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Article

Towards Sustainable Viscose-to-Viscose Production: Strategies for Recycling of Viscose Fibres

by
Diana Carolina Reyes Forsberg
1,
Jenny Bengtsson
2,*,
Nadine Hollinger
1 and
Tahani Kaldéus
3
1
MoRe Research AB (RISE), Box 70, 891 22 Örnsköldsvik, Sweden
2
Fiber Development, Department of Polymer, Fiber and Composites, RISE Research Institutes of Sweden, Argongatan 30, 431 53 Mölndal, Sweden
3
The Loop Factory AB, Industrivägen 6B, 432 40 Varberg, Sweden
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(10), 4127; https://doi.org/10.3390/su16104127
Submission received: 12 April 2024 / Revised: 7 May 2024 / Accepted: 11 May 2024 / Published: 15 May 2024
(This article belongs to the Topic Advances in Sustainable Materials and Products)
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Abstract

:
The potential for using discarded viscose textiles to produce high-quality viscose fibres is limited by the low molecular weight of the cellulose and its continued reduction in the recycling process. Herein, we present a straightforward approach of reprocessing discarded viscose textiles while achieving high-quality recycled viscose fibres. Discarded viscose textile was defibrated and centrifuged, and the resulting fibres were reprocessed under industrially relevant conditions. The produced viscose dope was fluid and resulted in viscose fibres with properties comparable to fibres made from commercial wood cellulose pulp (titer ~2 dtex; dry elongation ~16%, dry tenacity ~15 cN/tex). To explore the potential for a more environmentally friendly production process, the steeping step was performed twice (double-steeping), thereby producing a more homogeneous viscose dope. Through double-steeping, the consumption of carbon disulfide (CS2) could be reduced by 30.5%. The double-steeping method shows to be a suitable approach to reprocess discarded viscose textiles while reducing the environmental impact of the viscose process associated with the use of CS2. Our work demonstrates that discarded viscose textile has the potential to be part of a circular textile value chain.

1. Introduction

The textile industry is facing several sustainability challenges. Within this decade, it needs to transform its linear value chain to a circular one that champions a bio-based circular economy. It is a highly resource-intensive sector. Within the E.U., the textile industry is one of the top pressure categories for primary raw material and water usage, contributing to resource depletion, and textile production ranks fifth in greenhouse gas emissions, exacerbating the issue of climate change [1]. Moreover, the fashion industry has a history of encouraging fast fashion, leading to a culture of disposability with alarmingly low textile recycling rates, both in the E.U. and on a global scale. This results in vast quantities of waste being sent to landfills or being incinerated, and less than 1% of textiles are recycled into new textiles [2,3]. To address these challenges, the textile industry has started to adopt some sustainable practices, including the increased use of eco-friendly materials [4], improved manufacturing processes [5,6], and encouraging recycling and upcycling [7]. However, establishing effective recycling and circular economy systems for textiles is a challenge. Many textiles are difficult to recycle due to blended fibre textiles and complex chemical treatments.
For the past decade, substantial effort has been made to research how to reshape the textile industry and identify possibilities and challenges using cotton-based textiles in a circular value chain [8]. From an upscaling point of view, textile-to-textile fibre recycling has, in many cases, been the main goal of textile waste recycling, and several technologies and processes have been evaluated on how to obtain a high-quality recycled fibre from cotton [9,10,11].
Some of these technologies have evolved into commercial products. As late as 2023, the first commercial large-scale production of recycled cotton textiles was set up [12]. There are currently many projects and start-ups focusing on using cellulose pulp originating from cotton, wood, or agricultural waste to produce new textile fibres [13,14,15]; however, little to no effort has been directed towards utilising man-made cellulose fibres (MMCF), such as viscose fibres, as a recycling source. This is not without reason; viscose fibres are challenging to recycle due to the low molecular weight of the cellulose [16] and the continued reduction in the molecular weight in a recycling process, which hampers the utilisation of viscose fibres for high-quality recycled viscose fibres.
MMCF has been used for over 100 years to make textiles, with the primary process being the viscose process, which employs wood as the raw material [17]. The viscose process is a considerably versatile process, where changing process parameters or auxiliaries in the different steps will render viscose fibres of different properties, similar to natural fibres like cotton, which can be used in various textile applications [8,18,19,20]. The viscose process consists of roughly four main process steps, which, in simple terms, can be described as follows: formation of alkali cellulose by treating the cellulose with high concentrations of sodium hydroxide, ageing of the alkali cellulose to degrade it to achieve the desired degree of polymerisation, forming of sodium-cellulose xanthate by adding carbon disulfide and, finally, dissolution into diluted sodium hydroxide solution to form a viscous, orange-yellow solution known as cellulose-xanthate (viscose dope). The viscose dope is then extruded through spinnerets into a coagulating bath containing sulfuric acid. This bath initiates the regeneration process, converting the viscose dope into solid fibres. The fibres are further processed after being washed and neutralised to enhance their properties.
Despite its widespread use, the viscose process involves using chemicals like carbon disulfide in the xanthation step, which poses environmental and health concerns [21], necessitating careful management and adherence to safety protocols in manufacturing facilities. The efficiency of the xanthation, and thus the need for carbon disulfide in the viscose process, depends on the accessibility of the cellulose hydroxyl groups, which is partially set during the alkalisation of the cellulose in the steeping of the viscose process [18]. Different techniques have been applied to enhance the cellulose reactivity in wood pulp: enzymatic [22], irradiation [23], and beating [24]. The reactivity is also dependent on the type of raw material used [25,26] and may thus be altered when shifting from wood-based to textile raw materials.
Another important aspect of the viscose process is the predicted increased competition for wood used in the future [27]. Furthermore, the production of pulp for the viscose process makes up a significant share of the total energy demand in viscose fibre production [28]. A recent life cycle assessment of textile recycling via the cellulose carbamate process pinpointed the benefit of using a recycled material, specifically a post-consumer textile, instead of a primary in MMCF production [29]. Hence, there is an increasing need to use recycled material to produce viscose fibres.
In this work, we demonstrated the feasibility of reprocessing discarded pre-consumer viscose textiles in the viscose process. Moreover, we investigated how the viscose process may be adjusted when discarded viscose fibres are used as material input. Special attention was given to improving the sustainability of the process without foregoing the quality of the recycled fibres. Our approach involves utilising a double-steeping protocol that enables the reduction in carbon disulfide by improving cellulose accessibility without significant cellulose degradation. Additionally, this approach facilitates the removal of low-molecular-weight material that may consume carbon disulfide [25,30].

2. Materials and Methods

2.1. Materials

Pre-consumer undyed viscose fabric was provided by Imogo AB, Sweden. The pre-consumer undyed viscose had a viscosity = 166 mL g−1 (ISO 5351:2010 [31]), R 18 = 94.6% (ISO 692:1982 [32]), and a content of impurities, i.e., unhydrolysed synthetic fibres, of 0.34% T.S. (Tappi T222).
To prepare sodium hydroxide solutions (NaOH) of 18% and 12%, a stock solution of 30% NaOH (VWR chemicals, Suwanee, GA, USA) was diluted with Milli-Q H2O (Grade 1 purity water according to ISO 3696:1987 [33]). Additionally, a 4% NaOH stock solution was used together with Milli-Q H2O to dilute the concentration of NaOH in the sodium cellulose xanthate until it reached a concentration of 5%. All NaOH solutions were kept in vessels protected from atmospheric CO2. Deionised H2O was used to wash the mercerised samples. Liquid carbon disulfide (CS2, assay ≥ 99.9%, Honeywell, Charlotte, NC, USA) was used to produce sodium cellulose xanthate. The spin bath was composed of sulfuric acid 95–97% (H2SO4, Supelco, Bellefonte, PA, USA), sodium sulphate anhydrous (Na2SO4, assay ≥ 99.6%, WVR chemicals), and zinc sulphate heptahydrate (ZnSO4 · 7H2O, assay ≥ 100%, WVR chemicals).

2.2. Pretreatment of Pre-Consumer Undyed Viscose Fabric

Prior to viscose pilot testing, the pre-consumer viscose fabric was shredded using an 8 mm mesh at The Loop Factory, Varberg, Sweden. The shredded material was then diluted to 3% with deionised water and mechanically agitated at 3000 rpm with a retention time of 1 min. Following agitation, the samples were dewatered using a Heine centrifuge at MoRe Research, Örnsköldsvik, Sweden.

2.3. Viscose Dope Preparation and Spinning

Viscose dope preparation was carried out at the viscose pilot located at MoRe Research, according to Treiber 1962 [34]. Viscose dope was prepared using 22 g dry weight of shredded and defibrated pre-consumer material, here referred to as shredded textile. The shredded textile was mixed with 18% NaOH for 30 min at 50 °C, having a pulp consistency of 2.5%. After alkali treatment, the sample was pressed in a hydraulic press to remove excess NaOH and low-molecular-weight material that could consume carbon disulfide (CS2). Carbon disulphide is known to be consumed in the primary reaction to produce sodium cellulose xanthate and through side reactions, with, for instance, free NaOH and low-molecular-weight material [7,30,35,36].
After pressing, the composition of the alkali cellulose content, i.e., % cellulose and % NaOH, was calculated. For single-steeping, denoted as S.S., the pressed alkali sample was brought into a shredder for 30 min at 50 °C. This step was performed to open up the pressed alkali cellulose and facilitate the penetration of CS2 in the xanthation step [7].
For double-steeping, denoted as D.S., the steeping step was performed once again. Hence, the pressed alkali sample was gently divided into smaller pieces and immersed in 12% NaOH for 5 min. The experimental conditions were selected since 18% NaOH allows the material to swell and transform into alkali cellulose [6,37], while the use of 12% NaOH allows for decreasing the total amount of NaOH in the system and further increasing the swelling of the material, which facilitates additional removal of low-molecular-weight material during the pressing step [7]. After the second steeping step, i.e., the D.S. step, the sample was pressed and shredded, similar to S.S.
Carbon disulfide was added to the shredded alkali cellulose to produce sodium cellulose xanthate [17]. The xanthation was carried out at 32 °C for 150 min, and the concentration of the CS2 was varied between 19% and 36% (see Table 1). A total of 36% CS2 was chosen since it is typically used as a standard concentration in the viscose industry [35,38]; the lower amounts were chosen to challenge the convention. The reaction vessel was kept under vacuum during the xanthation. The obtained sodium cellulose xanthate was then transferred into the dissolution unit, caustic solution was added to target a viscose dope containing ~5% NaOH and ~9.5% cellulose, the temperature was decreased to 7 °C, and the sample was stirred for 3 h. Thereafter, the viscose dope was allowed to ripen for 16 h at room temperature to improve the spinnability of the dope. The described viscose process is depicted in Figure 1.
The viscose dope was extruded through a spinneret into a spinning bath to form rayon filaments. Detailed information on the composition of the spinning bath can be found in Table S1. The temperature was 50 °C. The spinneret contained a total of 90 holes, and each had a diameter of 80 µm. During the spinning process, the pump throw was set to 5 mL/min, rendering an extrusion speed of 11.1 m/min. The first godet speed was set at 18.8 m/min, and the second godet speed was set at 29.9 m/min, resulting in a draft in the coagulation bath of 70% following a 59% stretch in air at RT, in accordance with Equation (1):
S t r e t c h   % = V 2 V 1 − 1 * 100
where V1 represents the speed of extrusion or godet 1 (m/min) and V2 represents the speed of godet 1 or godet 2 (m/min), respectively.
After the spinning, the rayon filaments were removed from godet 2 and subjected to a washing sequence to ensure thorough cellulose regeneration and complete removal of any chemicals or by-products. First, the filaments were washed with sulfuric acid (18 g/L) at 90 °C and rinsed with deionised water. Second, the filaments were washed with a 1 g/L sodium hydroxide solution at 60 °C and, last, rewashed with deionised water. The mechanical properties of the rayon filaments, such as linear density, tenacity, and elongation, were measured according to the method SS-EN ISO 5079:2020 [39].

2.4. Filtration Test

The processability of a pulp in the viscose process is determined by the filtration test, where the filter clogging value is calculated. In the present work, the filter clogging value was determined according to Treiber 1962 [34] and modified by using a sheet of steel filter. The filter clogging value, Kw, was calculated according to Equation (2):
Kw = 2 × t 2 M 2 - t 1 M 1 × 10 4 t 2 − t 1
where t1 and t2 represent the filtration time in minutes for 20 min and 60 min, respectively. M1 and M2 refer to the amount of viscose dope filtered in grams during the first 20 min and 60 min, respectively. The calculated Kw value was then adjusted for viscosity to report the reduced filter clogging value, Kr, using Equation (3):
K r = K w ƞ 0.4
where ƞ refers to the ball fall time in seconds, determined by allowing a steel ball of 1/8 inch diameter to fall a distance of 200 mm in a glass tube of 20 mm diameter filled with viscose dope, and measuring the time of the fall.

2.5. Gamma Number

The gamma number indicates the degree of substitution in the anhydrous glucose units (AGU) by measuring the number of xanthogenate groups per 100 AGU. The AGU has three functional OH- groups, which allow a maximum achievable degree of substitution of 3, corresponding to a gamma number of 300. Prior to gamma number measurements, 1 g of viscose dope was diluted in 50 mL of 1% NaOH solution and stirred for 30 min. The solution was then transferred to a 100 mL volumetric flask that was filled with 1% NaOH solution up to the graduation mark. After shaking, 5 mL of the solution was transferred into a bottle containing 1.8 g of ion exchange resin (Amberlite, IRA 402) and allowed to react with the resin for 10 min. The solution was then filtered into a 250 mL volumetric flask. The flask was then filled with deionised water up to the graduation mark and shaken. The gamma number measurements were performed using a CARY 100Scan UV/visible spectrophotometer at a wavelength of 303 nm. A 1 cm quartz cuvette containing deionised water was used as the reference, and the gamma number was calculated according to Equation (4):
Υ = A × 546.8 B × C
where A represents the absorbance measured at 303 nm, B refers to the amount of viscose dope in g, and C represents the cellulose content in the viscose dope in wt%.

2.6. Determination of Caustic

The caustic content of pressed alkali cellulose and viscose dope samples was measured using the method described by Strunk 2012 [40]. The samples were diluted with deionised water and mixed with 0.5 M H2SO4 (1N). After mixing, phenolphthalein indicator drops were added, and the caustic content was determined by titration with 1.0 M NaOH (1N). The caustic content was then calculated using Equation (5):
C a u s t i c   c o n t e n t   ( % ) = a 1 × n 1 − a 2 × n 2 × 100 × 40 G × 1000
The terms a1 and a2 refer to the volume in millilitres of H2SO4 and NaOH, respectively, while n1 and n2 denote the normality of H2SO4 and NaOH, respectively. The variable G represents the amount of the sample, which can either be alkali cellulose or viscose dope.

2.7. Determination of Cellulose in the Alkali Cellulose and Viscose Dope

After determining the amount of caustic in the pressed alkali cellulose, the process of determining the cellulose content continued. To neutralise the pinkish solution obtained in Section 2.6, a few drops of H2SO4 were added, and the resulting liquid was filtered to collect the insoluble compounds. The insoluble compounds were then washed with deionised water, followed by a wash with acetone, and dried in an oven at 105 °C. Finally, the content of cellulose in the alkali cellulose was calculated according to Equation (6):
C e l l u l o s e   c o n t e n t   i n   a l k a l i   c e l l u l o s e   ( % ) = W e i g h t   o f   s a m p l e   a f t e r   o v e n   d r i e d   ( g ) I n i t i a l   w e i g h t   o f   p r e s s e d   a l k a l i   c e l l u l o s e   ( g )
To calculate the amount of cellulose content in the viscose dope, 1.5 g of the viscose dope was pressed between two glass plates. One of the plates was removed, and the remaining plate with the dope film was kept in a hydrochloric acid solution until the film loosened. The resulting film was then rinsed with deionised water and dried using a filter cloth. Then, the film was washed with acetone for 3 min followed by oven-drying at 105 °C until it reached constant weight. The cellulose content was then calculated using Equation (7):
C e l l u l o s e   c o n t e n t   i n   v i s c o s e   d o p e   ( % ) = W e i g h t   o f   s a m p l e   a f t e r   o v e n   d r i e d   ( g ) W e i g h t   o f   v i s c o s e   d o p e   ( g )

2.8. Particle Distribution in Viscose Dope

The particle size distribution of unfiltered viscose dope was analysed using the Beckman Coulter Multisizer 4, equipped with an aperture tube size of 280 µm. A total of 5 g of unfiltered viscose dope was dissolved in 200 mL of electrolyte solution (0.1 M NaOH). The sample was stirred for 15 min to ensure homogeneity before measurements. The particle size was measured in the range of 5.6 µm–168 µm.

2.9. Intrinsic Viscosity

The intrinsic viscosity of the shredded textile samples was determined using the standard ISO 5351:2010 [31]. The samples were weighed and then dispersed in water before dissolution in a 1 M cupri-ethylenediamine (CED) solution. The dissolved sample in CED was analysed by timing the flow through a capillary tube viscometer (Viscomat II) using a shear rate of 200 ± 10 s−1.

2.10. Chemical Oxygen Demand

Chemical oxygen demand (COD) analysis was conducted using a Lange COD cuvette tube test (150–1000 mg/L O2) and measured in the HACH LANGE DR 2800 spectrophotometer. To prepare samples, the first filtrate of the pressing step of the alkali cellulose was diluted with Milli-Q H2O until a dilution factor of 20. Next, 2 mL of the diluted sample was transferred into the test tube, which was then heated at 148 °C for 2 h. Following this, the sample was allowed to cool before measurements.

2.11. Molecular Weight Distribution

The molecular weight of shredded textile and viscose dope samples was determined by size exclusion chromatography (SEC) using an Agilent PL-GPC 220 with a refractive index detector. Three 20 µm Mixed-A columns (Polymer lab) were used, one guard column and two 30 cm columns in series. DMAc eluent containing 0.5% (w/v) LiCl was used at a flow rate of 1 mL/min and at 70 °C. A total of 25 mg dry weight of the sample was subjected to solvent exchange with methanol and DMAc, followed by dissolution in 8% (w/v) LiCl in DMAc. Samples were diluted to a concentration of 1.6% (w/v) and filtered before analysis. In the case of the shredded textile samples, a pre-swelling step with DMSO was performed before the solvent exchange, as described by Siller 2014 [41]. The pre-swelling step was performed to improve the swelling and solubility of the textile samples.

3. Results and Discussion

The undyed viscose textile was shredded and defibrated to open up the fabric and make it more accessible for viscose processing (see Section 2.2). The mentioned pretreatment showed no noticeable degradation of the material, as sample viscosity (viscosity of 166 mL/g) was consistent with earlier reported viscosity values for viscose textile fabric [42]. The molecular weight distribution of shredded textiles showed an inclination towards lower molecular weight and narrower polydispersity, which agreed with the low viscosity value (see Supplementary Materials Figure S1). After pretreatment, the processability of the shredded textile in the viscose process was evaluated using two different approaches, a single-steeping process (S.S.) and double-steeping (D.S.), as described in Section 2.3. The shredded textile was used directly after pretreatment in the viscose process, without any additional chemical treatment, such as bleaching, to reduce the environmental impact.

3.1. Viscose Production via Single-Steeping

The viscose dope produced using standard viscose process conditions, i.e., an S.S. step and 36% CS2 in the xanthation step, showed a similar process behaviour as normally observed in commercial dissolving cellulose pulp (DCP). After S.S. and pressing, the NaOH content in the alkali cellulose cake decreased to the expected concentration of 15% (see Table 2) and the COD in the filtrate was 2070 mg/L. However, the cellulose content in the alkali cellulose cake was slightly lower than the typical values seen in commercial DCP (see Table 2). Our explanation is the low viscosity of the shredded textile compared to commercial DCP, as low viscosity material most likely leads to higher losses of low-molecular-weight material that could pass through the filter while pressing the highly swollen alkali cellulose. This is expected since the high swelling of cellulosic fibres during alkali treatment facilitates the dissolution of low-molecular-weight material [43]. After pressing, the alkali cellulose cake was shredded, reacted with 36% CS2, and, lastly, dissolved to produce viscose dope. Note that in viscose processing, shredded alkali cellulose from, e.g., wood pulps, usually undergoes an ageing step to reduce cellulose molecular size and achieve a viscose dope of correct viscosity [44]. However, based on initial trials, no ageing was needed for the studied samples as the viscose textile had low viscosity and a narrow molecular weight distribution compared to, e.g., dissolving wood pulp (Figure S1). Importantly, by removing the ageing step, the overall processing time was reduced significantly. After the dissolution of the sodium cellulose xanthate, the obtained viscose dope was liquid-like, light-orange coloured, and slightly transparent (see Figure 2, sample SS-CS2/36). The produced dope had a narrower polydispersity than the starting material, i.e., shredded textile, with a slight shift towards a lower molecular weight, indicating minor cellulose degradation (see Figure 3).
In the viscose process, the Kr value is an important parameter used to evaluate pulp reactivity [30,38,45] and assess the suitability of the viscose dope for spinning [38]. Low Kr values are preferred to avoid clogging of the spinneret. Despite the relatively high Kr value obtained, the produced viscose dope still fell within the range of processable viscose dope for spinning, as indicated in Table 2. Another important quality parameter to assess the viscose dope by is the degree of substitution, i.e., the gamma number [38,46]. Typically, commercial DCP values between 30 and 45 are expected for a gamma number under the studied conditions [47], as shown in Table 2. In this study, a gamma number of 40 was achieved for the produced viscose dope. Collectively, these data indicate that shredded pre-consumer viscose textile material is suitable for reprocessing in the viscose process and further spinning. It is acknowledged that complications may arise if using post-consumer or dyed material, which remains to be further investigated.

3.2. Viscose Production via Double-Steeping

In an effort to not only encourage circularity in the viscose process but also minimise the environmental impact caused by the use of carbon disulphide, we conducted an evaluation of the steeping process and its effect on the consumption of CS2. We compared the S.S. and D.S. methods to determine which method allows the reduction in CS2 without compromising the quality of the viscose dope. The shredded textile was steeped using both methods, and the obtained alkali cellulose was processed in the xanthation step, using only a limited amount of CS2 (19%).
For the S.S. process, no changes in the alkali cellulose properties were expected as the steeping process was performed exactly as in the previous sample (Sample SS-CS2/36, Section 3.1). However, reducing the amount of CS2 in the xanthation step from 36% to 19% led to a significant decrease in the quality of the viscose dope. The Kr value greatly increased, to 4516, and the gamma number dropped to 23, as summarised in Table 2. Furthermore, a significant increase in the number of total particles in the viscose dope was observed, reaching a value of 171,900, previously being 33,434 (see Figure 4). As a result, the viscose dope could not be processed for spinning.
For the D.S. process, the first steeping stage underwent the same conditions as earlier samples: 18% NaOH, 50 °C, and a 30 min retention time, thus giving similar alkali cellulose properties after pressing (15% NaOH and 27% cellulose in alkali cellulose cake). The alkali cellulose cake was then subjected to a second steeping step with 12% NaOH for 5 min at 50 °C to reduce the concentration of NaOH in the system and further swell the sample for the additional removal of low-molecular-weight material during pressing [7,30]. Following the second pressing, the concentration of NaOH in the sample was reduced to approximately 12% and the cellulose content was measured to be 25% (see Table 2). The COD of the second filtrate was 1470 mL/g. It is worth noting that the double-steeping process did not significantly reduce sample viscosity. The initial viscosity of the raw material was 166 mL/g and, after D.S. and shredding, 161 mL/g, thus indicating no significant degradation of the material due to an additional steeping step.
The alkali cellulose cake was then shredded and reacted with 19% CS2 in the xanthation step followed by its dissolution to obtain a viscose dope. The viscose dope produced via D.S. showed a similar molecular weight distribution and gamma number as the viscose dope produced via S.S. with 19% CS2 (see Figure S2 and Table 2, respectively). Similar gamma numbers were expected as the gamma number relates to the degree of substitution, i.e., the degree of xanthation in the AGU [44,46]. The S.S. and D.S. samples were subjected to steeping conditions that allowed the conversion to alkali cellulose [37,48] and were treated with the same concentration of CS2. Consequently, obtaining similar gamma numbers was a reasonable outcome. Possibly, CS2 may be the limiting reactant, as our data clearly showed a correlation between CS2 concentration and gamma number, as shown in Figure 5. Therefore, lowering the CS2 concentration was found to decrease the degree of substitution.
The Kr value for the viscose dope produced via D.S. was half compared to the sample produced via S.S. (refer to Table 2 and Figure 5). This may appear surprising at first since both S.S. and D.S. samples showed similar gamma numbers. Similar discrepancies between pulp reactivity and gamma numbers have also been observed when softwood cellulose pulp pretreated with enzymes improved pulp reactivity, according to Fock’s test, but did not improve gamma numbers [45,46]. Östberg (2013) pointed out that there appears to be a distinction between the chemical reactions at the molecular level that result in a specific gamma value and those at a fibre level, i.e., the degree to which pulp reacts with chemicals that lead to a particular Fock reactivity or Kr value [45]. Hence, although the use of D.S. did not increase the gamma number, it significantly improved the reactivity of the shredded textile. In agreement with enhanced reactivity, shredded textile processed through D.S. showed a much lower total number of particles in the viscous dope (65,259) and appeared to be more fluid compared to the dope produced through S.S. (see the distribution of particles in the viscose dope in Figure 4). Based on these data, it implies that using the D.S. approach could result in a more homogenous and better-quality viscose dope and potentially help to reduce the amount of CS2 used in the xanthation step, as noted earlier by Sihtola, H. 1969 [30]. Additionally, these results suggest that the D.S. method might be suitable for reprocessing discarded textile material.
To further improve the quality of the viscose dope and keep the levels of CS2 low, an additional viscose dope was produced using double-steeping and a slightly higher concentration of CS2 (25%). The results showed that by using the D.S. approach and slightly increasing the CS2 concentration, the gamma number increased and the overall properties of the viscose dope improved significantly compared to the samples produced with 19% CS2 (see Table 2). The obtained viscose dope was fluid, with good filterability (Kr value of 1025) and a total number of particles of only 40,989. Although the viscose dope produced using D.S. and 25% CS2 showed a lower gamma number compared to the sample produced using 36% CS2, most likely due to a lower concentration of CS2, the obtained cellulose xanthate was soluble and the overall viscose properties such as Kr value, number of particles, and molecular weight distribution were similar to the sample produced using 36% CS2, making it suitable for the spinning process.
As found in this study, increasing the concentration of CS2 increased the gamma number. However, such an increase in CS2 concentration also seemed to favour by-product formation. Strunk pointed out that the orange colour in the viscose dope was most likely due to the formation of the by-product sodium trithiocarbonate [40]. In our samples, it was clear that, as the concentration of the CS2 increased, the intensity of the orange colour in the sodium xanthate also increased, as seen in Figure 2. The consumption of CS2 and by-product formation are unfavourable from an environmental and economic point of view due to the toxicity of CS2 and the amount of chemicals consumed during the process.
Collectively, our data suggest that by employing a D.S. protocol at the studied process conditions, the consumption of CS2 could be reduced by 30.5%, promoting both circularity when discarded textile is used and reducing the environmental impact due to the reduction in CS2 consumption.

3.3. Properties of Viscose Fibres

Fibres were spun from the samples SS-CS2/36 and DS-CS2/25, which displayed the most suitable properties for smooth spinning, including the lowest Kr values and the total amount of particles (see Table 2 and Figure 5, respectively). The mechanical properties of the spun fibres are summarised in Table 3. As may be concluded, both dopes, including the one with a lowered CS2 charge, produced viscose fibres with mechanical properties not inferior to comparable fibres made from dissolving wood pulp, i.e., primary material. This indicates that discarded viscose textiles are suitable for being included in a circular textile value chain. Photographs of the spinning and fibres produced from the DS-CS2/25 are displayed in Figure S3.
The marginal difference in elongation at break for the two different samples could be a result of the minor shift in MWD of the cellulose (Figure 3), although the link between MWD and elongational properties of the spun fibres is complex, and the trend may differ depending on the spinning process used [49,50]. Fibre elongation is also known to be sensitive towards defects within the fibre. From the particle distribution analysis of the dopes (Figure 4), it can be observed that the SS-CS2/36 contained somewhat fewer particles than the DS-CS2/25, which may explain the lowered elongation for the latter, since particles, i.e., undissolved fragments, act as weak points within the fibre.

4. Conclusions

The textile industry is currently dealing with sustainability challenges and is working towards transitioning to a circular value chain. One potential opportunity lies in utilising pre-consumer viscose textiles as input to the viscose process without additional chemical pretreatment. This approach could minimise chemical consumption by eliminating bleaching sequences. Instead, mechanical pretreatment, such as defibration followed by centrifugation, is a promising approach to making the material more accessible for viscose reprocessing without compromising the already low-viscosity material. As viscose textile fabrics have low intrinsic viscosity, the ageing step could consequently be removed and additional degradation of the material could be avoided. The mechanically pretreated material was suitable for direct reprocessing and showed satisfactory filter clogging values, enabling the spinning of produced dope. By applying a double-steeping protocol, the CS2 charge during xanthation could be lowered with minimal impact on viscose dope properties and with improved homogeneity. Viscose dope produced using pre-consumer viscose textile showed smooth spinnability and resulted in viscose fibres with properties comparable to fibres made from commercial cellulose pulp, even with reduced CS2 when using a double-steeping approach. We demonstrated the potential for discarded viscose textiles to be integrated into a circular textile value chain. Additionally, by employing double-steeping, it is possible to promote circularity in the viscose process (viscose to viscose) and reduce the environmental impact associated with CS2 used.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/su16104127/s1: Figure S1: The molecular weight distribution of shredded textile and reference dissolved cellulose pulp for comparison; Table S1: Composition of the spinning bath; Figure S2: Molecular weight distribution curve of shredded textile and viscose dope produced via single-steeping using 19% CS2 in the xanthation step (SS-CS2/19) and viscose dope produced using double-steeping and using 19% CS2 in the xanthation step (DS-CS2/19); Figure S3. Viscose filaments produced using double-steeping and using 25% CS2 in the xanthation step.

Author Contributions

Conceptualisation, T.K., J.B. and D.C.R.F.; methodology, investigation, and analysis, D.C.R.F. and N.H.; writing—original draft preparation, T.K. and D.C.R.F.; writing—review and editing, J.B. and T.K. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge BioInnovation and Vinnova (2021-04404) for financial support.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to acknowledge the project CITEX: Chemical recycling of MMCF, for great collaboration.

Conflicts of Interest

Authors Diana Carolina Reyes Forsberg and Nadine Hollingerwas employed by the research institute MoRe Research AB (part of RISE). Author Tahani Kaldéus was employed by the company The Loop Factory AB. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Sustainability 16 04127 g001
Figure 1. Schematic representation of the viscose process. S.S. refers to single-steeping and D.S. refers to double-steeping.
Figure 1. Schematic representation of the viscose process. S.S. refers to single-steeping and D.S. refers to double-steeping.
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Figure 2. Produced sodium cellulose xanthate (a) after the xanthation process and (b) after the dissolution in caustic soda.
Figure 2. Produced sodium cellulose xanthate (a) after the xanthation process and (b) after the dissolution in caustic soda.
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Figure 3. Molecular weight distribution curve of raw material (shredded textile) and viscose dope produced via single-steeping using 36% CS2 in the xanthation step (SS-CS2/36); viscose dope produced using double-steeping and using 25% CS2 in the xanthation step (DS-CS2/25).
Figure 3. Molecular weight distribution curve of raw material (shredded textile) and viscose dope produced via single-steeping using 36% CS2 in the xanthation step (SS-CS2/36); viscose dope produced using double-steeping and using 25% CS2 in the xanthation step (DS-CS2/25).
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Figure 4. Particle distribution of unfiltered viscose dope samples produced using single-steeping (S.S.) and double-steeping (D.S.) and at different concentrations of carbon disulfide (CS2).
Figure 4. Particle distribution of unfiltered viscose dope samples produced using single-steeping (S.S.) and double-steeping (D.S.) and at different concentrations of carbon disulfide (CS2).
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Figure 5. Comparison between carbon disulfide content for single-steeping (S.S.) and double-steeping (D.S.) samples versus both reduced filter clogging value, depicted as Kr value (bars, left-hand y-axis) and gamma number (%, dots, right-hand y-axis).
Figure 5. Comparison between carbon disulfide content for single-steeping (S.S.) and double-steeping (D.S.) samples versus both reduced filter clogging value, depicted as Kr value (bars, left-hand y-axis) and gamma number (%, dots, right-hand y-axis).
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Table 1. Overview of carbon disulfide, CS2, used in the xanthation step.
Table 1. Overview of carbon disulfide, CS2, used in the xanthation step.
SamplesLabelCS2 (%)
Single-steepingSS-CS2/3636
SS-CS2/1919
Double-steepingDS-CS2/2525
DS-CS2/1919
Table 2. Properties of viscose dope produced from pre-consumer undyed viscose textile and reference range of viscose properties of commercial dissolving cellulose pulp produced at the MoRe Research viscose pilot lab for comparison. (*) Indicates the alkali cellulose properties after first steeping and pressing.
Table 2. Properties of viscose dope produced from pre-consumer undyed viscose textile and reference range of viscose properties of commercial dissolving cellulose pulp produced at the MoRe Research viscose pilot lab for comparison. (*) Indicates the alkali cellulose properties after first steeping and pressing.
SamplesIntrinsic
Viscosity, mL/g		Alkali CelluloseViscose Dope
NaOH, %Cellulose, %KrNaOH, %Cellulose, %Gamma Number, %
SS-CS2/361661527111151040
DS-CS2/2515 *27 *
122510255933
SS-CS2/191526451651023
DS-CS2/1915 *27 *
1223217651024
Typical range, dissolving cellulose pulp400–60015–1629–36100–12004.0–4.59.4–10.530–45
Table 3. Properties of viscose fibres produced from pre-consumer undyed viscose textile, together with fibres from commercial dissolving cellulose pulp as raw material produced at the MoRe Research viscose pilot lab for comparison.
Table 3. Properties of viscose fibres produced from pre-consumer undyed viscose textile, together with fibres from commercial dissolving cellulose pulp as raw material produced at the MoRe Research viscose pilot lab for comparison.
SampleTiter (Dry), DtexElongation (Dry), %Tenacity (Dry), cN/tex
SS-CS2/362.12 (0.20)16.39 (3.68)15.51 (2.07)
DS-CS2/251.99 (0.24)11.66 (2.19)14.76 (2.47)
Typical range of viscose staple fibre (unbleached) [47]~2.0–2.2~15 ~15–19
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Forsberg, D.C.R.; Bengtsson, J.; Hollinger, N.; Kaldéus, T. Towards Sustainable Viscose-to-Viscose Production: Strategies for Recycling of Viscose Fibres. Sustainability 2024, 16, 4127. https://doi.org/10.3390/su16104127

AMA Style

Forsberg DCR, Bengtsson J, Hollinger N, Kaldéus T. Towards Sustainable Viscose-to-Viscose Production: Strategies for Recycling of Viscose Fibres. Sustainability. 2024; 16(10):4127. https://doi.org/10.3390/su16104127

Chicago/Turabian Style

Forsberg, Diana Carolina Reyes, Jenny Bengtsson, Nadine Hollinger, and Tahani Kaldéus. 2024. "Towards Sustainable Viscose-to-Viscose Production: Strategies for Recycling of Viscose Fibres" Sustainability 16, no. 10: 4127. https://doi.org/10.3390/su16104127

APA Style

Forsberg, D. C. R., Bengtsson, J., Hollinger, N., & Kaldéus, T. (2024). Towards Sustainable Viscose-to-Viscose Production: Strategies for Recycling of Viscose Fibres. Sustainability, 16(10), 4127. https://doi.org/10.3390/su16104127

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