Evidence of chemical bonding to hydroxyapatite by phosphoric acid esters

Phosphoric acid esters (PAEs) have been used as a self-etching primer for composite-to-enamel bonding in adhesive dentistry. However, the chemical mechanism of their interactions with hydroxyapatite (HA) is not clear. In the present study, HA particles were mixed with Resulcin AquaPrime (Merz Co.) priming agent that contains a mixture of PAEs, and dried. The primer, HA and the mixture of both were analyzed by FTIR. Primed and untreated HA discs were analyzed with attenuated total reflectance (ATR). After AquaPrime+D2O (1:1) was mixed with HA or Ca(OD)2 for 48 h, the primer and both mixtures were analyzed with 31P NMR in D2O. The solid and the liquid separated from both mixtures were analyzed with 31P NMR in CDCl3. The primer's characteristic bands (νCO, νCC) were found on the primer-subtracted mixture's spectrum and the primed HA disc. 31P NMR data indicated that the reactions of PAEs with HA produced PAEs–HA complexes, and were not a simple acid–base reaction like those with Ca(OD)2, either in liquid or in solid. It is concluded that phosphoric acid esters can decalcify and adhere to hydroxyapatite simultaneously.