Effect of a microwave cycle on properties of acrylic resins

Objectives: the aim of this study was to evaluate the effect of an additional microwave cycle, used as a disinfection method, on the Knoop hardness and glass transition temperature of one heat-polimerized acrylic resin (T), one autopolimerized (A) and one microwave-polimerized (M). Materials and methods: six specimens (2,2 x 2,2 x 2mm) from each denture base materials were fabricated, and three were used as controls and the other half was irradiated with an additional microwave cycle for 1 minute at 850W. Results: the mean values and standard deviations for the microhardness of each group, before and after the microwave treatment, were: T: 19,49 ± 0,19 e 20,35 ± 0,75; A: 18,62 ± 1,53 e 19,07 ± 0,83; M: 18,68 ± 0,98 e 22,58 ± 1,77. The p values obtained with the linked Student's t test were: T (p=0,14); A (p=0,37) e M (p=0,009). There was a significant increase on the mean microhardness values for the microwave-polimerized acrylic resin. The glass transition temperature (°C) values, determined by the Differencial Scanning Calorimetry (DSC), for the control and experimental groups, of each material were: T: 103,65 ± 1,12 e 94,66 ± 1,18; A: 98,88 ± 2,45 e 95,22 ± 3,11; M: 111,3 ± 5,32 e 108,38 ± 6. The not-linked p values of the Student's t test for this property, for the three different materials were 0,0006; 0,18 e 0,56, showing a significant decrease of the Tg for the heat polimerized resin group. Conclusions: according to the results, it is possible to conclude that the microwave additional cycle interfered on the microhardness and Tg of some acrylic resins and it is an alert to evaluate the cost/benefit of this technique.