Recycling textiles: the use of ionic liquids in the separation of cotton polyester blends

Abstract

Textiles are commonly made from intimate blends of polyester and cotton, which makes recycling very difficult. We report for the first time the use of ionic liquid in the separation of polyester cotton blends. By selective dissolution of the cotton component, the polyester component can be separated and recovered in high yield. This finding presents an environmentally benign approach to recycling textile waste.

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Introduction

One of the challenges facing society today is the recycling of textiles. It has been reported that the United States generates 25 billion pounds of textiles per year and only 15% of this is recycled with 85% being sent to landfills.^1–3 The U.S. EPA estimates that textile waste occupies 5% of land fill mass.^1,2 As such, many campaigns exist to improve the consumers' awareness towards recycling textiles. While donating or selling clothing has resulted in a decrease in the disposal of textiles this is not a complete recycling solution. Currently, only 15% of all textiles material is recycled back into the original fibre form.^2–4 A significant limitation associated with the recycling of clothing back into the original fibres is that the majority of clothing is comprised of blended fibres.³ The most common blend used in clothing is a polyester/cotton blend.^5,6 While the individual components, cotton and polyester, can be easily recycled, mechanical separation techniques cannot be used for intimately mixed polyester/cotton blends.^5–14 Limited attempts have been made to separate these blends chemically, however polyester is only soluble in very harsh solvents such as hexafluoroisopropanol¹⁵ and phenol/tetrachloroethane.¹⁶ High concentration acid solutions have also been investigated, such as 70–75% aqueous sulphuric acid¹⁷ and 85% aqueous phosphoric acid.¹⁸ However these are not environmentally sustainable solutions and the acid solutions promotes hydrolysis of the glycosidic linkage of the cellulose chain resulting in the degradation of the cotton fibre.¹⁹

Here we use ionic liquids, ILs, as environmentally friendly processing solvents for the chemical separation of polyester/cotton blends. The IL selected for this study is the cellulose dissolving 1-allyl-3-methylimidazolium chloride (AMIMCl).²⁰ The benefit of using ILs in this process is the ease with which the polyester and cotton can be separated. The AMIMCI selectively dissolves the cotton component, with the added advantage that the IL can be recycled and reused. We show that the polyester is recovered in high yield and that the cotton can be regenerated into various forms, such as wet spun into fibres or cast as films. In 2012, 88.5 million tonnes of fibres were produced,²¹ and this amount is expected to continue to increase with demand from developing countries. This technology reported here shows a facile route to the recycling of clothing in the form of polyester/cotton blends. We measure the material properties of the recovered polyester and cotton and show that no significant differences can be seen between the recovered polymers and the as received 100% polymers.

Materials and methods

Dissolution studies

1-allyl-3-methylimidazolium chloride AMIMCl and 1-butyl-3-methylimidazolium acetate BMIMAc where purchased from Sigma Aldrich and vacuum dried for 6 hours at 80 °C prior to each dissolution experiments. The water content of the ionic liquid (IL) was measured to be less than 0.2% by Karl Fisher titration. To determine if this IL would dissolve polyester, 100% polyester as received was submerged in the AMIMCI at 80 °C and monitored using polarising optical microscopy. Over a 48 hour period, no dissolution was observed.

Separation of cotton and polyester

All the yarn specimens were oven dried at 105 °C for 24 hours prior to dissolution to remove the moisture in yarns. All the yarn samples were obtained from Leading Textiles, Australia. The yarn was added to AMIMCI at 80 °C at 2 wt% increments up to 10 wt%. After 6 hours, the undissolved component was removed, rinsed with water and weighed. Table 1 shows the weight of the blend before cotton dissolution and the recovered amount. The IL was able to be recovered and recycled as previously reported as water is the coagulating solution.^22,23

Table 1 Dissolved cotton % and undissolved polyester of the blend samples in AMIMCL

Sample ID	Initial blend weight (mg)	Residue weight (mg)	Undissolved polyester from the blend %	Dissolved cotton from the blend %
A	1602.25	803.60	50.15	49.85
B	1598.23	798.85	49.98	50.02
C	1600.40	801.72	50.09	49.91

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Characterization of recovered polyester and regenerated cellulose

FTIR. FTIR spectra of the samples were measured on Bruker Alpha FT-IR spectrometer. Samples were scanned in the frequency range of 600 cm⁻¹ to 4000 cm⁻¹ at the scan resolution of 4 cm⁻¹ and background and sample scan time of 64 scans. FTIR measurement was repeated 3 times, taking over 3 places on the test specimen.

For the cotton films regenerated, the hydroxyl region 2990–3900 cm⁻¹ was deconvoluted, and curve was fitted adopting Gaussian model using OPUS 5.5 software. Finally, auto fitted spectrum was area normalized to obtain percentage conformations of three bands which represent inter molecular and intra molecular hydrogen bonding of the cotton films regenerated.

The intra-molecular hydrogen bonds for O(2)H⋯O(6), and O(3)H⋯O(5) (endocyclic oxygen) and the inter-molecular hydrogen bonds for O(6)H⋯O(3) in the cellulose of the bio films were appeared in 3455–3410, 3375–3340 and 3310–3230 cm⁻¹ respectively.^24,25

¹³C-CP-NMR. The solid state of ¹³C NMR spectra of the samples were recorded on Bruker AVANCE III 300 MHz Wide Bore using 4.00 mm MAS HX probe. The experiments were carried out at the spinning rate of 12.5 kHz at 25 °C for 5677 scans.

SEM. The morphology of samples was observed using Zeiss Supra 55VP scanning electron microscope (SEM) at an accelerating voltage of 5.00 kV. The sample surface was gold coated before observation.

DSC. The DSC experiments were performed on TA DSC Q 200 differential scanning calorimetry using 4–6 mg of the sample. Firstly the samples were heated to 100 °C at a heating rate of 10 °C min⁻¹ under N₂ atmosphere. Then the samples were held at 100 °C for 5 minutes to delete the thermal history. The samples were cooled down 0 °C at a cooling rate of 10 °C min⁻¹ and held for 5 minutes. Subsequently, the samples were heated from 0 °C to 300 °C at a heating rate of 10 °C min⁻¹ (second scan). The second scans of the samples are shown.

TGA. The samples were subjected to thermogravimetric analyses (TGA) on Netzsch STA 409 thermogravimetric analyser. Test was performed using 5–8 mg of the samples. The specimens were heated from 25 °C to 600 °C at a heating rate of 10 °C min⁻¹ under N₂ atmosphere. The decomposition temperature at which the weight loss begins (T_d) calculated as the onset.

Results and discussion

Fig. 1 shows the general scheme for the separation of the cotton/polyester blend using AMIMCI. The IL was able to be recovered and recycled as previously reported as water is the coagulating solution.^22,23

Fig. 1 Separation and utilisation of polyester/cotton blends using AMIMCI.

For this process we have studied both AMIMCI and BMIMAc, these ILs have been selected on the basics of their cellulose dissolving capabilities, both ILs show some of the highest cellulose dissolving capabilities.²⁶ Little difference in the separation capabilities was noted under the conditions investigated here. We also studied the impact of temperature on the separation process and found little difference in the time it took to dissolve the cotton component in either BMIMAc or AMIMCI at 80 °C, 100 °C and 120 °C. The dissolution of the cotton was considerably faster at 120 °C for both ILs as expected (see Table 1, ESI†).

Fig. 2a shows scanning electron image of the polyester cotton blend; prior to separation both cotton and polyester fibres can be observed. Fig. 2b shows the recovered polyester, only polyester fibres are observed.

Fig. 2 (2a)-SEM of polyester/cotton blend; (2b)-SEM of recovered polyester.

We also characterized the structure of the recovered polyester, using a combination of DSC, TGA, NMR and FTIR shown in Fig. 3a–d. For comparison we include the as received 100% polyester. It can be seen that no change in the melt temperature of the recovered polyester is measured, which is important since polyester can be recycled by melting the polyester and reshaping the polyester into the desired form (fibres, bottles, etc.). The ¹³C NMR spectrum and FTIR do indicate that some small amount (less than 2%) of cotton may remain with the recovered polyester as evidenced by the additional peaks observed at 60 and 110 ppm,^27,28 and the presence of the hydroxyl peak at 3300 cm⁻¹(ref. 25 and 29) in the FTIR spectrum.

Fig. 3 (a–d) Characterization of the recovered polyester (red curve) compared with the as received polyester (black curve) (a) DSC curves (b) TGA curves, (c) ¹³C-CP-NMR and (d) FTIR, all show little to no difference between the two polyester samples.

We now turn our attention to the regenerated cotton. We show that fibres and/or films can easily be prepared from the cotton/AMIMCI solution. Regenerated cellulose fibres have a considerable market share and are growing in popularity, common regenerated cellulosic fibres are viscose, rayon and lyocel.^30,31 Fig. 4a–c shows a series of material characterization experiments performed on the recovered cotton from the blend compared with as received cotton dissolved and regenerated from the same IL, AMIMCI. As can be seen no difference in the tensile properties are observed between the two cotton samples (Fig 4a, stress and stain at breakage and Young's modules provided in Table 2 ESI†). Similarly the thermal degradation temperature and FTIR show no appreciable differences (deconvolution of the OH region provided in Table 3 ESI†).

Fig. 4 (a–c) Characterization of the recovered cotton (red curve) compared with as received cotton (black curve) (a) DMA curves (b) TGA curves, (c) FTIR, all show little to no difference between the two cotton samples. Both samples are regenerated cotton.

Here we described the facile and efficient separation of polyester/cotton blends into their individual components. This was achieved by selective dissolution of the cotton component using the ionic liquid AMIMCI. We showed that the cotton could be regenerated using water as the coagulated solvent. Regenerated cotton is a material finding increased use in the textiles industry,^30–32 in the carbon fibre industry as low cost precursor materials^33,34 and as the starting material for bioethanol production.^20,35,36

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra04306e

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