Physical, morphological and chemical variability of industrial hemp hurd feedstocks for biocomposite manufacture

Abstract Industrial hemp hurd is an emerging feedstock for lightweight lignocellulosic composites. However, conventional supplier grading provides limited information on material variability relevant to manufacture, potentially leading to inconsistent processing and product quality. Six Australian hemp hurd feedstocks, comprising a mixed hurd, two fine fractions, two coarse fractions and a hurd–fibre mixture, were characterised using bulk density testing, sieve analysis, removable fibre quantification, two-dimensional digital image analysis, low-magnification scanning electron microscope, compositional profiling and hot-water extract pH. Bulk density ranged from 74.2 ± 4.2 to 115.7 ± 1.3 kg/m 3 , removable fibre from 0 to 8.14 ± 1.69%, sieve-derived D50 from 1.42 to 3.46 mm and normalised span from 0.82 to 2.59. Nominally similar fractions differed markedly. The two fine fractions differed in packing behaviour, fibre carry-over and particle regularity, while the coarse fractions had similar median lengths but contrasting particle geometries. The hurd–fibre mixture formed a distinct material profile, combining high fibre carry-over, low bulk density, broad particle size distribution and elevated ash and nitrogen. Inorganic ash ranged from 1.4 to 9.1%, amylase-treated neutral detergent fibre from 73.5 to 90.1%, and extract pH from 5.80 to 6.76. These results show that nominal particle size class alone is insufficient for specifying hemp hurd feedstock quality. Combining bulk density, removable fibre content, particle size distribution, image-derived morphology and selected chemical indicators enabled the development of an integrated feedstock trait map and a provisional hemp hurd classification and reporting framework. Together, these suggest a practical approach for more consistent hemp hurd characterisation, informed feedstock selection and binder-specific process development for hurd-based biocomposite manufacture.