Visualization of polyoxymethylene (POM) particle decomposition behavior in hydrothermal condition

Hydrothermal processing is of great concern as a prospective technology for solid plastic waste disposal. The knowledge of the decomposition behaviors of plastic particles in hydrothermal environments is vital to clarify the mechanism and improve the conversion efficiency. Polyoxymethylene (POM) is a common plastic with a pyrolysis starting temperature of 200 °C. This work conducted real-time and quantified visualization research on the whole decomposition process of a POM particle in saturated hot water (ranging from 148 °C to 250 °C) by a fixed camera and a calibration board. After being positioned at the visualized segment, the particle underwent first-step expansion (with a 108.3% the sectional area of the initial state at most), then became transparent and kept expanding (highest reaching 126.3% at 250 °C), followed by collapsing and dissolving into the hot water, which could be shortly described as thermal expansion, swelling expansion and pyrolyzed dissolution. The swelling effects were further studied and it was found the migration of water into the particle promoted the conversion. Finally, the analysis of the aqueous phase product figured out that over 90% of the carbon component was recovered in the liquid as formic acid, methanol, and ethers after processing. Through comparison with pyrolysis, it was found that the hydrothermal method could moderate the reaction conditions by decreasing the necessary temperature by over 200 °C, thus reducing the energy consumption of the reaction, and reform the products from formaldehyde the same time.