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cotton blended fabrics by 1-allyl-3-methylimidazolium chloride
Carbohydrate Polymers 123 (2015) 424–431
Contents lists available at ScienceDirect
Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Isolation and recovery of cellulose from waste nylon/cotton blended fabrics by 1-allyl-3-methylimidazolium chloride Fangbing Lv a,c , Chaoxia Wang a,∗ , Ping Zhu b,c,∗∗ , Chuanjie Zhang c a b c
Key Laboratory of Eco-Textile, Ministry of Education, School of Textiles and Clothing, Jiangnan University, Wuxi 214122, China State Key Laboratory of New Fiber Materials and Modern Textiles, Qingdao University, Qingdao 266071, China Key Laboratory of Biomass Fiber and Ecological Dyeing of Hubei Province, Wuhan Textile University, Wuhan 430073, China
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Article history: Received 9 May 2014 Received in revised form 12 January 2015 Accepted 15 January 2015 Available online 3 February 2015 Keywords: Cellulose Waste nylon/cotton blended fabrics Separation Recovery Ionic liquid
a b s t r a c t Development of a simple process for separating cellulose and nylon 6 from their blended fabrics is indispensable for recycling of waste mixed fabrics. An efficient procedure of dissolution of the fabrics in an ionic liquid 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) and subsequent filtration separation has been demonstrated. Effects of treatment temperature, time and waste fabrics ratio on the recovery rates were investigated. SEM images showed that the cotton cellulose dissolved in [AMIM]Cl while the nylon 6 fibers remained. The FTIR spectrum of regenerated cellulose (RC) was similar with that of virgin cotton fibers, which verified that no other chemical reaction occurred besides breakage of hydrogen bonds during the processes of dissolution and separation. TGA curves indicated that the regenerated cellulose possessed a reduced thermal stability and was effectively removed from waste nylon/cotton blended fabrics (WNCFs). WNCFs were sufficiently reclaimed with high recovery rate of both regenerated cellulose films and nylon 6 fibers. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nylon/cotton fabrics have attracted much attention due to their special handle and unique appearances (Malshe, Mazloumpour, ElShafei, & Hauser, 2012; Kim, Lee, Choi, & Lee, 2011). These textile products are indispensable in our daily life, but millions of tons of waste fabrics are generated every year (Ouchi, Toida, Kumaresan, Ando, & Kato, 2010). Generally, these waste fabrics are landfilled or incinerated (Miranda, Sosa-Blanco, Bustos-Martinez, & Vasile, 2007). Such disposing methods not only create environmental pollution but also lead to great waste of the valuable resources. If the waste fabrics are in the single component form of e.g. nylon or cotton fibers, different recycling or reusing techniques were developed (Wang, 2006, Chap. 2). During recent years, a number of methods have been pursued for recycling waste cotton fabrics (WCFs) to value-added products, such as preparing cellulose derivatives (Zhang, Zhang, Tian, Zhou, & Lu, 2013; Sun, Lu, Zhang, Tian, & Zhang, 2013) and for energy production. One potential way to high-value utilizing WCFs is extracting microcrystalline cellulose from waste
∗ Corresponding author. Tel.: +86 0510 85912105; fax: +86 0510 85912105. ∗∗ Corresponding author. Tel.: +86 027 87181347; fax: +86 027 87181347. E-mail addresses: [email protected] (C. Wang), [email protected] (P. Zhu). http://dx.doi.org/10.1016/j.carbpol.2015.01.043 0144-8617/© 2015 Elsevier Ltd. All rights reserved.
cotton fabrics (Xiong, Zhang, Tian, Zhou, & Lu, 2012) and to use them as reinforcement for composite materials due to its unique chemical and physical properties (Foulk, Chao, Akin, Dodd, & Layton, 2006). Commonly most waste textiles are blended fabrics. Attempts have been made to separate nylon blended fabrics by degradation or solubilization. The solvents used include aliphatic alcohol (Booij, Hendrix, & Frentzen, 1997), alkyl phenols (Frentzen et al., 2000) and hydrochloric acid (Sarian et al., 1998). The drawbacks of solvent extraction include the high volume of chemicals involved, specific temperature and pressure conditions required, and time consumed. The application of depolymerizations to nylon is limited by the cost associated with preparation, reaction and purification of products. Nylon is more expensive than polyester and much more expensive than polypropylene. This price difference suggests that, if it takes the same processing effort to convert the fiber into resin, an operation on nylon would be most profitable (Wang, 2010). Thus, separating and recycling the waste nylon/cotton blended fabrics as raw materials for different products are attractive. To our best knowledge few publications have been reported concerning on the separation and reclaim of the WNCFs. Most of these textile wastes are presently discarded without recycling due to lack of a simple and commercially acceptable separation process for cellulosic/polyamide mixed fabrics.
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Table 1 Isolation and recovery conditions of cellulose from WNCFs by [AMIM]Cl.a Entry
Waste fabrics ratio (wt%)
Treatment temperature (◦ C)
Exposure time (min)
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 3 3 3 3 3 3 3
110 110 110 110 110 90 100 120 110 110 110 110
80 80 80 80 80 80 80 80 40 60 100 120
a
Fig. 1. Scheme for isolation of cotton cellulose from WNCFs with [AMIM]Cl.
Ionic liquids represent a unique class of solvents that offer unprecedented versatility and tunability for the dissolution of biopolymers (Wilkes, 2002). Ionic liquids have been shown to be powerful solvents for processing biopolymers such as cellulose (Rogers & Seddon, 2003) and silk (Liu, Zhang, Xu, Liu, & Quyang, 2011). These have motivated research and led to various applications in many fields, for example as solvents in electrochemistry, organic synthesis, catalysis, photochemistry, etc (Plechkova & Seddon, 2008). In this paper, a novel process for separating cellulose and nylon 6 from their blended fabrics via ionic liquid was presented. The morphology of WNCFs before and after isolation was analyzed by SEM. Effects of treatment temperature, exposure time and waste fabrics ratio on the cellulose recovery were also investigated. The chemical structure and crystallization properties of control and extracted materials were characterized by FTIR and XRD, respectively. Moreover, the thermostabilities of the original WNCFs and recycled materials were characterized. All the works in this article indicated that this isolation process can be effectively used to recycle waste nylon 6 cellulosic blends.
All reactions were performed with 30 g [AMIM]Cl.
dried blended fabrics were added in three-necked round bottomed flask and mixed with 30 g ionic liquid completely. The mixtures were heated at 110 ◦ C and stirred with speed 200 rpm until the cotton cellulose completely dissolved, and worked up immediately by filtering off through a screener (pore size: 75 m). After filter, the cotton cellulose/[AMIM]Cl solution was obtained. The precipitation was washed with distilled water, and then dried at 50 ◦ C in vacuum oven for 48 h. 2.3. Regeneration of cotton cellulose The obtained cellulose/[AMIM]Cl solution could be coagulated in water as ionic liquid was completely miscible with water in any ratios. The regenerated cellulose (RC) films were prepared by casting the cellulose solution in between two glass plates and then soaked in the water bath to allow the ionic liquid to diffuse from the film. During the process, water is changed several times to ensure that the ionic liquid is removed from the sample. After the washing with deionized water several times for the complete removal of the ionic liquid, the coagulated cellulose film was then dried in a vacuum freeze dryer to get a brittle and flat film. And the residual [AMIM]Cl in coagulation bath was recovered by the distillation of the water/[AMIM]Cl mixture under reduced pressure both in rotary evaporator and distilling apparatus. 2.4. Design of experiment for cotton cellulose isolation
2. Experimental 2.1. Materials Nylon/cotton blended fabrics were collected from tailoring workshop and subjected to cutting and shredding processes. The ionic liquid 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) was purchased from Shanghai Chengjie Chemical Co., Ltd., China and dried in vacuum oven at 80 ◦ C for 48 h before use. Sodium hydroxide in analytical grade was supplied by Shanghai Medical Group Chemical Reagent Co. Ltd., China and used as received. 2.2. Isolation of cotton cellulose [AMIM]Cl was selected as the solvent for isolating cellulose from the WNCFs, with a scheme for isolation of cotton cellulose being described below and shown in Fig. 1. The WNCFs were first dewaxed in Soxhlet apparatus with sodium hydroxide solution (2 wt%) for 2 h at 80 ◦ C, subsequently immersed in 500 ml of boiling water for 2 h, and then dried at 80 ◦ C in vacuum oven for 24 h. The
Single factor experiments were designed to investigate the properties of cotton cellulose isolation and performed under various conditions as shown in Table 1. In this article, three predominant factors, including reaction temperature, reaction time and ratio of ionic liquid to WNCFs (m/m), were investigated respectively. The investigation of each factor’s effect on cotton cellulose isolation was performed with changing a single factor at one time, while keeping other variables constant. WNCFs were then added and mixed with the solvent according to ionic liquid/WNCFs mass ratio of 1, 2, 3, 4, and 5 wt%. The mixtures were heated at the temperatures of 90, 100, 110, and 120 ◦ C with stirring for 40, 60, 80, 100, and 120 min, respectively. After reacting for an enough long time, the resulting solutions were slowly poured in distilled water and washed until the [AMIM]Cl was removed from the sample as indicated by inability to detect its presence using transmission FTIR from KBr plates. The obtained cellulose was dried at 50 ◦ C for 48 h, and then weighed as mc. The filter residue was weighted as mr . In this article, the recovery rates of RC films and filter residues were calculated as follow: ˛ = mc /M × 100% and ˇ = mr /M × 100%, respectively, where ˛ was the recovery rate
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Fig. 2. Effects of treatment temperature, exposure time, and waste fabrics ratio on the recovery of RC films and filter residues.
of RC films, ˇ was the recovery rate of filter residues, and M was the mass of WNCFs. 2.5. Fourier transform infrared (FTIR) spectroscopy FTIR spectra were collected using a Thermo Nicolet 380 spectrometer. A certain amount of original cotton fabrics, RC films, filter residues, and pure nylon 6 fibers were respectively grounded with KBr (1:100, w/w) and pressed into transparent disks. The spectra were measured in the transmittance mode from an accumulation of 128 scans at a 4 cm−1 resolution over 4000–500 cm−1 range. 2.6. Scanning electron microscopy (SEM) SEM was used to investigate the microstructure and the surface morphology of original and treated WNCFs. Samples were embedded in conductive tapes and air-dried overnight. Transverse sections were cut using a rotary microtome. Thicker sections were coated with gold/palladium using an ion sputter coater and observed with a Phenom Desktop SEM microscope operated at 15 kV.
2.8. Thermal gravimetric analysis (TGA) Thermal stabilities of the WNCFs, RC films, and filter residues were determined using a NETZSCH TG 209. The samples were dried and ground into powder using a mortar and pestle. Then approximately 5 mg of the sample was loaded into a platinum pan and subjected to a 10 ◦ C/min temperature ramp over the temperature from 50 ◦ C up to 700 ◦ C. The chamber was continuously purged with N2 during the analysis. 2.9. Polarized light microscopy The solution was prepared by mixing yarns and [AMIM]Cl in a container made of two glass plates. No agitation was applied. One cotton yarn and one nylon 6 yarn were placed between the two glass plates. [AMIM]Cl contained in a pipette was introduced by capillary forces between the two plates. The morphologic variations of cotton and nylon fibers were observed by an Olympus BX-51 polarized light microscopy equipped with a Linkam TMS 91 hot stage. 3. Results and discussion
2.7. X-ray diffraction (XRD) 3.1. Factors affecting recovery rates of RC films and filter residues X-ray diffraction studies of the samples were carried out by high resolution (0.01◦ ) X-ray Diffractometer (X’ Pert PRO) Model RIGAKU MINISLEX with a scanning rate of 0.02◦ /min with nickel filtered Cu K␣ radiation ( = 0.154 nm) operating at 45 kV and 30 mA. The X’ Pert Highscore software was used to analyze the experimental data. The sampling ranges were from 5.00◦ to 45.00◦ . The regenerated cellulose films were cut into strips with 15 mm long and 10 mm wide for the measurement. The other samples were ground into powders for the measurement.
Generally, the isolation of cotton cellulose from WNCFs was primarily affected by the treatment temperature, time, and waste fabrics ratio. In the present study, isolation of cotton cellulose at different temperature was investigated. With increasing temperature, cotton components dissolved more rapidly. As shown in Fig. 2a, at an initial stage of 90–110 ◦ C, the dissolution occurred rapidly with an obvious increase in removal rate of cotton components, followed by a slower increase of removal rate at higher temperature.
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Fig. 3. SEM micrographs WNCFs of (a) 300× and (a ) 1000× original; (b) 300× and (b ) 1000× treated at 90 ◦ C for 80 min; (c) 300× and (c ) 1000× treated at 110 ◦ C for 80 min.
However, it is well known that the higher the treatment temperature, the lower the thermal stability of the regenerated cellulose (Liu et al., 2011). Dissolving at 120 ◦ C will cause the serious degradation of the cellulose. Similarly, the relationship between reaction time and isolation properties was illustrated in Fig. 2b. The cotton cellulose could be fully dissolved in [AMIM]Cl with stirring for 30 min at 110 ◦ C. Besides, with the increase of dissolution time, a higher removal rate of cellulose was achieved accordingly. However, it was characterized by a slight increase of removal rate and decreasing dissolution rate after reaching for 80 min. These phenomena might be explained by the relatively high crystallinity in residual cellulose fibrils and increasing viscosities were generated in [AMIM]Cl, weakening the free hydroxide anions association with hydrogen bonds formed in cellulose, which lead to the decrease of dissolution rate (Zhong, Wang, Huang, Jia, & Wei, 2013). The effects of the waste fabrics ratio on the cellulose recovery and mass were presented in Fig. 2c and d. As can be seen, the RC films recovery rate significantly decreased from 58% to 43% with the ratio shifting from 1 wt% to 5 wt%, which might be explained by the increase of cellulose concentration with increasing amount of waste fabrics, elevating the viscosity of solution and impeding the [AMIM]Cl effects on cotton cellulose. Compared to the reaction with ratio 3 wt%, in reaction with ratio 4 wt% and 5 wt%, the RC films recovery rate showed a dramatic decline, which was not appropriate of regeneration of cotton cellulose. In addition, the mass of regenerated cellulose was significantly small with waste fabrics ratio 1 wt%. Based on the data analysis, the optimal operation conditions were obtained as follows: reaction temperature 110 ◦ C, reaction time 80 min, waste fabrics ratio 3 wt%.
3.2. Morphological structure of WNCFs Fig. 3 displayed SEM photographs of the original and treated WNCFs at different temperature. As seen in Fig. 3a, cotton fibers were blended with nylon 6 fibers in original WNCFs. After treated by [AMIM]Cl at 90 ◦ C, the cotton components were dissolved and regenerated into the cellulose films. There was little appearance of structural change on the nylon 6 components during the dissolution and separation processes. Nylon 6 bundles could be recovered without apparent damage. Further, it can be clearly seen that some regenerated cellulose films remained on the surface of the nylon fibers. Complete separation of cotton and nylon 6 components, however, not possible, as some regenerated cellulose was found in the fiber bundles. To improve the separation degree, higher treatment temperature was required. For this purpose, the disintegration temperature was set at 110 ◦ C. Most of regenerated cellulose having remained in the nylon 6 fiber bundles was comparatively removed at 110 ◦ C and the purity of nylon 6 fibers was improved. At a higher treatment temperature, the cotton component can be more easily dissolved in ionic liquids and the viscosity of the obtained cellulose solution is relatively low, which makes the separation of the cotton and nylon 6 easier during the filtration process. 3.3. Spectroscopic analysis FTIR is a useful tool for obtaining rapid information about the chemical structure of separation products. Fig. 4 showed the FTIR spectra of the obtained RC films, filter residues, the pure cotton fabrics as well as pure nylon 6 fibers. In Fig. 4a, the spectrum of RC films was similar with that of pure cotton fabrics,
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Fig. 4. FTIR spectra of the original cotton fabrics, RC films, filter residues, and pure nylon 6 fibers.
indicating that no other chemical reaction occurred beside breakage of hydrogen bonds during the processes of dissolution and separation. The broad absorption band at 3300–3500 cm−1 corresponds to the stretching vibration of OH groups; the band at 2700–2900 cm−1 attributes to the vibration of CH2 groups. The appearance of an absorption band at 1640 cm−1 corresponds to the C O stretching vibration of C O H. This peak of RC films became intense compared with that of the pure cotton fabrics. The peak at 1350 cm−1 is attributed to the O H bending vibration and the peak at 1040 cm−1 attributes to the C O bond stretching vibration of C O C group in the anhydroglucose ring. The sharp peak at 810 cm−1 corresponds to the glycosidic C H deformation with ring vibration contribution and O H bending, which is characteristic of glycosidic linkages between glucose in cellulose and it is observed only in the filtrate films (Jin et al., 2009). This indicates that the crystal structure of cotton components in WNCFs was transformed from cellulose I into cellulose II through the separation process. In Fig. 4b, many characteristic peaks of the filter residue were identical with those of the pure nylon 6 fibers. It was indicative that the nylon 6 could not be dissolved in [AMIM]Cl and there was little influence on the chemical structure of nylon 6 fibers during the separation process. The characteristic peaks appearing at 1545 and 1650 cm−1 are assigned to the acylamino groups (Wu, Liu, & Berglund, 2002). The peak appearing around 3300 cm−1 was attributed to the NH stretching of amide groups of polyamide 6. In the wave number range of 2800–3000 cm−1 it can be found the band corresponding to valence vibrations of methyl and methylene groups, with two peaks at 2860 cm-1 and 2940 cm-1 . In this region there are peaks which correspond to antisymmetric C-H stretching vibration of methyl and methylene groups and also the bands attributed to symmetric C-H stretching vibration of above mentioned groups (Manito Pereira, Lopes, Timmer, & Keurentjes, 2005). Furthermore, there were small differences between the spectra of the filter residues and pure nylon 6 fibers. The sharp peak at 806 cm−1 was observed only in the filter residues. And the strong and broad absorption peak at 3400 cm−1 of the filter residues was compared with 3300 cm−1 of the pure nylon fibers. These differences could be attributed to the RC films remnant on the surface of the filter residues. The cotton fibers dissolved in [AMIM]Cl and the cellulose/[AMIM]Cl solution was formed with a certain viscosity. This viscous cellulose solution would stick on the surface of filter residues during the separation process. After washing in the water, the cellulose solution was coagulated into a small quantity of RC films.
3.4. XRD XRD patterns of the pure nylon 6 and filter residues were presented in Fig. 5a and b. Pure nylon 6 spectrum exhibited two intense high peaks at 20.1o and 23.9o . These two peaks were characteristic basal reflections of polyamide 6 corresponding to ␣-crystalline form. The filter residues showed similar crystal structure with pure nylon 6. Therefore isolation of cellulose from the WNCFs with ionic liquids had little influence on the crystal structure of the nylon 6 components. However, XRD spectra of the filtrate residue showed a prominent decrease in intensity at the characteristic 2 peak values compared with pure nylon 6. This was possibly due to the fact that a small quantity of RC films remained on the surface of the filter residues during the separation process. The crystal structures of original cellulose and RC films regenerated at 110 ◦ C were shown in Fig. 5c and d. Original cotton cellulose diffraction pattern (Fig. 5c) showed typical cellulose I structure, with a sharp peak at 22.7◦ and two broad peaks between 12◦ and 18◦ . After dissolution and subsequent separation, the RC
Fig. 5. XRD patterns of (a) pure nylon 6, (b) filter residues, (c) original cotton fabrics, and (d) RC films.
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Fig. 6. TGA and DTG curves of (a) RC films, (b) WNCFs and (c) filter residues.
films (Fig. 5c) showed typical cellulose II structure with wide peaks at 12◦ and 20◦ . The results illustrated that the crystal structure of cotton cellulose of WNCFs was transformed from cellulose I to cellulose II after the separation with nylon 6 by dissolving in [AMIM]Cl. The ionic liquids broke intermolecular and intramolecular hydrogen bonds of cotton cellulose as a result of the dissolution of the cotton components of WNCFs. 3.5. Thermal characterization TGA was conducted to investigate thermal stability of the RC films, filter residues as well as WNCFs, as shown in Fig. 6. In TG curves, the minor mass loss at temperature below 150 ◦ C was attributed to the evaporation of absorbed moisture. The mass
loss of water in this step is determined from 50 ◦ C to 150 ◦ C. It was about 1.7 wt% for RC films, 1.2 wt% for WNCFs, and 1 wt% for residue. The TG curves of WNCFs and the obtained RC films exhibited a main stage of decomposition which started around 280 ◦ C and 230 ◦ C, and showed maximum decomposition at 370 ◦ C and 290 ◦ C, respectively. The effective thermal degradation of the RC films began above 200 ◦ C and referred to the bond cleavage of cellulose macromolecules. The TGA diagram of the obtained RC films indicated that both the onset temperature and the temperature at maximum decomposition rate of RC were lower than those of the original cellulose which was in accordance with the literature (Cao et al., 2007). The results demonstrated that regenerated cellulose became thermally less stable, since the cellulose crystallinity decreased during the process of dissolving and
Fig. 7. Optical microscopic images of cotton and nylon 6 fibers in [AMIM]Cl at 110 ◦ C at different time.
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regeneration in ionic liquids (Jiang, Huang, Wang, Zhang, & Wang, 2012). For WNCFs, the first decomposition shoulder peak at about 372 ◦ C was attributed to the thermal degradation of the cotton component (Corrêa, Teixeira, Pessan, & Mattoso, 2010; Jonoobi, Khazaeian, Tahir, Azry, & Oksman, 2011; Roman & Winter, 2004). The major second decomposition peak around 432 ◦ C was associated with the nylon 6 component decomposition. In filter residue, the first peak disappeared but the second peak existed, indicating that most of cotton components were effectively dissolved in [AMIM]Cl solution. DTG showed a sharp peak at 440 ◦ C. This temperature agreed well with the value of around 450 ◦ C (Sekiguchi, Terakado, & Hirasawa, 2014) for the decomposition peak of nylon 6, determined at same heating rate. This result suggested that there was little effect on the thermalstability of the nylon 6 component during the isolation process. Moreover, the small tail peak at low temperature might be attributed to degradation of tiny RC films sticking on the filtrate residue surface. This was in accordance with the SEM results previously shown. At 700 ◦ C residues of about 16.9% for RC, 12.4% for WNCFs, and 8.8% for the filtrate residues can be determined from the TGA curves. As indeed evidence by the X-ray diffraction study, the crystallinity index of RC was lower than that of the pure cotton fabrics. 3.6. Isolation of cellulose from WNCFs The morphological variation processes of cotton and nylon 6 fibers in [AMIM]Cl were monitored and shown in Fig. 7. At first, the cotton and nylon 6 fibers were clearly observed. After 5 min original cotton fibers started to swell along the fiber and dissolve in [AMIM]Cl. Meanwhile, there was little influence on the morphology of nylon 6 fibers. Most of cotton fibrous materials disappeared after 10 min. At last, cotton fibers totally disappeared and only nylon 6 fibers were observed in the visual field. Multiple hydrogen bonds exist between two cellulose macromolecular chains. The [AMIM]Cl enters the space between cellulose chains. And Cl− anions attack the hydroxyl proton of the H O H bonds, meanwhile, [AMIM]+ cations associate with the oxygen atoms of H O H bonds. The two interactions result in the breakage of the extensive hydrogen bonding network between two cellulose chains, which further leads to the dissolution of cellulose (Zhang, Wu, Zhang, & He, 2005). However, nylon 6 is synthesized by ring opening polymerization of caprolactam and is composed of repeating units linked by amide bonds. The [AMIM]Cl cannot enter the space between the polyamide molecular chains and associate with ionize amide bonds, thus nylon 6 fibers cannot dissolve in [AMIM]Cl. Accordingly the cotton cellulose can be isolated from WNCFs by this selective dissolution mechanism. 4. Conclusions Recycling WNCFs into value-added products is an attractive option for environmental and economic benefits. A facile approach to extract cellulose from WNCFs with [AMIM]Cl has been demonstrated. The optimized condition was 3 wt% WNCFs in [AMIM]Cl at 110 ◦ C with successive mechanical stirring for 80 min. The highest yield of RC films was 58%; meanwhile, more than 39% of nylon 6 fibers were recovered. SEM and FTIR results showed that the morphology and chemical property of nylon 6 fibers remained unchanged and only a small quantity of RC films stuck on the surface of filter residues. TGA curves further verified that there was little influence on the thermostability of nylon 6 fibers during the separation process. This dissolving method provides a simple and efficient process for the separation of nylon/cotton mixed fabrics,
which points out a new route for the recycling of waste mixed synthetic/natural fabrics.
Acknowledgements The authors wish to thank National Natural Science Foundation of China (21174055), Graduate Students Innovation Project of Jiangsu Province in China (CXZZ1207246) and the Fundamental Research Funds for the Central Universities (JUDCF10038).
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Contents lists available at ScienceDirect
Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol
Isolation and recovery of cellulose from waste nylon/cotton blended fabrics by 1-allyl-3-methylimidazolium chloride Fangbing Lv a,c , Chaoxia Wang a,∗ , Ping Zhu b,c,∗∗ , Chuanjie Zhang c a b c
Key Laboratory of Eco-Textile, Ministry of Education, School of Textiles and Clothing, Jiangnan University, Wuxi 214122, China State Key Laboratory of New Fiber Materials and Modern Textiles, Qingdao University, Qingdao 266071, China Key Laboratory of Biomass Fiber and Ecological Dyeing of Hubei Province, Wuhan Textile University, Wuhan 430073, China
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Article history: Received 9 May 2014 Received in revised form 12 January 2015 Accepted 15 January 2015 Available online 3 February 2015 Keywords: Cellulose Waste nylon/cotton blended fabrics Separation Recovery Ionic liquid
a b s t r a c t Development of a simple process for separating cellulose and nylon 6 from their blended fabrics is indispensable for recycling of waste mixed fabrics. An efficient procedure of dissolution of the fabrics in an ionic liquid 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) and subsequent filtration separation has been demonstrated. Effects of treatment temperature, time and waste fabrics ratio on the recovery rates were investigated. SEM images showed that the cotton cellulose dissolved in [AMIM]Cl while the nylon 6 fibers remained. The FTIR spectrum of regenerated cellulose (RC) was similar with that of virgin cotton fibers, which verified that no other chemical reaction occurred besides breakage of hydrogen bonds during the processes of dissolution and separation. TGA curves indicated that the regenerated cellulose possessed a reduced thermal stability and was effectively removed from waste nylon/cotton blended fabrics (WNCFs). WNCFs were sufficiently reclaimed with high recovery rate of both regenerated cellulose films and nylon 6 fibers. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Nylon/cotton fabrics have attracted much attention due to their special handle and unique appearances (Malshe, Mazloumpour, ElShafei, & Hauser, 2012; Kim, Lee, Choi, & Lee, 2011). These textile products are indispensable in our daily life, but millions of tons of waste fabrics are generated every year (Ouchi, Toida, Kumaresan, Ando, & Kato, 2010). Generally, these waste fabrics are landfilled or incinerated (Miranda, Sosa-Blanco, Bustos-Martinez, & Vasile, 2007). Such disposing methods not only create environmental pollution but also lead to great waste of the valuable resources. If the waste fabrics are in the single component form of e.g. nylon or cotton fibers, different recycling or reusing techniques were developed (Wang, 2006, Chap. 2). During recent years, a number of methods have been pursued for recycling waste cotton fabrics (WCFs) to value-added products, such as preparing cellulose derivatives (Zhang, Zhang, Tian, Zhou, & Lu, 2013; Sun, Lu, Zhang, Tian, & Zhang, 2013) and for energy production. One potential way to high-value utilizing WCFs is extracting microcrystalline cellulose from waste
∗ Corresponding author. Tel.: +86 0510 85912105; fax: +86 0510 85912105. ∗∗ Corresponding author. Tel.: +86 027 87181347; fax: +86 027 87181347. E-mail addresses: [email protected] (C. Wang), [email protected] (P. Zhu). http://dx.doi.org/10.1016/j.carbpol.2015.01.043 0144-8617/© 2015 Elsevier Ltd. All rights reserved.
cotton fabrics (Xiong, Zhang, Tian, Zhou, & Lu, 2012) and to use them as reinforcement for composite materials due to its unique chemical and physical properties (Foulk, Chao, Akin, Dodd, & Layton, 2006). Commonly most waste textiles are blended fabrics. Attempts have been made to separate nylon blended fabrics by degradation or solubilization. The solvents used include aliphatic alcohol (Booij, Hendrix, & Frentzen, 1997), alkyl phenols (Frentzen et al., 2000) and hydrochloric acid (Sarian et al., 1998). The drawbacks of solvent extraction include the high volume of chemicals involved, specific temperature and pressure conditions required, and time consumed. The application of depolymerizations to nylon is limited by the cost associated with preparation, reaction and purification of products. Nylon is more expensive than polyester and much more expensive than polypropylene. This price difference suggests that, if it takes the same processing effort to convert the fiber into resin, an operation on nylon would be most profitable (Wang, 2010). Thus, separating and recycling the waste nylon/cotton blended fabrics as raw materials for different products are attractive. To our best knowledge few publications have been reported concerning on the separation and reclaim of the WNCFs. Most of these textile wastes are presently discarded without recycling due to lack of a simple and commercially acceptable separation process for cellulosic/polyamide mixed fabrics.
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Table 1 Isolation and recovery conditions of cellulose from WNCFs by [AMIM]Cl.a Entry
Waste fabrics ratio (wt%)
Treatment temperature (◦ C)
Exposure time (min)
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 5 3 3 3 3 3 3 3
110 110 110 110 110 90 100 120 110 110 110 110
80 80 80 80 80 80 80 80 40 60 100 120
a
Fig. 1. Scheme for isolation of cotton cellulose from WNCFs with [AMIM]Cl.
Ionic liquids represent a unique class of solvents that offer unprecedented versatility and tunability for the dissolution of biopolymers (Wilkes, 2002). Ionic liquids have been shown to be powerful solvents for processing biopolymers such as cellulose (Rogers & Seddon, 2003) and silk (Liu, Zhang, Xu, Liu, & Quyang, 2011). These have motivated research and led to various applications in many fields, for example as solvents in electrochemistry, organic synthesis, catalysis, photochemistry, etc (Plechkova & Seddon, 2008). In this paper, a novel process for separating cellulose and nylon 6 from their blended fabrics via ionic liquid was presented. The morphology of WNCFs before and after isolation was analyzed by SEM. Effects of treatment temperature, exposure time and waste fabrics ratio on the cellulose recovery were also investigated. The chemical structure and crystallization properties of control and extracted materials were characterized by FTIR and XRD, respectively. Moreover, the thermostabilities of the original WNCFs and recycled materials were characterized. All the works in this article indicated that this isolation process can be effectively used to recycle waste nylon 6 cellulosic blends.
All reactions were performed with 30 g [AMIM]Cl.
dried blended fabrics were added in three-necked round bottomed flask and mixed with 30 g ionic liquid completely. The mixtures were heated at 110 ◦ C and stirred with speed 200 rpm until the cotton cellulose completely dissolved, and worked up immediately by filtering off through a screener (pore size: 75 m). After filter, the cotton cellulose/[AMIM]Cl solution was obtained. The precipitation was washed with distilled water, and then dried at 50 ◦ C in vacuum oven for 48 h. 2.3. Regeneration of cotton cellulose The obtained cellulose/[AMIM]Cl solution could be coagulated in water as ionic liquid was completely miscible with water in any ratios. The regenerated cellulose (RC) films were prepared by casting the cellulose solution in between two glass plates and then soaked in the water bath to allow the ionic liquid to diffuse from the film. During the process, water is changed several times to ensure that the ionic liquid is removed from the sample. After the washing with deionized water several times for the complete removal of the ionic liquid, the coagulated cellulose film was then dried in a vacuum freeze dryer to get a brittle and flat film. And the residual [AMIM]Cl in coagulation bath was recovered by the distillation of the water/[AMIM]Cl mixture under reduced pressure both in rotary evaporator and distilling apparatus. 2.4. Design of experiment for cotton cellulose isolation
2. Experimental 2.1. Materials Nylon/cotton blended fabrics were collected from tailoring workshop and subjected to cutting and shredding processes. The ionic liquid 1-allyl-3-methylimidazolium chloride ([AMIM]Cl) was purchased from Shanghai Chengjie Chemical Co., Ltd., China and dried in vacuum oven at 80 ◦ C for 48 h before use. Sodium hydroxide in analytical grade was supplied by Shanghai Medical Group Chemical Reagent Co. Ltd., China and used as received. 2.2. Isolation of cotton cellulose [AMIM]Cl was selected as the solvent for isolating cellulose from the WNCFs, with a scheme for isolation of cotton cellulose being described below and shown in Fig. 1. The WNCFs were first dewaxed in Soxhlet apparatus with sodium hydroxide solution (2 wt%) for 2 h at 80 ◦ C, subsequently immersed in 500 ml of boiling water for 2 h, and then dried at 80 ◦ C in vacuum oven for 24 h. The
Single factor experiments were designed to investigate the properties of cotton cellulose isolation and performed under various conditions as shown in Table 1. In this article, three predominant factors, including reaction temperature, reaction time and ratio of ionic liquid to WNCFs (m/m), were investigated respectively. The investigation of each factor’s effect on cotton cellulose isolation was performed with changing a single factor at one time, while keeping other variables constant. WNCFs were then added and mixed with the solvent according to ionic liquid/WNCFs mass ratio of 1, 2, 3, 4, and 5 wt%. The mixtures were heated at the temperatures of 90, 100, 110, and 120 ◦ C with stirring for 40, 60, 80, 100, and 120 min, respectively. After reacting for an enough long time, the resulting solutions were slowly poured in distilled water and washed until the [AMIM]Cl was removed from the sample as indicated by inability to detect its presence using transmission FTIR from KBr plates. The obtained cellulose was dried at 50 ◦ C for 48 h, and then weighed as mc. The filter residue was weighted as mr . In this article, the recovery rates of RC films and filter residues were calculated as follow: ˛ = mc /M × 100% and ˇ = mr /M × 100%, respectively, where ˛ was the recovery rate
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Fig. 2. Effects of treatment temperature, exposure time, and waste fabrics ratio on the recovery of RC films and filter residues.
of RC films, ˇ was the recovery rate of filter residues, and M was the mass of WNCFs. 2.5. Fourier transform infrared (FTIR) spectroscopy FTIR spectra were collected using a Thermo Nicolet 380 spectrometer. A certain amount of original cotton fabrics, RC films, filter residues, and pure nylon 6 fibers were respectively grounded with KBr (1:100, w/w) and pressed into transparent disks. The spectra were measured in the transmittance mode from an accumulation of 128 scans at a 4 cm−1 resolution over 4000–500 cm−1 range. 2.6. Scanning electron microscopy (SEM) SEM was used to investigate the microstructure and the surface morphology of original and treated WNCFs. Samples were embedded in conductive tapes and air-dried overnight. Transverse sections were cut using a rotary microtome. Thicker sections were coated with gold/palladium using an ion sputter coater and observed with a Phenom Desktop SEM microscope operated at 15 kV.
2.8. Thermal gravimetric analysis (TGA) Thermal stabilities of the WNCFs, RC films, and filter residues were determined using a NETZSCH TG 209. The samples were dried and ground into powder using a mortar and pestle. Then approximately 5 mg of the sample was loaded into a platinum pan and subjected to a 10 ◦ C/min temperature ramp over the temperature from 50 ◦ C up to 700 ◦ C. The chamber was continuously purged with N2 during the analysis. 2.9. Polarized light microscopy The solution was prepared by mixing yarns and [AMIM]Cl in a container made of two glass plates. No agitation was applied. One cotton yarn and one nylon 6 yarn were placed between the two glass plates. [AMIM]Cl contained in a pipette was introduced by capillary forces between the two plates. The morphologic variations of cotton and nylon fibers were observed by an Olympus BX-51 polarized light microscopy equipped with a Linkam TMS 91 hot stage. 3. Results and discussion
2.7. X-ray diffraction (XRD) 3.1. Factors affecting recovery rates of RC films and filter residues X-ray diffraction studies of the samples were carried out by high resolution (0.01◦ ) X-ray Diffractometer (X’ Pert PRO) Model RIGAKU MINISLEX with a scanning rate of 0.02◦ /min with nickel filtered Cu K␣ radiation ( = 0.154 nm) operating at 45 kV and 30 mA. The X’ Pert Highscore software was used to analyze the experimental data. The sampling ranges were from 5.00◦ to 45.00◦ . The regenerated cellulose films were cut into strips with 15 mm long and 10 mm wide for the measurement. The other samples were ground into powders for the measurement.
Generally, the isolation of cotton cellulose from WNCFs was primarily affected by the treatment temperature, time, and waste fabrics ratio. In the present study, isolation of cotton cellulose at different temperature was investigated. With increasing temperature, cotton components dissolved more rapidly. As shown in Fig. 2a, at an initial stage of 90–110 ◦ C, the dissolution occurred rapidly with an obvious increase in removal rate of cotton components, followed by a slower increase of removal rate at higher temperature.
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Fig. 3. SEM micrographs WNCFs of (a) 300× and (a ) 1000× original; (b) 300× and (b ) 1000× treated at 90 ◦ C for 80 min; (c) 300× and (c ) 1000× treated at 110 ◦ C for 80 min.
However, it is well known that the higher the treatment temperature, the lower the thermal stability of the regenerated cellulose (Liu et al., 2011). Dissolving at 120 ◦ C will cause the serious degradation of the cellulose. Similarly, the relationship between reaction time and isolation properties was illustrated in Fig. 2b. The cotton cellulose could be fully dissolved in [AMIM]Cl with stirring for 30 min at 110 ◦ C. Besides, with the increase of dissolution time, a higher removal rate of cellulose was achieved accordingly. However, it was characterized by a slight increase of removal rate and decreasing dissolution rate after reaching for 80 min. These phenomena might be explained by the relatively high crystallinity in residual cellulose fibrils and increasing viscosities were generated in [AMIM]Cl, weakening the free hydroxide anions association with hydrogen bonds formed in cellulose, which lead to the decrease of dissolution rate (Zhong, Wang, Huang, Jia, & Wei, 2013). The effects of the waste fabrics ratio on the cellulose recovery and mass were presented in Fig. 2c and d. As can be seen, the RC films recovery rate significantly decreased from 58% to 43% with the ratio shifting from 1 wt% to 5 wt%, which might be explained by the increase of cellulose concentration with increasing amount of waste fabrics, elevating the viscosity of solution and impeding the [AMIM]Cl effects on cotton cellulose. Compared to the reaction with ratio 3 wt%, in reaction with ratio 4 wt% and 5 wt%, the RC films recovery rate showed a dramatic decline, which was not appropriate of regeneration of cotton cellulose. In addition, the mass of regenerated cellulose was significantly small with waste fabrics ratio 1 wt%. Based on the data analysis, the optimal operation conditions were obtained as follows: reaction temperature 110 ◦ C, reaction time 80 min, waste fabrics ratio 3 wt%.
3.2. Morphological structure of WNCFs Fig. 3 displayed SEM photographs of the original and treated WNCFs at different temperature. As seen in Fig. 3a, cotton fibers were blended with nylon 6 fibers in original WNCFs. After treated by [AMIM]Cl at 90 ◦ C, the cotton components were dissolved and regenerated into the cellulose films. There was little appearance of structural change on the nylon 6 components during the dissolution and separation processes. Nylon 6 bundles could be recovered without apparent damage. Further, it can be clearly seen that some regenerated cellulose films remained on the surface of the nylon fibers. Complete separation of cotton and nylon 6 components, however, not possible, as some regenerated cellulose was found in the fiber bundles. To improve the separation degree, higher treatment temperature was required. For this purpose, the disintegration temperature was set at 110 ◦ C. Most of regenerated cellulose having remained in the nylon 6 fiber bundles was comparatively removed at 110 ◦ C and the purity of nylon 6 fibers was improved. At a higher treatment temperature, the cotton component can be more easily dissolved in ionic liquids and the viscosity of the obtained cellulose solution is relatively low, which makes the separation of the cotton and nylon 6 easier during the filtration process. 3.3. Spectroscopic analysis FTIR is a useful tool for obtaining rapid information about the chemical structure of separation products. Fig. 4 showed the FTIR spectra of the obtained RC films, filter residues, the pure cotton fabrics as well as pure nylon 6 fibers. In Fig. 4a, the spectrum of RC films was similar with that of pure cotton fabrics,
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Fig. 4. FTIR spectra of the original cotton fabrics, RC films, filter residues, and pure nylon 6 fibers.
indicating that no other chemical reaction occurred beside breakage of hydrogen bonds during the processes of dissolution and separation. The broad absorption band at 3300–3500 cm−1 corresponds to the stretching vibration of OH groups; the band at 2700–2900 cm−1 attributes to the vibration of CH2 groups. The appearance of an absorption band at 1640 cm−1 corresponds to the C O stretching vibration of C O H. This peak of RC films became intense compared with that of the pure cotton fabrics. The peak at 1350 cm−1 is attributed to the O H bending vibration and the peak at 1040 cm−1 attributes to the C O bond stretching vibration of C O C group in the anhydroglucose ring. The sharp peak at 810 cm−1 corresponds to the glycosidic C H deformation with ring vibration contribution and O H bending, which is characteristic of glycosidic linkages between glucose in cellulose and it is observed only in the filtrate films (Jin et al., 2009). This indicates that the crystal structure of cotton components in WNCFs was transformed from cellulose I into cellulose II through the separation process. In Fig. 4b, many characteristic peaks of the filter residue were identical with those of the pure nylon 6 fibers. It was indicative that the nylon 6 could not be dissolved in [AMIM]Cl and there was little influence on the chemical structure of nylon 6 fibers during the separation process. The characteristic peaks appearing at 1545 and 1650 cm−1 are assigned to the acylamino groups (Wu, Liu, & Berglund, 2002). The peak appearing around 3300 cm−1 was attributed to the NH stretching of amide groups of polyamide 6. In the wave number range of 2800–3000 cm−1 it can be found the band corresponding to valence vibrations of methyl and methylene groups, with two peaks at 2860 cm-1 and 2940 cm-1 . In this region there are peaks which correspond to antisymmetric C-H stretching vibration of methyl and methylene groups and also the bands attributed to symmetric C-H stretching vibration of above mentioned groups (Manito Pereira, Lopes, Timmer, & Keurentjes, 2005). Furthermore, there were small differences between the spectra of the filter residues and pure nylon 6 fibers. The sharp peak at 806 cm−1 was observed only in the filter residues. And the strong and broad absorption peak at 3400 cm−1 of the filter residues was compared with 3300 cm−1 of the pure nylon fibers. These differences could be attributed to the RC films remnant on the surface of the filter residues. The cotton fibers dissolved in [AMIM]Cl and the cellulose/[AMIM]Cl solution was formed with a certain viscosity. This viscous cellulose solution would stick on the surface of filter residues during the separation process. After washing in the water, the cellulose solution was coagulated into a small quantity of RC films.
3.4. XRD XRD patterns of the pure nylon 6 and filter residues were presented in Fig. 5a and b. Pure nylon 6 spectrum exhibited two intense high peaks at 20.1o and 23.9o . These two peaks were characteristic basal reflections of polyamide 6 corresponding to ␣-crystalline form. The filter residues showed similar crystal structure with pure nylon 6. Therefore isolation of cellulose from the WNCFs with ionic liquids had little influence on the crystal structure of the nylon 6 components. However, XRD spectra of the filtrate residue showed a prominent decrease in intensity at the characteristic 2 peak values compared with pure nylon 6. This was possibly due to the fact that a small quantity of RC films remained on the surface of the filter residues during the separation process. The crystal structures of original cellulose and RC films regenerated at 110 ◦ C were shown in Fig. 5c and d. Original cotton cellulose diffraction pattern (Fig. 5c) showed typical cellulose I structure, with a sharp peak at 22.7◦ and two broad peaks between 12◦ and 18◦ . After dissolution and subsequent separation, the RC
Fig. 5. XRD patterns of (a) pure nylon 6, (b) filter residues, (c) original cotton fabrics, and (d) RC films.
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Fig. 6. TGA and DTG curves of (a) RC films, (b) WNCFs and (c) filter residues.
films (Fig. 5c) showed typical cellulose II structure with wide peaks at 12◦ and 20◦ . The results illustrated that the crystal structure of cotton cellulose of WNCFs was transformed from cellulose I to cellulose II after the separation with nylon 6 by dissolving in [AMIM]Cl. The ionic liquids broke intermolecular and intramolecular hydrogen bonds of cotton cellulose as a result of the dissolution of the cotton components of WNCFs. 3.5. Thermal characterization TGA was conducted to investigate thermal stability of the RC films, filter residues as well as WNCFs, as shown in Fig. 6. In TG curves, the minor mass loss at temperature below 150 ◦ C was attributed to the evaporation of absorbed moisture. The mass
loss of water in this step is determined from 50 ◦ C to 150 ◦ C. It was about 1.7 wt% for RC films, 1.2 wt% for WNCFs, and 1 wt% for residue. The TG curves of WNCFs and the obtained RC films exhibited a main stage of decomposition which started around 280 ◦ C and 230 ◦ C, and showed maximum decomposition at 370 ◦ C and 290 ◦ C, respectively. The effective thermal degradation of the RC films began above 200 ◦ C and referred to the bond cleavage of cellulose macromolecules. The TGA diagram of the obtained RC films indicated that both the onset temperature and the temperature at maximum decomposition rate of RC were lower than those of the original cellulose which was in accordance with the literature (Cao et al., 2007). The results demonstrated that regenerated cellulose became thermally less stable, since the cellulose crystallinity decreased during the process of dissolving and
Fig. 7. Optical microscopic images of cotton and nylon 6 fibers in [AMIM]Cl at 110 ◦ C at different time.
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regeneration in ionic liquids (Jiang, Huang, Wang, Zhang, & Wang, 2012). For WNCFs, the first decomposition shoulder peak at about 372 ◦ C was attributed to the thermal degradation of the cotton component (Corrêa, Teixeira, Pessan, & Mattoso, 2010; Jonoobi, Khazaeian, Tahir, Azry, & Oksman, 2011; Roman & Winter, 2004). The major second decomposition peak around 432 ◦ C was associated with the nylon 6 component decomposition. In filter residue, the first peak disappeared but the second peak existed, indicating that most of cotton components were effectively dissolved in [AMIM]Cl solution. DTG showed a sharp peak at 440 ◦ C. This temperature agreed well with the value of around 450 ◦ C (Sekiguchi, Terakado, & Hirasawa, 2014) for the decomposition peak of nylon 6, determined at same heating rate. This result suggested that there was little effect on the thermalstability of the nylon 6 component during the isolation process. Moreover, the small tail peak at low temperature might be attributed to degradation of tiny RC films sticking on the filtrate residue surface. This was in accordance with the SEM results previously shown. At 700 ◦ C residues of about 16.9% for RC, 12.4% for WNCFs, and 8.8% for the filtrate residues can be determined from the TGA curves. As indeed evidence by the X-ray diffraction study, the crystallinity index of RC was lower than that of the pure cotton fabrics. 3.6. Isolation of cellulose from WNCFs The morphological variation processes of cotton and nylon 6 fibers in [AMIM]Cl were monitored and shown in Fig. 7. At first, the cotton and nylon 6 fibers were clearly observed. After 5 min original cotton fibers started to swell along the fiber and dissolve in [AMIM]Cl. Meanwhile, there was little influence on the morphology of nylon 6 fibers. Most of cotton fibrous materials disappeared after 10 min. At last, cotton fibers totally disappeared and only nylon 6 fibers were observed in the visual field. Multiple hydrogen bonds exist between two cellulose macromolecular chains. The [AMIM]Cl enters the space between cellulose chains. And Cl− anions attack the hydroxyl proton of the H O H bonds, meanwhile, [AMIM]+ cations associate with the oxygen atoms of H O H bonds. The two interactions result in the breakage of the extensive hydrogen bonding network between two cellulose chains, which further leads to the dissolution of cellulose (Zhang, Wu, Zhang, & He, 2005). However, nylon 6 is synthesized by ring opening polymerization of caprolactam and is composed of repeating units linked by amide bonds. The [AMIM]Cl cannot enter the space between the polyamide molecular chains and associate with ionize amide bonds, thus nylon 6 fibers cannot dissolve in [AMIM]Cl. Accordingly the cotton cellulose can be isolated from WNCFs by this selective dissolution mechanism. 4. Conclusions Recycling WNCFs into value-added products is an attractive option for environmental and economic benefits. A facile approach to extract cellulose from WNCFs with [AMIM]Cl has been demonstrated. The optimized condition was 3 wt% WNCFs in [AMIM]Cl at 110 ◦ C with successive mechanical stirring for 80 min. The highest yield of RC films was 58%; meanwhile, more than 39% of nylon 6 fibers were recovered. SEM and FTIR results showed that the morphology and chemical property of nylon 6 fibers remained unchanged and only a small quantity of RC films stuck on the surface of filter residues. TGA curves further verified that there was little influence on the thermostability of nylon 6 fibers during the separation process. This dissolving method provides a simple and efficient process for the separation of nylon/cotton mixed fabrics,
which points out a new route for the recycling of waste mixed synthetic/natural fabrics.
Acknowledgements The authors wish to thank National Natural Science Foundation of China (21174055), Graduate Students Innovation Project of Jiangsu Province in China (CXZZ1207246) and the Fundamental Research Funds for the Central Universities (JUDCF10038).
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