Preparation And Properties Of Poly (Fluoro) Siloxane/TiO₂ Nanocomposite Self-cleaning Coatings

In recent years, self-cleaning coatings have received great interests because they can remove or decompose the contaminates or dust particles under the gravity, rain, wind, and so on. Until now, two kinds of self-cleaning coatings have been developed according to different self-cleaning principles. One is the self-cleaning superhydrophobic (water contact angle>150°) coatings, which remove the dust through rolling of water droplets, namely, in a way being similar to the "lotus leaf" effect. However, the current methods to fabricate superhydrophobic coatings have many shortcomings, such as, high complexicity, small area, high temperature treatment, expensive apparatus, and etc. The durability of the superhydrophobic is generally poor. Another self-cleaning coatings is prepared based on the photo-decomposition of organic pollutes by inorganic semiconductor materials. TiO2 coating is a typical example of this kind of self-cleaning coatings. Pure TiO2 coatings can be prepared by magnetic sputtering, chemical vapor deposition (CVD), sol-gel, and etc. However, these methods need high vacuum or high temperature conditions, being unsuitable for large-scale preparation. In this study, large-scale fabrication of both supra-amphiphilic and superhydrophobic nanocomposite self-cleaning coatings were conducted at room temperature by combinding TiO2 nanoparticles with special polymer binders. The structure and properties of the coating were studied. The research contents and results are as follows:(1) Supra-amphiphilic nanocomposite coatings were fabricated by mixing titania nanoparticles (Degussa P25) with a sol-gel derived silica sol and methoxysilane group-bearing styrene-co-acrylate (SA) oligomer, and curing with aminopropyltriethoxysilane (APS) at ambient temperature. The amphiphilic, mechanical and optical properties of the nanocomposite coatings with different titania contents and matrix compositions were investigated before and after sun-illumination. Supra-amphiphilic surfaces were achieved just after the coatings were placed in sunshine for a short time. The higher the amount of TiO2 nanoparticles or the lower the fraction of SA copolymer is, the shorter the time to reach super-amphiphilicity is. Accelerated weathering tests showed a good durability especially for the coatings with low fraction of organic phase while photocatalytic activity experiments with methylene blue demonstrated an excellent self-cleaning property of the coatings even with titania content less than 2.0 wt%. The mechanism for the rapid formation of supra-amphiphilic surface with self-cleaning performance was explained.(2) Fluorinated polysiloxane/TiO2 nanocomposite coatings were directly prepared at room temperature using a, co-bis(hydroxylpropyl)-terminated fluorinated polysiloxane oligomer (PMSF) and co-bis(hydrogen)-terminated poly(imethylsiloxane) (PDHS), Karstedt catalyst, and TiO2 nanoparticles. FT-IR spectra analysis shows that dehydrocoupling and hydrosilylation reactions took place between PMSF and PDHS, forming dried fluorinated polysiloxane coatings. SEM and AFM demonstrate that the micro-nano binary structure was formed by TiO2 nanoparticles self-assemble at TiO2 nanoparticle content beyond 25 wt%. Meanwhile, the coating exhibited a water contact angles (WCA) of as high as 168.7±2.4°and a sliding angle (SA) of as low as 0.7±0.3°at 35 wt% of TiO2 nanoparticle content. Moreover, the as-prepared superhydrophobic coatings also display excellent durability at various environmental conditions. For example, they show superhydrophobic properties (WCA>165°) within entire pH range (1-14), after treating at temperature ranging from -20 to 200?for 30 min, or after accelerated weathering tests for 4 weeks. Salad oil photocatalyis experiment reveals the as-prepared superhydrophobic coatings can decompose pollutes and recover its superhydrophobicity under UV illumination. The as-prepared superhydrophobic coatings have good anti-icing performance.(3) Robust superhydrophobic coatings were directly fabricated at room temperature using triethoxysilyl-terminated PMSF resin (FPU) and poly(methylphenylsilicone) resin (PMBS), APS curing agent, and TiO2 nanoparticles. The coatings have superhydrophobicity (WCA>150°and SA<10°) when the content of TiO2 nanoparticles is more than 35 wt%. SEM and AFM images reveal that the superhydrophobic coatings have micro-nano binary roughness. The pencil hardness and pendulum hardness first increase then decrease with increasing dosage of APS or TiO2 nanoparticle content, and steadily decrease with increasing amount of FPU. The coatings have robust mechanical properties and superhydrophobicity for the those coatings being composed of PMBS/FPU(9:1),35-42 wt% TiO2 nanoparticles and 20-30 wt% APS. The WCA and mechanical strength of the coatings did not show obvious variation even after 840 hours accelerated weathering test. Salad oil photocatalyis experiment reveals the as-prepared superhydrophobic coatings have photocatalytic activity and self-recovery property. (4) Superhydrophobic nanocomposite coatings were readily prepared by mixing of silicone resin (Dow Corning@ 3037), aminopropyltriethoxysilane and FesO4 nanoparticles, and subsequently curing at an ambient temperature. The surface wettability, surface morphology and composition, and long-term durability of the coatings were investigated by water contact angle analysis, filed emission scanning electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy and QUV accelerated weathering tests, respectively. WCA of 156.4°and 158.3°, and SA of 6.5°and 4.3°are obtained at 48 and 56 wt% of Fe3O4 content, respectively. The coatings display a pencil hardness of B, excellent weatherability, and electromagnetic shielding effectiveness beyond 60% in the frequency range of 10-3000 MHz.