The focus in this work has been to study if natural fibres can be used as reinforcement in polymers based on renewable raw materials. The materials have been flax fibres and polylactic acid (PLA). PLA is a thermoplastic polymer made from lactic acid and has mainly been used for biodegradable products, such as plastic bags and planting cups, but in principle PLA can also be used as a matrix material in composites. Because of the brittle nature of PLA triacetin was tested as plasticizer for PLA and PLA/flax composites in order to improve the impact properties. The studied composite materials were manufactured with a twin-screw extruder having a flax fibre content of 30 and 40 wt.%. The extruded compound was compression moulded to test samples. The processing and material properties have been studied and compared to the more commonly used polypropylene flax fibre composites (PP/flax). Preliminary results show that the mechanical properties of PLA and flax fibre composites are promising. The composite strength is about 50% better compared to similar PP/flax fibre composites, which are used today in many automotive panels. The addition of plasticizer does not show any positive effect on the impact strength of the composites. The study of interfacial adhesion shows that adhesion needs to be improved to optimise the mechanical properties of the PLA/flax composites. The PLA/flax composites did not show any difficulties in the extrusion and compression moulding processes and they can be processed in a similar way as PP based composites. Udgivelsesdato: July ; The focus in this work has been to study if natural fibres can be used as reinforcement in polymers based on renewable raw materials. The materials have been flax fibres and polylactic acid (PLA). PLA is a thermoplastic polymer made from lactic acid and has mainly been used for biodegradable products, such as plastic bags and planting cups, but in principle PLA can also be used as a matrix material in composites. Because of the brittle nature of PLA triacetin was tested as plasticizer for PLA and PLA/flax composites in order to improve the impact properties. The studied composite materials were manufactured with a twin-screw extruder having a flax fibre content of 30 and 40 wt.%. The extruded compound was compression moulded to test samples. The processing and material properties have been studied and compared to the more commonly used polypropylene flax fibre composites (PP/flax). Preliminary results show that the mechanical properties of PLA and flax fibre composites are promising. The composite strength is about 50% better compared to similar PP/flax fibre composites, which are used today in many automotive panels. The addition of plasticizer does not show any positive effect on the impact strength of the composites. The study of interfacial adhesion shows that adhesion needs to be improved to optimise the mechanical properties of the PLA/flax composites. The PLA/flax composites did not show any difficulties in the extrusion and compression moulding processes and they can be processed in a similar way as PP based composites.