Effect of different curing condition on material properties of acrylic resin for orthodontic appliances

The aim of this study was to evaluate the influence of different curing conditions on degree of conversion, glass transition temperature (Tg), microhardness and impact strength of an autopolymerizing acrylic resin. Twenty-one acrylic resin baseplates with wire clasps simulating removable appliances were produced according to three curing conditions and allocated in three groups (n = 7): Group A—curing in air during 1 h at room temperature and pressure; Group M—treated as in Group A plus additional heating by microwave energy (500 W; 3 min); Group TP—curing in water at 50 °C, during 15 min under 20 psi pressure. Microhardness was measured using a Knoop microhardness indenter, and the degree of conversion and Tg were determined by differential scanning calorimetry (DSC). For impact strength test, 12 non-notched specimens were fabricated per group according to ASTM D256 standard. Data were analyzed statistically by one-way ANOVA and Tukey's test (p < 0.05). Group TP presented the highest microhardness means (p < 0.05), followed by Groups M and A. Group TP showed statistically significant differences on impact strength means (p < 0.05) compared to Groups M and A, which, in turn, did not differ significantly to each other (p > 0.05). No significant differences were found among the Tg values of the three groups (p > 0.05). A better performance may be expected when autopolymerizing acrylic resins are cured under controlled heat and pressure conditions.