Effect of moisture absorption on the performance of flax fibre reinforced composites
Authors: Beelen Laurens 
Published: September 2022

The use of sustainable and environmentally friendly materials in today’s society is becoming increasingly important due to mankind’s increasing pressure on the carrying capacity of the planet. Natural fibre reinforced composites (biocomposites) can be used for this purpose as high-performance engineering materials. These biocomposites contain good specific properties and can therefore be used as an alternative to materials such as metals. Among natural fibres, flax has attracted high interest because it is among the highest performing natural fibres. However, these fibres have a hydrophilic character which makes them sensitive to humid environments. In this thesis, the extent to which water absorption takes place in these composites is investigated, as well as the impact this has on their mechanical properties.Several composites were produced for this research. There are three commercial types of flax fabrics that are combined with four matrices. These flax fabrics are FlaxTapeTM and two types of ampliTexTM. Then, there are the matrices, namely polypropylene, maleic anhydride polypropylene and polyoxymethylene, which are thermoplastic polymers. A final matrix that is used, is the thermosetting epoxy which is often used as reference due to the widespread use of as matrix for flax fibre reinforced composites. The selected fabrics are used to assess the impregnation quality due to their different structure. Therefore, optical microscopy images were taken of the various samples using a light microscope.To investigate the influence of humidity on the samples, they are first placed in environments with different moisture concentrations, varying from 23% to 97%. There, they remain until moisture saturation is achieved. One testing group of samples remained in the oven and are used as reference of a dry sample to investigate the properties directly after production and thus compare it with the composites after saturation in the various humidity levels.First, the diffusion speed of the moisture in the samples is analyzed. The samples are measured and weighed on a regular basis to determine the relative swelling and water absorption. Then the diffusion curves for the various composites are drawn up, whereby the diffusion coefficients are also calculated. In addition to the diffusion measurements, the maximum swelling and water absorption are measured for each moisture concentration at saturation. In the case of polypropylene and polyoxymethylene, diffusion takes place more rapidly than with the other matrices. The saturation values are close to each other for all composites with the same flax fabrics.After saturation, the samples were removed from the moist environments to measure the mechanical properties. Both longitudinal and transverse samples were analyzed. Based on these tests, flexural strength, flexural strain at failure and the flexural modulus were measured for the various composites. A decrease in properties can be observed with increasing moisture concentration. However, the decrease was shown to be depended on the type of fabric and matrix. Together with the microscopy images, it was noticed that the impregnation quality has a crucial role to reduce this drop on mechanical performance as function of humidity.Finally, after testing, the fractured surfaces were also studied using a scanning electron microscope. In this way, a larger influence of the compressive region was observed due to the appearance of buckling regions at high humidity levels.