Effect of storage time on microtensile strength of polyacid-modified resin composites

Objective. The purpose of this study was to determine the cohesive microtensile strength of four polyacid-modified resin composites (PAMRC: Dyract, Dyract AP, Freedom and F2000) after 24 h and 6 months of storage in water using the microtensile technique. Methods. Blocks measuring 1.5 cm×1.5 cm×1.0 cm were constructed incrementally for each material using a silicone matrix. The blocks were stored in distilled water at 37 °C for 24 h and were then serially sectioned in both x and y directions to obtain several beams measuring 1.0 cm×1.0 mm×1.0 mm. Each beam was gently trimmed from two sides with ultra-fine diamond burs to reduce the cross-sectional area to a ‘neck’ of approximately 0.4 mm2 located in the middle of their length. The beams (18–22 per material) were randomly divided and tested in tension either after 24 h or after 6 months of storage in water at 37 °C in a Vitrodyne testing machine at 0.6 mm/min. Results. Mean microtensile strengths (MPa) at 24 h were: 48.7±13.8 for Dyract AP; 36.5±14.1 for Dyract; 38.2±14.5 for Freedom and 48.6±7.2 for F2000. There was no statistically significant difference among the materials (p>0.05). At 6 months, only Dyract AP showed significant increase in microtensile strength (p<0.05) while the other materials showed no differences in microtensile strength with time (p>0.05). Significance. The results indicate that storage in water for 6 months caused no reduction of the microtensile strength of PAMRC materials.