Thermal stability of direct dental esthetic restorative materials at elevated temperatures

With increasing use of direct esthetic restorative materials, the identity of a body may rely upon knowledge of temperature effects on this class of dental restorations. This research examined the effect of atmospheric gas on thermal decomposition and color change of a wide variety of direct esthetic restorative materials. Cured discs (4 x 1 and 8 x 1 mm) were made using manufacturer's directions: traditional glass ionomer (Fuji II), light-curable resonomer (Fuji II LC), compomer (Geristore(TM)), and three types of resin composites - highly filled, urethane-based (Occlusin(TM)), and two Bis-GMA/TEGDMA resins: hybrid (Herculite® XRV(TM)) and microfill (Silux Plus(TM)). Three replications of each material were heated at 5°C/min in a thermogravimetric analysis unit using either room air or nitrogen purge to simulate different thermal environments. First derivative values of percent weight loss with respect to temperature were obtained to determine temperatures associated with increased decomposition rates. Room-air heating showed greater numbers of decomposition events than did nitrogen-heated discs. The only material decomposing less than 200°C in either atmosphere was traditional glass ionomer. The majority of decomposition occurred between 200°and 500°C for all materials. Only products containing glass ionomer components decomposed between 600°and 800°C. Room-air heating resulted in ash white discs at 800°C and higher. Specimens heated in nitrogen were gray to black at 600°C and higher. Heating atmosphere greatly affected color, and some products demonstrated distinguishing color changes: glass ionomers, in particular, showed characteristic color features. An atlas was constructed from color change of specimens recovered after 200°, 400°, 600°, 800°, and 1000°C compared with non-heated controls.