Reducing the negative effects of chemical polishing in acrylic resins by use of an additional cycle of polymerization

Abstract

Statement of problem The chemical polishing of dentures and orthodontic appliances is an efficient and time-saving technique. However, the process may adversely affect the physical properties of resin. Purpose This study evaluated the effect of an additional polymerization cycle in a microwave or water bath on the physical properties of an autopolymerized acrylic resin submitted to chemical polishing. Material and methods Control groups comprised acrylic resin specimens submitted to mechanical (group 1) and chemical (group 2) polishing. Experimental groups comprised chemically polished specimens submitted to an additional cycle of polymerization in a microwave at 450 W for 3 minutes (group 3) or in a hot water bath at 65° C for 1 hour (group 4). For the residual monomer test, 10 standardized half-disc specimens (30 × 3 mm) were fabricated for each of the test groups, and daily ultraviolet spectrophotometric (206 nm) analyses were assessed for 13 days (μg/cm 2). For transverse strength and internal Knoop hardness (KH) tests, 10 (65 × 10 × 2.5 mm) and 5 (32 × 10 × 2.5 mm) specimens were fabricated for each test group, respectively. Internal Knoop hardness (kg/mm 2) was assessed at depths of 100, 700, and 1500 μm. Transverse strength (MPa) was measured with a 3-point bending test in a universal testing machine with a 10-kg load cell at a crosshead speed of 5 mm/min. The data were analyzed with 1-way analysis of variance, and the means were compared with Student’s t test and Tukey-Kramer intervals ( P<.05). Results At the end of the first day of the test, all groups showed significantly different residual monomer levels ( P<.05). The groups in order of decreasing residual monomer were: group 2 (1315.7 ± 225.5 μg/cm 2), group 3 (848.2 ± 150.4 μg/cm 2), group 4 (295.1 ± 81.6 μg/cm 2), and group 1 (136.6 ± 45.5 μg/cm 2). After 13 days of water storage, the lowest residual monomer levels were recorded for group 1 (4.6 ± 1.6 μg/cm 2). There were no significant differences among group 2 (11.3 ± 3.1 μg/cm 2), group 3 (9.6 ± 1.5 μg/cm 2), and group 4 (12.3 ± 1.7 μg/cm 2). Group 1 demonstrated the highest transverse strength (78.1 ± 6.1 MPa); the other groups were not significantly different from each other. All groups exhibited lower KH values at the 100-μm depth than at 700- and 1500-μm depths. At the latter depths, group 4 specimens demonstrated the highest hardness values (18.0 ± 0.5 and 18.8 ± 0.7 kg/mm 2, respectively). Conclusion Within the limitations of this study, additional polymerization with hot water reduced residual monomer content of chemically polished acrylic resins on the first day. Although additional polymerization increased the internal hardness of the material, however, it did not recover the transverse strength that was decreased by the chemical polishing.