Epoxy-silica polymers as restoration materials

The epoxy monomer 3-glycidoxypropyltrimethoxysilane reacts, under non-hydrolytic conditions, with the primary amine (3- aminopropyl)triethoxysilane in 2:1 (1), 5:1 (2) and 10:1 (3) molar ratios leading to epoxy-silica materials which were characterized by TGA, DSC, Raman, and NMR investigations. The epoxy ring opening and the hydrolysis reactions of the Si(OR)3 groups were examined by means of 13C NMR before gelation occurs. The identification of the different types of silicate substructures present in the solids 1, 2 and 3 was accomplished by 29Si CP-MAS NMR spectroscopy which also provides a quantitative measure of the degree of condensation through the relative abundance of T0 [RSi(OR)3], T1 [RSi(OR)2OSi], T2 [RSi(OR)(OSi)2] and T3 [RSi(OSi)3] silicon nuclei. The combined results of the Raman and 29Si CP-MAS investigations clearly show that all the mixtures, especially the higher ratio ones, are characterized by a high degree of cross-linking; in addition, for 2 and 3, residual epoxy fragments are still present in the solid structure. The absence of residual enthalpy curing peaks in the calorimetric analysis indicates that the volatiles are simply released from all the samples. The thermogravimetric analysis shows that, among all the mixtures, 1 is featured by different structural properties which cause a higher degradation temperature and a longer decomposition time. To evaluate the effect of different experimental conditions on the properties of the final products, the reactions were also performed in the presence of small amounts of KOH and of a stoichiometric amount of water to effect hydrolysis of Si(OR)3 groups.