Durable Superhydrophobic Nanocoating for a Textile Substrate
Apart from the efficacy of the performance of superhydrophobic textiles, the durability on exposure to repeated washing cycles and weathering cycles is an important aspect. Current research studies are more focused on the durability of the nanocoating. Durable superhydrophobic coatings for textile fibers have been developed that can withstand several washing cycles. The fiber-based textiles possess a micro-/nanoscale roughness that is provided by the fibers' hierarchical structure.
Zimmermann et al. fabricated the superhydrophobic-coated poly(ethylene terephthalate) (PET) fabric by a simple, one-step gas phase coating procedure in which a layer of polymethylsilsesquiox- ane nanofilaments was grown onto the individual textile fibers. The coating imparted excellent abrasion resistance and enhanced water- repellent characteristics. Furthermore, important textile parameters such as tensile strength, color, and haptics are unaffected by the silicone nanofilament coating .
Wang et al. published research dealing with the production of durable superhydrophobic coated fabrics based on trideca- fluorooctyl-triethoxysilane and modification of silicon dioxide nanoparticles with polydimethylsiloxane (PDMS). Coated fabrics exhibited high abrasion resistance and excellent chemical resistance along with a sliding angle of 3° and a CA of 170°. The coated fabrics displayed durable superhydrophobicity where a sliding angle of less than 6° and a CA of 165° were obtained after repeated laundering for 500 cycles .
Xue et al. fabricated a durable self-healing superhydrophobic PET fabric by a convenient solution-dipping method using a material system consisting of PDMS and octadecylamine. The coating was stable after 20 cycles of laundry and 5000 cycles of abrasion without the superhydrophobicity being affected. The superhydrophobic PET fabric showed a CA of 161.3° ± 3° and a sliding angle of 4.5° ± 1.6° and also exhibited excellent durability on being subjected to washing and abrasion. A unique self-healing ability of coated fabric can significantly improve the service life of coated PET fabric .
Shaban et al. fabricated a durable superhydrophobic cotton fabric by using the sol-gel method. In this ZnO nanoparticles were applied to improve the water repellency of the cotton fabrics using a spin coating technique. The superhydrophobic coated cotton fabric exhibited a water contact angle (WCA) of 154°, which was achieved by coating the fabric with a 0.5 M ZnO precursor solution (pH 7 and 20 runs). The treated cotton fabric exhibited durability under UV illumination and outdoor environment and also showed excellent abrasion resistance .
Yazdanshenas et al. created a superhydrophobic cotton fabric by simple one-step ultrasonically assisted synthesis of octyltrie- thoxysilane-functionalized silica nanoparticles; subsequently, functionalized nanoparticles were applied onto the cotton fabric to get hydrophobic nanoroughness. Effective superhydrophobic coating was achieved by an ultrasonically assisted reaction of organic-inorganic hybrid precursors like octyltriethoxysilane and tetraethylorthosilicate. The superhydrophobic coated fabric showed a CA greater than 150° and a sliding angle of 8°. The application procedure did not significantly affect the physical and mechanical properties of the treated fabric .
Limitations of Superhydrophobic Nanocoating
Environmental safety is one of the major concerns in today’s high- technology era. The high demand for self-cleaning fibrous products that involve the use of a nanoroughened surface coatings leads to health and environmental safety concerns. During the preparation of a self-cleaning textile material, chemical residues are generated, which could lead to water pollution. Moreover, the leaching of Ti02 nanoparticles from the textile surface during laundering leads to the contamination of water. It must also be noted that nanoparticles can move and gather in the soil and could adversely affect the environment [82, 83]. The acute toxicity of a substance is represented by the lethal dose (LD50). It is the amount of a substance given in one go that kills 50% of the test population . For titanium dioxide, the value of LD50 is more than 12 g/kg on oral administration. It is studied and proven that nanoparticles are harmful to the human body and also for the lungs of mice . An ecological score of solvent-based coatings always remains inferior. Apart from this, the use of nonbiodegradable polymers for coating is also objectionable in terms of disposal ecology. This necessitates research in this important area of material science mainly on new polymers and techniques to generate ecofriendly superhydrophobic textile substrates.
Summary and Future Perspectives
This chapter has covered the recent studies on self-cleaning textile surfaces for the application of superhydrophobic nanocoating. It must be emphasized that very few studies have been carried out on the robustness of the treatment and the durability of superhydrophobic coatings. The attainment of good mechanical durability is the most difficult challenge for the application of superhydrophobic coatings. Pillar-type or needle-like structures are considered to be the most ideal geometry for superhydrophobicity. However, the low durability of these structures becomes their primary limitation. Therefore, from the mechanical durability point of view, a hemispherical or crater-like structure could be more preferable. There is a broad scope of products prepared with superhydrophobic coatings; however, the fabric surfaces should be carefully engineered to meet the technical expectations from such products. Apart from this, innovative techniques can be discovered to take advantage of the vast scope for the practical implementation of superhydrophobic surfaces in commercial products.
Ecological production of superhydrophobic textile materials should remain the quest of any research. The toxicity of nanoparticles, solvents, polymers, and other fillers used for imparting su- perhydrophobicity should be carefully tested before their utilization for modification of textiles. The preparation of superhydrophobic surfaces should consider the safety issues, and the use of fluoro- chemicals and organic solvents that pose potential risks to human health should be minimized. Therefore, further exploration of selfcleaning surfaces may provide upgraded technologies and reduce health and environmental issues [23, 86]. Such technologies must be thoroughly studied by the research community to explore the possibility of sustainable superhydrophobic textile materials.