Hygroscopic cycling of flax fibre composites: Simultaneous swelling and shrinking of fibre and matrix
Creators: name: Timmermans, Jakob
Publisher: name: Zenodo
Year: 2022
Subjects: Natural fibre reinforced composite
hygroscopic cycling
durability
surface energy
thermoplastic matrices
interface

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