Letting Bonds Flex Their Muscles

The materials used for bonded joints and waterproofing membranes that come under high dynamic stress need to be elastic and tear resistant. Novel silane-terminated polymers from the GENIOSIL® XT series open up the benefits of silane-crosslinking to such applications as well.

View over the roofs of a Spanish city: since the 1950s, flat roofs have become a well-established feature of commercial and multi-floor residential buildings.

When General Motors first bonded windshields into the body of test vehicles in 1963, the car company heralded the demise of prefabricated rubber gaskets. Adhesive joints became standard in the US car industry in 1978, and worldwide soon after – not only because they proved more cost effective, but also because they enhanced safety, making the glass less likely to shatter in the event of an accident. Elastic adhesive joints became a key technology in other sectors as well – particularly in cases where the bond is affected by forces that constantly vary over time.

Elastic bonding is widely used in road and rail vehicles to connect components to the load-bearing structure, such as the body or frame. The building sector, on the other hand, uses elastic waterproofing membranes to compensate for thermally induced stresses in the substrate while reliably bridging cracks.

The extent to which elastic adhesives and waterproofing membranes meet these requirements depends on the binders employed and the compounder’s knowledge. GENIOSIL® XT, a new product series from WACKER, yields, for the first time, silane-terminated polymers which can be formulated to shear-resistant materials of high tensile strength that are at once highly elastic and extremely tear resistant.

Dynamic loads cause adherends to move. They can only resist these loads if the adhesive layer is both strong enough and permanently capable of moving with the adherends, i.e. it must be capable of stretching elastically.

Shock and vibration are not the sole causes of dynamic loads. Fluctuating temperatures exert stress on bonds, too. Such stress arises when two adherends expand or contract to different degrees under the influence of heat or cold.

A waterproofing membrane is a prime example of this. It is applied in liquid form to the surface to be sealed, such as a balcony or terrace, where it cures to produce a seamless waterproof membrane. Because the substrate constantly moves as the temperature fluctuates, the seal experiences substantial dynamic stress. It has to accommodate the movement of crack edges and continue to bridge them under continual expansion and contraction.