Almost inseparable. Strong adhesion in metal-polymer compounds due to nanoporous adhesive layer
Although the use of different materials is one of the prerequisites for light and high-performance parts, the interface between metals and polymers is often the weak point of hybrid components. Now, however, nanoporous adhesive layers can be used to create permanent, reliable and practical joints. The direct application of a polymer onto a metal component, without the use of additional adhesives and a resource-intensive pretreatment of the surface, is desirable from a technical, economic and ecological perspective. The Fraunhofer Institute for Chemical Technology (ICT) in Pfinztal, Germany, has developed such a process. Plasma-enhanced chemical vapor deposition (PECVD) is used to deposit a thin porous adhesive layer onto a metal surface, in a quick and simple process step. The adhesion of the nanoporous layer results from the infiltration of liquid polymers into the pores. Where the polymer solidifies through cooling or cross-linking with the pores, the interlocking effect leads to very high levels of adhesion. An investigation of the interface strength of PPS metal hybrids shows that the adhesion can be improved by up to 400% by using nanoporous coatings rather than mechanical methods, and by approximately 200% compared to chemical pretreatment. Adhesion is achieved through the mechanical interlocking of the polymer with the pores in the layer. For this reason, in contrast to chemical activation methods, the nanoporous layer does not lose its adhesive strength over time, i.e. the layers can be stored for many weeks without any difficulty. Furthermore, this process is an economically viable alternative to established pretreatment processes. Advantages include the short process times of approx. 10 s and the possibility to coat large surface areas instantly. Both factors compensate for the higher investment costs of the plasma unit. The inexpensive consumable materials silicone oil and oxygen further improve the overall cost effectiveness. In addition, the coated areas are not required to undergo post-processing and can be stored without any adverse effects.