Coloration of nylon textiles with natural dyes: a critical review of recent progress, dye-fiber interactions, fastness, and functional performance.

Textile coloration is challenged by the use of petrochemical dyes, intensive auxiliary use, high water and energy consumption, and the production of complex effluents. Natural colorants offer renewable alternatives, but their environmental superiority cannot be assumed for durable synthetic fibers. Nylon requires dedicated evaluation because its amide backbone, terminal functional groups, semicrystalline morphology, and grade-dependent free volume make its dyeing behavior unlike that of other fibers. This critical review systematically searched Scopus and the Web of Science for English-language journal articles published during 2000-2026; screening and duplicate removal yielded 158 unique studies. The reviewed evidence supports chemical compatibility between nylon and several natural-colorant classes through ionic interactions, hydrogen bonding, dipole interactions, hydrophobic partitioning, coordination, and diffusion into amorphous regions. Acidic conditions commonly enhance uptake by protonating amino end groups, while crystallinity, free volume, temperature, pretreatment, and colorant aggregation control diffusion and fixation. Microwave, ultrasound, plasma, and enzymatic processing can improve extraction, mass transfer, surface accessibility, or mild fixation but are not inherently cleaner without quantified energy, chemical, and scale-up data. Adsorption and kinetic models are often statistically adequate yet mechanistically overinterpreted. Light fastness, extract reproducibility, wet durability, and retention of functional properties remain major weaknesses, while metallic mordants may conflict with cleaner-production objectives. Environmental inventories, toxicity, cost, and pilot-scale evidence remain substantially weaker than coloration data. By integrating nylon chemistry, processing, performance, sustainability, and industrial readiness within a structure-process-performance framework, this review identifies standardized extracts, repeated-use durability testing, environmental and economic assessment, and pilot validation as priorities for credible cleaner coloration.