The effect of hydroxyapatite presence on the degree of conversion and polymerization rate in a model self-etching adhesive.
OBJECTIVE
The effect of hydroxyapatite (HAp) content on photopolymerization of a model self-etching adhesive was studied by using attenuated total reflectance Fourier transform infrared (ATR/FT-IR) spectroscopy.


METHODS
The model adhesive contained two monomers: bis[2-(methacryloyloxy)ethyl] phosphate (2MP) and 2-hydroxyethyl methacrylate (HEMA) using a 1:1 mass ratio, representing an acidic formulation. Camphorquinone and ethyl 4-dimethylaminobenzoate were added to enable visible light photopolymerization in a constant concentration of 0.022 mmol per gram monomer. HAp [Ca(10)(OH)(2)(PO(4))(6)] powder were added to the test solutions to obtain mass fraction of 0, 1, 2, 3, and 4 wt%. The degree of conversion (DC) and the polymerization rate (PR) with/without HAp were determined using ATR/FT-IR with a time-based spectrum analysis.


RESULTS
Monomer DC and PR were significantly enhanced by addition of HAp. Incorporation of 4 wt% of HAp increased DC from 20.8 (±0.3) % to 93.4 (±1.1) %, and PR from 0.42 (±0.01) %/s to 3.21 (±0.07) %/s. The pH of adhesive solutions was measured and correlated with DC and PR. The pH of test solutions was also controlled using a base (sodium hydroxide, NaOH) to similar values as when using HAp. Results indicated that both the DC and PR increased with increasing pH, regardless of additive, confirming the role of pH on polymerization. From the IR spectral comparison, changes in molecular structures of the self-etching adhesive after the addition of HAp were observed, which were correlated with the specific interaction between 2MP and HAp. The effect of viscosity was also proposed to be another possible reason for the improved polymerization.


CONCLUSIONS
The photopolymerization of a self-etching adhesive was enhanced/accelerated in the presence of HAp. The results provide the critical information for understanding the interactions/bonding between self-etching adhesives and tooth substrates.
University of Missouri-Kansas City School of Dentistry, Kansas City, MO 64108, USA.
Ying
Y
Zhang
Yong
Y
Wang
D045662:Dental Etching / Q000379:methods; D017886:Durapatite; D006244:Hardness; D006863:Hydrogen-Ion Concentration; D055115:Light-Curing of Dental Adhesives; D008689:Methacrylates; D058105:Polymerization; D019279:Resin Cements / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared
C454021:2-(methacryloyloxy)ethyl phosphate; D008689:Methacrylates; D019279:Resin Cements; C005044:hydroxyethyl methacrylate; D017886:Durapatite