Natural fibre thermoplastic composites may be manufactured using a film stacking process, where fibre-preforms and matrix films are stacked alternately. The process has shown great potential due to the high flexibility referring to the stacking sequence and the using of simple tooling concepts, which may easily be scaled for industrial applications. As impregnation takes place through-the-thickness, the flow length is short and of particular interest when thermoplastic processing due to the high melt viscosities, particularly important when working with natural fibres and relatively low temperatures. In this study, flax fibres were impregnated by various thermoplastic matrix materials suitable to being processed at lower temperatures. Here we aim to study the impregnation of the natural fibres by varying the provided amount of matrix material within the film stacking process. Impregnation trials showed interesting potential of reducing the content of matrix material. This leads to a reduced thickness of the composite whilst maintaining the structural load bearing capability due to the unchanged presence of the fibre material. Starting from a fibre volume content of 50 % we demonstrate an increase it up to 77 %. Whilst we note no effect on the strength, we observed notable improvement in the flexural moduli.@en

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.@en

The objective of this paper was to understand the damping of carbon- and flax-fibre reinforce polymers. We analysed carbon and flax symmetric angle ply laminates for a series of layup angles of 0°, ±10°, ±20°, ±30°, ±60°, 90°. We measured both series using dynamic mechanical analysis (DMA) and vibration beam measurements (VBM). Our results showed that the VBM and DMA elastic results can be converted from one into the other, and we found a mathematical formulation to relate the damping results of both. We discussed the underlining mathematical models of each measurement method and the physical interpretation. For both methods, we relate the elastic properties to the elastic modulus and the damping properties to the specific damping capacity (SDC), ψ. We concluded that within a certain frequency range micro-mechanical (DMA) and macro-mechanical (VBM) damping may be compared and even converted into one another.@en

The damping behaviour of continuous carbon fibre and flax fibre reinforced polymer (CFRP and FFRP) composites was studied by comparing angle-ply laminates. Using logarithmic decrement measurements, dynamic mechanical analysis and vibration beam measurements, the damping was described as the specific damping capacity ψ in order to compare data using the different methods. Our results show approximately 2–3 times better damping of FFRP compared to CFRP at low frequency and low strain. We show that the damping of both materials increases with increasing angle-ply orientation below 300Hz. While the matrix and interface seems to contribute mainly to damping at lower frequencies, the fibre shows an increasing contribution with ψ = 64.4% for unidirectional FFRP at 1259Hz in the 5th mode of vibration, without a notable change in the elastic modulus. This work demonstrates that the FFRP may be simultaneously stiff and efficient at damping.@en

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