The effects of leaching and silicones on the surface conductivities of sealing glasses

GT V01 1960 069-079

In a previous paper (J. Soc. Glass Tech., 1958, 42, 227 T), the authors showed the necessity for removing a proportion of the alkali from the surface of a glass of high alkali oxide content before applying a silicone film to ensure resistance to atmospheric weathering. The durability of the silicone film in severe climatic conditions was tested using the surface conductivity of the glass at room temperature as a criterion. The present paper is concerned with the various factors which con- tribute to the measured values of this surface conductivity, for example the time of exposure to humidity, polarization due to continuous direct current, and the influence of leaching and baking procedures employed for the surface pre treatment. Four types of glasses were studied: a high alkali glass, a soda glass, a lead glass and a borosilicate glass. The initial conductivity of the untreated glass surfaces was dependent upon the alkali oxide content of the glass composition. The initial rate of change of conductivity with exposure time was also influenced by the alkali content in a similar manner. Siliconed samples of these glasses were subjected to an accelerated climatic test at 55OC and 95% relative humidity. In all cases the breakdown of the silicone film was shown to be inevitable in these severe conditions, but a leach-bake procedure definitely improved the performance of the soda glass and gave a marginal improvement with the lead glass, but produced a deleterious effect with the borosilicate glass.

J. Edge & L. F. Oldfield