Sure, plastic straws in your iced coffee are largely a thing of the past. But have you ever wondered what happens to that black plastic lid that sits atop your hot latte?

According to the American Chemical Society, not all plastics are created equal, and some types and colors are easier to recycle than others.

Those black foam or plastic coffee lids, often made with polystyrene, typically end up in landfills. They are made with a color additive that leads to ineffective sorting in recycling facilities that cannot detect black colors in their scanners. 

Instead of reflecting light, these items absorb it, making it nearly impossible for automated recycling processes to catch black polystyrene.

Plus, breaking down the molecular bonds of these plastics requires intense heating that is costly and ineffective, making it easier to toss these items instead of properly recycling them. 

Fortunately, scientists from Cornell and Princeton Universities have leveraged the chemistry behind these stubborn plastics to create a solution.

Mixed with a little sunlight or white LED light, the scientists say they can convert black and colored polystyrene into reusable materials.

While it’s not novel science to utilize light to help break down plastic into new, more useful materials, this strategy requires an additional compound to help convert light into heat to break apart the plastics’ chemical bonds.

“Simple, visible light irradiation holds the potential to transform the chemical recycling of plastics, using the additives already found in many commercial products,” said the paper’s authors, Sewon Oh, Hanning Jiang, and Erin Stache, in a statement. 

However, finding those “helper compounds” or additives can be difficult, as they may create more waste or are hard to easily incorporate into recycled materials.

But Jiang and Stache utilized a compound that is already found in black polystyrene: Carbon black.

Carbon black is an additive found in most black plastics, like food containers and packaging. The researchers first tested this method through lab-made black polystyrene.

They ground a mixture of the plastic and carbon black into a fine powder, placing it into a sealed glass vial. From there, they set the vial of powder under a high-intensity white LED light for 30 minutes. The carbon black converted that light into heat, breaking apart the polystyrene’s molecular structure.

This led to the creation of shorter one-, two-, and three-styrene units, which makes them easier to separate and use in new plastic products, creating a circular recycling method.

“By taking advantage of existing photothermally active additives in black plastics, we use light to depolymerize commercial plastics back to monomers,” the researchers wrote in their study, which was recently published in ASC Central Science.

Applying the technique to post-consumer food materials and coffee cup lids, the researchers found that up to 53% of the polystyrene turned into styrene monomer under LED lights.

Though waste samples contaminated with things like canola oil or soy sauce broke down slightly less efficiently under LEDs, under sunlight outdoors, they found a higher rate of efficiency for all colored polystyrene. 

This is especially exciting because the method’s use of sunlight and existing materials makes it a cost-effective and sustainable approach.

While it’s unclear what comes next for this method, the scientists are eager to be one step closer to creating a closed-loop recycling process for this type of plastic.

“Our work addresses the poor recyclability of black plastics and is amenable to mixed plastic waste,” they write in the conclusion of their research, “and our mild yet powerful technique makes the circular economy of black plastics more viable.”

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