Viscosity and glass transition temperature of hydrous float glass

Viscosity of water-bearing float glass (0.03–4.87 wt% H2O) was measured in the temperature range of 573–1523 K and pressure range
of 50–500 MPa using a parallel plate viscometer in the high viscosity range and the falling sphere method in the low viscosity range. Melt
viscosity depends strongly on temperature and water content, but pressure up to 500 MPa has only minor influence. Consistent with
previous studies on aluminosilicate compositions we found that the effect of dissolved water is most pronounced at low water content,
but it is still noticeable at high water content. A new model for the calculation of the viscosities as a function of temperature and water
content is proposed which describes the experimental data with a standard deviation of 0.22 log units. The depression of the glass tran-
sition temperature Tg by dissolved water agrees reasonably well with the prediction by the model of Deubener [J. Deubener, R. Mu¨ ller,
H. Behrens, G. Heide, J. Non-Cryst. Solids 330 (2003) 268]. Using water speciation measured by near-infrared spectroscopy we infer that
although the effect of OH groups in reducing Tg is larger than that of H2O molecules, the difference in the contribution of both species is
smaller than predicted by Deubener et al. (2003). Compared to alkalis and alkaline earth elements the effect of protons on glass fragility is
small, mainly because of the relatively low concentration of OH groups (max. 1.5 wt% water dissolved as OH) in the glasses.