Influence of curing modes on crosslink density in polymer structures

Objective. This study investigates the influence of curing modes on the crosslinking density of dental composites. Methods. A light-cure unit (BISCO VIP) that allowed for independent command over time and intensity was selected. Four different light-curing modes with constant light energy density were investigated (control (C), pulse delay (PD), soft-start (SS) and pulse cure (PC)). The degree of crosslinking was assessed directly by measuring the glass transition temperature of 1 mm thick composite (Z100, 3M-ESPE) specimens using differential scanning calorimetry (DSC 2920). Polymer softening in ethanol was used as an indirect method for assessing the degree of crosslinking. After light-curing, specimens were stored in air at 37 °C for 24 h and subjected to hardness testing using a digital microhardness tester (n=6, LOAD=500 g; dwell TIME=15 s). The specimens were then placed in 75% ethanol–water solution at 37 °C for 24 h and post-conditioning hardness was determined. Mean hardness (KHN)/hardness deterioration (ΔKHN) was computed and data was subjected to analysis using one-way ANOVA/Scheffeʹs test. Results. Ranking of degree of crosslinking density by DSC was as follows: C>PC>SS>PD. For the indirect method of determining crosslinking density, ΔKHN ranged from 10.8 to 12.9 and ranking was PC>SS>C>PD. Conclusions. Specimens polymerized with PD were significantly more susceptible to softening in ethanol than specimens cured with PC. Results of this study suggest that polymerization with PD resulted in a lower crosslink density and gave rise to polymers with an increased susceptibility to softening in ethanol.