Current challenges and solutions for the recycling of (mixed) synthetic textiles

The environment in particular suffers from the negative consequences of high clothing consumption. The production of textiles releases emissions such as carbon dioxide (CO₂). The greenhouse gas (GHG) accelerates global warming and thus climate change. Additionally, synthetic fibers, which make up a large part of the mart, are now mostly derived from petroleum. The extraction of this finite resource is sharply criticized by many environmental organizations. Leaky extraction techniques or accidents cause contamination to enter the environment, threatening biodiversity.
As clothing consumption increases, so does the amount of used textiles. In Germany, approx. 1.5 million tons of used textiles from private households were separately collected in 2019 [2]. Furthermore, there are textiles disposed of through residual waste as well as post-industrial waste, which are not collected separately today. About half of the collected textiles can be re-used (second-hand). However, the share of reusable textiles has been decreasing in recent years, according to collectors and sorters. If re-use is not an option due to the condition of the clothing, recycling is an approach to counteract – at least partially – the negative ecological impacts. Resources such as water, energy and chemicals can be saved by using recycled materials and overflowing landfills can be relieved.

Fiber blends and challenges in textile recycling
Textile products often have a high diversity of materials: Particles can be introduced into the material, fibers consist of bi- or multi-components, and yarns of mixed materials. With the help of such material mixtures, products are functionalized and/or properties of fiber materials are combined. The variety of materials in the product is also increased in the construction of, e.g. clothing textiles. Examples of this are:
-    Multilayer constructions
•    Example: membranes, linings or wadding in jackets
-    Haberdashery
•    Example: zips, buttons, bias binding
-    Colors, prints and applications
•    Example: dyed textiles, printed or embroidered logos
-    Functional coatings
•    Example: Water-repellent coating

The pre-condition for sustainable recycling is material flows that are as pure or consistent as possible. The types and quantities of contaminants are defined very differently depending on the technology. Material mixtures can pose challenges or make recycling unecological, uneconomical or technologically unfeasible.
Recycling technologies for synthetic textiles
Today, in addition to re-use (second-hand), used textiles are predominantly found in processes where the textile fabrics are recovered. For example, cleaning fabrics, insulation or filling materials are produced and used in other branches of industry. Although these types of use have ecological and economic advantages, the final disposal task is not solved, but merely postponed.
Table 1 shows various recovery and recycling processes for textiles. The example of polyester (PET) textiles shows which products can be manufactured in the different processes.

Table 1: Overview of existing recycling processes for polyester textiles (Source: ITA)

Mechanical and thermo-mechanical recycling processes in particular are used today for used textiles and for post-industrial waste. Various chemical recycling technologies are being developed for textile waste. The processes are described below using the example of polyester-containing used textiles.

Mechanical recycling (tearing)
Mechanical recycling processes change materials in their form, but not in their chemical structure. Textile fabrics are loosened with the aim of exposing fibers and spinning them into a new yarn or producing nonwovens. In the case of mixed used textile waste, the fibers are usually dyed. The decolorization of the fibers is possible with the help of chemicals before or after the tearing process.
The actual tearing of the textiles is done with the help of equipped rollers. The textiles are mechanically stressed and shredded by a rotating movement. Due to the mechanical stress, the fibers produced usually have a shorter length than primary fibers and may additionally be contaminated with dust. In order to produce new yarns from the torn fibers, a high proportion of raw materials usually has to be added. A large proportion of the torn fibers is therefore processed into nonwovens. Products are, for example, painter's felts or insulation material. Very short fibers, so-called fiber flocks, can also be used in applications such as seat filling for furniture upholstery.
Both mono and mixed materials can be processed in mechanical recycling processes. The energy demand of the process is low compared to the processes described below.  However, the market for the broken fibers is limited and high-quality recycling into new yarns is usually not possible without adding new raw materials.

Thermo-mechanical recycling (re-granulation)
In thermo-mechanical recycling, plastic is melted and extruded. The molecular structure of the plastic remains intact. Pure and predominantly uncontaminated waste, such as post-industrial waste, can be recycled in this way.
The material is drawn into the re-granulator by a screw conveyor. The material is melted by the built-in heating elements and the resulting frictional heat. A degassing system is often connected to the extruder to remove volatile substances. The melt is also filtered. After extrusion, the melt is cooled, if necessary via a water bath, and cut into short strands by a rotating knife. The re-granulate can then be returned to the production process, for example. This can not only reduce raw material costs, but also waste disposal costs [3, 4].

Sketch of an extruder for processing pure waste plastics (Source: ITA)

The regranulation process is susceptible to interfering substances. For example, even small amounts of cotton or elastane can greatly disrupt the recycling process of PET and make recycling technologically difficult or impossible.

Chemical recycling
In chemical recycling, plastics are broken down into chemical intermediates (oligomers) or their basic building blocks (monomers) by adding chemicals [5]. Various processes are described in the literature for chemically recycling PET textiles:
-    Hydrolysis
-    Alcoholysis (glycolysis/methanolysis)
-    Aminolysis

The oligomers or monomers must be purified by means of filtration and/or separation processes. Dyes in particular may be difficult to remove. After purification, the purified oligomers or monomers can be processed into new plastics or other chemical products. In this way, high-quality secondary raw materials can be obtained, which also serve as raw materials for the spinning process of new textile fibers. Usually, only individual fiber types can be dissolved and processed from a textile mixture in chemical recycling. Consequently, large quantities of residual material are produced, which must either be fed to further recycling processes or disposed of [6].

In chemical recycling, PET is broken down into its monomers e.g. MEG and TPA (source: ITA)

The economic efficiency of chemical recycling is only given with larger mass flows. In addition, it must be taken into account that today's processes are usually very energy intensive and the economic viability depends strongly on the quality of the purification [7].

Outlook: collecting, sorting and recycling of textile waste in europe
The media presentation of many textile companies suggests the image of increased clothing production from recycled materials. However, these do not come from recycled old clothes, as might be assumed, but from other waste products such as PET bottles or post-industrial waste. According to the Ellen MacArthur Foundation, only 1% of collected old clothing was sent for so-called textile-to-textile recycling in 2015. The underlying processes are currently costly as well as technologically not fully mature.
According to the European Union's "Waste Framework Directive", which will come into force in 2025, textiles will have to be collected separately and compulsorily throughout the EU in the future. For this reason, it is assumed that within a few years the separately collected quantity of used textiles in Europe will increase to approx. 5.5 million tons by 2025 [10]. Further future laws on extended producer responsibility will oblige companies in the future to also take care of recycling after the use phase of products. The responsibility already starts with the design of a new product, since e.g. material mixtures as described above have a significant influence on the subsequent recycling path.

Increase in the amount of used textiles collected in the EU in million tons [8] (Source: ITA)

In order to (further) develop recycling technologies and sustainably manage material flows, the interdisciplinary and networked work of different actors is of great importance. At the Institut für Textiltechnik at RWTH Aachen University (ITA), Aachen/Germany, solutions are developed together with partners along the entire process and recycling chain in public or bilateral research projects. One example of this is the Industry Research Group Polymer Recycling (IRG). The IRG is a consortium of 11 companies along the textile value chain and the ITA. The long-term goal of the consortium is to research methods and processes for obtaining high-purity recycling fractions from contaminated and mixed textile waste containing chemical fibers. The focus is on waste textiles made of polyester and polyamide. Within the framework of the IRG, experimental trials or literature research, among other things, are carried out by ITA. The results are discussed with the partners. The IRG sees itself as a platform and promotes exchange between the companies and the research institute.  Interested companies from industry and research can subsequently join the project consortium.

The identification of high-quality recycling routes that enable economically and ecologically competitive value-added cycles for textiles is important, especially in view of the expected legal changes in the coming years. At ITA, end-of-life scenarios for textile waste streams are modelled and evaluated using the example of polyester-containing used textiles with different qualities by means of Life Cycle Assessment. The model developed enables companies to classify their existing or future technologies and/or textile polyester waste streams in the textile recycling market. It can also provide a basis for political decisions. Companies and research institutes are invited to participate in order to fill the model with valid data. If you are interested in participating here or in the IRG, or for more information, please contact Ms. Amrei Becker (amrei.becker@ita.rwth-aachen.de).

References
[1] Ellen MacArthur Foundation (ed.): A new textiles economy: Redesigning fashion's future, 2017, URL: https://www.ellenmacarthurfoundation.org/assets/downloads/publications/A-New-Textiles-Economy_Full-Report_Updated_1-12-17.pdf, Accessed on March 1, 2022
[2] bvse e.V: Bedarf, Konsum, Wiederverwendung und Verwertung von Bekleidung und Textilien in Deutschland, 2020, abrufbar unter: https://www.bvse.de/dateien2020/1-Bilder/03-Themen_Ereignisse/06-Textil/2020/studie2020/bvse%20Alttextilstudie%202020.pdf, Accessed on March 1, 2022
[3] Hopmann, C., Michaeli, W.: Einführung in die Kunststoffverarbeitung. 8. Aufl.- München: Carl Hanser Verlag, 2017
[4] Woidasky, J.: Kreislaufwirtschaft und Recycling.  In Elsner, P., Eyerer, P., Hirth, T.: Polymer Engineering. 1. Aufl.- Berlin, Heidelberg: Springer-Verlag, 2008, S. 610-618
[5] Martens, H., Goldmann, D.: Recyclingtechnik – Fachbuch für Lehre und Praxis. 2. Aufl.- Wiesbaden: Springer Verlag, 2016
[6] Bartl, A.: Wissenschaftliche Untersuchungen zur stofflichen Verwertung der textilen Restfraktion durch mechanische Verfahrensschritte und Entwicklung eines geschlossenen Gesamtkreislaufes, 2011, URL: https://nachhaltigwirtschaften.at/resources/fdz_pdf/endbericht_1106_stoffliche_verwertung.pdf, Accessed on March 1, 2022
[7] Gries, T.; Wulfhorst, B.; Veit, D.: Textile technology. 2nd edition. - Cincinnati: Hanser Publications, 2014
[8] Fachtagung zum Thema “Alttextilien – Akteure im Dialog“, URL: https://textile-zukunft.de/wp-content/uploads/2014/10/Euratex-Future-of-used-textiles-across-Europe-16.11.2021.pdf, Accessed on March 1, 2022

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