关键词: 纤维素纳米晶, 废旧纺织品回收, 脱色, 靛蓝染料, 靛蓝牛仔布, 复合膜, 抗紫外线性能

Abstract:

Objective To achieve the high-value recycling of waste indigo denim and the preparation of functional materials, this study aims to clarify the role of indigo dye in the recycling process and explore a high-value technical pathway without complex decolorization. The feasibility of directly preparing functional cellulose nanocrystals (CNCs) without decolorization was successfully demonstrated, providing both a theoretical basis and technical support for the recycling of waste denim.

Methods Three types of CNCs (named W-CNCs, D-CNCs, and T-CNCs) were prepared by sulfuric acid hydrolysis from undyed denim, waste indigo denim and waste indigo denim decolorized by N, N-dimethylformamide (DMF). The microstructure, physicochemical properties and thermal stability of the three CNCs were characterized by SEM, XRD, FT-IR and TGA. The films were prepared by compounding the three types of CNCs with polyvinyl alcohol (PVA) at different mass ratios (1%, 3%, 5%, 7%, 10%), respectively. The mechanical properties, thermal stability and UV resistance of the resulting composite films were subsequently evaluated.

Results All three CNCs exhibited a rod-like morphology with no significant difference in dimensions and maintained a cellulose I crystal structure. Due to the retention of the characteristic functional groups of indigo dye, the D-CNCs suspension displayed a distinct blue color, whereas the W-CNCs and T-CNCs suspensions appeared milky white. Compared with raw cotton fibers, the thermal stability of all CNCs decreased significantly, and the presence of residual indigo dye did not markedly influence this trend. Functional analysis demonstrated that the tensile strength of the PVA composite film increased by 20.4% with the incorporation of 5% D-CNCs. When the D-CNCs content was elevated to 10%, the initial decomposition temperature of the composite film was raised by 8%. Moreover, with increasing D-CNCs loading, the transmittance of the composite film in the 200 - 400 nm UV range decreased substantially, resulting in a gradual enhancement of UV-blocking performance. Specifically, the UV absorption rates reached 96.39% and 98.5% at D-CNCs loadings of 5% and 10%, respectively.

Conclusion The residual indigo dye not only imparted distinctive chromatic properties to CNCs, but also demonstrated notable advantages in enhancing the mechanical properties, improving thermal stability, and conferring UV resistance to PVA composites. The technology proposed in this study for directly preparing functional CNCs from waste indigo denim can be extended to the field of textile waste recycling, providing low-cost raw materials for UV-resistant packaging materials and flexible composite materials. Furthermore, this study presented a viable technical pathway for the high-value recycling and reuse of waste indigo denim, offering promising environmental benefits and application prospects.

Key words: cellulose nanocrystal, recycling of waste textiles, decolorization, indigo dye, indigo denim fabric, composite membrane, UV resistance