Adhesives for Structural Applications
Fundamental Material Properties
Most of the adhesives belong to the group of organic polymer compounds. The polyreactions polymerization, polycondensation, and polyaddition produce molecular structures that through their configuration have a major impact on the properties of a layer of adhesive. The most important factors that influence the final properties are the chemical structure of the monomers, the reactions that take place in order to create the polymers, and the resulting structure of the adhesive. Adhesives are very similar to plastics in terms of their chemical structures and material properties. Consequently, they frequently exhibit a material behavior dependent on duration and temperature. Their heat resistance and durability are generally inferior to those of metals. Environmental influences can damage the adhesive and the boundary layers between the components, and thus can reduce the strength of a joint.
Of all those adhesive systems conceivable and commercially available, a few systems have proved to be particularly suitable for glass applications in buildings owing to their chemical, physical and mechanical properties. These include UV – and light-curing adhesives, epoxy resin, and polyurethane adhesives, as well as silicones. The material behavior of these adhesives varies considerably, ranging from high-strength adhesives with low elongation at failure to highly deformable elastic adhesives that have only a low tensile shear strength (Fig. 6).
According to their material properties, epoxy resin adhesives are thermosets. These products exhibit high strengths but at the same time only very low elongation at failure . Especially important among the UV – and/or lightcuring adhesives are various acrylates and methacrylates. Most of them are classed as thermoplastics and are characterized by their high strengths and comparatively low elongation at failure . Adhesives based on polyurethane include a wide range of materials. Reactive two-part polyurethanes represent the type most often used for glass in building. This group of adhesives includes both thermosets and elastomers. Their strengths are similar to those of acrylates, although much greater elongations are possible . Chemically, MS polymers are very similar to polyurethanes. On the other hand, their curing behavior is similar to the silicones. And in terms of strength and elongation at failure, they fill the gap between polyurethanes and silicones [34,35].
Owing to their chemical structure, silicones are fundamentally different from the other organic, polymer adhesives. In contrast to these systems formed by chains of carbon atoms, in the silicones it is silicon-oxygen compounds that form the elements linking the molecules. The cross-linking at room temperature to form a high-molecular polymer (room temperature vulcanizing, RTV) for the adhesive and sealing systems used for glass in building applications takes place either through the action of the moisture in the air (1-part) or through the addition of a hardener (2-part). The adhesives obtained in this way are classed as elastomers and exhibit very low strengths with extremely high elongation at failure .
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