TEMPO-Mediated Oxidation of Cellulose and Preparation of Cellulose Nanofibrils

Hardwood Kraft pulp was oxidized by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) using sodium hypochlorite as the primary oxidant and sodium bromide as the catalyst at pH 10.0. The effect of the added amount of primary oxidant on the degree of oxidation was studied. Sanning electron microscope (SEM) and X-ray diffraction (XRD) were applied to study morphology and crystalline structure of the oxidized cellulose. Carboxylate content and water retention value (WRV) of the oxidized cellulose were also analyzed. The results showed that the recovery of oxidation was more than 90%; the carboxylate content reached a maximum of 0.81 mmol/g (cellulose) when sodium hypochlorite added was 3 mmol/g (cellulose). WRV of the cellulose were significantly increased to 224% after oxidation while WRV of the unoxidized cellulose was only 106%. Cellulose nanofibrils of 5-20 nm in width were obtained by homogenization of oxidized cellulose using an ultrasound wave generator. A stable homogeneous suspension of cellulose nanofibrils was also formed. After the oxidation the cellulose crystallinity increased from 61.10% to 63.84%, but the crystallinity of the nanofibrils was reduced to 55.89%. Films with a flat surface and transmittance of 30 similar to 43% in the visible light range (390 nm similar to 780 nm) were prepared from the obtained nanofibrils.