A neutron reflection study of the effect of water on the surface of float glass

Abstract

Specular neutron reflection has been used to investigate the incorporation of water into the surface of float glass in a number of different environments. For float glass soaked in water for up to 6 months, two different layers were identified. A surface layer, whose thickness remained constant at approximately 30 Å, contained water at about 40% of the density of pure water. A second layer contained water at about 10% that had penetrated deeper into the sample as the immersion time increased reaching approximately 500 Å in 6 months. An isotope exchange experiment indicated that the lifetime of a water molecule in this layer was about one day. There was a reduction of the glass density in these layers due to the removal of sodium and evidence for gel formation at the surface. When the glass was exposed to a saturated water vapour at temperature up to 80 °C, the water was also found to penetrate into the glass surface over a period of about an hour, but there was no loss of material from the glass. The speed of water penetration was a strong function of temperature. On application of high temperature (150 °C) and pressure (120 bar) the glass underwent rapid ageing when in contact with water. The formation of a visible gel layer was observed, however the penetration depth of the water apparently reduced as the treatment time increased because of dissolution of the gel layer at the surface. Float glass with higher levels of alumina shows a small reduction in the water penetration suggesting that its increased durability resulted from stabilising the sodium ions rather than preventing ingress of water.