Discovery of VOC-compliant TEOS sol and its application to SiO2/novolac hybrid coatings
The solvent effect and gelation property of TEOS based sol–gel process by decreasing co-solvent (alcohols and acetone) content are investigated to obtain VOC-compliant TEOS sol. A solution with the fixed amount of TEOS and co-solvent, adding the maximum possible amount of water while maintaining homogeneity, is regarded as “precursor solution”, and more water causes a liquid/liquid phase separation in the precursor solution. The quantitative relationship between water and co-solvent amount in the precursor solutions is obtained. The gel time prolongs with decreasing water content in a precursor solution until a peculiar TEOS/H2O molar ratio at which the system does not gelatize. This TEOS/H2O molar ratio is called critical gelation point (CGP). The molar ratio of TEOS/H2O at CGP in acetone precursor solution is extremely higher than that in alcohols precursor solutions. The sols below CGP do not gelatize and have little co-solvent, and the significance of the sols below CGP lies in that the sols behave as solvent or thinner of the coatings and are used to mix with suitable polymers to prepare VOC-compliant hybrid coatings. Novolac (thermoplastic phenolic resin) is deliberately synthesized to prepare SiO2/novolac hybrid coatings in the proposed way. The ethanol content reduces from 67.5 wt.% prepared by the conventional method to 12.9 wt.% prepared by the new way. The hybrid coatings show excellent adhesion and hardness, and can adopt conveniently spray application. They can be used to substitute for conventional solvent-based coatings. âº The critical gelation point (CGP) is discovered and applied to hybrid coatings. âº The sols below CGP have little co-solvent as a solvent or thinner of hybrid coatings. âº SiO2/novolac hybrid coatings are prepared with sols below critical gelation point. âº VOC-compliant SiO2/novolac hybrid coatings have excellent mechanical performance. âº A new way is proposed to prepare VOC-compliant inorganic–organic hybrid coatings.