Determination of the optimal photoinitiator concentration in dental composites based on essential material properties

Objectives: The aim of this study was to determine the concentrations of the photosensitizer (camphoroquinone, CQ) and coinitiator (ethyl-4-dimethylaminobenzoate, EDMAB) that resulted in maximum conversion but generated minimum contraction stress in experimental composites. Methods: Experimental composites were prepared with an identical resin formulation [TEGDMA:UDMA:bis-GMA of 30.25:33.65:33.65]. Five groups of resin were prepared at varied CQ concentrations (0.1, 0.2, 0.4, 0.8 and 1.6 wt% of the resin). Five subgroups of resin were prepared at each level of CQ concentration, by adding EDMAB at 0.05, 0.1, 0.2, 0.4 and 0.8 wt% of the resin, resulting in 25 experimental resins. Finally, strontium glass (∼3 μm) and silica (0.04 μm) were added at 71.5 and 12.6 wt% of the composite, respectively. Samples (n = 3) were then evaluated for Knoop hardness (KHN), degree of conversion (DC), depth of cure (DoC) and contraction stress (CS). Results: There was an optimal CQ and EDMAB concentration that resulted in maximum DC and KHN, beyond which increased concentration resulted in a decline in those properties. KHN testing identified two regions of maxima with best overlaps occurring at CQ:EDMAB ratio of 1.44:0.42 and 1.05:1.65 mol%. DC evaluation showed one region of maximum, the best overlap occurring at CQ:EDMAB ratio of 2.40:0.83 mol%. DoC was 4 mm. Overall, maximum CS was attained before the system reached the maximum possible conversion and hardness. Significance: (1) Selection of optimal photoinitiator/amine concentration is critical to materials' formulation, for excessive amounts can compromise materials' properties. (2) There was no sufficient evidence to suggest that contraction stress can be reduced by lowering CQ/EDMAB concentration without compromising DC and KHN.