Surface characterization of float glass related to changes in the optical properties after reheating

Abstract

Commercial soda-lime float glass samples, exhibiting different levels of their haze indices after reheating, have been investigated in terms of their surface morphology and surface composition. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations confirm that the origin of the increase in the haze indices is due to the formation of roughness on the bottom face of the glass, which has been described as ‘wrinkling’. Tin depth profiles in the bottom face of the float glass, with and without reheating, were measured by means of SIMS and XPS. It was found that a tin-enriched layer was formed at the surface, due to the migration of tin towards the surface during reheating. There was a linear correlation between the amount of tin enriched at the surface and the haze index after reheating. This observation indicates that the wrinkling is caused by the surface segregation of tin. Furthermore, SIMS depth profiles of iron at the bottom face have revealed that the amount of tin enriched at the surface is inversely correlated to the surface iron concentration. The surface iron concentration is dependent not only on the concentration in the bulk glass but also on the iron concentration in the tin bath and on the atmosphere used during the float process. In particular, iron as an impurity in the tin bath can penetrate into the bottom face of float glass. These analytical results, which have led to the successful improvement of the thermal durability of float glass, are discussed in detail. Copyright © 2001 John Wiley & Sons, Ltd.