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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
Stoffschlüssige Metall-Kunststoff-Verbindungen durch Verwendung von latent-reaktiven Pulverlacken als Klebstoff - Werkstoff- und Technologieentwicklung
: Gedan-Smolka, Michaela; Kühnert, Ines; Scholz, Peter; Craz, Sebastien le; Garray, Didier

Präsentation urn:nbn:de:0011-n-5158381 (1.9 MByte PDF)
MD5 Fingerprint: 18303bcef92fa231720466554bbf322a
Erstellt am: 7.11.2018

2018, 21 Folien
European Technical Coatings Congress (ETCC) <2018, Amsterdam>
Vortrag, Elektronische Publikation
Fraunhofer IWU ()
reaktiver Pulverlack; Metall-Kunststoff-Verbund; Haftvermittler; Hybridbauteil; umformstabil; Stoffschluss

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.