Influence of pretreatment on properties of cotton fiber in aqueous NaOH/urea solution

In this study, cotton fiber was pretreated in aqueous NaOH/urea solution instead of dissolution. According to the increasing severity degrees of pretreatment, three kinds of pretreated cotton fibers (S-PCF, M-PCF and H-PCF) were obtained for characterization and analysis. For comparison, pretreatment using a single NaOH solution without urea was also conducted under the same conditions to obtain other pretreated fibers (S-NaCF, M-NaCF and H-NaCF). The morphology of unpretreated (UCF) and pretreated cotton fibers (PCFs and NaCFs) was analyzed by SEM, and the results showed that when pretreated by NaOH aqueous solution with or without urea, the cotton fiber presented both a convolution structure and a coarser surface, resulting from the decreasing of fibrils on the fiber surface. A further morphological analysis of various fiber samples from FQA indicated that this pretreatment process could lead to fibers with a shorter length, larger width, more distinct kink and curl, as well as less fine content. By FTIR, it was found that there were no new changes in the chemical structure of PCFs and NaCFs, but the intensity decrease of some major peaks indicated that the fiber structure might have been partially destroyed. XRD results suggested that the crystallinity degree of PCFs decreased dramatically from 81.52 to 33.41 % and more cellulose II appeared, with a maximum value of 13.68 %. The changes of the structure and property of NaCFs were all much weaker when compared with PCFs. Cellulose acetate was prepared by various fiber samples. Compared with UCF, the degrees of substitution of PCAs and NaCAs were all improved, and the biggest value of PCAs was 2.88, which was 2.66 in NaCAs.