Pt.111-The Chemical Durability of Glasses

JSGT 1951 V35 T462-T468

The Significance of the Co-ordination Requirements of the Cations in the Constitution of Glass. III. The Chemical Durability of Glasses

In its present form the random network theory of the constitution of glass fails to account for the experimental fact that the substitution of Li+ for Na+ ions in an existing soda–lime–silica glass causes the breakdown of the glass; nor can it account for many of the phenomena observed on a study of the chemical corrosion of glass. The author shows that the reactions between a glass and water can be explained on the basis of the coordination requirements of the silicon ion and its dependence on the polarisability of the surrounding O^2- ions. The action of water on a soda–lime–silica glass leads to the replacement of some Na⁺ and Ca²⁺ ions by H⁺ ions, with the formation of a film of hydrogen glass in which, at first, the Si⁴⁺ ions are incompletely screened. In consequence, H₂O molecules are attracted and the film swells, giving at least partial protection to the glass surface and causing the corrosion to slow down. Two basically different processes are in operation when a solution of orthosilicic acid is allowed to age, namely, polymerisation to satisfy the coordination requirements of the Si⁴⁺ ion and condensation as the result of water being cleaved off. Explanations are offered of the contraction during the formation of Vycor glass; of the action of alkaline solutions on glass; and of the results of Dimbleby and Turner's work on the effect of different ions on the resistance of glasses to attack by water.

 
W. A. Weyl