Changes in Degree of Conversion and Microhardness of Dental Resin Cements

There are few studies available on the post-light activation or post-mix polymerization of dental resin cements as a function of time. This in vitro study evaluated the successive changes in the degree of conversion (DC) and microhardness during polymerization of six commercial resin cements (light-cured [Choice 2, RelyX Veneer], chemical-cured [Multilink, C&B Cement] and dual-cured [Calibra, RelyX ARC]) within the first 24 hours and up to seven days. Resin specimens were prepared for Fourier transform infrared (FTIR) spectroscopy and microhardness testing to determine the DC and Vickers hardness (VH), respectively. The light-cured materials or mixed pastes of the dual-cured materials were irradiated with a light-curing unit (Elipar TriLight) through a precured composite overlay for 40 seconds. The FTIR spectra and microhardness readings were taken at specified times: 1, 2, 5, 10, 15, 30 and 60 minutes; 24 hours and after two days and seven days. According to the FTIR study, most of the curing reaction of Choice 2 and RelyX Veneer occurred within 10 and 30 minutes, respectively. Multilink, C&B Cement and Calibra exhibited gradual increases in the DC up to 24 hours, with no further statistically significant increase (p>0.05). RelyX ARC attained a DC value within five minutes, similar to that at seven days (p>0.05). Choice 2 and RelyX ARC showed gradual increases in the VH, up to 15 minutes, with no further significant change over the remaining observation time (p>0.05). For RelyX Veneer, Multi link, C&B Cement and Calibra, there were no significant increases in the VH value after 24 hours (p>0.05). The light-cured materials produced significantly higher DC values than the chemical-cured materials (p<0.05). The DC values of the two dual-cured resin cements were significantly different from each other (p<0.001). The results suggest that the significant polymerization reaction was finished within 24 hours post-mix or post-light activation for all resin cements tested.