THERMAL-EXPANSION OF COMPOSITES

Coefficient of thermal expansion (CTE) has been determined for selected composite materials using differential thermal analysis. Variables evaluated were: type of material, with particular emphasis on filler content; annealing; thermal history, with particular attention being payed to the effects of multiple heating and cooling cycles; ageing in wet or dry conditions. Filler content was a major factor involved in controlling CTE, although clearly other factors such as the type of filler, resin and degree of conversion are important. For an inlay material, annealing at 120 degrees C significantly reduced the value of coefficient of thermal expansion and this is most likely due to an increase in conversion of methacrylate groups. The findings of this study confirm those of previous studies regarding the reduction in CTE following an initial heating. This is most likely due to the relief of internal stress. New information reported here relates to the fact that stress release can occur slowly without heating and that rapid stress release can be achieved through water storage at mouth temperature. These results suggest that, clinically, internal stresses induced by polymerization will be dissipated rapidly. A further finding was that long-term water storage causes an increase in CTE, which may reflect changes at the resin-filler interface.