Technology: Adhesive Bonding in Ventilated-cavity Facade Elements
For decades adhesive bonds have played a major role in the tooling and automotive industries, as well as in aeronautics. But does it make sense to use adhesives in facades? Large, high-profile projects employing adhesives are rare. For the Disney Concert Hall in Los Angeles (ill. 5), the structural calculations foresaw employing acrylic-foam tape to mount the 6100 stainless-steel panels (1.20 x 3.0 m), but, in the end, the weight of the facade was borne by clips. In the meantime, adhesives have been developed further, and when structural calculations and codes are met, they constitute a viable option. There are three types of adhesive systems: a. chemically curing (ill. 1); b. self-adhering (ill. 2); c. physical bond. The first two types are more prevalent in the construction industry. Chemically curing adhesives are viscous and react when the second component is introduced (e.g. the ambient humidity), to an elastic or solid material. Single- and multiple-component silicones and polyurethanes, as well as epoxy resins, belong to this group. Self-adhering systems are typically tapes, e.g. of polythene (PE) or polyurethane (PU). The latter possess an elastic foam core that can develop restoring forces that can cause stresses. The adhesive's thickness is in the nanometre range. Thus, compensation for irregularities is only possible to a very limited degree. Double-sided tapes are economical and compress well, but are not suited to structural applications. In acrylic-foam tapes, both the adhesive surfaces and the core are made of permanently adhering polymers. They behave visco-elastically: the adhesive behaviour is dependent on the velocity of the load. When the change occurs slowly, for example, a thermal expansion, the adhesive tape yields to the forces, it is viscous and absorbs stresses. A dynamic load, e.g. a gust of wind, causes an elastic reaction. The two processes can be superimposed upon each other. Acrylic-foam adhesives are durable, very strong, and suited to structural applications. Physically setting systems such as dispersions or adhesion agents are at first viscous. Once they have been applied, the solvent evaporates and the pure adhesive remains and solidifies. Hot-melt adhesives, which re-solidify as they cool, also belong to this group. In their viscous state, chemically curing and physical-bond adhesives are capable of compensating for tolerances and can contribute to the adhesion. Bonding tapes are typically already cured and their adhesive strength increases when pressure is applied.
