Hydroxyapatite effect on photopolymerization of self-etching adhesives with different aggressiveness.

OBJECTIVE: To understand the correlation of the acidic monomer/hydroxyapatite (HAp) reaction with the photopolymerization behaviour of self-etching adhesives with different aggressiveness.
METHODS: Two commercial self-etching adhesives the strong Adper Prompt L-Pop (APLP, pH ~ 0.8) and the mild Adper Easy Bond (AEB, pH ~ 2.5) were used. HAp powders were incorporated into both adhesives to acquire solutions with concentrations of 0, 1, 3, 5, 7 wt%. The attenuated total reflectance Fourier transform infrared (ATR/FT-IR) technique was employed to collect the in situ spectra during light-curing, from which the degree of conversion (DC) and polymerization rate (PR) were calculated. The pH of each tested solution was also measured.
RESULTS: Without HAp incorporation, the DC and PR of the strong APLP (7.8% and 0.12%/s, respectively) were much lower than those of the mild AEB (85.5% and 5.7%/s, respectively). The DC and PR of APLP displayed an apparent increasing trend with the HAp content. For example, the DC increased from 7.8% to 58.4% and the PR increased from 0.12 to 3.8%/s when the HAp content increased from 0 to 7 wt%. In contrast, the DC and PR of AEB were much less affected by the HAp content. The observations were correlated well with the spectral and pH changes, which indicated that APLP underwent a higher extent of chemical reaction with HAp than AEB.
CONCLUSIONS: The results disclosed the important role of the acidic monomer/HAp chemical reaction in improving the photopolymerization of the strong (low-pH) self-etching adhesives such as APLP. The phenomenon of polymerization improvement strongly depended on the adhesive aggressiveness.