Methods: Four composite materials were studied, formulated with three different matrix structures. These were, respectively: silorane (Hermes, 3M-Espe); Ormocer (Experimental, Voco) and conventional dimethacrylate (Tetric-CeramHB, Ivoclar; X-trafil, Voco). Four groups of cylindrical specimens (4 x 6 mm) were prepared in molds, by complete light irradiation, and then aged in 3 solvents: methyl ethyl ketone (MEK), ethanol, and water for 1 month at 37oC. The creep-strain under 35 MPa compressive stress in 37oC 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: The materials all exhibited classic creep and recovery curves. For all matrix structures, the principal creep parameters were significantly different (p = 0.001) for each solvent condition. All materials showed lower creep-strain in water than in ethanol or MEK solvents. Maximum creep-strain and permanent-set gave significant (p = 0.01) negative linear-regressions (r2>0.98) with logarithm of the solvent solubility-parameter. The percent mean (SD) creep-strain ranged from minima of 0.72 (0.01) for the silorane and 0.82 (0.01) for the Ormocer, both in water, to the maximum of 3.48 (0.30) for Tetric-CeramHB in MEK. Similar trends were found for permanent-set. The dimethacrylate-based composites X-trafil had similar stability to the Ormocer.
Conclusion: Two new-matrix composite types: silorane and Ormocer, exhibited viscoelastic stability in food-simulating solvents. But this behaviour was closely matched by one highly-filled dimethacrylate material.