Influence of drying plaster in some properties of acrylic resin

Objectives: The aim of this study was to verify the influence of previous dried plaster used in an acrylic resin polymerization method, evaluated by flexural strength and dimensional changes of material. Method: A heat-activated denture base resin, Wave Vipi (Vipi Dental Ltda), was used. The following groups were formed: Group 1A - Vipi-Wave microwave polymerization, untreated gypsum inclusion; Group 1B - Vipi-Wave microwave polymerization, prior treatment with the inclusion of gypsum (plaster dried); and Group 2 - Vipi-Wave heat-cured in water bath. For flexural strength analysis, we manufactured acrylic resin bars (65 mm x 10 mm x 3.0 mm), and dimensional change was accomplished by previous construction of a gypsum cast simulating an edentulous maxillary arch, from which we made upper maxillary complete dentures (n = 6). The cast was dried in a microwave oven (10 min/600 W) in Group 1B. The flasks remained for 2 hrs at 37°C ± 2°C, and then the acrylic resin processing method was undertaken. The specimens were stored in distilled water (37°C ± 2°C for 48 hrs ± 2 hrs), the three-point bending test was applied (n = 9), and results were obtained (MPa). Analysis of dimensional change was measured by the paths between 6 pre-determined points on the occlusal surfaces of teeth, with the software Image Tool. Data analysis was evaluated by ANOVA and Tukey's test (p < 0.05). Results: Means and standard deviations of differences in measured linear distances, before and after prosthesis polymerization (molar to molar, M-M'), in mm, were: G1A (3.48 ± 0.35), G1B (0.48 ± 0.44), and G2 (2.07 ± 0.54); and for flexural strength, means and standard deviations were: G1A (94.40 ± 16.72), G1B (104.15 ± 21.38), and G2 (147.28 ± 36.48). Conclusion: It was concluded that the previous dried plaster increases the flexural strength of acrylic resin, and reduces dimensional shrinkage resulting from the polymerization procedure.