Dyeing and fastness properties of a series of 4-fluorosulfonylphenylazo-5-pyrazolone dyes on polyester were investigated in this study. The 4-nitrophenylazo-5-pyrazolone dyes were also synthesized to compare their dyeing and fastness properties on polyester with those of fluorosulfonyl-substituted analogues. The substantivity of 4-arylazo-5-pyrazolone derivatives containing a p-fluorosulfonyl group in the diazo component was lower than that of their nitro analogues which have a higher extinction coefficient and higher affinity because of the polar nitro group. They showed relatively hypsochromic color and lower chroma on polyester compared with their nitro analogues because of the relatively weaker electron-accepting power of the fluorosulfonyl group compared to the nitro group. Disperse dyeing of polyester with 4-fluorosulfonylphenylazo-5-pyrazolone disperse dyes achieved high color fastness and reduces the adverse environmental impact of the dyeing process by providing the option of performing alkali clearing instead of reductive clearing, which has high biological oxygen demand when discharged into the dyeing effluent and generates carcinogenic aromatic amines.

Blends of polyethylene terephthalate (PET) with spandex are widely used in sportswear and outdoor apparel. However, dyeing PET/spandex fabrics remains challenging due to the high energy required at elevated dyeing temperatures and persistent problems with poor color fastness caused by dye staining on the spandex component. In this study, we investigated the dyeing behavior of a chemically modified poly(ethylene terephthalate-co-polyethylene glycol) (PCP) blended with spandex and compared it with conventional PET/spandex blends. The PCP/spandex fabrics exhibited significantly improved dyeability, showing higher dyebath exhaustion and greater color strength than PET/spandex blends, particularly at sub-conventional dyeing temperatures. The optimal dyeing condition for PCP/spandex blends was identified as 110 °C for 60 min, which provided a balance between enhanced dye uptake and minimized spandex staining. Moreover, PCP/spandex fabrics demonstrated improved color fastness at lower dyeing temperatures (110–120 °C), primarily due to the reduced staining tendency of the spandex component when blended with PCP fibers. This reduction in spandex staining minimized dye migration during washing. Overall, these findings suggest that PCP/spandex blends offer a promising, energy-efficient alternative to conventional PET/spandex fabrics. They enable effective dyeing at lower temperatures while achieving improved color fastness, thereby addressing key challenges in the dyeing of elastic fiber blends.