Making plastics more recyclable

Despite interest in recycling, the U.S. has seen a significant decline in plastic recycling rates due to increased plastic production. Polypropylene (PP) is the most common landfill-bound plastic, and high-density polyethylene (HDPE) is expensive, so both are key plastics for improving recyclability.

Mechanical recycling plays a critical role in reducing landfill-bound plastics. Tillinghast et al. compared rheological properties of virgin PP and HDPE with recycled PP and HDPE from three recycling plants to determine if rheology could effectively way to sort these into suitable applications and to reduce landfill waste.

Traditionally, melted polymers are characterized by one overarching parameter, their “melt flow rate.” Instead, the researchers measured both the shear viscosity and the extensional viscosity of the melted plastics — how they flow and stretch.

“How waste facilities sort and clean plastic waste is the largest source of contamination, as it can be very challenging to remove food particulates, adhesives, and separate different plastic types,” said author Guinevere Tillinghast. “This is why we chose to characterize via rheology. If the plastic is unclean or missorted it can cause failure during the extensional rheology or change the shear rheology.”

They found rheology measurements were effective at distinguishing HDPE for creating blow-molded products, like bottles, from HDPE for injection molding, which is used for more intricate structures. The measurements also distinguished PP for thermoforming applications, like food containers, from PP for injection molding. Incorporating a secondary sorting step by looking for visual clues about the material’s initial manufacturing methods increased this substantially, particularly for finding blow-molding HDPE.

Next, the researchers will investigate how contamination affects rheology and processing and note recyclability will improve if rheology measurements are adopted more widely.

Source: “Recovery of postconsumer mechanically recycled polymers,” by Guinevere Tillinghast, H. Henning Winter, and Jonathan P. Rothstein, Journal of Rheology (2025). The article can be accessed at https://doi.org/10.1122/8.0001004 .