Photoinitiator content in restorative composites: influence on degree of conversion, reaction kinetics, volumetric shrinkage and polymerization stress

Purpose: To determine the influence of rate of polymerization, degree of conversion and volumetric shrinkage on stress development by varying the amount of photoinitiators in a model composite. Methods: Volumetric shrinkage (with a mercury dilatometer), degree of conversion, maximum rate of reaction (RP _max) (with differential scanning calorimetry) and polymerization stress (with a controlled compliance device) were evaluated. Bis-GMA/TEGDMA (equal mass ratios) were mixed with a tertiary amine (EDMAB) and camphorquinone, respectively, in three concentrations (wt%): high= 0.8/1.6; intermediate= 0.4/0.8 and low= 0.2/0.4. 80 wt% filler was added. Composites were photoactivated (400 mW/cm² x 40 seconds; radiant exposure=16J/cm ²). A fourth experimental group was included in which the low concentration formulation was exposed for 80 seconds (32 J/cm²). Results: For the same radiant exposure, conversion, RP_max and stress increased with photoinitiator concentration (P< 0.001). When the low concentration group exposed to 32 J/cm² was compared with the high and intermediate groups (exposed to 16 J/cm²), RP_max still increased with the photoinitiator concentration between all levels (P< 0.001) but conversion and stress did not vary (P> 0.05). Shrinkage did not vary regardless of the photoinitiator concentration or radiant exposure. For the photoinitiator concentrations used in this study. Polymerization stress was influenced by conversion but not by rate of reaction.