Circularity for Sustainable Textiles: Aligning Fiber Compositions of T-Shirts with Ecodesign and Recyclability ^†

Abstract

The sustainability transition of the textile industry requires amongst other strategies circular approaches. Ecodesign guidelines and design for recycling are approaches that reduce resource consumption and textile waste. Garments are made of a large variety of different materials, from blended fibers to haberdashery items, colorants, and finishings, making it challenging to predict the composition of post-consumer textile waste. This mix of materials complicates recycling efforts, contributing to globally less than 1% of fiber-to-fiber recycling. This study investigates material compositions of one of the most popular and widespread garments: T-shirts. While about half of our sample contains cotton only, the other items contain two or more fibers, revealing huge variations in fiber blends, including varying degrees of elastane contents, which are not linked to functional requirements. These blends, especially the varying levels of elastane, increase costs and efforts for recycling, making fiber-to-fiber recycling less attractive and more expensive than new fiber production. They also contribute to avoidable microfiber pollution. Accordingly, this study underlines the requirements for providing detailed ecodesign guidelines and applying the extended producer responsibility to incorporate environmental lifecycle costs, to help shift the industry towards a circular economy.

1. Introduction

With a growing world population and a steadily increasing demand for new clothes, especially affordable mass-produced fashion, the production of virgin fibers continues to grow and is estimated to reach a new high with 160 million tons of annual fiber production by 2030. Two types of fibers continue to dominate the market, polyester fibers and cotton fibers [1]. With weekly changing offers in fashion stores in high-income countries, the expectation of consumers for new and cheap garments is high, and the resulting negative environmental impacts of the textile and clothing production and trading as well as waste are generally not thought about. To reduce the negative impacts of garment production on the environment, we require better waste management, turning waste into resources and reducing the extraction of new resources. One way of solving the issues of the industry is fiber-to-fiber (F2F) recycling. At the moment, only about 8% of the fibers produced worldwide are recycled fibers, of which under 1% originate from pre- and post-consumer textile waste, with the rest mainly originating from the recycling of PET bottles [1].

With its Strategy for Sustainable and Circular Textiles, anchored in the European Green Deal, the European Union aims to improve F2F recycling efforts in the industry and reduce the recycling of bottles-to-fibers, as this practice is removing valuable feedstock from the packaging industry. Another goal of the strategy is the implementation of a minimum content value of recycled fibers for new garments by 2030 [2]. Currently, the cost of recycled fibers is still too high compared to virgin fibers [3], which is one of the factors for low production volumes and utilization.

To increase the viability of F2F recycling processes, we need to overcome several challenges currently inhibiting the economic and ecological upscaling of the technologies. Missing information about fiber blends and contaminants, both internal and external, is just one of the inhibiting factors [4]. In the future, initiatives like the Digital Product Passport (DPP) aim to store and communicate this data across the whole value chain, providing accurate information whenever necessary [2]. This will provide textile sorting and recycling companies with valuable information regarding the material composition, enabling fast decision-making and sorting towards the most viable recycling route. Until the DPP is properly implemented and also extensively available in the post-consumer textile waste stream, we need to base our decision-making on other factors. Furthermore, we should look at the fiber blends utilized for the garments. Applying an extended producer responsibility concept to garments may also enable circularity by making garments that are more difficult to recycle and with less longevity more expensive for producers and consumers, hence internalizing some the environmental costs.

Fiber shedding during the use of garments, either released in the air when they are worn or in the water when they are washed, releases non-degradable synthetic microfibers. Exceptions to this are that in some textile and garment mills, microfibers released during manufacturing may be collected with filtration systems; also, in industrialized countries, residual sludge from household laundry can be dried and incinerated. Otherwise, fiber shedding during wear is considered a source of ubiquitous environmental pollutants [5]. Even though there is no international standard testing method agreed and established up to now to record microfiber shedding during the textile lifecycle, unnecessary use of synthetic fibers, which do not degrade in the environment, should be avoided.

Currently, the blending of fibers is not regulated in any way and guidelines are missing, resulting in a large variety of different blends of fibers with different values. This is another obstacle for recycling, as we are not able to accurately predict the material composition of the textile waste. The blending of fibers is carried out for various reasons, mainly to improve the comfort of the garments but also to save costs [6]. The fabric composition is one of the main pieces of information needed for high-quality textile recycling [7]. To make the processes more viable, the regulation of blends in the form of, e.g., mandatory ecodesign guidelines would improve the overall productivity and quality of F2F recycling efforts. Elastane fibers are hard to detect both manually by hand and eye and also automatically by technologies like Near-Infrared (NIR) Spectroscopy, as they are usually only present in small quantities and often deeply imbedded in the yarns, making them one of the most challenging fibers in blends [8]. Imaging technologies in combination with Artificial Intelligence also require solid data for predicting the fiber composition and suitable recycling method.

This study presents a comprehensive analysis of the fiber composition and the correlation between the elastane content and the garment fit, based on an extensive sample of over 1200 T-shirts. By focusing on fit-dependent elastane thresholds, this research provides actionable ecodesign guidelines that support an extended producer responsibility, automation in textile sorting, as well as the creation of closed-loop material systems. Unlike previous works, it offers quantitative insights into blend diversity as well as fiber usage in fast fashion, and most importantly, the correlation of fit and elastane usage, laying the groundwork for material standardization and enhancing the recyclability of textile waste. Knowing the composition of the currently available textile waste streams furthermore allows us to predict fiber compositions during sorting. These findings can accelerate the transition toward fiber-to-fiber recycling even before regulatory tools like the Digital Product Passport are fully implemented.

2. Circularity and Recycling of Post-Consumer Textiles

In this section, we provide an overview of the current legal framework for textiles within the European Union. We explain the significance of T-shirts and elastane fibers and provide an overview of available recycling technologies, including their challenges and economic relevance.

2.1. Regulations to Increase Circularity and Recyclability of Textiles

The European Union is aiming for more sustainable products and greener production processes for products within the EU. This includes textiles. With the release of the European Green Deal, the EU proposed a number of directives and regulations that are supposed to be in action by the year 2030. The Ecodesign for Sustainable Products Regulation (ESPR) is one of them. It aims to establish a framework for the setting of ecodesign requirements for sustainable products and includes the Digital Product Passport, Green Public Procurement, as well as the ban on the destruction of unsold items. It also obliges companies to design their products sustainably. Textiles are currently one of the product groups prioritized in this regulation, next to steel and furniture items. The ecodesign requirements generally aim to improve the product quality in terms of durability, longevity, reusability, upgradability, and reparability. They should also be designed in a way that allows product maintenance and refurbishment and improve remanufacturing and recycling of the product at the end of its lifecycle. This entails reducing the presence of recycling-inhibiting substances. Products are also requested to become more energy and resource efficient, generate less waste, and to be manufactured using recycled content [9]. While durability, reusability, and repairability are all part of sustainable garment design and production, as they allow an extension of the use phase of the products, in this study, we focus on what comes after a product is deemed unusable and is collected as post-consumer waste. Textile recycling should be the end as well as beginning of the value chain or cycle, while other measures aim to extend its length. The use of synthetic, non-degradable fibers is not regulated in any way. However, studies to understand the toxicity of microfibers from textiles as an environmental pollutant are conducted and there is some understanding that this issue needs to be tackled [5]. Accordingly, as research is still underway, synthetic fibers are not regulated yet, but will likely be subject to the future ecodesign requirements of garments.

Another building block of the European Green Deal is the Circular Economy Action Plan (CEAP). It proposes initiatives along the whole lifecycle of products, aiming to make sustainable products the norm within the EU and to empower consumers as well as public buyers to make more ecofriendly choices. By targeting sectors with a high requirement for virgin resources and a high potential for circularity, the action plan also includes the textile and garment industry. The aim is to ensure less waste generation and lead global efforts on circular economy practices, making them more accessible for everyone [10].

Another part of the Green Deal is the EU strategy for sustainable and circular textiles, which is addressing the production and consumptions of textiles specifically. The vision of the EU Commission is that by 2030, all textile products within the European Union are durable, repairable, and recyclable. They should be made to a great extent from recycled fibers, free from hazardous substances, and produced in respect of social rights and the environment. The strategy also aims to make reuse and repair services widely available and improve the overall product quality. As a direction for action, the strategy also addresses design requirements for longer-lasting, repairable, and recyclable garments, introducing minimum recycling contents and a Digital Product Passport, which is supposed to communicate important information during the whole lifecycle of a garment to different stakeholders. Overproduction and overconsumption as well as the destruction of unsold goods and greenwashing (Green Claims Directive) by the companies are aimed to be reduced [2]. The Digital Product Passport (DPP) is an important initiative across all directives. As mentioned before, it is part of the ESPR and the European Green Deal. It is supposed to store various information for different stakeholders across the textile value chain and its lifecycle. The DPP can be attached to a garment, e.g., by QR code or RFID/ NFC chip, and store all product-related data. The data can be public or restricted for certain stakeholders. By providing all this product-specific data, the aim of the DPP is to improve transparency along the value chain, enhance consumer trust, and facilitate reuse, repair, and recycling processes [9]. The DPP is still facing a number of challenges before it can be introduced to textiles and become the industry norm. Key challenges include the capturing of all the relevant data due to the complex supply chains within the textile and garment industry, standardized formatting of the data, as well as the attachment and durability of the DPP on the garments. If the DPP in the form of a QR code or an RFID chip is damaged, data retrieval becomes impossible. In particular, laundering processes are potential risks, and depending on the type of attachment, also the removal by consumers, as bulky tags might be uncomfortable during wear or consumers may fear supervision or data leaks through the tags.

The legal framework and requirements are constantly evolving towards a more sustainable textile industry, aiming for more ecofriendly textile products in the future. It is often difficult to consider all stages of a garment’s lifecycle, and a lot of decisions have huge effects later in a product’s life. With design requirements like the ecodesign guideline, we can try to prevent the production of textiles unsuitable for recycling, reducing negative effects on the environment, e.g., caused by landfilling or incineration. With improved recycling techniques and growing knowledge regarding material composition and requirements of different textile product groups, we can create solid ecodesign guidelines that will not restrict the actual visual design of our garments, while still improving their recyclability and repairability. Thus, it is highly important to know what materials are currently being used in the production of our clothes and to question whether this is still compatible with the knowledge and production techniques we have today as well as the goals we have set for a more sustainable future. To gain insight into one of the most important product groups, T-shirts, we have conducted this study as a basis for the analysis of more product groups and the proposal of tailored ecodesign guidelines to create a more ecofriendly garment industry.

2.2. Product Group T-Shirts

As one of the most popular and widespread types of garments among all age and gender groups, T-shirts make up a large portion of the produced clothing worldwide. They are staple pieces in basically every wardrobe and available in a large variety of different colors, styles, and fits as well as made from a variety of different fibers and blends. Due to their popularity, they in turn also make up a large portion of the post-consumer waste. In a previous study, we found that of the analyzed post-consumer garments, around 25% were T-shirts, followed in second place by underwear, with around 16%. The study analyzed 2293 garments collected in two German cities in the year 2022 [4]. Other data also suggests that T-shirts are one of the most commonly bought items of clothing in Germany [11]. Due to their large volume and given that they are usually single-layered and do not have fastening systems such as buttons and zippers, T-shirts are an interesting product group for recycling companies. Gaining in-depth knowledge about their material composition can make them an attractive input product for the different recycling technologies.

The European Union defines T-shirts, for classification for import and export matters, as “a lightweight knitted or crocheted garment, of a vest type, of cotton or man-made fiber, […], with or without pockets, with long or short close fitting sleeves, without buttons or fastenings, without collar, without opening in the neckline” [12]. The classification is summarized under the heading code 6109. T-shirts can have all kinds of adornments and prints, except lace. If the garment has fasteners like buttons or zippers, it is classified under the category Jerseys and Pullover with the heading code 6110 [12]. This distinction is not made in the market or by consumers, where the term T-shirt is used more loosely, describing above all the shape of the garment. T-shirts can thus be also made from lace or even woven fabrics, though this is less common. In this study, we consider all garments listed by the selected brands in their online shops under the label T-shirt as such; this does include garments like, e.g., polo shirts that by the official definition would fall under the category jerseys.

Traditionally, T-shirts are made from cotton knits. With the rising consumer demand for cheap garments, blends of polyester and cotton (polycotton) have become more popular, reducing the costs of the raw materials. There are no limitations to fiber usage in the production of T-shirts, so T-shirts made from man-made fibers, both synthetic and cellulose-based, are common as well. As T-shirts are made from knit fabrics, elastane fibers are sometimes added to improve the dimensional stability of the fabric during and after wear as well as during laundry processes and to add more comfort for tighter fits [13]. The fabric structure of knits is already rather stretchable, depending on the knit pattern, though as mentioned, not very dimensionally stable when worn over long periods of time [13,14]. Added elastane fibers thus allows the fabrics to return to their original shape when relaxed, leading to the lengthening of the lifecycle. By definition, they consist of knitted fabrics, which, according to recent insights into textile-specific design features by the Microfiber Consortium, show higher fiber shedding than wovens [15].

T-shirts usually come without any adornments, and due to the knitted structure of the garments, do not need any closures, though it is possible to add them for design purposes. Special styles like the polo T-shirt usually come with a small button placket. Big prints are common, and additional haberdashery items include pearls, sequins, ribbons, or embroidery both directly onto the fabric or as a patch. In a previous study, we analyzed 618 T-shirts. The results show that around 67% of the analyzed garments did not have any added haberdashery items. The most documented haberdashery items or adornments were big prints, with around 19%, and buttons were present in around 6%. Haberdashery items can also be present in combinations, e.g., a print paired with sequins [16].

2.3. Elastane Fibers

Even though elastane fibers are not used in large quantities and make up just 1.1% of the world fiber production, it is a highly relevant fiber for the garments- industry [1]. Mostly used as a functional fiber in blends, elastane is usually present up to 20% in a fabric, depending on the intended purpose of application. Higher elastane contents are possible and common especially for medical textiles like support bandages or shapewear [17]. Cotton or wool fabrics usually have elastane contents up to five percent, while polyester or nylon textiles can reach values up to twenty percent [18].

Elastane is made from 85% or more polyurethane polymers and is the most popular elastomeric fiber. The fibers have a high elastic recovery, which, as mentioned before, allows fabrics with an elastane content to return into their original shape after wear or laundering, and thus the garments become baggy less quickly [17]. Elastane fibers are also characterized by their high elasticity as well as an elongation at break of 400–650% [19]. Added elastane also increases the bursting strength as well as reduces the shrinkage of cotton fabrics [20]. However, increasing the elastane content also contributes to increasing fiber shedding of the elastane fiber as well as the blended component, contributing to microfiber pollution [21], probably because it creates more friction.

Elastane often forms the core of core-spun yarns as they can be sensitive to UV rays or exposure to oils. The sheath yarns thus protect the elastane fibers from possible degradation [17]. As elastane fibers have high luster due to their structure and thus appear to have a slightly orange sheen to them, processing them in a core-spun yarn will make the fabric more visually appealing as well [22]. Another technique for processing elastane fibers is called plating, where elastane fibers are blended with other fibers during knitting. The loops will be formed by one yarn of just elastane and one composed of the other fibers, e.g., cotton [20]. Usually, the elastane yarn will be fed in a way that allows it to lay on the back of the fabric, providing protection from UV radiation as well as reducing the visibility from the outside. Both methods of processing elastane fibers can be challenging for their detection during sorting though. Automated technologies like Near-Infrared Spectroscopy can only analyze the fiber composition at the surface of the fabric, so the core-spun elastane will stay undetected [8]. Elastane could be detected in plated fabrics by analyzing the left side of the garments, though post-consumer garments usually have the right side facing out, and turning them inside out would be too time and resource consuming. Identifying elastane fibers via touch is also not a viable and reliable option, which, paired with the low content values, makes elastane a hard-to-detect contaminant during recycling processes.

To shortly summarize, added elastane fibers improve the dimensional stability of a garment and thus prolong its lifecycle. They offer added comfort, especially for tight-fitting garments, and allow knitted fabrics to stretch more without becoming baggy or risking bursting. For material-based textile sorting, the fibers pose a challenge, as they are usually deeply ingrained into the fabrics, making the detection by eye or NIR Spectroscopy difficult. For textile recycling, the fiber counts as a contaminant, and low to non-elastane fiber contents are preferred. Improving the detection or making the occurrence of elastane fiber values in blends more predictable will greatly improve the economic and ecological viability of textile recycling technologies.

2.4. Textile Recycling Technologies

Textile recycling is an essential part of the circular economy, enabling us to transform post-consumer garments, which nowadays are mostly regarded as waste and being sent to landfills, into a valuable resource. With several technologies available targeting different fibers and blends, we are able to recover fibers from the textile waste in various qualities and can reuse them in the production of new products, including new garments. On paper, textile recycling sounds easy, but in practice, several challenges inhibit the upscaling of those technologies, thus preventing a quick transformation of the textile and garment industry towards more circularity and sustainability. For high-quality textile recycling, knowledge about the material composition of the textile input is crucial [7], information that often is either not provided by the producers or is not communicated along the whole lifecycle of a garment, meaning it is effectively missing at the end-of-life. To tackle the lack of information, initiatives like the Digital Product Passport are emerging. Before products with a DPP are on the market, we need to bridge the information gap differently, e.g., by analyzing currently available products regarding their material composition and looking for similarities. These analyses, like those performed in this study, can also provide valuable insights for the proposition of ecodesign guidelines.

As a preparation to the actual recycling processes, we need to sort the textile waste. First, reusable garments will be identified to be resold as secondhand garments. In the European Union, this is regulated by law, marking the second step of the Waste Hierarchy in the Waste Framework Directive 2008/98/EG [23]. Garments whose fabrics can still be reused for other applications will be sorted out for further processing, e.g., cleaning rags or insulation material. Some garments will be shredded for the production of non-wovens for other industries [24]. Fiber-to-fiber recycling of post-consumer textiles is not widespread yet. The amount of F2F recycled fibers has a market share of about 0.6% of the world fiber production [1]. As mentioned before, the lack of knowledge regarding the material composition is a huge factor in this low number. The classical sorting system is not focused on material-based sorting but rather on sorting for secondhand usage. There are technologies emerging that allow us to detect the material composition of the fabrics and sort them accordingly. One of the most popular methods is Near-Infrared Spectroscopy.

NIR Spectroscopy is a non-destructive analysis method for material identification. After calibration, it is quick and easy to use, does not require any chemicals, and can be utilized in a running, automated sorting process. During NIR analysis, material-specific spectra are measured and compared with a database with recorded reference spectra of known material samples. When exposed to near-infrared light (800–2500 nm), the organic molecules of the sample will absorb certain wavelengths of the NIR light based on bond vibrations. The amount of light absorbed at the different wavelengths creates a spectrum, which reflects the chemical composition of the sample. This works for all organic molecules containing hydrogen, though major bonds like C–H, O–H, and N–H are the most reactive, producing material-specific spectra collected and analyzed by the NIR device [25,26]. NIR Spectroscopy only works on the surfaces of the samples, so for multi-layered textiles like lined jackets, this identification method will not deliver accurate information. As mentioned before, elastane fibers embedded in the fabrics are also hard to detect. Low content values of fibers in general often stay undetected, and with heavily coated textiles, only the surface of the coating material will be analyzed. Furthermore, ageing processes, e.g., due to prolonged wear, laundering, or UV exposure, might alter the materials too much for accurate identification. In particular, worn-out cotton garments are affected by this and may no longer be reliably analyzed via NIR Spectroscopy [8]. Material-based textile sorting by NIR Spectroscopy can help create waste streams with higher homogeneity, preparing the textile waste for more efficient textile recycling. As it presents challenges of its own, NIR sorting should be used in combination with other technologies to further improve the quality of the textile waste streams.

Additional methods like the use of Artificial Intelligence (AI) paired with Image Recognition for the detection of, e.g., the product group or haberdashery items can be useful. AI models can pair the collected product group data measured in the sorting process with the material databases built for the product groups and help predict the material compositions. Paired with the NIR data, we can predict, e.g., elastane contents based on the product group and fit, which might not be detected by NIR alone. This can also allow us to not only speed up the sorting process but also make it more reliable and cost-efficient when compared with manual sorting. The generated material streams can be tailored specifically to the demands of recycling companies and directed to the different recycling processes, enhancing their economic and ecological viability by reducing contamination and the presence of foreign fibers in the input [7].

We will now briefly present the different recycling technologies currently available. They are in different stages of development, some being broadly applied for decades, others still on a lab scale. They all have advantages and disadvantages, which we will highlight as well. Before all recycling technologies can be conducted, certain pre-treatments can be carried out to reduce the amount of contamination present in the textile waste. This includes practices like washing and drying, removal of haberdashery items, bleaching or decoloring, as well as treatment with selective solvents to target specific materials within the waste. Generally, all textiles will be cut into smaller pieces to enhance the effectiveness of the recycling processes by providing a larger surface area.

We will start off with the most common recycling process, mechanical recycling. For this technique, the cut-up garments will be shredded in a series of rotating drums equipped with spikes, disentangling the fabrics until only the fibers remain [27]. Heavier contaminants like left-over haberdashery items and unshredded textile parts will be automatically removed during this process, and the unopened textile parts are fed back into the process. Contaminants like colorants, coatings, or dirt remain in the fibers, if not removed prior. This mechanical process is highly damaging to the fibers, reducing their properties, e.g., fiber length, abrasion resistance, and tensile strength, resulting in low-quality fibers that usually are utilized in other industries [28]. For application in the garment industry, the blending with virgin fibers is necessary to improve the fiber quality [27,29]. If the textile material is sorted by color before mechanical recycling, additional bleaching or recoloring steps can be omitted [30]. If no color sorting is being conducted, the color of the regained fibers is usually grey with random spots of color in between.

Almost all fibers can be fed into the mechanical recycling, as long as the tensile strength and cut resistance are not too high and do not damage the machines. Filament fibers or textiles with high elastane content values are potentially damaging as they pose the risk of winding around the moving parts of the machinery, causing heat development by friction and damaging the machinery. Heavily coated or laminated textiles are also unsuitable, as the fibers are too stuck together and the shredding drums do not have any real leverage point for disentangling. To sum it up, mechanical recycling is a widespread and low-cost method of textile recycling, producing low-quality fibers that still have a wide field of application in other industries. For usage in garment production, they need to be blended with new fibers. Fiber blends can be recycled by mechanical recycling practices. Generally speaking, this recycling process is considered a downcycling process, turning the materials into lower-value products than before but still keeping them in use. Although, in terms of costs, energy, and chemical consumption, it is currently the best and thus most used process. For the cutting and shredding process itself, the fiber composition does not need to be known. But when processing the fibers into new garments, it is important to declare the materials. The fibers can only be processed into staple yarns with a lower staple length and would therefore shed more fibers, including microfibers [15]. The mechanical recycling itself could be processed in mills with precautionary measures and filtration systems to capture shredded fibers.

The second textile recycling technology is thermo-mechanical recycling. In this process, shredded thermoplastic textiles, like polyester or nylon, are fed into a heated screw extruder, slowly melting the polymers. The molten polymers are then extruded into long strands, hardened, and cut into pellets. These pellets can then be reheated and reformed into a variety of different products, including fibers [31]. This recycling process is widespread in the recycling of packaging waste. PET bottles are often turned into fibers using this process [32]. Around 98% of all recycled PET (rPET) fibers produced in 2023 are bottle-to-fiber (B2F) recycled, and the remaining 2% of produced rPET is recycled PET from other sources [1]. The large share of B2F recycled fibers is due to the simplicity of the recycling process and the availability of the well-sorted waste stream. Thermo-mechanical recycling requires high-purity input, something that is hard to achieve with textiles, but easy to achieve with PET bottles [29], which, e.g., in Germany, are already sorted as a separate waste stream to begin with using a deposit system for buying and returning bottles. Textiles, compared to bottles, are a highly heterogenous waste stream with a large variety of different materials blended together. Sorting this waste stream to a single-variety material is hard to achieve and not yet economically viable [27].

If garment producers design their products using mono-material design strategies, they could be suitable for this recycling process; otherwise, thermo-mechanical recycling is not viable for textile recycling. On top of the difficulties of homogenous textile input streams, the polymer quality generally stays the same during thermo-mechanical recycling processes, unless additional chemical steps are induced, meaning degraded polymers, e.g., due to prolonged UV exposure or harsh laundering conditions, will stay degraded and reduce the quality of the regained polymers. This degradation as well as contaminants that may have been in the input will lead to decreased properties and may cause breakage during fiber spinning, damage the recycling machinery, and higher maintenance, e.g., due to clogged filters [33]. To summarize, thermo-mechanical recycling is widespread in the packaging industry and for the production of B2F fibers. It requires high-quality, homogenous input streams of thermoplastic polymers. For the recycling of post-consumer garments, it is not suitable due to the heterogeneity of the waste stream as well as degraded polymers. If garments are produced from mono-materials and collected separately, closed-loop recycling systems could be established with additional steps to improve the polymer quality during each cycle. Closed loop refers to a system in which few virgin materials need to be introduced; instead, waste materials are utilized as input without the loss of quality like in downcycling processes.

The next two recycling technologies are often summarized under the name chemical recycling. We think it is necessary to differentiate them from each other as the processes work differently. One process is deconstructing the fibers to their polymers utilizing selective solvents, while the other is deconstructing them all the way to their monomers by depolymerization, resulting in different end-products and qualities. Both processes are suitable to produce filament fibers, which have less shedding than staple fibers [34]. Let us start off with solvent-based recycling technologies, also known as polymer recycling. As the name already gives away, selective solvents are used for separating textile waste, dissolving specific fibers from the shredded textiles [27]. The dissolved polymers are in solution and can be regained by filtration [35]. This process is often performed with cotton or other cellulosic fibers. The recovered cellulose polymers can be turned into man-made cellulosic fibers like viscose. Due to the selective nature of the utilized solvents, recycling of fiber blends is possible [36]. The most common inputs are either post-consumer textile waste with a high cotton content or polycotton textile waste. Fibers that are unaffected by the solvents remain as solids in the mixture and can be recovered and reused separately [28]. The utilized solvents are tailored to the fibers that are supposed to be recovered and can be reused. Depending on the machinery, closed-loop solvent cycles are possible. In particular, for the recovery of cellulose from cotton, NMMO is used as a solvent, and the same solvent is also used during the production of Lyocell [27,28]. The polymer quality will stay the same during this process, meaning that highly degraded fibers are not suitable. It is also not possible to recycle cotton or man-made cellulosic fibers an unlimited number of times with this recycling process, as the polymer qualities will reduce with each cycle, resulting in a reduced degree of polymerization (DP) and thus, e.g., reduced tensile strength [29]. Solvent-based recycling is promising for fiber blends, as the solvents selectively dissolve certain fibers, leaving the other fibers mainly untouched [36]. It could thus also be used as a pre-treatment step to remove low values of fibers that would inhibit other recycling processes. The solvents can be used in closed solvent cycles, and polymer recovery rates vary depending on the utilized solvents and fibers. Cotton or cellulosic fibers cannot be recycled in a closed-loop system by this technology, as the polymer properties will degrade. Although, we can still produce higher-quality cellulosic fibers with this recycling technology compared to mechanical recycling and thus keep reusing the materials for longer periods of time. This recycling technology still has limited relevance in the textile industry, mainly for pre-consumer waste, and is predominantly used at small pilot plants for post-consumer textiles.

Chemical monomer recycling is one of the most promising textile recycling technologies in terms of the quality of the regained fibers. Synthetic polymer fibers, like PET or nylon, are depolymerized into their monomers using different techniques. Common depolymerization processes are hydrolysis, methanolysis, or glycolysis. Depending on the process, we can produce different monomers or oligomers, and different levels of contamination or foreign fibers are possible. Depolymerization by methanolysis has the highest impurity tolerance of up to 30%, though generally speaking, the lower the contamination, the better. The most widely applied depolymerization process for PET is glycolysis, which will produce BHET oligomers, suitable for the production of new PET. The recovered monomers are of virgin quality, and they can be reused for polymer formation and then for the production of a variety of different products, including textile fibers [33,37]. These recycling technologies can produce high-quality products and thus are very promising for closed-loop fiber-to-fiber recycling [29]. As mentioned before, they are only suitable for closed-loop recycling of synthetic polymers. Natural polymers like cellulose can be depolymerized, e.g., into glucose. Repolymerization to cellulose is not possible, but the utilization of the regained monomers in other industries is possible, e.g., turning glucose into bio-fuels [27]. Polycotton textiles can be recycled by this method, creating different products, as described before [27,35]. Depolymerization processes, depending on the method used, require harsh chemicals, high temperatures and pressure, as well as long reaction times, making the process rather resource-, energy-, and cost-intensive [28]. Despite this, chemical monomer recycling is preferable to monomer production from fossil fuels, as it allows us to keep already-available products in the cycle and prevent further destruction of the environment during fossil fuel extraction. We can recover monomers very similar to new monomers and continue to produce high-quality products using available resources that would otherwise go to waste. The process is as well only on small pilot plants applied to post-consumer textiles.

Lastly, biological recycling has to be mentioned. Similar to chemical monomer recycling, the polymers of the fibers are depolymerized, this time by natural means. Enzymes or microorganisms deconstruct the polymers into their monomers or oligomers. These biological processes usually work selectively, so the recycling of mixed fabric waste is possible [38]. Due to the nature of the enzymes or microorganisms, they require very tailored and stable reaction conditions. The right temperature, pH level, and also reaction time are crucial for efficient high-quality recycling as well as extensive pre-treatment steps to increase the accessible surface area of the fibers [29]. Compared to chemical monomer recycling processes, biological depolymerization is usually conducted under milder conditions [28]. Contaminants can also act as inhibitors, so the removal of all additional substances before biological recycling processes is advised. Certain depolymerization products like glucose might be consumed by the microorganisms utilized if the reaction times are too long, reducing the yield. Continued supervision is thus required [39]. The recovered monomers are also of virgin quality and can be reused as described earlier [27]. Biological recycling processes are still on a lab scale, though they seem very promising, especially for the recycling of mixed textile waste, as the enzymes and microorganisms selectively depolymerize fibers from the textile input and produce high-quality monomers. Further research and upscaling of the processes are required before these technologies can be broadly applied.

All recycling technologies introduced have advantages as well as challenges, and there is no perfect technique. Every process has its own requirements regarding material input and contamination levels and requires certain conditions and machinery. Generally speaking, it is preferable to have a minimum amount of contaminants or foreign fibers in the textile waste stream to improve overall recycling processes and output [40]. As explained before, material-based sorting and automated contamination detection as well as removal can act as facilitators for more efficient recycling processes. Another big facilitator can be legally defined design regulations like ecodesign guidelines.

3. Methodology

We analyzed T-shirts of two popular brands and post-consumer data collected by students at the University of Applied Sciences (HTW) Berlin, Germany. The students were studying Garment Technology in the first semester and were tasked with documenting the material compositions of the T-shirts they had in their own closets. By definition, these garments are not post-consumer waste yet, as they are still in use. Nevertheless, this data allows us to gain insights into the material composition of future post-consumer material streams. The selected brands are Primark and H&M and have been randomly selected from the statistic “The giants of large-scale distribution” from the Modaes.es (2023) report titled “El Mapa de la Moda 2023 Facts & Figures” [41]. The brands ranked in 6th and 2nd, respectively. We collected data from the German online shops of both brands at two different times about three months apart. Items listed in the German online shops have been analyzed, as the authors are based in Germany. This study as a whole is placed in the context of European laws and restrictions, and the authors do not expect drastic changes with the offered garments in other European countries. The data collected should therefore be able to reflect the European market.

Table 1. Applied search criteria for analyzed garments in the category T-shirts.

Origin	Main Category	Additional Filters	No. of Total Results	Notes
Post-consumer	T-Shirts	-	258 (15 November 2023)	Data collected by Garment Technology students at HTW Berlin 20 garments missing material information
Primark	Women Clothes Tops and T-Shirts	Product type: T-Shirts	234 (3 March 2025)	Includes tops, vests, long sleeves, and bodies Data from [43]
	Women Clothes Tops and T-Shirts	Product Type: T-Shirts Style: Polo shirts, Classic T-Shirts, T-Shirts with Print, Short T-Shirts, Long T-Shirts, Oversized Look, Uni T-Shirts, T-Shirts with Straight Cut, Adaptive T-Shirts, Performance T-Shirts	210 (10 June 2025)	Includes bralettes 20 garments missing material information
H&M	Women Clothes Tops and T-Shirts	Product type: T-Shirts Brand: H&M	281 (3 March 2025)	Includes long sleeves Data from [43]
	Women Clothes Tops and T-Shirts	Product type: T-Shirts Brand: H&M	452 (10 June 2025)	Includes some bralettes, some long sleeves, sets

showcases the applied filters and documented number of garments for each sample. For easier visualization, we will combine both samples of each brand into one in the Results. We have analyzed the results by fit of the garment and fiber composition, putting an extra focus onto elastane fibers and how the fit might influence elastane content values in blends. Possible contaminants for recycling, like haberdashery items, have not been taken into account in this study. Primark did not provide material data for 20 T-shirts, which have been excluded from the sample. In the post-consumer garment sample, 20 garments did not have any documented material composition either, and they have been excluded as well. In a study from Circle Economy by Wilting and Van Duijn (2020), 24% of analyzed post-consumer garments did not have any labels anymore [42]. Consumers may remove the labels to improve the comfort of the garment, as they sometimes are rough against the skin. Garments marked as sportswear by the brands are also not included in the results, leading to 59 items by Primark and 62 by H&M being removed from the results. Sportswear items usually have higher elastane values due to more specialized requirements, which could justify the higher content values, and should be analyzed separately. This resulted in a total of 1278 T-shirts spread over the five separate samples.

For each garment, we (a) documented the fabric composition listed by the brands on the website or, in the case of the post-consumer garments, on the care label, (b) classified the fit of the garment by the guideline provided in Figure 1, as well as (c) classified whether it is marked as sportswear or not. For the post-consumer garments, there was no differentiation between sportswear and “casual” garments. The fit was differentiated by three different options: (1) Tight, (2) Regular, and (3) Loose. The garment and sleeve length shown in Figure 1 are symbolic and not taken into account for the decision-making. Assigning a fit to a garment is a subjective matter, and to keep the margin of error as small as possible, all garments of one sample have been graded by one person on one day. In the case of post-consumer garments, the students received instructions beforehand on how to classify the T-shirts.

The fabric composition paired with the fit of the garment allows us an overview of the typical fiber blends and the typical elastane values of different garment cuts. We can connect the percentage of elastane with the fit of the T-shirts and thus propose ecodesign recommendations for fiber blends and elastane content values for the different fits, making content prediction during sorting more accurate when simultaneously using technologies like NIR Spectroscopy and AI-based Image Recognition tools. By knowing the different requirements provided by the fits, we can formulate more accurate ecodesign guidelines and thus keep contamination or recycling inhibitors at a minimum for each product and fit, rather than having generic guidelines for a product group.

It was found in the study by Wilting and Van Duijn (2020) that in around 41% of garments they analyzed, the material information provided by brands on clothing labels is not correct, with fabrics made from fiber blends particularly affected [42]. This study relied on publicly available care label data provided by the brands. The material compositions proposed by the brands cannot be tested for correctness in a lab during this study, and they will thus be treated as being correct. Laboratory validation vis spectroscopy or chemical analysis would improve data reliability. Future work should combine label-based datasets with lab-confirmed material analysis.

The data collected and analyzed in this study is included in the Supplementary Materials.

During the preparation of this study, the authors used ChatGPT-3.5 for the purposes of additional data interpretation and partial text generation; the generated text was not used in the final version of the manuscript and only functioned as a rough guideline for Section 4. The authors have reviewed and edited the output and take full responsibility for the content of this publication.

4. Results and Discussion: Fiber Content and Blends

A total of 1278 T-shirts have been analyzed. We collected data samples from two major fashion brands at two different times three months apart as well as a sample of post-consumer textiles. This resulted in 258 post-consumer garments, 365 garments by the brand Primark, and 675 garments by the brand H&M available in their German online shops.

As shown in Figure 2, about half (55%) of all analyzed T-shirts were made from a single fiber type, followed by 36% made from two different fiber types. Three or four fiber blends accounted for only 9% of garments combined. For all separate samples, the majority of documented T-shirts were composed of one type of fiber. Among the mono-material garments, cotton was as expected overwhelmingly dominant, representing 97.5% (693 pcs) of all mono-fiber garments, given that we excluded sports T-shirts. We assume that there are more polyester mono-material T-shirts in the sport segment.

The most popular mono-fiber used is cotton, which may be due to a preference for cotton and cellulosic fibers by German consumers, especially in summer, when the comfort, breathability, as well as moisture management properties of cotton fibers make it a favored material [44]. In addition, cotton remains the most popular fiber for the production of T-shirts in general. The high share of mono-material T-shirts, especially those made from cotton fibers, presents a promising opportunity for F2F recycling. Cotton garments are especially suitable for solvent-based recycling technologies, which require clean, homogenous input streams in order to yield high-quality recycled fibers. In this study, we did not distinguish between different cultivation conditions of cotton fibers, e.g., organic or conventional cotton. As many labels are faded during washing or cut out, a distinction is not possible in post-consumer garments.

To better understand which fibers are used in the production of T-shirts, we analyzed how often each fiber was present. This encompasses the usage of each fiber as a mono-material or within a blend. The results can be seen in Figure 3. As seen before, cotton (CO) was by far the most common fiber, present in a total of 1042 garments. The second most frequently used fiber was elastane (EL), found in 439 garments, followed by PET, which was present in 172 T-shirts. Viscose (CVI) and nylon (PA) are also notable, appearing in 151 and 96 garments, respectively. Animal fibers like wool (WO) and silk (SI) are rarely used in T-shirts by these two brands and in the post-consumer sample, only present in 34 garments combined.

The dominance of cotton fibers reflects their widespread use and popularity among consumers, especially due to the physiological properties already mentioned. Elastane fibers are often used to improve the comfort of garments, especially close-fitting ones. Elastane is also added to knitted fabrics to improve the dimensional stability of the fabrics, resulting in a large number of T-shirts containing the fibers. The high frequency of elastane fibers across the samples highlights its role as a functional additive in knitted garments. Polyester is widely used in blends, particularly with cotton, where it contributes to improved wrinkle resistance, mechanical strength, and lower production costs. However, due to its low moisture absorption, PET is not considered to be a very comfortable fiber for prolonged everyday wear. It is unsurprising that only 10 garments were made from 100% PET fibers. The share in sports garments is expected to be higher. Viscose, being made from cellulose, has similar properties to cotton. Since it is typically made from wood pulp, it can serve as a cost-effective alternative to cotton while maintaining comparable comfort characteristics.

Figure 4 presents the different two-fiber blends identified in the analyzed T-shirts. A total of 60 distinct two-fiber blends were documented, of which only one blend was used by both brands at both documented times and was found in the post-consumer waste. In total, 21 blends appeared at least in two of the five collected samples. The most frequently used two-fiber blend is a combination of cotton and elastane, which accounted for more than 50% of all two-fiber blends. This is followed by PET and elastane blends.

Figure 4 illustrates only the two types of combined fibers, disregarding their values. In this study, cellulosic fibers comprice viscose, modal, Lyocell, and linen (LI). The following

Table 2. Different 2-fiber blends utilized with no. of T-shirts of each blend.

CO + EL Total 232 pcs (50.7%)	CO95/EL5	184	Cellulose + EL Total 12 pcs (2.6%)	CVI92/EL8	8
	CO96/EL4	22		CVI93/EL7	3
	CO97/EL3	13		CVI95/EL5	1
	CO94/EL6	4	PA + EL Total 39 pcs (8.5%)	PA92/EL8	17
	CO90/EL10	4		PA95/EL5	9
	CO93/EL7	3		PA90/EL10	6
	CO98/EL2	1		PA98/EL2	2
	CO92/EL8	1		PA93/EL7	2
PET + EL Total 52 pcs (11.4%)	PET95/EL5	14		PA97/EL3	1
	PET89/EL11	14		PA94/EL6	1
	PET91/EL9	11		PA80/EL20	1
	PET87/EL13	5	PET + Cellulose Total 23 pcs (5.0%)	PET65/CVI35	11
	PET90/EL10	2		PET85/CVI15	5
	PET88/EL12	2		CVI70/PET30	3
	PET80/EL20	2		CMD66/PET34	2
	PET93/EL7	1		CMD63/PET37	2
	PET92/EL8	1	Others Total 56 pcs (12.2%)	CVI80/PA20	14
Polycotton Total 29 pcs (6.3%)	CO70/PET30	11		CVI70/PA30	13
	PET65/CO35	6		CO50/SI50	10
	CO59/PET41	3		CVI65/PA35	7
	CO80/PET20	2		PA65/CVI35	3
	CO65/PET35	2		CO96/PA4	2
	CO50/PET50	2		CO67/PA33	2
	CO60/PET40	1		PET78/PA22	2
	PET59/CO41	1		SI90/CO10	1
	PET61/CO39	1		CVI79/PA21	1
Cellulose Total 15 pcs (3.3%)	CO75/CVI25	5
	CO87/CVI13	2
	CO80/CVI20	2
	CO77/CVI23	2
	CO60/CVI40	2
	CO97/CVI3	1
	CVI85/LI15	1

provides more detailed information on their individual values and their frequency of occurrence. The blend written in bold is the blend utilized in all analyzed samples, and the italicized blends appear in at least two of the five samples.

As discussed previously, cotton and cellulosic fibers are favored for their comfort-related properties, including softness, breathability, and the ability to absorb moisture, making them comfortable to wear even during warm months [44]. It is therefore unsurprising that blends of cotton and elastane fibers dominate, with four other categories containing cellulosic fibers as well. In total, 79.7% of all two-fiber blends, including those summarized in the category Others in Figure 4, contain cellulosic fibers like cotton or viscose, and 65.5% of all two-fiber blends contain elastane fibers. Table 2 also reveals that 16 blends were used only for one garment, respectively, and 17 blends were used for two different garments each. These rarely used blends make up over 50% of all documented two-fiber blends, raising important questions about material diversity in textile production. Specifically, it prompts a critical reflection on whether such a wide variety of blends is truly necessary, what led to the decision of using these specific, uncommon blends, and whether the garments could have been manufactured from a more common blend in the first place. Reducing the number of distinct blends in circulation can simplify sorting and recycling processes and enhances the circularity potential of garments, though it might raise concerns regarding limiting product differentiation or design freedom. However, fiber blends are usually not a unique selling point of a brand, so regulating them for easier end-of-life processing will not majorly restrict the brands’ design freedom or style. Blend restrictions can rather help simplify the design and sourcing processes, as fewer options are available, but they will be tested and improved accordingly for their specific suggested products. During the use phase of the garments, blend restriction will have no negative effects on consumers. From our point of view, the restriction of blends will not have any negative impacts, but rather help to promote a more sustainable textile industry by improving the end-of-life handling of post-consumer garments and enhancing recycling possibilities.

The following Figure 5 illustrates the variety of three-fiber blends identified in the analyzed T-shirts. A total of 32 distinct three-fiber blends were documented, with nine blends (spread over 45 garments) appearing in more than one of the five samples. The most commonly used three-fiber blends are combinations of two different cellulosic fibers (e.g., cotton and viscose) with elastane. This trend aligns with the findings from the two-fiber blends, where cellulosic fibers and elastane are also predominantly paired.

Table 3. Different 3-fiber blends utilized with no. of T-shirts of each blend.

Cellulose + EL Total 40 pcs (47.1%)	CO48/CVI47/EL5	11	PET + Cellulose + EL Total 30 pcs (35.3%)	CO57/PET33/EL10	6
	CO75/CVI20/EL5	8		CO57/PET38/EL5	5
	CO48/CMD47/EL5	4		PET49/CVI47/EL4	5
	CO90/CVI5/EL5	3		PET60/CVI35/EL5	4
	CO85/CVI10/EL5	3		PET69/CVI26/EL5	2
	CO93/EL5/CVI2	2		PET52/CVI46/EL2	2
	CO86/CVI9/EL5	2		CO90/PET5/EL5	1
	CO58/CMD38/EL4	2		CO68/PET27/EL5	1
	CO48/CVI48/EL4	2		PET89/CVI9/EL2	1
	CO82/CVI13/EL5	1		CO96/PET3/EL1	1
	CMD57/CO35/EL8	1		PET48/CO45/EL7	1
	CLY66/CO29/EL5	1		PET71/CO26/EL3	1
PA + Cellulose + EL Total 9 pcs (10.6%)	CVI63/PA32/EL5	4	Cellulose + PET Total 3 pcs (3.5%)	CO67/CVI23/PET10	2
	CVI56/PA40/EL4	3		PET60/CO30/CVI10	1
	CVI63/PA31/EL6	1	Others Total 3 pcs (3.5%)	CVI52/PET28/PA20	2
	CVI60/PA34/EL6	1		PAN79/PET20/EL1	1

provides detailed information on the distinctive blend ratios and their frequency of occurrence. The italicized blends were found in at least two of the five sample groups. Notably, 95% of all three-fiber blends contain elastane fibers, with only five garments not utilizing the fiber in their blends. Additionally, nearly all three-fiber blends include cellulosic fibers, with just one exception documented in the Others category. Among the cellulose–elastane blends, 92.5% contain 5% elastane fibers, and the remaining elastane values are 4% and 8%. In PET–cellulose–elastane blends, 50% use 5% elastane contents, while in nylon–cellulose–elastane blends, the share is 44.5%.

The majority (43.5%) of the three-fiber blends are composed of 95% cellulosic fibers and 5% elastane, a portion even higher than in two-fiber blends (32.8% with the same ratio). The prevalence of cellulosic fibers blended with elastane suggests a continued focus on comfort and performance in T-shirt design. The substitution of parts of the cotton content with man-made cellulosic fibers such as viscose, modal, or Lyocell may reflect an effort to reduce raw material costs while maintaining the desirable properties of cotton fibers. In Germany, fibers like modal or Lyocell, especially branded by Lenzing, are also perceived as high quality by consumers, potentially increasing their attractiveness to both brands and consumers. Approximately 39% of the three-fiber blends are composed of PET and cellulose, with or without added elastane fibers. These blends may serve to balance cost efficiency with functional improvements, such as reduced wrinkling of cotton fabrics, while also enhancing durability and ease of care.

Only a small portion of analyzed T-shirts were made from four-fiber blends. In total, four distinct four-fiber combinations were documented, spanning 24 garments. One of these blends was used in two of the five samples, accounting for 19 garments (79%). Two of the blends contain wool fibers paired with polyester and elastane, the fourth fiber being either polyacrylic or viscose. The remaining two four-fiber blends combine cotton, viscose, PET, and elastane with different values and have been used for just one garment, respectively. Although wool fibers are often associated with high-quality garments, their values in the documented blends were relatively low, at 15% and 6%, respectively, and most likely added to making the garments appear of higher quality. From a material-sorting perspective, four-fiber blends are not really relevant fraction-wise in T-shirts. Separating more than two fiber types during recycling poses considerable technical challenges, especially when several types of synthetic fibers are combined or when individual fiber fractions are small. Given these limitations, the necessity of using such complex blends in the production of T-shirts should be critically evaluated. It may be possible to achieve similar fabric performance properties with fewer fiber types, which would improve recyclability.

As previously mentioned, we differentiated between three different cuts of the T-shirts: tight, regular, and loose. Figure 6 summarizes the distribution. Approximately 41% of all analyzed garments were classified as tight-fitting, followed by 34% of T-shirts with a loose fit. The remaining 25% were regular-fitting. The fit distribution varied between brands. Primark T-shirts were predominantly tight-fitting (52%), with only 21% classified as loose. In contrast, H&M T-shirts showed a more balanced distribution, with 38% tight- and 39% loose-fitting garments. Post-consumer garments are primarily mostly loose, followed by tight and then regular fits.

There are no prominent differences between the March and June samples regarding the fit distribution of the brands, suggesting that the season does not significantly influence the fit of the garments. Instead, fit appears more closely linked to fashion trends, brand positioning, and target demographics. Based on the two brands that we analyzed, it is not possible to predict the fit of a T-shirt. Despite brand tendencies, each sample included all three fit types, each accounting for at least 20%, respectively.

A total of 14 different elastane content values were recorded in the analyzed T-shirts. As shown in Figure 7, the most common elastane content value is 5%, followed after a significant gap by values of 3%, 4%, and 8%. Among all garments containing elastane, nearly 59% of the garments featured elastane values of 5%. Higher elastane content values of 10% or more were only utilized for a total of 42 garments (9.6%) combined.

As previously discussed, elastane functions primarily as a performance-enhancing fiber in garments, contributing to stretch, dimensional stability, and comfort. It is typically used in low percentages in blends, which aligns with the observed distribution. The dominance of the 5% content value, and the fact that over 90% of elastane content values are under the 10% threshold, are consistent with standard textile industry practices, particularly in T-shirt production, where flexibility and comfort are desired, without compromising fabric stability.

Figure 8 illustrates the relationship between elastane content values and the fits of the garments. All three fit categories included T-shirts with a 0% elastane content, with loose-fitting garments representing the highest share of elastane-free T-shirts. Notably, 197 (37.7%) of the 522 tight-fitting T-shirts also contained no elastane fiber. Tight-fitting T-shirts feature nearly all documented elastane content values, except the 12% value, with 5% being the most common elastane value. For regular and loose fits, elastane was present in 82 and 32 garments, respectively. However, the majority, 244 and 398 garments, respectively, do not have any elastane content.

The results show that there is a clear correlation between garment fit and the likelihood of elastane use: the tighter the fit, the higher the chance of elastane utilization in blends. However, it is only an indication, as there are also tight-fitting elastane-free T-shirts, but also loose-fitting ones with unexpected elastane contents. Accordingly, by using image data for automatic sorting, it is not possible to sort for an elastane content or exclusion according to the fit. The exact rationale behind fabric selection decisions remains speculative and cannot be conclusively determined from the available data.

5. Discussion: Proposing Elastane Content Levels and Reduced Fiber Blends for Ecodesign Guidelines

Current circularity efforts emphasize the need for improved design-for-recycling and better sorting of post-consumer waste. One of the key barriers identified in this study is the widespread use of low-percentage elastane blends, which are difficult to detect and can inhibit recycling processes. By analyzing the elastane content in relation to the garment fit, this research contributes practical data to guide ecodesign regulations and optimize future material flows for circular textile systems.

To make recycling more efficient, accurate sorting needs to be more feasible and material streams more predictable and homogenous. In general, the more homogenous the waste stream, the better the processing into new high-quality textile products. Based on our findings of various fiber blends in T-shirts, we propose ecodesign guidelines for elastane usage depending on the garment fit and recommend an overall reduction in the number of blends and alignment of the percentages. By choosing fibers more consciously, potential microfiber shedding of synthetic, non-degradable fibers can also be reduced.

We have documented excessive variability in fiber blends, often without functional justification. Sixty different two-fiber blends, thirty-two three-fiber blends and four four-fiber blends have been documented in this study. Of the 60 two-fiber blends, over 50% were used just for one or two garments each, suggesting that these fiber combinations are not industry standard. Wool contents in four-fiber blends were low and not essential for the functionality of the T-shirts, but rather posing a major recycling barrier. Certain fabric blends appear overengineered, with unnecessarily complex fiber compositions, potentially driven by marketing (e.g., wool for quality) or cost-saving rather than true functional necessity. Using complex multi-fiber blends for garments increases the environmental burden without clear performance benefits. This diversity and lack of standardization complicates automated sorting, reduces recycling compatibility, and increases process inefficiencies, especially when the values of the utilized fibers are low. The different recycling technologies rely on relatively homogenous input streams or require clear composition data to be effective. In the absence of standardized labeling or traceability, this inconsistency in blends becomes a barrier to circularity.

The high proportion of tight-fitting T-shirts without elastane is somewhat surprising. While knitted fabrics inherently offer stretch due to their fabric structure, they are also more prone to permanent deformation over time. To maintain dimensional stability, elastane is commonly added to close-fitting garments expected to stretch during wear. There are three possible reasons for that. First, elastane is not necessary when fibers, yarns, and knit structures provide sufficient elasticity, such as yarns with high twist and fibers with high elasticity, such as high-quality cotton or wool. The absence of elastane in tight-fitting garments can also be due to a lack of knowledge or a conscious strategy, particularly relevant for “fast-fashion” brands, where shorter garment lifespans might align with business models focused on frequent replacement cycles.

Notably, our results show that loose-fitting garments, which do not stretch significantly during wear and thus carry less risk of deformation, have elastane fibers present. The presence of elastane in 7.4% (32 of 430) of loose-fitting garments could result from bulk fabric purchasing, where the same material is used across various product types without taking the actual requirements of the specific garments into account. Blending more than two fibers, especially when one or more are used in low amounts, should be critically assessed. If the functional benefit of an additional fiber is negligible, they should be avoided. Each added fiber type reduces recyclability, and many recycling technologies are limited to processing just one or two fiber types per waste stream [36]. A limited palette of design-approved recyclable blends could be recommended within ecodesign regulations. Garments with blends outside that list should require justification based on functional necessity, such as weather resistance, compression, or insulation. Otherwise, their use represents overengineering, which contradicts the principles of circularity and efficient resource usage. “Fast-fashion” design practices may furthermore intentionally undermine garment durability. Tight-fitting garments without an elastane content may deform more quickly, losing their dimensional stability and becoming baggy. This could potentially accelerate garment obsolescence and push faster rebuying. This underlines the importance of policy-driven ecodesign regulations, not just voluntary guidelines. While absolute standardization across all product categories is unrealistic, developing fit- and function-specific ecodesign criteria for the different product groups is both feasible and desirable.

While from the material and performance or aesthetics side, it is not necessary to combine viscose or modal instead of cotton with polyester, or use nylon instead of polyester in combinations (see Table 3); it is probably due to the availability of the fabrics that they are chosen in technical product development by apparel engineers and fashion designers. A first step in reducing this huge variety would be to combine the EPR with ecodesign guidelines, making it more expensive to market T-shirts with fiber blends containing more than two different materials. The second step would be defining preferred material combinations, such as cotton–elastane instead of cotton–viscose–elastane or polyester–cotton instead of nylon–cotton. If brands chose a less-preferred option, they may have to include a higher price, as the higher variability may increase the difficulties in post-consumer textile sorting and recycling.

In order to predict the elastane content, we propose standard fiber blends with fixed percentages, as in

Table 4. Average elastane content values based on the fit with current possible elastane values and proposed elastane values.

Fit	Current Possible Elastane Content Values by Fit	Current Average Elastane Content in Fit	Current Average Elastane Content for Only Elastane T-Shirts	Proposed Possible Elastane Content Values by Fit	Average Elastane Content with Proposed Regulated Elastane Content Values in Fit	Average Elastane Content with Proposed Regulated Elastane Content Values for Only Elastane T-Shirts
Tight	Unregulated	3.7%	5.7%	0% 2%; 5%	2.9%	4.7%
Regular	Unregulated	1.2%	4.7%	0%; 2%	0.5%	2%
Loose	Unregulated	0.4%	5.2%	0%	0%	-
Total	Unregulated	1.9%	5.7%	0%, 2%, 5%	1.3%	4.1%

. While the ecodesign guideline for T-shirts should favor elastane-free garments, and hence the use of yarns with high twist and fibers with high elasticity and an adequate knit density to increase the dimension stability, there will probably be fibers and knits left where elastane has a functional requirement. For example, higher-weight knits with coarser yarns would require a higher percentage, and a tight fit may need more elastane, whereas a loose fit does not require any.

To calculate the average elastane content across T-shirts, each garment containing elastane was multiplied with their specific elastane value. The resulting products were then summed to determine the total elastane contribution in the analyzed T-shirts. To derive the average elastane content, the sum was divided by the total number of garments, either including the elastane-free garments or leaving them out of the calculation. The general equation can be seen below.

$$Average elastane content value={\displaystyle \frac{\sum (Elastane value\times number of T-shirts)}{\sum number of T-shirts}}$$

This method ensures that the average elastane content accurately reflects the distribution of elastane values across the sample. For the “average elastane content with proposed regulated elastane content values only for elastane T-shirts” in Table 4, we calculated all T-shirts currently containing 5% elastane or higher with the new value of 5% and all T-shirts with elastane values of 4% or less with the proposed value of 2%. This results in an average of 4.7% elastane content.

If garments with elastane are produced using only the recommended elastane levels outlined in Table 4, when including and also leaving the amount of elastane-free T-shirts unchanged, the average elastane content could be reduced to 1.3%. The average elastane content of only T-shirts containing elastane fibers can be reduced from 5.7% to 4.1%. The impact is especially notable for regular and loose fits, where elastane is often not necessary. For regular-fitting garments, we suggest a maximum of 2% elastane content to support dimensional stability, especially in the chest area of women and the shoulder area of men, enhancing the length of the possible lifecycle of the garments. This would reduce the average elastane content in regular-fitting T-shirts to 0.5%. For loose fits, we propose eliminating elastane fibers entirely. For tight fits, where elastane fibers can be functionally required, the suggested standard is 5%, which is already the most common value observed. Additionally, we propose a 2% elastane value. An elastane content of 2% should be sufficient for lightweight knits, providing adequate elasticity and dimensional stability. Heavier, more structured knit fabrics may require up to 5% elastane to maintain their shape and performance characteristics during wear for longer periods of time. Consequently, reducing the elastane content will also reduce fiber shedding, including microfiber pollution of the environment, and significantly increase the recyclability of post-consumer T-shirts.

Most T-shirts analyzed in this study are made from cotton or other cellulosic fibers, either as mono-materials or in a blend with elastane fibers. In particular, mono-cotton garments are promising candidates for solvent-based fiber-to-fiber recycling. However, elastane is difficult to detect through visual inspection, manual touch, or even automated technologies like NIR Spectroscopy, complicating the sorting of mono-material waste streams. Applying fit-based elastane content guidelines would enhance predictability and the sorting efficiency. For instance, if a recycling process is not able to tolerate any elastane content, loose-fitting T-shirts can serve as an ideal input, as they are not allowed to have any elastane contents under our proposed ecodesign guidelines. If the technologies can deal with 1% elastane contamination, regular-fitting T-shirts could be used as an input. With predictable average elastane content values, and with material-based sorting paired with AI-driven classification of the product group, fit of the garment, and contaminant detection, we can improve the quality of the textile waste streams after sorting, making them less heterogenous and tailoring their composition to the requirements of the recycling technologies. Realizing this vision will require further research and collaboration across the industry.

6. Conclusions

To summarize the findings of this study, we conclude that while the fit distribution of T-shirts is relatively balanced and cannot be reliably predicted by brand or season, it does correlate with the elastane content. This correlation allows for the approximate prediction of the elastane content based on the fit of the garment, which has meaningful implications for textile sorting and recycling. Across all 1278 analyzed T-shirts, the average elastane content was 1.9%. When separated by fit, significant differences emerged, as shown in Table 4, where tight-fitting T-shirts have an average elastane content value of 3.7%, regular-fitting T-shirts of 1.2%, and loose-fitting garments just 0.4%. Due to the high number of garments without any elastane fibers, the overall average remains relatively low. These variations are critical, as recycling technologies are able to tolerate certain amounts of contaminants. Knowing that the different fits have different average elastane content values allows us to match the sorted textile waste better with the proper recycling technologies and to tailor the reaction conditions as well as the amount of chemicals needed, which will make the processes more economical and ecologically viable.

In the current landscape of “fast fashion”, material selection is still largely driven by cost, aesthetics, and production convenience, rather than recyclability or lifecycle impact. This study demonstrates that many of the complex or rarely used fiber blends found in just 1278 T-shirts are neither functionally necessary nor properly recyclable. Unless urgently addressed, this design inconsistency threatens the viability of F2F recycling at scale. To enable a truly circular textile economy, ecodesign frameworks must provide more than just suggested recyclability: they must also actively shape material usage in order to reduce microfibers from garments as environmental pollutants.

Policies like the EU Ecodesign for Sustainable Products Regulation and the Textile Strategy are now beginning to address this. Notably, the DPP, expected to become mandatory for textiles as part of the ESPR, will play a critical role in more efficient and sustainable textile sorting and recycling. Once fully implemented, this system will allow automated, high-accuracy sorting of garments based on their recyclability profiles and requirements provided by recycling companies. It may also hold producers accountable for unnecessary material complexity, such as a low content of third fibers or inconsistent blends, which currently obstruct closed-loop textile recycling. By combining material regulations in design with data transparency enabled through the DPP, the fashion industry can move toward a system where product recyclability is not a post hoc consideration but a default design constraint. Until this shift becomes the industry standard, research like this is essential to highlight which garments and material decisions enable or inhibit progress toward textile circularity. According to WRAP (2017), reusing 1 kg of clothing can save up to 25 kg of CO₂-equivalent emissions. Given the scale of T-shirt consumption, even marginal improvements in recyclability and lifespan can lead to significant environmental benefits [45]. To further support circularity, policymakers should promote production from recycled and sustainably sourced materials, invest in F2F recycling infrastructure, and incentivize repair, reuse, and resale markets. Clear design for recycling standards and eco-modulated fees can further accelerate the industry’s sustainability transition.

Beyond the fit of a garment, the fabric structure also influences a garment’s stretchability and dimensional stability and thus influences the potential need for added elastane fibers. Future work will explore the interplay between fabric construction, elastane content, and garment fit, refining the proposed ecodesign guidelines further. In addition to the fiber composition, other materials and components like haberdashery items, colorants, and finishings or coatings can present challenges both during automated material detection as well as recycling processes. Systematically analyzing these elements will help identify recycling inhibitors, guide the development of ecofriendlier alternatives, and support the creation of comprehensive ecodesign guidelines for the different product groups. Furthermore, this study does not address biodegradable fibers, as their degradation-focused end-of-life pathway is incompatible with closed-loop F2F recycling systems. However, further research is needed to explore how biodegradable materials might fit within hybrid circularity frameworks, especially in cases where F2F recycling is technically or economically infeasible.