Effect of fibre orientation on the mechanical properties of polypropylene–lyocell composites

Natural fibre reinforced plant structures are widely used in nature. These plant structures combine light weight with superior mechanical properties. The fibre orientations in plants are optimized to the occurring forces, especially to the bending of plants by wind forces. Therefore it is important to study the effect of fibre orientation to the mechanical properties of materials in a systematic experimental approach. In this study the effect of reinforcement fibre orientation on mechanical properties of bio-based lyocell-reinforced polypropylene composite was analysed. For this purpose, special technique to produce composites with defined fibre orientation and fibre wetting was developed consisting of the production of intermingled hybrid yarn followed by defined yarn laying and thermoforming processes. The formed composites were subjected to tensile strength tests and dynamic mechanical analyses. The experimentally determined E-modulus was compared with values, calculated from the modified rule of mixture of Virk and Krenchel. The analysis showed that the experimental E-moduli were somewhat smaller than the theoretical values, which is indicative of a less than perfect interfacial bonding between the fibres and matrix. The influence of water on the composite performance was also analysed. It was shown that the composites sorb approximately 30% water by weight, and it has a strong influence on the E-modulus and other performance parameters. Graphical abstract