Meaning
Industrial purification processes that remove absorbed volatile organic compounds and heavy metals from recycled polymer flakes prepare the material for food-contact applications. In the reclamation of polyethylene terephthalate and high-density polyethylene bottles, post-consumer flake decontamination uses high temperatures, vacuum, and gas stripping to extract deep-seated contaminants. This process is restricted to washed, sorted flakes before they are remelted and pelletized.
The efficiency of the purification step is determined by the challenge test protocol required by food safety agencies.
Cleaning Mechanism
Diffusion of volatile compounds from the solid polymer into the gas phase is driven by heat and vacuum. The flakes sit in solid-state polycondensation reactors for several hours to allow the contaminants to escape. This also increases the molecular weight and viscosity of the resin.
Process Variable
Residency time and temperature must be tightly controlled during the decontamination run. Higher temperatures accelerate the diffusion rate but can cause yellowing of the polymer flakes if oxygen is present. Nitrogen gas flows through the reactor to maintain an inert atmosphere.
Recycling Qualification
Food-contact approval depends on the complete removal of a specific set of test chemicals. Challenge tests involve deliberately contaminating flakes with surrogate chemicals and measuring the remaining concentrations after decontamination. Successful qualification allows the recycled plastic to be reused in beverage bottles.
