Effect of soy milk pre-treatment on the dyeability of polyamide with annatto dye

Purpose The purpose of this work is to investigate the optimal conditions for dyeing polyamide fabrics using a natural dye extracted from annatto seeds, both with and without pre-treatment of the fibers with soy milk, to evaluate its potential as a plant-based mordant. The study encompassed analyses of color intensity and uniformity, wash and rubbing fastness tests, as well as characterizations through spectrophotometry, SEM and Fourier transform infrared spectroscopy (FTIR). The research aimed to understand the behavior of the carotenoids bixin and norbixin under different pH ranges, with an emphasis on sustainability and the replacement of metallic mordants with environmentally friendly alternatives. This study aims to evaluate the effect of soy milk pre-treatment on the dyeability of polyamide using annatto dye. Design/methodology/approach Polyamide fabrics were pre-treated with soy milk and dyed by exhaustion using annatto extract at different pH levels. Analyses included color strength (K/S), color difference (ΔE), washing and rubbing fastness, SEM and FTIR. Soy milk improved color uniformity and intensity, especially at alkaline pH (K/S = 40.30; ΔE = 0.77). It acted as a binder rather than a mordant. The study introduces a sustainable method for dyeing synthetic fibers with natural pigments, replacing metallic mordants with plant-based alternatives. Findings The main results indicated that soy milk pre-treatment did not act as a traditional mordant but enhanced color uniformity and intensity under alkaline pH conditions. The best experimental outcome was achieved with polyamide pre-treated with soy milk and dyed at basic pH, resulting in a K/S value of 40.30 and a ΔE of 0.77. Wash fastness was satisfactory (grade 4–5), and color transfer was lower in synthetic fibers. Although dye exhaustion was higher at acidic pH, uniformity was inferior. SEM and FTIR analyses suggested that soy milk acts as a binder rather than forming direct chemical bonds with the fiber. Originality/value This study presents originality by applying soy milk as a potential plant-based mordant for dyeing polyamide with natural annatto dye – an approach rarely explored on synthetic substrates. The research combines analytical methods such as spectrophotometry, SEM and FTIR to evaluate the interaction between fiber, dye and mordant, providing novel data on soy milk performance across different pH ranges. The work contributes to the advancement of sustainable alternatives in the textile industry by promoting the replacement of metallic mordants with eco-friendly solutions, and it demonstrates the technical and environmentally viable potential of natural dyes for application on synthetic fibers.