Breaking down black plastic: US scientists use sunlight to transform hard-to-recycle polymers

Simple visible light can be used to break down black plastic into “chemically useful” materials that can then be turned into new products, according to a new study by researchers from the universities of Princeton and Cornell, US. This finding is particularly crucial considering the difficulties traditionally associated with recycling black plastics. 

“Carbon black is a photothermal agent which converts light into heat. Generally speaking, the more photons (light particles) it absorbs, the higher the temperature,” Erin E. Stache, assistant professor of chemistry at Princeton University and a co-author of the study, tells Packaging Insights. 

“For a comparison, think of a black versus a white car on a very sunny day — the black car will be much hotter to the touch because it’s promoting photothermal conversion. Carbon black absorbs a wide range of light (almost all visible light). Hence, it possesses extremely high temperatures at its surface when irradiated under focused sunlight, which exceeds the degradation temperature of polystyrene and achieves effective depolymerization.”

According to the scientific paper published in ACS Central Science, the research team recycled a lab-made black polystyrene by grounding a mixture of polystyrene and carbon black to a fine powder. They then placed it in a sealed glass vial and set the vial under high-intensity white LEDs for 30 minutes. 

The carbon black converted the LED light into heat, which broke apart. In experiments, the scientists recycled the leftover carbon black and styrene monomer into polystyrene, showing the method’s circularity.

Industrial application

Discussing the scalability of this method for industrial applications, Stache says that her team is “very excited” about their research-level results, which achieve an 80% styrene monomer recycling rate without any additional chemicals.

“We’re actually relying on the existing commercial additives currently used in product production, which is a huge benefit to our approach — not expensive, exogenous catalysts or reagents. For industrial applications, the reaction setups (such as container, gas flow, light collection, etc) and conditions need to be re-optimized for scale and reaction efficiency, and we are pursuing some very novel approaches with our collaborators.”

Erin E. Stache, assistant professor of chemistry at Princeton University (Image credit: Princeton University).The researchers applied the technique to post-consumer black plastic from food containers and coffee cup lids by cutting the waste into small pieces. They found up to 53% of the polystyrene converted to styrene monomer. 

Concerning the cost of the large-scale application of the method, Stache says: “For a fair comparison, we could assume the separation processes (using nitrogen gas flow and condensers) take the same amount of energy.”

“Then we are really comparing the energy they take to heat up the polystyrene. Under traditional chemical recycling methods like pyrolysis, the reactors need to be preheated to a high temperature (like 400 to 800 degrees Celsius) before polystyrene starts to depolymerize,” she continues.

“Luckily, photothermal heating is almost instantaneous, where the surface temperature could rise to several 100 degrees Celsius in nanoseconds under light irradiation. This not only largely reduces the preheating time, but also relies on harnessing solar energy, one of the most renewable resource we have at our disposal.”

Stache explains that photothermal conversion, like any light-driven process, requires large surface areas to allow sufficient light penetration, and in this case, heat diffusion, which she says will require further engineering and is currently underway in her research program.

Addressing concerns

The researcher further tackles issues related to the real-world application of the process, including concerns about secondary waste or emissions, as well as material contamination. 

“The proper scale-up, especially using sunlight, can largely reduce the use of fossil fuels needed for pyrolysis, which could significantly reduce CO2 emissions,” says Stache.

“The other byproducts from pyrolysis, such as the styrene monomer product, are the same and, with proper engineering controls, can be managed safely. We have shown that the leftover carbon black material can be recycled repeatedly without loss of efficiency in depolymerization.”

The research team investigated the impact of the presence of food contaminants by adding them to the plastic prior to depolymerization. 

Carbon black absorbs a wide range of light and possesses extremely high temperatures when irradiated (Image credit: Princeton University).“According to our results, even adding 20 wt % of canola oil, sugar or soy sauce did not significantly reduce the efficiency of our reaction, so our method can be quite tolerant to real-world plastic wastes.”

“However, food containing anti-oxidative compounds (or radical scavengers in chemistry language) might lower the recycling yield of polystyrene, or at least slow down the process, but this is under further study. The simplicity of our approach should lend it to be applicable to real-world plastic waste streams, with associated contaminants.”

The next steps

Stache expresses her excitement about the potential of her team’s approach as a real recycling solution. 

“Because we are using the existing commercial additives, manufacturers don’t need to incorporate new catalysts or components that they are unfamiliar with,” she highlights.

“In addition to scale up studies, we are very interested in further studying the impact of other additives on the efficiency of depolymerization, such as plasticizers, surfactants or other composite materials that might be added to commercial products.”

“With systematic studies, we hope to create a blueprint for good and bad additives (as they pertain to photothermal depolymerization) so that products can be designed cost-effectively with end-of-life recycling in mind,” Stache concludes.