Abstract:

Objective Cotton waste accounts for 35%-40% of the global textiles waste of more than 100 million tons annually, with a low recycling rate of less than 30%. In particular, the fallen cotton fibers less than 7 mm during the mechanical opening process are not used because they are too short to be processed into fiber nets or other forms of textiles. It is imperative for such short cotton fibers with or without dyes to be recycled to avoid environmental pollution and recycle these high-quality cellulose resources.

Method Aiming at the problems that the waste cotton short fibers produced after mechanical opening and recycling have poor mechanical properties and are difficult to reuse, the part recovered waste cotton short fibers were oxidized in 2,2,6,6-tetramethylpiperidine-1-oxide radical (TEMPO)/NaBr/NaClO system to prepare cellulose nanofiber (CNF). CNF suspension with different concentrations and recycled waste cotton short fibers were thoroughly mixed to prepare all cellulose pulp. Finally, the full cellulose fiber self-reinforced composite paper (CCP) was formed by wet-laid forming and hot-pressing technology. The CCP was prepared from waste denim to maximize the use of very short cotton fibers. The morphology, mechanical properties, and reinforcement mechanism of CPP were characterized and analyzed.

Results CNF was successfully prepared by the TEMPO oxidation method, and the obtained CNF had a regular long strips with the length of about 0.8-1.2 μm and the diameter of about 10-35 nm. The FI-TR showed that there were absorption peaks corresponding to —COOH or its derivatives at 1 755 cm^-1 and 1 670 cm^-1. This indicated that the cotton short fiber was successfully oxidized by the TEMPO/NaBr/NaClO oxidation system to introduce carboxyl groups and transferred into CNF. In the aspect of CCP morphology, the surface pores of CCP decreased significantly with the increase of CNF concentration, leading to dense structure of CCP, and the pores on the surface and cross section of CCP with 5% CNF were filled. This indicates that CNF as a reinforcing agent in the pulp can bind closely to the pulp fibers. It effectively fills the pores between the cellulose fibers, making the paper form a dense structure. Moreover, the mechanical properties of CCP kept increasing with the increase of CNF concentration. The tensile strength of CCP with 0% CNF was 11.20 MPa, and that of CCP with 5% CNF was 4.72 MPa. It was also found that the maximum increase of CCP with 5% CNF in bursting strength and in tearing strength was 2.5 times and 1.8 times higher than those of 0% CNF. In addition, the XPS results showed that the relative content of C2 (C—OH) increased continuously from 37.03% for 1% CNF to 72.03% for 5% CNF and the relative content of O1s1 (C—OH…O) increased from 52.44% for 1% CNF to 72.29% for CCP with 5% CNF. This indicates that the hydrogen bonding content of CCP increases with with higher CNF concentration.

Conclusion The TEMPO/NaBr/NaClO oxidation system was utilized to introduce carboxyl groups on the cellulose surface, and CNF was successfully prepared. The addition of CNF effectively improved the mechanical properties of CCP. The CNF formed an extensive cross-linked network with the recycled cotton short fibers through hydrogen bonding, which facilitated the self-reinforcing effect of cellulose. Since CCP is composed of all-cellulose material, it has good sustainability and can be applied in green packaging. This work can be further explored in the future to develop special papers (e.g., banknote paper and other security papers), different sensors as components of smart packaging.

Key words: waste cotton, cellulose nanofiber, recycling, composite paper, self-reinforcing, wet-laid web, hot pressing technique