Optimization of supercritical carbon dioxide decolorization for recycling colored recycled PET pellets using response surface methodology

Colored waste polyethylene terephthalate (PET) commonly requires decolorization before high-value recycling. In contrast to our previous single-factor study on red PET pellets, the present work investigates a heterogeneous, fabric-derived mixed-color recycled PET feedstock in a continuous-flow supercritical carbon dioxide (scCO2)/N,N-dimethylformamide (DMF) system. A four-factor Box-Behnken response-surface design was used to evaluate time (20–40 min), temperature (50 °C–90 °C), pressure (9–15 MPa), and DMF flow rate (0.1–0.3 mL/min). The response was the reduction in integrated K/S color strength and should therefore be interpreted as an optical decolorization ratio rather than a component-specific dye mass balance. Model optimization identified 40 min, 90 °C, 15 MPa, and 0.24 mL/min as the optimum conditions. Three validation experiments gave a decolorization ratio of 81.382 ± 1.146% (mean ± SD, n = 3), compared with a predicted value of 82.528%. Recovered DMF gave a lower decolorization ratio of 65.132% under the same conditions. The viscosity-average molecular weight decreased by approximately 4.2%, indicating limited molecular degradation, while FTIR and TGA showed no major detectable change in the principal PET functional groups or thermal-degradation pattern. The results establish a multivariable operating window for lower-temperature, reduced-solvent decolorization, but they do not provide a dye-specific mass balance or establish unchanged crystallinity, molecular-weight distribution, or mechanical properties.