An Improved Photo-initiator System for Dental Composites

Objective: While significant improvement has been made over the past several years, inefficient polymerization and in-vivo discoloration in composites remain a challenge. Most of the commercially available composites comprise of camphorquinone (CQ) and ethyl-4-dimethylaminobenzoate (EDMA) as photo-initiator system. The objective of this research is to use diphenyl phosphine oxide (DPO) for possible dental applications.
Materials: Ethoxylated-bisGMA (BisEMA), triethyleneglycol dimethacrylate (TEGDMA), CQ, EDMA and DPO were obtained from Sigma-Aldrich, USA.
Methods: To compare the two Photo-initiator systems, 1% CQ-EDMA and 1% DPO were added in BisEMA, BisEMA-TEGDMA mixture (50:50 by weight) and TEGDMA. These six combinations were subjected to isothermal (37°C) photo-polymerization in a DSC to measure heat of cure. ATR-FTIR spectra were also obtained for BisEMA-TEGDM mixture before and after curing to measure %conversion. CIE L*a*b* values were measured by Chroma-Meter before and after UV (24hr) and humidity (37°C) exposure (24-hr, 15 and 30-days). Curing for all the samples was carried for 30-sec at 300-MW/cm².
Results: Mean heat of cure for BisEMA, BisEMA-TEGDMA and TEGDMA with CQ were 62.8, 66.3 and 56.4kJ/mol, while 58.2, 71.0 and 77.0-kJ/mol with DPO respectively (p<0.03). Mean %conversion for BisEMA-TEGDM with CQ was 69.8 and 76.6% with DPO (p<0.0095). Analysis of exotherms in DSC and %conversion measurements indicated that DPO significantly accelerated the curing and enhanced the overall curing by 30-sec light cure vis-à-vis CQ. ΔE≤3 (p<0.0095) for the DPO samples corresponded to its improved color stability of cured composite when exposed to both water as well as to UV light.
Conclusion: Recent studies have demonstrated that Bisphenol A can cause xenoestrogenic effect and TEGDMA may promote the proliferation of cariogenic microorganisms like Lactobacillus acidophilus and Streptococcus sobrinus. The use of DPO as photo-initiator can limit these adverse effects by increasing %conversion in composite. Polymerization using DPO initiator also confers improved color stability.