Scientists have recently found a new method of recycling black polystyrene, which is simply using sunlight and an ingredient that already exists in plastic.
Not all plastics are equal since some types and colors are easier to recycle than others. Black polystyrene, commonly used in single-use items such as product packaging, food containers, and take-out coffee cup lids, is notoriously hard to recycle.
Its dark color from carbon black pigment keeps it from being imaged by optical scanners in recycling plants. Those scanners cannot detect black polystyrene because it absorbs light instead of reflecting it, making automated identification and sorting nearly impossible.
In addition, breaking polystyrene’s molecular bonds traditionally needs a temperature of over 300 ºC (572 ºF) in oxygen-free chambers making it costly, inefficient, and unsuitable for large-scale recycling.
An emerging strategy for plastic recycling is known as photothermal conversion, using light to help break down plastic into chemically useful materials that can be recycled into new products. This process requires another compound to help convert light into the heat needed to break apart polymer bonds, causing concerns about generating additional waste or altering the properties of the plastic.
Researchers at Cornell and Princeton University have developed a method that leverages carbon black, already embedded in black polystyrene, as the heat-converting agent. They ground lab-made black polystyrene into a fine powder and placed it under high-intensity white LED lights for half an hour.
After the carbon black absorbed the light, it generated localized heat to break the polystyrene into smaller molecules called styrenes. These styrenes and the leftover carbon black were then recycled into new polystyrene.
Next, the scientists cut black polystyrene food containers and coffee cup lids into small pieces and then subjected those to the light treatment. Up to 53% of the polystyrene was converted back into styrene. Moreover, they also found that it was up to 80% when much more intense focused sunlight was used as the light source.
In addition, a multicolored mixture of black, yellow, red, and colorless polystyrene waste converted at a 67% efficiency in sunlight, which is higher than white LEDs (45%).
