Morphological and rheological behaviors of micro-nanofibrillated NaOH-pretreated Aspen wood

This research was aimed at fibrillating wood (Aspen) particles using ultra-fine friction grinding, facilitated by a sodium hydroxide pretreatment step. Its objective was to examine the effect of grinding on the extent of fibrillation and the rheological behaviors of the products. The morphology of the ground samples were characterized by optical (OM) and scanning electron microscopy. The extent of fibrillation was evaluated by measurements of particle size distribution (PS), specific surface area, and water retention value. The rheological behavior of the sample suspension was examined by oscillatory and rotational flow testing. Morphological results revealed a reduction in particle size with increasing mechanical fibrillation. The fibrillated samples contained micro- and nano-sized fibers of approximately 50–1000 nm in width, and lengths between hundreds of nanometers to micrometers. Rheological results revealed an increase in storage moduli (G′) and loss moduli (G″) of the fibrillated samples as the grinding was more severe. A shift in the cross-over point (G′ = G″) towards higher strains was also observed, indicating a more stable network structure with a higher extent of grinding. The fibrillated samples showed shear thinning (reduced viscosity) and a decreased thixotropic behavior when subjected to increasing shear rates. A higher extent of fibrillation led to aqueous suspensions of higher shear viscosity, which would also increase when increasing the solid content of the measured suspension. Overall, this study offers a low-cost and simple means for producing fibrillated particles from wood materials, and also benefits their downstream processing through the acquired understanding of their rheological behavior.