Complete separation of colorants from polymeric materials for cost-effective recycling of waste textiles

•Complete color separation from waste textiles to realize textile recycling.•Solvents with good dye solubility incapable of separating 100% dyes from fabrics.•Reducing fiber density raises μdye in all parts of fibers higher than that in solutions.•Solvents with solubility parameter close to polymer raise dye desorption equilibrium.•Molecular weights of polymers and structures of dyes were not changed. Currently, only 1% of waste textiles, mainly whites, are recycled because dyes on fibers caused various problems. Unseparated colors might sublime during the fiber regeneration process and pollute the working environment. More seriously, final color quality of regenerated fibers is uncontrollable. Technologies for complete color removal from waste textiles either by dye-destruction or extraction have major challenges impeding their industrialization. Dye-destruction damages polymers, changes dyeability of regenerated textiles, and pollutes the environment. Since the differences in solubility parameters between polymers and dyes are large, and dye extractions focuses on finding solvents with high dye solubility, dye-extraction fails to remove all dyes from textiles. Finding solvents is almost impossible to lower the chemical potential of dyes in solutions than that in fibers where dyes are accessible, and polymers are tightly arranged. Lessening fiber density substantially disrupts physical interactions between dyes and fibers, and thus raises chemical potential of dyes in all parts of fibers higher than that in solutions, although reducing fiber density might increase the chemical potential of dyes in solvents. Here we demonstrate that minimization of fiber density by solvents and temperatures completely removes disperse dyes, acid dyes and direct dyes from polyethylene terephthalate, nylon and cotton fibers, respectively. The chemical structures of dyes and average molecular weight polymers did not change after dye removal.