Photoinitiator chemistry and concentratrion: Effect on polymerization-characteristics and mechanical-properties

Objective: The aim of this study was to determine the influence of photoinitiator chemistry and concentration on polymerization characteristics (degree of conversion, crosslink density) and mechanical properties (hardness, elastic modulus) of experimental composites photocured by different methods. Methods: Composites based on BisGMA/TEGDMA and 65% of feldspar filler were prepared with different type and concentration of photoinitiators (camphorquinone (CQ), phenylpropanedione (PPD), bisacylphosphine oxide (BAPO) or CQ/PPD, CQ/BAPO at concentrations of 1% or 2%). Bar specimens were prepared (n=10) and all tests were conducted at the same specimens. Degree of conversion (DC) was determined by Fourier Transform Infrared Spectroscopy (FTIR). Knoop Hardness (KHN)was assessed using un indenter Shimadzu HMV with load of 50g for 5s. Elastic modulus (E) was evaluated through a three point bending flexural test in a Universal testing machine. An indirect evaluation of crosslink density (CLD) was performed by hardness measures before and after storage in absolute ethanol for 24h. Data were submitted to two-way ANOVA and Tukey's test (pre-set α=0.05). Results: The evaluation of DC, KHN, E and DLC revealed no difference among CQ, PPD and CQ/PPD composites, regardless the concentration. BAPO composites showed the significantly higher values in all tested properties. No difference was observed between 1% and 2% concentrations. Conclusions: Photoinitiator concentration of 1% is sufficient to provide adequate polymerization, similar to that observed for composites with 2%. CQ and PPD photoinitiators showed similar efficacy of polymerization, and no synergic effect was observed when they are associated. Composites containing BAPO show superior quality of polymerization when compared to CQ and PPD. BAPO is a suitable substitute for CQ, as a photoinitiator in resin composites.