Abstract
Thermoplastic composites offer new manufacturing prospects, thanks to the ability to melt the matrix. Welding, tape placement, 3D printing, overmoulding, or even stamping involve adhesion of the thermoplastic polymer at high temperature. First, under heat and pressure, contact at the microscopic scale is ensured by the deformation of surface roughness, this is the intimate contact step. Then, the development of the mechanical strength of the assembly is controlled by the diffusion of macromolecules at the interface which is defined as the healing step. Nowadays, continuous manufacturing processes tend to be faster and present very short residence time which could limit the adhesion development. A good understanding of these mechanisms is therefore very important to control and predict such industrial processes. Welding tests at different temperatures and contact pressures were carried out over a large range of residence times using a controlled welding bench enabling very short welding times (down to 1 second). The mechanical adhesion between PEKK-carbon composite samples was characterized using double cantilever beam fracture tests. Adhesion was found to develop in two steps which could be described as an intimate contact-healing coupled step and a pure healing step. From this, the healing kinetics was identified and an empirical model was developed to account for the effect of pressure on adhesion build-up. This model could then be compared with existing models to describe the establishment of intimate contact between the coupons.
Highlights
Thermoplastic composites have become more and more popular over the last few years mainly tanks to the ability to melt the matrix and because they require shorter processing cycles compare to thermoset composites
Three adhesion regimes have been identified [12]: The first one (I), at the shortest contact times, is a coupled regime where intimate contact is not fully achieved and healing has already started at some spots
In the terminal regime (III), other adhesion phenomena like fiber bridging occur which induce higher values of GIC which are not associated to pure healing
Summary
Thermoplastic composites have become more and more popular over the last few years mainly tanks to the ability to melt the matrix and because they require shorter processing cycles compare to thermoset composites. New forming and assembling processes have recently been developed such as overmoulding, automated tape placement, 3D-printing or welding. In all of these processes the end-quality of the part is ensured by the adhesion step between the thermoplastic pieces. The first step, where the assembly has no mechanical strength, is generally referred to the establishment of intimate contact at the interface which accounts for the initial roughness of both the adherents to explain that contact is not immediately perfect. The most used model was developed by Lee and Springer [3] which simplified Dara and Loos model [4] by considering that the surface of each adherent can be modelled as a succession of identical rectangles that spread under pressure and temperature.
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