Thermoplastic fiber reinforced composites (FRP) exhibit a number of advantages that have led to the increased use of thermoset FRP materials in aerospace structural material applications. In particular, the high-performance thermoplastic polyetheretherketone (PEEK) fulfills the strict and demanding material requirements of the aerospace industry. To make the best possible use of the lightweight construction potential of FRP, structural adhesive bonding has been explored as a potential alternative to conventional joining methods. Unfortunately, PEEK has so far been considered to be too difficult to bond adhesively. Furthermore, no consistent explanation or model in the literature exists that can completely describe adhesion between the joining part and the adhesive. This understanding of adhesion is, however, a prerequisite for airworthiness certification and the implementation of structural bonds in order to create safe and reliable adhesive bonds. At first, the influence of different pretreatment processes (vacuum blasting, vacuum UV radiation, low-pressure plasma) on PEEK was investigated with respect to the modification of the surface properties. The physical pretreatment processes and especially the VUV treatment, led to high-strength and ageing-stable adhesive bonds. For a detailed understanding of a surface with good adhesive properties, the pretreatment processes were further varied (variation of the process gases, rinsing the samples with polar liquids and a fluorine coating of the samples). Contrary to the published textbook theory and popular opinions in the current literature, the results experimentally confirmed that adhesion does not take place through the formation of covalent bonds, but primarily determined by Van der Waals (VdW) interactions. Thus, the adhesive bond strength was found to be determined by the surface roughness instead of the surface chemistry. In order to allow as much VdW interaction as possible by increasing the surface area accordingly, the scale of this roughness was decisive. This scale lies in the nm or molecular range, for example, caused by a chemical etching process, which can be initiated e.g. by physical pretreatment processes. The results could also be successfully substantiated through tests with other thermoplastics (polyetherimide, polyethersulfone) and thermosets (RTM6, 8552/IM7). The developed, generally valid model for adhesion can completely describe all regarded samples that showed high adhesive strength, regardless of the substrate material, the treatment method, the process gas used, or whether the samples were additionally rinsed or coated with fluorine.    «

Thermoplastic fiber reinforced composites (FRP) exhibit a number of advantages that have led to the increased use of thermoset FRP materials in aerospace structural material applications. In particular, the high-performance thermoplastic polyetheretherketone (PEEK) fulfills the strict and demanding material requirements of the aerospace industry. To make the best possible use of the lightweight construction potential of FRP, structural adhesive bonding has been explored as a potential alternativ...    »