Aqueous chemi-mechanical recycling for blending, decolorizing, and purifying mixed polyolefins

In this work, aqueous chemi-mechanical recycling is introduced as a process to overcome challenges with chemical recycling and mechanical recycling. In chemi-mechanical recycling, water near its critical point (325–400 °C, 9–25 MPa) promotes the intermixing of polyethylene (PE) and polypropylene (PP), a phenomenon that exploits the unusual solvent properties of near critical water including low dielectric constant. Processing at optimal temperature and reaction times preserves the molecular weights and crystallinities of both the PE and PP phases, while simultaneously producing well-dispersed droplet morphologies with less coalescence than a corresponding melt-blend prepared through twin-screw extrusion performed at a temperature of 200 °C. The extractive nature of the aqueous process reduces volatile organic compounds by 96% and reduces the color of the recycled resin. Preservation of molecular weight overcomes the polymer degradation limitation of mechanical recycling, and the water environment reduces exposure to molecular oxygen, eliminating polymer oxidation. Economic and energy analysis indicate that chemi-mechanical recycling is cost competitive with virgin material and results in lower emissions than incineration. Under conservative assumptions, the CO 2 abatement cost of chemi-mechanical recycling is 56 US dollars per ton of CO 2 , which compares favorably to other proposed methods of emissions abatement such as direct air capture of CO 2 . A mechanism is proposed wherein the chemi-mechanical process extracts additives and colorants and leads to partial polymer chain miscibility to promote blending. • Near-critical water enables the intermixing and purification of PE and PP wastes • Plastic properties are preserved, and well-dispersed droplet phases are produced • 96% of volatile organics are extracted and color of PCW is reduced • Economically & environmentally competitive compared to current technologies