It appears that the black plastic lid on your coffee cup has a remarkable ability. The Stache Lab at Princeton Chemistry has discovered this feature and is utilizing it to recycle at least two significant types of plastics.
This innovative technique for enhancing depolymerization utilizes an additive commonly found in many plastics: a pigment known as carbon black, which gives plastics their black hue. By employing a method called photothermal conversion, researchers direct intense light onto the carbon black pigment in plastic, initiating the degradation process.
To date, studies have demonstrated that carbon black can effectively break down polystyrene and polyvinyl chloride (PVC), both of which are among the least recycled plastics in the waste stream. Through photothermal conversion, concentrated light is used on plastics with this pigment to trigger degradation.
Recent publications highlight this potential. A study published in ACS Central Science late last year presented a proof-of-concept for the depolymerization of polystyrene using a standard Fresnel lens to focus the light. Earlier this month, the lab released their method for upcycling PVC in the Journal of the American Chemical Society (JACS).
In both instances, carbon black acts as the catalyst for the breakdown, a finding Assistant Professor of Chemistry Erin Stache recently discovered, which even surprised the industrial partners she consulted. The lab’s technique has been tested on a variety of post-consumer waste, including PVC pipes, black construction pipes, trash bags, credit cards, and even the familiar yellow rubber duckies.
“What is particularly surprising, especially in the case of black polystyrene depolymerization, is that these materials have been produced for decades, yet it seems no one realized this was achievable,” noted Stache. “Under normal sunlight conditions, the energy isn’t sufficient to break down these polymers. But if you increase the light intensity significantly, then depolymerization begins to occur.”
“We can certainly modify our habits to reduce the amount of plastic we consume. However, it’s unlikely that we will completely eliminate our reliance on plastic. So, can we treat it as a resource instead? Can we convert it into other essential commodity chemicals? We’ve discovered that we can.”
In their ACS publication, researchers demonstrated that unmodified post-consumer black polystyrene could be successfully depolymerized into styrene monomer without any additional catalysts or solvents. By simply applying targeted radiation to the plastic, they achieved monomer yields of up to 80% in just five minutes.
“I believe this combination of photothermal energy and depolymerization techniques is truly groundbreaking. Black plastic constitutes about 15% of all plastics, and we’ve found that incorporating just 10% black polystyrene into a plastic mixture yields satisfactory results,” stated Hanning Jiang, co-first author of the paper.
“Carbon black is capable of absorbing light across a range from UV to infrared, which is beneficial because we aim for this agent to capture as much light as possible and convert it into heat.”
Subsequently, the lab adapted their process for PVC, achieving promising results. They expanded the method by mixing post-consumer polystyrene into the PVC-carbon black composition—”We essentially spatula it in,” Stache explained—and were able to upcycle the material into several common consumer products.
A major hurdle in recycling PVC is its carbon-chlorine bonds, which produce hydrochloric acid (HCl) whether the material is being mechanically or chemically recycled. HCl is corrosive and extremely toxic.
“We utilized carbon black to kickstart the thermal degradation of PVC, which produces HCl along with an acceptor for HCl that reacts to form a new compound,” Stache elaborated. “This way, we can access a new commodity chemical through the process. We utilize what is typically a negative aspect—the HCl—and combine it with another commodity chemical to create 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 was published in the Nov. 25, 2024, edition of ACS Central Science.
