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 hydrotase at pH 7.4, or dilute HCl at pH 1.4. The stability of conventional dioxiranes 3,4-epoxycyclohexylmethyl-3,4-epoxycyctohexane 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 hydrotase, 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. (C) 2005 Elsevier Ltd. All rights reserved.