Kinetic fragility of hydrous soda-lime-silica glasses

Abstract The effect of hydration on the kinetic fragility of soda-lime-silica glasses was investigated by viscometry in the glass transition range. Water-bearing glasses were prepared from industrial float glass (FG) and a ternary model glass (NCS = 16Na 2 O 10CaO 74SiO 2 in mol%) by bubbling steam through the melt at 1480 °C and up to 7 bar. Additionally, a sodium borosilicate glass (NBS = 16Na 2 O 10B 2 O 3 74SiO 2 in mol%) was hydrated under equal conditions. As detected by infrared spectroscopy water dissolves in the glasses exclusively as OH-groups. The hydration resulted in a total water content C W up to ≈ 0.2 wt% for FG, NCS and NBS glasses. Kinetic fragility, expressed by the steepness index m , was determined from the temperature dependence of η at the glass transition. Viscosity data from previous studies on hydrous float glasses ( C W > 1 wt%) were surveyed together with literature data on the (H 2 O)–Na 2 O–CaO–SiO 2 , (H 2 O)–Na 2 O–SiO 2 and (H 2 O)–SiO 2 systems to expand the range of water concentration and bulk composition. We could demonstrate that m decreases for all glasses although water is dissolved as OH and should depolymerize the network. An empirical equation of the general type m = a − b log C W where a , b are fitting parameters, enables m to be predicted, for each glass series as function of the water content C W . The enlarged data base shows that the parameter B of the Arrhenius viscosity-temperature relation decreases much stronger than the isokom temperature at the glass transition.