Short Poster Lecture
A well known problem of cellulose fibre-reinforced plastics is the poor fibre/matrix adhesion, especially in polyolefin-based composites which results in a low composite tensile strength. Experiments have shown a better adhesion for bast fibre-reinforced composites compared to regenerated cellulose fibre-reinforced composites. This phenomenon is based on the one hand on the rougher surface and on the other hand on the lignin content in the cell-wall. It is well known that bast fibre bundles are more difficult to separate into smaller bundles and single fibres as the degree of maturity and thus the lignin content increases, indicating a better adhesion between the fibre cells. Lignin is missing in regenerated cellulose fibres. The presented study investigates the influence of lignin as a bio-inspired adhesion promoter on the interfacial shear strength (IFSS) of regenerated cellulose fibres (lyocell) in bio-based and petrochemical matrices. The IFSS was measured with microbond, pull-out and fragmentation tests. The use of lignin leads to a significant improvement of the IFSS and has the same – or even better – effect as a commercial available, petrochemical-based adhesion promotor. Injection moulded polypropylene-based composites have shown significant higher tensile strength values as compared to the composites containing the untreated fibres. Another bio-inspired approach to improve the fibre/matrix adhesion of lyocell fibres was carried out by using plant root systems as a natural role model. For surface area enlargement of lyocell fibres a chemical treatment was developed that induced fibre-fibrillation. It could be shown that the fibre surface area was increased while the cross-sectional area was only reduced slightly by fibrillation. Therefore, fibres were embedded in a maleic anhydride grafted polypropylene matrix. The IFSS was measured with a single fibre fragmentation test and has shown a significant improvement compared to non-fibrillated fibres. Hence, fibre-fibrillation caused an improved fibre/matrix-interaction which can improve the mechanical properties of the composite depending on the loading condition and the state of stress. In the overall consideration these examples show that bio-inspired interphases are a basic possibility to improve the fibre/matrix adhesion and at the same time the mechanical composite characteristics.