Absorption and removal of contaminants from recycled high-density polyethylene

A major concern in the processing and use of recycled plastics is the variable, and unknown, nature of the feedstock. Post-consumer and post-industrial waste may have been used for a variety of applications not originally intended by the manufacturer. This research was designed to probe the migration potential of common, yet possibly hazardous, substances which might be introduced into a high density polyethylene (HDPE) beverage container during consumer misuse. After determining the feasibility of container contamination, a systematic study was conducted to determine the likelihood of removing potential contaminants under the conditions of a typical recycling process.^ The substances selected and the contaminant each represents is as follows: (1) xylene, paint thinner or gasoline; (2) methyl salicylate, oil-based household cleaner; (3) isopropanol, common organic solvent; and (4) ethylene glycol, antifreeze. These substances represent a range of physical properties.^ The recycling process was divided into four steps: absorption of consumer introduced contamination, desorption during the wash cycle, desorption during drying, and desorption during extrusion. Model experiments were performed to measure the migration of the contaminants in each step.^ The solubility and an effective Fickian diffusion coefficient were calculated for each step of the recycling process and each surrogate. In all steps except absorption, the diffusion coefficient represents a combination of the rates of diffusion in the HDPE and removal from the HDPE surface, whether by dissolution or by vaporization. The rate of migration was dependent on the polarity, vapor pressure, and solubility of the contaminants as well as the density of the HDPE. The results from the laboratory results were compared to results from a pilot plant recycling operation.^ In this worst case scenario of 100% consumer contamination of the HDPE, none of the contaminants were removed to levels acceptable by the U.S. Food and Drug Administration (FDA) for food contact applications. However, this research demonstrated that increases in the residence time in and temperature of the desorption environment could significantly increase the amount of contaminant removed during a typical recycling. Improvements in the extruder, such as an applied vacuum for removal of volatiles, could also have a significant effect.