Pre-heating time and exposure duration: Effects on post-irradiation properties of a thermo-viscous resin-composite

Objective: To evaluate the effects of pre-heating time and exposure duration on the degree of conversion (DC), maximum rate of polymerization (RP max), polymerization shrinkage strain (PS) and surface micro-hardness (VHN) of Viscalor. Methods: Viscalor syringes were pre-heated using a Caps Warmer (VOCO, Germany) in T3 mode (at 68 °C) for 30 s (T3-30s) and 3 min (T3-3min) and then the composite paste was extruded into appropriately sized molds. Light irradiation was applied at zero distance from the upper surface with a LED-LCU of mean irradiance 1200 mW/cm 2 for either 20 or 40 s. The real-time polymerization kinetics and DC at 5 min and 24 h post-irradiation (DC 5min and DC 24h) were measured using ATR-FTIR (n = 3). PS was obtained with the bonded-disk technique (n = 3). Top and bottom Vickers micro-hardness (VHN top and VHN bottom) were measured at 5 min post-irradiation and after 24 h dry storage (n = 5). Data were analysed using one-way ANOVA, two-way ANOVA, independent t-test and Tukey post hoc tests (p < 0.05). Results: Polymerization kinetic curves of Viscalor from 0 to 15 min were similar for different pre-heating times and exposure durations. Pre-heated Viscalor (T3-30s and T3-3min) with 40 s exposure had greater VHN top and VHN bottom than for Viscalor (no heat) (p < 0.05). Exposure duration did not significantly affect DC, RP max and PS (p > 0.05). After 24 h storage, DC and VHN increased. Pre-heating did not increase the DC 24h, relative to no pre-heating (p > 0.05). Two-way ANOVA showed that there was no significant interaction between pre-heating time and exposure duration (p > 0.05). Significance: Increasing irradiation time from 20 to 40 s did not affect DC, RP max or PS, but increased VHN top. Composite pre-heating had no adverse effect through any premature polymerization. For Viscalor, 3 min pre-heating and 20 s irradiation were sufficient to provide adequate hardness, without increasing PS or compromising polymerization kinetics.