Effects of Polymeric and Inorganic Additives on the Rheology and Dyeability of Multiple Recycled <scp>PET</scp> Fibers

ABSTRACT The influence of a chain extender (CE), Cloisite 30B (an organo‐modified nanoclay), and a halogen‐free flame‐retardant (FR) on the rheology, morphology, dyeability, and color fastness of multiple‐recycled polyethylene terephthalate (rrPET) fibers was investigated. Four formulations—virgin PET (v‐PET), CE040, FRNC4500, and FRNC4510—were melt‐spun and characterized. Rheological analysis confirmed that CE040 enhanced viscosity and spinnability, whereas FRNC4500 exhibited increased yield stress due to rigid filler–polymer interactions. Observations revealed that nanoclay promoted microchannel formation and improved additive dispersion, thereby facilitating dying. Dyeing with Disperse Red 60 demonstrated that FRNC4510 achieved colorimetric coordinates (L, a, b) and K/S values closest to v‐PET, with minimal Δ E ab (&lt; 2), confirming superior dye uptake. Fastness tests (ISO‐105‐series) further indicated that FRNC4510 exhibited wash fastness of 4–5, rubbing fastness of 4–5 (dry) and 3–4 (wet), and light fastness up to 5–6, outperforming CE040 and FRNC4500, which showed more surface‐dominated dye adsorption and consequently lower durability. Improved performance of FRNC4510 is attributed to the synergistic role of nanoclay and FR in enhancing amorphous accessibility and stabilizing dye–polymer interactions. These findings demonstrate that molecular architecture and additive compatibility are decisive not only for rheology and spinnability but also for long‐term color durability.