Closed-loop recycling of colored regenerated cellulose fibers from the dyed cotton textile waste

Worldwide, 45 million tons of waste cotton textiles are produced annually, of which 75% is burned and buried, leading to serious environmental pollution. In this study, a method for directly preparing colored regenerated cellulose fibers (CRCFs) from dyed cotton textile waste (DCTW) was demonstrated. The tensile strength of CRCFs reached 226 MPa, which was equivalent to that of commercial viscose fibers. CRCFs exhibited excellent color fastness and hydrophilicity. In addition, CRCFs can be reprocessed into secondary CRCFs. The tensile strength of secondary CRCFs was 14.64% less than that of the primary CRCFs due to the reduction in the polymerization degree of secondary CRCFs; however, it also can be woven into fabrics. The exploration of the secondary utilization of CRCFs provides an experimental basis for prolonging the service life of DCTW. This approach of preparing CRCFs achieves closed-loop recycling of waste colored cellulose textiles and prevents environmental pollution caused by decoloring and dyeing.


Introduction
According to international reports, more than 150 million tons of waste textiles are generated annually around the world; cotton textile waste accounts for 30% of the total waste (De Silva and Byrne 2017). Theoretically, 95% of these waste textiles can be recycled (Turemen et al. 2019). In fact, the recycling utilization rate is extremely low (Burton 2018). In developed countries such as Germany and Britain, the recovery rate of waste cotton textiles is less than 30% (Wu et al. 2016;Palme et al. 2014), while the recovery rate in China is less than 10% (Wu et al. 2015) (Fig. 1). At present, the main treatment methods of the waste cotton textiles are burning and burying (Weber et al. 2017). However, incineration produces a high amount of toxic gases as well as atmospheric pollution, and burial leads to the release of chemical dyes from waste clothing into the soil, leading to soil pollution and water pollution (Sezgin et al. 2021).
According to statistics, the production of 1 ton of cotton fibers requires 46 GJ energy, 5730 m 3 water, and 2680 m 2 land, which generates 2 tons of carbon dioxide emissions (Burton 2018;Muthu 2014). Besides, the dyeing of 1 ton of textiles leads to the discharge of more than 120 tons of textile coloring wastewater. Textile printing and dyeing wastewater account for 51% of the total wastewater (Kasavan et al. 2021). Textile wastewater exhibits toxicity as well as endangers life and health (Palacios-Mateo et al. 2021). A considerable amount of printing and dyeing mud is primarily dumped into landfills (Ren et al. 2017), leading to major pollution of the soil ecosystem (Ning et al. 2014). With the increase in the world population, the above situation is getting increasingly worse (Chapagain et al. 2007;Zhou et al. 2019). However, the higher the population, the more land is required to grow food; Hence, the amount of land required for cotton is continuously decreasing. Therefore, the efficient utilization of waste cotton textiles and emission reduction of harmful substances have become research hotpots.
Currently, mechanical recovery and chemical recovery are mainly employed for the recycling of waste cotton textiles (Yu et al. 2021;Haule et al. 2016). Mechanical recycling exhibits advantages of cost-effectiveness, facile processing, and mass production. However, currently, most of the products recycled by mechanical recovery enter the secondary market with poor performance and low economic value, including filling materials, sound insulation materials, and heat insulation materials. To increase the added value of products, Fan et al. have produced three-dimensional fiber needle-punched composites from denim waste as furniture materials (Meng et al. 2021;Wang et al. 2022;Fan et al. 2022;Lu et al. 2020). However, the recycling of composites is difficult. In chemical recovery, dyes and finishing agents from clothes often need to be removed. For example, Yousef et al. (2019) have removed dyes and dissolved polymer parts of colored cotton fabrics as well as obtained cotton fibers. Two issues must be solved. First, the waste liquid generated by the fading of dyed cotton textiles pollutes the environment; Second, faded cotton fibers need to be dyed before or after weaving, thereby producing waste liquid again and consuming a large amount of energy. In recent years, some researchers have attempted to directly prepare colored regenerated cellulose fibers (CRCFs) from dyed cotton textile waste (DCTW), which is an effective route to prevent secondary pollution. For example, Liu et al. (2018) have dissolved blue cotton textile waste to prepare blue regenerated cellulose fibers Fig. 1 The global distribution of production and recovery rate of waste textiles in 1 year using lithium hydroxide/urea and sodium hydroxide/ urea systems. Nevertheless, fibers only can be dissolved in an alkali/urea system at ultra-low temperatures, which is challenging to the operation. Haslinger et al. (2019) successfully upcycled dyed pre-and post-consumer cotton waste to new man-made cellulose fibers via dry-jet wet spinning. Ma et al. (2019) dissolved red cotton textile waste in a binary solvent (dimethyl sulfoxide and 1-butyl-3-methylimidazolium acetate) and produced red regenerated cellulose fibers via wet spinning. However, the mechanism of fiber structure change, the difference in the physical and chemical properties of fibers before and after the recycling, and the possibility of recycling times of CRCFs need to be further explored. Hence, it is imperative to recycle DCTW with zero pollution as well as a high utilization rate.
In this study, a permanent recycling route for the direct preparation of CRCFs from DCTW by a wet spinning process was proposed. First, DCTW was dissolved in a binary solvent of 1-butyl-3-methylimidazolium chloride and dimethyl sulfoxide, and dissolved solutions were directly spun into CRCFs under different spinning process parameters. Second, CRCFs prepared in the first time were recycled and reprocessed into the secondary CRCFs under the same optimal spinning process parameters as those used in the first time. Third, physicochemical properties of the CRCFs prepared in both the first and the second time were carefully analyzed and compared. In addition, the mechanism of the difference in properties between the first and second regenerated fibers was revealed.

Materials
Waste cotton textiles used in the experiments are textile scraps free from the Guangzhou Xinyang Textile Co., Ltd, China. The ionic liquid 1-butyl-3-methylimidazolium chloride (IL) was purchased from Aladdin (Shanghai, China). Dimethyl sulfoxide (DMSO) and sulfuric acid were purchased from Xi'an Ruili Jie Experimental Instrument Co., Ltd. Copper (II)ethylenediamine complex was commercially available from Shanghai Taixuan Industrial Co., Ltd. (Shanghai, China). All materials were not processed before use.

Pretreatment and dissolution
Waste cotton textiles were classified according to color. The metal, zippers, and other sundries were removed (Fig. 2a). Then, the waste cotton textiles were cut into fiber powders (< 2 mm), as shown in Fig. 2b, followed by being immersed in 0.5% dilute sulfuric acid solution with a total content of 2 wt% in a 75 °C water bath at different treatment times of 1, 1.5, and 2 h. Next, the fiber powders were cleaned with deionized water until the pH was neutral. After pretreatment, the fiber powders were dried in a 60 °C oven. Regenerated cellulose solutions (2 wt%) (Fig. 2c) were obtained by the dissolution of fiber powders in a mixture of IL and DMSO at ratios of 1:1, 1:1.5, 1:2, and 1:2.5 at 90 °C under different times. The wet spinning device is comprised of an injection pump with a spinneret, a coagulation device, a draw slot, and a rotating collector (Fig. 2d). Spinning solutions were extruded through the spinneret with a diameter of 0.7 mm into an aqueous coagulation bath at a drawing rate of 40 mm/s. The fibers passed through a 140-cm-long coagulation bath and then entered 60 °C hot-water draw slots at drawing ratio from 1.0 to 1.40 times. Fibers were wound up and washed using deionized water to remove solvents from the dissolution procedure. CRCFs were dried at normal temperatures for more than 24 h prior to characterizations or weaving. In all experiments, waste red cotton textiles were used as an example. CRCFs with different colors (Fig. 2e) were prepared according to the optimal process utilized for the regenerated red cellulose fibers.
Preparation and recycling of regenerated fabric Regenerated fabric from red regenerated cellulose fibers with optimal parameters was prepared using a weaving loom. Regenerated fabric was used for color fastness and hydrophilicity tests. Then, the regenerated fabric was cut into fiber powders without any treatment. The second regenerated fibers were prepared according to the optimum process of the first CRCFs prepared from waste cotton textiles at an IL:DMSO ratio of 1:2 and a draw ratio of 1.35.

Rheological test and optical imaging of spinning solutions
A rotational rheometer (Anton Paar MCR 302) was employed for measuring the viscosity of solutions with different proportions of IL and DMSO at shear rates from 1 to 1000 s −1 at 1 Hz. Rheological measurements were performed on a stress-controlled rotational rheometer, which was equipped with a plateplate geometry (diameter Ф of 25 mm with a gap of 1 mm). The dissolution process of waste cotton fibers was photographed using a light microscope (UB 100i).

Measurement of the polymerization degree of waste cotton and CRCFs
The polymerization degree of all fibers was determined by the copper-ethylenediamine method (ASTM D1795-13). The polymerization degree of fibers was calculated using the intrinsic viscosity obtained by using the Ubbelohde viscometer with an internal diameter of 0.8 mm. The intrinsic viscosity is calculated by the average of five test results according to the following Eq. (1) where [η] is the intrinsic viscosity; t 1 and t 0 are the outflow time of the copper ethylenediamine cotton solution and 0.5 M copper ethylenediamine solution, respectively; c, the actual concentration of the copper ethylenediamine cotton solution, which is 0.1 g/100 mL.
Finally, the polymerization degree of all fibers was obtained by Eq. (2) (De Silva and Byrne 2017).

Mechanical performance test
The tensile test of CRCFs was completed on a single-fiber electronic tensile strength tester with a preload of 0.2 cN. The effective length of the measured fibers was 20 mm, and the tensile speed was 10 mm/min. An average of 10 valid tests was conducted for each sample. The tensile strength of the fibers was calculated by dividing the load by the fiber cross-section area. A bundle of fibers were frozen in liquid nitrogen before being cut off. The cross-section of the regenerated fibers was measured with an area measuring tool of a light microscope (VHX-500).

Morphology observation
The micromorphology of CRCFs prepared using optimal parameters was characterized by scanning electron microscopy (SEM). For cross-sectional imaging, the fibers were frozen in liquid nitrogen before being cut off. The surface roughness of fibers (3 × 3 μm 2 ) was observed by atomic force microscopy (AFM). The root-mean-square roughness (R q ) was estimated by the device software.

Chemical analysis
The chemical characteristics of CRCFs were analyzed by fourier transform infrared (FTIR) spectroscopy. The surface chemical structure of CRCFs was determined by X-ray photoelectron spectroscopy (XPS) using monochromatic radiation. The element content of CRCFs were obtained in the mapping mode of Energy Dispersive Spectrometer (EDS). X-ray diffraction (XRD, Dmax-Rapid II, Rigaku Industrial Corporation, Japan) patterns were recorded in the 2θ range from 0° to 100°. CRCFs do not require any treatment before chemical analysis. The crystallinity index of the fibers was calculated according to the following equation (Li et al. 2021).
where Xc is crystallinity. I c and I a are the sum of integral areas of crystalline peaks and amorphous peaks, respectively.

Thermal gravimetric analysis
A thermogravimetric analyzer (Mettler TGA/ SDTA851e) was utilized to investigate the thermal stability of waste cotton textiles and CRCFs. The tests were conducted at a heating rate of 10 °C/min from room temperature to 600 °C under nitrogen. In each test, ~ 5 mg of fibers were loaded.

Color fastness test
CRCFs were woven into a plain weave fabric with a size of 40 × 100 mm 2 . The plain weave fabric obtained was attached to a standard multifiber adjacent fabric, stitching and testing the washing fastness, according to ISO 105-C10:2006(E). The fabric with a size of 50 × 140 mm 2 was prepared to investigate the rubbing color fastness of fabrics, according to ISO 105 X12:2002.

Wettability test
OCA40 Micro contact angle tester was utilized to measure the water contact angles of waste cotton textile and regenerated fabric. The initial water contact angle and the water contact angle 6 min later were measured at room temperature with a humidity of 60%.

Chromaticity index test
The fabric chromaticity index was determined by X-Rite Colori7 spectrophotometer with D65 light source. The sample was folded into 4 layers for testing. Each sample was tested three times.

Effect of pretreatment time on fiber dissolution
Acid or alkali treatment is typically employed to decrease the polymerization degree of cotton (Ma et al. 2019). However, alkali treatment leads to discoloration because the strong covalent bonds between groups from the dye and cellulose groups are broken due to a long pretreatment time (Wedin et al. 2018). Therefore, acid treatment is employed to decrease the degree of polymerization of waste cotton fibers. Figure 3a shows the different dissolution of waste cotton fibers at pretreatment time of 1 h, 1.5 h, and 2 h in the solvent (IL:DMSO = 1:2). Different swelling time reflects the influence of polymerization degree on the dissolution of waste cotton fibers. Fibers swelled rapidly in the solvent in the initial stage and slowed down in the latter stage. At 1 h pretreatment, the swelling time of fibers was long. The fibers did not decompose after 10 h of dissolution. With the increase in the pretreatment time to 1.5 h, the fibers were completely dissolved after 10 h of dissolution. When the pretreatment time was 2 h, the fibers were dissolved completely within 6 h. With the increase in the treatment time, the swelling time of fibers became shorter, and the fibers started to disintegrate rapidly. Figure 3b shows the polymerization degree of the fibers after acid treatment. The longer the pretreatment time, the lower the polymerization degree, mainly resulting from the change in the molecular chain of the fibers. With the further decrease in the polymerization degree, the tensile strength of CRCFs became worse (De Silva and Byrne 2017). Hence, 2 h pretreatment time and 6 h dissolution time are selected for the dissolution of DCTW because under this treatment condition, the dissolution time of regenerated fibers is considerably shortened without causing an excess decrease in the degree of polymerization of the fibers.

Viscosity effect of the spinning solution on tensile strength of CRCFs
The rheology of spinning solutions is of significance, which affects the formation and properties of CRCFs (Olsson 2013). The fluidity of regenerated cellulose spinning solutions was affected by the ratio of the binary solvent. With the increase in the shear rates, the spinning solutions exhibited characteristic shear thinning performance (Fig. 4a). The shear thinning of cellulose solutions is caused by the arrangement of the cellulose macromolecular chains near the shear field, and with the increase in the shear rate, the entangled macromolecular chains were disentangled (Zhen et al. 2011). With the increase in the DMSO content, the viscosity of the spinning solutions was lower, and the fluidity became better. Anions are key to dissolving cellulose as they aid in breaking the hydrogen bonds between and within the cellulose  (Lei et al. 2018). Figure 4b shows the stress-strain curves of CRCFs at different solvent ratios. With the increase in the DMSO concentration, the tensile strength of CRCFs increased first. The tensile strength of CRCFs increased from 111 MPa at an IL:DMSO ratio of 1:1 to 179 MPa at an IL:DMSO ratio of 1:2. Then, at an IL:DMSO ratio of 1:2.5, the tensile strength decreased. Therefore, the IL:DMSO ratio is selected as 1:2 in subsequent experiments.

Effect of draw ratio on tensile strength of CRCFs
The tensile strength and elongation of CRCFs at different draw ratios were obtained. With the increase in the draw ratio (< 1.35), the tensile strength of CRCFs increased (Fig. 4c). This result is related to the fact that the increase in the draw ratio is conducive to the orientation of cellulose macromolecules and formation of dense structures. Drafting increases the crystallinity of CRCFs. The crystallinity of CRCFs is 20.18% when the draft ratio is 1.0. After 1.35 times of drafting, the crystallinity of the regenerated fiber increases to 24.34% (Fig. 4d). However, with the increase in the draw ratio to 1.4, the tensile strength of CRCFs decreased, and the fibers were liable to break during spinning. The tensile strength of CRCFs reached 226 MPa at a draw ratio of 1.35 times, which is equivalent to that of commercial viscose fibers (Mendes et al. 2021). This result indicates that the strength of CRCFs meets the weaving requirement.

Morphology analysis of CRCFs
The SEM images in Fig. 5a-c revealed the morphology of CRCFs at a draw ratio of 1.35. CRCFs exhibited nearly circular cross sections, and a surface structure with grooves, which is a common structural feature by wet spinning. In addition, holes or defects were not observed in the cross section of CRCFs in Fig. 5b, indicating that the internal structures are uniform and dense. In contrast, natural cotton fibers exhibited a rough surface, and convolutions were observed along the fiber length (Fig. 5d). The roughness decreased from 30.8 nm (Fig. 5f) for the natural cotton fibers to 17.3 nm (Fig. 5e) for CRCFs, revealing that the surface of CRCFs was smoother than that of the natural cotton fibers.
Effects of dyes on the chemical structure and properties of CRCFs

Spectroscopic analysis
FTIR spectroscopy was employed to examine the chemical structures of waste colored cotton fibers and CRCFs. Both the red and the white waste cotton fibers exhibited the same characteristic peaks (Fig. 6a). Five main functional groups were observed at 3336 cm −1 , 2918 cm −1 , 1164 cm −1 , 1109 cm −1 , and 1032 cm −1 , corresponding to intramolecular hydrogen-bond stretching, CH 2 asymmetrical stretching, anti-symmetrical C-O-C stretching, anti-symmetric in-plane stretching, and C-O bond stretching (Dassanayake et al. 2021), respectively. Compared with that observed from the colored waste cotton fibers, Fig. 6 a FTIR patterns, b XRD patterns. c TGA, and d DTG of waste cotton as well as CRCFs. e Element content of CRCFs. f Stressstrain diagram of CRCFs new peaks were not observed from both red and white regenerated cellulose fibers, indicating that most of the solvent is removed during coagulation and solvent exchange. However, the peaks of regenerated cellulose fibers at 1109 cm −1 and 1054 cm −1 disappeared, and the peak at 3336 cm −1 became wider, suggesting that the cellulose I crystalline structure is transformed to cellulose II during spinning.

XRD analysis
XRD characteristics of waste cotton and CRCFs were further analyzed for the purpose of investigating the crystalline structure. Five crystalline peaks are identified in waste cotton and they are the most observed crystalline peaks for cellulose I as shown in Fig. 6b. The diffraction peaks of CRCFs is obviously different from that of waste cotton. Two crystalline peaks (20.6°and 34.6° 2θ) are observed in CRCFs, which is consistent with the experimental results of French (2014French ( , 2020 and Yao et al. (2020), and. It is hypothesized that short range order within a glucose unit and between adjacent units survives regenerating and generates the characteristic amorphous XRD profiles. The XRD results show the effects of the regeneration process on crystalline structure of cellulose. The crystalline structure revealed a transformation from cellulose I to cellulose II during the recycling process because the original crystalline structure of cellulose and the glycoside linkages between the glucose units were partially destroyed or modified, leading to the change of crystalline structure caused by the interaction between the cellulose molecule and solvent during dissolution and spinning. This result confirmed FTIR test results. The diffraction peaks of red and white regenerated cellulose fibers exhibited the same trend.

Thermal analysis
TGA was employed to estimate the thermal performance of waste cotton and CRCFs. Figure 6c-d shows TGA and DTG curves of CRCFs and waste cotton with different colors. The weight of waste cotton and CRCFs decreased slightly between 50 and 100 °C due to the loss of retained water and decreased sharply between 300 and 400 °C due to thermal decomposition of cellulose. The thermal behavior of waste cotton was different from that of CRCFs. The initial decomposition temperature (Ti) of CRCFs was decreased in comparison with that of waste cotton. Compared with waste cotton, the weight loss of CRCFs reduced due to the low crystallinity of CRCFs (Liu et al. 2018). The variance in the weight loss among the different colors of waste cotton might be caused by impurities that are not removed. Table 1 lists the thermal decomposition data. The highest decomposition temperature (Tmax) shown on the DTG curve also revealed the same trend.

Element contents and tensile strength analysis
There are no Cl, N and Na elements in the white regenerated fibers (Fig. 6e), indicating that the solvent has been completely removed after the regenerated fibers have been soaked in deionized water for a long time. Therefore, the solvent in the red regenerated fibers prepared under the same process can be completely removed. However, red regenerated fibers contained traces of Cl, Na, and N, which are components of reactive dyes, because reactive dyes are most used in natural fibers (Chen et al. 2015). The red regenerated cellulose fibers exhibited a stress-strain curve like that of the white regenerated cellulose fibers (Fig. 6f), and the average tensile strength of red and white regenerated cellulose fibers were 224.8 MPa and 226.3 MPa, respectively. This result indicated that the dyeing of the waste cotton fibers does not affect the tensile strength of CRCFs. Hence, the dyed waste cotton without decolorization can be used directly as raw materials to produce colored cellulose fibers. This method can prevent the environmental pollution caused by discoloration and dyeing, achieving the recycling of dyes and cellulose fibers.

Analysis of the properties of regenerated cellulose fabric
Color fastness of the regenerated cellulose fabric Figure 7a-c shows an optical microscopy image of regenerated cellulose fabric woven from recycled fibers before and after testing of washing and rubbing color fastness. Fading was not observed after the fabric was washed in Fig. 7b and the regenerated cellulose fabric exhibited an excellent color fastness score of 5. After severe friction, the surface morphology of the fabric was damaged, but color change did not occur (Fig. 7c). The excellent color properties of the regenerated fabric can considerably reduce the high pressure on the environment caused by the printing and dyeing of textiles.

Hydrophilicity of regenerated cellulose fabric
Figure 7d-e show the hydrophilicity test of regenerated cellulose fabric and waste cotton textiles. The initial water contact angles of the waste cotton fabric and regenerated cellulose fabric were 105° and 80°, respectively. The remarkable difference in the initial contact angle revealed that the regenerated cellulose fabric exhibits better hydrophilicity. The water contact angle of the waste cotton fabric was considerably greater than that of regenerated cellulose fabric after 6 min. Cross-links between the active groups were formed in the crystalline region and high-order region of the regenerated cellulose molecule, so it was difficult for the water molecule to penetrate the crystalline region but only passed through the amorphous region. The hygroscopicity of regenerated cellulose fabric was improved. The more the number of amorphous areas, the easier the adsorption of water molecules, which was consistent with the lower crystallinity of CRCFs than that of the waste cotton fibers.

Comparative analysis of the first and second regenerated fibers
The tensile strength of the second regenerated fibers decreased by 14.64% in comparison with that of the first regenerated fibers (Fig. 8a), which is caused by the reduction in the degree of polymerization of CRCFs during second regeneration. As shown in Fig. 8b, the polymerization degree of the second regenerated fibers decreased by 18.01% in comparison with that of the first regenerated fibers. From aspect of chemical structure, glycoside bonds of cellulose break and form new groups during chemical recovery. Chemical analysis of regenerated fibers was carried out by XPS. The carbon atoms in cellulose can be divided into three binding forms, which were indicated by the three corresponding energy spectrum peaks in XPS spectra (Fig. 8c). The chemical bond types of cellulose macromolecules of different binding forms of carbon remain unchanged, but their relative contents vary considerably as shown in Table 2. Compared with first CRCFs, the relative content of C-O exhibited a decrease tendency in second CRCFs (from 39.77% to 21.23%). The relative content of C=O exhibited an increase trend (from 27.92% to 43.74%). O 1s XPS spectra and test data on the CRCF surface represented the bonds of oxygen atoms in cellulose macromolecules (Fig. 8d). The O 1s XPS spectrum can be fitted into two lesser peaks, corresponding to the C-OH-O and C-OH peaks. C-OH Fig. 7 Light microscope images of a regenerated fabric, b regenerated fabric after washing, c regenerated fabric after rubbing; Water contact angle between d waste cotton and e regenerated fabric represents the number of free hydroxyl oxygen atoms in cellulose. The number of free hydroxyl groups in the second CRCFs increased. In contrast, owing to the electron attraction of the oxygen atom to the hydrogen bond, the corresponding the C-OH-O peak in second CRCFs was slightly reduced and the relative content of C-OH-O decreased from 62.32 to 42.24% compared with first CRCFs (Table 3). The binding energy of the bonds closely related to hydrogen bonds (C-OH-O) shows a decreasing trend, because the regeneration process interferes with the binding state of bonds, effectively reducing the overall chemical bond energy of cellulose macromolecules. This is consistent with the previous results of reduced degree of polymerization of secondary regenerated fibers. Figure 9a-c show the color of waste cotton fabric and regenerated fabric by spectrophotometer. The color of waste cotton is different from the first regenerated fabric, but the color of the second regenerated fabric is similar to the first regenerated fabric. Figure 9d shows the chromaticity index of fibers. The color parameters of waste cotton fabric and the first  regenerated fabric were different. During the pretreatment of cotton, the dilute sulfuric acid solution created an acidic environment, which caused the breaking of the covalent bonds between cellulose and dyes and subsequently affected the ultimate chromaticity index of regenerated fabric. The second regenerated fabric exhibited the nearly same color parameters as the first regenerated fabric, indicative of excellent color retention in regeneration and potential of the permanent use of dyes in the textile cycle.

Conclusions
In this study, CRCFs can be directly prepared using DCTW by wet spinning. The CRCFs prepared at the first time mostly retain the color of DCTW, and the tensile strength of CRCFs was not affected by dyes and reached 226 MPa under the optimum spinning parameters, which was equivalent to that of commercial viscose fibers. Besides, CRCFs exhibited better hydrophilicity than that of DCTW because the fibers were transformed from cellulose I to cellulose II during dissolution and spinning. CRCFs without any treatment could be reprocessed directly into secondary CRCFs under the same optimal spinning parameters that were used at the first time. The color of CRCFs did not exhibit any difference before and after re-spinning, and the color fastness of CRCFs was excellent and reached the highest level 5. The tensile strength of secondary CRCFs was 14.64% less than that of primary CRCFs due to the reduction in the polymerization degree of secondary CRCFs during regeneration, but it also could be woven into fabrics on the weaving loom.
This new strategy of using DCTW to directly prepare CRCFs not only prevents environmental pollution caused by decolorization and dyeing but also cuts recovery cost. Moreover, the secondary recycling of CRCFs has demonstrated potential for multiple reuses, which will prolong the service life of DCTW and reduce human demand for the land to grow cotton. Hence, this study demonstrates immense significance for reducing global environmental pollution, land resource crisis, and wastewater problem via the close-loop recycling of DCTW.
Author contributions HL: conceptualization, methodology, data curation, formal analysis, investigation, writing-original draft. WF: conceptualization, project administration, resources, visualization, conceptualization, validation, writing-review and editing. YM: methodology, data curation, formal analysis, investigation. HD: visualization, conceptualization, validation. YS: validation. SW: visualization, validation. XZ: formal analysis. LH: validation. XY: conceptualization. SSL: formal analysis. SG: data curation, investigation.  Fig. 9 The color shown by a spectrophotometer of a waste cotton fabric, b the first regenerated fabric, and c the second regenerated fabric, d Chromaticity index of waste cotton and regenerated fabric (L* corresponds to brightness; a* is the redgreen coordinate; b* is the yellow-blue coordinate; C* is color saturation; h° is the hue angle)