Advances in Recycling and Reuse Technologies for Textile Fiber Material Products

Recycling fiber materials is an important means to achieve resource circulation and reduce environmental pollution. Different forms of fiber recycling and reuse are classified in Figure 2 [22]. Depending on the recycling approach, current fiber material recycling technologies can be primarily categorized into three types: mechanical recycling, chemical recycling, and biological recycling. Due to differences in physical and chemical properties, different types of fiber materials require distinct recycling techniques. A comparison of the advantages and disadvantages of these three recycling technologies is presented in Table 1. A systematic understanding of the principles and application scope of different recycling methods is crucial for promoting the efficient recycling and reuse of fiber materials. 2.1. Mechanical Recycling Mechanical recycling technology is currently the most mature and widely used method for fiber material recovery. The core principle involves physically processing waste textiles into fibers or yarns through mechanical means, then reprocessing these fibers into new textile products. As this process involves no chemical reactions, it offers distinct advantages, including operational simplicity and relatively low energy consumption. However, research by Filho [24] has demonstrated that mechanical recycling leads to the progressive degradation of the regenerated fiber quality with each recycling cycle, ultimately affecting yarn and fabric production. This technology is particularly suitable for natural fibers such as cotton, linen, and wool (with the specific recycling process illustrated in Figure 3 [25]) and certain synthetic fibers, including polyester and nylon. The standard mechanical recycling process for post-consumer textiles is shown in Figure 4. According to statistics, mechanical recycling accounts for approximately 43% of total fiber recovery [26] representing the highest proportion among the three major recycling technologies. The typical process consists of several key steps: sorting, cutting, fiber opening, carding, and re-spinning. The mechanical recycling process begins with rigorous sorting, which helps ensure a certain level of recycling quality. Modern sorting centers combine manual sorting with automated technologies. First, near-infrared spectroscopy is used to quickly analyze fiber composition [27] followed by color recognition via high-resolution cameras, and finally, manual verification to ensure accuracy. After sorting, the textiles undergo pre-treatment, including the removal of impurities such as plastic components, buttons, and zippers. Next, cutting and shredding equipment processes large pieces of waste textiles into smaller fragments or fibrous materials. As shown in Figure 5 [28] rotating blades shear the fiber materials as they pass through the cutting device. Subsequently, the fragments undergo fiber opening, carding and drawing to break them down into individual fibers. Finally, the processed fibers are spun into recycled yarn or developed into nonwoven fabric products. Different fiber types exhibit significant variations in recyclability. As the second most commonly recycled fiber after cotton, acrylic fiber possesses unique application value. Ahu et al [29] successfully obtained recycled acrylic fibers by blending acrylic fabric waste with covered yarns and polybutylene terephthalate (PBT) elastic yarns. For mixed-material waste, Hussien and Nachiappan [30] explored separation and recycling techniques for cotton-acrylic blended knitwear. Through mechanical shredding and further refinement using metal pins on feed rollers, they successfully extracted recycled cotton and acrylic fibers for nonwoven fabric development, demonstrating the feasibility of recycling mixed materials. The diversity of waste types has given rise to innovative recycling solutions. Sanches et al [31] addressed waste materials with different structural characteristics by combining shredded fibers extracted from woven, knitted, and nonwoven waste with virgin cotton fibers and recycled polyester. They employed conventional ring spinning to produce yarns, which were subsequently processed into blended knitted fabrics using small-diameter circular knitting machines. This integrated utilization of waste materials from different sources effectively expands the source of recycled raw materials, provides new ideas for the comprehensive utilization of waste textiles, and enhances the performance diversity of the final product. In-depth research on the production process of recycled yarn from waste textiles has revealed directions for technological optimization. Yu et al [32] systematically investigated the complete process from waste cotton fibers to yarn-reinforced composites, which sequentially includes opening, carding, drawing, roving, spinning, and final twisting operations, as illustrated in Figure 6. Particularly noteworthy is their finding that twist parameters decisively influence product performance. The optimal twist for single-yarn reinforced composites was determined to be 0.76 times that of conventional single yarns. Furthermore, plied yarns exhibited significant strength variations across different twist ranges: in the low-twist range (550–850 TPM), the tensile strength reached 148.52 MPa, while in the high-twist range (850–1050 TPM), the strength decreased with increasing twist. These precise measurements provide a scientific basis for process parameter optimization. Recycled fibers require appropriate blending ratios with virgin materials in practical applications. Wanassi et al [33] produced a blended yarn by mixing waste cotton fibers with virgin cotton fibers in a 50:50 mass ratio, following a manufacturing process that included opening, carding, drawing, roving, and spinning. This blended yarn demonstrated a 33.5% cost reduction compared to 100% virgin cotton yarn. Following this, Arafat et al [34] successfully produced medium-count (30 Ne) yarns suitable for knitwear production by blending pre-consumer and post-consumer recycled cotton fibers with virgin cotton using ring spinning technology. Meanwhile, Ütebay [35] investigated an optimal blend ratio of 50% recycled cotton, 30% virgin cotton, and 20% polyester fibers specifically for pre-consumer cotton knitwear waste, while also confirming the suitability of rotor spinning technology for processing recycled materials. Collectively, these studies provide practical formulations to facilitate the industrial-scale application of recycled fibers. The mechanical recycling of waste textiles has shown unique value in the development of functional materials. Mohamed et al [36] used needle-punching technology to produce nonwovens from acrylic and wool wastes. The result exhibited thermal insulation properties superior to those of conventional glass and mineral wool, Figure 7 [37] shows a high-speed needle punching machine. Çay [38] converted a variety of textile wastes into biochar through low-temperature carbonization and applied it to cotton fabrics to obtain functional textile materials that provide high thermal and physiological comfort. These studies show that waste textiles have unique advantages in the field of functional materials. Mechanically recycled fibers also show good reinforcing effects in construction and composite applications. Patricia et al [39]. added cotton fibers extracted from discarded denim to polyester concrete, then irradiated the concrete with γ-rays. Eventually, the compressive and flexural strengths of the discarded cotton fiber polyester concrete were enhanced. Kamble and Behera [40], meanwhile, used pre-consumer cotton textile waste to develop thermoset composites suitable for furniture and automotive components. Petrucci et al [41] developed polypropylene with moisture-resistant properties by washing, removing impurities, and garneting waste denim to obtain denim fibers, which were added to a polypropylene matrix. Finally, composite panels were obtained by injection molding to develop a polypropylene composite suitable for automotive door panels. These high-value-added applications provide new economic growth points for waste textile recycling. Quality assessment studies of recycled materials have provided unexpected discoveries. Julia and Anders [42] compared post-consumer waste with different degrees of abrasion. They found that more abraded materials lost less fiber length during the recycling process. This overturned traditional understanding and provided a new perspective for screening recycled materials. Taken together, the mixed application of waste textile fibers with other materials is the mainstream technical route at present. It shows broad application prospects in the fields of yarn production, functional material development, and composite material enhancement, opening up a diversified development path for the recycling of textile resources. The mechanical recycling methods for different types of waste textile materials are shown in Table 2. In summary, the advantages of mechanical recycling technology are its simple process, lower cost, and lack of chemical pollution, but its main disadvantage is that the length and strength of recycled fibers are usually reduced. This affects the performance of the final product, and it is difficult to reach the original state, even though the blending of multiple fibers can enhance the performance of the product to a certain extent. Therefore, mechanical recycling is usually used for low-end products, such as building insulation, car interiors, and carpet padding, and is difficult to use for high-end textile production. 2.2. Chemical Recycling Chemical recycling technology refers to the decomposition of waste fiber materials into monomers, oligomers, or reusable polymers through chemical reactions, which are then re-polymerized to produce new fiber polymers for high-quality recycling [48,49]. Compared with mechanical recycling, chemical recycling can effectively remove impurities such as dyes, auxiliaries, and coatings from textiles and restore the original properties of the fibers to a certain extent, thus producing recycled materials with properties close to or even better than those of virgin fibers. However, the process of chemical recovery usually involves high temperatures, high pressures, or the use of chemical reagents, making it more technically complex and costly. 2.2.1. Chemical Recycling Technology for Waste Cotton Fibers Significant breakthroughs have been made in research on the chemical recycling of waste cotton fibers, especially in environmental management and material regeneration. Researchers have developed a variety of innovative technologies to convert waste cotton fibers into high-value-added materials. Ma et al [50] developed dual network hydrogels (Cellulose/PAM DNHs) based on waste cotton fabrics and polyacrylamide with excellent pore structure and laminar architecture. These can efficiently adsorb a variety of heavy metal ions, such as Cd, Cu, Pb, Zn, and Fe, and offer a new approach for industrial wastewater treatment. Meanwhile, Zeng et al [51] successfully prepared porous, biocompatible, and highly conductive electrode materials through the in situ polymerization and carbonization of common waste cotton textiles. The nitrogen-doped carbon nanoparticles encapsulated on the surface provided a large specific surface area for bacterial growth, which significantly improved the performance of the microbial fuel cell and expanded the application of waste cotton fibers in the energy field. In the fields of cellulose separation and regeneration, researchers have developed various green solvent systems and catalytic methods. Wang et al [52] developed a deep eutectic solvent (DES) based on choline chloride (ChCl) and p-toluenesulfonic acid (TsOH) to successfully treat waste polyester cotton blend fabrics by simultaneously extracting polyester (PET) and microcrystalline cellulose (MCC) under the optimal conditions (75% DES, 110 °C, and 10 min) with yields of 99.20% and 69.46%, respectively. Yousef et al [53] developed a deep eutectic solvent based on nitric acid and p-toluenesulfonic acid (TTB) to extract polyester and microcrystalline cellulose (MCC) under optimal conditions (75% DES, 110 °C, 10 min) with yields of 99.20% and 69.46%, respectively. In addition, Yousef et al[53] efficiently recovered cotton fibers and polyester from waste denim fabrics by de-dyeing with nitric acid solution and separating the fabrics with switchable hydrophilic solvents, achieving a recovery rate of 96%. In terms of fiber regeneration, Liu et al [54] successfully converted degraded waste cotton fabrics into regenerated fibers with strengths of up to 1.11–1.29 cN/dtex using environmentally friendly alkaline/urea solvent systems (LiOH/urea and NaOH/urea) combined with wet spinning. Hou et al [55] used phosphotungstic acid (HPW) to catalyze the hydrolysis of waste cotton fabrics to prepare microcrystalline cellulose, with a yield of up to 83.4% and a crystallinity of 85.2% under optimal process conditions. Wei, Robinson, Huang et al [56,57,58] used ionic liquids as catalysts, providing new ideas for the high-value utilization of waste cotton fibers. 2.2.2. Chemical Recycling of Polyester Fibers Polyester fiber (PET) is one of the most common synthetic fibers, and its main component is polyethylene terephthalate (PET). The chemical recycling of PET mainly relies on depolymerization reactions, and common processes include hydrolysis, alcoholysis, ammonolysis, and glycolysis, as shown in Figure 8a [59]. The aim is to degrade PET into its monomers or oligomers, then re-synthesize PET through polycondensation reactions to create high-quality recycled polyesters. Different methods of PET chemical recycling and their derived value-added products are shown in Figure 8b. Hydrolysis is the reaction of PET with water under acidic, neutral, and basic conditions to produce terephthalic acid (TPA) and ethylene glycol (EG), which can then be reused for polymerization by purification [60]. Pereira et al [61]. explored the hydrolysis of PET catalyzed by a variety of acid catalysts (zeolites, inorganic acids, ionic liquids, carboxylic acids, metal salts, and carbon dioxide) at 200 °C for 2 h. Carboxylic acids and metal salts gave greater than 80% TP yields, and zeolite had a negligible effect on the TPA yield. CO₂ as a catalyst did not significantly increase the TPA yield, and its TPA yield was higher when acetic acid was used as a catalyst. Onwucha et al [62] succeeded in hydrolyzing PET in a neutral environment without the use of catalysts, resulting in higher TPA yields. This method avoids the use of acids, toxic solvents, complex TPA purification/recovery processes, and the generation of large amounts of wastewater. However, it requires a long reaction time (6–24 h) and a high PET/water ratio to obtain a TPA yield of 85–98%. Peterson et al [63] performed alkaline hydrolysis of mucilage/PET, pure mucilage, and pure PET samples at a solid-liquid ratio of 1:100 in aqueous NaOH. Yan et al [64] obtained alkaline banana peel extract (PBPE) through the calcination of waste banana peels. Hydrolysis of PET using K₂CO₃ contained in BPE was carried out, and nearly 100% of the content of the TPA was obtained through reactions at 150 °C for 4 h. This method is suitable for the recovery of high-purity PET, but it has high energy consumption and requires the precise control of the reaction conditions in order to prevent the formation of by-products. Li et al [65] proposed a method that combines hydrolysis, reactive processing, and decolorization, which was very effective in converting colored PET fabrics into high-purity TPA under alkaline conditions. The final monomer yield (88.51%) and decolorization rate (94.22–97.65%) were higher than those of TPA produced by conventional hydrolysis. Alcoholysis refers to the degradation of PET to dimethyl terephthalate (DMT), diethyl terephthalate (DET), or low polyesters through the reaction of alcohols, such as methanol (methanolysis) and ethylene glycol (glycolysis), with PET, which is then further purified and re-polymerized to PET. Ma et al [66] synthesized an ionic liquid using Bronsted–Lewis dibasic acid [HO₃S(CH₂)₃-NEt₃]Cl–[ZnCl₂]_0.67, which was then used as a catalyst to efficiently catalyze the methanolysis of PET at atmospheric pressure, 195 °C, and a reaction time of 30 min, with a yield of about 78.4% of DMT. Furthermore, the results showed that two methanol molecules were needed to depolymerize to obtain DMT and ethylene glycol (EG). Lozano-Martinez et al [67] investigated the subdegradation of PET by subcritical ethanol and achieved 94% PET degradation through reaction for 30 min at 275 °C, with a pressure of 40 bar and a 5% weight ratio of PET to ethanol. Tang et al [68] successfully prepared MgO/NaY catalysts with different MgO contents by using the incipient wet impregnation method. The results were used as modified mesoporous catalysts to catalyze the methanolysis of PET. The results showed that the MgO/NaY catalyst had the best catalytic effect when the MgO content was 21%, and the PET conversion and DMT yield were as high as 99% and 91%, respectively, at a methanol-to-PET mass ratio of 6, a catalyst dosage of 4wt%, and a reaction time of 30 min (at 200 °C). Ammonolysis refers to the reaction of PET with ammonia or amines (such as ammonia gas or ethylene diamine) to produce terephthalic acid diamide and EG. A key feature of amination is its focus on upgrading or functionalizing PET rather than simply recycling it. Liang et al [69] developed an innovative one-step method that does not require catalysts or solvents, enabling PET amination at mild temperatures (40–120°C). This technology can directly recover high yields (>90 mol%) and purity (>95%). Although amination holds great potential for the upcycling and functionalization of PET, it requires large amounts of non-environmentally friendly amines and is prone to side reactions that generate impurities. The glycolysis method depolymerizes PET into bis(2-hydroxyethyl) terephthalate (BHET) using diols (usually diethanol) as degradation agents [2]. Chen et al [70]. used a combination of Zn(OAc)₂/DBU catalysts to depolymerize waste PET to bis(2-hydroxyethyl) terephthalate (BHET). The reaction took 77 min at 180 °C with a weight ratio of PET/EG of 1:3 and a Zn(OAc)₂/DBU molar ratio of 1:2. The results showed that the purity of BHET was close to 80%, and the purity of the glycolysis product was 94.85%. When using a DBU molar ratio of 1:2 for 77 min, the BHET purity was nearly 80% and the purity of the glycolysis product was 94.85%. Kim et al [71] used a miniature MgO-doped SiO₂ catalyst to promote the glycolysis of PET, and the final extracted BHET had a purity of more than 99.85%, which indicated that this catalyst has significant benefits for promoting the glycolysis of PET. In addition to the above-mentioned catalysts, other researchers have explored various catalysts, including ionic liquids [72,73], Mn₃O₄ [74], and Fe₂O₄ [75]. Although glycolysis requires relatively high temperatures (close to 200 °C), it possesses the advantage of being less affected by contamination, making it the most widely used method for recycling PET in industry. Chemical recycling technologies for different types of waste textile materials are shown in Table 3. Overall, chemical recycling technologies can achieve high-quality recycling of fiber materials, but their industrial application still faces several challenges. First, chemical recycling processes usually involve high temperatures, high pressures, or the use of chemical reagents, resulting in high energy consumption and, in some cases, the potential generation of hazardous waste streams or by-products. The question of how to optimize the process to reduce its environmental impact is thus a focus of current research. Second, the recycling of mixed fiber materials remains a difficult issue. For example, blended fabrics contain a variety of components, such as PET, cotton, and nylon. Identifying a method of efficiently separating the different components in order to improve the efficiency and purity of chemical recycling is an important challenge for the industry. In addition, the economic viability of chemical recovery is a key issue. As chemical treatment involves expensive reagents, solvents, and catalysts, which are often more costly than mechanical recovery, the process needs to be further optimized to improve the economics of recovery. 2.3. Biological Recycling The biological recycling of waste textiles has emerged as an eco-friendly option in recent years. Compared to traditional mechanical and chemical recycling, biological recycling offers significant advantages such as low energy consumption, mild processing conditions, and environmental friendliness. This technology primarily uses specific enzymes or microorganisms to break down polymer materials in waste textiles, converting them into monomers or other valuable products like cellulose and bioethanol. Unlike chemical recycling, biological recycling typically operates under gentler conditions, consumes less energy, and uses more environmentally friendly reagents. Its high selectivity makes it a promising solution for separating mixed textile waste. However, biological recycling still faces challenges, including high costs and slow reaction rates. Biological recycling is primarily applied to natural fibers such as cotton, linen, and wool, which can be broken down by enzymes into basic chemical compounds like sugars and amino acids. Enzymes can be used to decompose waste textiles into monomeric building blocks, which can then be utilized to produce various high-value-added products, such as sugars and bioethanol. In biological recycling processes, cellulases are typically employed to break down cellulose-based materials like cotton fibers, while proteases are used to degrade protein-based materials such as wool and silk. Due to the specificity of these enzymes, they enable the separation and recycling of mixed textile waste. Figure 9 illustrates various value-added products extracted from cellulose-based waste using biological recycling technology [25]. For the biological recycling technology of textile waste, Hu et al [84] used cellulase enzyme produced by Aspergillus niger fungus for the hydrolysis of textile waste, and then obtained glucose with a recovery rate of 70.2% after technological treatments, including autoclaving, freezing alkali/urea treatment, and milling. The resulting glucose can be further used for the production of ethanol or other chemicals. In addition, the PET fibers remaining after hydrolysis can be reused in textile production through melt spinning, forming a closed-loop resource utilization system. For the biological recycling of protein fibers such as wool, this is mainly achieved by keratinases. Navone et al [85] investigated the selective digestion of wool fibers in wool/polyester blends using enzymatic digestion and the recovery of the blends by keratinases, which exclusively degrade the keratin proteins in the wool, and by sodium thioglycolate, a reducing agent used to disrupt disulfide bonds in the wool keratin proteins. The polyester fibers recovered using this method showed no significant difference in elastic modulus or hardness compared to virgin polyester. The recovered wool degradation products can be used to produce biofertilizers, animal feeds, or cosmetic raw materials. Due to the large number of disulfide bonds in wool fibers, it is difficult for ordinary proteases to completely degrade them. Zhang et al [86] investigated this issue and found a green and highly efficient alternative method: a mixture of Es protease, L-cysteine, and urea was used to degrade wool, with a weight loss of up to 99.5%. Although biological recycling is primarily applied to natural fibers, recent years have seen progress in research on the biodegradation of synthetic and man-made fibers. Yoshida et al [87] reported a bacterium capable of degrading and assimilating PET. Quartinello et al [88] investigated the enzymatic hydrolysis of PET in textile waste. Under neutral aqueous conditions at 250 °C and 40 bar for 60–90 minutes, they utilized Humicola insolens cutinase (HiC) to hydrolyze PET. This enzyme specifically cleaves the ester bonds of PET, achieving an 85% recovery rate of terephthalic acid (TA) during the chemical pretreatment stage, which increased to 97% after enzymatic treatment. The recovered TA can serve as a raw material for PET resynthesis. This method offers a new approach to closed-loop recycling of PET textile waste, combining environmental friendliness with economic potential. Kawai et al [89] reviewed the current state of PET enzymatic degradation and its potential applications in waste stream management. While these studies provide new insights for plastic bottle recycling, they have not yet been widely applied to textile fibers. Due to the large molecular size of enzymes, their penetration into the interior of PET materials is limited, confining the hydrolysis reaction primarily to the surface, which significantly restricts the reaction rate. Researchers have explored enzymatic hydrolysis for fibers such as nylon using mixtures of proteases and lipases [90], Nagai et al [91] conducted a study to quantify the reaction rates of nylon hydrolase acting on thin nylon layers. These studies have suggested potential pathways for the biological recycling of polyamide fibers, though practical applications still face challenges. Vecchiato et al [92]. investigated the enzymatic hydrolysis of flame-retardant-pigmented rayon fiber waste. At 50 °C and pH 4.8, with an 8 h reaction time, the waste was decomposed into glucose and flame-retardant pigments, achieving recovery rates as high as 98% and 99%, respectively. The recovered glucose and flame-retardant pigments can be reused in ethanol and rayon fiber production. This demonstrates that biological recycling technology is not only suitable for pure natural fibers but can also be applied to regenerated cellulose fibers with specialized functional treatments. In summary, biological recycling technology offers an environmentally friendly approach to textile waste recovery, which is particularly suitable for natural fibers and certain regenerated fibers. Biological recycling techniques for different types of waste textile materials are summarized in Table 4. Compared with chemical recycling, biological recycling typically operates under milder conditions with lower energy requirements, employing benign solvents and chemicals. The high specificity of enzymes makes biological recycling an excellent choice for separating mixed textile waste. However, biological recycling still faces limitations such as restricted applicability, demanding pretreatment requirements, and commercialization challenges. This technology is primarily effective for natural polymers but comes with higher costs and slower reaction rates than chemical recycling. Textile waste often requires pretreatment before biological recycling, as antimicrobial or insect-resistant treatments on fabrics may inhibit enzyme activity.