Chemical Degradation of Composite Resins by pH Cycling

Objectives: To evaluate the influence of different pH conditions on the mechanical properties of three composite resins (Tetric EvoCeram [T], Proclinic [P], R&S [R]). Methods: Fifteen rectangular-shaped samples (25×2×2mm) of each composite resin were randomly assigned to three groups (n=5): [1] specimens were immersed in deionized water for 10 days; [2] specimens went on deionized water for 24h and then on 9 pH cycles, during 9 days (6h/day at pH 2.5); [3] specimens were first submitted to the 9 pH cycles and then immersed on deionized water for 24h, always at 37ºC. After conditioning, Vickers hardness (HV) and flexural strength were measured. The microhardness test was realized with Duramin, using 981.2 mN for 15 sec.. The flexural strength was measured using an universal testing machine with 1 KN and a crossheadspeed of 0.75 mm/min. Data were analyzed with ANOVA and Student-Newman-Keuls tests. Results: The HV mean values (HVN) ranged between 15.23±0.97 (group [P3]) and 21.27±0.86 (group [R1]). Proclinic's HV was statistically lower than the two other composites (p<0.05). The mean values of flexural strength (MPa) ranged between 70.68 ±8.20 (group [T2]) and 109.81±20.32 (group [R1]). ANOVA revealed statistical differences just between composites (p<0.001). R&S yielded significant higher flexural strength results than Proclinic and Tetric (p<0.05). Conclusions: Submitting specimens to 9 pH cycles followed by immersion in water for 24h softened the composite resins, however didn't affect their flexural strength. (Work performed at UICOB, I&D unit nº4062 of FCT). filipach@gmail.com