It turns out that the black plastic lid atop your coffee cup has a superpower. And the Stache Lab at Princeton Chemistry, which uncovered it, is exploiting that property to recycle at least two major types of plastic.
Their startling mechanism for promoting depolymerization relies on an additive that many plastics already contain: a pigment called carbon black that gives plastic its black color. Through a process called photothermal conversion, intense light is focused on plastic containing the pigment that jumpstarts the degradation.
So far, researchers have shown that carbon black can depolymerize polystyrene and polyvinyl chloride (PVC), two of the least recycled plastics in the planet's waste stream. Through a process called photothermal conversion, intense light is focused on plastic containing the pigment that jumpstarts the degradation.
Two recent papers highlight the potential. First, in ACS Central Science at the end of last year, there was a proof-of concept for the depolymerization of polystyrene using a common Fresnel lens to focus photonic energy. Then, earlier this month, the lab published their method to upcycle PVC in the Journal of the American Chemical Society (JACS).
In both cases, carbon black serves as the trigger of the breakdown, a quality Assistant Professor of Chemistry Erin Stache discovered recently and that even industrial partners she has spoken with were unaware of. The lab's method has since been tried out on such post-consumer waste as PVC pipes, black construction pipes, trash bags, credit cards, even those ubiquitous yellow rubber duckies.
"The surprising thing, especially with the black polystyrene depolymerization, is that they've been manufacturing these materials for decades and it seems no one recognized that this was possible," said Stache. "Under ambient sunlight, the energy is not sufficient to break down these polymers. But if you increase the light intensity enough, then you start seeing the depolymerization.
"We can certainly change our habits to help alleviate the amount of plastic we use. But we're not going to get rid of our dependence on plastic. So can we think of it instead as a resource? Can we turn it into other commodity chemicals that we have to make anyway? We have found that we can."
In their ACS paper, researchers showed that unmodified post-consumer black polystyrene samples were successfully depolymerized to a styrene monomer without adding catalysts or solvent. Simple, focused radiation on the plastic provided monomer yields of up to 80% in just five minutes.
"I think this marriage between photothermal and depolymerization strategies is really groundbreaking. Black colored plastic accounts for ~15% of all plastics, and we found that 10-weight percent of black polystyrene in plastic mixture is enough to give good yield," said Hanning Jiang, co-first author on the paper.
"Carbon black absorbs all the way from UV to IR, and that's great because what we want is for this agent to take as much light as possible and transform light into heat."
Next, the lab adapted their method to PVC and received strong results. They extended the process by adding polystyrene into the PVC-carbon black mixture—"We basically spatula it in," said Stache—and were able to upcycle the material and then derivatize it into a couple of common consumer products.
Part of the challenge of recycling PVC is that the material has carbon-chlorine bonds that generates hydrochloric acid (HCl) whether it's being recycled mechanically or chemically. Hydrochloric acid is corrosive and highly toxic.
"We used carbon black to initiate the thermal degradation of PVC, generate HCl with an acceptor for HCl that reacts to make an adduct," Stache explained. "So you can basically access a new commodity chemical from the process. We take advantage of what is normally a bad process - the HCl - and add it to another commodity chemical, and then we get a new product."
Recycling of Post-Consumer Waste Polystyrene Using Commercial Plastic Additives was authored by Sewon Oh, Hanning Jiang, Liat Kugelmass, and Erin Stache and appeared in the Nov. 25, 2024 edition of ACS Central Science.
Upcycling Poly(vinyl chloride) and Polystyrene Plastics Using Photothermal Conversion was authored by Hanning Jiang, Erik Medina, and Erin Stache and appeared in the Jan. 13, 2025 edition of the Journal of the American Chemical Society.
