Thermal analysis of visible-light-activated dental composites

Abstract

In this study the complex cure behaviour of a commercial dental composite, activated by visible light, is analysed by differential scanning calorimetry (DSC) and thermo-mechanical analysis (TMA). Flat plane samples must be used in both techniques in order to prevent the temperature increase due to the exothermic nature of the curing reaction and to avoid the effect of light absorption through the thickness. The isothermal cure of the studied material leads to low values of the final degree of reaction and glass transition temperature that may adversely affect the physical and mechanical properties and the environmental resistance of the composite. The linear shrinkage measured by TMA is a function of the aspect ratio of the sample as a consequence of the adhesion between the sample and the glass plates. True linear shrinkage can be measured only if a low-friction coupling agent between the sample and plates is employed. Isothermal TMA and DSC data, collected at the same temperatures, are processed in order to obtain the degree and the rate of reaction. The comparison of the kinetic behaviour resulting from DSC and TMA indicates that volume relaxation effects due to the viscoelastic nature of the composite matrix may be responsible for the delayed response in TMA. The dark reaction detected by TMA after interruption of light exposure suggests a higher sensitivity of this technique at low rates of reaction compared with DSC.