Impact strength and fracture morphology of denture acrylic resins

Statement of problem. Microwave-polymerization cycles may affect the impact strength and fracture morphology of denture base acrylic resin, and the microstructural effects of these processes have not been fully determined. Purpose. This study evaluated the impact strength and fracture morphology of denture base acrylic resins processed by microwave energy and hot water bath. Material and methods. Twenty specimens measuring 65 x 10 x 2.5 mm were fabricated from each of 4 acrylic resins processed according to the manufacturers' recommendations: Lucitone 550 (control; 9 hours at 74 degrees C); Onda Cryl (3 minutes at 360 W + 4 minutes pause + 3 minutes at 810 W); Acron MC (3 minutes at 500 W); and Vipi Wave (20 minutes at 180 W + 5 minutes at 540 W). The impact strength was evaluated in an impact testing machine using the Charpy method with a load (impact action) of 3.95 J. Mean values Of impact strength were compared by Tukey honestly significant difference test (alpha = .05). Fractures were classified as brittle or intermediate. Fractographic analysis was performed for all fragments by angle analyses of crack propagation, and the microstructural morphology characterization was accomplished with scanning electron microscopy (SEM). Data from the fractography analysis were submitted to the Kruskal-Wallis test for angles and radius (alpha = .05). Results. Significant differences (P < .001) were found in the impact strength for Vipi Wave and Acron MC acrylic resins, which demonstrated the lowest values (0.19 +/- 0.04 and 0.21 +/- 0.02, respectively). Most fractures were classified as brittle (Lucitone 55%; Onda Cryl 75%; Acron MC 90%; Vipi Wave 65%). Fractographic angle analysis of brittle fractures showed no differences among acrylic resins studied; however, angle values of intermediate fractures for Onda Cryl were lower in comparison with those from Lucitone 550 and Vipi Wave (P = .03). The SEM observations revealed that brittle fractures showed defined and organized crystallographic planes, whereas the intermediate fractures had a disorganized appearance. Conclusion. Within the limitations of this study, it was observed that impact strength in microwave-polymerized acrylic resins varies according to the period of irradiation. Acrylic resins exhibited a high number of brittle fractures, irrespective of the processing technique.