Effect of thermal treatment on flexural strength of bis-acrylic resin

Abstract

Objective: The aim of the present study was to evaluate the effect of post-polymerization heat treatment on the flexural strength (FS) of two bis-acrylic resins. Materials and Methods: Two commercially available autopolymerized bis-acrylic resins were tested: Structur 3 (S3), (VOCO GmbH, Cuxhaven, Germany) and Protemp 4 (P4), (3M ESPE, Minnesota, USA). A stainless-steel mold was used to fabricate the specimens with a dimension of 25x2x2 mm according to ISO 4049:2000 standards. A total of 50 specimens were fabricated using each material (N=100). After fabrication, specimens were randomly allocated to 10 experimental groups (n=10) and immediately subjected to the respective post-polymerization heat treatment used: no treatment, nt; conventional hairdryer 2200 W at a distance of 20 cm at a temperature of 60°C for 1 minute, hd1; microwave at 750 W for 1 minute, mw1; water bath at 60ºC for 1 minute, wb1; water bath at 60ºC for 5 minutes, wb5. Specimens were stored in a dry environment at room temperature (20±2ºC) until testing. Three-point bending test was performed at 30 minutes after initiating the mixing process. Additionally, a positive control group was made for each material in which the specimens were not submitted to any type of post-polymerization heat treatment and were tested at 24 hours after the beginning of the mix (S3-nt-24 and P4-nt-24). Data was submitted to ANOVA (α = 0,05). Results: All post-polymerization treatments significantly (P<0.001) increased the FS compared to the untreated group measured at 30 minutes for the two resins. The S3-wb5 group had statistically similar values (P> 0.05) to S3-nt-24. P4-hd1, P4-mw1 and P4-wb5 did not show significant differences (P >0.05) compared to P4-nt-24. Conclusion: Post-polymerization heat treatments increased FS of the two resins tested. It is possible to obtain FS values at 30 minutes similar to those obtained at 24 hours with specific treatments.