Continuous Chemical Recycling of Polystyrene with a Twin – Screw Extruder

Abstract

The increasing scarcity of resources and growing environmental awareness require higher recycling rates for plastic waste. Common techniques to do that are mechanical recycling, thermal recycling and chemical recycling, which is also called feedstock recycling. From all three techniques, chemical recycling is the only one which can produce new materials that correspond to the quality of conventional virgin material. However, the technique is limited to suitable polymers, e.g. polystyrene, which can be depolymerised at elevated temperatures. For an efficient industrial scale-up, a continuous process is desirable. In our work, we present such a continuous process for the recycling of polystyrene from post-industrial waste. A co-rotating, tightly intermeshing twin-screw extruder in high-temperature design is used together with a vacuum separation system with three degassing openings. By determining a stable process point a continuous depolymerisation of polystyrene is technically realised.

The atmospheric oxygen and moisture are removed via the first degassing opening of the extruder. The degradation products of the depolymerisation process are then degassed through the second and third opening. The degradation products are passed through a water-cooled condenser where they are liquefied. The styrene yield is maximised by tuning the process parameters barrel temperature, screw speed and configuration, mass throughput and degassing design. Analysis of the products reveals a considerable influence on increasing recovery rates with increasing barrel temperature, decreasing throughput and longer residence time. A longer residence time is realised by a lower throughput and an optimised screw configuration. We anticipate our process as a very promising technique to efficiently and economically scale-up the chemical recycling of poly-styrene waste.