Coating film having water repellency and low refractive index

Claim 1

A process for forming a coating film on a substrate surface, which comprises preparing a reaction mixture comprising a silicon compound (A) of the following formula (1): Si(OR).sub.4 ( 1) wherein R is a C 1-5 alkyl group, a silicon compound (B) of the following formula (2): CF.sub.3 (CF.sub.2)nCH.sub.2 CH.sub.2 Si(OR.sup.1).sub.3 ( 2) wherein R 1 is a C 1-5 alkyl group, and n is an integer of from 0 to 12, an alcohol (C) selected from the group consisting of diethylene glycol monomethyl ether, diethylene glycol monoethyl ether and an alcohol of the following formula (3): R.sup.2 CH.sub.2 OH (3) wherein R 2 is a hydrogen atom, or an unsubstituted C 1-12 alkyl group or a C 1-12 alkyl group substituted with hydroxy, methoxy or ethoxy, and oxalic acid (D), in a ratio of from 0.05 to 0.43 mol of the silicon compound (B) per mol of the silicon compound (A), in a ratio of from 0.5 to 100 mol of the alcohol (C) per mol of the total alkoxy groups contained in the silicon compounds (A) and (B), and in a ratio of from 0.2 to 2 mol of the oxalic acid (D) per mol of the total alkoxy groups contained in the silicon compounds (A) and (B); heating this reaction mixture at a temperature of from 50° to 180° C. until the total amount of the silicon compounds (A) and (B) remaining in the reaction mixture becomes at most 5 mol %, while maintaining a SiO 2 concentration of from 0.5 to 10 wt % as calculated from silicon atoms in the reaction mixture and maintaining absence of water, to form a polysiloxane solution; then coating a coating fluid comprising the polysiloxane solution on a substrate surface to form a coating; and heat-curing the coating at a temperature of from 80° to 450° C. to form a coating film having a refractive index of from 1.28 to 1.38 and a contact angle of water of from 90° to 115°, as adhered on the substrate surface.