The Comprehensive Stability of Silorane-Matrix Composites

Objectives: The aim was to compare several measures of material stability of a silorane-matrix composite with representative dimethacrylate-matrix composites. These included setting shrinkage and stress, viscoelastic creep/recovery and stability in solvents.

Methods: Filtek Silorane-matrix composite (3M-Espe) was investigated along with representative high-performance dimethacrylate composites, which may be comparable as regards (i) filler volume-fraction and/or (ii) filler size-distribution. Materials were thoroughly light-cured by established protocols. Shrinkage was measured by the bonded-disk method and shrinkage-stress by the Bioman method, both at 23 ^oC. Four groups of cylindrical specimens (4 x 6 mm) were prepared in molds, by complete light irradiation, and then conditioned in 3 solvents: methyl ethyl ketone (MEK), ethanol, and water for 1 month at 37 ^oC. The creep-strain under 35 MPa compressive stress in 37 ^oC water was recorded continuously for 2 h and then the unloaded recovery-strain for 2 h. The data were statistically analysed by ANOVA, Bonferroni's test and by linear regression.

Results: A plot of maximum stress (MPa) versus maximum strain (%) for shrinkage showed a highly-significant (p< 0.01) differentiation of Silorane from dimethacrylate-matrix materials; < 1 % strain and < 2MPa stress. Maximum viscoelastic creep was < 1 % for Silorane with high recovery, and for all 3 solvents studied. With dimethacrylates, max-creep strongly correlated with solubility-parameter of conditioning solvents: MEK> ethanol > water, up to 4 % strain. Similar trends were found for permanent-set.

Conclusion: A Silorane-matrix composite exhibited high dimensional and host-stress stability during and following photopolymerisation. This benefit was also found in viscoelastic-strain stability in compression after extended exposure to conditioning solvents of increasing power. This solvent-stability may be attributable to the highly-reactive silorane cationic setting chemistry and the resulting hydrophobic silorane network structure.