V.A. Kreisberg, V.P. Rakcheev, I.H. Smith
XIX International Congress on Glass, July 1-6, 2001, Extended Abstracts, Oral and Poster Sessions, Society of Glass Technology, Edinburgh, Great Britain (Scotland), p. 669
ABSTRACT. In the total problem of gas incorporation in glass the questions of degassing, gas transport and gas diffusion are considerably less studied than equilibrium aspects of the problem. The increasing interest in gas-transport characteristics is also connected with necessity of modelling the processes in glass on annealing. The processes of degassing and water release from float glasses of different melting methods and various water contents were investigated by kinetic thermodesorption mass spectrometry. The primary impurity released from glass grains of two fractions in temperature range from 450 upto 1050 K is water. At higher temperature upto a temperature of 1650 K other gases (SO2, O2, CO2) are mainly evolved. The quantitative relation of SO2 and O2 are consistent with equations of decomposition of SO3 and sulfates in terms of oxidation of carbon-containing substances by O2 with the formation of CO and CO2. The main desorption of water takes place in two temperature ranges. Dehydration and dehydroxylation of surface and near-surface region of glass cause the first low-temperature thermodesorption peak at 450 K. The position of second thermodesorption peak of water at 850 and 950 K depends on the particle size range of the glass grains. Water desorption in this temperature range proceeds through the mechanism of diffusion. Close agreement between water content values obtained by mass spectrometry and IR spectroscopy was achieved for the glass melted electrically, whereas such agreement for glass melted in a gas air fired furnace is not as good. The coefficients of water diffusion in industrial float glasses were determined from the isothermal parts of kinetic curves of water desorption at temperatures from 750 upto 1050 K. Our data for transition temperature region and the results of McGinnis and Shelby for removal water at high temperatures from float glass melt fit a straight line in Arrhenius co-ordinates. The temperature dependence of water diffusion from float glasses in wider temperature range from 750 upto 1623 K is described by the following equation D[m2/s] = 5.8e-6 exp(-137[kJ/mol]/RT). Water diffusion coefficient values are dictated by the glass composition and do not depend on float glass melting conditions. The full degassing of a glass proceeds under condition of certain optimal relation between temperature, diffusion coefficient value and geometrical size of glass sample.