Circular clothing – recycling of polyester-containing textiles

The overall eco-balance of polyester is better than that of natural fibres like cotton. Nevertheless, textiles containing polyester do not have a good reputation. Today, one property of the material in particular is criticised: it does not decompose naturally. This makes a functioning collection and high-quality recycling of such textiles all the more important. But a circular economy for polyester does not yet exist.

In an interview with K-MAG, Carsten Eichert talks about the challenges of recycling textiles containing polyester, the new revolPET® technology and its contribution to climate protection.

Mr Eichert, up to now textiles have not been recycled – at least not to a high standard. Why is that?

Carsten Eichert: Textiles usually consist of a variety of materials. Even a shirt made of 100% cotton is usually sewn with a nylon thread. In addition, different colours, additives and finishing materials are used, which make material recycling, i.e. mechanical recycling, impossible. Especially in the post-consumer waste stream, there is an absolutely heterogeneous mix of materials that cannot be separated by type with the existing sorting techniques (almost exclusively manual sorting!) in such a way that mechanical recycling would be possible.

Mechanical recycling is dependent on very clean input streams, such as those found in PET bottle recycling. The textile industry cannot offer this. Therefore, this type of recycling is largely ruled out, especially if a larger proportion of synthetic fibres is involved. For pure cotton products, recycling is now possible, but also under strict quality requirements for the supplier industry.

You have now developed a process with which mixed materials containing polyester can be recycled. How does the process work?

Eichert: Our revolPET® process is based on a simple chemical reaction: the reaction of the polyester polymer with a metal hydroxide. This reaction splits the polyester polymer into a monoethylene glycol and a metal terephthalate. The polyester polymer is depolymerised. Both products obtained are basic chemicals needed for the production of new polyester. The reaction of the polyester with the metal hydroxide is described in various patents and publications, so it is nothing new. Our innovation is that we were able to integrate this reaction into a continuous process. In this way, we realise advantages in process control as well as in terms of efficiency.

What in particular was the challenge in the development?

Eichert: The particular challenge was to identify the technology approach for continuous depolymerisation in order to enable the option of economies of scale.

Only through economies of scale, i.e. the reduction of the relative costs per target unit by increasing the throughput, for example, does it become realistic to realise depolymerisation and thus the recycling of polyester fibres economically. The depolymerisation realised by us in an extruder also enables the direct use of reaction energy that is released. This reaction energy is generated by the reaction of the metal hydroxide with the polyester molecule. In our revolPET® process, we do not let this released energy escape, but use it directly in the reaction chamber for the subsequent reactions. This makes our process extremely energy-efficient and thus also ecologically advantageous.

What potential does this technology have? Where can it be used in the future?

Eichert: The revolPET® technology can process different input streams. It can also be a mix of different PET/polyester products that are to be brought to the depolymerisation of the polyester. The reaction is a strictly selective one, which means that the non-polyester parts in the input stream do not react or react only very little and are present as solids after the depolymerisation step. This allows for easy and good separation with the result that other materials in the input mix, for example cotton fibres, can be isolated and sent for further recycling.

Our technology is to be used globally. Wherever PET or polyester waste is produced, a revolPET® plant can be located. Thanks to advantageous process parameters, very large capacities are not necessary to operate the technology profitably. From an annual capacity of about 15kt/a onwards, it can be assumed that the operation is profitable. One prerequisite is that at least 75% of the input stream is polyester or PET.

To what extent do you contribute to climate protection in this way?

Eichert: The production of new polyesters and thus also their basic chemicals, which we produce with our technology, is currently done from fossil raw materials. The production of one tonne of terephthalic acid causes 1.8t of CO2 equivalents. The revolPET® technology produces it with only 0.7t CO2 equivalents. In addition, we get the raw material that can be used again for new products. The recycling of PET with this technology can be repeated as often as desired. This is how we realise complete circularity of polyester.

The complete substitution of fossil-based raw materials by recycled raw materials or secondary raw materials is in itself climate-friendly. By saving energy and thus reducing CO2 emissions compared to the crude oil-based production route, our process contributes to climate protection. The results have been confirmed by an LCA carried out by the TU Braunschweig according to strict scientific criteria.

You also participated as an exhibitor at K 2022. How did the trade fair go for you?

Eichert: The feedback and interest in our technology at K 2022 confirmed our efforts. We had a very good response and a lot of international interest in our revolPET® technology. Now we have to turn the contacts we made into possible business relationships. The work has begun after the fair. Our conclusion after 8 days at the fair: We will be back!