Soot and sunlight paved the way for recycling non-recyclable plastics

The emergence of robotic waste sorting systems has been a boon for the recycling industry, but at the same time it has removed black plastic from the list of recyclable items. Optical systems are simply not able to recognize it on the conveyor belt. But even if they could cope with this, the question of direct recycling remains, which is hampered by the special chemical composition of black plastic. Scientists from the USA were able to get around the problem.

Crushed black polystyrene in a test tube under the focused rays of the sun. Image source: Hanning Jiang

Black plastic, such as the caps on disposable coffee cups or food containers, must be both durable during direct use and recyclable. Its decomposition typically occurs in an oxygen-free chamber when heated to 300 °C, which cannot be considered energy efficient on a large scale. The use of other dyes or additives that would help black plastic disintegrate under milder conditions is not suitable for the manufacture of shelf-stable packaging.

Researchers from the USA proposed coloring polystyrene with carbon black additives. To some approximation, this is carbon black, but obtained within strict technical conditions. When illuminated by bright sunlight and even under artificial light, carbon black in the composition of polystyrene begins to participate in the photothermal conversion reaction. It absorbs photons and converts them into heat, helping the plastic break down – converting bound polystyrene molecules into liquid styrene. Liquid solutions can be reused to make plastic products.

In a series of experiments, scientists have shown that carbon black additives help convert up to 53% of black polystyrene into styrene under bright light. In the case of focused sunlight, the decomposition rate reaches 80%. If black polystyrene based on carbon black is mixed with waste polystyrene of other colors, then in total up to 67% of the recycled raw materials are converted into styrene.