Engineering and characterisation of the interface in flax/polypropylene composite materials

The main objectives of the present PhD project were to study the interface in flax/iPP composites, to develop and optimise an appropriate surface treatment for improving the interface in flax/iPP composites, and to assess the effect of the applied surface treatments on the properties of flax fibres (both physical and chemical). Two surface treatments, acetylation and stearation, were developed and optimised in the present study. The effect of these two surface treatments upon the structure of flax fibres was studied using XRD, SEM, ATR-FTIR, ToF-SIMS, XPS, IGC, DVS and single fibre tensile testing. It was found that the treatments did not significantly change the fibre strength, but they altered the fibre surface characteristics (chemical and physical). The water absorption was also significantly reduced after treatment, especially for green (as-received) flax, as was shown using DVS. In all cases the fibre surfaces were found to be very heterogeneous (from a chemistry point of view). The effect of the treatments and the processing conditions on the interfacial bond was studied using the single fibre fragmentation test, and it was shown that both treatments resulted in a stronger interface. The development of transcrystallinity was also found to produce a stronger interface, along with the use of slower cooling rates. However, an examination of the tensile properties of short flax fibre composites revealed that acetylation did not significantly change the tensile strength, in comparison with the untreated fibres, probably due to the manufacturing route followed in the present study that resulted in fibre lengths lower than the critical length. SEM post-mortem examination of the composites' fractured surfaces revealed that acetylation improved adhesion with iPP. (author)