A comprehensive study on preparation of nanocellulose from bleached wood pulps by TEMPO-mediated oxidation

Abstract

In this work, nanocellulose (NCs) was prepared from bleached wood pulps by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) - mediated oxidation method using sodium hypochlorite (NaClO) as oxidizer. By this method, both the chemical modification of the cellulose structure and the mechanical treatment played a very important role in obtaining NCs. Thus, the effects of pH values, oxidizer content on the oxidation degree of the cellulosic molecules and the effects of both the oxidation degree and the ultrasonic times during the mechanical disintegration of oxidized cellulose were investigated. The structure and properties of NCs were characterized by Fourier - transform infrared spectroscopy (FTIR), X- ray diffraction (XRD) analysis, phase - contrast microphotograph and scanning electron microscopy (SEM). The oxidization degree of cellulosic molecules was determined by the carboxylate content in the cellulosic molecules. The higher the carboxylate content, the higher was the oxidation degree. The results revealed that the optimal reaction conditions to get high carboxylate content and reaction yield were NaClO content of 10 mmol/g, pH of 10, and oxidation reaction time of 253 min. At these reaction conditions, the carboxylate content and reaction yield were 1.18 mmol/g and 87 %, respectively. With an ultrasonic treatment time up 60 min, NCs with 10–30 nm in widths and lengths of 40–60 nm were obtained. The crystalline degree of NCs was 65.2 % and it was nearly unchanged compared to that of pristine cellulose.