Model reaction of epoxy-containing siloxane

Abstract

3-glycidyloxypropyltrimethoxysilane (GPTS), which contains hydrolizable silicon alkoxide bonds and a ring-openable epoxy group, was used as the model substance for studying the reaction conditions in producing transparent and non-shrinking sol-gel epoxy-SiO2 composites. When GPTS reacts with the mixture of H2O and n-butyric acid (nBA, used as a model curing agent) using triethylamine (TEA), pyridine or triphenylphosphine (TPP) as a catalyst at 50°C, the GPTS undergoes both hydrolysis and ring-opening reaction simultaneously and independently. The ring-opening reaction shows an induction period in the initial stage of the reaction when TEA was used as catalyst, while no induction period was observed when pyridine and TPP were used as catalyst. The rate of ring-opening reaction of epoxide group catalyzed by TPP and pyridine is faster that that catalyzed by TEA, but is still slower than the hydrolysis of SiOMe bonds. When 2-hydroxyalkylmercaptan was used as the curing agent (TEA as catalyst), the ring-opening reaction of GPTS was so reactive that the epoxide group not only reacted with mercaptan, but also reacted with H2O, and hence it is of no use to the synthesis of transparent and non-shrinking sol-gel epoxy-SiO2 composites. From the detailed study of the influence of the structure of siloxanes on the reactivity of hydrolysis, it is suggested that RSi(OR′)3 is the better type of precursor for the synthesis of transparent and non-shrinking sol-gel epoxy-SiO2 composites. The structures of R and R′ are also discussed in this paper.