The reactions of 3-glycidoxypropyltrimethoxysilane in acidic solutions on polymerization and in the presence of silica

The epoxy group in 3-glycidoxypropyltrimethoxysilane (GPS) readily undergoes hydrolysis in acidic conditions to form a solution containing principally the vicinal diol or a reaction product between the vicinal diol and the silanol groups formed during the hydrolysis (an alkoxysilane). Additional reaction products include alkene, enol, low concentrations of carbonyl compounds, intermolecular condensation products, e.g. ethers, and reaction products specific to the acid used to promote the hydrolysis of the alkoxysilane and epoxy groups. On evaporation of acidified GPS solutions at room temperature, a polymerized form of GPS is produced. This polymer is found to be readily oxidized on heating in air and from simulated outdoor weathering trials (SODW), but not when formed on silica powders from a 1-2 wt% solution to form a coating. In the absence of silica, unreacted hydroxy groups are readily available for oxidation, a result that is consistent with the reported molecular structure of polymerized GPS.