Temperature and Aging Effects on Hysteresis in Dental Composites

Hysteresis or strain cycling in dental composites results from their viscoelastic behavior. Objectives: This paper examines hysteresis and stiffness when composite specimens at different degrees of maturation and temperature are mechanically loaded. Two hypotheses were tested: 1) hysteresis is higher at elevated temperature and at lower maturation levels; 2) elastic modulus is higher at higher maturation levels. Methods: Rectangular beam specimens (1.0 mm thick x 5 mm wide x 25 mm long) of a commercial BisGMA/TEGDMA hybrid composite (Z250, 3M ESPE) were made and tested in single cantilever beam flexure in a dynamic mechanical analyzer (DMA 2980, TA Instruments). Specimens (n=6) were fabricated in a glass fixture and irradiated at 600mW/cm2 (VIP, Bisco, Inc) in increments across the beam for a total of six minutes. The specimens were preloaded to 0.1N then an additional load was applied by ramping at 10N/min up to 18N then back to 0N, repeated six times. Tests were completed at room temperature and at 37C. Specimens were tested over a period of eight days after fabrication. Results: Hysteresis, as measured by residual strain after each load cycle, was statistically higher by 1.5 to 4.6X at 40C vs room temp (t-test, p=0.014). Residual strain per cycle was lower in specimens with longer periods of aging. Elastic modulus measured 8 days after fabrication was double the value at 5 min and was statistically lower at 40C than at room temp (t-test, p=0.009). Conclusions: Composites have lower stiffness and greater residual strain at oral temperatures than at room temperature. Composite specimens do not demonstrate full strain recovery when unloaded and residual strain increments with each load cycle. NIH/NIDCR 09431.