A diffuse-interface formulation for melting and vaporization of plastics*

Thermo-chemical recycling of plastics, which uses heat and oxidizer-reduced atmospheres to convert waste into syngas, is an emerging technology for recycling multi-polymer materials.

Plastics undergo molecular breakdown during melting and vaporization. Capturing this complex phenomenon in simulations necessitates accurate interface-resolved simulations to account for phase changes and gas phase transport with compressibility effects. Additionally, a surrogate representation of the thermophysical properties for each phase is required. Diffuse Interface (DI) method formulations are a viable alternative for scenarios involving mass transfer, as DI allows solving mass balance equations in each phase, thus satisfying mass conservation at a discrete level. 

This work aims to derive a diffuse interface formulation for multiple solids and multi-component systems under non-dilute conditions for liquid and vapor phases. The formulation further includes melting and vaporization terms to account for phase change.  The model's capabilities are demonstrated using multi-layer films commonly found in food packaging, consisting of low-linear polyethylene and polypropylene.