Natural materials such as wood exhibit high mechanical properties through cellulose structured at multiple length scales and embedded in a matrix of similar chemical structure. These hierarchical materials have inspired the design of lightweight composites composed of naturally occurring polymers. However, the close proximity of melt and decomposition temperature remain a challenge. In this work, cellulose propionate (CP) is modified to reduce its glass transition temperature and melt viscosity, allowing its use as a matrix in a natural fibre-reinforced composite. Through better impregnation, the modified CP matrix composites showed an increase in stiffness and strength of ∼10% and 20%, respectively, in comparison to unmodified CP matrix composites. The impact properties also increased by up to 28%, showing that modified CP is a credible matrix for realising sustainable all-cellulose natural fibre composites with high stiffness, strength and toughness.

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Natural fibre (NF) reinforced composites offer high specific mechanical properties and are an ecological alternative to synthetic fibre-reinforced composites. While having great potential, their use today is limited to non-structural applications, mostly with epoxy or polypropylene matrices. This work studies suitable high-performance thermoplastic matrices and characterises their bulk properties, fibre-wetting and composite mechanical behaviour. Thermoplastic polymers such as poly-L-lactide (PLLA) and polyoxymethylene (coPOM) are matrices with bulk properties similar to epoxy. The results show that PLLA matrix NF-composites have a longitudinal modulus and strength of 27 GPa and 308 MPa. The tougher coPOM matrix NF-composites show both high transverse stiffness and strength of 2.6 GPa and 41.5 MPa and show that even the drawback of creep can be overcome by the use of hierarchically structured coPOM. The developed NF-composites demonstrate in-plane properties comparable to those with epoxy matrices and can outperform them by up to 26% in the transverse direction.

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Natural fibres (NF) have shown to be a suitable alternative to glass fibres as reinforcing material in polymer composites since they offer similar specific strength and stiffness. Thus, those composites may be readily used in the automotive, sport and leisure. However, most of today's natural fibre applications are based on discontinuous fibre architectures, underrunning the performance of continuous synthetic fibre composites. Flax fibres show a great potential to be used as continuous reinforcing fibres in thermoplastic matrix composites. The use of high performance engineering polymers and the application of preceding plasma-based fibre surface treatments may further enhance the mechanical properties, making flax fibre thermoplastic composites an environmentally friendly alternative. We investigated the mechanical behaviour of pure flax fibres, introduced an engineering polymer as matrix system, compared this material to commonly used matrices such as epoxy or polypropylene and studied the effect of two different plasma treatments on the mechanical performance of natural fibre composites (NFC). The influence of the plasma methods was found through composite failure testing. Results have shown, that composites with plasma treated fibres and engineering polymers along with a high fibre volume content offer a great potential as environmentally sustainable substitutes for synthetic composites in many applications.@en