Abstract
The novel use of aqueous suspensions of cellulose nanofibrils (CNF) as an adhesive/binder in lignocellulosic-based composite manufacture requires the removal of a considerable amount of water from the furnish during processing, necessitating thorough understanding of the dewatering behavior referred to as “contact dewatering”. The dewatering behavior of a wood-CNF particulate system (wet furnish) was studied through pressure filtration tests, centrifugation, and characterization of hard-to-remove (HR) water, i.e. moisture content in the wet furnish at the transition between constant rate part and the falling rate part of evaporative change in mass from an isothermal thermogravimetric analysis (TGA). The effect of wood particle size thereby particle specific surface area on the dewatering performance of wet furnish was investigated. Permeability coefficients of wet furnish during pressure filtration experiments were also determined based on Darcy’s law for volumetric flow through a porous medium. Results revealed that specific particle surface area has a significant effect on the dewatering of wet furnish where dewatering rate significantly increased at higher specific particle surface area levels. While the permeability of the systems decreased over time in almost all cases, the most significant portion of dewatering occurred at very early stages of dewatering (less than 200 seconds) leading to a considerable increase in instantaneous dewatering when CNF particles come in contact with wood particles.
Highlights
cellulose nanofibrils (CNF) consists of nano and micro-scale cellulosic fibers suspended in water and is mostly available in the form of a low-consistency aqueous suspension
The results revealed that increasing the micro and nanofibrillated cellulose (MNFC) fibrillation decreased the dewatering performance, this could be tuned by in situ precipitation of precipitated calcium carbonate (PCC) to achieve a desirable combination of strength and processing performance[23]
Samples of pure CNF 10 wt.% exhibited considerably lower amounts and rates of water removal within the same period of time compared to wood particles (WPs)-CNF mixes with the same solids content (Fig. 4a,b)
Summary
CNF consists of nano and micro-scale cellulosic fibers suspended in water and is mostly available in the form of a low-consistency (less than 5 wt.%) aqueous suspension. Understanding and controlling the water removal behavior of the CNF suspension, both solely and in the form of a mix with other materials is a critical step to optimize the production process. Dimic-Misic et al studied the effect of shear stress as well as swelling (expressed as the water retention value at a relatively low consistency) of micro and nanofibrillated cellulose (MNFC) on the dewatering behavior of the cellulose furnishes. 0.01 s−1), including the addition of colloidally unstable mineral particles (such as undispersed calcium carbonate), acid dissociation of the surface water bound to the nanofibrils of cellulose by adding ultrafine calcium carbonate nanoparticles, and controlling the rheological properties with respect to length and aspect ratio of fibrils[24,25,26] Further assessments have been done to enhance the dewatering capability of MNFC suspensions and furnishes under an ultra-low shear rate (approx. 0.01 s−1), including the addition of colloidally unstable mineral particles (such as undispersed calcium carbonate), acid dissociation of the surface water bound to the nanofibrils of cellulose by adding ultrafine calcium carbonate nanoparticles, and controlling the rheological properties with respect to length and aspect ratio of fibrils[24,25,26]
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