Investigation on changes in surface composition of float glass––mechanisms and effects on the mechanical properties

Changes brought about in the mechanical properties on the top and bottom faces of float glass by weathering were compared, and factors governing these properties were investigated. Resistance to the formation of cracks was shown to be improved by weathering, and the level of improvement was significantly higher for the top face compared with that for the bottom face. Secondary ion mass spectrometry depth profiles have shown that a hydrated layer, which originates from the ion exchange reaction between sodium and hydrogen ions, forms more easily on the top face than on the bottom face during weathering. The significantly improved resistance to the formation of cracks on the top face can be explained by the low hardness of the hydrated layer. Furthermore, it has been proved that the Sn2+ ion suppresses the ion exchange reaction between sodium and hydrogen ions on the glass surface. Consequently, the thinner hydrated layer on the bottom face can be ascribed to the existence of the polarisable Sn2+ ion, which penetrates into the bottom face of the float glass during the float process. These results lead to a method for the successful control of the ion exchange reaction, which is important for controlling the mechanical properties of the float glass.