Nettle fibre (Urtica dioica L.) reinforced poly(lactic acid): A first approach

Stinging nettle (Urtica dioica L.) is a bast fibre plant ideally suited to cultivation in central Europe, producing fibres of remarkable high tensile strength and fineness. Only little literature is available about nettle-reinforced standard plastics. The present study represents a first approach to produce nettle-reinforced poly(lactic acid) (PLA) with fibre loads of 20-40 wt-% to assess the technical potential of this material compared to 30 wt-% nettle/polypropylene. The tensile strength could only be increased in case of 30 wt-% nettle/poly(lactic acid) from 52 of the pure PLA to 59 MPa. This is far away from the real potential of the nettle fibres used here with a single element tensile strength of 930 +/- 500 MPa. Concerning the Young's and flexural modulus, a clear reinforcement effect was found for all poly(lactic acid) composites. The effect was strongest in case of 30 wt-% nettle/PLA: both moduli increased from < 3500 MPa of poly(lactic acid) to > 5,000 MPa. This is as well far below the single element value of the pure fibres (26,451 +/- 14,445 MPa). As known from PLA reinforced with other bast fibres, the unnotched Charpy impact strength is lower than that of the pure polymer. The nettle-reinforced samples were found to have Charpy impact values < 50% of the pure PLA. In general, the results show a good potential for nettle as reinforcement for PLA. The crucial point for the future development will be to improve the fibre-matrix interaction in order to increase especially the tensile strength of the composites by closing the large gap between fibre and matrix strength.