Method: 100 specimens of Fuji II LC RMGI were prepared in 25x2x2mm silicone moulds. Five groups (n=20) were tested: a) control group with no light-curing; b) light-curing delayed 1min after mixing; c) light-curing delayed 5min; d) light-curing delayed 10min; e) light-curing delayed 15min. The specimens were removed from the moulds 15 min after mixing and were immersed in water. Flexural strength and microhardness were tested at 24h (n=10, for each group) and 1 year (n=10, for each group). Data were subjected to two-way ANOVA and Tukey tests at a 0.05 significance level.
Result: At 24: a short delay in light-activation caused a significant increased of flexural properties (highest value at 5min) and microhardness (higest values at 10min). A more important delay (>10min) involved a significant decreased of these properties. However, at 1 year the highest values for flexural strength and microhardness were obtained for the shortest delay (1min).
Conclusion: RMGI would be structurally different according to the extent of acid-base or resin polymerization reactions. At 24h: a short delayed light-activation (acide-base reaction extended) increased mechanical properties. However, this gain decreased with time: at 1 year, the minimum delay (polymerization reaction extended) involved the highest values. The hydrolytic degradation (related to polymerization reaction) would outweigh the maturation (related to acide-base reaction): the highest long-term mechanical properties concerned the groups for which an early polyHEMA network was fostered.