Volume 1, Issue 7 (July 2004)
Effect of Float Glass Surface Composition on Silicone Sealant Adhesion Tested After Simultaneous Exposure to Hot Water and Ultraviolet Light
The objective of this study is to evaluate the effect of float glass surface composition on silicone sealant adhesion in a wet environment. Float glass samples obtained from six different European glass manufacturers were characterized analytically (XPS) and tested for their hydrolytic stability (DIN 52296). Fifteen tensile/adhesion joints (H-pieces) were prepared for each float glass sample using a two-part, tin-catalyzed, condensation-cure silicone sealant containing a controlled amount of free aminosilane adhesion promoter. Five H-pieces each were tested after four weeks of ambient cure conditioning (reference specimens) as well as after an additional 500 h and 1000 h immersion in 45°C hot water with simultaneous UV exposure. As expected, all reference specimens failed cohesively in the tensile/adhesion test. All specimens exposed to the additional 500 h of hot water immersion and UV also showed cohesive failure. However, after 1000 h of exposure, differences in adhesion behavior could be observed. While the sealant failed cohesively (all five specimens) on one float glass, partial adhesive failure was observed on the other glass samples. For these glasses, the average extent of adhesive failure varied between 4–19%. Because of the limited amount of float glass samples studied, the experimental results must be interpreted cautiously. No correlation is observed between the hydrolytic stability of float glass samples and sealant adhesion. However, the chemical surface composition of the float glass appears to affect sealant adhesion. Low levels of Na and K (added to the glass composition as alkaline oxides) result in better sealant adhesion. Higher surface concentrations of these ions can be tolerated if the glass surface is also rich in Al ions. Poorer adhesion results, however, when high levels of alkaline Na or K oxides are combined with a low level of Al.