silver depends on its concentration, surface area, and the interaction rate of ions with fungus and bacteria [54]. The silver ions (Ag+) are generated in the presence of water and body fluid and can instantly react with proteins, amino acid residues, free anions, and receptor groups on cell membranes and deactivate them by denaturizing [55]. The efficiency of silver in eradicating the bacteria and fungus is also pertinent to its role in disrupting the enzymatic systems of microorganisms [54]. The Ag+ cations attach to electron donor groups containing sulfur (—SH), oxygen, phosphates, carboxylates, and nitrogen in the cell structure [54, 56] The possible alterations caused by silver in cell permeability, respiration, and DNA replication of microorganisms such as E. coli all are contributing factors that can deplete the growth of bacteria [57–59].

Mahltig et al. [60] thoroughly investigated the antimicrobial finishing of textiles using the sol–gel method. In one of their research studies, they produced antimicrobial viscose fabric through embedding the organic and inorganic biocidal compounds such as silver and copper compounds and hexadecyltrimethyl-ammonium-p-toluolsulfonat (HTAT) in silica sol [60]. They assessed the efficiency of coating in hindering the growth of Aspergillus niger fungus and two types of bacteria including Bacillus subtilis (a gram positive) and Pseudomonas putida (a gram negative) [60]. The silver modified coatings were successful in eliminating both bacteria and fungus; however, the coating containing copper did not show a good performance against fungus. They reported that 100% efficiency against bacteria and fungus was observed for samples coated with sols containing silver/copper/HTAT [60]. Busila et al. [61] coated cotton/polyester fabric by Ag:ZnO/chitosan colloids and assessed the antibacterial activity against S. aureus, E. coli, and M. luteus bacteria. It was observed that chitosan–metal ion chelation enhanced the positive charge density of coatings leading to a higher interaction with the negatively charged cell surface [62]. They employed two functionalizing agents of GPTMS and TEOS to have a better dispersion of nanoparticles in chitosan and better adhesion of hybrid coatings to the substrates. The results showed that Ag-doped ZnO/chitosan composite systems had better efficiency compared with Ag/chitosan and ZnO/chitosan. All samples showed very good antimicrobial activity with the ability to reduce up to 50–95% of the viability of bacteria [61]. Daoud and Xin [17] synthesized the TiO₂ colloid and applied it to cotton fabric and analyzed the antimicrobial property of samples against K. pneumoniae, a gram-negative bacterium, and S. aureus, a gram-positive bacterium [19, 63]. They incubated the plates for 24 h at 37°C under ambient cool white fluorescent light having a slight portion of UV. It was reported that TiO₂ coating layer efficiently inhibited the growth of bacteria. Pakdel et al. [30, 31] investigated the impact of different types of noble metals in introducing permanent antimicrobial activity to wool and cotton fabrics. Their research compared the efficiency of three types of noble metals including Ag, Au, and Pt in improving the functionality of TiO₂/SiO₂ coatings against E. coli bacteria [30, 31]. It was observed that silver was more efficient in improving the antimicrobial activity of coatings. Au and Pt improved the antimicrobial property of coatings only in their highest concentrations. Figure 8.15 shows the antimicrobial activity of cotton samples coated with Ag- and Au-modified colloids [30].

8.3.3 UV-Protective Nanocoatings

Improving the UV protection features of textiles has attracted great attention in recent years. It is proven that the UV section of solar spectrum can trigger some unfavorable degradation reactions in polymeric structures such as fibers [64]. Solar spectrum can be classified into three main UV wavelength sections including UVC (220–280 nm), UVB (280–320 nm), and UVA (320–400 nm) [65]. The ozone layer of our planet prevents the UVC rays to reach the earth surface protecting creatures against the harmful impacts. Therefore, only UVA and UVB can pass through the stratosphere and reach the earth. But some parts of UVB with lower wavelengths than 310 nm can still be harmful for human skin causing some problems such as skin reddening, skin burning, and even skin cancer [66]. More and more recently, due to the depletion of the earth’s ozone layer, an increasing rate of harmful solar radiation hits the earth. Therefore, numerous research has been devoted to enhancing the UV protection property of textiles [67]. Some factors such as the presence of dyes, textile structure, and finishing history, among others, can impact on the ultimate UV protection characteristic of fabrics [68–70]. The UV protection capability of fabrics is evaluated by ultraviolet protection factor (UPF) according to Australia and New Zealand standards [27]. This scale defines the amount of UV radiation that can pass through the fabrics and reach the skin. This simply means only 1/UPF of UV radiation can pass through the fabric. Also, it gives an idea of how long a person can stay under sunlight while wearing protective clothes without skin reddening. UPF is classified in three main groups including good (15<UPF<24), very good (25<UPF<39), and excellent (40<UPF<50, +50) UV protection levels [27].

There are numerous publications regarding the UV protective clothes particularly cotton [71]. Daoud et al. [16, 18] demonstrated that the anatase crystallite structures of TiO₂ nanoparticles grown through the low-temperature sol–gel method on cotton fabric surface can give birth to an excellent (+50) UV protection. Likewise, Abidi et al. [72] applied titania nanosols to the cotton fabrics and produced self-cleaning and UV-protective cotton fabrics. Apart from cotton, the UV absorption property of TiO₂ (P-25) nanoparticles was used to increase the photostability of other types of fabrics such as wool, polyester, and polypropylene against harmful impacts of UV [73–75]. Pakdel et al. [76] studied the impact of increasing photocatalytic activity of TiO₂ nanoparticles applied to wool on fabric’s photoyellowing. Based on the obtained results, an inverse relation was established between the photocatalytic activity of TiO₂-based nanoparticles and the photoyellowing rate of wool [76]. They employed the photoinduced chemiluminescence (PICL) method to compare the efficiency of synthesized coating in blocking UV irradiation [76]. PICL is a delicate and feasible approach to study the photodecomposition of polymeric structures such as textiles and fibers. It measures the population of free radicals generated in polymeric structures exposed to light illumination at a controllable atmosphere. The samples were exposed to the light source under the N₂ gas atmosphere. The initial sharp peak was related to phosphorescence emission and charge recombination of free radicals [76]. After decaying the first peak and switching the test environment to oxygen, the second sharp peak appeared, which was related to PICL. The intensity of PICL peak was used to study the photodegradation pace of polymeric substrates [76]. Figure 8.16 shows that TiO₂-based coatings have suppressed the PICL peak intensity implying a very good UV blocking of nanocoating applied to wool fabrics through the sol–gel method.

It was reported that reducing the photocatalytic activity of photoactive nanoparticles such as ZnO can make them suitable choices for UV protection applications [77, 78]. Zhang et al. [77] reduced the photocatalytic activity of ZnO nanoparticles by providing a silica shield and applied them to the wool fabric surface. It was realized that the modified nanoparticles could further retard the photoyellowing rate of wool fabrics through reducing the free radicals in the system [77]. This method prevented any potential damage caused by the presence of photoactive nanoparticles on wool [77].

8.4 Impregnation and Cross-Linking Method

One of the most important aspects of nanocoatings on textiles is their endurance against washing processes and mechanical tensions. Using cross-linking agents and chemical spacers in the impregnating bath has been reported effective to improve the nanoparticle stability through facilitating the establishment of linkages with functional groups on textile surface [79]. Using some polycarboxylic compounds such as 1,2,3,4-butane tetracarboxylic acid (BTCA), maleic acid, succinic acid, citric acid, and 1,2,3-propanetricarboxylic acid as cross-linking agents was effective in improving the stability of nanoparticles on textile surfaces [2, 80]. The most commonly used substrates for this method are cellulosic and proteinous fibers thanks to their intrinsic functional groups that exist in their structure. Natural cotton possesses plenty of hydroxyl groups in its structure, all of which can be considered as potential sites for reaction with cross-linking agents. However, proteinous fibers have fewer functional groups on their surfaces compared to cellulosic ones. Therefore, a pretreatment step should be introduced prior to surface coating. Some pretreatment methods such as surface oxidation with KMnO₄ in mild acidic conditions or treatment with succinic acid solution in DMF have been suggested to increase the population of negatively charged functional groups on the surface and in turn increase the durability of nanoparticles [20, 71]. The reaction between the cross-linking agents and substrate was mostly performed in the presence of sodium hypophosphite (SHP), which played a role as a catalyst. Also, it was observed that the photoinduced radicals generated from TiO₂ can trigger the cross-linking reaction by activating the carboxylic groups and the vinyl double bond of the cross-linking agents and facilitate the reaction with cellulose chains [81].

The cross-linking method has been used to apply different types of nanoparticles such as TiO₂, CNT, ZrO₂ and silica nanoparticles to the fabric surface [71, 82–84]. Meilert et al. [85] applied TiO₂ nanoparticles to cotton fabrics and affixed them to cellulose fibers using chemical spacers. They used three types of chemical spacers including succinic acid, 1,2,3-propanetricarboxylic acid, and BTCA in their research. The chemical spacers had at least two free carboxylic acids, one of which could be involved in the esterification process with hydroxyl groups that exist in the cellulose fiber surface. TiO₂ nanoparticles were attached to the second free carboxylic acid through the electrostatic interaction. Figure 8.17 shows the mechanisms of esterification reactions between cellulose and chemical spacers. The commercial TiO₂ nanoparticles were employed for coating the cotton samples. The self-cleaning property of fabrics was compared through monitoring the red wine stain degradation under simulated sunlight and the amount of released CO₂ gas from each sample.

In a research study conducted by Montazer et al. [71, 86], the TiO₂ Degussa P-25 nanoparticles were stabilized on the surface of wool samples using cross-linking agents of citric acid and BTCA under sonication. The former one had three carboxylic acid groups and the latter one had four carboxylic acid groups in their structures. Different aspects of coated samples were assessed including photoyellowing, self-cleaning, wettability, antimicrobial property, and antifelting [86–89]. Prior to coating, the wool samples underwent a surface oxidation process in a mild acidic solution of KMnO₄. Figures 8.18 and 8.19 show the possible route of stabilizing TiO₂ nanoparticles on wool using BTCA and SHP. It was observed that the presence of TiO₂ (P-25) coating brought about self-cleaning, antifelting, and reduced photoyellowing for wool samples [71]. A photoinduced hydrophilicity was also observed on coated wool samples [88]. A similar study was carried out to stabilize the TiO₂/Ag nanoparticles synthesized through the photoreduction method on wool samples [90]. A significant improvement in the antimicrobial activity of samples against E. coli and S. aureus was observed after incorporating silver in the impregnating bath [90]. The application of this approach was tried for other types of nanoparticles such carbon nanotubes (CNTs) [91]. Conductive wool samples were produced through applying CNTs to fabrics in an ultrasonic bath in the presence of cross-linking agents [91].

Gashti et al. [84] used the cross-linking method to stabilize dimethyldichlorosilane modified silica nanoparticles on the surface of cotton to produce hydrophobic fabric. They used BTCA and SHP in their research as a cross-linking agent and catalyst, respectively. It was reported that the water contact angle on treated cotton samples was 132.4°. In another study, ZrO₂ nanoparticles were applied to wool samples by the cross-linking method, and some features such as self-cleaning, electromagnetic reflection, and fire retardant of samples were studied [82, 92]. The presence of nanoparticles on the surface of wool enhanced the fire-retarding property and electromagnetic reflection of wool samples [92]. The positive impact of CNTs, stabilized on cotton by BTCA, on thermal properties, flammability, and antimicrobial activity of cotton samples were demonstrated [93].

8.5 Plasma Surface Activation

In order to achieve an efficient durable coating of nanoparticles, some methods such as laser irradiation, electron beam, UV irradiation, ion beam, microwave irradiation, and plasma have been used to physically modify the structures of textile substrates. Among these techniques, plasma surface modification of textiles is mostly considered as an effective pretreatment approach in the nanocoating field. In general, plasma is considered as the fourth state of matter and is defined as an excited, ionized, and equally charged state of gas that consists of photons, electrons, atoms, molecules, and ions [94, 95]. Plasma by itself has some applications in the textile finishing field [96]. Some features such as enhanced mechanical characteristics, antistatic finish, hydrophilic treatment, hydrophobic finishing, improving the dye uptake, bleaching, and flame retardant have been reported [97]. There are several advantages for plasma processing such as being clean, dry technology, and no need to use solvent in comparison with common conventional treatment methods. Depending on the gas pressure, two methods of low-pressure plasma and atmosphere-pressure plasma can be categorized. The latter, in turn, is divided into three categories of corona discharge, dielectric barrier discharge, and atmospheric-pressure glow discharge [98]. Low-pressure plasma equipment usually uses more energy and less gas compared with atmosphere-pressure plasma. It is much easier to obtain a uniform treatment on textiles using low-pressure plasma equipment [98]. Basically, the impact of plasma processing on textiles depends on some factors such as the type of textiles, the type of used gas, the power and frequency of electrical supply, the temperature of the process, and the duration. The generated plasma has the capability to alter the composition of chemical groups on polymeric substrates by introducing free radicals that resulted from dissociating the chemical bonds [99]. One of the advantages of plasma method is modifying and optimizing the features of superficial layers of substrates while sustaining the intrinsic features of the bulk material [100]. Usually, plasma process impacts the depth of <100 nm of substrate surface, and it can modify the surface morphology as well as chemical composition of the material surface. The generated free radicals react with the gaseous compounds in the atmosphere leading to introducing new functional groups on the substrate. New functional groups on the surface will increase the adhesive strength of nanoparticles promoting the durability of nanocoating layer. Cold plasma is employed for surface modification of textiles. The plasma is named cold or low temperature because the overall temperature of plasma is at the ambient level. The generated electrons have very high temperature, but due to their low heat capacity, the plasma is not hot [95, 100]. Therefore, it can be used for surface modification of textiles. Depending on the employed gas, there are four major plasma processes on textiles: cleaning (ablation), activation, grafting, and deposition [95, 100]. In a cleaning process, an inert gas such as He, Ar, and N₂ plasmas will be used. In this case, the generated plasma can break apart the polymeric structure of contaminants such as oil adsorbed on the substrates and then completely get them removed in the vacuum condition [100, 101]. The ablation process can be done based on either physical sputtering or chemical etching depending on the type of employed gas [95, 101]. For surface activation purposes, some gases without any carbon such as oxygen and ammonia can be employed. The generated species of these gases can react with the outer layer of substrates and produce functional groups such as hydroxyl, carbonyl, peroxyl, carboxylic, amino, and amines on the surface [100, 101]. The activated surfaces have better interactions with the coating layers applied to their surface, hence more stable composites and coating structures. In grafting, an inert gas such as Ar reacts with the outer surface of polymeric substrate to introduce some free radicals on the surface of the material. Then by introducing some allyl alcohol to the system, the grafting of monomers to the active sites of substrate will be completed [98, 100, 101]. By using other types of gases such as methane or carbon tetrafluoride, the plasma process can be used for material deposition. This method is also called as plasma polymerization or plasma-enhanced chemical vapor deposition (PECVD) method [98]. Through this method, a very thin film of polymers deposits on the textile surface [98, 101].

Plasma processing has widely been used by researchers as a pretreatment to introduce active sites on different types of textiles such as polyester, cotton, wool, cotton/polyester, and wool–polyamide before coating with nanomaterials. This process increases interactions between the applied nanoparticles and substrate surface. It has been reported that through some modification processes such as radio frequency plasma and vacuum–UV light irradiation, some negatively charged groups such as —COO and —O—O can be introduced to the fabric surfaces increasing the tendency of positively charged nanoparticles for deposition on fabric surfaces (Figure 8.20) [102]. The TiO₂ nanoparticles can be anchored to modified fabrics through ionic interaction between the negatively charged groups and positively charged Ti⁴⁺ [103]. Despite the plasma processing, the UV light does not introduce any cationic and anionic groups on the textile surfaces. However, the exposure of textiles to UV with wavelength below 241 nm results in breakage of O═O bonds leading to producing new reactive sites on the fiber surface.

The application of different types of plasma has been reported in literature [104, 105]. Qi et al. [103] used the low-temperature plasma of oxygen gas to modify the surface properties of polyester fabrics prior to coating with TiO₂ colloid. The application of oxygen gas resulted in introducing negatively charged groups of COO^– and —O—O^– on the polyester surface. Bozzi et al. [102, 106] employed radio frequency plasma (RF-plasma), microwave plasma (MW-plasma), and vacuum–UV light irradiation for pretreatment of polyester, cotton, and wool–polyamide fabrics. Interactions between RF-plasma and the carbon of the substrate resulted in producing new functional groups on the surface such as C—O, C═O, —O—C═O, —COH, and —COOH. The pretreated samples showed higher hydrophilicity and were able to degrade red wine and coffee stains [106]. Yuranova et al. [107] used RF-plasma and vacuum–UV to activate the surface of polyester–polyamide fabric. They applied silver nanoparticles on the modified substrate and assessed the antimicrobial activity against E. coli bacteria. Tung et al. [108] employed a microwave-generated plasma afterglow (MWGPA) treatment to modify wool fiber surface prior to coat with TiO₂ nanoparticles. They investigated the impact of some parameters such as gas mixtures, gas flow, treatment distance, treatment duration, and power flow of the plasma treatment on wool by evaluating the photocatalytic activity of coated fabrics [108]. They used different mixtures of gases such as argon/oxygen, argon/hydrogen, and argon/oxygen/hydrogen to produce plasma. The plasma pretreatment significantly increased the uptake of TiO₂ nanoparticles on wool surfaces leading to 70% improvement in photocatalytic activity compared with untreated wool sample.

8.6 Polymer Nanocomposite Coatings

The application of nanoparticles to textiles mostly does not result in a robust coating layer to withstand multiple washing cycles. All treatment methods mentioned in previous sections aimed at increasing the stability of nanocoatings on fabrics to provide safe and eco-friendly products for end users. One of the most efficient methods of fabricating a durable nano-coating layer on textiles is applying the functional coatings in the form of polymer nanocomposites. In this method, nanoparticles will be embedded into the dispersion of polymers, which play a role as carriers in surface coating process. This method will significantly enhance the stability of coatings on substrates. Through selecting appropriate functional polymer matrices as medium and nanoparticles as fillers, some features such as desired wettability, ultraviolet (UV) resistance, antimicrobial, conductivity, and flame retarding can be imparted to textiles [109]. In a sense, the polymer nanocomposites containing nanoparticles can possess more functionalities compared with each individual component of composite systems. The ultimate functionalities of polymer nanocomposites depend largely on the type of polymer, type of nanoparticles, and their shape and size [110]. The nanoparticles intensely tend to agglomerate during the production of nanocomposites [110]. However, polymers can be effective in reducing the aggregations among the fillers [109]. The surface modification of inorganic nanoparticles with some polymer surfactants or other types of modifiers such as silane coupling agents has been suggested to induce a repulsion among the dispersed nanoparticles. Figure 8.21 demonstrates surface modification of an inorganic nanoparticle with 3-methacryloxypropyl trimethoxysilane molecules [110]. This will result in a better compatibility between nanoparticles and polymer dispersion.

The nanoparticles that exist in the composite systems can react with functional groups of textile substrates covalently resulting in good wash fastness of applied coatings. Various techniques have been reported to prepare the polymer nanocomposites such as sol–gel, in situ polymerization, and blending the polymer and fillers together. There are two ways to use polymer nanocomposites: incorporating them into the fiber polymer in melt spinning or developing functional coatings. Among coating methods, the most common methods are dip coating and blade coating followed by drying and curing. Using polymer nanocomposites as a coating can produce stable, thin, transparent, and multifunctional layers on textiles. Different types of micro- and nanosize materials such as metals, metal oxides (TiO₂, ZnO), graphene, carbon nanotubes (CNTs), and phase change materials (PCMs) can be used in preparing the polymer nanocomposites. In sections below, some of the reported applications of nanocomposite coatings on textiles will be highlighted.

8.6.2 Thermal Regulating Coatings

Heating the indoor living environment takes approximately around 42% of the overall consumed residential energy bills [121]. This underlines the necessity of introducing new inventory measures and products to mitigate the load of energy consumption in the residential heating systems [121]. Therefore, new insulation techniques for building materials should be introduced such as new products with high thermal resistance (R-values) and low emissivity [122]. One of the alternative methods of reducing the household heating can be using textiles that are capable of preventing the loss of body heat or in some cases regulating the body temperature. The main aim of producing textiles with personal thermal management (PTM) capability is preventing the loss of thermal radiative energy in the winter and facilitating its release in the summer [123]. These types of textiles should be wearable, stretchable, and mechanically robust and at the same time efficient in preventing the energy loss. Sections below focus on some of the methods that have been reported in the literature to fabricate textiles with PTM function.

8.6.2.1 Phase Change Materials (PCMs)

One of the materials that have been used in surface coating of textiles to produce thermoregulating smart textiles are phase change materials (PCMs). PCMs, which are also called latent heat storage materials, are capable of storing and releasing the heat by changing their physical state [124]. Different types of PCMs such as solid–solid, liquid–gas, and solid–gas have been reported, but the solid–liquid type is the most common version of PCMs. The use of solid–liquid PCMs has been hailed mostly due to their lower volume change during phase transition and at the same time higher heat storage capacity [125]. When the temperature of environment is higher than the melting point of PCMs, the solid material encapsulated in the thin polymeric shell starts absorbing the heat energy and therefore transforms to the liquid form. The liquid form of PCMs again changes to the solid form when the surrounding environment temperature falls to lower than its melting point [125]. Through the phase transformations, a substantial amount of heat can be stored and released [125]. In a sense, these materials can absorb heat in the warm condition, store it through transition of their physical state from solid to liquid, and then release the stored energy to the environment during the cooling process by changing their phase from liquid to solid. When the wearer’s body temperature increases, the PCMs absorb the extra heat, and when the wearer’s body temperature drops, they can release the absorbed heat (Figure 8.22). The insulation effect that PCMs can provide relies largely on temperature change and its pace, which takes place over the narrow temperature range [126].

There are two main types of PCMs including organic (paraffin and nonparaffin-based materials) and inorganic [127]. Paraffin waxes can be used as PCMs and have a changing phase temperature of 18–36°C [128]. Different types of paraffin with various numbers of carbon in their structure and characteristics such as melting temperature and crystallinity can be utilized [124]. Of these alkyl hydrocarbon paraffin PCMs, heptadecane, hexadecane, octadecane, nonadecane, and eicosane are noteworthy [128]. Fatty acids, alcohols, and glycolic acids are among the common non-paraffin materials, and hydrated inorganic salts are classified as inorganic PCMs [127, 129]. These materials are confined in a thin layer of polymer shell through microencapsulation (for particles between 1 and 1000 μm) and nanoencapsulation (for sizes <1 μm) processes [128]. It is worth mentioning that a higher efficiency can be obtained from nanocapsules compared to microcapsules due mostly to their faster heat transfer rate and smaller particle size [127]. There are several methods for incorporating the PCMs in textiles such as adding PCMs in coating formulation, spinning PCMs–fiber polymer mixture solution, cross-linking, and laminating pre-prepared PCM–polymer film to the substrate [130]. PCMs can be embedded into different smart textile coatings including various types of binders such as acrylic and polyurethane solution/foam before applying to textiles. There are some influential parameters that determine the final quality of coating and its efficiency. These factors are polymer binder type, the mass ratio between the binder and PCM, the type and mechanical stability of the PCM shell, affinity between the fabric and the binder, and curing conditions [131]. Efficiency of PCMs in producing thermoregulating sport products such as ski wear, hunting clothing, boots, gloves, and ear warmers has already been reported [128].

In a research carried out by Shin et al. [128], the thermoregulating polyester fabric was prepared. They synthesized melamine–formaldehyde microcapsules containing eicosane through an in situ polymerization. The polyurethane binder was used to apply the prepared PCMs to polyester fabrics through a conventional pad–dry–cure process [128]. They added 5–23% wt. of prepared microcapsules into the coating formulation and reported that the coated fabrics had the heat storage capacity of 0.91–4.44 J/g. However, the washing test results revealed that the synthesized microcapsules did not have good durability on fabrics, and only 40% of heat storage capacity could be retained after five cycles of laundering. With increasing amount of add-ons on fabrics, it was observed that some characteristics such as air and vapor permeability, flexibility, and shear properties of fabrics were adversely affected. However, the coating layer increased the tensile linearity, roughness, and moisture regain of polyester fabrics. Therefore, there should be a balance between the formulation of coating that is applied to fabrics and the expected properties of products considering their ultimate performance and application [132]. Sánchez et al. [133] synthesized PCM microcapsules of paraffin wax with polystyrene shell and applied to cotton fabrics using different types of commercial binders. Their results demonstrated that using 35 wt% microcapsules to binder ratio in the coating formulation provided a thermal storage capacity of 7.6 J/g. The performance of coated fabrics dropped to 3.6 J/g after undergoing a washing durability test [133].

Specific types of fabrics containing PCMs can also be employed as tensile structures in architecture and construction applications [134]. The thermal insulation capacity of architecture fabrics plays a very important role in preventing the structure’s interior space from being overheated by solar radiation. Some practical applications of these types of fabrics are the covers over sport halls, greenhouses, tennis arenas, stadiums, military shelters and tents, and airport ceilings and roofs, [134]. Therefore, the PCM embedded textiles are important due to their insulating properties. When the outside temperature rises and the construction is exposed to the solar radiation, PCMs that exist in the structure of fabrics absorb the latent heat energy by changing their phase; as a result, the surrounding temperature variation will be negligible [134]. For these types of fabrics, the silicon rubber binders are appropriate carriers for applying PCMs to the surface of fabrics [134]. The salt hydrate PCM can be mixed with silicone rubber binder and be applied to the fabric surface by a simple knife-over-roll coating approach. The smart coated fabrics with PCM-silicon rubber are able to reduce the heat flux into the interior space of buildings and alleviate the overheating of structures significantly. Regulating the heat flux in and out of constructions, the PCM-silicone rubber coatings on fiberglass fabrics can contribute to saving consumed energy for cooling and heating systems [134].

8.6.2.2 Nanowire Composite Coatings

Coating textiles with metal nanowires has been suggested as an effective approach to prevent the waste of body heat. The nanowires are right candidates for this purpose because of their high aspect ratio leading to high electrical conductivity and mechanical characteristics. The nanowire coatings on textiles can reflect around 40% of heat radiation generated by body inwards leading to keeping the wearer warm. This product is suitable to be used in winters when keeping the body heat is a crucial factor. Conversely, the coatings that are transparent to infrared radiation and opaque to the visible light can be a suitable option for summer thermal clothes [123]. The spaces between the metallic nanowires applied to textiles can be controlled to less than the wavelength of the infrared emitted from human body to prevent the loss of thermal energy. These textiles with personal thermal management capabilities will retain their flexibility, breathability, and wearability like a normal cloth [121].

In a paper published by Hsu et al. [121], different aspects of personal thermal management textiles have been thoroughly discussed. Among various types of nanowires, silver nanowires (AgNWs) have been found promising for personal thermal management applications in textiles. The advantages of AgNWs are their high conductivity and yield strength. What is more, applying an electricity source to the AgNWs can provide the function of Joule heating capability for textiles, and the wearer can even feel warmer by this method. CNT nanowires have also been used for this purpose, and it was reported that they had lower rate of IR reflection and higher emissivity compared to the products coated with AgNWs. The assessment of coatings demonstrated that the AgNW-coated textiles can provide a better insulation compared with CNT-coated samples [121]. Figure 8.23 shows the thermal images of fabrics coated with AgNWs and CNT as well as the Joule heating functionality of coated fabrics. It can be seen that the AgNW-coated samples are shown in dark blue implying their lower temperature. This indicates that the AgNWs can prevent the emittance of body IR radiation to the ambient environment, hence providing a lower emissivity and better personal thermal management functionality. At the same time, fabrics coated with CNT needed more voltage (12 V) to generate heat to reach to 38°C compared with AgNW-coated fabrics that needed just 0.9 V [121].

Stabilizing the nanowires on textiles is a challenging issue mostly due to their weak bonds with the substrate [121]. Among different reported methods for improving the adhesion of nanowires on substrates, the application of coatings to textiles in the form of nanocomposites has been found promising and resulted in a durable layer of nanomaterials on textiles. In a research study conducted by Yu et al. [135], the AgNWs were applied to cotton surface in the form of AgNW/polydopamine nanocomposite through dip-coating method. They reported fabrication of a flexible and washable cotton cloth, which was capable of reflecting the middle-to-far IR radiation emitted from the wearer’s body by 86%. Also, the coatings provided a rapid Joules heating function to the products. They first soaked the cotton fabric into the polydopamine solution and then immersed into AgNW dispersion for three times. Figure 8.24 shows the surface morphology of coated samples and IR reflectance spectra of samples coated with AgNW/polydopamine (ANDC) [135]. In another research, Guo et al. [136] prepared a textile-based product with two functionalities of energy harvesting and energy saving. They coated the nylon fabrics (FPAN) by multilayer coating composed of silver nanowires (AgNWs), polydimethylsiloxane (PDMS), and fluoroalkylsilanes (FAS) (Figure 8.25) [136]. The gap between the AgNWs was adjusted around 300 nm, which was much narrower than the wavelength of the IR radiation (9 μm) emitted from human body. PDMS played a role as a triboelectric material and protective layer on AgNWs preventing them to peel off. FAS, which had a low surface energy, was applied simply through a dip-coating method and bounded to the PDMS via silane groups and increased the output performance by enhancing the surface charge. The devised multilayer fabric could behave like a triboelectric material of a triboelectric nanogenerator (TENG) by saving the movement energy and using it for powering the LEDs.

8.6.3 Conductive Coatings

With the emerging application of a vast number of personal electronics, sensing, and health care devices, wearable electronics have attracted wide attention from both academia and industry [137]. In recent years, many new wearable products (such as smart watches, soft displays, wearable sensors, etc.) have been introduced to the consumer market. It has been forecasted that the wearable technology market will be worth more than US$50 billion by 2022. Among these wearable electronic devices, conductive textiles will be playing more and more important roles along with current and future developments [138].

Traditionally, conductive textiles are produced mainly for antistatic and electromagnetic shielding applications. Common methods of preparing conductive textiles include adding conductive fillers into the polymer solution before synthetic fiber spinning, spinning metal fibers into normal yarns, metal plating on yarns or fabrics, converting polymer filaments to carbon fibers, and dip-coating conductive materials onto fabrics. However, the conductive textiles resulted from these conventional fabrication processes and microsized conductive materials normally suffer the disadvantages of low electrical conductivity, poor structure stability, or severe performance deterioration under wearing and washing conditions.

Modern wearable electronics require all functional components with high electrical conductivity to reduce power consumption. For integrated textile-based devices, structural and electrical stabilities should be high enough to maintain a high performance level during practical wearing conditions and survive a certain number of laundry cycles. In some of the cases, stretchability is also important to corporate stretchable devices with human body movements. Traditional conductive textiles have not been able to meet the requirements for most of the wearable applications; therefore, the development of new materials and structures will be critical for the future of wearable electronics. In this regard, various nanomaterials that have been developed for other uses can find their application in conductive textiles.

8.6.3.1 Carbon-Based Conductive Coating

Carbon is an earth-rich and highly conductive material that can be fabricated into many formats and dimensions, such as buckyballs, carbon quantum dots, carbon nanotubes, carbon fibers/nanofibers, graphene, and various three-dimensional porous carbon structures [139]. Compared with other conductive nanomaterials, carbon nanomaterials have the advantages of lower cost and relatively simpler material preparation.

Hu et al. [140] used a simple dip-coating process to prepare highly conductive cotton fabrics using single-walled carbon nanotubes (SWNTs), as shown in Figure 8.26a. By immersing cotton fabrics into a well-dispersed aqueous SWNT solution (Figure 8.26b), a black-colored conductive fabric can be formed after drying (Figure 8.26c). Due to the large van der Waals forces between carbon nanotubes and textile fibers, strong hydrogen bonding formed between carboxyl groups on nanotube surface and hydroxyl groups of cotton. Also an ideal adhesion of flexible nanotubes to fiber surface (Figure 8.26d) was achieved, and the coated conductive fabric exhibited great mechanical properties. The carbon nanotubes withstood both tape and washing tests, without obvious degradation of electrical conductivity. Through multiple coating cycles, acid treatment, and mechanical pressing, electrical conductivity of the coated fabric reached as high as 125 S/cm, with a sheet resistance lower than 1 Ω/sq. Unusually, it has also been found that the fabric conductivity increased by stretching the coated fabric to 2.4 times the original length. This phenomenon is probably contributed by the enhanced physical contact between interconnected carbon nanotubes among stretching. Supercapacitor devices were fabricated using CNT coated cotton fabrics as both electrodes and current collectors. With a 0.24 mg/cm² CNT loading on the fabric, the device achieved a specific capacitance of 140 F/g at 20 μA/cm².

In wearable electronics, conductive textiles can work not only as electrodes but also as stretchable electrical conductors in some cases. For these applications, a minimum possible conductivity variation is desirable to maintain stable function of electronic devices. Using a superelastic fiber made of styrene–ethylene–butylene)–styrene block copolymer as the core, carbon nanotube sheets were wrapped on the stretched fiber to form a core-sheath conducting fiber [141]. A multilevel buckling effect of the carbon nanotube sheet after releasing the tensile strain was observed on the fiber surface. This stretchable conductive fiber maintained efficient conductive pathway to enable a less than 5% resistance change with a 1000% tensile deformation, which makes it an ideal candidate for elastic electronics and artificial muscles.

8.6.3.2 Metal-Based Conductive Coating

Compared with carbon material, metals have much higher electrical conductivity; therefore, they are more effective to form conductive layers. Many metals, such as gold, silver, and platinum, have been made to nanowires for the application of transparent conductive coating, and mainly benefited from mesh like nanowire network and partial coverage on the substrates. Using silver nanowires through a dip-coating process, conductive nylon, cotton, and polyester threads were prepared (Figure 8.27) [142]. Due to the hydrophilic feature of cotton surface, silver nanowires from the aqueous coating solution had good interaction with cotton thread, while the other two hydrophobic fibers required chemical treatment to achieve ideal nanowire coverage. By increasing the coating cycle number to adjust silver nanowire loading, the unit resistance of 0.8 Ω/cm was achieved on nylon thread.

It has been found that a thermal annealing post-treatment at 150°C would effectively reduce electrical conductivity of the coated threads, through removing organic surfactant existing in the coating solution and creating interwire junctions with thermal fusing of the nanowires. The excellent mechanical stability of the silver nanowire coated threads was demonstrated by a bending test. In this test, both nanowire-coated nylon threads and commercial silver-coated conductive yarns were repeatedly bent on a metal rod with a radius of 6 mm. The results revealed that the resistance of the commercial conductive yarn increased from 2.8 to 12.2 Ω/cm after 200 bending cycles; at the same time, the nanowire-coated thread had only a slight resistance increase to 3.2 Ω/cm, from the same starting resistance of 2.8 Ω/cm. It should be pointed out that due to significantly lower coverage on thread surface than conventional particle or dense film-based metal coating, metal nanowires can be more efficient to achieve flexible and low-cost conductive coating on textiles.

Through using a novel template method, conductive coating has been applied on single fibers to form parallel electrode pairs for fabricating dual functional electronic devices [143]. As shown in Figure 8.28a, four parallel stripes of sticky tape were used to closely wrap a single fiber. After removing two stripes from the fiber surface, gold sputter coating was applied to introduce a conductive layer onto the fiber. By unwrapping the remaining two stripes, a parallel electrode pair was formed on the fiber. The single fiber device was completed after depositing a layer of poly(3,4-ethylenedioxythiophene) (PEDOT) (Figure 8.28b) on the gold electrodes through electrochemical polymerization and finally covering the fiber with a layer of gel electrolyte. The as-prepared single fiber device showed electrochromic effect by applying a low voltage (±0.6 V) across the electrodes. Clear color change of the PEDOT-covered gold layer to dark blue can be found in Figures 8.28c and d after switching the applied voltage from 0 to 0.6 V, while the color change happened on the neighboring electrode once the voltage changed to –0.6 V, with a fast response time shorter than 5 s. In addition to stable electrochromic behavior, the single fiber device can also function as a supercapacitor with a specific capacitance of 20.3 F/g. It has also been reported that both electrochromic and supercapacitor functions can work simultaneously without any interruption.

8.7 Conclusion and Future Prospect

This chapter tried to shed light on some aspects of nanocoatings that are commonly employed to fabricate smart textiles. Different methods of coating and associated mechanisms in introducing novel functionalities to textiles were discussed. The focused methods include sol–gel, cross-linking, plasma, and nanocomposite coatings, which are among the most used methods reported in literature. The mechanisms of different functionalities of smart fabrics including self-cleaning, UV protection, hydrophobicity, antimicrobial activity, flame retardancy, personal thermal management systems, and conductive coatings were thoroughly discussed. There are numerous publications in each of these fields, all of which could not be covered in one book chapter. There are significant achievements on the concept of smart textiles, but yet again researchers are exploring new methods to promote the efficiency and durability of novel functionalities and nanocoatings. Also, investigating the safety aspects of nanomaterials on human health and their potential impacts on environment is among the important fields that scientists should explore in their future work.

Acknowledgements

We wish to acknowledge support of an Alfred Deakin Postdoctoral Fellowship for the first author. This work was also supported (partially) by the Australian Government through the Australian Research Council’s Industrial Transformation Research Hub scheme (IH140100018) and Centre of Excellence scheme (CE140100012).

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