Curing efficiency of 2,4,6-trimethylbenzoyldiphenylphosphine oxide initiator in composite resins

Methods: Unfilled resins were prepared by mixing 1 mmol of BisGMA and 1 mmol of TEGDMA (Esstech Inc., Essington, PA, USA). Increasing concentrations of Lucirin^® TPO in the range of 1.25*10^-3 – 12*10^-2 mmol (0.05wt%-4.97wt%) were added to each mixture. Filled resins were prepared by adding 75wt% Tetric EvoCeram fillers (Ivoclar Vivadent, Schaan, Liechtenstein) to the unfilled resins. Standardized samples in Teflon moulds, 5 mm in diameter and 2 mm thick, were cured using a polywave LED LCU (bluephase^® G2; Ivoclar Vivadent, Schaan, Liechtenstein) at a 1 mm tip-to-surface distance. The DC was determined using micro-Raman spectroscopy based on changes in peak intensity of 1640 cm^-1 and 1610 cm^-1 peaks, assigned to the aliphatic and aromatic groups in the cured and uncured materials, respectively. The data were analysed using ANOVA and regression analysis (α=0.05).

Results: Increased initiator concentrations increased logarithmically the DC of unfilled and filled resins. The DC of unfilled resins was 73-91% at the top and 63-81% at the bottom surfaces and that of filled resins was 53-81% at the top and 47-70% at the bottom surfaces. Unfilled resins containing more than 1.08wt% Lucirin^® TPO showed similar DC values (p>0.05). The same was found in filled resins with more than 1.50wt% Lucirin^® TPO (p>0.05). The bottom-to-top ratios were in excess of 85% in all groups.

Conclusions: Lucirin^® TPO showed a high reactivity upon irradiation with a polywave LED LCU resulting in clinically acceptable range of DC values. Fillers significantly reduced conversion but had no effect on the logarithmic relationship between initiator concentration and the DC.