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Recycling

New process recycles black plastics using sunlight

Carbon black pigment helps drive recycling of black polystyrene

by Payal Dhar, special to C&EN
December 17, 2024

 

Various clean black plastic containers for carryout or delivery food spread neatly across a gray surface.
Credit: Shutterstock

Nearly all black plastics get sent to the landfill or get incinerated because their pigment makes them difficult to sort in plastics recycling plants. As a result, the recycling industry pays little to no attention to black plastics when devising strategies for recycling waste material. But a new study describes a simple method to break down polystyrene that takes advantage of the very pigment that makes these plastics a problem in the first place (ACS Cent. Sci. 2024, DOI: 10.1021/acscentsci.4c01317).

The process uses a chemical recycling method called photothermal conversion that converts light into heat. That concentrated heat breaks down a plastic’s polymer chains into smaller molecules that can then be re-used. Usually, the process needs an additive that can absorb light efficiently, but in this case, a pigment already present in black polystyrene, carbon black, does the job.

The researchers collected postconsumer items made of polystyrene—such as food containers, coffee cup lids, plant pots—and exposed them to light from LEDs for 30 min. The heat generated from the photothermal reaction broke down the polystyrene into styrene units, converting up to 53% of the postconsumer plastic to styrene monomer.

Outdoors on a brick surface, a Fresnel lens clamped to a ring stand focuses sunlight onto a vial full of chips of black plastic.
Credit: Hanning Jiang
Focused sunlight breaks down a sample of black polystyrene.

What’s more, they repeated the experiments with focused sunlight instead of light from LEDs and achieved even higher yields—up to 80% conversion of a black foam tray to styrene in just 5 min. But in some postconsumer samples, the presence of orange juice, canola oil, soy sauce, or other contaminants lowered the efficiency.

This method also worked if black and other colored polystyrenes were mixed. The researchers found that as long as 10% of the mix was black plastic, they could get yields similar to those of all-black batches when using the sun as the light source. After the light has broken down the black polystyrene, the leftover carbon black continues the photothermal reaction on the remaining polystyrene. At the end of the process, the carbon black can be recovered and reused for further recycling, while the plastic monomers can be recycled into polystyrene.

“The incredible feature of this work is its simplicity,” says Erin Stache of Princeton University, the study’s lead investigator. Rather than relying on catalysts and solvents, it uses an additive already present in commercial plastic to promote end-of-life recycling.

Also, industrial chemical recycling methods usually rely on pyrolysis, requiring temperatures above 400 °C. The method described in this paper achieves high efficiency with localized heating at temperatures below 150 °C.

This paper demonstrates a novel approach to recycling black polystyrene, says Jinxing Chen of Soochow University, who wasn’t involved with the study. “[It] addresses long-standing challenges in recycling black plastics and aligns with goals for a circular plastic economy,” he says.

The study is a proof of concept. Stache’s team is looking for ways to reconfigure the reaction setup to make it more scalable, to rely more on sunlight to reduce costs, and to depolymerize other plastics.

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