Optimal process conditions to recycled polyester dyeing using natural annatto dye

The growing interest in replacing synthetic dyes with natural sources is a result of environmental concern, since natural dyes are less aggressive and more biodegradable. Despite being a sustainable alternative, natural dyes have some limitations. In this context, this paper evaluates the natural annatto dye application in conventional (PES) and recycled polyester (rPES) fabrics, with a view to industrial application. For this purpose, operational parameters (temperature and pH) were varied to achieve the optimum dyeing process condition, regarding the color and quality of fabrics. The temperature oscillated from 98.3 °C to 136.2 °C, while the pH from 4.17 to 9.83 in the liquid bath during dyeing, at a fixed concentration of 1.5% of natural annatto dye, for 60 min. FTIR analysis was performed to characterize the presence of the dye molecule in the dye bath. The dyeing that showed the best results, for the exhaustion (95.6%), K S (2.21) and fastness scores (4/5), was carried out at 115 °C and pH 4.17. The dyed samples showed low light fastness values after 5 h of exposure. Further analysis to show the effect of dyeing time, presented that in 10 min the maximum fabric coloring is reached, with a K S value of 3.58. Also, the color parameters changes of the dye in water solution were evaluated at different pH values. In conclusion, the conditions evaluated for natural annatto dye application presented good dyeability for both fabrics (PES and rPES), demonstrating great potential for use in the textile industry . • Temperature, time and pH parameters in recycled PES dyeing were investigated. • Lower pH and higher temperature provide notable dyeing results in recycled polyester. • Although the absorbance was lower at acidic pH, higher K⁄S was obtained for the fabric. • Reduced time for dyeing recycled PES was reported. • A cleaner production process using natural dye in recycled PES dyeing was developed.