Rheological properties and isochomal workability of lead silicate glass melts

Four different industrial lead silicate glass melts were investigated by means of the cylinder compression method with respect to the flow and stress generation behaviour, the critical deformation rate, h(c), and the high-temperature tensile strength, sigma(ts). Increasing PbO concentration has usually a large influence on the Newtonian viscosity, it makes the melt ''shorter'' (more modernly speaking: more ''fragile''). On the other hand, the influence on the non-Newtonian flow and stress generation behaviour and therefore, on the brittleness is very small, however, the high-temperature tensile strength is markedly negatively influenced. At low viscosities the melts with large PbO content show larger critical deformation rates and therefore, a better isochomal workability than those with smaller PbO contents. As compared with soda lime silicate glass melts the lead silicate melts reach clearly smaller h(c), and sigma(ts) values at comparable degrees of deformation and therefore they have a worse isochomal workability. This unexpected behaviour is discussed under structural aspects.