Transforming post‑consumer cotton waste textiles into viscose staple fiber using hydrated zinc chloride

Large amounts of cellulose-based waste textiles are generated every year, yet little is done to recycle this waste. Alternatives such as fiber-to-fiber recycling, where a significant part of the value of the waste textiles is recovered, are attractive possibilities. In this study, we have investigated the viability of using hydrated zinc chloride (ZnCl 2 ·4H 2 O) as a solvent and swelling agent to convert cotton waste tex-tiles (the most abundant cellulose-based waste textile) into a dissolving pulp that can be used as raw material for the production and spinning of viscose fibers. The solvent produced an accessible dissolving pulp and exhibited excellent recyclability, maintaining good dissolving power even after repeated recycling. The dissolving pulp was subsequently used to produce viscose dope, a spinning solution which was spun and cut into viscose staple fibers. The viscose dope exhibited good properties (moderate filter clogging value and gamma number), and the resulting staple fibers were strong and of good quality (high linear density, elongation, and tenacity). These results illustrate the potential of using hydrated zinc chloride for the production of viscose grade dissolving pulp from cotton waste textiles.


Introduction
Cellulose-based waste textiles are an abundant and underutilized source of material with significant potential for recycling and valorization.The global fiber production in 2020 was 109 million tonnes, of which plant-and manmade cellulosic fibers accounted for 36%.Cotton constitutes the largest fraction by far, with an annual production of 26.2 million tonnes (Opperskalski et al. 2021).According to the Ellen MacArthur Foundation, only 14% of all post-consumer clothes were recycled in 2017, the remainder mainly being incinerated or landfilled (Ellen MacArthur Foundation 2017).Most of these recycled textiles can be more appropriately described as downcycled, since less than 1% of the fibers used for clothing production originated from recycled fibers (some experts maintain that the fraction is even less, e.g.below 0.1%) (Ellen MacArthur Foundation 2017).Of this meager 1%, most was assumed to be

Cotton waste textiles and chemicals
The waste textiles used in this study were white waste textiles consisting of 100% cotton.The textiles were sorted based on fiber type and color, and were provided by a Swedish textile sorting facility.The textiles were cut into smaller pieces, approximately 5 × 5 cm, and non-cotton components such as buttons, seams, and labels were removed at the textile sorting facility before delivery.The ZnCl 2 used in this study was purchased in its anhydrous form from Chemos GmbH (Altdorf, Germany), and the chemicals used for the viscose process and fiber spinning process were obtained from VWR Chemicals (Radnor, PA, USA) and Honeywell International Inc. (Charlotte, NC, USA).

Low-consistency treatment
A batch of 578 kg of fresh ZnCl 2 •4H 2 O was prepared by mixing 200 kg of water at room temperature with 378 kg of ZnCl 2 in an enameled vessel with a volume of 330 L, equipped with a heating jacket and an impeller stirrer (BÜCHI AG, Uster, Switzerland).
In the low-consistency ZnCl 2 •4H 2 O treatment, 403 kg of the prepared ZnCl 2 •4H 2 O was heated to a temperature of 81 °C in the same 330 L enameled vessel, while being vigorously stirred to ensure rapid heating.Upon reaching 81 °C, 4 kg of cotton waste textiles was added to the vessel.Another 2 kg of cotton waste textiles was added to the vessel after 10 min, and a further 2 kg after an additional 15 min.Thus, a total of 8 kg of cotton waste textiles was added, equivalent to a consistency of 2 wt% (mass textiles/mass ZnCl 2 •4H 2 O).After 1 h, 121 kg of water, at ambient temperature, was added to the vessel to precipitate the dissolving pulp.The liquid was pumped out of the vessel into a storage vessel and the dissolving pulp was washed with an abundance of water (ambient temperature) 3 times, to wash out the remaining ZnCl 2 .After the washing, the dissolving pulp was evacuated from the vessel and washed once more in a collection tray.

High-consistency treatment
In the high-consistency ZnCl 2 •4H 2 O treatment, 174 kg of the remaining ZnCl 2 •4H 2 O was heated to 80 °C in the same 330 L enameled vessel while being vigorously stirred to ensure rapid heating.Upon reaching 80 °C, the cotton waste textiles were added in a fed-batch manner according to Table 1, eventually reaching a consistency of 4.3 wt%.After the last addition of cotton waste textiles, the treatment was continued for an additional 35 min, leading to a total treatment time of 1.5 h.After completion of the treatment, the mixture was evacuated from the vessel into a collection tray to which 52 kg of water, at ambient temperature, was added to precipitate the dissolving pulp.The diluted ZnCl 2 was separated from the dissolving pulp and the dissolving pulp was washed three times with an abundance of water (ambient temperature).

Recycling of ZnCl 2
A series of experiments were performed during which the ZnCl 2 was recovered and recycled between each experiment, in order to investigate the recyclability of the hydrated zinc chloride.The series consisted of ten experiments, during which fresh cotton waste textiles was treated with ZnCl 2 •4H 2 O.Each experiment was conducted in a 20 L Rettberg reactor equipped with a heating jacket and a turbine stirrer (Gebr.Rettberg GmbH, Göttingen, Germany).Fresh ZnCl 2 •4H 2 O was prepared by mixing water and anhydrous zinc chloride at the same ratios as described in the procedure for the preparation of ZnCl 2 •4H 2 O.All ten experiments were carried out at 80 °C, with a consistency of 2.5 wt%, for 1 h.After the treatment with ZnCl 2 •4H 2 O, 0.3 g of water was added for every g of ZnCl 2 •4H 2 O in the reactor to precipitate the dissolving pulp, which was separated from the liquid.The liquid was recovered after each experiment and the water added during precipitation was removed using an industrial rotavapor (120 °C, 400 mbar at the start, 50 mbar at the end) to concentrate the diluted ZnCl 2 to the desired hydration level (ZnCl 2 •4H 2 O).The regenerated ZnCl 2 •4H 2 O was then mixed with fresh cotton waste textiles and subjected to an additional cycle of treatment, in a total of 10 experiments (including the first experiment with fresh ZnCl 2 •4H 2 O).
Viscose process and the spinning process A sample of the dissolving pulp prepared according to the low-consistency ZnCl 2 •4H 2 O treatment was washed once with 0.5% w/v sodium hydroxide solution and once with water to decrease the content of any residual ZnCl 2 .The purified dissolving pulp was air-dried at room temperature.The purified dissolving pulp was converted into viscose dope according to the viscose process based on literature methodology (Woodings 2001;Strunk 2012), with some minor modifications: A lower temperature was used in the pre-aging step (40 °C instead of 50 °C), a longer residence time was required in the mercerization step (1 h instead of 30 min), and a lower amount of carbon disulfide was necessary in the xanthation step (32% instead of 36%) (Strunk 2012).The first two modifications were used to avoid unnecessary depolymerization of the material and thus a decrease in yield in the process, while the last modification was needed to ensure good properties of the resulting dope.
A spin bath of 5 L at 50 °C was prepared, with the composition described in Table 2. Filtered and de-aerated viscose dope was added to a syringe, which was placed at the spinning pilot (Fig. 1) and the spinning was initialized.The spinneret contained 90 holes, each with a diameter of 80 µm.The pump throw was set for 5 ml/min which resulted in an extrusion speed of 11 m/min according to Eq. 1 (1) Waste textiles added (kg) 3.6 0.9 0.9 0.9 0.9 0.4 where v 0 = extrusion speed (m/min), Q = pump throw (ml/min), a = number of holes in the spinneret, and d = diameter of the holes in the spinneret (m).The speed of godet 1 was set to 18.8 m/min, which gave a draw ratio of 1.7, according to Eq. 2 where v 0 = extrusion speed (m/min) and v 1 = velocity of godet 1 (m/min).The speed of godet 2 was 29.9 m/min resulting in a stretch of 59%.The stretch was calculated according to Eq. 3 (2) where v 1 = velocity of godet 1 (m/min) and v 2 = velocity of godet 2 (m/min).The spinning was terminated when almost all viscose in the syringe had been utilized.The spun filaments were removed from godet 2, cut and washed in 1 L washing solution according to a washing sequence (Table 3), and dried in room temperature overnight.
Analyses of cotton waste textiles, dissolving pulp, viscose dope and staple fibers

Limiting viscosity number
Samples of the cotton waste textiles and the dissolving pulps produced using the low-and high-consistency treatments (Sects."Low-consistency treatment" Fig. 1 The spinning pilot setup and "High-consistency treatment") were analyzed to determine the limiting viscosity number in cupriethylenediamine solution, according to ISO 5351 (ISO 2010).

Wide-angle X-ray scattering (WAXS)
The influence of the ZnCl 2 •4H 2 O treatment, and the washing of the dissolving pulp with dilute NaOH solution, on the crystallinity of the cellulose was investigated.The crystallinity of samples of the cotton waste textiles and the dissolving pulp produced with the low-consistency treatment, before and after washing with dilute NaOH solution, as well as samples of the precipitate after washing (as described in Sect."Viscose process and the spinning process"), were studied using wide-angle X-ray scattering (WAXS) analysis, as described previously (Ruuth et al. 2022).

Sodium, zinc and chloride ion amount determination
Samples of cotton waste textiles, dissolving pulp before and after washing with dilute NaOH solution, and solid residue recovered from the washing water were analyzed for sodium, zinc, and chloride ion content.The samples subjected to elemental analysis were dissolved following a method for dissolution of lignocellulosic materials developed by Sluiter et al. (2011) with one exception: Instead of subjecting the samples to autoclavation, each sample was divided in two equally sized vessels and treated in a MARS 6 microwave digestion system (CEM, Matthews, NC, USA) at 120 °C for one hour following a ramp-up time of 15 min.The content of sodium and zinc in the samples were analyzed using an Optima 8300 Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) Optical System equipped with a Segmented-array Charge-coupled Device (SCD) detector (PerkinElmer, Waltham, MA, USA).The wavelengths used to detect sodium and zinc were 589.892 nm and 206.200 nm respectively.The chloride ion content of the samples was analyzed using an 861 Advanced Compact Ion Chromatography unit equipped with a Metrosep A Supp 5-150/4.0 column (Metrohm AG, Herisau, Switzerland).

Evaluation of recycling of ZnCl 2 •4H 2 O
During the experiment on recycling of ZnCl 2 , the dissolving power of the liquid (i.e. its capacity to produce dissolving pulp), and its color, was visually evaluated after each run.The viscosity of the liquid was measured after 5 and 10 experiments, and compared to the viscosity of the freshly prepared ZnCl 2 •4H 2 O. Any change in appearance or characteristics of the dissolving pulp after recycling the ZnCl 2 •4H 2 O once, five times, and nine times was monitored visually.

Viscose dope characterization
Samples of the viscose dope produced as described in Sect."Viscose process and the spinning process" were subjected to various analyses to determine the ball fall time, filterability, filter clogging value (K r ), gamma number (defined as the number of xanthonate groups per 100 anhydroglucose units), as well as the NaOH and cellulose content in the alkali cellulose and viscose dope.The methods of analysis and the calculations have been described in detail in the literature (Strunk 2012).

Mechanical properties and appearance of viscose staple fiber
Prior to analysis, 20 staple fibers were placed in a conditioned room with a temperature of 25 °C and a humidity of 50% for over 4 h.The staple fibers were analyzed with regard to their linear density (titer), elongation, and tenacity using a Vibroskop 400 and a Vibrodyn 400 (Lenzing Instruments GmbH & Co. KG, Gampern, Austria).The appearance of the viscose staple fibers was studied visually.

Results and discussion
Production of dissolving pulp from cotton waste textiles using ZnCl 2 •4H 2 O The macrostructure of the cotton waste textiles was completely disrupted after both low-and high-consistency ZnCl 2 •4H 2 O treatment.The result was a dissolving pulp with a loose and evenly distributed fibrous structure, and the dissolving pulp was easy to pull apart using minimal force (Fig. 2).The results of the viscosity measurements on the cotton waste textiles and the dissolving pulps showed that both kinds of ZnCl 2 •4H 2 O treatment had a significant depolymerizing effect on the cotton waste textiles, and the limiting viscosity number of dissolving pulp produced with both low-and high-consistency treatment was similar (Table 4).This similarity in viscosity indicates that ZnCl 2 •4H 2 O treatment is capable of transforming cotton waste textiles into dissolving pulp over a wide range of consistencies.The significant decrease in limiting viscosity number of the cotton waste textiles during treatment facilitated the subsequent conversion into viscose.In fact, the aim of the aging process in standard viscose production is to achieve a limiting viscosity number of the dissolving pulp of about 240 ml/g, corresponding to a degree of polymerization (DP) of approximately 300, before xanthation can take place (Strunk 2012).Due to the much higher DP of cotton cellulose compared to wood pulp cellulose (McLean and Allen 2000), the demand for decreasing the DP of cotton waste textiles in order to meet the target DP could be significantly higher compared to dissolving pulp made from wood, depending on the amount of wear and tear the waste textiles have been exposed to during their lifecycle (Gehmayr et al. 2011;Duan et al. 2016;Haslinger et al. 2019;Wedin et al. 2019).The limiting viscosity number of the dissolving pulp produced in this study (Table 4) was only slightly above the limiting viscosity number of 240 ml/g optimal for xanthation (Strunk 2012), providing a clear indication that there are synergies between the method to produce dissolving pulp proposed in the present study and the standard viscose process.Consequently, the pre-aging step during the viscose production in this study could be conducted at a lower temperature compared to the standard process (Strunk 2012), as the limiting viscosity number of the dissolving pulp barely required further adjustment.The process described in this paper is thus capable of producing dissolving pulp and adjusting the DP of the dissolving pulp simultaneously, without the need for a separate DP adjustment step before the production of viscose dope.
WAXS measurements showed a distinct pattern of crystalline cellulose I (French 2014) in the raw cotton waste textiles and the dissolving pulp, before and after washing with dilute NaOH (Fig. 3).However, the diffractogram for the dissolving pulp that had not been washed with dilute NaOH is different in the sense that the central part is elevated, as if there is an amorphous hump underneath.Amorphous scattering diffractograms have been observed experimentally for cellulose, going back to Ellefsen et al. (1957), and were recently discussed by French (2020).The peaks from crystalline cellulose remain but are smaller, also indicating the reduced amount of crystalline material.This suggests that the degree of cellulose crystallinity in the cotton waste textiles was decreased by treatment with ZnCl 2 •4H 2 O. Since fully dissolved cellulose precipitates as the more thermodynamically stable cellulose II upon regeneration (Donald 2001), it is unlikely that the cellulose was completely dissolved during ZnCl 2 •4H 2 O treatment.Our interpretation is that the de-crystallization during treatment was partial leaving crystalline cellulose I regions in the samples that seeded re-crystallization into cellulose I  upon precipitation and washing.Moreover, the crystallite size was found to be similar for cellulose I in the cotton waste textiles and in the dissolving pulp washed with NaOH, i.e., no significant changes were observed in the line widths of the crystalline signals in Fig. 3.Alternatively, the overall high signal intensity in the dissolving pulp sample before washing with dilute NaOH could originate from the presence of non-crystallizing monomeric and oligomeric sugars resulting from depolymerization by ZnCl 2 •4H 2 O treatment, which were removed by subsequent washing.In fact, the mass yield of dissolving pulp was 90%, which means that 10% of the material was lost during treatment with ZnCl 2 •4H 2 O.This explanation is supported by the fact that aqueous ZnCl 2 is acidic, and has been claimed to partially hydrolyze cellulose (Cao et al. 1995).Regardless of the reason behind the change in appearance in the diffractograms for dissolving pulp before and after washing with dilute NaOH, Fig. 3 shows that it is possible to obtain dissolving pulp with a cellulose I lattice after ZnCl 2 •4H 2 O treatment.While most of the ZnCl 2 is separated from the dissolving pulp during the precipitation and primary washing with water, residual ZnCl 2 was detected in the dissolving pulp following analysis of sodium, Fig. 3 WAXS diffractograms of cotton waste textiles, dissolving pulp produced with and without washing with dilute NaOH, and the solid residue recovered in the washing water (offset by 300, 200, 100, and 0 a.u.respectively) zinc and chloride ion content (Table 5).It was possible to decrease the content of residual ZnCl 2 in the dissolving pulp through more thorough washing (one washing step with dilute NaOH followed by one washing step with deionized water), but the washing efficiency was uneven with regards to zinc and chloride.While more than 95% of the chloride was removed during more thorough washing, almost 50% of the zinc remained in the washed dissolving pulp (Table 5).ZnCl 2 (and free zinc ions (Zn 2+ )) are known to react with aqueous alkali to produce various zinc containing species, of which several readily precipitates, depending on the concentrations of zinc and alkali.These species include zinc oxide (ZnO), zinc hydroxide (Zn(OH) 2 ), zincate (Zn(OH) 4 2− ), and Simonkolleite (Zn 5 (OH) 8 Cl 2 •H 2 O) (Cain et al. 1987;Cousy et al. 2017;Junuzović et al. 2020).Due to the multitude of possible reaction pathways, it is probable some occurred during the washing step with dilute NaOH.This would have allowed most of the chloride ions to wash off as sodium chloride (NaCl) while precipitating zinc-containing compounds remained with the washed dissolving pulp.Furthermore, zinc species such as Zn(OH) 4 2− have been shown to form strong hydrogen bonds with cellulose (Yang et al. 2011).Such interactions would contribute even further to the uneven removal of chloride and zinc from the dissolving pulp during the washing.
A distinct signal is visible at about 11° in the diffractogram of the dissolving pulp washed with dilute NaOH (Fig. 3).This corresponds closely to a prominent signal in the diffractogram obtained from the solid residue found in the washing water.This could indicate a compound/complex formed by the addition of NaOH which was not completely removed from the dissolving pulp by washing (Table 5).The signal could originate from the compound itself.Alternatively, the compound could, through interaction with cellulose, disrupt the crystal structure of the cellulose such that the disturbed crystal structure produced this response.
Further research is required to elucidate the interactions between ZnCl 2 •4H 2 O and cotton waste textiles, the effects on the dissolving pulp thus produced, and the effects of different washing conditions.Apart from providing insight into how ZnCl 2 •4H 2 O affects the structure and morphology of the materials; such studies could help in designing an efficient method for the separation of ZnCl 2 from the dissolving pulp.

Recycling of ZnCl 2
No deterioration was observed in the dissolving power of the ZnCl 2 •4H 2 O after repeated recycling.No changes in visual appearance or characteristics were observed between the dissolving pulps produced in different experiments during the recycling trials (Fig. 4).
However, the recycled liquid became more viscous as the experiments were repeated: the viscosity increasing from 4 mPa s for freshly prepared ZnCl 2 •4H 2 O to 13 mPa s after 10 experiments (Table 6).The recycled liquid steadily became darker in color over the course of repeated experiments (Fig. 5).This indicates that although the dissolving properties of the liquid remained unaffected, recycling led to accumulation of impurities in the liquid.These are probably cellulose degradation products, such as monomeric or oligomeric sugars, and their corresponding degradation products, resulting from the cellulose-degrading properties of aqueous ZnCl 2 (Cao et al. 1995).Removal of these impurities would be necessary in an industrial process.Increasing viscosity usually results in higher energy consumption, and the viscosity may exceed levels suitable for pumping and proper mixing.

Viscose production and fiber spinning
The viscose dope produced from the dissolving pulp made from cotton waste textiles had very similar properties to typical viscose dope produced from wood pulp (Table 7) (Barthelemy and Williams Table 5 The sodium, zinc and chloride ion content in cotton waste textiles, dissolving pulp produced before and after washing with dilute NaOH, and the solid residue recovered in the washing water Na (mg/g) Zn (mg/g) Cl (mg/g) 1945; Strunk et al. 2011).In particular, the combination of a moderate K r value and gamma number suggested that the dope was suitable for viscose spinning, without the need for any additives.Indeed, the dope could be spun into fine, white filaments which in turn were cut into staple fibers (Fig. 6), all without any complications.
The linear density (titer) of the fibers was slightly higher than the typical values for viscose fibers (Table 8).The elongation was also slightly higher, while the tenacity corresponded to values in the upper range for typical viscose fibers (Table 8).This means that high quality, albeit somewhat coarse, viscose fibers can be readily produced from dissolving pulp obtained by ZnCl 2 •4H 2 O treatment of cotton waste textiles.Other researchers have succeeded in spinning regenerated fibers directly from a solution of bacterial cellulose and ZnCl 2 •3H 2 O, but the resulting fibers exhibited very poor mechanical properties (Lu and    Shen 2011), in stark contrast to the fibers produced in the present study.In fact, should the spinning process be optimized, the quality of the fibers could be improved even further.
The precise overall yield of the process could not be determined in this study due to some practical issues.However, since the production of dissolving pulp had a mass yield of about 90%, and no major losses of material were observed during the production of the staple fibers, it could be expected that the material efficiency of this process would be sufficient for its commercialization.
The suitability and quality of a dissolving pulp intended for viscose fiber production depends on several factors related to the source of the dissolving pulp and the processes involved in producing it (Strunk et al. 2011).The fibers produced during this study were obtained with minor adjustments to the standard method for viscose dope preparation (Strunk 2012), and without additives or any significant optimization of the process.Nonetheless, the adjustments led to a decrease in consumption of carbon disulfide by 11%, compared to the standard process.Such a decrease is noteworthy, as carbon disulfide is a toxic pollutant and as such, a lowered consumption during viscose production is highly advantageous.The favorable fiber properties, and the fact that only minor adjustments were required to the standard viscose production method suggest the possibility of substituting current raw materials in viscose production with materials recycled from cotton waste via ZnCl 2 treatment.Furthermore, the process used to produce the dissolving pulp was rapid and could be operated at moderate temperatures (low energy consumption), and the solvent is cheap and exhibited excellent reusability.

Conclusions
The treatment of cotton waste textiles using ZnCl 2 •4H 2 O in the production of viscose proved to be successful.ZnCl 2 •4H 2 O is a very stable liquid with cellulose-dissolving capabilities very similar to those of ionic liquids, and was shown to be easy to recover and recycle, at least nine times.The similar limiting viscosity number of dissolving pulp produced at low and high consistencies indicates that ZnCl 2 •4H 2 O is capable of transforming cotton waste textiles into dissolving pulp over a wide range of consistencies.The dissolving pulp produced had the same crystal lattice as native cellulose (cellulose I lattice) and was shown to possess properties such as limiting viscosity number and chemical accessibility suitable for viscose manufacturing.High-quality viscose staple fibers were spun using the dissolving pulp as raw material, with a decrease in carbon disulfide consumption by 11% compared to when using dissolving pulp made from wood.The benefits and opportunities of using ZnCl 2 •4H 2 O in the dissolution step in viscose production from cotton waste textiles suggest that further studies should be carried out.

Fig. 2
Fig. 2 Cotton waste textiles before treatment (a) and the resulting dissolving pulp after low-consistency treatment with ZnCl 2 •4H 2 O (b)

Fig. 4
Fig. 4 Dissolving pulps produced from cotton waste textiles after recycling of the ZnCl 2 •4H 2 O once (a), five times (b) and nine times (c)

Fig. 6
Fig. 6 Viscose staple fibers produced from cotton waste textiles-derived dissolving pulp

Table 1
Addition of cotton waste textiles to the ZnCl 2 •4H 2 O in the high-consistency treatment

Table 2
Spin bath composition

Table 3
Fibre washing scheme, which chemicals for each washing step, concentrations, temperature, residence time and their functions Vol.: (0123456789)

Table 4
The limiting viscosity number of cotton waste textiles and dissolving pulps obtained with low-and high-consistency ZnCl 2 •4H 2 O treatment Vol.: (0123456789)

Table 6
Viscosity of fresh and recycled ZnCl 2 •4H 2 O. Viscosity was measured at 80 °C

Table 7
Properties of the viscose dope produced from cotton waste textiles-derived dissolving pulp in the present study and typical viscose dope derived from wood pulp(N.Hollinger, personal communication, December 27, 2021) a AC = Alkali cellulose Vol:.(1234567890)

Table 8
Mechanical properties of viscose staple fibers obtained in this study and values for the mechanical properties typical of viscose staple fibers(N.Hollinger, personal communication, December 27, 2021)