Hygroscopic cycling of flax fibre composites: Simultaneous swelling and shrinking of fibre and matrix

Abstract

When natural fibres are exposed to humid environments, they absorb moisture and swell.Often, the natural fibre reinforced composites are produced in a combination with the widelyused non-/low-swelling matrices, like maleic anhydride grafted polypropylene (MAPP) andepoxy. The water uptake resulting in fibre swelling causes the build-up of internal stresseswhen the fibres are surrounded, and thus constrained, by the typically used matrices. Theseinternal stresses eventually lead to damage formation, resulting in loss of mechanicalproperties. The scope of this thesis is to investigate a potential strategy to decrease theaforementioned sensitivity of the natural fibre composites when exposed to hygroscopiccycling conditions. In this strategy the hydrophilic matrix polyamide-6 (PA6) will be used dueto its ability to absorb water and swell, resulting in a composite with decreased stress build-upby the hygroscopic expansion of the fibres, due to a matrix that swells and shrinks togetherwith the fibres.The hypothesis of having a good fibre-matrix interface when adding the hydrophilic matrix PA6to the hydrophilic flax fibres, was evaluated with a wettability analysis. A wettability analysisimplies using contact angle measurements to get an idea of the surface energy componentscalculated following the Van Oss model of both fibre and matrix. These surface energycomponents are then used to calculate the theoretical work of adhesion which shows thequality of the interface. Hygroscopic cycling is being applied on the traditional compositesflax/MAPP and flax/epoxy together with flax/PA6 composites to analyze their behavior after along durability cycling. The moisture content and sample thickness were monitored during theentire cycling process. After every cycle, a group of specimens was removed from the cyclingprotocol and mechanically tested in bending by using the three-point bending method.Therefore, mechanical degradation was monitored every cycle with a total of nine hygroscopiccycles performed. It is of great interest to take a closer look at the sample zone where failurehas occurred and investigate the failure mode. By using scanning electron microscopy (SEM),different damage mechanisms can be detected for the composites between the different testedcycles showing degradation of the materials.This study revealed that the hydrophilic matrix PA6 and flax fibres result in a high work ofadhesion value which shows a strong interfacial bonding and leads to a good impregnation ofthe thermoplastic. Furthermore, this was additionally supported by the high longitudinal andtransverse strength of the flax/PA6 composites as by the investigation of the fractured surfacesof the reference-unaged samples. The flax/PA6 samples obtain higher strength values thanthe flax/MAPP composites at the end of the long durability study. These observations aresupported by the analysis of the SEM pictures that indicate less damaged interfaces within theflax/PA6 specimens. In conclusion, the use of a hydrophilic matrix in natural fibre reinforcedcomposites shows promising results for composite materials exposed to humid environmentsfor a significant period