Getting plastics into the circular economy

Driven by environmental and sociopolitical developments, the issue of plastics recycling is currently attracting an enormous amount of attention. The problem is that the mostly linear value chains – especially with short-lived plastic products such as packaging – do not ensure at the end of the chain that the plastic is necessarily sent for the most appropriate method of recycling. Consequently, considerable quantities of plastic waste are used (merely) for energy recovery or are exported to South-East Asia. 

Solutions for reuse

When it comes to returning plastics to the circular economy, a distinction is made between material recycling and chemical recycling. Material recycling is a multi-step process (sorting, shredding, cleaning, drying, regranulation), in which the polymer structure of the plastic is not influenced at all or only slightly. Chemical recycling on the other hand, breaks down the polymer structure into its basic monomer elements. These are subsequently built up again into a polymer through a synthesis process. As a rule, chemical recycling is used when material recycling is not possible. If chemical recycling does not appear expedient either, there is still the option of energy recovery, which, under such circumstances, may be the best alternative.

Technical innovations for meeting political targets

The aim of the EU and the still young German packaging legislation is to drastically increase the recyclate content of plastics. To achieve the set targets, an enormous increase in recycling capacities is needed and, apart from that, various technological innovations will be absolutely essential:

- in the field of packaging design,
- in the choice of materials,
- in the use of new renewable resources,
- in avoiding material composites and/or
- in the development of separable material composites
- in the further development of sorting techniques,
- in the processing technology etc.  

IKV has been dedicating a considerable amount of work to issues of this kind since the 1980s. Over the decades, the number of research projects in the field of plastics recycling has increased and continued to evolve, frequently in direct cooperation with partners from industry. 

IKV can support companies in many matters concerning recycling and the circular economy of plastics. This could be, for example, study projects, training or consulting, but also concrete trials on processability and the testing of new materials and compounds. 

Ongoing research projects in recycling and the circular economy

In view of climate change, environmental pollution, population growth and resource dependency, the transition from a linear to a circular economy is both ecologically and economically necessary. In the 1980s and 1990s, intensive efforts were being made to develop processes for the chemical recycling of plastic waste, although unresolved process problems and economic reasons (such as the inaccessibility of material flows) prevented large-scale applications. However, the growing problem of the quantities of plastic waste, in particular the discharge of plastic waste into seas and waters, as well as a greater awareness of the environment and a need for sustainable solutions have led to a renewed interest in the recycling of raw materials.

The current research focus of the IKV is on the use of a twin screw extruder for the continuous depolymerisation of polystyrene. First results show a condensate yield of over 60 %. The condensate obtained can be separated by subsequent distillation and fractionation and used for subsequent polymerisation. 

Sustainable production and processing of plastics is essential for the energy revolution. Thus, it is necessary to replace petroleum-based plastics with alternatives from renewable sources and to reduce the amount through lightweight techniques. As a biobased material, thermoplastic cellulose acetate (CA) represents a promising alternative to conventional plastics such as polystyrene. The raw materials wood and cotton do not compete with food production. To use CA for technical applications like foamed insulation boards, the compound consisting of polymer, plasticiser and additives has to be adapted. In order to pass a standardised flammability test, the foamed insulation boards have to be equipped with flame retardant additives.

An innovative flame-retardant system for CA based on halogen-free additives, specially adapted to foam production, was developed and investigated in foam extrusion with regard to processability and the resulting foam properties. The higher viscosity requires higher melt temperatures compared to the reference compound without flame-retardants. The additives also affect the foaming process due to the reduced melt strength. By using blowing agent mixtures based on HFO-1234ze, foam boards with a density of less than 80 kg/m³ were produced. 

Climate protection through Carbon Capture and Utilisation (CCU) focuses on CO2 use, energy saving and low carbon footprint chemistry, which has the potential to replace established mass products with CO2-containing compounds. Polyether polyols (PETs) are such a mass intermediate (global production: 6,600,000 tons per year; key component for polyurethanes (PUR)).

The first CO2-containing PETs are about to enter the market. Crosslinkable CO2-PETs have the potential to significantly expand the product portfolio. The Climate-KIC innovation project "Dream Products" has successfully taken the first decisive steps in the production of crosslinkable PETs on a 50 kg scale. In order to turn these compounds into real drop-in solutions, a matrix of material properties and specifications of existing products has to be met. The focus is on five product groups: rubber, composites, fibers, inorganic catalysts and special polyurethanes used, for example, as materials for shoes. The frequently refined evaluation of technical possibilities, life cycles, economic potentials and entrepreneurial potentials facilitates decision-making in the implementation of results and market launches. The focus of the IKV is on investigating the processability of these rubbers in internal mixers, extruders and injection molding machines.

Thanks to the PECVD barrier technology, non-refillable PET bottles were also able to assert themselves for certain areas of sensitive beverages. However, this is not yet the case for returnable PET bottles, which is due to a number of technical challenges that have not yet been solved. The barrier coatings of silicon oxide (SiOx layers) developed and used for non-refillable PET bottles cannot withstand the cleaning process with sodium hydroxide solution (NaOH) in the refillable process and are completely or partially removed. In principle, one solution would be to recoat the cleaned bottles, but the PET would also be attacked by NaOH and the surface would be roughened considerably so that the layers deposited on it would no longer be able to form a closed surface and the achievable barrier would remain marginal after several washing processes.

In comparison with disposable PET bottles and returnable glass bottles, returnable PET bottles perform significantly better in the ecobalance. The switch from the use of disposable PET bottles and returnable glass bottles for sensitive beverages to returnable PET bottles would lead to a significant reduction in climate-damaging CO2 emissions in the eco-balance for this sector. The collection and washing of returnable PET bottles requires less energy than new production and disposal or the recycling process of non-refillable PET bottles. The low weight in relation to glass bottles leads to lower energy consumption during transport. The low weight, together with the low breakage sensitivity of plastic bottles, also makes it easier for the consumer to handle the bottles in relation to the glass.

One aim of this project is therefore to use the expertise of the SFB-TR 87 to make these advantages of returnable PET bottles available for sensitive beverages as well. 

PLA is a biobased plastic, which is mainly obtained from corn starch. Currently, the mechanical properties are not sufficient to produce technical components. In addition, pure PLA is not sufficiently flame retardant, so that its use in electrical components such as switch housings is out of the question. The aim of this project is to equip the material with flame retardants on the one hand and to control crystallization in such a way that the mechanical properties can be improved through the targeted use of nucleating agents on the other. In this way, a sustainable raw material is to be "made fit" for industrial applications.

In the Cluster of Excellence "Integrative Production Technology for High-Wage Countries", the IKV and other RWTH institutes have been researching the development of self-optimizing production processes. This research will be continued in the Internet of Production cluster.

For the injection moulding process, this means that the machine can react autonomously to external disturbances and still produce high-quality components. Since recycled plastic has greater fluctuations in material properties due to the preceding processing process, the utilization phase and the lower purity, this plastic represents a disturbance variable for the processing process and often no or only a small proportion of recycled material can be used for high-quality plastic products. The higher process robustness of the self-optimising injection moulding process enables the processing of larger proportions of recycled regrind or regranulate and thus contributes to the recycling economy.

Prof. Dr. rer. nat. R. Dahlmann

Scientific director Contact for cooperations in research and development +49 241 80-25928 rainer.dahlmann@ikv.rwth-aachen.de