Properties of Poly (Methyl Methacrylate) Polymerized by Waterbath vs. Microwave

OBJECTIVE: Poly (methyl methacrylate) (PMMA) was introduced to dentistry for denture fabrication in 1937. Recent advances now allow denture fabrication with improved resin polymers using injection molding, and the option for polymerization by conventional water bath or microwave radiation. In 1999, Dentsply International introduced an injection molding system (Success) that used its water bath cured PMMA (Lucitone 199®, Dentsply International). That same year, Dentsply also revealed that their PMMA could be polymerized by microwaves using a specialized composite flask. This study compared the flexural strength and porosity of PMMA blocks polymerized for 15 minutes in a microwave oven to a conventional 9 hour water bath cycle. METHOD: The study design was a blinded, observational, laboratory study. Forty eight thin (10mm x 3mm x 40mm) and thick (10mm x 6mm x 40mm) PMMA (Lucitone 199®) blocks were prepared using an injection molding system (Success). Four groups (n=12) of thick and thin PMMA blocks were polymerized either by 9 hours in a water bath or by 15 minutes in a calibrated microwave oven following all manufacturer instructions. Flexural strength was determined using a static load tester (Instron) in 3-point bend mode. Sample porosity was evaluated under light microscopy (30X). Data were analyzed using an independent t-test and two-way ANOVA (SPSS, α<0.05). RESULTS: A significant difference in the flexural strength (p=0.026) was found only between the microwave thick PMMA block (125.63MPa) and water bath thick PMMA block (112.17MPa) groups. No significant differences in porosity were noted between the groups. The flexural strength and decreased porosity of microwave polymerized PMMA equaled or exceeded water bath processed PMMA. CONCLUSION: Polymerization of PMMA in calibrated microwave ovens may be a viable option for acrylic dentures based on the flexural strength and reduced processing time. Funded by the NNMC.