Ion exchange of monovalent ions in float glass

Stefan Karlsson

Ion exchanged glasses are made for several different purposes, e.g. chemical strengthening, colouration and optical waveguides. Chemically strengthened float glass is considerably more expensive to produce than thermally strengthenened, but will likely find applications in future building and interior constructions where strength demands, design and shape prevent the use of thermal strengthening. The aim of the presented work is to study ion exchange processes on float glass surfaces. In longer terms, the chemical strengthening is planned to be applied to specific critical area e.g. around a drilled hole which without treatment deteriorates the overall strength of the glass. In this study, the less frequently studied approach single-side ion exchange of different ions of commercial float glass is described. The concentration vs. depth profiles were determined either by the use of the Surface Ablation Cell (SAC), which allows the continuous removal of the material from a flat glass surface by slow controlled isotropic dissolution. The results show that similar diffusivities and concentration vs. depth profiles are achieved with single-side ion exchange as from the traditional way of immersing glass in molten salt bath. Ion exchange of Ag+ stains the float glass on both sides giving it a yellow or amber-brownish colour. Unlike Ag+ ion exchange of Cu+ stains the float glass on the tin-side only, giving it a yellow, red or red-brown colour. Determining the concentration vs. depth profiles of ion exchanged float glasses with the SAC was convenient except for Ag+ which was determined with SEM-EDX. The work confirms that the procedure and equipment of the SAC are cheap, easy to use and gives data similar to those gained by other surface analytical techniques. The Calculated diffusion coefficients of K+, Ag+ and Rb+ are in accordance with literature data while Cu+ and Cs+ diffusion coefficients were slightly lower. The diffusion coefficients of the different ions follow the order Ag+>K+>Cu+>Rb+>Cs+ and ranges between 9.4E-10 and 4.8E-13 cm2s-1. The calculated activation energies for diffusion of K+, Ag+ and Cu+ corresponds with reported literature data and were calculated to: Ag+(air-side) 152 kJ/mol, Ag+(tin-side) 185 kJ/mol, K+ 108 kJ/mol and Cu+ 115 kJ/mol.