Viscosity of dental porcelains in glass transition range

Changes in the viscosity of dental porcelain as a function of temperature are the controlling factor determining incompatible stresses in porcelain-fused-to-metal restorations. A new method was used to estimate viscosity using a viscoelastic analytical model that considers the relation between the heating rate and deformation temperature (Td) of dental porcelain with stress applied during heating. The activation energy of viscous flow and the viscous constant in the Arrhenius equation were calculated for six kinds of commercial body and opaque porcelains. The effect of the heating rate on the instantaneous coefficient of thermal expansion was also measured. Tg onset was graphically determined using data on thermal deformation and agreed well with the strain point as defined by viscosity. The data gathered in this study was also compared to the experimental results of previous works by others.