Cationic cure of epoxy resins via benzylsulfonium salts covalently bound to glass surfaces

The cure behavior of epoxy resins in the presence of glass fillers was investigated using differential scanning calorimetry (DSC). A novel benzylsulfonium salt capable of covalently bonding to glass surfaces through a trialkoxysilane moiety were synthesized. Coupling of the salt to silica gel (as a model glass surface), characterization of the bound material, and its ability to initiate the cationic cure of DGEBA resins were investigated. The bound material was characterized by solid-state ¹³C and ²⁹Si CP/MAS NMR, FTIR, and TGA. The sulfonium salt was coupled with silica as the Br anion form because of the insolubility of the SbF₆ salt. After anion exchange, silica-bound salt with SbF₆ counterion was shown to initiate cure of epoxy resins but only at temperatures much higher than with an analogous unbound salt (>200°C and < 100°C, respectively). The inability to get complete anion exchange of Br anions for SbF₆ (necessary for cationic initiation activity) after coupling allowed formation of excess tetrahydrothiophene (THT) during heating, through decomposition of the residual Br salt, causing temporary termination and a large delay in cure. The temporary termination mechanism involved reaction of THT and the active oxonium ion to give a primary alkylsulfonium salt. In addition, it was discovered that the silica gel itself had an inhibiting effect on the cure of epoxy resins cured with unbound initiator, giving low T_g materials. This was due to inherent surface interaction with the salt and not to chemical reaction with the surface or with a physically adsorbed contaminant (such as water). The degree of inhibition increased with increasing filler content. Low surface area glass beads also inhibited cure, although surface modification of the glass beads with bound benzylsulfonium salt (SbF₆ form) improved cure significantly, reducing onset delay and giving high T_g materials. The degree of delay was inversely dependent on the amount of silane coupled to the surface and varied with counterion.