Dual-Cure Adhesive for Fiber-Reinforced Composites

Fiber reinforced composites on the resistance to fracture of vertically fractured and reattached fragments

Thermosettng resins are frequently used as the matrix for fiber-reinforced composites. Since these materials no longer melt after cross-linking, parts made from them cannot be welded. This is why adhesive bonding is used as the joining technique. For this reason, the Fraunhofer Institute for Structural Durability and System Reliability LBF investigates so-called structural adhesives that bond parts together permanently and provide a certain dimensional stability as well. In motor vehicles, such adhesive bonds are considerably more crash-resistant than other types of joints. Moreover, the layer of adhesive exhibits good damping characteristics. Vibrations that generate audible noises or detectable vibrations are reduced by the adhesive.

For one and a half years, the LBF has studied so-called dual-cure adhesives that cure in two steps. Until now, these adhesives were characterized by a constant elasticity. It has now succeeded in producing an innovative dual-cure adhesive with variable elasticity. This means that the layer of adhesive exhibits high strength at the center and behaves elastically at the edges. Thanks to the stiffness gradient, the service life of adhesive bonds is extended noticeably. The increased durability is attributed to the fact that the adhesive can deform better at the edges. Sudden stresses that occur at the edges can, as a consequence, be absorbed. The high strength at the center of the layer of adhesive results in long-lasting dimensional stability of the bond and the adhesively bonded vehicle body.

The initial curing mechanism in the newly developed dual-cure adhesives is initiated by heat. It takes places throughout the entire layer of adhesive and yields a flexible product. A second curing step is then initiated by irradiating certain areas with UV light. The polymer chains cross-link further only at the UV-irradiated areas, increasing the local stiffness.