Abstract:

Objective In order to increase the diversity of yarns, a composite yarn was produced using a polyurethane (TPU) nanofiber film strip as the core and cotton fibers as the sheath (TPU-film-strip/cotton) through ring spinning. During the spinning process, core exposure emerged as an issue that affects the quality of the composite yarn. Basic theoretical research on fully covered composite yarns is limited. Therefore, the fundamental principles and key parameters for achieving a completely covered structure were theoretically investigated. Three types of TPU-film-strip/cotton composite yarns were prepared, and their mechanical properties were evaluated.
Method For obtaining a fully covered yarn, a theoretical model was developed to describe the relationship between the film strip size and the parameters of the outer fibers under ideal conditions. Using this model, the theoretical linear density equation for sheath layer was established. The width of the cotton strand at the nip of front roller was experimentally investigated, and the actual linear density equation of the sheath was derived through linear fitting. Guided by both the theoretical and actual linear density equations, TPU-film-strip/cotton composite yarns were fabricated. Based on the spinning results, the theoretical linear density equation was subsequently revised.
Results Based on the theoretical model, the mathematical relationship between the film strip size and the parameters of the outer fibers was established. The theoretical linear density equation of the composite yarn sheath is given. Linear fitting was applied to derive the relationships between the yarn linear density and the 1/3 width of the strand at the nip of front roller, resulting in the actual linear density equations of the composite yarn sheath for the single roving feeding mode and the double roving feeding mode. The double roving feeding was found an satisfactory method. +++Using a TPU nanofiber film strip measuring 2.4 mm×0.1 mm as the core and cotton fibers as the sheath, the theoretical linear density of the sheath was calculated to be 32.33 tex, while the actual linear density was 17.21 tex. Therefore, the design linear density of the composite yarn sheath needed to be greater than 32.33 tex. Three types of TPU-film-strip/cotton composite yarns were spun using the double roving feeding method under the following process parameters: (1) design linear density to be 33.33 tex, which is close to 32.33 tex; (2) design linear density to be 50.00 tex, about 1.5 times of 32.33 tex; and (3) design linear density to be 66.70 tex, about twice of 32.33 tex. The results indicated that core exposure occurred in the yarns produced under situations (1) and (2), whereas a fully covered composite yarn was successfully achieved under situation (3). The spinning trials revealed that the theoretical linear density was insufficient in practice. Achieving a completely covered structure, the actual amount of sheath needed to be more than twice the theoretical linear density. Thus, the coefficient in theoretical linear density equation was doubled, resulting in a modified equation. During the actual spinning process of the composite yarns with strip as core, both the modified theoretical and actual linear densities of the composite yarn sheath should be calculated first. The maximum of these two values was defined as the critical linear density. A necessary condition for producing a fully covered composite yarn is that the linear density of the sheath must be greater than this critical value.Under situation (3), the produced TPU-film-strip/cotton composite yarn using TPU strip 33 tex and cotton 66 tex exhibited no core exposure and demonstrated a breaking strength of 13.43 cN/tex and an elongation at break of 45%.
Conclusion By integrating an electrospun film strip into cotton fibers, fully covered TPU-film-strip/cotton composite yarns were successfully produced using the ring spinning method. The fully covered yarn improves the evenness uniformity of the composite yarn and provides a certain protection to the core strips. This spinning approach overcomes the bottleneck of low mechanical properties that has limited the application of nanofiber membranes. Furthermore, electrospun films can be easily functionalized to exhibit a wide range of properties. The theory of strip-spinning may offer a new pathway for producing micro-nano composite yarns, particularly for specialized multi-functional yarns and fabrics.

Key words: thermoplastic polyurethane, nano-fiber film, strip spinning, composite yarn, completely covered structure, cotton yarn, ring spinning