18Plant fibres have a unique set of properties ranging from being stiff and brittle, such 19 as hemp and flax, to more ductile, such as coir, combining these properties with their 20 cost and availability makes them attractive alternative reinforcements for the 21 production of greener composites. This article reviews the tensile properties of 22 various plant fibre or plant based natural fibre-reinforced polymers reported in the 23 literature. We critically discuss the use of plant fibres as reinforcement for the 24 production of bio-based, renewable or green polymer composites, showing the 25 2 evolution of the properties of plant fibre composites. The reported tensile properties 1 of plant fibre-reinforced polymer composites are compared against various renewable 2 and non-renewable engineering/commodity polymers as well as the tensile properties 3 of commercially available randomly oriented glass fibre-reinforced polymers (GFRP). 4Green composites containing random short plant fibres do have similar properties to 5 randomly oriented GFRP at a lower overall part weight. Unidirectional plant fibre-6 reinforced polymers offer better performance than randomly oriented GFRP and could 7 have the potential to be adapted in applications requiring even higher mechanical 8 performance, especially in areas where the use of costly synthetic fibres might be less 9 attractive. Furthermore, plant fibres can also be regarded as effective fillers to replace 10 more expensive polymers and improve the green credentials of final composite parts. 11These features may motivate the industry to introduce more plant fibre-based products 12 to the market. 13

Loose hierarchical flax fibres/polypropylene composites were manufactured in a simple way based on a paper-making process in order to include nanocellulose and allow the hornification of the nanofibres in a controlled manner. The effect of flax fibre content on the flax/polypropylene composites and the influence of nanocellulose on the properties of these composites are discussed. By increasing the flax content a slight decrease of the tensile strength and an increase of the Young´s modulus were observed. On the other hand, no significant effect was noticed when increasing the bacterial cellulose content in the composites.