Use of (glycidoxypropyl)trimethoxysilane as a binder in colloidal silica coatings

Colloidal silica coatings were produced from suspensions of silica modified with (glycidoxypropyl)trimethoxysilane (GPS). Coating dispersions were prepared by adding GPS to a silica colloid (12 nm) suspension. Adsorption of hydrolyzed GPS species on silica surfaces was monitored by attenuated total reflection Fourier transform infrared spectroscopy. The addition of GPS to a basic silica suspension (pH 9.5) favored condensation among hydrolyzed GPS species over adsorption. By contrast, more adsorption on the silica colloids occurred in acidic suspensions (pH 4) and condensation among hydrolyzed GPS species was slower. The interaction between GPS and colloidal silica was also reflected in the aggregation and gelation behavior of the suspensions and the coating microstructure. Suspensions prepared by addition of GPS at low pH resulted in coatings that were less prone to cracking. In addition, polyamine could be added to these suspensions in order to cure the coatings. Compared with unmodified silica coatings, coatings prepared with GPS modification were denser, adhered better to the polymer substrate, and could be made thicker (up to 20 mu m). Coatings were also transparent to the eye.