Recycled fiber-based stretch denim yarns with semi-elastic cores for reduced bagging tendency

Cotton-elastane core-spun yarns are widely used in stretch denim and comfort apparel, as they combine the natural aesthetics of cotton and the high stretch and recovery of elastane. However, fabrics produced with these yarns are prone to bagging under repeated and dynamic loading during wear. To overcome this limitation, two semi-elastic filaments, T400 (a bicomponent PET/PTT filament) and polybutylene terephthalate (PBT), were evaluated as alternative core materials. Denim production typically requires coarser yarns, resulting in high cotton consumption. Considering sustainability constraints and the rising cost of cotton, a portion of virgin cotton was replaced with recycled cotton derived from pre-consumer fabric waste. A portion of colored Ecovero (a new eco-friendly viscose) was incorporated into blends to enhance the spinnability and mélange aesthetics in yarn. Finally, a series of core-spun yarns was produced using varying blends of recycled cotton, Ecovero, and virgin cotton, combined with elastane, T400, and PBT cores. Their structural characteristics, tensile properties, plastic deformation, and stress relaxation were then evaluated. Elastane-based yarns exhibited the highest plastic deformation and stress relaxation, followed by PBT, while T400 showed the lowest values. Specifically, at 20% recycled fiber content, T400 core-spun yarns demonstrated approximately 68% lower plastic deformation and 37% lower stress relaxation compared to elastane-based yarns. The statistical analysis confirms that both recycled cotton content and core filament type play a decisive role in governing the yarn behavior.