Effect of preheating on the viscosity of composite resins

To determine the effects of time and temperature on the viscosity of preheated composite resins.

Eleven composite resins were heated to 60°C, and temperature analyses were performed at intervals of 1 min until they had cooled to 25°C. The permanent oscillatory shear test was performed at 25°C, 35°C, 50°C, and 60°C for three composite resins under a shear rate of 1s^-1. One- and two-way analysis of variance were used for the analysis (α = 0.05).

There was no significant interaction between the composite resin and time (P = 0.9304), and only the main effect time was significantly different (P < 0.0001). A difference was observed between T0 and T6 (P < 0.001), but not after T7. The increase in temperature resulted in a viscosity reduction (P < 0.05). At 25°C, Beautifil II presented higher viscosity. Palfique LX5 showed a significant viscosity reduction with increasing temperature compared with the others (P < 0.05). For Beautifil II and Z100, there was no difference at temperatures of 50°C and 60°C, while for Palfique LX5, no statistical difference was observed at 35°C, 50°C, and 60°C.

Ten minutes of preheating were sufficient to reach a temperature of 60°C, reducing viscosity by at least 84%. However, 5 min after removal, the composite resin cooled to room temperature.

Preheating composite resin has potential benefits. To determine how this approach will work in clinical practice, it is important to define the effects of time and temperature in the protocol of this technique and understand its limitations.