Structure-property relationships in Japanese knotweed – The potential of using the stem for composite applications

The Japanese perennial knotweed (Fallopia japonica) is a globally widespread neophyte whose usability is being investigated, e.g., to use knotweed for biogas plants and as a substitute for firewood. The present study investigates the potential of Japanese knotweed for material use. Morphological studies were carried out on the stem cell structures and arrangements (microstructure) and the external stem structure (macrostructure) and showed that Japanese knotweed is a plant species with several hierarchical morphological levels being a highly complex fibre-matrix composite with a low density. Mechanical properties were investigated using tensile, bending, compression and impact tests for fresh and dry specimens and then mathematically converted in density-related lightweight construction indices and compared with other materials using Ashby maps. Particularly under compression, properties are close to woods and wood composites, making the plant an interesting material for lightweight sandwich panels, where assembled slices of the stalk could serve as core elements. Fibre bundles, extracted from the stalk, show relatively low mechanical properties (tensile strength: 93 MPa; Young’s modulus: 4.77 GPa) compared to bast fibres such as hemp. The shredded stalks could be compounded into homogeneous granulates directly after harvesting without other separation processes. Therefore, the study presents a proof of concept for Japanese knotweed to apply the shredded stalks in injection-moulded PLA composites (tensile strength: mean = 54 MPa; Young’s modulus: mean = 5.61 GPa) comparable or even better than wood fibre-reinforced polymers.