Valorization of byproducts of hemp multipurpose crops: short non-aligned bast fibers as a source of nanocellulose

Nanocellulose was extracted from short bast fibers, from hemp (Cannabis Sativa L.) plants harvested at seed maturity, non-retted, mechanically decorticated in a defibering apparatus giving non-aligned fibers. A chemical pretreatment with NaOH and HCl allowed the removal of most of the non-cellulosic components of the fibers. No bleaching was performed. The chemically pretreated fibers were then refined in a beater and treated with a cellulase enzyme, before the mechanical defibrillation in an ultrafine friction grinder. The fibers were characterized by microscopy, infrared spectroscopy, thermogravimetric analysis, and X-ray diffraction after each step of the process, to understand the evolution of their morphology and composition. The obtained nanocellulose suspension was composed of rod-like fibrils with widths of 5-12 nm, stacks of nanofibrils with widths of 20-200 nm, and some larger fibers. The crystallinity index was found to increase from 74% for the raw fibers to 80% for the nanocellulose. The nanocellulose retained a yellowish color indicating the presence of some residual lignin. Properties of nanopaper prepared with the hemp nanocellulose were similar to those of nanopapers prepared with wood pulp derived rod-like nanofibrils.