Branched polystyrenes as recycling additives to upgrade mechanical recyclates for reuse in foam applications

• Applicant: Professor Dr. Manfred Wilhelm
• Subject Area: Preparatory and Physical Chemistry of Polymers
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 558696271

Thermoplastic particle foams made of polystyrene, known as expanded polystyrene (EPS), are widely used in the building sector in insulation applications as well as in protective and food packaging. The important area of energy saving in the building sector in particular demonstrates the considerable ecological and economic relevance of expanded polystyrene. Around 7.2 million tonnes of EPS were used worldwide in 2022. While recycling rates in Germany of nearly 50 % are achieved in the packaging sector, over 80 % of EPS in the construction sector is only recycled energetically. Even in the recycling processes already established in the packaging sector, less than half of the recycled polystyrene (rPS) is reused to produce EPS. The thermal and mechanical stress in the recycling process generally results in a reduction in material quality, e.g. due to a reduction in the chain length of the polymers. This will have a negative impact on the foaming process as well as on the final properties of the product. Due to these changes in processing and final properties, downcycling to lower-quality products usually takes place. In EPS formulations, rPS can currently only be used in small proportions; one example is an EPS product from the application partner BASF, in which a maximum of 10 % recycled material is currently used. The basic idea of the “PS-Rezy-Foam” project presented here is to use the existing fundamental knowledge of the project partners on the relationship between polymer topology, strain hardening and foamability. With this knowledge, industrial polystyrene foams based on mechanically recycled material will be improved. By adding defined polystyrenes with complex topologies (as PS recyclate additives, mass fraction < 10 %), the elongation hardening properties and thus also the foaming properties of the raw material will be substantially improved. Therefore, we will be able to produce high-quality foamed PS products with high recyclate content without loss in quality. The general aim of the collaboration is to improve sustainability in the plastics industry. The developed branched polymer topologies and related optimized foaming processes are expected to significantly increase the amount of recyclate used in EPS formulations. In addition, this provides the basis for the use of lower quality recyclate. This will increase the amount of recyclate available from previously unused waste streams or low waste qualities, thus making significant impact towards a circular economy.

• DFG Programme: Research Grants (Transfer Project)
• Application Partner: BASF SE
• Cooperation Partner: Dr. Carl-Christoph Höhne