TEMPO-mediated oxidation of softwood thermomechanical pulp

Abstract A softwood thermomechanical pulp (TMP) was suspended in water and oxidized with sodium hypochlorite and catalytic amounts of sodium bromide and 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) at pH 10. When the NaClO addition levels were 20–26 mmol g^-1of the TMP, the yields of water-insoluble TEMPO-oxidized TMPs fractions were approximately 40%. Sugar composition and other analyses revealed that most of the lignin and hemicellulose components in the TMP were removed as water-soluble fractions by the oxidation. Thus, almost pure TEMPO-oxidized celluloses can also be prepared from TMP, although the addition levels of NaClO are much higher than for the TEMPO-mediated oxidation of pure celluloses. The water-soluble fractions were analyzed by NMR, showing that polyuronic acids formed from glucomannan and cellulose by oxidation were the main compounds. The water-insoluble fractions of TEMPO-oxidized TMPs prepared with NaClO of 20–26 mmol g^-1had carboxylate contents of approximately 1.2 mmol g^-1, and had the same cellulose I allomorph and the same crystal widths as in the original TMP. Transparent and highly viscous gels were obtained by disintegration of these TEMPO-oxidized TMPs in water, and the gels consisted of individual nanofibers 4–6 nm in width. Thus, TEMPO-oxidized cellulose nanofibers can also be prepared from TMP.