The task of technologies is not only to recover raw material otherwise destined for waste-to-energy or landfill: it is also to avoid the formation of waste itself and to save valuable materials, water and energy
We continue our journey into the world of textile recycling by publishing an examination of technologies for recycling textile materials, extracted from the study carried out by Dr. Aurora Magni for ACIMIT – Italian Textile Manufacturers Association – entitled “Textile Recycling: Scenarios and State of the Art,” with financial resources made available by ICE – Agency for the Promotion Abroad and Internationalization of Italian Companies and the Ministry of Foreign Affairs and International Cooperation, as part of the ITMA Milan 2023 Project, aimed at supporting the participation of Italian companies in this event.
The full version can be downloaded from www.acimit.it
With growing global consumption, increasing demand for fiber and increasing textile waste urgently pose the development and deployment of technologies for sorting and recycling end-of-life textiles and pre-consumer waste.
According to a study by research firm McKynsey, as urgently as it is needed, the potential of the recycling process is actually limited by objective factors, including the presence of multiple materials in a single garment that are often not easily separable and the absence of an efficient recycling preparation chain. It reads, “Our analysis indicates that by overcoming these barriers, fiber-to-fiber recycling could reach 18 to 26 percent of gross textile waste in 2030 with capital investments of 6 to 7 billion euros. This sector could, once matured and downsized, become self-sustaining and profitable with a profit pool of between 1.5 and 2.2 billion euros by 2030.
The environmental and social results would be decidedly positive: creation of about 15,000 jobs and reduction of CO2 emissions by about 4 million tons, equivalent to the cumulative emissions of a country the size of Iceland. The effects on the economy would also be positive: quantifying the secondary effects on GDP from employment, CO2 reduction, and water and land use, the industry could reach 3.5-4.5 billion euros in total annual holistic impact by 2030, reaching a return on investment of 55 to 70 percent annual holistic impact.”
Digital technologies to support systemic actions
Technologies will play an important role both in preventing waste formation and, of course, in recycling, but also in countering illegal and improper behavior. In fact, the illegal dispersion of textile material in the environment requires targeted interventions consistent with the principle launched by the EU ‘polluter pays’. In Italy, combating this phenomenon will require advanced investments to monitor the territory with satellites, drones and artificial intelligence systems, an experience already tested to intercept plastic abandonment on beaches.
But it is in production processes that digital technologies will play a decisive role in the collection, sorting and selection of materials for reuse and recycling and in particular in:
– Implementation of platforms to support industrial symbiosis programs aimed at facilitating matching textile waste supply and demand: a formula already tested in EU programs, such as the Material Match Making platform created by UNIVA and Centrocot, together with corporate partners under the Life M3P program;
– Monitoring the collection and processing flows of materials destined for recycling by making efficient and documented the processing process itself, with reliable values regarding percentages indicating the component from recycling present in new products and environmental declarations;
– Development of business models based on circularity and sharing: from the sharing economy to the sale of used garments and accessories life on line;
– Awareness-raising activities and active involvement of consumers in assuming responsible behavior from the purchase phase to the disposal of the good.
Collecting and preparing for recycling
The steps that a textile waste has to go through to enter the reuse chain are: 1. Collection of textile fraction carried out by home collection or roadside containers; 2. Temporary deposit/storage; 3. First selection by type of article with opening of bags deposited by users; 4. Second selection by type and quality, carried out manually by specialized personnel, to extract the fraction of greatest value and to create homogeneous batches of reusable products; 5. Sanitization of the product sent for reuse.
The portion of municipal textile waste discarded because it is not suitable for reuse is in turn sorted by type of material and sent to production of rags for industrial use (absorbent and washing cloths and mops) and for floor protection; to textile recycling; or to incineration-termination.
Research has focused on the sorting stage with technologies that speed up and make the activity more reliable, helping the industry to transition from manual-craft modes of work to more industrial models. These are mainly material reading systems using infrared spectroscopy (NIR) to identify color and composition of textile products. Natural, man-made or synthetic fibers have, in fact, differentiated chemical and molecular structures that react differently to electromagnetic waves.
By way of example, we cite two recent experiences:
–SIPTex, a Swedish consortium led by IVL, the Swedish Institute for Environmental Research, has developed a pilot plant with a sorting system using NIR and VIS technologies that recognizes and separates fabrics by prevailing fiber type and color and sends via a compressed air system to designated collection areas.
-Prato-based Next Technolgy Tecnotessile has developed a prototype semiautomatic machine that recognizes materials running on an automatic belt based on structure (knit, orthogonal, warp-knit), color (7 reference colors) and fiber composition. The machine consists of a sensing station with advanced cameras using near-infrared hyperspectral technology, assisted by an artificial intelligence system with self-learning algorithms and image storage, a conveyor belt, and garment collection baskets.
Before recycling, it is also necessary to remove components such as soft-rubbery or metal elements such as buttons and zippers. While soft-rubbery parts can bind to the fiber being recycled making it unusable, hard parts can damage machines during processing. And a spark caused by a metal component hitting machine parts can cause easily flammable fibers such as cotton to ignite.
The recycling step
There are different types of textile recycling, as outlined by the European Commission in the document ‘Study on the technical, regulatory, economic and environmental effectiveness of textile fiber recycling’: mechanical, thermo-mechanical, thermo-chemical and chemical recycling.
Mechanical recycling
Mechanical recycling has a long history. In fact, it is an established technology that has been adopted for decades, especially in the processing of woolen fabrics, as the Prato district demonstrates. Once freed from metal parts and linings, fabrics are sorted according to fiber composition and color shade (which will reduce or even avoid the use of dyes on the recycled yarn). In the case of woolen fabrics, carbonization is provided, a chemical treatment to remove any cellulosic fibers present that would compromise the quality of the regenerated product. The materials undergo tearing and defibering, mechanical treatments aimed at untangling and releasing the fibers. The resulting material is then ready for carding, which is the machine that parallels the fibers and aligns them into slivers, which will then be made into yarn by stretching and twisting. The coloring of the final yarn can be achieved by selecting and blending fabrics based on the color shade already present or by ennobling processes that can be adopted either on pre-spinning materials or on the product at the end of the cycle. This regeneration system is used both to produce yarns ready for re-entry into the market and as a preparatory step for subsequent heat or chemical treatments; in the case of mixed or low-quality fibers, it is used to fray materials to obtain padding, filler material, and reinforcement for composites.
While mechanically treated wool maintains a good level of quality, more critical issues relate to the treatment of other fibers in which there may be loss of quality, so much so as to require insertion of virgin fibers to raise the level of the yarn produced. The preservation of dyes and chemical additives in the material obtained by mechanical recycling can be interpreted as an advantage (sorting the waste stream by color avoids dyeing processes), but it can be a problem due to the presence of undesirable contaminants, so much so that it can be difficult to claim compliance of the material obtained with REACH parameters.
However, mechanical recycling allows, albeit in a downcycling logic, to treat a wide range of materials and fabrics that cannot be recycled by other technologies, and it is a process with low energy consumption (between 0.3 and 0.5 kW per kg of input material) and low water consumption, limited if necessary to a cleaning process as pretreatment; compared to chemical recycling, it requires lower investments and an average level of technical expertise of the employees.
Technological research focuses mainly on improving the quality of the materials obtained and on reconfiguring the machines for the subsequent thermal and chemical treatment stages starting from a more effective selection of the input materials.
Thermo-mechanical recycling
Like mechanical recycling, thermo-mechanical recycling is a cost-effective and well-established process. Based on grinding and melting materials, it is particularly useful for recycling man-made fiber production waste and some consumer waste collected at specialized centers (e.g., plastic bottles), but it is not suitable for thermoset polymers. The possible presence of polymers incompatible with the recycling process can cause problems in processing and penalize the quality of the output, so careful selection of the input material is an important prerequisite.
Research related to this type of recycling is focused on reducing the immiscibility of polymer blends. The same is true for color since pigments remain together with other contaminants such as wash residues, flame retardants, coatings, etc., present in or on the fiber or fabric (potentially in conflict with REACH regulation). It should also be kept in mind that the properties of the polymer/fiber deteriorate after each cycle. Therefore, despite the similarities with melt processing of virgin or waste plastics, specialized equipment or components are required to ensure a stable and continuous process and no changes in the degree of stickiness of the resulting polymer. Also, because the dyes remain in the polymeric material, only dark colors can be obtained unless the input is color-selected and the ability of some thermochromic dyes to change color at a certain temperature is exploited.
Thermo-chemical recycling
The process uses the partial oxidation reaction of polymers or heat to degrade polymers into monomers that can be used as raw materials for the chemical industry, and is a useful technology for the reduction of textile waste that cannot be treated by other methods, but not for fibre-to-fibre recovery. It is considered a mature technology, although developments to enable the production of raw materials for the chemical industry as an alternative to energy recovery or fuel production are very recent.
The main output of the process, syngas, has many possible applications in chemical synthesis reactions leading to a whole range of products.
The energy requirement for thermochemical recycling is high. In thermochemical recycling, pyrolysis and gasification processes, combustion takes place at temperatures varying between 800 and 1200°C with sufficient oxygen to completely oxidise the material. The output products (gas and oil) generated by gasification and pyrolysis can be used for thermal and energy purposes. However, with subsequent purification/upgrading steps, they can also be converted into chemical intermediates and thus serve as raw material for the chemical industry.
Chemical recycling
Uses chemical dissolution or chemical reactions to disassemble used fibers, extract polymers for new uses or break them down into their constituent monomers for reconstruction into new polymer fiber structures.
The study cited indicates three cases where this recycling technology is applied:
– The recycling of cotton to obtain pulp that can be used in the production of cellulosic fibers (viscose, lyocell) by sulfate, sulfite and sulfur-free process. The sorting of textile wasteis very important: a high cotton content (at least 50 percent) is required to optimize the process. Tolerance to dyed fabrics depends on the process, but most technologies include a bleaching and/or bleaching step. The resulting pulp can be mixed with wood pulp before spinning.
– Â Recycling of PA6 and PET monomers is a depolymerization process in which polymer chains are broken down into monomers by different technologies and various reaction conditions (temperature-pressure-time-catalysts). The solvents applied are typically water (i.e., hydrolysis, used for PA6), alcohols (i.e., methanolysis) or glycols. All three reaction mechanisms can be used for PET, although glycolysis is the most common. In addition to the three mentioned methods of solvolysis, a fourth method has recently become available, namely, an enzymatic depolymerization reaction. In this case the chemical reaction is mediated by a biological catalyst. Although the final output depends on the reactant, PTA (terephthalic acid) and MEG (ethylene glycol) are the traditional monomers obtained from PET that can be repolymerized to obtain virgin polymer of high purity, while for PA6 the output is caprolactam that can be repolymerized into virgin PA6. The efficiency of chemical recycling of synthetic fibers is highly dependent on the purity of the input material. For economic reasons, the PET or PA content of the input should be around 80-90%, which is why there is a tendency to obtain polyester fiber from recycling of PET packaging and industrial waste. The use of PET for textile fiber production, however, removes polymer from the repeat recycling loop since polyester is not easily recyclable and is, therefore, discouraged by the EU Commission, which urges the adoption of fiber-to-fiber recycling. Polyamide, on the other hand, is usually obtained from carpet tiles, fishing nets and other plastic waste, with recovery estimated at around 65 percent of the total input stream.
– The poly-cotton case: Using solvent-based dissolution and filtration processes, it is possible to separate the different materials and extract the desired components: the recovered cellulose can undergo the previously described process to produce cellulosic man made while the PET polymers remain largely intact and can be made into filaments, although in today’s practice they are incinerated for energy recovery. A second type of technology consists of a hydrothermal approach to degrade (partially) cotton or PET or both. It is carried out by bathing in water, pressure, temperature and chemistry. A third approach focuses on partial degradation through enzymatic treatment to obtain glucose (which can be used for other industrial processes), cellulose powder, and PET. To obtain PET fibers through a melt spinning process suitable for textile applications, however, virgin PET must be incorporated to improve its quality.
– Recycling through biotechnology: The biological cycle relies on resources that can decompose and build up nutrients to be transformed into new renewable resources. Processes can be distinguished into: biological, i.e., end-of-life biological decomposition in which microorganisms metabolize textile materials into simple molecules (compost and anaerobic digestion), biochemical, i.e., enzymatic depolymerization using enzymes to deconstruct polymer fabric into monomers, and fermentation by microorganisms.
Textile waste is seen as a cellulose-rich feedstock for the second-generation biorefinery in an open-loop recycling process (cotton to cellulosic man made), however much contaminants that may be present, elastomer and chemicals used for dyeing and finishing, as well as the high crystallinity of cotton fibers, may be constraints to the recycling itself. Wool and silk are made of keratin, a protein whose recycling has been investigated by the food industry for recovery of biopolymers from post-slaughter plumage, but the severe associated costs and negative environmental impacts have currently discouraged this line of research.
Studies are underway for the enzymatic recycling of PET, although this approach is difficult to apply to textiles today. There is, however, a growing idea that methodologies developed in the area of biotechnological management of materials from renewable and nonrenewable sources may be an avenue to be explored further or be functional for chemical recycling processes, for example, in the pre-recycling preparation stage. This is one of the most innovative fronts of textile materials recycling research, although still not very scalable at the industrial level.
Aurora Magni – Presidente Blumine srl
