Comparison of residual monomer content and flexural strength of two auto-polymerizing reline resins subjected to microwave post-polymerization treatment

Abstract

Statement of Problem : Microwave post-polymerization has been suggested as a method to improve the flexural strength of auto-polymerizing denture relines resin. However, the effect of microwave post-polymerization on residual monomer content and its influence on flexural strength has not been investigated. Purpose : This study analyzed the effect of microwave post-polymerization on the residual monomer content and its influence on the flexural strength of two auto-polymerizing reline resins (Kooliner and Denture Liner) and compared its flexural strength. Materials and Methods : For each material, 70 specimens (64x10x3.3 mm) were polymerized according to manufacturer's instructions and divided into seven groups (n is equal to 10). Control group specimens remained as processed. Before testing, the specimens were subjected to post-polymerization in a microwave oven using different power (550 and 650W) and time (three, four and five minutes) settings. The 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%. The specimens were subjected to three-point bending device with a span of 50mm and crosshead speed of 5mm/min and flexural strength was determined in MPa. Data analysis included Student 't' Test and One-way analysis of variance. Results : For Kooliner and 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 post-polymerization irradiation can be an effective method to increase the flexural strength of Kooliner (at 550 W for 5 mins) and Denture Liner (at 650 W for 5 mins) by reducing the residual monomer content by further polymerization at free radical sites. The Kooliner reline resin had superior flexural strength compared to Denture Liner reline resin.