Cascaded floatation of mixed waste polyesters based on a hydrophilic gradient induced by methanol, ethanol, and ethanolamine modification

Plastics are extensively used in packaging, automobiles, sanitation, agriculture, and architecture due to their lightweight, malleability, corrosion resistance, and low cost. However, their short lifespan leads to the rapid accumulation of plastic waste. Therefore, recycling waste plastics is crucial to conserve resources and protect the environment from "white pollution." This study developed a flotation process to facilitate the recycling of common polyesters by separating polyamide (PA), polycarbonate (PC), polyethylene terephthalate (PET), polymethyl methacrylate (PMMA), and polyurethane (PU). The flotation process was based on a hydrophilic gradient induced by modification with methanol, ethanol, and ethanolamine. The floatability sequence of the polyesters was PA < PU < PET < PC < < PMMA in the methanol-based system, PA < PU < PET < < PC ≈ PMMA in the ethanol-based system, and PET < PU < PA < < PC < PMMA in the ethanolamine-based system. The floatability of the polyesters was closely related to the reactivity of ester exchange. Based on the hydrophilic gradients, a cascaded flotation process was designed, following the ester exchange mechanism, to sequentially recover PET, PA, PC, and PMMA. PA, PET, and PU were the sinking plastics after modification with methanol, ethanol, and ethanolamine, respectively, while only PC remained hydrophilic after methanol modification. The purity of PA, PC, PET, and PMMA was 95.10 %, 93.68 %, 89.39 %, and 86.42 %, respectively, with recovery rates of 82.96 %, 83.90 %, 96.08 %, and 100 %. This study presents a promising approach for the flotation separation of waste polyesters.