Dry and chemical-free chromatic homogenization of mixed-colored waste polyester textiles using supercritical CO₂

Abstract

Herein, disperse dyed mixed-colored (blue, yellow, orange) polyester textile (PET) was homogenized into a common final color using a supercritical carbon dioxide (scCO₂) system following Le Chatelier's equilibrium principle and partition equilibrium between fabric–scCO₂–dye phases and subtractive color mixing principles. For that, mixed-colored PET were placed together in the scCO₂ system at predetermined weight ratios and treated under controlled temperature and pressure conditions. Under supercritical conditions, dye molecules exhibited enhanced mobility and partial redistribution between textile substrates via the scCO₂ medium, enabling chromatic equilibration without the use of auxiliary chemicals or solvents. The resulting PET demonstrated a new and consistent color, confirmed by colorimetric analysis (K/S values) and visual color variance compared to the initial mixed-color feedstock. Detailed data analysis of treated PET revealed that the standard deviation of K/S values within each group ranged from 0.06 to 0.38, confirming homogenized shade across all sample, which is in line with scCO₂’s unique superlevelling capacity, enabling chromatic harmonization. The samples were further analyzed for their color fastness to wash and rubbing as well as tensile strength properties. Results showed that, the color fastness of both wash and rubbing ranges from 4/5–5 with full strength retention before and after the treatment. The results of this study are of great importance, as it presents a new, dry and chemical-free approach for homogenization of mixed-colored waste polyester into a common color to facilitate sustainable repurposing and recycling of textile waste, reducing the need for re-dyeing.