Relative photon absorption determination and the influence of photoinitiator system and water content on C=C conversion, water sorption/solubility of experimental self-etch adhesives

The present study aimed to determine the influence of photoinitiator/co-initiator system and the water content on polymerization efficiency, water sorption and solubility of experimental self-etch adhesives; as well as to understand the relation between photoinitiators and the curing unit on the resultant properties. Experimental adhesive formulations were formulated by mixing bis-2-methacyloyloxy-ethyl phosphate (2MP, Sigma Aldrich, USA) and 2-hydroxyethylmethacrylate (HEMA, Esstech Inc., USA) in equal parts by weight. Four photoinitiators were used: camphorquinone (CQ), phenantrequinone (PQ), trimethylbenzoyl-diphenyl-phosphine oxide (TPO) and bisacyl-phosphine oxide (BAPO). The tertiary amine ethyl-4-dimethylaminobenzoate (EDMAB) and the salt diphenyliodonium hexafluorphosphate (DPI) were tested as co-initiators. The following groups were tested: CQ+EDMAB, CQ+EDMAB+DEPI, PQ, PQ+EDMAB, PQ+DPI, PQ+EDMAB+DPI, TPO TPO+DPI, BAPO and BAPO+DPI. Materials were tested both without and with the addition of 10 wt% of deuterium water (D2O). Photoinitiator absorption and the light-curing emission spectra were determined with a spectrophotometer to calculate the relative photons absorption (RPabs). Degree of conversion was obtained with Fourier-transformed infrared spectroscopy (FTIR/ATR). Water sorption and solubility were determined after successive weighting procedures. Results were analyzed by two-way analysis of variance (ANOVA) followed by Tukey׳s post-hoc test (α=0.05). PQ exhibited the highest RPabs values (334.9), followed by CQ (52.8), BAPO (33.7) and TPO (8.4). TPO-based material presented the highest DC in the neat state. Considering the formulations with water addition, TPO- and BAPO-based materials presented the highest values, whereas CQ- and PQ-based ones produced conversion values below 4.6. TPO- and BAPO-based materials presented superior performance than CQ- and PQ-based ones for water sorption and solubility. It was possible to conclude that self-etch adhesives ability to polymerize was not only dependent on the relation between light and photoinitiators spectra, but also individual characteristics of free radicals development and that groups formulated with TPO and BAPO presented superior properties as those formulated with CQ and PQ.