Continuous Processing of Cellulose Nanofibril Sheets Through Conventional Single-Screw Extrusion

Abstract

Extrusion-based processes are one of the most widely used continuous polymer processing techniques because of their commercial availability, versatility, and cost-effective scalability. For these reasons, conventional single-screw extrusion was investigated and used for continuously processing mechanically fibrillated cellulose nanofibrils (CNFs) into wet sheets. A high shear mixing procedure was used to process highly loaded CNF pastes with up to ∼25 wt % solids, composed of ∼91 wt % CNF and ≤ ∼9 wt % of a processing aid like carboxymethyl cellulose (CMC), xanthan gum (XG), or anionic polyacrylamide (aPAM). Validation of the mixing procedure proved that highly loaded CNF/processing aid pastes can be processed in under 40 min. The higher solid loadings significantly reduced the preparation and drying time. The water-retention ability and stability of CNF suspensions containing different processing aids were assessed through centrifugation and zeta potential analysis. Extrusion of the prepared pastes showed that cohesive sheets could be produced continuously at output rates of 7.45 ± 0.47 kg/h (or 1.14 ± 0.072 dry) without the introduction of surface defects. Wet extrudates with an average width of ∼5 cm and thickness of 1.46 ± 0.05 mm were continuously processed, with the length limited only by the lack of collection rolls. A viscosity−shear rate dependence analysis linked extrudate homogeneity and reduced defects to a stronger Newtonian response for CNF/CMC pastes when compared to pure CNFs. On the other hand, CNF/XG and CNF/aPAM pastes experienced a much stronger shear thinning response, which correlated with a stronger appearance of observable aggregates and pinhole defects in the extrudates. Tensile testing of the pressed and heated CNF/CMC extrudates revealed equivalent mechanical properties to cast CNF films prepared through conventional solution casting. Lastly, preliminary calendering results for CNF/CMC extrudates showed that full consolidation can be achieved, thus providing a way to continuously dry and press the wet extrudates.

Controlled Dispersion and Setting of Cellulose Nanofibril (CNF) - Carboxymethyl Cellulose (CMC) Pastes

“…In the wet state (i.e., suspension), the adsorbed CMC carboxylates reduce bril-bril contacts and bril-bril friction and entanglements (Schmid and Klingenberg 2000). A reduction in apparent viscosity has been reported in highly loaded CNF + CMC and corn-stover + CMC pastes which agrees with the stated theories (Samaniuk et al 2012;El Awad Azrak et al 2020). This absorption behavior has also been observed for low solids (1-3 wt.%) pulp systems that contain CMC as a wet end additive (Beghello and Lindström 1998;Watanabe et al 2004;Yan et al 2006;Liimatainen et al 2009).…”

Deconstruction and Reassembly of Renewable Polymers and Biocolloids into Next Generation Structured Materials

“…CNFs, for instance, have been slot-die extruded at a high solid content from an aqueous slurry into wet sheets that were then hot-pressed and dried. 676 Sheets with ultimate tensile strength of ∼110 MPa and Young’s modulus above 9 GPa were produced in a continuous fashion (>1 kg/h dry mass). The introduction of processing aids was necessary, since pure CNF underwent severe dewatering, which in turn increased the solid content and led to die clogging.…”

“…The (e) resistance and (f) morphology of the final construct is affected by the composition of the blend. Adapted with permission from ref ( 676 ). Copyright 2020 American Chemical Society.…”

Process Intensification of Thermoplastic Lignocellulose Production through High-Solids Reactive Extrusion Enabled by a Novel Recycle Loop

“…Direct screw extrusion of forestry biomass is challenging because the lignocellulosic fibers exhibit low deformability and no melting point below their decomposition temperature, thus lacking the necessary flowability for most polymer processes. The most common solution currently is melt compounding wood fibers into a thermoplastic matrix to generate wood-plastic products. − With typically high fiber loadings (60% or higher is not uncommon), the thermoplastic matrix serves as a binder and lubricant in the process. , Besides thermoplastic lubricants, using excessive liquid is another way to aid the extrusion of lignocellulosic fibers for preparing biobased products; for example, the preparation of cellulose nanofibers by twin-screw extrusion usually uses a high water content to assist the convey of fibers inside the extruder. − In some cases, soluble sugars or polymers are added along with the liquid to act as a lubricant for the fibers, thereby allowing extrusion at higher solids contents. − In cases of reactive extrusion, because many added lubricants can produce undesirable side reactions, an excess of reactants, if liquid, or added solvents is ideal for conveying lignocellulose inside the twin-screw extruder. However, this significant excess of reactants requires complex separations afterward from the product, leading to increased production capital and energy costs.…”