For 20 years now, car makers have been reinforcing the welds in car bodies with high-performance adhesives. By making the car body considerably stiffer, these reinforcing adhesives have made it possible to use thinner metal panels – making the cars lighter. Lighter car bodies mean improved performance and lower fuel consumption, and hence reduced output of climate-damaging carbon dioxide.
 The conventional reinforcing adhesives do, however, have one major drawback: Although they easily cope with the mechanical stresses of everyday motoring, they are unable to withstand a serious crash. Because of their brittleness, they tear open like a zipper. This is why car makers don’t use these adhesives in critical crash-safety components. For the safety-critical areas in the car body, car manufacturers therefore need structural adhesives that are so tough that they won’t rupture in a crash – and still perform equally well in all other respects.

Safe, safer, crash-resistant
To  develop such  high-performance adhesives, a team of scientists and product developers systematically investigated the relationship between the chemical composition of the adhesives, their internal structure (technically referred to as morphology), and the resultant mechanical properties.
Structural adhesives usually consist of a brittle adhesive matrix with spherical islands of soft, rubber-like polymers. These islands only begin to form when the adhesive is curing (hardening). The experts use the term ‘conventional rubber-toughened’ for this kind of morphology. Crash-resistant adhesives, however, call for a much more complex structure.
The key to crash resistance is the morphology of the cured adhesive. Henkel has developed a method of distributing soft particles, only a few hundred nanometers in size, uniformly throughout the matrix before curing of the adhesive. This is an entirely novel technology. Using this method, the experts can precisely control the adhesive’s morphology and internal interfaces and thus adjust its mechanical properties.

With this technology, the Henkel specialists have created a new generation of crash-resistant structural adhesives with especially high thermal stability, between -40 and +90 degrees Celsius, and a crash resistance that is more than ten times better than that of reinforcing adhesives. This has been demonstrated by drop tower experiments in which the experts test the mechanical strength of metal-to-metal bonds.