Extended creep behavior of dental composites using time–temperature superposition principle

Objectives. The purpose of the study was to characterize long-term creep deformation behavior of selected dental composite resins. Method. Two hybrid composite resins (one midifill type Clearfill and the other minifill type Charisma) and one microfill type (Durafill) were evaluated. 25 mm×10 mm×2 mm rectangular bar specimens were subjected to single cantilever mode of flexural loading and their creep compliance changes determined over 10 min durations in the temperature range of 32–120 °C at 5 °C intervals. The reference temperature selected was the body temperature of 37 °C for the oral environment. All creep compliance curves at other temperatures were shifted along the log time axis to superimpose them on the reference temperature (37 °C) curve to generate an extended isothermal master curve over eight decades of time (seconds). The shift factors were determined as a function of temperature and fitted by the least square method to the William–Landel–Ferry model. Results. The data showed excellent fit to the model, indicating the validity of the model to characterize the long-term behavior using the time–temperature superposition principle. The creep compliance changed from initial mean values of 76, 245 and 605 μm2/N to 3 year mean values of 217, 880 and 1653 μm2/N for Clearfill, Charisma and Durafill systems, respectively. These means were significantly different both as a function of composites and time (p<0.05). Significance. The results indicate that resin volume fraction and filler parameters may play a critical role in the long-term deformation and energy release processes, and these processes may be an important influence on the long-term clinical durability of composite restorations.