Is it possible to obtain plastic recyclates from previously unused waste streams in order to produce high-quality fibres and films? And how can bio-based polymer fibres with adjustable biodegradability be produced? Researchers at the Fraunhofer Cluster of Excellence Circular Plastics Economy CCPE are addressing these questions in the Zirk-Tex project. Together, they are developing innovative recycling methods to complement mechanical processes for producing sustainable roofing membranes and geosynthetics. In doing so, they are investigating the entire value chain on a pilot scale.

The quality of available recyclates must improve

The market for recycled plastics, especially polypropylene (PP) and polyethylene terephthalate (PET), still has a lot of potential. Demand for sustainable solutions is growing, but the quality of available recyclates must improve. Many recycling processes fail due to contaminants that make processing difficult. As a result, recycled materials have often been unable to compete with virgin materials. The six institutes of the Fraunhofer CCPE cluster, together with the Fraunhofer Institutes for Molecular Biology and Applied Ecology IME and for Optronics, System Technologies and Image Exploitation IOSB, want to refute and change this. In the Zirk-Tex project (short for circular textile fabrics), they are answering the question of whether it is possible to produce recycled plastics for high-quality films, nonwovens and fibres from previously unused plastic waste, which can then be further processed into roofing membranes. Several hundred million square metres of roof underlay are laid in Europe every year.

Production of biodegradable geotextiles

At the same time, biopolymers are often still stigmatised as not having the necessary properties for demanding technical applications. In the second use case of the project, the project partners succeeded in producing pollutant-free fibres from biopolymers – specifically polylactide (PLA) and polybutylene succinate (PBS) – for the manufacture of biodegradable geotextiles whose degradation in the environment can be adjusted. Such biodegradable textiles are needed, for example, for the temporary stabilisation of slopes and embankments or for the construction of access roads.

Economical production of roofing membranes from PP and PET

"It is less problematic to produce injection-moulded components from recycled plastics, for example. But manufacturing textile products such as nonwovens for roofing membranes from them is much more difficult, as thread formation processes place very high demands on the recycled materials. The starting material must be homogeneous and completely free of impurities in order to be extruded evenly through the finest capillaries and to withstand very high tensile forces after extrusion,‘ explains Dr Evgueni Tarkhanov, scientist at Fraunhofer IAP, describing the challenge. ’Processing stability is the be-all and end-all for production." Even the smallest dirt particles or minor amounts of foreign polymers in extruded filaments represent defects in the spun mass and increase the likelihood of filament breakage during the manufacturing process. Replacing the filament bundles on the thread guide mechanisms takes time and can sometimes result in the production machines being shut down, which is associated with immense costs.

Rethinking the entire value chain

"In order to manufacture roofing membranes from post-consumer waste, we are able to map the entire process chain for PP and PET on a pilot scale: from sorting (Fraunhofer IOSB) to innovative recycling methods (Fraunhofer ICT and Fraunhofer IVV) to application. Recycling suitable waste plastics produces a granulate or compound that is additivated (Fraunhofer LBF) and then further processed by spinning into nonwovens, films or membranes," summarises Dr Christian Schütz, project manager and scientist at Fraunhofer LBF. The research team evaluated the potential of two recycling processes: the PET fraction underwent glycolysis, a chemical recycling process followed by repolymerisation, while both PP and PET fractions were recovered using a solvent-based recycling process involving purification. The residues from both recycling processes were further recycled by pyrolysis (Fraunhofer UMSICHT). The practical work was accompanied by a life cycle assessment (Fraunhofer UMSICHT) and a material flow analysis of available material streams (Fraunhofer IML).

Innovative recycling processes for pure recyclates

Using the Fraunhofer IVV's solvent-based recycling process, the research team was able to separate PP from unwanted polymers and additives, resulting in a virtually pure material. The waste stream contained 33 per cent PP and 67 per cent foreign plastics. After the process, polyethylene (PE) was the only significant unwanted polymer, accounting for less than 2 per cent. The team also ensured the stability of the PP recyclate (rPP) at high temperatures during processing. The rPP obtained was then spun into multifilament yarn at Fraunhofer IAP. ‘By using the right additive strategy, we also succeeded in significantly improving the processing stability of both PP and PET recyclates,’ says Schütz.

Multifilament yarn with 48 filaments

For the glycolysis of PET, carried out at Fraunhofer ICT, the researchers used PET trays containing 13 percent unwanted foreign substances. Glycolysis is a form of solvolysis in which ethylene glycol is used to depolymerise PET into bis(2-hydroxyethyl) terephthalate (BHET). ‘In solvolysis, the plastics are specifically broken down into their respective monomers using a depolymerisation reagent,’ says Schütz. The resulting BHET was repolymerised into rPET at Fraunhofer IAP and processed into a multifilament yarn with 48 filaments on a pilot melt spinning line. In addition, the researchers were able to show that the same PET fraction can be recycled using a solvent-based recycling process. ‘We were able to obtain fibres for the production of nonwovens from both PP and PET, and PP was also suitable for membrane production,’ summarises Tarkhanov.

Promising further utilisation

The pyrolysis of residues from the solvent-based recycling process of the PP fractions yielded a high proportion of pyrolysis gas and a low proportion of coke. Pyrolysis oil fractions with high oil content and low coke content were obtained from the residues of the solvolysis of PET fractions. The products from both raw material streams can be further recycled in a promising manner.

Better carbon footprint

In addition, the accompanying work showed that suitable and sufficient quantities of PP and PET are available, but that the logistics and sorting for accessing them still need to be established. The aggregated results of the LCA showed that the value chain of the Fraunhofer CCPE process has a better climate balance than the use of new plastics for both biopolymers and recyclates. ‘We were able to successfully demonstrate that previously unused material flows are also a real option for the production of high-quality materials based on recyclates,’ says Schütz, summarising this part of the project.

Biodegradable geotextiles made from PLA and PBS

Is it possible to produce plastics for landscaping from bio-based polymers (PLA and PBS)? Can products be manufactured without environmental problems and with controlled degradation? How can we ensure that PLA and PBS remain stable during use and then degrade quickly and completely afterwards? The Fraunhofer CCPE is addressing these questions in the second use case of the project, which focuses on geotextiles that are designed for short-term use of less than ten years and therefore need to degrade quickly. The focus was on the degradability and ecotoxicity of the bioplastics, with tests carried out on virgin PLA and PBS. To test degradability, the project partners stored fibres of two PBS and three PLA types in moist soil at Fraunhofer UMSICHT for 25 weeks at 40 degrees Celsius and 90 per cent relative humidity. Using tailor-made additives from Fraunhofer LBF, the researchers were able to adjust the timing and course of the degradation of the PLA and PBS fibres, significantly accelerate it and verify it in decomposition tests, while largely preserving the material properties until the onset of degradation. Schütz: ‘We were able to produce fibres with controlled and adjustable degradation behaviour for both PLA and PBS. The ecotoxicity tests carried out by Fraunhofer IME showed no evidence of any effects giving cause for concern. Our results open up concrete development prospects for geotextiles for real-life applications, which we now intend to pursue further together with industry partners.’

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