The Effect of Chemical Surface Treatments on the Flexural Strength of Repaired Acrylic Denture Base Resin

Purpose: To investigate the effect of the selected chemical surface treatment agents on the flexural strength of heat-polymerized acrylic resin repaired with autopolymerized acrylic resin. Materials and Methods: Ninety heat-polymerized acrylic resin specimens (Meliodent) were prepared according to ISO1567 and randomly divided into nine groups: positive and negative control groups (groups I and II), and seven experimental groups (groups III to IX). Specimens in groups II to IX were cut in the middle and beveled 45 degrees. Group III was then treated with methyl methacrylate (the liquid part of Unifast TRAD) for 180 seconds. Group IV was treated with Rebase II adhesive according to the manufacturer's instructions. Groups V to IX were treated with methyl formate, methyl acetate, and a mixture of methyl formate-methyl acetate at various concentrations (75: 25, 50: 50, 25: 75% v/v, respectively) for 15 seconds. They were then repaired with autopolymerized acrylic resin (Unifast TRAD). A three-point loading test was performed using a universal testing machine. One-way ANOVA and post hoc Tukey's analysis at p < 0.05 were used for statistical comparison. Failure analysis was then recorded for each specimen. The morphological changes in untreated and treated specimens were observed by scanning electron microscopy. Results: The flexural strengths of groups III to IX were significantly higher than that of group II (p < 0.05). The flexural strengths of groups IV to IX showed no significant difference among them (p > 0.05). All specimens in groups V to IX showed 100% cohesive failure, while groups II, III, and IV showed cohesive failure of 10%, 60%, and 60%, respectively. From scanning electron micrographs, the application of methyl formate, methyl acetate, and a mixture of methyl formate-methyl acetate solutions on heat-polymerized acrylic resin resulted in a 3D honeycomb appearance, while specimens treated with methyl methacrylate and Rebase II adhesive developed shallow pits and small crest patterns, respectively. Conclusion: Treating surfaces with methyl formate, methyl acetate, and a mixture of methyl formate-methyl acetate solutions significantly enhanced the flexural strength of heat-polymerized acrylic denture base resin that had been repaired with autopolymerized acrylic resin.