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 3to 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 lm 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 b-spodumene (Li2O AE Al2O3 AE 4SiO2)/b-eucryptite (Li2O AE Al2O3 AE SiO2) with a measured crystallite size of 235–380 A ˚ . 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 lm.