Preliminary Study to Select Adhesive
There are more than 250000 industrial adhesives available worldwide, which suit different types of applications and fulfill diverse requirement criteria . Taking into account the specific application profiles for the use of glass as a structural material, it is possible to reduce the wealth of different adhesives to just a few candidates. The material of glass is characterized by the hydrophilic surface. The moisture of the surface cannot be removed permanently as it is build up constantly by the humidity. Concerning the bonding of glass, that surface acts as a barrier. The adhesive must be able to break through (for example, by adding promoters or primers) and to build up stable adhesion forces. Furthermore, the specifications for the applications are diverse, which means that adhesives for structural glass include types with medium modulus, e. g., silicones, as well as adhesives with high and ultra-high modulus, e. g., epoxy resin and polyurethane adhesives. Bearing in mind the specific needs of this project, drawing up a shortlist requires a differentiated analysis and the compilation of the requirements with which the adhesives must comply:
• suitability for glass-glass joints,
• full-bond joints (300 x 300 mm2),
• minimum joint thickness of 1.5 mm (compensation of tolerances, pouring),
• highly transparent and clear for a joint thickness up to 2 mm,
• curing possible in joints up to 2 mm thick,
• viscosity of 15000 to 50000 mPas,
• low shrinkage in volume,
• few air bubbles in the joint,
• resistant to moisture and UV radiation, and
• load-bearing over a temperature range from —25 to + 75°C.
As an example, the joint thickness was influenced by conflictive factors: the tolerances of the laminated glass elements, the curing behavior of the adhesive, the filling process, the structural behavior of the flexible corner joint, and,
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finally, the visual appearance. Flat glass panels with high aspect ratios as for glass beams tend to warp during the tempering process. Hence, a minimum gap of 1.5 mm was necessary to avoid contact between the individual glass panes of the post, as well as to provide sufficient space for a nozzle to fill in the adhesive. In contrast, a thick bond line may reduce the stiffness and the transparency of the joint and impair the curing of the adhesive.