Highlights on the mechanical pre-refining step in the production of wood cellulose nanofibrils

This work investigates the effects of PFI mill refining and especially, its contribution to the defibrillation of cellulose fibers, as the mechanical pretreatment step in the production of cellulose nanofibrils (CNFs). Hardwood and softwood kraft pulps were refined at varying PFI mill revolutions and further processed using an ultrafine-friction grinder at predefined low specific energy conditions to produce aqueous CNF suspensions. The degree of fibrillation of the CNFs was assessed by studying the properties of the suspensions and anisotropic foams prepared by ice templating. The differences in aspect ratios of the hardwood and softwood CNFs refined at different PFI mill revolutions resulted in anisotropic foams of distinct porous structures and mechanical performance. The lower the CNF aspect ratio, the weaker the anisotropic foams. Softwood CNF foams demonstrated higher mechanical performance than hardwood foams, mainly due to the inherent differences in structure and morphology between these two fiber types. The presence of hemicelluloses in the CNF suspensions, however, also explained the observed differences in compressive behavior, by their action on the fibril-fibril interactions in the foam cell walls. This study thus sheds light on the role of PFI mill refining as a mechanical pretreatment for CNF production. Low PFI mill revolutions were preferred for optimal fibrillation of both hardwood and softwood fibers prior to the high-shear mechanical treatment. Conversely, a too intense pre-refining resulted in CNF materials with poor performance, due to a sharp decrease in the fiber aspect ratio that inhibited the formation of a percolating network and fibril-fibril interactions.