Recycling of Spandex: Broadening the Way for a Complete Cycle of Textile Waste

Abstract

With the continuous growth of the global textile and apparel industry, coupled with the increasing demand for comfort in clothing, the use of spandex in blended fabrics has become increasingly widespread. Spandex, a high-elasticity synthetic fiber, is extensively applied in apparel and medical products. However, due to its typically low content in textiles and complex blending with other fibers, the recycling process becomes significantly more challenging. This review focuses on the recycling and utilization of waste spandex and its blended fabrics, analyzing the importance of spandex recovery from blended fabrics. It provides an overview of existing recycling technologies for spandex and its blended textiles, discussing the advantages and disadvantages of physical, chemical, and combined methods. This review emphasizes that the physical dissolution method, due to its simplicity, efficiency, and low cost, is currently the preferred strategy for recycling spandex-blended fabrics. Finally, this review outlines the pathways for reusing spandex after dissolution, offering new insights for enhancing the added value of regenerated materials and promoting the green recycling and utilization of spandex-blended fabrics.

1. Introduction

Spandex is a synthetic fiber known for its exceptional elasticity [1], with its invention dating back to the 1950s. In 1958, DuPont, an American company, successfully synthesized spandex for the first time, branding it “Lycra” [2]. The invention aimed to fulfill the growing demand for highly elastic fibers, particularly in sportswear and dancewear. Spandex significantly enhanced the comfort and stretchability of fabrics, quickly gaining popularity in the market. With advancements in technology, the production process of spandex gradually matured and found widespread use in various applications such as apparel, medical textiles, and sports equipment. Since the 1990s, spandex has become an indispensable component of the global textile industry [1], with continuous market growth. To this day, spandex is often referred to as the “MSG” of the textile industry, and the saying “No spandex, no fabric” has been widely accepted in the textile and garment sectors [3]. With increasing consumer demand for comfort and functionality in clothing, along with advancements in spandex production technology and cost reduction, spandex-blended fabrics have become more widespread in the modern textile market.

Spandex (also known as Lycra or Spankx) is a synthetic fiber based on polyurethane, with its chemical structure consisting of polyurethane segments, typically produced through the reaction of diisocyanates with polyols. The unique block copolymer structure of spandex, alternating between soft and hard segments, imparts exceptional properties such as a tensile elongation of 400% to 800% and a recovery rate of over 95%. These properties significantly enhance the comfort and elasticity of fabrics [4]. Despite its high mechanical strength, the unique molecular structure of spandex allows it to exhibit extremely high elongation when stretched.

Even a small amount of spandex can significantly enhance the elasticity of fabrics. Spandex is rarely used alone in textiles; it is typically blended with other fibers such as cotton, polyester, and nylon to create blended yarns, including types such as twisted yarn, core-spun yarn, and covered filament yarns [5] (Figure 1a,d). This blending process allows for a synergy between the elasticity and comfort of different fibers, thereby complementing and enhancing the overall fabric performance [6] (Figure 1b,d).

The textile industry’s growth reflects the population increase and the rapid development of human society [8,9]. However, the large volumes of textile waste generated have become a critical issue for global environmental protection and waste management [10]. Over 90 million tons of textile waste is produced worldwide annually [11]. However, the recycling rate is less than 20% [12] (Figure 1c). The main reason for this situation is the high proportion of blended fabrics, which accounts for approximately 85% of textile waste [13,14,15]. Only a tiny portion of single-fiber materials can be directly recycled using textile recycling technologies, while the complex composition of blends greatly complicates identification, sorting, and separation processes. The global production of spandex fibers is estimated to be about 3–5% of total textile fiber production, However, the total amount of spandex waste blends reaches 25 million tonnes, which is about 20% of total fiber production, and will grow to 29% by 2030 [1,16,17]. Traditionally, spandex fibers have followed a linear, unsustainable production model, where petroleum-based raw materials are used for manufacturing, and the fibers are discarded after use [18], either in landfills or incinerated. This linear model leads to the depletion of petroleum resources and significant waste generation. Moreover, spandex is a non-biodegradable synthetic fiber that exhibits excellent chemical resistance, particularly in acidic and alkaline environments [19], where it maintains high chemical stability. The chemical stability of spandex complicates its recycling process.

Thus, spandex’s unique physical and chemical properties, complex blend structure, and widespread use in blended fabrics pose significant challenges for recycling textile waste [3,20,21]. During mechanical recycling, spandex’s high elasticity and thermoplasticity can cause fiber entanglement [22,23], leading to blockages in fiber transport channels, cotton ball formation, or the clogging of needle rollers due to molten adhesion [24], which hinders equipment operation and increases processing costs. In chemical recycling, the high chemical stability of spandex makes it challenging to degrade effectively. The complex degradation products can negatively impact the environment and interfere with the recovery of other fiber components. For instance, in the study of the alcoholysis of polyester/spandex-blended fabrics, it was found that even with only 6% spandex content, alcoholysis products from spandex reduced the polyester degradation monomer recovery rate by 32.4% and caused issues with color quality [25].

Some studies have focused on the recycling and utilization of pure spandex, such as physical dissolution and alcoholysis methods [26,27]. However, research on the recycling of spandex-blended fabrics remains limited. Existing studies often overlook the recovery potential of spandex, opting to remove it first when processing waste textiles to recover other components. For example, in the case of nylon/spandex-blended fabrics, thermal treatment is employed to remove spandex and recover the nylon [3,28]. While this process is simple and effective, it removes the relatively higher-value spandex component and retains the lower-value nylon, thereby failing to maximize the recovery value and achieve full-component recycling of the fabric. Albitres et al. [29] demonstrated that the joint recycling of three textile materials could be achieved by melting and blending waste nylon/spandex fabrics with PET. However, the properties of this blend may make it difficult to apply in higher-value applications. Therefore, due to the complexity of the components and structure of spandex-blended fabrics [30], separating spandex from other fibers and realizing green, high-value recycling of blended fabrics still face significant scientific and technological challenges.

As global attention to environmental protection and sustainable development continues to grow, the role of spandex in future textiles will become even more significant. With its outstanding elasticity and comfort, spandex will remain a key position in high-performance sportswear, medical textiles, and smart fabrics used in wearable technology. With the development of new recycling technologies, the spandex recycling process is expected to improve further, reducing environmental pollution and enhancing the resource reuse rate. Therefore, the recycling and reuse of spandex is both necessary and important.

This review begins with an overview of the current status of spandex waste recycling and utilization, followed by a discussion of the latest advancements in recycling methods for spandex and its blended fabrics. It also discusses the key challenges in recycling waste spandex-blended textiles. Furthermore, this thesis provides an overview of the recycling methods for spandex fabrics with different blending components, focusing on various recycling approaches’ technological levels and application values. Finally, suggestions and prospects for recycling spandex-blended fabrics into high-value-added chemicals are proposed.

2. Recycling and Utilization of Spandex Waste Yarns

As the primary source of spandex waste yarns is production-generated scraps [31], their relatively uniform composition results in lower technical requirements for processing. Consequently, this area has already developed significant research and technological applications. Recycling methods for spandex waste yarns typically include mechanical processing, chemical degradation, and biological degradation (

Table 1. Description of spandex waste recycling methods.

Recycling Methods		Advantages	Disadvantages	Citations
Physical Method	Fiber Regeneration	Low cost, high recycling efficiency, mature technology.	Relative molecular weight and solution viscosity decrease.	[19,22,23]
	Additives	Low process requirements, high recovery rate.	The surface finish is poor, and the application scenarios are limited.	[32,33,34]
	Composites	Avoid the problem of uniformity of spinning liquid, and the product is widely used.	High cost of equipment and materials.	[35,36]
Pyrolysis	High-temperature pyrolysis	Can be used to treat mixed waste, and the products (oil, carbon, gas) are widely used.	The separation and purification are difficult, and the energy consumption of the equipment is high.	[37,38]
Chemical Method	Alcoholysis	The decomposition conditions are mild, and the polyols can be recovered for recycled polyurethane synthesis.	Solvent recovery is costly, and the catalyst can introduce metal contamination.	[39]
	Acidolysis	Efficient separation of soft segments (polyols) and hard segments (diamines) for rigid PU (polyurethane) foams.	strong acids (e.g., succinic acid) are required; corrosion equipment; after hydrolysis, it needs to be neutralized.	[40,41]
	Aminolysis	Selectively cleaves urethane bonds, and the product is of high purity.	Amine reagents are highly toxic (e.g., ethylenediamine); residual amines may contaminate the recovered product.	[42]
Biological Method	Esterolytic Degradation	Has a fast degradation rate and is suitable for polyester PU; works well in a humid environment.	Polyether-type PU cannot be effectively processed.	[43,44]
	Urethanolytic Degradation	Suitable for PU of urethane and amide bonds; degradable urethane urea.	The rate of hydrolysis is slow, and the effect on bonds embedded in deep layers is poor.	[45]
	Synergistic degradation	Some microorganisms are assisted by oxidase and peroxidase to break the chain and enhance the degradation efficiency.	The process is complex, and it is necessary to optimize the enzyme combination and reaction conditions.	[46]

and Figure 2), with notable progress achieved in separation, degradation, and reuse technologies.

2.1. Physical Methods

The physical recycling methods involve mechanically breaking down or grinding the waste spandex into spandex fragments. These regenerated fibers can then be used as raw materials for further processing into new materials or products. Depending on the application field of the regenerated spandex waste, it can be categorized into three types: re-spinning waste fibers, reuse as additives, and preparation of composite materials.

Some researchers have developed a process that involves adding spandex waste fibers to the polyurethane spinning solution for re-spinning [18,22]. The spandex waste fibers are crushed into short fibers and introduced into a diluted polyurethane solution, which is then stirred and dissolved to create a spinning solution with the required concentration and viscosity. Through dry spinning, spandex with properties identical to those of conventional spandex products can be obtained [1,21]. Spandex can be dissolved in solvents such as dimethylformamide (DMF) or dimethylacetamide (DMAc) to form solutions, which can be used as plasticizers or fillers to improve the flexibility and elasticity of other polymers [47]. For example, spandex solutions can be incorporated into polyvinyl chloride (PVC) to enhance its toughness and tensile properties [33]. However, this method may not be suitable for applications in food packaging or other fields where recycled products are restricted [32].

Additionally, recycled spandex (polyurethane) can be used in coating applications, such as fabric coatings. In this context, it not only provides additional durability and water repellency but also serves a physical shielding function, mitigating the photodegradation of PET. However, due to polyurethane’s isocyanate and polyether structures, degradation products such as quinones and aromatic compounds may form under ultraviolet (UV) exposure [34]. These degradation by-products could potentially have adverse effects on ecosystems. The excellent elasticity and wear resistance of spandex give it advantages in the field of composites, such as blending with epoxy resins (EP) or polypropylene (PP) to enhance the tensile properties and recovery ability of the materials [35]. Moreover, spandex can be blended with thermoplastic polymers and processed through extrusion or injection molding to form composite materials, improving the toughness and impact strength, which makes them suitable for sports equipment and elastic products [3,48].

The physical recycling methods of spandex can effectively improve the utilization rate of spandex waste and, through relatively simple processes, allow it to re-enter the textile or composite materials industries. These methods are suitable for promotion and application in industrial settings. However, further consideration may be needed regarding their potential environmental impact. The recycling of spandex-blended fabrics can refer to the recycling methods for spandex waste fibers. Suitable solvents for dissolving spandex, such as DMF and DMAc, can be selected while also considering the use of spandex’s melting and thermoplastic properties for the recycling and reuse of its blended fabrics.

2.2. Pyrolysis Methods

The pyrolysis of textile waste involves reactions such as the macromolecular bond cleavage, isomerization, and polymerization of small molecules under anaerobic conditions, redistributing elements such as carbon, hydrogen, and oxygen from organic polymers into the three-phase pyrolysis products: gaseous, liquid, and solid carbon-containing compounds [36]. Pyrolysis can convert organic solid waste into high-value products, including pyrolysis oil, biochar, and combustible gases, showing promising development prospects.

The pyrolysis of spandex can be divided into two main stages: first, the decomposition of spandex’s amine groups and polyether/polyester segments, followed by the further decomposition of soft segments and other polymers [49]. During pyrolysis, volatile products such as nitrogen compounds, aromatic amines, diisocyanates, and various oxides are generated [50]. Eschenbacher et al. [51] investigated the pyrolysis products of highly elastic ether-based PU and semi-rigid PU at 600 °C, finding that ethylene, propylene, various oxides, and organic nitrogen compounds were the primary products. The pyrolysis products from these materials were complex and challenging to separate and purify [37]. The temperature and oxygen concentration during pyrolysis significantly influence the nature of the products. Morado et al. [52] reported a polymer degradation mechanism, termed Cyclization Triggered Chain Cracking (CATCH), applied to polyurethane degradation. The recovered polyols from the degradation products were successfully used in polyurethane adhesives and photochromic coatings. The experimental results showed that this degradation mechanism is not limited to polyurethane and could potentially be applied to other polymers, offering long-term environmental benefits.

Since pyrolysis generally does not consider the purity and separation of raw materials, it can be used to treat mixed, hard-to-separate waste. Anceschi et al. [53] conducted pyrolysis of PET-PU mixtures, converting them into microporous carbon materials. The pyrolysis of PU contributes to forming aromatic compounds and volatile gases while promoting the formation of microporous structures in the final carbon material, which can be used as a high-performance adsorbent for treating industrial wastewater.

The advantages of pyrolysis include its ability to rapidly and efficiently convert waste spandex and complex, mixed, and structurally challenging blended fabrics into usable chemical products [38]. However, some products generated during pyrolysis (such as aromatic compounds) may have negative environmental impacts. Furthermore, there is still limited systematic research on the pyrolysis conversion mechanisms and products, and developing key core technologies and high-value products remains crucial.

2.3. Chemical Methods

The chemical recycling of spandex involves selectively breaking the chemical bonds in its molecular chains (such as carbamate bonds, ester bonds, or ether bonds) through chemical reactions, converting the waste spandex into reusable monomers, oligomers, or high-value-added chemicals. The chemical recycling methods for spandex include alcoholysis, aminolysis, and acid hydrolysis, among others [27,30,54] (Figure 3).

Wang et al. [55] used ethylene glycol as an alcoholysis agent to study the effects of alcoholysis conditions on the product yield, structure, and performance and successfully obtained the raw material polytetrahydrofuran for spandex. Gu et al. [39] employed ethylene glycol and propylene glycol as alcoholysis agents, in combination with bimetallic and alkali metal catalysts, to conduct alcoholysis of waste polyurethane foam. They proposed optimal reaction conditions, which provide guidance for the regeneration of waste polyurethane.

Figure 3. Chemical degradation reaction scheme of spandex (polyurethane) [56].

Figure 3. Chemical degradation reaction scheme of spandex (polyurethane) [56].

In addition, Gama et al. [40] used succinic acid to depolymerize flexible polyurethane foam waste, and the regenerated polyols obtained could partially replace conventional polyols (up to 20%) in the production of rigid foams. Liu et al. [57] employed a Friedel–Crafts alkylation reaction to convert waste polyurethane foam plastics and aniline into multifunctional hyper crosslinked polymers (HCP-PUF), which can be used as adsorbents. After five cycles, the material retained more than 80% of its regeneration capacity.

Bech et al. [41] proposed a polyurethane recycling method combining acid hydrolysis and hydrolysis, effectively recovering polyols and diamines from soft and hard PU foams. By using diacids (such as succinic acid) to break down polyurethane, the hard and soft segments are successfully separated, allowing for the recovery of polyols and diphenylamine. After acid hydrolysis, the resulting imides can be hydrolyzed or hydrogenated to recover diamines. This method achieved a recovery rate of up to 83%.

The chemical recycling methods for spandex primarily rely on the cleavage of chemical bonds. However, the degradation products are complex and are significantly influenced by the reaction conditions, degradation agents, and catalysts, potentially including amines, alcohols, acids, aromatics, and other substances [42]. Therefore, these factors must be considered when recycling spandex-blended fabrics.

2.4. Biological Methods

Biological recycling methods utilize microorganisms or enzymes to degrade spandex, enabling its reuse or harmless treatment [44,58]. The degradation of polyurethane primarily relies on hydrolytic enzymes secreted by microorganisms that cleave specific chemical bonds in the material. For example, esterases and lipases break down ester bonds (R-O-CO-O-R’) in polyester-based polyurethanes [59], leading to a fast degradation rate, especially in humid environments. However, these enzymes are ineffective in breaking down polyether-based polyurethanes [43,44]. Urease and proteases target the carbamate groups (-NH-CO-O-) in polyether-based polyurethanes. However, the hydrolysis rate is slower, and their effectiveness is limited in deeper layers of the material [45]. Some microorganisms enhance degradation efficiency through auxiliary enzymes such as oxidases and peroxidases, which facilitate oxidative chain cleavage [46]. These methods are environmentally friendly and operate under mild reaction conditions. However, the current research indicates that under both environmental and laboratory conditions, the biodegradation rate of polyurethane is relatively low. Similarly, biological methods have not been clearly applied to the degradation and recycling of spandex.

Fuentes-Jaime et al. [60] proposed that polyurethane functional groups in polyurethane films may undergo hydrolysis, causing the carbon backbone to break after cultivation. Similar changes in PU after microbial cultivation have been reported. Some purified esterases, such as polyurethanease A (PueA) and B (PueB) from Pseudomonas aeruginosa and polyurethane esterase PudA from Pseudomonas acidovorans, have been shown to degrade PU, indicating that the presence of specific esterases in microbial communities plays a crucial role in the degradation of PU.

Su et al. [61] used polyurethane powder as the sole carbon source and enriched three microbial communities from landfill leachate, which efficiently degraded polyurethane films within one week. Microbial composition and extracellular enzyme analysis revealed that the community can secrete esterases and ureases, both of which may be involved in the degradation of polyurethane. Tournier et al. [62] reported that three enzymes, UMG-SP-1, UMG-SP-2, and UMG-SP-3, respectively hydrolyze low-molecular-weight diamines formed through chemical glycolysis of polyether-based polyurethane foam. This study provides a new perspective on the enzymatic degradation of polyurethane.

Compared to physical methods, chemical methods, and thermal treatments, biological methods have the advantage of lower energy consumption and do not produce harmful by-products, aligning with the principles of green and sustainable development [63]. Additionally, they can selectively degrade polyurethane without compromising the integrity of other fibers or materials, making them suitable for complex blended waste. However, the drawbacks include slower degradation rates and lower efficiency, particularly when dealing with more robust polyurethane structures. Optimization of enzyme use or microbial cultivation conditions may be necessary, and large-scale industrial application faces significant challenges [64]. Biodegradation technology varies in effectiveness for different types of polyurethane and still presents numerous technical issues in practical operation, such as the need for specific environmental conditions (humidity, temperature, etc.).

Nevertheless, biodegradation remains an environmentally friendly and promising method for solid waste treatment. Further research is needed to explore the biocatalytic degradation mechanisms of spandex, improve the catalytic efficiency and stability of enzymes, and optimize enzyme production costs, durability, and industrial-scale degradation efficiency. The goal is to develop efficient, economical, and sustainable biocircular methods.

3. Recycling and Utilization of Spandex-Blended Fabrics

Compared to spandex waste fibers, spandex-blended fabrics possess more complex physicochemical properties and blended structures [28] (Figure 4), thus facing more significant technical challenges during separation and recycling. The macromolecular structures of spandex and other fibers differ significantly. For example, the main component of cotton fibers is cellulose, which consists of glucose units linked by β-1,4-glycosidic bonds [65]; polyester is formed by the ester bond linkage of terephthalic acid and ethylene glycol [66]; and nylon primarily contains amide bonds [67]. These structural differences not only determine the distinct physicochemical properties of the fibers but also lead to significant differences in their solvent selectivity and reactivity, which presents higher technical demands for efficient separation and recycling. Additionally, the interactions between different fiber components must be carefully considered during recycling to optimize the solvent selection and precise control reaction conditions. Therefore, the subsequent sections will explore the advantages and disadvantages of various recycling methods and fiber blends (Figure 4 and

Table 2. Description of waste spandex-blended fabric recycling methods.

Recycling Methods		Advantages	Disadvantages	Citations
Physical Method	Physical dissolution	Simple, economically viable.	Limited solvent options, many not environmentally friendly.	[22,26,68]
	Mechanical separation	Suitable for initial fiber sorting, simple, eco-friendly.	Low fiber separation efficiency.	[69]
	Compression molding	No complex pretreatment, reduces solvent use, suitable for large-scale recovery.	Low economic value of recovered materials, potential fiber property loss.	[22,70]
Pyrolysis	High-temperature pyrolysis	Can treat mixed waste, products (oil, carbon, gas) widely used.	Complex products, difficult separation and purification, high energy consumption.	[37,38]
Chemical Method	Alcoholysis	Complete spandex decomposition, polyols suitable for new materials.	High technology demand, harsh conditions, expensive catalysts.	[71,72]
	Acidolysis	Effective for complex polyurethane degradation.	Requires high temperature, corrosive chemicals, potential environmental and equipment damage.	[73,74]
	Co-Degradation	Suitable for mixed textiles, improves fiber recovery rate.	Complex, requires optimization for different fibers.	[29]
Comprehensive Method	Multi-Step Processing	Combines physical and chemical methods, enhances recycling efficiency, more product possibilities.	Complex, high technical and equipment requirements, increased costs.	[75,76]

), evaluating each technique’s separation effects, environmental impacts, and market value when applied to spandex-blended fabrics. This will provide important references for future improvements in the recycling technology of blended fabrics.

3.1. Physical Methods

Physical dissolution is a common strategy in the recycling process of spandex-blended fabrics [69]. The core principle is to leverage the differences in the solubility properties of various fiber materials for separation and recovery. By selecting appropriate solvents, spandex or other fiber components can be effectively dissolved, thereby separating the remaining fibers for resource reuse [22]. Additionally, simple physical blending can also be employed to reprocess and utilize waste spandex-blended materials [70].

The selection of solvents is typically based on various solubility parameter theories, including but not limited to the Hansen Solubility Parameters (HSPs) [68], Hildebrand Solubility Parameters (a single cohesive energy density index) [77], and Fedors’ group contribution method [78]. These parameter systems guide the selection and optimization of solvents by quantifying the interactions between the solvent and solute (such as dispersion forces, polarity, hydrogen bonding) [79]. The Hildebrand parameter is generally used for the preliminary screening of nonpolar or weakly polar systems, such as evaluating the compatibility of hydrocarbon solvents with spandex. The Hansen solubility parameters are more precisely used to predict the solubility matching between polar solvents (such as DMF and DMAc) and spandex. The Fedors parameter, calculated using the group contribution method, can quickly predict solvent performance when experimental data are unavailable. Solvents with good spandex solubility include DMF, DMAc, and dimethyl sulfoxide (DMSO) [69].

The solvent-based spandex recycling method is the most commonly used approach. Boschmaier et al. [26] used Hansen Solubility Parameters to assess the interactions between solvents and various fibers (such as spandex, polyester, and polyamide). Through analysis, the authors selected DMSO as the optimal solvent from six solvents (Cyrene, DMAc, DMF, DMSO, GVL (γ-Valerolactone), and NMP (N-Methyl pyrrolidone)). At 120 °C, DMSO effectively separated spandex from polyester/spandex and polyamide/spandex-blended fabrics while maintaining the integrity of the other fibers. The solvent recovery rate was 99%, indicating the method’s potential for economic feasibility and environmental sustainability. Lukas Vonbrul et al. [80] also used Hansen Solubility Parameters to select solvents for dissolving spandex. However, due to the poor performance of single solvents such as DMF, especially when dealing with more tightly woven fiber structures during recovery, they developed an efficient, low-toxicity mixed solvent system. By combining DMSO with other solvents, they developed four solvent mixtures (SB1, SB2, SB3, and SB4) that successfully removed over 90% of spandex from nylon/spandex-blended fabrics, achieving efficient separation from nylon. This system follows green chemistry standards. Currently, extensive research has been conducted on developing green solvents for spandex, yet discussions regarding the subsequent recovery and reuse of spandex remain limited.

Xia et al. [81] used a dissolution method to fully recycle cotton and spandex from fabrics. They employed 1-allyl-3-methylimidazolium chloride (AmimCl) ionic liquid to dissolve cotton into a cellulose solution, which was then used to form an in situ composite with graphene dispersed in AmimCl, resulting in the preparation of uniform and UV-resistant cellulose/graphene films. Meanwhile, the undissolved spandex fibers were dissolved in DMF to create transparent, flexible PU films.

Additionally, it is possible to focus on dissolving other fibers. For example, in the treatment of nylon/polyester/spandex composite fabrics, Lei et al. [82] used a calcium chloride dihydrate/methanol system and an alcohol/alkali combined system to dissolve and separate nylon and polyester, with the remaining spandex being directly recovered for reuse. Under specific conditions, multiple fiber components can also be dissolved simultaneously. For instance, Wang et al. [55] treated cotton/spandex fabrics by utilizing H₂SO₄ aqueous solution, Sodium hydroxide (NaOH)/urea aqueous solution, and Lithium chloride (LiCl)/DMAc solvent systems to successfully convert the waste elastic fibers into regenerated films, which hold potential applications in packaging materials.

Moreover, compression molding technology can also be employed to directly recycle nylon/spandex-blended fabrics to prepare thermal insulation materials. Research by Dissanayake et al. [31] indicates that this process requires neither large amounts of solvents nor complex pretreatment steps, thus reducing the environmental impact. However, the economic value of the recycled material is relatively low.

Although significant progress has been made in developing green solvents for spandex in recent years, research on the recycling and reutilization of spandex materials after dissolution remains insufficient. From the perspectives of solvent purity, efficiency, and cost-effectiveness, methods that prioritize dissolving spandex for recycling have notable advantages regarding separation efficiency and operational simplicity [83]. This is particularly true when spandex typically constitutes a small proportion of the blended fabric, as these methods better preserve the properties of the other fibers, simplify the subsequent recycling processes, and ensure the quality and high-value utilization of the regenerated materials. In contrast, processes involving the dissolution of other fibers or co-dissolution are usually accompanied by more complex procedures and solvent systems [5,84,85]. Although these methods may hold high-value potential in certain specific applications, there is still room for improvement in solvent usage and treatment costs. Therefore, future research should focus on combining the advantages of both recycling methods to establish a more efficient and green spandex-blended fabric recycling system, thereby achieving greater resource utilization and environmental benefits.

3.2. Chemical Methods

When applying chemical methods to treat spandex-blended fabrics, in addition to fully considering the chemical properties of spandex itself, particular attention should be paid to the chemical degradation characteristics of the blended fibers [86]. Due to the highly crosslinked structure of the polyurethane in spandex, chemical degradation is challenging, and the complete chemical degradation-based recycling of spandex-blended fabrics is still fraught with difficulties and challenges at this stage. Existing chemical treatment methods for polyurethane-based composites [73], such as alcoholysis and acid hydrolysis, provide theoretical support and valuable insights for the chemical recycling of spandex-blended fabrics.

There is no fully developed or feasible technological solution for recycling blended fabrics designed to degrade spandex. Previous studies have shown that waste materials from polyurethane and polyester or polyamide mixed systems can be alcoholized under conditions of alcohols and catalysts to obtain a viscosity-homogeneous polyol mixture [71], which can then be used to prepare regenerated polyurethane foam materials. Johansen et al. [72] also successfully degraded various soft and rigid polyurethane materials using tert-pentanol as a solvent (Figure 5), breaking them down into primary-grade polyols and diphenylamine. While these studies provide valuable insights into the degradation and reuse of spandex, they face significant limitations when dealing with complex blended fabrics containing polyester or cotton fibers, potentially leading to the partial depolymerization or damage of other fiber components. Therefore, applying chemical methods focused on spandex degradation in recycling spandex-blended fabrics presents significant technical challenges, which require further exploration and optimization.

Suppose the choice is made to prioritize the chemical degradation of other fibers. In that case, it is necessary to comprehensively consider the complex structures and degradation characteristics of spandex, polyester, cotton, and other fiber components. Since the alcoholysis or hydrolysis of polyester and nylon typically requires higher temperatures and conditions [74], spandex is often already in the degradation range under such conditions. Therefore, choosing a joint chemical degradation treatment for such blended fabrics is more feasible than degrading a single fiber component. For example, Albitres et al. [29] used melt extrusion blending technology to mix nylon/spandex-blended fabrics with waste PET, successfully generating a partially miscible blend through ester–amide exchange reactions, as well as acid hydrolysis, alcoholysis, and ammonia hydrolysis. This material has potential for reuse and aligns with the principles of sustainable development [87].

Theoretically, chemical treatment methods for spandex fibers can enable high-value reuse [88]. However, due to the complexity of the macromolecular structures in blended fibers, achieving the controllable degradation and recombination of individual components in practical applications remains a significant technical challenge [89]. Therefore, future research should focus on reducing the reaction conditions for chemical degradation to adapt to more complex blended fibers while also enhancing the degradation process’s selectivity and efficiency. In addition, the environmental friendliness and economic feasibility of chemical recycling methods [90] are aspects that urgently need improvement in order to achieve more sustainable industrial applications.

3.3. Comprehensive Methods

Due to the complex fiber structure and the significant differences in the physical and chemical properties of different fibers in blended fabrics, a single recycling method is often insufficient for efficiently processing these blended fibers (Figure 4). Therefore, combining multiple recycling technologies into a comprehensive method is an effective strategy for achieving the complete separation and recovery of fibers [75,76]. The comprehensive method typically combines physical and chemical methods, first using physical techniques such as mechanical grinding and filtration to remove easily separable components and then employing chemical dissolution or degradation technologies to further recover the remaining fibers. This multi-step approach enhances the overall efficiency of the recycling process, making it more effective for handling complex blended materials.

Since achieving complete chemical degradation of the blended components in spandex fabrics is challenging, combining physical and chemical methods can help reduce the reaction conditions and simplify recycling. Erha Andini et al. [69] employed a hybrid physical–chemical approach for blended cotton, polyester, nylon, and spandex fabrics. First, they used microwave-assisted glycolysis with ethylene glycol in the presence of a ZnO catalyst. The textile waste was heated to 210 °C with the glycol solution, successfully depolymerizing polyester and spandex into their respective monomers within 15 min. Subsequently, nylon was dissolved using 90% formic acid at room temperature, allowing for the separation from cotton fibers. Finally, a purification process was used to recover the individual fiber components. This method combined physical and chemical techniques to recover all components in the blended fabric completely.

In addition to combining physical and chemical hybrid recycling strategies, some studies achieve direct conversion recycling of spandex-blended fabrics. Yelin Ko et al. [84] proposed an innovative technique that allows waste polyester/spandex-blended textiles to be directly converted into University of Oslo-66 (UiO-66) metal-organic framework (MOF) coatings without separation (Figure 6). This method involves alkaline hydrolysis of the polyester/spandex-blended fabric using NaOH and ethanol, followed by a surface activation technique that enhances the adhesion of the MOF coating to the fabric. This novel chemical conversion pathway not only enables the recycling of the blended fabric but also transforms it into a functional material with superhydrophobic properties [91,92], thus achieving the high-value reuse of discarded spandex fabrics. However, the method does not discuss the potential impact of residual dyes and additives in the blended fabric on the final recycled product [21,93], which could be an important issue that requires further investigation in practical applications.

The integrated method, by combining physical and chemical approaches, allows for the flexible design of optimal recycling paths based on the component characteristics of blended textiles, ensuring the efficient separation and recovery of various fibers. Its advantage lies in the multi-step processing approach, which maximizes each material’s recovery rate and economic benefits. Additionally, recycling systems that combine physical, chemical, and biological treatment technologies can provide more feasible solutions for the recycling and reuse of complex textiles [94], driving the development of recycling technologies toward more environmentally friendly and economically viable directions.

4. Conclusions and Future Outlook

This article reviews the recycling methods of spandex waste and its blended fabrics, analyzing the advantages and disadvantages of existing recycling technologies. The widespread use of spandex in textiles has presented challenges in waste recycling, particularly in recycling blended fabrics, which are more difficult to process. Through a comprehensive evaluation of the recycling technologies for spandex and its blended textiles, the following conclusions are drawn.

The technology for spandex recycling currently faces significant challenges: the high elasticity, thermoplasticity, and chemical stability of spandex make it prone to fiber entanglement or degradation difficulties during the recycling process, especially in complex blended fabrics. These properties increase the difficulty of recycling technologies, particularly when recycling waste with mixed components, as the separation and processing tend to be more complicated.

In practical production, the applicability of different recycling methods varies. When spandex makes up a significant proportion of the textile, or when recycling costs are low, or when only spandex or a single component from a blended fabric needs to be recycled, physical recycling methods are the simplest and most economical choice. These methods are suitable for situations requiring low energy consumption and simple operation, especially in recycling spandex or other fibers. Pyrolysis is suitable for handling waste with complex mixed components, particularly multi-fiber mixtures that are difficult to separate. Pyrolysis not only effectively transforms waste but also provides energy-related by-products for recycling. Therefore, pyrolysis is an effective technology when faced with difficult-to-separate mixed fabrics or when the target recycling products are energy-related by-products. When the recycling target is small molecules or specific monomers, chemical recycling methods offer advantages. In particular, chemical recycling methods such as alcoholysis and acidolysis demonstrate high efficiency and precision when recycling polyols or other chemicals from polyurethane. These methods are ideal for scenarios where high-value-added recycling products are sought. Integrated methods are designed for complex blended fabrics, and when the recycling target is to recover and grade all components of the fabric, combining physical and chemical recycling methods can better achieve fiber separation and recovery. This approach can improve the recycling efficiency and ensure all fiber components’ effective recovery and reuse.

With the increasing demand for environmental protection, innovations in spandex recycling technologies are significant. Future research should focus on developing green and environmentally friendly recycling methods and efficient technologies, especially in low-energy recycling and the application of green solvents. A combined recycling system using both physical and chemical methods will improve the recovery rate of waste spandex-blended fabrics and increase their market application potential.

Spandex’s excellent elasticity and comfort make it highly applicable in smart textiles, wearable devices, and functional apparel. In the future, spandex is expected to integrate with smart sensing technologies, promoting the development of smart clothing and medical textiles. Additionally, through functional treatment, spandex can be used in high-tech fields such as conductivity and thermosensitivity, further expanding its application scenarios.