THERMAL-EXPANSION COEFFICIENT OF DENTAL COMPOSITES MEASURED WITH STRAIN-GAUGES

Objectives. A simple test method was developed to determine the coeffi cient of thermal expansion of prevailing restorative resin composites and to study the transient behavior as a function of temperature and r epeated thermocycles. Methods. Strain gauges were used to determine th e thermal expansion for seven commonly used restorative resin composit es by measuring the instantaneous strain along with temperature change . The temperature was measured by means of a thermocouple, the tip of which was embedded in the composite.The differences among the test gro ups were analyzed using ANOVA, followed by Scheffe's multiple comparis ons test. Results. The coefficient of thermal expansion determined for the composites tested was: 22.5 +/- 1.4 x 10(-6)/degrees C (Z-100), 2 3.5 +/- 1.4 x 10(-6)/degrees C (P-50), 32.6 +/- 1.6 x 10(-6)/degrees C (Herculite XR), 34.1 +/- 1.8 x 10(-6)/degrees C (APH): 35.4 +/- 1.4 x 10(-6)/degrees C (Conquest), 41.6 +/- 1.5 x 10(-6)/degrees C (Silux P lus), 44.7 +/- 1.2 x 10(-6)/degrees C (Heliomolar). The coefficient wa s almost linear in the considered temperature range (26-75 degrees C) for ail composites (r > 0.99) and decreased with each consecutive ther mocycle (p < 0.1). Significance. Thermally induced loads. introduced i nto restored teeth by the mismatch of the coefficient of thermal expan sion of the tooth and the restorative material, may be related to micr oleakage and wear problems. A highly filled hybrid composite such as Z -100 had a coefficient of thermal expansion closest to that of the too th crown, confirming other studies which demonstrated the benefits of high filler loading in matching the properties of the dental hard tiss ues.