Effect of microwave postpolymerization treatment on residual monomer content and the flexural strength of autopolymerizing reline resin.

BACKGROUND Microwave postpolymerization has been suggested as a method to improve the flexural strength of an autopolymerizing denture reline resin. However, the effect of microwave postpolymerization on the residual monomer content and its influence on flexural strength have not been investigated. OBJECTIVES This study analyzed the effect of microwave postpolymerization on the residual monomer content and its influence on the flexural strength of an autopolymerizing reline resin (Denture Liner). MATERIALS AND METHODS A total of 70 specimens (64 Chi 10 Chi 3.3 mm) were polymerized according to the manufacturer's instructions and divided into 7 groups (n = 10). Control group specimens were not subjected to any further processing. Before testing, the specimens were subjected to postpolymerization in a microwave oven using different power (550 and 650 W) and time (3, 4, and 5 min) settings. Two specimens of each group were then manually ground into fine powder and samples extracted from the specimens using reflux method. The samples were then subjected to gas chromatography for residual monomer determination in area%. Eight specimens were subjected to a three-point bending device with a span of 50 mm and crosshead speed of 5 mm/min, and the flexural strength was determined in MPa. Data analyses included Student's t-test and one-way analysis of variance. RESULTS For the Denture Liner reline resin, the residual monomer content decreased and the flexural strength increased significantly with the application of microwave irradiation using different time/power combinations. The specimens with the lowest residual monomer content were the similar specimens which presented with the highest flexural strength. CONCLUSION Microwave postpolymerization irradiation can be an effective method for increasing the flexural strength of denture liner (at 650 W for 5 min) by reducing the residual monomer content by further polymerization at free radical sites.