Preparation and characterization of high-yield cellulose nanocrystals (CNCs) from softwood kraft pulp

The preparation of CNCs in yield higher than 50% from wood pulp is challenging and requires fine tuning of the reaction conditions in order to avoid degradation of cellulose but to remove hemicelluloses and traces of extractives found in the original pulp.The pulp used was ECF-bleached softwood kraft pulp than contained about 26 wt-% easily hydrolysable carbohydrates. The dry pulp was ground and freeze-dried to remove all moisture. Batches of 10, 25, 35 and 40 g of freeze-dried pulp were weighed into glass bottles, and 250 mL of cold 60 wt-% H₂SO₄ was added to each bottle. The pulp suspension was shaken manually for 10 min at room temperature in order to homogenize the mixture. The temperature of the suspension did not increase, since the pulp was completely dry when the acid was added. The glass bottle was sealed and placed in a 45°C water bath for 120 min under agitation. The hydrolysis was stopped after 120 min by diluting the suspension ten-fold with distilled water. The following day, the cellulose fractions were washed by centrifugation, and dialyzed against distilled water for 4-5 days or until the pH of the dialysis water remained unchanged over the course of a day. The CNCs were then disintegrated mechanically using a kitchen blender and with a homogenizer. The disintegrated suspension was next filtered through a 280 mesh polyamide fabric in order to remove any large fibre fragments that may have remained intact.The yield of the 25 g batch was 61.4% and for the 35 g batch 65.3%, i.e. very close to the theoretical maximum value of 74%, since the easily hydrolysable carbohydrates should be removed during the hydrolyses. About 1% of xylose and 1% of mannose were detected in the resulting CNCs, and only traces of other sugar units.Dynamic light scattering gave a z-average size of approximately 100 nm. The content of sulfur was lower for the 10 g batch, about 0.35 %S, than for the 35 and 40 g batch, about 0.7 %S. The zeta-potential was about -30 mV and the electrophoretic mobility was approximately -2 µmcm/Vs. CP/MAS ¹³C-NMR analysis gave a crystallinity of 53% compared to 55% for a commercial CNC sample. The properties of the prepared CNC from softwood kraft pulp in high-yield were similar to other CNCs. It was also shown with flow cytometry that CNCs prepared from softwood kraft pulp contained less hydrophobic material than CNCs from hardwood kraft pulps.