Various Methods to Fabricate Superhydrophobic Textile Substrates

There are various methods to fabricate superhydrophobic textile surfaces that are commercially available, such as sol-gel processing, CVD, layer-by-layer (LBL) assembly, nanoparticle-composite coating, hydrothermal synthesis, and polymer film roughening. The primary methods to create superhydrophobic textile substrates are mentioned in Table 21.1 [40, 41].

Method

Interactions between substrates and coatings

Roughness formation

Hydrophobization

Ref.

LBL assembly

Stable Covalent binding

Nanoparticle/nanostructure

coating

Low surface free energy chemical

42,43

Polymer film roughening

Polymer binding or polymer film itself

Phase separation creating a permeable structure

Hydrophobic polymer itself

44,45

CVD

Polymer binding or covalent binding

Polymerization helping to grow nanostructures

Structure Polymerized itself

46

Sol-gel method

Adhesion or covalent binding

Roughened surface coating

Low surface free energy chemical

47,48

Hydrothermal

synthesis

Stable covalent binding

Nanostructure growing through the hydrothermal effect

Low surface free energy chemical

49

Nanoparticles composites coating

Covalent binding and polymer binding

Nanoparticles creating a micro-/nanostructure

Low surface free energy chemical

50

21.3.1 LBL Method

The LBL strategy is a most flexible method to develop thin film nanocomposites and to change textile surfaces [51]. Xue et al. developed superhydrophobic cotton surfaces by generating a dual-size rough surface via a complex coating of epoxy- and amino-functionalized silica nanoparticles on the cotton fabric. Further hydrophobization was achieved by utilizing 1H,1H,2H,2H- perfluorodecyltrichlorosilane, stearic acid, and a combination of both. The functionalized cotton fiber showed effective interaction with silica. Functionalized silica nanoparticles facilitate hydrophobization and also generate durable dual-size surface roughness. The fabricated silica coatings were durable and show stable hydrophobicity [41].

Amigoni and coworkers fabricated hybrid inorganic/organic surfaces by different alternating layers of epoxy- and amino- functionalized silica nanoparticles. After that, fluorinated aldehyde was grafted to obtain a hydrophobic top layer of amino- functionalized silica nanoparticles, thus constructing a monomo- lecular layer by means of imine formation. The hydrophobicity and stability of the surface increased with the number of layers

[52] .

Manca et al. prepared surfaces with antireflective and robust superhydrophobic properties. The sol-gel process was employed to create a double layer consisting of silica nanoparticles with a trimethylsilanized surface. Firstly, a homogeneous organo-silica gel layer was applied onto a glass substrate, and secondly, a trimethylsiloxane-functionalized-nanoparticle-based layer was coated onto it. A hydrophobic monolithic film was obtained after thermal curing of two layers onto a glass substrate. The coated glass substrate showed a CA of 168 with excellent superhydrophobicity

[53] .

21.3.2 Polymer Film Roughening/Phase Separation

In view of their film-forming ability and adaptability, polymers are increasingly used for fabricating superhydrophobic surfaces. Phase separation of the multicomponent blend can be used to form polymer-based rough surfaces [87]. Zhang et al. depicted a basic and conservative technique for getting a superhydrophobic surface on wool with a brush-like polymer through the migration of organic siloxane segments in the acrylate side chains to the outer layer. Initially, the increase in film-forming properties and cohesiveness could be contributed by acrylate polymer chains. Finally, low surface free energy characterized by long Si-O-Si chains could be employed to improve water repellency [88].

21.3.3 CVD Method

This method helps in depositing gaseous reactants on the surface, which results in the formation of a nonvolatile solid film. A superhydrophobic textile was prepared by growing a layer of nanofilaments of polymethylsilsesquioxane onto textile fibers through a one-step gas phase coating process [46].

21.3.4 Sol-Gel Method

This method, indeed, is a perceived technique for synthesizing nanoparticles and gels. Leng et al. created a superoleophobic cotton fabric based on a multiple-length scale structure. The multilength scale roughness was on the base of the woven structure, with an additional two layers of silica microparticles and nanoparticles, bonded covalently to the cotton fabric. The produced superoleophobic cotton fabric showed a high CA (153° for 5 pL droplets) and a low sliding angle [9° for 20 pL droplets) [54].

Yu et al. synthesized a perfluorooctylated quaternary ammonium silane and produced a silica-sol. The coupling agent and silica- sol particles were coated onto cotton fabrics by a pad-dry-cure method. The finished cotton fabrics showed high oleophobicity and hydrophobicity [55]. Wang et al. prepared superhydrophobic textile substrates by applying a particulate silica-sol solution from cohydrolyzed tetraethoxysilane/fluorinated alkyl silane with NH3H20 on the fabric. The treated textile substrates exhibited CA greater than 170° and sliding angles lower than 7° with high superhydrophobicity [56].

21.3.5 Hydrothermal Synthesis Method

The hydrothermal technique is well recognized for the preparation of micro-/nanoscale surfaces. There have been significant studies on the preparation of superhydrophobic surfaces by ZnO materials, which can grow with various structures on the surface. Xu and Cai fabricated the superhydrophobic surface of the cotton fabric by using a hydrothermal synthesis method. Firstly, nanocrystals of ZnO were produced and incorporated on to the cotton fibers. Then, situated ZnO nanorods were created onto the cotton fibers to create nanoroughness. Further modification of the acquired cotton fibers was carried out by dodecyltrimethoxysilane to get superhydrophobic ZnO-coated surfaces [49].

21.3.6 Nanoparticles Composite Coating

Nanoparticle composite coating is a well-known coating technique to create a rough surface on textiles and involves a coating of nanoparticles on fibers. To enhance the durability of superhy- drophobicity and coating rate, the textile substrates and particles are generally modified by incorporating functional groups, for example, amino, carboxyl, hydroxyl, and epoxy groups.

Ramaratnam et al. fabricated ultrahydrophobic textile surfaces by depositing a monolayer of nanofluorinated polymer and reactive silica nanoparticles on the fibers. Firstly, silica nanoparticles coated with an ultrathin layer of poly(glycidyl methacrylate) polymer containing epoxy were applied on a fiber surface. Then, the silica nanoparticles coated with epoxy functional groups could readily react with the functional groups of the fibers and the functional groups of the hydrophobic polymers. A thin hydrophobic rough layer was further generated by grafting the hydrophobic polymer on to the surface of the fiber [50].

Application of Superhydrophobic Textile Surfaces

Commercially several self-cleaning products based on the lotus effect are available, and some more products are in the development phase. Currently, various possible self-cleaning surface applications have been patented and exploited [57]. Various functional applications drive the research on superhydrophobic textile surfaces. This section will cover the applications of a superhydrophobic surface and also discuss the role of surface modification in the improvement of textile performance through superhydrophobic coatings.

21.4.1 Water Repellency

In many research studies, the main potential application of super- hydrophobicity is waterproofing of textiles. The superhydrophobic coated textiles can be used as water-resistant garments and for outdoor applications. The textile structure can be maintained in such a way that it can be superhydrophobic with breathability, which overcomes the limitations of traditional waterproof textiles finished with sealing agents, plastics, rubbers, and fluorocarbons chemicals

[46].

21.4.2 Self-Cleaning

The natural environment gradually contaminates the textile surface. Cleaning of such contaminated textile surfaces requires a lot of effort and makes the process economically expensive. Moreover, surfactants used for cleaning adversely affect the aquatic environment. These problems create the opportunity to fabricate substrates that have anticontamination properties and can clean themselves. For several decades, associated research and technology for developing self-cleaning substrates have been a hot topic [58]. David Soane, an inventor, introduced Nano-Care, which is a fabric finish. Fabrics developed from Nano-Care-coated threads and fibers show exceptional repellency to liquid molecules, perspiration, and dirt particles. This Nano-Care idea was inspired by the peach fuzz and is known as the "Nano-Care effect.” Nano-Care’s "fuzz" is created by tiny scale whiskers and is incorporated on to the cotton fibers [59].

21.4.3 Antibiofouling and Anticorrosion

The operational and maintenance costs increase because of biofouling of submerged structures and ships’ hulls. Biofouling can be suppressed via underwater superhydrophobicity, which can be achieved by developing a rough hydrophobic surface that creates an air pocket between the coating and water [60].

The possibility of creating surfaces that repel water opens the doors to the field of erosion restraint for metal segments [61, 62].

21.4.4 Multifunction

The accomplished self-cleaning properties with new functionalities must be consolidated for the next generation of superhydrophobic coatings, for example, optical transparency, in a coating that is produced using reasonable materials and is adequately durable for regular use [63]. Tomsic et al. utilized siloxane functionalized with fluoroalkyl, a reactive binder, and silver nanoparticles for the preparation of oil-/water-repellent and antibacterial cotton fabric [64].

Current Developments in Superhydrophobic Nanocoatings and Their Significance for Self-Cleaning Textile

Special wettability has contributed to the development of the textile industry, which is concerned with clothes in daily life. Everyday textiles with superhydrophobicity can be endowed with oil/water separation and self-cleaning properties, which needs the achievement of micro/nanostructure roughness or low-surface- energy materials. The key to effectively creating superhydrophobic nanocoatings on textile surfaces lies in successful coating and fixing of hydrophobic components onto a hydrophilic textile surface [65].

Schoeller Textil AG prepared, patented, and commercialized the lotus effect-based textile finishes. The fabric surface can be made superhydrophobic and oleophobic by nanosphere formation on the surface of the treated fabric and, therefore, the fabric can be endowed with self-cleaning properties, as reported for the lotus effect [66]. A patent from Waeber et al. revealed that the finish was composed of two water-/oil-repellent components. One contained a gel-forming compound, while the other one had nonpolar water-/oil-repellent components. To insolubilize the finish, a cross- linking agent was utilized. Shrinkage in the film occurred during drying of the treated fabric, which resulted in an anisotropic distribution of a gel-forming component of the finish, and a microstructure was created on the fabric surface. The phase transitions and phase instability of the components determined the self-organization of the gel-forming component and the creation of the microstructure [67].

Different iron oxide nanoparticles, like FeO and magnetic Fe304 and Fe203, were synthesized via a coprecipitation method. FeO and Fe304 were successfully coated on textiles to make superhydrophobic surfaces. The water droplets on the textile with Fe203 nanocoatings modified with thiol gradually reduced and were finally absorbed by the textile, as shown in Fig. 21.7A, while the textile with FeO retained its superhydrophobicity after modification, as shown in Fig. 21.7A. It was revealed that the interaction between thiol and Fe(II) plays an important role in the formation of a superhydrophobic surface [68]. On the other hand, iron oxides were successfully fabricated on a fabric substrate by in situ growth and superhydrophobic surfaces were acquired after modification with n- octadecyl thiols [69].

Yu et al. used a sol-gel method based on perfluorooctylated quaternary ammonium silane and silica nanoparticles to fabricate superhydrophobic cotton surfaces. A combination of both components provided excellent nanoroughness and low surface energy, which resulted in the lotus effect [55]. Bae et al. created a superhydrophobic cotton fabric by utilizing a nanocomposite coating of silica nanoparticles in a perfluoroacrylate-based water-repellent agent. The combined effect of nanoroughening and low-surface- free-energy components helped in decreasing the consumption of nonecofriendly fluorocarbons to 0.1 wt% [70].

Joshi et al. successfully used nanosilica and nanoclay to produce a superhydrophobic cotton fabric with CA >150°, wherein the water droplets rolled across the fabric surface and carried away the dust

Water droplets on textiles coated with (a) Fe0 and (b) FeO after thiol modification [68]

Figure 21.7 Water droplets on textiles coated with (a) Fe203 and (b) FeO after thiol modification [68].

particles. Two methods were used to prepare superhydrophobic cotton. The LBL method with nanosilica particles (1 wt%) showed the best results because it gave some preferred benefits regarding CA or robustness; however, it was less practical on an industrial scale than the dip coating method. Furthermore, the LBL method maintained the comfort characteristics of the fabric by retaining air permeability. The nanosilica deposited on the cotton surface was visible in the SEM images [Fig. 21.8) [15].

Nakajima et al. fabricated transparent superhydrophobic thin films with ТЮ2 by using a sublimable material and resulting coating of a fluoroalkyl silane that fulfills the prerequisites of superhydrophobicity, transparency, and durability. Valpey and Jones patented a process and composition of aqueous systems having Ti02 for producing a self-cleaning surface. The finish is composed of nanoparticles with a particle size <300 nm and a hydrophobic

SEM images; (a) cotton fiber surface and (b) LBL nanosilica- deposited cotton fiber surface after five bilayers [15]

Figure 21.8 SEM images; (a) cotton fiber surface and (b) LBL nanosilica- deposited cotton fiber surface after five bilayers [15].

film-forming polymer. This innovation gives a procedure and composition that consolidates roughness and hydrophobicity of the surface for fabricating a self-cleaning substrate. The fabricated substrates have numerous characteristics that incorporate selfcleaning and water repellency [71].

Self-cleaning property can be developed mainly by superhy- drophobicity and also by photocatalytic action. Photocatalytic action is the promotion of photoreaction in the presence of a catalyst that will decompose the organic molecules by using ultraviolet (UV) light [72]. The electrons on the surface get excited when photocatalysis occurs, and they escape from the valence band to the conduction band, forming electron-hole pairs on the surface. This gives rise to a positive charge (H+) in the valence band and a negative charge (e~) in the conduction band [73-75]. The formed pairs get trapped and react with other molecules that get absorbed on the photocatalyst. Titanium dioxide (Ti02), thus, can act as a photocatalyst and catalyze the photodegradation of organic molecules and dust pollutants. Presently, titanium dioxide is used to get self-cleaning properties, and commercially several self-cleaning products are available, such as bathroom and kitchen tiles, textiles, window glass, and air filters [76]. Pisitsak et al. prepared a self-cleaning cotton fabric by using Ti02 nanoparticles. An excellent self-cleaning effect was investigated for cotton fabric finished with higher Ti02 concentrations. The results showed that the coated cotton fabric appeared cleaner after one wash than uncoated samples [77].

Xu et al. created a superhydrophobic ZnO nanorod array film on a cotton substrate by using wet-chemical processing and did further modification using an n-dodecyltrimethoxysilane (DTMS) layer. The combination of the surface roughness created by the ZnO nanorod arrays and the low surface free energy developed by the DTMS layer adsorption resulted in superhydrophobic cotton. The treated substrate showed superhydrophobicity with a sliding angle of 9° (40 pL water droplet) and a CA of 161° (8 pL water droplet) [49].

 
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