Using sunlight to recycle black plastic

Hanning Jiang

This innovative method successfully breaks down previously difficult-to-recycle plastics

Spotted: Every year, a staggering 19-23 million tonnes of plastic waste pollute our waterways and oceans, posing a serious threat to marine life. While plastic’s affordability and versatility have made it indispensable in modern life, its widespread use has led to a pressing environmental crisis. 

Recycling offers a potential solution, but not all plastics are easily recyclable. Black plastic, a common material in consumer goods, presents a particular challenge. Its dark colour makes it hard to detect by sorting technologies, meaning it can’t be processed and recycled via normal channels. Now, however, chemists from Princeton and Cornell may have found a way to overcome this obstacle. 

The distinctive characteristic of black plastic is the presence of carbon black, a form of elemental carbon commonly used as a pigment or UV light absorber. The chemists discovered that carbon black’s properties also make it an ideal additive for converting light into heat, enabling the breakdown of polymer bonds for a conversion-based chemical recycling process. Exposing black plastics to high-intensity white LEDs or focused sunlight achieved significant results: a 53 per cent conversion rate with LEDs and an impressive 80 per cent conversion rate with sunlight. 

This technology can also be applied to multi-coloured and colourless polystyrene waste, but that’s just one of its additional benefits. According to Erin Stache (Princeton) and Hanning Jiang (Cornell), two of the lead chemists, since the heat is derived from light, “this allows the sole use of solar energy and could potentially save tremendous amounts of energy and help reduce CO2 emissions. Additionally, photothermal conversion generates heat only near the surface of the black pigment, which helps avoid side reactions and allows higher recycling efficiency.” 

The technology requires further refining. Assistant Professor Stache says, “We see varying efficiencies of depolymerisation from product to product. We’d like to probe the reason behind these changes by systematically studying the commercial products to understand the effects of other additives (…) At the same time, engineering a scalable system to be compatible with the scale of plastic production will be very important to see this technology become economically viable.”

Written By: Joshua Solomon