Chapter 14: Improving superhydrophobic coatings for textiles through chemical modifications – Functional Textiles for Improved Performance, Protection and Health

14

Improving superhydrophobic coatings for textiles through chemical modifications

C.H. Xue,     University of California, Davis, USA

Abstract:

This chapter provides an overview of superhydrophobic textiles with detailed illustrations of key principles of superhydrophobic textiles, chemical modification coating on fibers, and hydrophobization for improving superhydrophobicity and surface robustness of textiles. Applications of superhydrophobic textiles in practical and potential fields are discussed. Future trends of superhydrophobic textiles are also presented.

Key words

superhydrophobic surface

coating

textile

chemical modification

surface roughness

14.1 Introduction

Superhydrophobic textiles recently attracted a lot of attention due to their simple fabrication, easy availability of raw materials, large-scale production of superhydrophobic surfaces (Xue et al., 2008b) obtained possibly by industrialized techniques (Wang et al., 2007; Xue et al., 2009), and the potential applications of such surfaces in a variety of areas. They literally repel water, making water hardly stick to the surface and bounce off after an impact, and thereby such treatment can remove dust and surface contaminants very effectively, showing a self-cleaning effect like that of the lotus leaf (Barthlott and Neinhuis, 1997). Superhydrophobicity of textiles can not only provide protection from a wide variety of liquids but also prolong the lifetime of the fabrics due to the prevention of water wetting that causes fiber degradation (Xue et al., 2008a). Superhydrophobic textiles with superoleophobic properties are desirable for oil repellency in industrial or household environments (Hoefnagels et al., 2007; Leng et al., 2009).

In the following context, I will discuss the key principles of superhydrophobic textiles, various ways to produce superhydrophobic coatings on textiles, and the role of chemical modifications in improving superhydrophobicity and robustness of textile surfaces. Then, commercially available superhydrophobic textile products and their corresponding applications are reviewed. Future trends and suggestions in superhydrophobic surfaces on textiles are also presented.

14.2 Key principles of superhydrophobic textiles

A superhydrophobic surface is defined as having a water contact angle greater than 150° and a roll-off angle lower than 5° (Michielsen and Lee, 2007; Wu and Shi, 2006). There are now abundant evidence that combinations of topography and hydrophobic chemical structure produce surfaces with high contact angles with very low roll-off angles (Roach et al., 2008; Zhang et al., 2008b).

Theoretical models for liquid drops on topologically rough and chemically non-homogeneous surfaces have been established by different researchers (Cassie and Baxter, 1944; Wenzel, 1936; 1949) and these models have recently been used to rationalize the lotus effect (Cheng et al., 2006; Crick and Parkin, 2009; Gao and McCarthy, 2006; Hou and Wang, 2009; Marmur, 2003; Qu et al., 2008; Roach et al., 2008). Water can interact with rough surfaces by contacting only the peaks of the roughened surface, wetting the peaks while leaving the valleys with air trapped, or by wetting the entire surface, both the peaks and valleys, as shown in Fig. 14.1(b–d). When water interacts with the surface by contacting only the peaks of the roughened surface, as shown in Fig. 14.1(c), the apparent contact angle, θCB, is given by the Cassie–Baxter equation for wetting on composite surfaces made of the solid and air (Cassie and Baxter, 1944),

14.1 Schematic of the behavior of a droplet on a perfectly flat surface (a), and rough surfaces of wetting states: (b) Wenzel, (c) Cassie–Baxter, and (d) combined models.

[14.1]

where fs is the fraction of projected planar area of the drop in contact with the solid. In the limit of fs → 0, the macroscopic contact angle θCB approaches 180°, leading to ideal superhydrophobic behavior. When water wets both peaks and valleys of a rough surface with uniform chemical composition, as shown in Fig. 14.1(b), the apparent contact angle of the drop, θw, is given by Wenzel’s formula (Wenzel, 1936, 1949),

[14.2]

where γ is the ratio of the actual area of liquid–solid contact to the projected area on the horizontal plane of the rough surface, and θ is the equilibrium contact angle of the liquid drop on the corresponding flat surface. Wenzel’s equation predicts that roughness will amplify the intrinsic wetting behavior of a surface as determined by its surface chemistry. If the contact angle on the smooth surface is larger than 90°, roughness will further increase the observed contact angle. If it is less than 90°, roughness will reduce the observed contact angle. Equations [15.1] and [15.2] represent two possible equilibrium states of liquid drops on rough surfaces. Drops in the Cassie–Baxter state can easily roll because of low resistance from the air pockets. In contrast, drops in the Wenzel state can become ‘sticky’ from contacting the rough surface (Cheng et al., 2006; Roach et al., 2008) in that the interfacial area between solid and liquid is much larger than that on a flat surface, as shown schematically in Fig. 14.1(a) and (b). It is, however, possible to generate ‘sticky’ surfaces in the regime of Cassie and Baxter, usually by using a surface with high intrinsic contact angle hysteresis; the combined Cassie-Baxter/Wenzel state can also have large hysteresis as the interfacial area between solid and liquid can be at least as large as on a flat surface, as shown schematically in Fig. 14.1(d). For the surfaces with self-cleaning effect, it is generally believed that the two-level roughness, in combination with low surface energy, amplifies the apparent contact angle, lowering the hysteresis, and is responsible for the rolling behavior of the drops.

Most textiles are textured woven or knit structures consisting of nature or synthetic fibers with organic, elastic and usually micro-sized features, thus exhibiting some distinction from other solid surfaces in terms of creation of superhydrophobic surfaces. According to the main requirements for a superhydrophobic surface, there are two major methods of producing superhydrophobic textiles. The first method is to construct textiles with a big roughness factor through physical methods. The materials consisting of the substrate can be initially hydrophobic or created afterwards. The second method is to enhance the roughness of an intrinsic textured textile through chemical modifications, and then hydrophobize the roughness-enhanced textile surface by changing the surface chemistry or applying a hydrophobic material to lower the surface energy, or to generate a layer of nanoscopic fibrous or particle coating of materials with low surface energy on the microscopic fibrous textiles. In addition, nanoscaled coating of materials with low surface energy through chemical modification can also be done to impart roughness and hydrophobicity onto fibers simultaneously, making textiles superhydrophobic. Superhydrophobic rough coatings for textiles through chemical modifications will be discussed in the following section.

14.3 Chemical modifications for fabricating rough surfaces on textiles

Rough surfaces can be constructed on textiles by attachment of micro or nanoparticles on fibers through sol-gel processing, hydrothermal synthesis, complex coating, layer-by-layer methods, and others. The particles used may be inorganic, organic or organic/inorganic hybrid. It is widely known that inorganic substances, such as ZnO, SiO2, TiO2 and the like, have lower affinity for organic substances, for example textile fibers. Therefore, for application of superhydrophobic textiles, it is very important to compatibilize the particles with the textile substrates through chemical modification to enhance the stability of the superhydrophobic coating on textiles as well as the robustness of superhydrophobicity during service.

14.3.1 Sol-gel processing

Sol-gel processing is a well recognized method for preparing gels and nanoparticles. Researches based on the sol-gel method to develop applications have resulted in technologies to obtain multilayered films, porous pillars, thin films, nanocrystalline materials, nanopowders, rough coatings and clusters for uses in paints, antiseptics, nanocomposites, drugs, biomedical implants and military components (Bae et al., 2009; Daoud et al., 2004; Xue et al., 2008a). The surface roughness of the textiles fabricated via the sol-gel method can be easily tuned through changing the protocol of the method and the composition of the reaction mixture. Daoud (2004) prepared transparent and durable superhydrophobic surfaces on knit and woven cotton substrates of various dimensions at low temperatures using a modified silica sol. The silica sol is produced via cohydrolysis and polycondensation of a hexadecyltrimethoxysilane, tetraethoxyorthosilcate and 3-glycidoxypropyltrimethoxysilane mixture. Leaching behavior of the hydrophobic properties by comparing the contact angle and water gain values before and after washing showed that after ten cycles of washing, the hydrophobic properties maintained well in the later wash cycles. The maintenance of the hydrophobic properties of the coating after repeated washing was attributed to the linking ability of 3-glycidoxypropyltrimethoxysilane that promoted a high level of adhesion to the cotton substrates. Hoefnagels et al. (2007) reported the fabrication of biomimetic superhydrophobic cotton textiles by either one-step or two-step reactions to generate in situ silica particles with amine groups on their surfaces, which were covalently bonded to the cotton fibers; the amine groups were then utilized to hydrophobize the surface via the reaction with mono-epoxy-functionalized polydimethylsiloxane. When a perfluoralkyl silane was used for the surface modification, oleophobic textiles were obtained. Xue et al. (2008a) prepared superhydrophobic cotton fabrics by sol-gel coating of TiO2 and surface hydrophobization. It was found that the superhydrophobicity of the obtained fabrics relied on the roughness caused by the sol-gel coating rather than the concentration of TiO2. Yu et al. (2007) prepared a silica sol with appropriate particle size via alkaline hydrolysis of tetraethoxysilane in a mixture of ethanol and water, and synthesized a perfluorooctylated quaternary ammonium silane coupling agent. The silica sol and perfluorooctylated quaternary ammonium silane coupling agent were applied to cotton fabrics by conventional pad-dry-cure process. The fabrics treated with both silica sol and perfluorooctylated quaternary ammonium silane coupling agent showed high hydrophobicity and oleophobicity. Li et al. (2008) used water glass, a cheap and common industry product, as the precursor to prepare silica sol under acid catalyzed hydrolysis and condensation. After dip-coating the silica sol onto cotton surfaces, the surfaces were then modified with hexadecyltrimethoxysilane to gain a thin layer through a self-assembled process. As a result, the cotton fabrics treated with such processes showed superhydrophobicity. The superhydrophobic surfaces were made at low temperatures without expensive equipment and tedious processes.

Bae et al. (2009) have imparted superhydrophobicity to the hydrophilic cotton fabric by a combined treatment of silica nanoparticles and a costeffective commercial water-repellent agent. For the cotton fabrics treated with silica nanoparticles of average diameter 378 nm, water contact angles above 130° could be easily obtained even with a very low water-repellent agent concentration of 0.1 wt% at which no hydrophobicity was exhibited for the neat cotton fabric treated with the water-repellent agent only.

Wang et al. (2008) have produced stable superhydrophobic surfaces with water contact angles over 170° and sliding angles below 7° by simply coating a particulate silica sol solution of co-hydrolyzed TEOS/fluorinated alkyl silane with NH3 H2O on various substrates, including textile fabrics (e.g. polyester, wool and cotton) and electrospun nanofiber mats.

It should be mentioned that the above-mentioned superhydrophobic coatings through sol-gel method were mostly conducted on cotton textiles. It is well recognized that there are a lot of reactive hydroxyl groups on cotton fibers. Thus covalent bonds can be formed between coating and cotton substrate by condensation in sol-gel process and/or dehydration in curing process, making sol-gel coating robust on textiles.

14.3.2 Hydrothermal synthesis

Hydrothermal synthesis is a well-known method for the fabrication of nano/microscale materials. Researches on growing inorganic materials with all kinds of structure on organic materials to fabricate superhydrophobic surfaces on textiles were conducted. Xu and Cai (2008) employed a hydrothermal method on cotton fabrics to create superhydrophobic surfaces. First, ZnO nanocrystals were prepared and applied on the cotton fibers. Subsequently, oriented ZnO nanorod arrays were fabricated on the fibers to form nanoscale roughness. Finally, the as-obtained fabrics were modified by n-dodecyltrimethoxysilane to obtain superhydrophobic surfaces. Their method showed a high degree of experimental reproducibility, and the fabrication processes were applied using inexpensive laboratory equipment usually employed for conventional textile processing. The researchers proposed that two factors contributed to the enhancement of the attachment between the cotton fiber substrate and the ZnO nanorods. N-dodecyltrimethoxysilane was infiltrated above and below the ZnO seed layer as binding agents when the cotton fabrics were immersed into n-dodecyltrimethoxysilane ethanol solution. The as-produced Si–OH groups in hydrolyzed n-dodecyltrimethoxysilane could react with both the ZnO seeds and the cotton fiber substrates since there are abundant hydroxyl groups on their surfaces. As a result, the ZnO seeds and the fiber substrate were tightly bound with each other. Furthermore, there are three hydroxyl groups in one hydrolyzed n-dodecyltrimethoxysilane molecule and hydrolyzed n-dodecyltrimethoxysilane could easily form cross-linked chains, thus the nanorods were firmly bundled and fixed on the ZnO seeding layer. The two layers created in this process successfully improved the fastness and stability of the ZnO nanorods on the cotton substrate.

14.3.3 Complex particle coating

Well-defined coating of nano/micro particles on fibers is a promising way to make rough coatings on textile. In order to make the coating fast and the superhydrophobicity function durable, particles and/or textile substrates are usually modified by introduction of functional groups, such as carboxyl, amino, epoxy, hydroxyl, and so forth. By doing so, the particles and the textile fibers can compatibilize with each other by forming covalent bonds. In complex coating, particles with different functional groups can attach to each other by reaction, forming firm coating. And the remaining functional groups on the coating surface facilitate further hydrophobization with low-surface-energy materials.

Ramaratnam et al. (2007) prepared ultrahydrophobic textile surface via decorating fibers with a monolayer of reactive nanoparticles and non-fluorinated polymer. In their first step of the surface modification, silica particles covered with an ultrathin reactive layer of poly(glycidyl methacrylate), an epoxy containing polymer, were deposited on the fiber surface. The silica particles covered with epoxy functional groups are capable of reacting with the fiber surface containing complementary (e.g. carboxy, hydroxy) functionality and with hydrophobic polymers possessing the functional groups, leading to affinity to the epoxy modified surface. During the second step, a hydrophobic polymer was grafted to the surface of the fibers and nanoparticles, and an ultrathin rough hydrophobic layer chemically anchored to the fiber boundary was generated.

We prepared superhydrophobic surfaces on cotton textiles by a complex coating of amino- and epoxy-functionalized silica nanoparticles on epoxy-functionalized cotton textiles to generate a dual-size surface roughness, followed by hydrophobization with stearic acid, 1H,1H,2H,2H-perfluorodecyltrichlorosilane, or their combination (Xue et al., 2009), as shown in Fig. 14.2. The epoxy-functionalization of the cotton enhances the interaction between the fiber and the silica coating. The incorporation of the functionalized SiO2 particles not only generates a firm dual-size rough surface but also facilitates the further hydrophobization of the surface. The achieved coating surfaces are robust and the superhydrophobicity of the cotton textiles is long lasting.

14.2 Schematic illustration of procedure for preparation of superhydrophobic surfaces on cotton substrate roughened by functionalized silica nanoparticles. Adapted from Xue et al. (2009).

Leng et al. (2009) introduced raspberry-like, dual-size structures onto woven cotton fibers, leading to a triple-size surface structure. Relatively large silica particles were generated 3n situ and covalently bonded to the cotton fibers. After treatment with 3-aminopropyl-triethoxysiloxane and hydrochloric acid, the surface charge was turned positive due to the protonation of amine groups. Negatively charged silica nanoparticles were then electrostatically adsorbed onto the fiber surface. The obtained roughened structure was stabilized by SiCl4 cross-linking, followed by surface modification with a perfluoroalkyl silane. The modified textiles were completely non-wettable by both water and hexadecane, which both showed high contact angles and low roll-off angles.

14.4 Hydrophobization for lowering the surface energy of roughened textiles

After application of a roughening coating, most textiles cannot reach superhydrophobic state. It is necessary to lower the surface energy of the rough coated textiles. Until now, various low-surface-energy coatings have been developed to modify organic and inorganic rough surfaces, leading to fabrication of superhydrophobic surfaces, many of which are suitable for superhydrophobic coating on textiles. The commonly used reactive molecules for low-surface-energy modifications are long alkyl chain thiols, alkyl or fluorinated organic silanes, perfluorinated alkyl agents, long alkyl chain fatty acids, polymer, and polydimethylsiloxane, and so forth as shown of some examples in Fig. 14.3, or their combinations. Among others, commercially available products such as water-repellent agent and poly(acrylate-g-siloxane) textile finishing agent can also be used.

14.3 Examples of surface reactive molecules for low-surface-energy modifications: (a) long alkyl chain with R1 groups, (b) long alkyl chain organic silanes with R2 groups, and (c) long alkyl chain fluorinated silanes with R3 groups, in which R1 could be –SH, –OH, –COOH, –NH2, and so forth; R2 and R3 could be –Cl, –OCH3, –OCH2CH3; the length of these chains might be variable from C8 to C18.

14.4.1 Hydrophobization with fluorinated molecules

In hydrophobization, the most commonly used chemicals were fluoroalkylsilanes due to their extremely low-surface-free energy and easy reaction of the silane groups with the hydroxyl groups on coatings. Large amount of superhydrophobic surfaces were based on the surface modification with fluoroalkylsines. Examples in this chapter focus mainly on superhydrophobic textiles. Hoefnagels (2007) in situ introduced silica particles to cotton fibers to generate a dual-size surface roughness, followed by introduction of a perfluoroalkyl chain of 1H,1H,2H,2H-perfluorodecyl trichlorosilane to the silica particle surface. The substrate becomes highly oleophobic, as demonstrated by a static contact angle of 140° and a roll-off angle of 24° for a 15 μL sunflower oil droplet. By introducing a nano/microparticle dual-size structure to the woven cotton fiber network followed by surface perfluorination with 1H,1H,2H,2H-perfluorodecyl trichlorosilane, the modified textiles were completely non-wettable by both water and hexadecane (Leng et al., 2009). It should be noted that many superhydrophobic surfaces are poor in oil repellency. Up until now most superoleophobic surfaces, including superoleophobic textiles, are based on perfluorinated alkyl hydrophobization. Choi et al. (2009) developed a simple dip-coating process for delivering a conformal coating of fluorodecyl polyhedral oligomeric silsesquioxane molecules on commercial fabrics that exhibit reversible, deformation-dependent, tunable wettability, including the capacity to switch their surface wetting properties (between superrepellent and superwetting) against a wide range of polar and non-polar liquids.

14.4.2 Hydrophobization with alkyl molecules

Alkanethiol is a kind of active surfactant bearing a hydrophobic alkyl chain and a thiol group as a surface anchor. Zhang et al. (2004) employed n-dodecanethiol to modify the rough surface of gold or silver aggregates, fabricated by electrochemical deposition to prepare superhydrophobic surfaces. Their experiment shows that the contact angles can reach around 156 °C with a droplet of 4 μL after modification by SAMs of n-dodecanethiol on the silver aggregates (Shi et al., 2007). Wang et al. (2007) modified normal commercial cloths with suitable gold micro/nanostructures. Surface hydrophobization was then carried out by immersing the as-prepared product in an ethanol solution of n-dodecanethiol, forming superhydrophobic cloths with the highest water contact angle of close to 180°.

Fluorochemicals have extremely low-surface-free energy. However, such compounds are high cost and have a potential risk for human health and for the environment. Hence, the development of non-fluorinated modifying agents or lowering the concentration of fluorochemicals used is very important for the fabrication of environmentally friendly coatings on textile substrates. In order to lower the risk and cost of fluorochemicals, stearic acid was combined with 1H,1H,2H,2H-perfluorodecyl trichlorosilane to lower the content of fluorochemical in hydrophobization of the TiO2 particle or silica particle roughened cotton textiles, which is useful for industrial application of superhydrophobic textiles (Xue et al., 2008a, 2008b, 2009).

Li et al. (2008) modified the silica coating with hexadecyltrimethoxysilane on cotton substrate prepared by sol-gel method using water glass. The achieved product presented superhydrophobicity with a water contact angle higher than 151°. Xu and Cai (2008) modified the ZnO nanorod array film on cotton fabrics with a layer of n-dodecyltrimethoxysilane. The modified cotton fabrics exhibited superhydrophobicity with a contact angle of 161° for 8 μL water droplet and a roll-off angle of 9° for 40 μL water droplet. Most importantly, the hydrolyzed n-dodecyltrimethoxysilane can react with the ZnO seeds, the cotton fibers, and the nanorods. Therefore, the fastness and stability of the ZnO nanorods to the cotton substrate were largely improved.

14.4.3 Hydrophobization with non-fluorinated polymer

Ramaratnam et al. (2007) deposited a thin film coating of non-fluorinated hydrophobic polymer, which contains 29 wt% of styrene and 1.4 wt% of reactive maleic anhydride groups, on the poly(ethylene terephthalate) fabric covered with the epoxidized silica nanoparticle, leading to the generation of an ultrahydrophobic textile surface. The coating was permanently anchored to the fiber boundary due to the chemical attachment of the nanoparticles and polymers to the surface.

14.4.4 Hydrophobization with silicon compounds

Hoefnagels (2007) turned normally hydrophilic cotton superhydrophobic by in situ introducing silica particles to cotton fibers to generate a dual-size surface roughness, followed by hydrophobization with polydimethylsiloxane. The obtained superhydrophobic textile exhibits a static water contact angle of 155° for a 10 μL droplet. The roll-off angle of water droplets depends on the droplet volume, ranging from 7° for a droplet of 50 μL to 20° for a 7 μL droplet. Li et al. (2007) prepared superhydrophobic cellulose-based materials coupled with transparent, stable and nanoscale polymethylsiloxane coating by a simple process via chemical vapor deposition, followed by hydrolyzation and polymerization. In this research, polymethylsiloxane plays the role as not only hydrophobization but also nano-scaled silicone coating.

14.5 Nanoscaled coating of materials with low surface energy

Researches on nanoscaled coating of materials with low surface energy on fibers were also conducted. In this way, roughness and hydrophobicity can be imparted simultaneously onto fibers, making textiles superhydrophobic. Li et al. (2007) transformed hydrophilic cellulose into superhydrophobic cellulose by a process via chemical vapor deposition, followed by hydrolyzation and polymerization. First, a sheet of cotton fabric was cleaned by ultrasonic washing in ethanol and water, respectively, and then dried. Second, the cotton fabric was placed in a sealed chamber for a set time, into which a saturated atmosphere of trichloromethylsilane was introduced. Next, the cotton fabric was withdrawn from the chamber and immersed into an aqueous solution of pyridine at room temperature to hydrolyze the remaining Si–Cl bonds. The cotton fabric was washed with water carefully to remove the excess reagents. Finally, the cotton fabric was treated in an oven at 150 °C for 10 min. Subsequent polymerization of Si–OH results in a nanoscaled silicone coating tightly attached to the surface.

Zimmermann et al. (2008) prepared superhydrophobic textile fabrics by a one-step gas phase coating procedure by which a layer of polymethylsilsesquioxane nanofilaments was grown onto the individual textile fibers. A total of 11 textile fabrics made from natural and manmade fibers were successfully coated and their superhydrophobic properties evaluated by the water shedding angle technique. A thorough investigation of the commercially relevant poly(ethylene terephthalate) fabric revealed an unparalleled longterm water resistance and stability of the superhydrophobic effect. Because of the special surface geometry generated by the nanoscopic, fibrous coating on the microscopic, fibrous textiles, the coated fabric remains completely dry even after two months of full immersion in water and stays superhydrophobic even after continuous rubbing with a skin simulating friction partner under significant load. Furthermore, important textile parameters such as tensile strength, color and haptics are unaffected by the silicone nanofilament coating.

Zhang et al. (2008a) described a simple and economical method of obtaining a superhydrophobic surface on wool textile by a comb-like polymer comprising acrylate and organic siloxane. The combination of acrylate and organic siloxane could exhibit some unique characteristics: first, the acrylate polymer chains could contribute to the increase of the cohesiveness and film forming properties; second, the long Si–O–Si chains, characterized by their low surface energy, could be utilized to enhance the water repellency; third, the Si–O–Si chains could immigrate towards the surface of the outer layers, resulting in forming a surface with roughness in nanosize scale. To achieve this goal, the poly(acrylate-g-siloxane) was prepared by emulsion copolymerization of acrylate with silicone oligomers containing a double bond. After being treated with the resulting emulsion, the wool textile could exhibit excellent superhydrophobicity.

Liu et al. (2007) fabricated artificial lotus-leaf structures on cotton substrates via the controlled assembly of carbon nanotubes onto the surface of cotton substrates. Both pristine carbon nanotubes and surface modified carbon nanotubes with poly(butylacrylate) shells were used as building blocks to form artificial structures on cotton similar to a lotus-leaf surface at the nanoscale. Cotton fabrics have been endowed with superhydrophobic properties.

14.6 Applications

The researches on superhydrophobic textile surfaces are driven by various functional applications. Superhydrophobic coatings on textiles not only can improve the performance of conventional textile materials by surface modification, but also bring about new functions which are not available for the textiles themselves.

14.6.1 Water repellence

In most studies, the waterproofing of textiles is considered to be among the primary potential applications for the superhydrophobic effect. Textiles with a superhydrophobic coating could find applications as water-resistant apparel and be useful for any kind of application where textile surfaces are exposed to the environment. An advantage of superhydrophobic textiles is that the fibrous structure can be maintained to keep the substrate breathable, which is preferable to the traditional waterproof textiles coated with rubbers, plastics, sealing agents or conventional wet-chemical finish using fluorocarbons. Additional benefits of the superhydrophobic effect on textiles could include a plastron layer (Zimmermann et al., 2008). This thin layer of air forms on many natural superhydrophobic surfaces upon immersion in water. On the one hand, the ability to support a plastron layer would prevent wetting of the textile even upon full immersion in water; on the other hand, it would significantly reduce the frictional drag in water. And it is convenient for users to get their mobile marquees, awnings, umbrellas, and so forth with water shedding effect easily stored even after a shower.

14.6.2 Self-cleaning

After some time textile surfaces in a natural environment usually get contaminated. Cleaning them requires extensive effort; additionally, often surfactants are applied with negative effects on the environment. The creation of textiles that can clean themselves or have anti-contamination properties has long been a dream for human beings. These textiles are usually called ‘self-cleaning clothes’ and the research related to the creation of self-cleaning clothes has been a hot topic for several decades (Liu et al., 2007). In fact, the technology for creating self-cleaning textiles has developed rapidly in recent years. Currently, there are two main techniques used in the production of self-cleaning textiles, which are similar to that of self-cleaning coating (Parkin and Palgrave, 2005). One is the production of surfaces that can break down, decompose or even ‘digest’ dirt, and another is the production of surfaces with repellent properties. The common route to fabricate both of these two types of self-cleaning textiles is to treat the target textile substrates with self-cleaning coatings. Self-cleaning textiles are divided into two categories: superhydrophobic textiles and hydrophilic textiles with photocatalytic properties for the destruction of organic dirt.

Daoud and Xin (2004) reported the surface treatment of cotton textiles with titania nanoparticles. After using a simple dipping-drying procedure, a thin film of titania nanoparticles was formed on the cotton fiber surface. Owing to the high photocatalytic activity of titania nanoparticles in breaking down dirt molecules, pollutants and micro-organisms, these treated cotton fabrics were expected to be ideal materials for self-cleaning fabrics. This method has been employed by many researchers to fabricate self-cleaning cotton (Bozzi et al., 2005a; Yuranova et al., 2006), polyester (Bozzi et al., 2005b) and wool (Daoud et al., 2008; Tung and Daoud, 2009) textiles.

However, the high photocatalytic activity of the titania nanoparticles also presents several potential problems that must be taken into consideration before these types of self-cleaning textiles are made into commercial products. The first problem with this approach is that photocatalytic coatings generate free radicals under UV irradiation (Linsebigler et al., 1995), and these free radicals might damage human skin, accelerating the ageing of skin and even causing skin cancers (Liu et al., 2007). The other problem with this approach is that these free radicals would probably damage the textile fibers, causing the degradation and decomposition of textiles. As an alternative, a safer strategy is the utilization of repellent coatings to create self-cleaning textiles.

Superhydrophobic textiles with self-cleaning property are prepared mostly based on the ‘lotus effect’ of plants. As shown in Fig. 14.4. when a droplet of water rolls off the surface of superhydrophobic textiles, it takes away the dust on the surfaces, while on normal hydrophobic textiles, the dust remains on the surfaces.

14.4 Water droplets rolling through surfaces with dust on: (a) normal hydrophobic textiles and (b) superhydrophobic textiles with selfcleaning properties.

Superhydrophobic textile surfaces with self-cleaning effects have been fabricated by mimicking the behavior of the lotus-leaf and other desert plants, which have rough surfaces combined with hydrophobic surface chemistry. Several approaches described in previous sections of this chapter have been industrialized and transformed into some proprietary technologies. Products with self-cleaning property such as shirts, blouses, skirts and trousers that shrug off ketchup, mustard, red wine and coffee (Forbes, 2008) have already been produced.

With Mincor TX TT, BASF for the first time prepared textiles with a genuine self-cleaning effect based on nanostructured surfaces like its model in nature (Anon, 2007. Steigelmann, 2007). This finishing material endows technical textiles for awnings, sunshades, sails and tents with the same self-cleaning effect as the lotus plant. What on the surface of the plant leaves are tiny papillae, on treated textiles are innumerable particles with a diameter of under 100 nm embedded in a carrier matrix. Whether in nature or technology, the effect is the same: these tiny particles keep water droplets and particles of dirt at bay. The dirt particles are carried along by the water droplets and are washed away without the need for detergents or scrubbing. In 2006, polyester awning fabrics finished with Mincor TX TT were very successful in achieving the transition from the laboratory to practical applications, and fabrics for sunshades and sails treated with Mincor TX TT are also promising. Applications of Mincor to washable fabrics, such as cotton, to produce dirt-repellent clothing are also welcome.

Nano-Care (Forbes, 2008) is a finish applied to fabrics developed by inventor and entrepreneur David Soane, now made by his company NanoTex. The fabrics made of Nano-Care treated fibers or threads repel molecules of liquid, dirt or perspiration. This idea was inspired by the fuzz on a peach, which was called the Nano-Care effect. Nano-Care’s ‘fuzz’ is made of minuscule whiskers attached to the cotton threads.

Another company, Swiss firm Schoeller Textil AG, calls its technology NanoSphere (Forbes, 2008). The system has nanoscopic particles of silica or of a polymer on the clothing fibers and these particles provide the lotus-like bumpy roughness, making textiles superhydrophobic with self-cleaning effect.

When rain falls on lotus leaves, water beads up with a high contact angle. The water drops promptly roll off the leaves, collecting dirt along the way, leaving the substrates in the original dry state. When self-cleaning textiles become widely available, marquees, awnings, sails, textile roofs in modern architecture, and so forth with lotus effect are expected to constitute the biggest market.

14.6.3 Multifunctionalization

Textiles with more than one function would have their value added, as is true with superhydrophobic textiles. Xue et al. (2008a) prepared superhydrophobic textiles with UV-shielding property through incorporation of TiO2 particles by titania sol coating. Tomši et al. (2008) prepared multifunctional, water- and oil-repellent and anti-microbial finishes for cotton fibers from a commercially available fluoroalkylfunctional water-borne siloxane, nanosized silver and a reactive organic–inorganic binder. Vilnik et al. (2009) fabricated hydrophobic and oleophobic sol-gel coatings with a long-lasting passive anti-bacterial effect for cotton fabrics without the addition of any anti-bacterial agents. The prepared fabrics have long-lasting low-surface-energy values, owing to the excellent adherence of the finishes on cotton fabrics and to the structure of the coatings, with 1H,1H,2H,2H-perfluorooctyltriethoxysilane acting in a specific role. The main cause of the passive anti-bacterial effect of the unwashed fabrics was the ability of the coatings to repel water. In fact, superhydrophobic surfaces have found their application in anti-biofouling (Marmur, 2006; Zhang et al., 2008b) on submerged surfaces, e.g. ships’ hulls, because the ability of organisms to adhere to superhydrophobic surfaces is much decreased. Hence, superhydrophobic textiles might also find future application as anti-microbial coatings.

14.7 Future trends

With increased research activities on superhydrophobic surfaces, many efforts have been devoted in the fabrication of superhydrophobic coatings on textiles. Although several publications have revealed the robustness of the superhydrophobic coating on textiles and the durability of superhydrophobicity, there is still much work to be done to put superhydrophobic textiles into real application.

The advantage of using textile substrates to fabricate superhydrophobic surfaces relies on the easy manipulation of a large production area and rich raw materials with many different properties. It is necessary to maintain the properties of the substrate in a way that makes it suitable for its original purpose. Such techniques could result in excellent robustness of coatings with durable superhydrophobicity, without altering the physico-chemical properties, color and hand of the textiles, which would be highly beneficial to the final products.

Again, talking about self-cleaning surfaces, many publications take the example of removing dust by water. But for textiles in application, it is not so simple. First, the contamination is complex, which may not be fly ash but water slurry or kitchen oil. In this case washable or oil-repellent textiles are preferable. Second, the contamination may be caused by heavily pressed attachment of smudge. In this case, the self-cleaning function will not perform properly on that spot again until the smudge is removed. A lotus-leaf repairs itself after scratch because it has tiny wax crystals on the surface which grow back. Self-healing materials have been prepared in other fields. Fabricating textiles with self-healing superhydrophobicity may not be impossible.

Superhydrophobic textiles with other functions have been fabricated as described earlier in this chapter. Multifunctional textiles are greatly appreciated by a more discerning and demanding consumer market due to the addition of high value, and will be one of the primary investigations on textile functionalization. Surfaces with tunable superhydrophobicity (Bhushan, 2009; Choi et al., 2009) have been reported, which might trigger the investigation of superhydrophobic coatings on textiles with stimulus tunable wettability to fabricate smart textiles.

Finally, environmental issues should be taken into account when preparing superhydrophobic textiles. It has been recognized that some fluorochemicals have potential risks for human health and for the environment. Hence, the development of non-fluorinated modifying agents is preferable for the fabrication of environmentally friendly coatings on textile substrates.

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