In the past decades extreme weather events have become more common as a result of climate change, which is brought on by the increasing concentrations of greenhouse gasses in the atmosphere. Climate change and sustainable development are motivators for the research topic of this thesis, since the prevention of a climate crisis is favourable to the mitigation of its consequences. Emissions and plastic waste can, in part, be decreased by substituting synthetic materials with degradable and sustainably sourced ones since their embodied energy is lower than that of synthetic materials. Flax fibre composites are thought to be capable of competing with glass fibre in various product applications, such as non-essential structures, sports equipment and designer products. Because of the hydrophilic nature of plant fibres, they tend to be subject to incompatibility when paired with hydrophobic polymers such as epoxy, which ultimately means that they have below average interfacial properties. The studies performed on this issue suggest that hydrolysation of the hydrophilic hemicellulose, which is a one of compounds making up plant fibres, with dilute alkali solutions leads to an increment in tensile and transverse properties of flax fibre composites. The interfacial properties of synthetic fibres are best evaluated by performing the pull-out test, the micro-bond test and the single fibre fragmentation test. Since plant fibres are prone to scattered tensile properties, the single fibre fragmentation test is found to be most suitable as it consists of a fully epoxy embedded fibre, increasing the chances of the test to succeed. According to literature, an increase in adhesion manifests as an increased amount fibre fragments in the sample, as well as shorter fragmentation length. Due to the more ductile nature of elementary flax fibre bundles, their fragmentation did not resemble the one observed for synthetic fibres in literature. It was however noticed that the birefringence patterns forming as a consequence of stress concentration, matrix cracks or debonding, could be used as a qualitative indication towards the improvement or deterioration of the interfacial properties. Results were affected by the uncontrollability of independent variables such as fibre diameter, the presence of kink-bands and other naturally occurring fibre defects. Alkalisation was found to affect the birefringence patterns in the single fibre fragmentation test in two opposing ways: by affection adhesion and fibre failure strain. Low intensity treatments showed and increased manifestation of birefringence patterns due to the improved adhesion, while the higher intensity treatments were found to hinder the nucleation and propagation of birefringence patterns because of the decreased failure strain difference between the two materials. Tensile tests of the technical fibres showed that higher intensity treatments led to an increase in failure strain of the fibres due to decrease in micro-fibril angle in elementary fibre bundles and swelling in technical fibres. The applicability of the single fibre fragmentation test in combination with elementary flax fibre bundles is limited due to the hardly controllable independent variables, and is unlikely to provide an accurate quantification of the interfacial shear strength of plant fibres.