Synthesis and properties of a green and self-cleaning hard protective coating

Polysiloxane coatings prepared by pre-hydrolysis/condensation of methyltrimethoxysilane (MTMS), etraethoxysilane (TEOS), methyltriethoxysilane (MTES) and polydimethylsiloxane (PDMS) in the presence of acid solution were presented in this paper in order to explore their potential use as protective and self-cleaning agents for car surface. The hardness and adhesion on the substrate properties of coatings were thoroughly examined by pencil hardness tester and adhesion tester. Contact angle meter was used to measure the hydrophobic angle properties of coatings. All the properties of coatings were related closely to the composition of coating and the hydrolysis degree of organoalkoxysilane. We found that the Scratch hardness can be improved by the controlling of TEOS quality ratio and acidity conditions, while more dosage of MTMS, MTES and PDMS favor raising the hydrophobic angle. Excellent adhesion property of paint coat was obtained with the presence of γ-(2,3-epoxypropoxy) propytrimethoxysilane (KH-560). Whatever, the best self-cleaning coatings with excellent hardness and adhesion on the substrate was sample A4 synthesized by a sol–gel process of low-cost organoalkoxysilane, prepared at the quality ratio of (MTMS):(TEOS):(MTES):(PDMS):(KH-560) of 8:7:4:4:4 and water-to-precursor quality ratio of 1:7. Finally, ambient-curable self-cleaning coating suitable for large area application was obtained. The scratch hardness and adhesion strength of the coatings was 3H and 0 after hardening with a high speed, respectively. The contact angle after UV aging of the coating was 92°. The transmittance of the coating exceeded 98.5%. Meanwhile, the coating has good resistance to solvent and weather resistance.