In‐Process Melt Separation of Depolymerized PET in PET / PE Blends for Upcycling via Twin‐Screw Extrusion

ABSTRACT Our previous work focused on depolymerizing polyethylene terephthalate (PET) in twin‐screw extrusion, as part of a broader project to continuously separate PET from polyolefins in the melt. This study focused on linear low‐density polyethylene (LLDPE) and PET films and the use of ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), and bis(2‐hydroxyethyl) terephthalate (BHET) to depolymerize the PET in the extruder to levels above 90% Mw. In this work, the focus will shift to achieving separation of the two polymers in the twin‐screw extruder, which is made possible due to a 90% reduction in the Mw of the PET, which caused a decrease of its viscosity by several orders of magnitude. Owing to the viscosity difference and pressure buildup in the die, the low‐viscosity PET preferentially exited a degassing vent instead of going through the die. This is because the flow of the PET would travel through a non‐pressure vent rather than through a high‐pressure die. However, owing to the higher viscosity of the LLDPE, the pressure was too high to pass through such a small diameter vent hole. Supercritical CO 2 (SCCO 2 ) was used to assist in this extraction, but SCCO 2 negatively impacted the overall degree of separation. Through analysis of the separated materials, it was concluded that a high separation of the two materials was achieved. TGA and FTIR confirmed that the material separated from the vent was 100% PET. The material removed from the die was composed of 95% LLDPE and 5% PET.