Mechanical PU Recycling: A New Path for Rigid Foam?

Rigid polyurethane foam is used wherever insulation, lightweight construction, and stability are important. But until now, its life has usually ended in the waste incineration plant. A team at the Institute of Plastics Technology (IKT) at the University of Stuttgart wants to change that: a new process should make it possible to mechanically recycle PU rigid foam and return it to new products as a filler – an approach that is more energy-efficient than previous processes and could enable the closing of material cycles.

In K-Mag, Lukas Fritsch, research assistant in the Processing Technology Department at the IKT, talks about the challenges, potential, and next steps.

Lukas Fritsch: Polyurethane (PU) is a thermoset – in other words, a plastic that cannot be remelted. For this reason, PU has mostly been thermally recycled or chemically broken down into its basic components. Mechanical recycling would actually be the simplest and most energy-efficient form of recycling. However, this has only been possible to a limited extent so far: The waste material is processed into flakes, granules, or powder and used, for example, as filler for cushions or toys. Returning it to its original area of application – such as the production of new insulation panels from recycled PU – has not yet been realized.

Fritsch: In our research project, we want to integrate mechanically recycled PU rigid foam as a filler in the production of new goods. To do this, the recycled material is mixed with the two starting components, polyol and isocyanate, during the processing stage. However, conventional mixing units are not designed for high filler content. That is why we are using a twin-screw extruder that was developed specifically for this application. This allows us to process significantly higher filler content than was previously possible.

Fritsch: The biggest challenge is to enable a continuous process with consistently high reproducibility and mixing quality, despite possible fluctuations in dosing and varying particle sizes in the recyclate. The prototype must therefore be precisely tailored to the process. At the same time, we need to investigate which filler proportions can actually be processed effectively. The goal is to produce insulation panels that meet industrial standards. In preliminary tests, we are examining how the recycled particles affect the properties of the PU foam and how these can be improved through process optimization. Finally, scaling up to an industrial scale is on the agenda – this poses both process engineering and material challenges.

Fritsch: Basically, we see potential applications wherever PU rigid foam is used. There is particularly great potential in the construction sector – for example, in the manufacture of insulation panels. Here, we can reduce the proportion of new material and close material cycles. We see further applications in refrigeration and air conditioning technology, for example for insulating cooling containers, as well as in the automotive and transportation industries.

However, it is not only users who would benefit from successful implementation, but the entire value chain: PU processors can offer more sustainable products, raw material manufacturers can adapt their systems to new applications, and machine builders can expand their portfolios with new plant technologies.

Fritsch: The decisive step is to transfer the process from laboratory scale to industrial scale. To do this, the process must be transferred to larger plants – with the aim of ensuring stable production with consistently high product quality. It is also important to collaborate with industrial partners who can integrate the process into existing processes. In this context, economic efficiency and competitiveness must also be evaluated and possible approvals for safety-relevant applications clarified.

And last but not least, market acceptance of the new process is crucial. This depends above all on the consistently high quality of the PU foams with recycled content.