Efficient separation of polyester/cotton blends using a deep eutectic solvent–NaOH hybrid system for textile recycling

Abstract

To overcome the key challenges in polyester (PET)/cotton blend separation—such as difficulty in separation, high energy use, and performance degradation—a green, innovative and efficient choline chloride/ethylene glycol deep eutectic solvent–sodium hydroxide (DES–NaOH) hybrid system was developed. Benefiting from the selective structural loosening of PET fibers by DES, confirmed by Raman imaging technology, which significantly accelerates degradation under alkaline conditions, this work achieves complete degradation and recycling of PET fibers under mild reaction conditions (dissolution temperature of 98 °C, duration time of 60 min, NaOH dosage of 5% w/v, and liquor ratio of 1 : 30). Importantly, high-purity terephthalic acid was recovered through acidification, precipitation, and filtration, while cotton fibers maintained structural integrity and properties with minimal mass loss (<3%), confirming the valuable, effective and selective separation process. This work offers an innovative, environmentally friendly, and cost-effective solution to the challenge of PET/cotton blend waste recycling.

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Green foundation

1. This work developed a mild and recyclable DES–NaOH system for efficient PET/cotton separation, reducing energy and chemical usage and achieving selective degradation of PET fibers with a TPA yield of 98.95%, while preserving cotton fibers (<3% mass loss) and promoting green and sustainable processes for textile waste recycling.

2. We developed a recyclable DES–NaOH system enabling efficient PET/cotton separation under mild conditions (98 °C, 1 h), reducing energy and chemical use. 100% PET is recovered as terephthalic acid and cotton fibers remain intact. The reusable system minimizes waste and emissions and is further enhanced by its excellent water recycling capability, offering a practical green solution for textile recycling.

3. Future research will focus on designing and optimizing diverse DES–NaOH hybrid systems to enhance the separation efficiency of PET/cotton blends. Additionally, the research could explore separation processes of PET mixtures/cotton blends and enhance TPA valorization and expand applications.

Introduction

Global textile waste has escalated exponentially in recent years, propelled by burgeoning population growth and living standards. As the largest textile consumer worldwide,¹ China processes above 50 million metric tons of textile fibers annually, yielding over 20 million tons of post-consumer waste. Alarmingly, recycling rates remain critically low at a mere 15%, with the vast majority of waste relegated to landfills or incineration practices that lead to substantial resource depletion and severe environmental pollution.^2,3 However, textile waste harbors immense potential for sustainable recycling.⁴ Life cycle assessments have demonstrated that recycling merely 1 kilogram of textile waste can curtail CO₂ emissions by 3.6 kilograms, preserve approximately 6000 liters of water, and reduce pesticide consumption by 0.2 kilograms.⁵ These metrics vividly illustrate the profound environmental benefits inherent in textile recycling.

Polyester/cotton blended textiles represent a substantial proportion of the global textile market. Polyethylene terephthalate (PET), the predominant petroleum-derived polyester polymer, necessitates approximately seven tons of crude oil to produce a single ton of virgin fiber and is characterized by an exceptionally protracted natural degradation timeline.^6,7 Conventional disposal methods, including landfilling and incineration, inadequately mitigate the enduring environmental persistence of PET and concomitantly engender significant ecological detriment.⁸ In contrast, cotton fibers present a naturally derived, cellulose-based alternative distinguished by inherent biodegradability and remarkable versatility. Beyond their key role in textiles, recycled cotton fibers show great potential as feedstocks for value-added industrial applications, including microcrystalline cellulose production, cellulose xanthate synthesis, carboxymethyl cellulose preparation, bioethanol generation, and carbon fiber fabrication.^9,10

The efficient recycling of polyester/cotton (PET/cotton) blended textile waste faces major challenges in developing technologies that are both environmentally friendly and capable of efficient separation, while maximizing the high-value reuse of the recovered components.^11,12 Current strategies for separating PET/cotton blends primarily involve selective reaction of one fiber type with specific chemicals to degrade it into small molecules for removal and recovery,^13,14 including methods such as alcoholysis,¹⁵ acid hydrolysis,^2,16 alkaline hydrolysis,¹⁷ enzymatic treatment,^18,19 hydrothermal processes,^20,21 and ionic liquid dissolution techniques.^22,23 However, these methods commonly suffer from technical challenges such as harsh reaction conditions, high processing temperatures, and lengthy reaction times. For example, alcoholysis requires high temperatures (190–240 °C) and is limited by catalyst deactivation;²⁴ acid hydrolysis poses corrosion risks and lengthy reaction cycles (>24 h);² ethanol/alkali systems enable degradation of the polyester component but are costly and compromise cotton fiber quality;²⁵ enzymatic methods are restricted by low yield (<30%) and strict pH/temperature requirements;²⁶ hydrothermal processing necessitates extreme operating conditions and yields complex byproducts.^27,28 Although ionic liquid technology has been explored and applied on an industrial scale in the field of textile recycling, there is still room for optimization in terms of processing temperature, solvent recovery rate, and cost control for large-scale applications.^23,29 Deep eutectic solvents (DESs) have emerged as promising eco-friendly alternatives to organic solvents and ionic liquids, enabling sustainable chemical processes.^30–34 Recently, our research team has developed a novel hybrid system of DES and sodium hydroxide, which achieves efficient separation and recycling of waste PET fiber textiles under mild conditions,³⁵ addressing key limitations of mainstream methods in reaction temperature, processing time, and separation efficiency. The hybrid system is recyclable and adheres to principles of green sustainability, offering an innovative, environmentally friendly, and cost-effective solution for the green and high-value recycling of PET/cotton blended textiles.

In this work, we innovatively employed a choline chloride/ethylene glycol deep eutectic solvent (DES) combined with sodium hydroxide (DES–NaOH) for the efficient separation and recovery of polyester/cotton (PET/cotton) blended textiles. Key process parameters—including dissolution temperature, treatment time, alkali dosage, and bath ratio—were systematically investigated. Under optimized conditions, polyester fiber components were efficiently dissolved and recovered under mild conditions, while the structure and properties of cotton fibers were well preserved. The mechanism of selective degradation was further explored, and specificity was verified using pure cotton fabrics. This method features low carbon emissions, low energy consumption, and high efficiency, offering new insights and a technical foundation for sustainable separation and resource utilization of polyester–cotton blends.

Results and discussion

Construction of an efficient separation strategy for PET/cotton blends

As illustrated in Fig. 1a, over 20 million tons of waste textiles are generated annually in China, with polyester (PET)/cotton blends constituting more than 50% of this waste stream. The conventional disposal methods for such PET/cotton blend fabrics typically involve landfilling and incineration, posing significant environmental and economic challenges. In contrast, an advanced open-loop recycling process enables the highly efficient separation of PET and cotton, facilitating the intact recovery of cotton fiber while depolymerizing PET into terephthalic acid (TPA). Given its versatile utility, TPA is a critical feedstock in diverse fields, including textile dyeing, medical, coating, electrical, agricultural, synthetic rubber etc. This work introduces a novel choline chloride/ethylene glycol deep eutectic solvent (DES)–NaOH hybrid system for the efficient separation of PET/cotton blends (Fig. 1b and c), which achieves selective and effective PET dissolution into TPA while preserving the structural integrity of cotton fiber. The environmental friendliness of this process primarily stems from the compliance of both the raw materials (i.e., DES and NaOH) and the process flow of the DES–NaOH hybrid system with the development requirements of green chemistry. Furthermore, the hybrid system exhibits excellent recyclability and reusability. Experimental results showed that the recycled DES–NaOH system achieved complete degradation of PET fabric in each cycle under conditions of 98 °C for 50 min, with no significant decline in system performance even after five consecutive cycles (Fig. S2). No residual PET fibers were detected in the degradation solution after five cycles, confirming the sustained high degradation capability of the system after being reused five times (Fig. S3). Moreover, the system maintains high degradation efficiency under scale-up conditions, demonstrating its potential for subsequent industrial applications (Fig. S4). Notably, the process consumes approximately 200 ml of water per gram of processed PET/cotton blended fabric, and this volume can be further minimized through the implementation of water recycling systems, substantially diminishing the environmental footprint and wastewater treatment costs. Consequently, the DES–NaOH process achieves enhanced economic viability, operational efficiency, and sustainability. In addition, compared to alternative processing methods such as alkaline hydrolysis,³⁸ DES treatment,³⁴ ethanolamine treatment,¹⁵ hydrothermal processing,³⁹ ethanol-alkaline aqueous processing,²⁵ and ionic liquid degradation,⁴⁰ the proposed strategy demonstrates super advantages including complete PET degradation (100% conversion efficiency), production of high-purity and high-value TPA, selective and efficient separation, and nondestructive recycling of cotton fiber. Notably, the DES–NaOH hybrid system demonstrates several advantages over previously reported work, including a higher recycling rate, shorter recycling time and lower operating temperature (Fig. 1c and Table S1).

Fig. 1 (a) Comparison of the traditional and recyclable disposal of PET/cotton blend textile waste; (b) an efficient and sustainable strategy for separating PET/cotton blends via the DES–NaOH hybrid system proposed in this work; (c) main parameters including recycling rate, recycling time, and temperature between DES–NaOH treatment and other previously reported work.

We conducted systematic experiments using pure PET and cotton fibers with the DES–NaOH system to assess the feasibility of the proposed separation strategy. As shown in Fig. 2a, the dissolution kinetics of PET in the DES–NaOH hybrid system were investigated by treating 1 g of PET samples in 31.5 g of hybrid solvent at 98 °C under magnetic stirring. The time-dependent dissolution profiles demonstrated rapid degradation kinetics, with an efficiency of 87.7% achieved within just 20 min and complete dissolution occurring within 50 min. In contrast, pure DES exhibited limited effectiveness, dissolving only 4.0% of PET after 50 min and plateauing at 12.8% after 300 min, while NaOH alone achieved only 26.8% dissolution under identical conditions. These results highlight the superior efficiency of the DES–NaOH hybrid system for PET separation. Additionally, sequential treatment protocols (DES followed by NaOH or NaOH followed by DES) produced only marginal improvements, achieving dissolution rates of 31.9% and 28.8%, respectively (Fig. S5 and 6). Quantitative gravimetric analysis (Fig. 2b) confirmed the selectivity of the process, with cotton fibers experiencing minimal mass loss (<3%). Furthermore, SEM images (Fig. 2c) illustrated the progressive structural degradation of PET fibers: after 20 minutes, partially dissolved fibers exhibited significant surface erosion characterized by noticeable thinning, roughness, and defect formation; after 40 minutes, only discontinuous fiber fragments with multiple fracture points remained; and after 50 minutes, no residual fibers were detected, confirming complete dissolution.

Fig. 2 (a) Time-dependent dissolution behavior of PET fibers in the DES–NaOH hybrid system, DES system alone, and NaOH system alone, respectively; (b) quantitative gravimetric curve of cotton treated with DES–NaOH for different times; (c) structural evolution of a PET fiber with increasing duration time in the DES–NaOH hybrid system; (d and e) in situ Raman spectroscopy and 2D Raman imaging of the PET and cotton fibers, respectively; (f) micro-structural evolution of PET/cotton blends in the DES–NaOH hybrid system.

To elucidate the selective degradation mechanism of PET/cotton blended fibers by the DES–NaOH system, in situ Raman spectroscopy was employed to monitor structural changes in cotton and PET fibers upon mixing with DES (Fig. 2d and e). PET fibers exhibited progressive decreases in characteristic peaks related to ester bonds (860 cm⁻¹), trans and gauche ethylene glycol (1097 and 1120 cm⁻¹), and benzene rings (1616 and 1729 cm⁻¹) during DES exposure (Fig. 2d₁),⁴¹ reflecting fiber loosening and reduced packing density. The structural disruption facilitates subsequent alkali-induced PET fiber degradation. Two-dimensional Raman imaging comparisons between pristine and 30-minute DES-treated PET fibers (Fig. 2d₂) corroborated the loosening effect induced by the DES. Conversely, cotton showed negligible changes in the characteristic cellulose peaks (e.g., 1096 cm⁻¹ for glycosidic C–O–C stretching and 1462 cm⁻¹ for HCH and HOC bending),⁴² indicating that the DES does not alter cotton's functional groups or packing structure (Fig. 2e₁). Large-area 2D Raman imaging of pristine and 30-minute DES-treated cotton further confirmed the preservation of the fiber structure and chemical integrity (Fig. 2e₂). Based on the above analysis, we propose a synergistic degradation mechanism for the DES–NaOH hybrid system (Fig. 2f). The process starts with DES-induced surface activation, where the DES selectively modifies the PET fiber surface through confined chemical reactions, disrupting the dense surface layer to create porous structures while converting hydrophobic surfaces to hydrophilic surfaces. This activated surface architecture subsequently facilitates rapid NaOH penetration into the amorphous regions of the PET. And then, the concentrated alkaline solution efficiently catalyzes ester bond cleavage in PET macromolecules under remarkably mild conditions, following a “wall-breaking and fast-penetration” accelerated degradation pathway.

Parameter optimization for PET/cotton blend separation

Building upon the selective separation capability of the DES–NaOH hybrid system for PET and cotton, we established a comprehensive parameter optimization framework for PET/cotton blends. The separation efficiency was quantitatively assessed by measuring the fiber dissolution rate while systematically investigating key variables such as dissolution temperature, treatment duration, NaOH dosage, and solid-to-liquid ratio. As shown in Fig. 3a, the dissolution efficiency exhibited pronounced temperature dependence, rising from 26.1% below 70 °C to complete dissolution (100%) at 98 °C, in agreement with previously reported PET degradation kinetics.⁴³ Increasing the temperature to 110 °C did not improve the performance but led to higher energy consumption, establishing 98 °C as the optimal temperature. Fig. 3b presents the time-dependent dissolution profile at 98 °C, showing rapid initial kinetics with 67.06% dissolution achieved within 30 minutes and complete dissolution reached at 60 minutes. Extending the process duration beyond 60 minutes offered no additional benefit but increased operational costs, making 60 minutes the ideal treatment time. Regarding NaOH dosage (Fig. 3c), a critical threshold of 5% (w/v) was identified for achieving complete dissolution; dosages below this level resulted in incomplete reactions, while higher dosages increased reagent costs and complicated post-treatment without improving yield.

Fig. 3 (a)–(d) Effect of dissolution temperature, duration time, NaOH dosage, and solid–liquid ratio on the fiber dissolution rate of PET/cotton blends, respectively; (e) morphological evolution of PET/cotton blends with elevated treatment times.

Similarly, the solid–liquid ratio (Fig. 3d) demonstrated saturation characteristics, reaching optimal performance at 1 : 30. Further increases in the ratio resulted in diminishing returns while simultaneously elevating operational costs. In addition, the micromorphological evolution of PET/cotton blends under prolonged treatment durations is illustrated in Fig. 3e. The images reveal that complete degradation of PET occurs at approximately 50 min, which aligns well with the findings in Fig. 3b. These systematic investigations conclusively established the optimized parameters: temperature of 98 °C, duration of 60 min, NaOH dosage of 5% (w/v), and solid–liquid ratio of 1 : 30, achieving maximum separation efficiency of PET/cotton blends while maintaining operational economy.

Furthermore, additional separation experiments were performed on PET/cotton blended fabrics with varying polyester/cotton ratios, including common blends such as 60/40 and 50/50. The results demonstrate that under identical optimized conditions, efficient separation of the polyester component was achieved across all tested blend ratios, with cotton fiber loss consistently remaining below 3% (Table S2). These findings indicate that the process shows good adaptability and broad universality for diverse PET/cotton fabric compositions.

In addition, the effect of dyeing on process and technical scalability was examined. White PET fabric was dyed with 2% (owf) Disperse Red 92 under standard conditions (130 °C, pH 5, heating rate 2.5 °C min⁻¹, 60 min), followed by reduction, washing, and drying. Both dyed and undyed fabrics underwent complete degradation with 100% efficiency, confirming that dyeing does not inhibit polyester degradation (Fig. S7). UV-vis spectroscopy and HPLC analysis further indicated no adverse effect on TPA recovery (Fig. S8). These results demonstrate the robustness of the process for both white and dyed waste textiles, confirming that dyeing does not impair polyester degradation and TPA purity—supporting its applicability in recycling waste blend textiles.

Dissolution and separation of PET/cotton blends in the DES–NaOH system

As shown in Fig. 4a and b, morphological analysis revealed distinct structural changes in PET/cotton blends. The untreated sample displayed smooth, cylindrical PET fibers alongside naturally twisted cotton fibers with textured surfaces. Following DES–NaOH treatment, microscopic examination confirmed the complete removal of the PET component, while the cotton fibers remain fully intact, preserving their original twisted morphology and surface characteristics. Notably, no detectable structural damage was observed in the retained cotton fibers, demonstrating the effective separation of the blended fibers and the nondestructive recovery of cellulose using the DES–NaOH system. Furthermore, respinning experiments were conducted using the recycled cotton fibers (i.e., the remaining cotton fibers) to assess their reusability. The results indicate that DES–NaOH treatment does not adversely affect the spinnability or yarn quality of the fibers, as confirmed in Fig. S9. These findings indicate that the recycled cotton fibers can be directly used in the production of regenerated textiles, highlighting their strong potential for practical reutilization.

Fig. 4 (a and b) Comparison of morphological examination of polyester/cotton blends before and after treatment with the DES–NaOH hybrid system with optimized processing parameters; (c and d) FTIR and XRD comparison of polyester/cotton blends before and after DES–NaOH treatments.

FTIR spectroscopic analysis (Fig. 4c) provided definitive evidence of chemical changes occurring in the PET/cotton blends during DES–NaOH treatment. The untreated blends exhibited a characteristic C=O stretching vibration peak at 1720 cm⁻¹, attributed to the PET component, which was entirely absent in the post-treatment spectra. Conversely, the treated sample retained only cellulose-specific absorptions: the O–H stretching at 3340 cm⁻¹, C–H stretching at 2900 cm⁻¹, and the C–O–C glycosidic bond vibration at 1050 cm⁻¹—all corresponding precisely to the fingerprint region of pristine cotton. These results confirm the complete removal of PET while maintaining the structural integrity of the recovered cellulose, highlighting the selectivity and efficacy of the DES–NaOH hybrid system.

Additionally, XRD analysis (Fig. 4d) illustrated the structural evolution of PET/cotton blends during DES–NaOH treatment. The untreated sample exhibited characteristic PET diffraction peaks at 17.5° and 25.6°, whereas the treated sample displayed only cellulose I diffraction peaks at 14.9° (1−10), 16.6° (110), 22.5° (200), and 34.3° (004), matching those of reference cotton cellulose.⁴⁴ These findings confirm the complete dissolution of PET and the preservation of the cellulose I crystalline structure.

UV-Vis spectroscopic analysis of the PET fiber dissolution product obtained via the DES–NaOH system revealed a characteristic absorption at 240 nm (Fig. 5a), corresponding to π–π* transitions of aromatic rings or double bonds, consistent with the reported spectrum of terephthalic acid (TPA). The yellow-brown powder derivatives, obtained through acid precipitation and filtration (Fig. 5a), exhibited exceptional purity, reaching 99.85% as determined by HPLC analysis. This was evidenced by a single symmetric peak with a retention time of 7.01 min (Fig. 5b). FTIR spectroscopic analysis (Fig. 5c) further confirmed the identity of the product by showing characteristic peaks at 3063 cm⁻¹ (carboxylic O–H stretch), 1683 cm⁻¹ (C=O stretch), and 1287 cm⁻¹ (C–O of –COOH stretch), all matching the standard spectra of TPA.⁴⁵ Mass spectrometry analysis identified a characteristic peak at m/z 165 (Fig. 5d), corroborating the formation of TPA. Together, these results conclusively demonstrate that the dissolution product is high-purity TPA.

Fig. 5 (a) UV-Vis spectroscopic curve of the recycled TPA from PET fibers. Inset: photograph of TPA; (b–d) HPLC (b), FTIR (c) and mass spectrometry (d) analysis of TPA, respectively.

Stabilization of cotton in the DES–NaOH hybrid system

To further validate the selectivity and specificity of the DES–NaOH hybrid system for polyester degradation, experiments were conducted at two levels: commercially available white pure cotton fabric and red-colored pure cotton yarn were treated under identical optimized process conditions, followed by a comparison of their properties before and after processing.

With respect to white pure cotton fabric, SEM analysis (Fig. 6a) demonstrated excellent structural preservation of the cotton fibers, showing only superficial swelling while retaining their native morphological features. Notably, no chemical degradation or mechanical fragmentation was observed, confirming the mildness of the processing conditions. Moreover, a detailed comparative UV-Vis spectral analysis was performed between pristine and cotton-treated DES–NaOH solutions after 60 min processing. The nearly identical spectral profiles confirm the absence of detectable cellulose dissolution, indicating that the minor mass loss observed was attributable solely to physical fiber detachment (Fig. 6b). The XRD patterns maintained all characteristic cellulose I diffraction peaks at 14.9°, 16.6°, 22.5°, and 34.5° (Fig. 6c), confirming the preservation of the native crystalline structure of the cotton fabric after DES–NaOH treatment.⁴⁶ Thermogravimetric (TG) analysis showed closely matching decomposition profiles between pristine and treated cotton fabric samples, and negligible differences were observed in the onset temperature, the temperature at the maximum decomposition rate, and the terminal degradation temperature, as evidenced by Fig. S10. In addition, tensile testing (Fig. 6d) revealed comparable stress–strain behavior. The breaking force decreased marginally from 346.52 N (untreated) to 324.61 N (treated), indicating a minimal reduction in tensile strength of only 6.3%. Collectively, these results show that the cotton fabric exhibits exceptional stability in the DES–NaOH hybrid system, as evidenced by the preservation of crystallinity, thermal stability, and tensile performance.

Fig. 6 (a) Structural evolution of cotton fibers in the DES–NaOH system with elevated treating times; (b) comparison of UV-Vis spectra between the pristine DES–NaOH system and the cotton-treated DES–NaOH system after 60 min processing; (c) comparison of the XRD patterns between pristine and DES–NaOH-treated cotton fabric; (d) comparison of tensile behavior between pristine and treated cotton fabric; (e) flowchart of red-dyed cotton yarn treatment with the DES–NaOH system; (f and g) digital images of red-dyed cotton yarn before and after DES–NaOH treatment; (h) K/S value curves of red-dyed cotton yarn before and after DES–NaOH treatment.

In addition, the compatibility with red-colored pure cotton yarn was evaluated to assess the effect of dyeing on process scalability. The pure cotton yarn, dyed with C.I. Reactive Red 88, was pre-treated with Na₂CO₃ and soap solution to remove surface dye and was subsequently processed under identical optimized process conditions (Fig. 6e). The treated yarn retained its original appearance and color integrity (Fig. 6f and g), with only minor changes in the K/S value before and after treatment, as measured using a Datacolor colorimeter, indicating a negligible loss in color depth (Fig. 6h). These results confirm that the process is compatible with both white and dyed cotton textiles. Dyeing does not impact the color of the cotton yarn, supporting the potential application of this method in recycling dyed blended textile waste.

Conclusions

In summary, this work developed an innovative and sustainable DES–NaOH hybrid system for the efficient separation and recycling of PET/cotton blends. Leveraging the selective structural loosening of PET fibers induced by DES, combined with mild reaction conditions—specifically, a dissolution temperature of 98 °C, a reaction time of 60 minutes, a NaOH dosage of 5% w/v, and a liquor ratio of 1 : 30—we achieved complete dissolution of the PET component while preserving the structural integrity of cotton fibers, with the mass loss kept below 3%. Raman imaging further confirmed the selective degradation of PET, facilitating accelerated degradation under optimized conditions. Based on these findings, a “wall-breaking + rapid penetration” degradation mechanism was proposed for the PET/cotton blends. Additionally, high-purity and high-value terephthalic acid was successfully recovered through acidification, precipitation, and filtration procedures, highlighting the potential for open-loop recycling of PET. This work presents an eco-friendly and facile approach for the efficient and sustainable separation of PET/cotton blends, simultaneously delivering environmental benefits, energy efficiency, and cost-effectiveness. In the next stage, we will focus on the cost of the process and Life Cycle Assessment (LCA), which is critical for transitioning this technology from the pilot-scale to industrial application.

Experimental

Raw materials

The experimental materials employed in this work consisted of industrially produced woven fabrics including pure PET fabric (100 g m⁻²), pure cotton fabric (150 g m⁻²), and PET/cotton blended fabric (80/20, 100 g m⁻²). All these fabrics underwent thorough pretreatment processes, including desizing, scouring, and bleaching, but were not subjected to subsequent processing such as dyeing or printing. Choline chloride (ChCl) and ethylene glycol (EG) were purchased from Macklin Biochemical Co., Ltd, Shanghai. Sodium hydroxide (NaOH) was purchased from Aladdin Reagent Co., Ltd, Shanghai. Hydrochloric acid (HCl) was purchased from Sinopharm Chemical Reagent Co., Ltd, Shanghai.

Preparation of the DES–NaOH hybrid system

The preparation of the DES–NaOH hybrid system was carried out in two necessary steps. First, the choline chloride/ethylene glycol (ChCl/EG) deep eutectic solvent was synthesized according to previously reported protocols.^30,36,37 Specifically, ChCl and EG were precisely weighed in a 1 : 2 molar ratio and sequentially transferred to an Erlenmeyer flask. The mixture was homogenized using magnetic stirring at 80 °C until complete dissolution, followed by further stirring for 30 min to yield a clear, transparent DES solution. The resulting DES was stored in a sealed container until use. For the preparation of the DES–NaOH hybrid system, 1.5 g of NaOH was gradually introduced into 30 mL of the as-prepared DES in a separate Erlenmeyer flask under continuous magnetic stirring at 90 °C for 25 min, yielding a homogeneous DES–NaOH hybrid system that was subsequently employed in the experiments.

Separation of polyester/cotton fabrics

The fabric samples were cut into squares with dimensions of 0.5 cm × 0.5 cm. Each specimen was precisely weighed using an analytical balance (accuracy: ±0.1 mg) to ensure the reliability and reproducibility of the experimental data. Subsequently, the samples were immersed in the DES–NaOH hybrid system, and the effects of some key process parameters including dissolution temperature, treatment time, alkali dosage, and bath ratio on the dissolution behavior were systematically investigated. Following dissolution, the reaction mixture was filtered to recover the undissolved fiber fraction, which was sequentially washed with deionized water, dried, weighed, and reserved for subsequent analysis. Simultaneously, the filtrate was collected, acidified using 36% hydrochloric acid, and subjected to precipitation. The resulting precipitate was recovered by filtration, rinsed with deionized water, dried, and weighed for further characterization.

To evaluate the dissolution and separation behavior of the DES–NaOH system on PET/cotton blends, parallel treatments were performed using pure DES or NaOH alone on the same fabrics under identical conditions (Fig. S1a and b). This comparative design allowed quantitative analysis of dissolution efficiency and separation selectivity. Sequential treatments with alternating DES and NaOH were also applied to investigate the stepwise dissolution mechanism (Fig. S1d). These comparisons highlighted the enhanced separation efficacy of the DES–NaOH system (Fig. S1c), indicating the synergistic effect of the combined solvent strategy in polyester/cotton recycling.

Characterization

Testing items and the corresponding time points of treated blends, fiber dissolution solution, recycled cotton fibers, and the PET derivative (terephthalic acid, TPA) are summarized in Table S3, and the detailed testing procedures are given as follows:

Fiber dissolution rate

The dissolution and separation efficiency of various treatment systems mentioned in Fig. S1 for fabric samples was quantitatively evaluated based on the fiber dissolution rate, which was calculated using the following equation:where R% represents the fiber dissolution rate; m₀ is the mass of the fiber before dissolution; and m₁ is the mass of recycled fiber after dissolution and separation.

Characterization of Raman spectra and two-dimensional (2D) mapping

The Raman spectra and 2D mapping of the cotton and PET fibers with DES solvent were measured using a Raman spectrometer (Renishaw, inVia, UK) equipped with a 785 nm laser and a 50× objective lens. In time-resolved lattice scanning, 16 spectra were continuously scanned, with an exposure time of 5 seconds and 100% laser power for each spectrum and an interval of 2 minutes between scans. During the Raman imaging test, spectral data were collected within the selected testing area using a spatial spacing of 500 nm, an exposure time of 0.2 seconds, and 100% laser power. The collected spectra were preprocessed using WIRE 5.3 software for cosmic ray removal, noise filtering, and baseline flattening in sequence and then subjected to two-dimensional and three-dimensional analysis.

Characterization of the fiber dissolution solution

UV-Vis spectral analysis of the fiber dissolution solution was performed on a UV-2600 spectrophotometer (Shimadzu, Japan) with 400-fold deionized water-diluted samples and a 200–400 nm scanning range.

Characterization of recycled fibers

The PET/cotton blends were dissolved and separated in the DES–NaOH hybrid system, followed by filtration to recover the undissolved fiber components. In detail, the surface morphologies of the treated PET/cotton fabric were examined using a VHX-970F ultra-depth-of-field 3D microscope (Keyence, Japan) at different processing times. Further morphological analysis was examined using an S-4800 scanning electron microscope (Hitachi, Japan) at 5 kV after gold sputtering. Fourier transform infrared spectroscopy (FTIR) spectra were acquired on a Prestige-21 spectrometer (Shimadzu, Japan) via the KBr pellet method, with 20 cumulative scans at 9 cm⁻¹ and resolution over 4000–400 cm⁻¹. The crystalline structure was analyzed by X-ray diffraction (XRD, SmartLab SE, Rigaku, Japan) using CuKα radiation (λ = 0.145 nm) at 40 kV, scanning from 5° to 60° (2θ) at 2° min⁻¹ with 0.02° s⁻¹. Thermal stability was studied by thermogravimetric analysis (DTG-60H, Shimadzu, Japan) under a nitrogen atmosphere, with a 5 °C min⁻¹ heating rate from 25 °C to 600 °C. Tensile tests were performed according to GB/T 3923.1-2013 using a YG(B)026G tensile tester with specimen dimensions of 200 mm × 50 mm, a gauge length of 150 mm, and a crosshead speed of 50 mm min⁻¹; three replicates were measured for each sample.

Characterization of TPA

The PET derivative—terephthalic acid (TPA)—was comprehensively characterized. High-performance liquid chromatography (HPLC) analysis was conducted using a Waters HClass system, employing 90% acetonitrile (containing 0.1% formic acid) as the mobile phase, a Waters BEH C18 column (1.7 μm, 2.1 × 50 mm), a column temperature of 40 °C, and a flow rate of 0.6 mL min⁻¹. Mass spectrometry analysis was carried out using a Waters G2-XS QTOF instrument in the negative ionization mode (capillary voltage of 3 kV), with parameters including an ion source temperature of 110 °C, a desolvation temperature of 400 °C, and a nitrogen gas flow rate of 800 L h⁻¹. FTIR analysis was performed on the TPA product, with parameters set to 20 scans at 9 cm⁻¹ and resolution over 4000–400 cm⁻¹.

Author contributions

Lele Zhang: methodology, investigation, writing – original draft, data curation; Tengfei Wang: investigation, data curation; Yong Wang: writing – review & editing, funding acquisition, supervision; Dongdong Ye: conceptualization, writing – review & editing, supervision; Zongqian Wang: conceptualization, writing – review & editing, funding acquisition, supervision.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The authors declare that all the data related to this study are available within the paper or can be obtained from the authors on reasonable request.

Supplementary information (SI) includes: the experimental procedures and testing details; a comparison of representative recycling strategies for polyester/cotton blends; an evaluation of solvent recycling; the scale-up recycling experiment; a comparison of PET degradation properties under different solvent conditions; an assessment of different recycling pathways and their effectiveness; and the re-weaving performance of the recovered cotton fibers is available. See DOI: https://doi.org/10.1039/d5gc04525h.

Acknowledgements

This work was financially supported by the National Key R&D Program of China (2024YFB4709304), the Key Research and Development Plan Project of Anhui Province (202423i08050057, 2023t07020001, 2022a0502029), the Forestry Research Project of Guizhou Province (2024-18), the Natural Science Foundation of Anhui Province (2308085ME144), the University Synergy Innovation Program of Anhui Province (GXXT-2023-035, GXXT-2022-027), the Key Research and Development Plan Project of Wuhu (2023yf002), and the Open Project of the Key Laboratory of Textile Fiber and Products (Ministry of Education) (Fzxw2024005).

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