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, t1/2, and the layer thickness. An optimum layer thickness of 20–70 μm 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 (Tg) and melting temperatures (Tm) 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 β-spodumene (Li2O · Al2O3 · 4SiO2)/β-eucryptite (Li2O · Al2O3 · SiO2) with a measured crystallite size of 235–380 Å. 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 μm.