Photo-polymerization efficiency of self-etch dental adhesives composed of camphorquinone or trimethylbenzoyl-diphenyl-phosphine oxide

Unique design of self-etch adhesive monomers combines acidic functional group and double bond into a single molecule, which presents complexity as using the environmentally-sensitive photoinitiators in such a system. The objective of this study was to evaluate the kinetics of polymerization for self-etch adhesives cured with two different photoinitiators camphorquinone (CQ) and trimethylbenzoyl-diphenyl-phosphine oxide (TPO). Model self-etch adhesives contained bis[2-(methacryloyloxy)ethyl] phosphate (2MP) and 2-hydroxyethyl methacrylate (HEMA) (mass ratio of 2MP:HEMA=1:1). Different contents of water from 0 wt% to 60 wt%, or NaOH from 0.2 wt% to 2 wt% were incorporated to the system. The adhesives were cured using quartz–tungsten–halogen (QTH) or light-emitting diode (LED) light curing units. The degree of conversion (DC) and polymerization rate (PR) of the model self-etch adhesives were determined using ATR/FT-IR with a time-based spectrum analysis. TPO generally outperformed CQ/4E to initiate the self-etch adhesives when using the QTH curing unit. In addition, the DC of the TPO-initiated adhesives displayed increasing trend with water content, in contrast to a decreasing trend for the CQ/4E-initiated system. Furthermore, polymerization of the TPO-initiated system was independent of pH of the self-etch adhesives, whereas the DC and PR of the CQ/4E-initiated system were strongly influenced by the pH. As the LED curing unit was used, polymerization of both TPO and CQ/4E-initiated adhesives was greatly depressed. However, the DC and PR of the CQ/4E initiated adhesives could be improved by increasing pH of the system. In summary, the TPO-initiated system exhibits superior polymerization performance to the CQ/4E system as well as water-favorable and pH-independent features as using QTH curing unit.