Nina Graupner

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.

Impact and hardness optimisation of composite materials inspired by the babassu nut (Orbignya speciosa)

The babassu nut is the fruit of the babassu palm Orbignya speciosa. The combination of hardness and impact strength is difficult to acquire for artificial materials, making the babassu nut a promising source for biomimetic inspiration. Unnotched Charpy impact tests, Shore D hardness tests and scanning electron microscopy were used for mechanical and microscopical analysis of the pericarp. Four major principles were found for a biomimetic approach: a hard core ((1); endocarp) is embedded in a soft outer layer of high impact strength ((2); epicarp) and is reinforced with fibres of variable fineness (3), some of which are oriented radial to the core (4). Biomimetic fibre-reinforced composites were produced using abstracted mechanisms of the babassu nut based on regenerated cellulose fibres (lyocell, L) with two different fineness values as reinforcement embedded in a polylactide (PLA) core matrix and polypropylene (PP) based outer layers. The biomimetic fibre composite reaches a significantly higher impact strength that is 1.6 times higher than the reference sample produced from a PLA/PP/L-blend. At the same time the hardness is slightly increased compared to PP/L.

Cellulose Fiber-Reinforced PLA versus PP

The present study focuses on a comparison between different cellulose fiber-reinforced thermoplastics. Composites were produced with 30 mass-% lyocell fibers and a PLA or PP matrix with either an injection (IM) or compression molding (CM) process. Significant reinforcement effects were achieved for tensile strength, Young’s modulus, and Shore D hardness by using lyocell as reinforcing fiber. These values are significantly higher for PLA and its composites compared to PP and PP-based composites. Investigations of the fiber/matrix adhesion show a better bonding for lyocell in PLA compared to PP, resulting in a more effective load transfer from the matrix to the fiber. However, PLA is brittle while PP shows a ductile stress-strain behavior. The impact strength of PLA was drastically improved by adding lyocell while the impact strength of PP decreased. CM and IM composites do not show significant differences in fiber orientation. Despite a better compaction of IM composites, higher tensile strength values were achieved for CM samples due to a higher fiber length.

Three-dimensional braiding of continuous regenerated cellulose fibres

A growing interest in biocomposites leads to the extension of commonly used three-dimensional braiding processes for composite preforming to cellulose-based fibres. A rayon fibre (Cordenka™) is processed on an Institut für Textiltechnik 3D rotary braiding machine, generally used for the processing of stronger and stiffer glass and carbon fibres. A rectangular profile was produced from 32 yarns and the braiding angle of the yarn was analysed. Analysis of the fibre tensile properties during the different processing steps revealed only a minor reduction in fibre strain. The fibre strength and Young’s modulus were unaffected by the braiding process showing that 3D rotary braiding can be extended to biobased fibres without any required changes.