Stability of silorane dental monomers in aqueous systems

Siloranes (silicon-based monomers with oxirane functionality) are investigated as matrix resins for new low shrinkage/stress dental composites. Compounds containing oxirane groups are known to be reactive with water, which could impart instability to the composite. Objective To test the stability of siloranes by measuring changes in the chemical structure of the oxirane group in aqueous environments. Methods Two siloranes (PH-SIL and TET-SIL) and their 1:1 mixture (SIL-MIX) were evaluated (n=2–3). Siloranes were mixed in aqueous solutions with and without 1% tetrahydrofuran (THF) containing either liver esterase or epoxide hydrolase at pH 7.4, or dilute HCl at pH 1.4. The stability of conventional dioxiranes 3,4-epoxycyclohexyl-methyl-3,4-epoxycyclohexane carboxylate (ECHM-ECHC), and bisphenol A diglycidyl ether (BADGE) were also monitored under similar conditions. NMR was used to estimate the extent of reaction and give structural information about reaction products. Results Siloranes were found to be stable for 24 h in all aqueous environments tested. In contrast, ECHM-ECHC reacted at pH 1.4 to form species containing oxirane, ester, hydroxyl and carboxylic acid groups. Water hydrolyzed the ester group of ECHM-ECHC in the presence of liver esterase. In the presence of epoxide hydrolase, BADGE oxirane groups were hydrolyzed to diols, hydrolysis ranged from 0 to 34% depending on the aqueous environment. Conclusion The stability and insolubility of siloranes in biological fluid simulants suggests that these may be more suitable for use in the oral environment than conventional oxirane-functional monomers.