Scientists use sunlight to break down the most recyclable plastic

Black polystyrene, commonly used in food containers, coffee lids and packaging, is notoriously difficult to recycle. Its dark color, achieved with carbon black pigments, defeats optical scanners at recycling plants, and the pigment complicates traditional recycling methods.

Now scientists at Cornell and Princeton universities have developed a groundbreaking method that uses sunlight and soot itself to convert this challenging waste into reusable materials.

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

The challenge with black polystyrene

While bans on single-use plastics increase, black polystyrene remains widely available. The challenges of recycling arise from two main problems: sorting difficulties and an energy-intensive breakdown.

Optical scanners, which are crucial for sorting plastics in recycling plants, cannot detect black polystyrene because it absorbs light instead of reflecting it. This makes automated identification and sorting almost impossible.

Additionally, breaking polystyrene’s molecular bonds traditionally requires heating to over 572°F (300°C) in oxygen-free chambers. This process is costly and inefficient and is therefore not suitable for large-scale recycling.

A promising alternative, photothermal conversion, uses light to heat plastics via an auxiliary compound that converts light into local heat. However, the inclusion of these compounds raises concerns about creating additional waste or altering the properties of the plastic when it is first used.

A new approach with Carbon Black

Instead of introducing external compounds, the researchers used carbon black, already embedded in black polystyrene, as a heat conversion agent. This approach eliminates the need for additional additives and at the same time takes advantage of a component that was previously considered a hindrance.

In their experiments, the team ground lab-made black polystyrene into a fine powder and placed it under high-intensity white LED light. The soot absorbed the light and produced localized heat that broke the polystyrene into smaller molecules called styrenes.

These styrenes were then recycled into new polystyrene along with the remaining carbon black, achieving a closed-loop process.

When tested on consumer waste such as food containers and lids, the results were promising. Up to 53% of the material was converted back into styrene. In concentrated sunlight, the efficiency increased to 80%. Thanks to the heat conversion properties of carbon black, even mixed waste containing black, yellow, red and clear polystyrene is converted with 67% efficiency in sunlight.

A step towards sustainable plastic recycling

This method’s reliance on sunlight and existing materials makes it both cost-effective and sustainable. Concentrated sunlight offers the highest efficiency, allowing the process to be adapted to regions with a lot of sunlight.

Even when contaminated with substances such as rapeseed oil or orange juice, the method showed only minimal losses in efficiency, demonstrating its robustness.

By addressing the sorting and energy challenges of recycling black polystyrene, this innovation represents a significant step towards reducing landfill waste. Additionally, it could pave the way for the recycling of other difficult-to-process colored plastics, thus creating a circular economy for plastic use come closer.

As environmental regulations on single-use plastics tighten, solutions like these could be critical to managing plastic waste.

The study was recently published in ACS Central Science.