Scientists at the universities of Princeton and Cornell in the United States have developed a method to address two issues at once: black plastic pollution and limited recycling of polystyrene (PS) and polyvinylchloride (PVC).
In a series of papers, the team explained how carbon black, an additive that makes black plastics difficult to recycle, promotes the depolymerisation of PS and PVC under intense LED or solar light.
The process, called photothermal conversion, chemically recycles post-consumer PS and PVC under low bulk temperatures compared to pyrolysis and produces fewer, milder side reactions.
Carbon black is used to jump-start the degradation under intense light due to its ability to absorb radiation from ultraviolet (UV) all the way to infrared (IR), thereby converting more light into more heat. What makes it so hard to sort black plastics – black carbon’s near-infrared absorbance - thus makes this pigment the perfect catalyst for PS and PVC depolymerisation, themselves hard to recycle polymers.
PS depolymerisation
The team first developed the method for PS depolymerisation. They showed that carbon black facilitates localised heating under visible light, enabling selective depolymerisation of polystyrene into styrene monomer.
Using polystyrene-carbon black composites with 10% carbon black by weight, the method achieved up to 60% monomer yield under LED light. The process temperature did not exceed 150 C, much lower than the 395 C usually required to chemically recycle PS.
Under these same conditions, the researchers then turned to unmodified post-consumer black polystyrene samples, including PS foam, food containers, and coffee cup lids. No additional carbon black or metal catalysts were added to the process. The method successfully depolymerised eight black polystyrene plastics with up to 53% yield.
In addition, the team melt processed white or clear polystyrene samples with carbon black to achieve 5 weight percentage carbon black films. Under the same depolymerisation conditions, the method produced styrene with up to 54% yield.
To account for food contamination and impurities common in post-consumer waste, the researchers conducted photothermal depolymerisation of black PS foam in the presence of various food contaminants in 20 weight percentage concentration.
With sucrose contamination, monomer yields were almost identical to the uncontaminated sample. With canola oil and soy sauce contamination, yields lowered to 46%. With orange juice, to 37%.
Finally, the academics tested their method using sunlight as the sole irradiation source rather than LED lights. Using a Fresnel lens, they found PS foam fully depolymerised with 80% styrene yield after 5 minutes.
“The surprising thing, especially with the black polystyrene depolymerisation, is that they’ve been manufacturing these materials for decades and it seems no one recognised that this was possible,” said co-author Erin Stache, assistant professor of chemistry at Princeton. “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 depolymerisation.”
PVC depolymerisation
Next, the lab adapted their method to PVC depolymerisation. PVC recycling is challenging, amongst other things, because when broken down it releases hydrochloric acid (HCl), a corrosive and toxic by-product.
The team actually used this to its advantage, capturing the HCl released during photothermal degradation and using it to synthesise new valuable chemicals.
Using post-consumer PVC waste samples including pipes, containers, credit cards, tubes, and toy ducks, PVC upcycling with photothermally recycled styrene achieved 84% (1-chloroethyl)benzene under white LED light in one hour, and co-upcycling of PS and PVC achieved 42% yield under focused sunlight irradiation in just 4 minutes.
Subsequent nucleophilic substitution on the chloro-adduct formed 1-phenylethanol (a fragrance additive) and fendiline (a heart disease drug) in high yields.
“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.”
The team shared their findings in two papers: Recycling of Post-Consumer Waste Polystyrene Using Commercial Plastic Additives, published in ACS Central Science; and Upcycling Poly(vinyl chloride) and Polystyrene Plastics Using Photothermal Conversion, published in the Journal of the American Chemical Society.