Influence of different initiators on the degree of conversion of experimental adhesive blends in relation to their hydrophilicity and solvent content

Objectives: The aim of this study was to compare the curing reaction of five experimental adhesive blends containing different photo-initiating systems. The hypothesis tested was that degree of conversion (DC) of resin blends is affected by resin type, solvent content and photo-initiating system. Methods: The experimental methacrylate resin blends were ranked from hydrophobic (R2) to hydrophilic (R3 and R4) and tested as neat, or solvated with 10% or 20% ethanol, or 10% ethanol and 10% water. Three different photo-initiators were used: IS-1 = 0.25% CQ (camphorquinone) + 1% EDMAB (ethyl 4-dimethylaminobenzoate); IS-2 = 1.25% TPO (diphenyl(2,4,6-trimethylbenzoyl)-phosphine oxide); IS-3 = 0.25% CQ + 0.50% EDMAB + 0.50% TPO. DC of resin blends was measured with a differential scanning calorimeter. Data were analyzed with a three-way ANOVA. Results: Neat resin type influenced DC, as R4 showed the highest values compared to R2 and R3 (p < 0.05). Solvent had a significant effect on DC (p < 0.05): dilution of resin blends with 10% or 20% ethanol or 10% ethanol + 10% water increased the DC of all resins, except for R4. Initiators influenced the polymerization since neat resins and mixtures solvated with 10% or 20% ethanol showed their highest DC values when polymerized with IS-1 or IS-3 (p > 0.05), while IS-2 or IS-3 increased the DC values of resins diluted with 10% ethanol and 10% water (p < 0.05). Conclusions: Water-compatible photo-initiators such as TPO should be included in the hydrophilic solvated adhesive formulation to ensure an appropriate DC of the adhesive layer.