Effects of Temperature on Primary Curing Kinetics of Light Curing Dental Composites Investigated by Dielectric Analysis (DEA)

Objectives: Many publications show clearly that temperature affects the final degree of cure (DC) of visible light curing resin based composites (VLC RBC). However, there is a non-uniform situation concerning the temperature dependent reaction rate during the first stages of curing. Some researcher e.g. Daroch et al found a “significant” increase of reaction rate by a factor 1.5 to 2 if the temperature is raised from 20°C to 50°C. Price et al found an increase of factor 1.5 being a not significant (p=0.16). These small changes are surprising as the viscosities of TEGDMA-BisGMA based resins change typically by a factor 5 to 10 leading to the conclusion that the reaction rate should correspondingly increase due to the higher molecular mobility.
Methods: DEA has a good time resolution of 10 to 20 data points per second if a frequency of 1,000 Hz is used and was thus applied to investigate the reaction kinetics using a Netzsch DEA 231. The investigated VLC RBC were camphorquinone initiated Arabesk Top OA2 and Grandio, VOCO, Germany.
Results: It was found that there is almost no significant temperature dependency of both reaction rate and initiation time during the phase of primary curing in the temperature range 30 to 50°C. This can be explained either by the low activation energy of 20 kJ/mole for photo-initiation or by the assumption that the rate of radical annihilation increases in a way that compensates for the viscosity decrease.
Conclusions: For dentists this is in principle good news as they need not to bother much about the temperature effects on reaction rate of VLC RBC if it is camphorquinone initiated.