Optimization of the concentration of photo-initiator in a one-step self-etch adhesive

Objectives. The objective of this study was to determine the optimal concentration of photo-initiator (camphorquinone) in an experimental one-step self-etch adhesive and to investigate the role of the photo-initiator. Methods. Seven experimental one-step adhesives with a varying amount of camphorquinone ranging from 0 to 5.25 wt% were prepared. Their micro-tensile bond strength to enamel and dentin was determined. In addition, the bond strength was also determined when the adhesive was not light-cured prior to the application of the composite. SEM and TEM were used for further evaluation of the resultant interfacial ultrastructure. Results. The bond strength to enamel was not influenced by the amount of photoinitiator, whereas the bond strength to dentin dropped significantly when concentrations below 0.35 wt% camphorquinone were used. Besides phase-separation droplets, electron microscopy revealed the presence of many small droplets at the bottom of the adhesive layer when the adhesive contained no or only a low concentration of initiator, or when the adhesive was not light-cured. Significance. Since polymerization is severely hampered by oxygen inhibition in thin layers, one-step self-etch adhesives depend greatly on the polymerization of the first layer of lining composite to achieve their ultimate mechanical strength. Consequently, the bond strength to enamel is not influenced by the amount of photo-initiator, but on dentin, bond strength is compromised by droplets, probably due to water absorption, and additionally by the negative effect of water on polymerization and by continuing demineralization of unpolymerized acidic monomers. Overall, it was found that minimally 0.7 wt% camphorquinone was needed. (C) 2009 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.