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

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 similar to 25 wt % solids, composed of similar to 91 wt % CNF and <= similar to 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 similar to 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.