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.