In a groundbreaking discovery, researchers at Princeton University’s Stache Lab have found an unexpected superpower in black plastic. The unassuming black plastic lid on your coffee cup—like many other plastic items—contains a pigment called carbon black, which has now been identified as a trigger for a revolutionary new recycling method. This novel technique has the potential to recycle two of the most difficult-to-recycle plastics: polystyrene and polyvinyl chloride (PVC).
While carbon black has long been used to give plastic its black color, scientists at Princeton have now revealed that it plays a crucial role in depolymerizing plastics, breaking them down into their monomers, the basic building blocks of these materials. Through a process known as photothermal conversion, intense light focused on the plastic triggers the degradation, offering a promising solution to some of the world’s most persistent plastic waste problems.
The Science Behind the Breakthrough
In essence, the process developed at the Stache Lab uses carbon black as a catalyst for photothermal conversion. When plastic containing carbon black is exposed to high-intensity light, the pigment absorbs light across a wide spectrum—from ultraviolet (UV) to infrared (IR)—and converts this energy into heat. This localized heat is sufficient to break the chemical bonds in the polymer, initiating the process of depolymerization—the breakdown of the complex polymer chains into their simpler monomers.
This new discovery focuses on two major types of plastic—polystyrene and PVC—both notorious for being difficult to recycle. Polystyrene, commonly used for products like packaging materials, disposable cups, and food containers, and PVC, found in pipes, flooring, and a range of other products, are often discarded because of the complexity and cost involved in recycling them. The Stache Lab’s method could potentially revolutionize the way these materials are processed and reused.
Polystyrene Depolymerization: A Groundbreaking Demonstration
One of the most notable achievements came at the end of 2024 when researchers demonstrated the depolymerization of black polystyrene using just a Fresnel lens to focus light on the material. Published in the journal ACS Central Science, this proof-of-concept showed that unmodified post-consumer black polystyrene could be successfully broken down into styrene monomers—the basic building block of polystyrene—without the need for catalysts or solvents.
Remarkably, this process achieved monomer yields of up to 80% in just five minutes, showing the incredible potential of the method for fast and efficient recycling. The fact that carbon black could trigger such a transformation, a process previously unrecognized in industry, astonished even the researchers involved. Assistant Professor Erin Stache, who led the research, noted that this was a major discovery, particularly considering that no one had identified carbon black’s catalytic properties for plastic degradation in the decades of its use in manufacturing.
Stache emphasized that while ambient sunlight was not powerful enough to break down these polymers, the increased intensity of the light focused on the plastic was sufficient to initiate the chemical reaction. “Under normal sunlight, the energy isn’t enough to break down the polymers. But if you increase the intensity of the light, you start to see the depolymerization.” This simple yet effective method could become an important tool in the global effort to reduce plastic waste.
Upcycling PVC: Turning Waste Into Value
Following the success with polystyrene, the researchers turned their attention to PVC, a material notorious for its difficult recycling process. PVC has carbon-chlorine bonds that, when broken, release hydrochloric acid (HCl), which is both corrosive and toxic. Handling PVC recycling is complicated because the generation of HCl requires additional steps to safely manage or neutralize it.
In a subsequent study published in the Journal of the American Chemical Society, Stache’s team demonstrated how they could use carbon black to initiate the thermal degradation of PVC and handle the corrosive byproducts more safely. By adding an HCl acceptor to the mix, the researchers were able to neutralize the hydrochloric acid and transform it into a new commodity chemical. This innovative approach allowed them to upcycle PVC, turning it into useful materials rather than just breaking it down into waste.
“What we’ve done is take advantage of what is normally a bad process—the HCl release—and turn it into something productive,” explained Stache. “We’re taking PVC, which has been a huge challenge for recycling, and creating new products from it.”
This upcycling technique offers a two-fold solution: not only can PVC be depolymerized, but it can also be transformed into new products, contributing to a circular economy and reducing the environmental burden of this challenging plastic.
Carbon Black’s Role in Recycling: A Game-Changer
What makes this discovery particularly exciting is the role of carbon black—a common but often overlooked additive used in a wide range of products. From tires to inks and plastics, carbon black is ubiquitous in consumer goods, particularly those made of black plastic. Stache and her team found that even small amounts of carbon black (as little as 10% by weight in a plastic mixture) can dramatically enhance the efficiency of the recycling process.
The use of carbon black as a catalyst is a game-changer for recycling. This additive has been in widespread use for decades, but its potential as a catalyst for photothermal conversion had not been recognized until now. Carbon black’s ability to absorb light across a broad spectrum and convert it into heat is what makes this process so effective. With its wide availability and relatively low cost, carbon black could become a key component in a more sustainable approach to plastic recycling on a global scale.
Scaling Up: From Lab to Industry
While the laboratory results have been promising, one of the main challenges now facing the Stache Lab is scaling the process to make it viable on an industrial level. In an industrial setting, the key question is whether enough light can be focused on a large volume of plastic waste to effectively drive the depolymerization process. This involves overcoming challenges such as the penetration of light through layers of waste plastic and ensuring that the photothermal conversion works efficiently at scale.
However, Stache is optimistic about the future. She explained, “We’re using post-consumer waste and depolymerizing it just by shining light on it. That’s the most applied thing you can do.” With continued research and collaboration with engineers, the team is working to refine the technique and find ways to adapt it for large-scale applications. “If the chemistry works, you’re going to find a way to scale it,” Stache said confidently.
The Role of Undergraduate Researchers: A New Generation of Innovators
The research team also includes students like Erik Medina, a Princeton undergraduate who joined the Stache Lab after hearing Stache present her work at a colloquium. As a co-author on the JACS paper, Medina contributed to overcoming technical challenges in the project, including optimizing the reaction setup. His determination and passion for the project highlight the importance of engaging young scientists in cutting-edge research.
Medina’s perspective on the work is insightful: “There is a need to fully utilize the chemical resources present in the raw starting materials,” he said. “I’d love to see people find more creative ways to leverage common additives like carbon black to do this.” His enthusiasm for the project and belief in the real-world applications of the technique speaks to the growing interest in sustainable technologies among younger generations of scientists.
Looking Ahead: A Sustainable Future for Plastic Recycling
The Stache Lab’s breakthrough offers a hopeful outlook for the future of plastic recycling. With an innovative process that uses light to break down tough plastics like polystyrene and PVC, the team is paving the way for a more sustainable and efficient method of dealing with plastic waste. While challenges remain, particularly around scaling the process for industrial use, the results so far are promising.
As Stache aptly put it, “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 the coming years, this approach to plastic recycling could become an integral part of efforts to tackle the global plastic waste crisis. By turning waste plastics into valuable chemicals and products, the Stache Lab’s work could mark the beginning of a new era in recycling—one where carbon black, light, and innovative thinking offer a powerful solution to a pressing environmental challenge.
References: Sewon Oh et al, Recycling of Post-Consumer Waste Polystyrene Using Commercial Plastic Additives, ACS Central Science (2024). DOI: 10.1021/acscentsci.4c01317
Hanning Jiang et al, Upcycling Poly(vinyl chloride) and Polystyrene Plastics Using Photothermal Conversion, Journal of the American Chemical Society (2025). DOI: 10.1021/jacs.4c16145