Effects of the conditions of the TEMPO/NaBr/NaClO system on carboxyl groups, degree of polymerization, and yield of the oxidized cellulose

Dissolving pulp from Pinus caribaea was oxidized by means of the TEMPO (2,2,6,6-tetramethylpiperidine-1-oxide radical)/NaBr/NaClO system. Effects of the conditions including pH, NaClO dosage, and time on the carboxyl group content, degree of polymerization (DP), and solid recovery of oxidized cellulose were determined. A pH of 10 to 10.5 was found to be optimum for selectively oxidizing cellulose, and carboxyl groups were up to 0.75 mmol/g. However, increasing pH from 9 to 12 facilitated cellulose depolymerization by consistently indicating a gradual drop in DP, thus resulting in cellulose loss. In addition, oxidation was accelerated by the addition of 1 mmol/g to 6 mmol/g NaClO; however, further addition did not enhance the carboxyl groups. The maximum value of carboxyl groups was more dependent on NaClO dosage and governed by the crystal structure of the raw material. To obtain oxidized cellulose with a higher yield and DP, NaClO dosage could be controlled at 4 mmol/g to 6 mmol/g, while the reaction time was limited to 6 h to 8 h.