Fibre reinforced plastics (FRPs) possess many favourable material properties but at the same time they are also susceptible to weathering effects. This is especially true for epoxy matrix based composites, which are still the composite material of choice in aerospace applications and are renowned for their limited resistance to weathering effects. The common way to protect FRPs from adverse weathering effects is the application of primer and/or paint layers subsequent to the main manufacturing process. Current coatings contain a large fraction of organic solvents which are released into the environment during the coating process. The emission of volatile organic compounds (VOC) from solvents has recently come under scrutiny, leading to the introduction of stringent environmental regulations around the world.
This research investigates new methods to reduce the emission of VOC in the coating process of aerospace FRPs. The investigation is based on the manufacturing process for FRP components for the MH90 and AHR Tiger military helicopters, for which composite parts are manufactured by the Eurocopter subsidiary Australian Aerospace Composites. An analysis of the manufacturing process has revealed that the primer coating is the coating comprising the greatest potential to reduce VOC emissions. Based on literature research, in-mould priming using thermoplastic polymer materials was identified as the coating method with the greatest potential of reducing VOC emissions. Suitable methods to apply the thermoplastic primer coatings and ways of integrating the new coating process in an aerospace manufacturing process are discussed. It was shown that the proposed in-mould priming process not only reduces VOC emissions by up to 205 g/m2 compared to current aerospace coatings, but also reduces labour intensive surface preparation.
Subsequent to the selection of an in-mould coating method, thermoplastic polymer materials as a feedstock for the new coating were selected based on requirements derived from military coating specifications and information provided by Eurocopter. In order to determine the suitability of the thermoplastics as a potential feedstock for in-mould primer coatings, the interface adhesion strength of the interface between the primer coating and the FRP substrate, as well as the interface strength between the topcoat and the primer coat are measured using tape, single lap-shear and pull-off adhesion tests. Polybutylene terephthalate (PBT), Polyvinylidene fluoride (PVDF) and the two Polyamidea PA11 and PA12 were found to be the most promising candidate materials. A demonstrator component based on a real helicopter component was manufactured using the Australian Aerospace Composites manufacturing facility. The successful manufacturing of this component demonstrates the practical feasibility of the developed in-mould priming process.
Motivated by difficulties encountered when attempting to measure the interface adhesion strength of thin films adhering to rigid substrates, the shaft loaded blister test was investigated as an alternative test method for the characterisation of thin film interface adhesion. Finite element analysis (FEA) is employed to study the effects of non-linearities on the accuracy of the analytical solution for the shaft loaded blister test. The FEA model was validated using constrained blister test measurements showing a good correlation between the experimental and the FEA data. The analytical solution was then compared with the energy release rate obtained from J-integral evaluation in the FEA. Simplifications of the analytical solution are discussed using a random sampling method and it is shown that the thickness ratio between film and substrate can be ignored for thin films on rigid substrates. Further, values for the angular quantity, ω , which is required to calculate the mode mix phase angle are tabulated for the case of thin, elastic films on stiff substrates using a crack surface opening displacement extrapolation method.
Previous research conducted by the Corporate Research Centre for Advance Composite Structures (CRC-ACS) has suggested that selected thermoplastic polymers form an interphase region when co-curred with epoxy resins. Where an interphase is present the thermoplastic mixes with epoxy resin leading to a gradual transition between the two materials. It has been assumed that this interface is in the micrometer range. These previous results have motivated the investigation of the interface morphology of the interface between the epoxy resin and the selected thermoplastic polymers. It was found that the previously reported interphase dimension in the micrometer scale was based on incorrect energy-dispersive X-ray spectroscopy (EDX) measurements. Monte Carlo simulation is used to estimate the size of the error. Based on these new findings it is shown that EDX has insufficient resolution for the investigation of interphases smaller than 1 µm. Ultra low acceleration voltage scanning electron microscopy and phase angle atomic force microscopy techniques were developed as part of this research as better suited alternatives. The results obtained with these techniques show that all semi-crystalline thermoplastics investigated do not exhibit interphases in the micrometer range (> 1µm) with the epoxy resin and the processing conditions used in this research. However a phase separated interphase was discovered between the epoxy resin and the amorphous, thermoplastic, Polyetherimide (PEI).