Abstract:

Objective With the improvement of life quality and hence the increasing demand for textiles, a large amount of textile waste is generated globally each year. Recycling and reusing this waste is crucial for developing low-cost, sustainable, high-performance, and renewable materials. This study focuses on creating humidity-responsive composite materials from recycled waste contton and polyurethane (PU) textiles, aiming for high-end applications.

Method A cellulose/polyurethane composite film with humidity-responsive properties was developed using cellulose extracted from waste textiles. The process began with the pretreatment of waste cotton fabrics through bleaching and activation. Afterward, the fabrics were dissolved in LiCl/dimethylacetamide(DMAc) solvents, regenerated into a cellulose film, and dried. The cellulose film was then mechanically crushed to produce uniform cellulose powder (the average particle size is 0.17 mm). Next, the cellulose powder and polyurethane material were separately dispersed in a dimethylformamide(DMF) solution. Among them, the mass fraction of cellulose powder is 0, 10%, 20%, 30%, 40%, respectively. These were then mixed in a mold to form the cellulose/polyurethane composite film. Last, the morphology characteristics of the composite film were observed using scanning electron microscopy and the mechanical properties of the composite film were tested using a universal testing machine. In addition, based on the mechanism of humidity response of the cotton/PU composites, the humidity responsiveness of the composite film was analyzed.

Results Humidity-responsive cellulose/polyurethane films were successfully prepared using the mold forming method. The microstructure of the composite film shows rough surface characteristics, which is because the addition of cellulose filler changes the internal structure of the composite film and generates a new surface area on the surface of the composite film. Furthermore, chemical structural analysis further confirmed the effective incorporation of cellulose into the PU matrix during making the cellulose/polyurethane composites. A cellulose/polyurethane film with a thickness of (0.18±0.02) mm was shown to easily withstood a weight of 1 000 g without being damaged. This capacity is 34 000 times higher than the weight of the film itself, and showed good mechanical properties. This is due to the tight bond between the polyurethane matrix and the cellulose powder, which can support the good mechanical properties of the composite film. In addition, the cellulose/polyurethane composite film demonstrated a tensile strength of 19.23 MPa. This strength is believed to be from the abundant hydrogen bonds present in the film, which enhance its overall integrity. By introducing cellulose materials, the composite film exhibited excellent humidity driving performance. Cellulose polymer chains form layered networks through intermolecular and intramolecular bonds, facilitated by the abundant hydroxyl groups in cellulose macromolecules. This unique structure imparts hygroscopic and expansive properties to cellulose fibers, causing them to react to water molecules. As humidity increases, water molecules quickly diffuse into the composite film, which allows cellulose to absorb significant amounts of water, causing it to expand, providing the driving force for the deformation of the film. As the content of cellulose continued to increase, the response bending angle of the film became larger, while the time needed to reach the maximum bending angle was shortened. When the mass fraction of cellulose was 30%, it showed good molding effect and driving performance. Specifically, it showed a rapid response time of 15 s, a recovery time of 32 s, and a bending angle of 136.3°. This excellent humidity response is due to the hydrophilic nature of cellulose and the elasticity of polyurethane, which together facilitated the absorption and desorption of water molecules.

Conclusion The humidity switch can expand or contract as cellulose absorbs or desorbs water molecules. This process generates a driving force for film deformation. Consequently, it effectively enables reversible shape actuation and recovery. This occurs by desorbing or absorbing water molecules, allowing for reversible shape driving and recovery. Additionally, the composite film was applied to simulate a "mechanical gripper" and successfully completed the grasping operation of objects. This method of reusing waste textiles has opened up new avenues for the application of cellulose materials in the fields of intelligent driving. The rapid humidity-responsive composite films have great application potential in intelligent drive structures.

Key words: waste textile, regenerated cellulose, polyurethane, humidity response, composite material