Hydroxyapatite Effect on Photopolymerization of Self-etching Adhesives with Different Aggressiveness

Objective: To understand the correlation of the acidic functional monomer/hydroxyapatite (HAp) reaction with the photopolymerization behavior of self-etching adhesives with different aggressiveness.

Methods: Two commercial self-etching adhesives Adper Easy Bond (AEB, pH~2.5) and Adper Prompt L-Pop (APLP, pH~0.8) were used. To mimic the demineralization or reaction, 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 APLP (7.8% and 0.12%/s, respectively) were much lower than those of AEB (85.5% and 5.7%/s, respectively). Depending on the amount of HAp incorporated, the photopolymerization behaviors of the two self-etching adhesives were significantly different. The DC and PR of APLP displayed an apparent increasing trend with the HAp content. For example, the DC increased from 7.8% to 60.3% 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 changes, which indicated that APLP underwent a higher extent of chemical reaction with HAp than AEB. The observed pH changes with the HAp incorporation suggested that the chemical reaction/interaction of the acidic monomers with HAp strongly depended on the aggressiveness of the adhesives.

Conclusion: The phenomenon of improved polymerization was determined by the aggressiveness. The results disclosed the important role of the acidic monomer/HAp chemical reaction in improving the photopolymerization of the low-pH self-etching adhesives such as APLP. This work was supported by NIH T32-DE07294.