Strengthening of container glasses by ion-exchange dip coating

The strengthening of industrial grade sodium aluminosilicate glasses was investigated using halide base ion-exchange by dip coating. Li ion-exchange showed a significant flexural strength enhancement of 16-163% up to a maximum value of 506 MPa for container glass composition with progressively increasing network modifier additions of alumina from 3 to 10 wt%. The results showed a linear relationship between square root of exchange duration, t(1/2), and the layer thickness. An optimum layer thickness of 20-70 mu m was found to be adequate to counteract the surface flaws responsible for container glass fracture. Differential thermogravimetry experiments showed a shift in both the transformation (T-g) and melting temperatures (T-m) with concomitant change in viscosity with alumina additions. The surface crystallization phase responsible for strength increase was identified by X-ray diffraction as a low thermal expansion co-efficient (TCE) phase of beta-spodumene (Li2O - Al2O3 - 4SiO(2))/beta-eucryptite (Li2O - Al2O3 - SiO2) with a measured crystallite size of 235-380 angstrom. Some experimental runs of electrostatic spraying were found to have a limited impact in controlling the process parameters in producing a surface crystallized layer thickness of 20 mu m. (c) 2005 Elsevier B.V. All rights reserved.