Substance to substance bonded metal-plastic joints by the use of latent reactive powder coatings as adhesive - Material and technology development: Presentation held at European Technical Coatings Congress, ETCC 2018, Towards a bright future, new coating technologies, June 26 29, 2018, Amsterdam

Caused by the demands of lowering the fuel consumption and the aimed e-mobility the automotive and transportation industry enhance their efforts of weight-reduction. For that reason structures in multi-material design, e.g. polymer-metal composites, and polymeric or polymeric fiber reinforced structures are used in increasing amounts. Large differences of thermal expansion coefficients make it difficult to join metals and polymers. So the connection of incompatible materials is mainly performed by form- or force-fit so far. The steadily increasing use of those material combinations and additional requirements regarding with corrosion protection and impermeability make it necessary to change the joining technologies from mechanical joining to adhesive bonding. Therefore usually a further technology step is needed whereas the glue is applied, followed by the assembling. Our goal was to develop powder coating materials with an additional well defined latent adhesive function in combination with an efficient technology which enable highly efficient production lines for metal-plastic hybrids. As found in the IPF earlier, in the presence of Zn-II-acetylacetonate the reaction between uretdione crosslinkers and OH-group polyester results in a two-step curing mechanism. At lower crosslinking temperatures up to 150°C a polyallophanate network is formed selectively. At temperatures higher than 190°C the polyallophanate network can be transferred into polyurethane structures very fast and completely. Already in the polyallophanate state the powder coating material resists high deformation and complex forming operations and therefore the powder can be applied to a metal substrate, in our case aluminum, as a coil or cutted board application. Based on this two-step curing mechanism various low temperature curing and highly flexible powder coating formulations were developed in this study. Furthermore, the shaping and assembling behavior of an adhesive polymer-metal composite was examined whereby the polyallophanate layer acts as latent reactive adhesive and high quality surface finish simultaneously. To generate the metal-polymer hybrid, a thermoplastic polyurethane (TPU) was over-molded onto the pre-coated aluminum substrate by injection molding. In this manner metal-polymer joints based on flexible polymers can be kept. Furthermore, on top of the TPU a polyamide 6 layer can be over-molded to generate a stiff composite structure. The TPU in between the precoated metal and the polymer top layer acts as stress and strain compensation which can result from thermal expansion as well as external deformations. A covalent bonding between the powder coating film and the thermoplastic polyurethane generates substantially higher adhesive strength in contrast to physical adhesion for the hybrid composite. The described metal-polymer hybrids can be fabricated in a shortened process chain, which was another key feature of the project. The prefinished coil or board materials could be formed (e.g. by incremental forming, deep drawing) and inserted to the over-molding procedure. In this manner an effective coating process with less cleaning steps and a high versality in the follow up composite production can be achieved

Sheet Metal Design Approach for 3D Shaped Facade Elements with Integrated Solar Thermal Functionality

This paper provides an overview of the development of a 3D formed and metal-based facade element that combines a custom design and solar thermal functionality. To achieve this, a novel simplified solar thermal collector structure was developed using formed sheet metal half-shells with an integrated channel structure on the inside and a special absorber coating on the outside. The sheet metal half-shells were manufactured by highly innovative incremental sheet forming (ISF), which allows seamless integration into existing facades. As a part of this paper, the initial test results on thermal efficiency and the energy accumulation of the new collector type are presented

PreFiHy - Prefinished light metal polymer hybrid parts

Within the framework of the European collective research project the project consortium consisting of the Fraunhofer Institute of Machine Tools and Forming Technology IWU together with the Leibniz Institute of Polymer Research Dresden (IPF) and the Belgian Centre for Research in Metallurgy (CRM) and The Collective Centre for the Belgian Technology Industry Sirris deals with the development of relevant technologies and process routes for the efficient manufacturing of ready to use hybrid parts. The innovative approach of the project is the connection between the metal and the plastic that shall consist of adhesive bond in the form of a 2-in-1 (adhesion promoter + surface finishing) powder-coating system. In the novel functional powder coating formulation a latent adhesive function is integrated which can be used for chemical bonding in subsequent efficient industrial processes (e.g. injection moulding) in order to create structural parts. Thus, additional thermoplastic materials can be connected to the pre-coated metal resulting in multi-material parts with high-bond-strength in a cost-efficient way. Product costs can be lowered by a quarter of current final product costs by introducing this high-performance, 2-in-1 coating system to the market saving several pre-treatment steps and additional glue application. On that basis several powder coating systems were produced by the Leibniz Institute of Polymer Research Dresden using the new crosslinking mechanism. The aim of the coating development was to achieve a maximum bonding strength with retention of the forming stability at the same time. In parallel the Belgian Centre for Research in Metallurgy developed a large-scale method which makes it possible to pre-treat the aluminum sheets in an efficient way using roll coating technology. In order to evaluate the forming stability of the various powder coatings as a function of the pre-treatment method, the Fraunhofer IWU developed a testing concept from forming point of view. With the aid of this method, preferred coating systems could be selected by applying an evaluation matrix. Using an interior demonstrator component from the automotive sector, the applicability and the advantages of the novel joining technology could be demonstrated by the example of a 3D complex geometry. In summary, it can be seen that a low-cost method has been developed on the basis of the project results, enabling the industry to efficiently produce hybrid composites with excellent bonding strength. By eliminating cost-intensive pre-treatment steps, the process chain can be shortened and costs can be saved