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

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.

Viscalor syringes were pre-heated using a Caps Warmer (VOCO, Germany) in T3 mode (at 68°C) for 30s (T3-30s) and 3min (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 1200mW/cm² for either 20 or 40s. The real-time polymerization kinetics and DC at 5min and 24h 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 5min post-irradiation and after 24h 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).

Polymerization kinetic curves of Viscalor from 0 to 15min were similar for different pre-heating times and exposure durations. Pre-heated Viscalor (T3-30s and T3-3min) with 40s 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 24h 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).

Increasing irradiation time from 20 to 40s 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, 3min pre-heating and 20s irradiation were sufficient to provide adequate hardness, without increasing PS or compromising polymerization kinetics.