Chemical Strengthening of Float Glasses: Effect of Composition and Structure

Two silicate float glasses were considered in the present work: a typical clear soda lime silicate glass and a soda magnesia silicate glass. Both materials underwent chemical strengthening in KNO3, and a different behavior was pointed out in terms of potassium penetration and corresponding surface compression. The soda magnesia silicate glass demonstrated a significantly higher propensity for reinforcement through the exchange of sodium with potassium owing to a much higher interdiffusion coefficient, although the activation energy for the process was substantially identical for the two materials. The difference in performance in terms of Na/K exchange can be only partially correlated to the slightly larger amount of sodium in the soda-magnesia silicate glass, which is instead characterized by a more open structure, richer in five-, six-membered or larger rings, as revealed by NMR and FT-IR/micro-Raman spectroscopy. This structural difference appears to be dictated by the chemical composition of the glasses, soda magnesia silicate material being richer in network formers such as silica, alumina, and, very likely, magnesia, which causes the presence of more abundant Q3 units and more limited Q2 and Q4 ones for a faster movement of the alkaline atoms within the structure.