Improvement and analysis of fibre/matrix adhesion of regenerated cellulose fibre reinforced PP-, MAPP- and PLA-composites by the use of Eucalyptus globulus lignin

The presented study investigates the influence of Eucalyptus globulus lignin as a natural additive on the fibre/matrix interaction of lyocell (regenerated cellulose fibre) in different matrices (polylactide - PLA, polypropylene - PP, maleic-anhydride-grafted-PP - MAPP). For this purpose, lyocell fibres were treated with a lignin-ethanol solution. It was shown that the fibre tensile strength was not affected by the lignin-treatment. The viscosity of the PLA matrix was reduced by adding lignin. This effect might lead to a better wettability of the fibres but the mechanical characteristics of PLA were not affected by the treatment. Apparent fibre/matrix adhesion was assessed using a single fibre pull-out test and a single fibre fragmentation test. Both testing procedures revealed a trend of improved interfacial shear strength for the lignin-treated fibres compared with untreated fibres for all investigated matrices. Additionally the apparent interfacial shear strength was investigated with short bending tests and double-notched tensile tests on lyocell/PLA composites. A significant increase was observed for lignin-treated composites compared to untreated composites. Microscopic investigations revealed a rougher fibre surface with lignin particles. The higher specific fibre surface can also lead to improved apparent fibre/matrix interaction. Furthermore, it is hypothesised that van-der-Waals forces between the hydrophilic fibres and the more hydrophobic matrix are favoured by the less hydrophobic character of lignin.