Researchers from Cornell University and Princeton University demonstrated in ACS Central Science that a specific additive in black plastics can be utilized, using sunlight or white LEDs, to transform black and colored polystyrene waste into reusable raw materials.
Inside this reaction vial, spotlit by concentrated sunlight, a piece of black polystyrene from a foam tray breaks down into a recyclable material. Image Credit: Hanning Jiang.
Plastics vary in recyclability, with certain types and colors being easier to process than others. Black foam and black coffee lids, typically made from polystyrene, often end up in landfills due to color additives that hinder effective sorting.
“Simple, visible light irradiation holds the potential to transform the chemical recycling of plastics, using the additives already found in many commercial products,” say the authors Sewon Oh, Hanning Jiang, and Erin Stache.
A developing approach to plastic recycling employs light to break down plastics into chemically valuable materials that can be repurposed into new products. This method relies on a helper compound to transform light into heat, facilitating the breaking of polymer bonds.
Identifying a suitable helper that avoids generating additional waste and integrates seamlessly into recycled materials presents a significant challenge. To establish a circular economy for plastic recycling, Stache and a team of researchers utilized an existing component in black polystyrene waste—an additive called carbon black.
Researchers evaluated a technique for recycling lab-produced black polystyrene by grinding a blend of polystyrene and carbon black into a fine powder. This powder was placed in a sealed glass vial and exposed to high-intensity white LEDs for 30 minutes. Carbon black converted the LED light into heat, breaking down the polystyrene’s molecular structure into a mixture of shorter one-, two-, and three-styrene units. These components were cleanly separated within the reaction setup. The team successfully reused the resulting carbon black and styrene monomer to produce new polystyrene, showcasing the circular nature of the approach.
When the method was applied to post-consumer black plastic from food containers and coffee cup lids, the waste was cut into small pieces, and up to 53% of the polystyrene was converted to styrene monomer. Waste samples contaminated with canola oil, soy sauce, and orange juice showed reduced efficiency.
Switching the light source from LEDs to concentrated sunlight outdoors increased the reaction efficiency to 80%. A multicolored mix of black, yellow, red, and clear polystyrene waste converted to styrene at a higher rate under sunlight (67%) than white LEDs (45%).
The researchers attribute this improvement to the higher light intensity of focused sunlight. This demonstration of sunlight’s capability to break down colored polystyrene waste suggests the potential for creating a closed-loop recycling process for these materials.
The authors acknowledge support from Cornell University, Princeton University, and the US Department of Energy’s Catalysis Science Early Career Award.
Journal Reference:
Oh, S., et al. (2024) Recycling of Post-Consumer Waste Polystyrene Using Commercial Plastic Additives. ACS Central Science. doi.org/10.1021/acscentsci.4c01317.