The diffusion mechanism of tin into glass governed by redox reactions during the float process

The diffusion mechanism of tin into glass was investigated using a lab-scale float apparatus, with which it was possible to independently vary the heating parameters, in order to determine the reasons for the characteristic tin penetration profile of float glass. Tin penetration profiles of glass samples with heating at various temperatures, times and atmospheres were measured by means of SIMS. The tin enriched inner layer, which is characteristic of the tin penetration profile of float glass, was seen to be formed by heating at more than 800°C. It was found that the depth of the tin enriched inner layer was proportional to the holding time at the maximum temperature during the heat treatment, and was inversely proportional to the Fe3+ concentration in the glass. It was also proven that the tin enriched inner layer was formed by penetration of hydrogen from the atmosphere through the molten tin into the glass. These facts indicate that the reaction between hydrogen and Fe3+ is involved with the formation of the tin enriched inner layer. Consequently, it has been proposed that the formation mechanism of the tin enriched inner layer is governed by two redox reactions and the diffusion behaviors of both Sn2+ and Sn4+. Namely, one of these two redox reactions is the reduction of Fe3+ to Fe2+ due to hydrogen, resulting in the formation of a reduced surface layer. Another is the oxidation of Sn2+ to Sn4+ due to Fe3+ in the glass. Furthermore, it was revealed by the analysis of commercial float glass with various thicknesses that the depth of the tin-enriched inner layer is also influenced by the stretch coefficient for the attenuation of the glass during the float process. These analytical results leading to a successful control of tin penetration into glass during the float process are discussed in detail.