Vacuum pyrolysis depolymerization of waste polystyrene foam into high-purity styrene using a spirit lamp flame for convenient chemical recycling

Waste plastic pollution is a pressing global issue. In particular, waste polystyrene (PS) foam causes significant damage to marine ecosystems due to its toxic decomposition products. However, effective recycling methods for waste PS foam have not yet been developed. With the aim of establishing an efficient chemical recycling process for waste PS, this study presents a simple and convenient method for recovering high-purity styrene from waste PS foam by vacuum pyrolysis depolymerization. The vacuum pyrolysis of waste PS foam was performed in a Pyrex tube, ignited using a spirit lamp flame for 20 min, resulting in a recovery of 98%-purity styrene in a 55% yield. The yield was further increased to 67% by conducting the pyrolysis in the presence of molecular sieves, which prevented side reactions such as backbiting and radical coupling. Additionally, increasing the temperature enhanced the styrene yield. This vacuum pyrolysis depolymerization process for chemical recycling requires no temperature controller or monomer fractionation. It can be applied to various waste plastics with carbon-carbon backbones, thus offering a promising solution for reducing plastic waste. Furthermore, the closed loop system, which includes monomer recovery without the need for purification and its reuse for reproducing the original products, also contributes to a reduction in natural fossil fuel consumption. Vacuum pyrolysis of waste polystyrene foam over a spirit lamp flame for 20 minutes produced 98% pure styrene without needing fractionation or purification, which promises a convenient closed-loop chemical recycling system.