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Diese Studie untersucht die Quell- und Flüssigkeitsrückhalteeigenschaften von Zellstoffen aus Textilabfällen und konventionellen löslichen Zellstoffen und konzentriert sich dabei auf ihr Verhalten während des Viskoseprozesses. Die Studie vergleicht die Leistung der Auflösung von Holzfasern, Baumwolllinterfasern und zwei Chargen Baumwollabfallfasern. Schlüsselergebnisse zeigen, dass Fasern auf Baumwollbasis unter alkalischen Bedingungen eine höhere Quell- und Flüssigkeitsretention aufweisen als Fasern auf Holzbasis. Die Studie untersucht auch den Einfluss der Fasermorphologie und chemischer Eigenschaften auf diese Verhaltensweisen, wobei der Einfluss der Zellwanddicke und des Lumenraums hervorgehoben wird. Darüber hinaus untersucht die Forschung die Pressbarkeit der Zellstoffe nach der Mercerisierung und zeigt, dass mehr Quellung in NaOH-Lösung mit höherem Retentionsverhalten und geringerer Pressbarkeit einhergeht. Die Studie kommt zu dem Schluss, dass Baumwollfasern zwar eine stärkere Quellung und Retention aufweisen, die Aktivierung von Zellulose und die Reaktivität in weiteren Prozessen jedoch noch untersucht werden müssen. Diese vergleichende Analyse bringt die Industrie der erfolgreichen Umsetzung der Viskoseproduktion aus Baumwollabfällen näher und ebnet den Weg für weitere Optimierungen.
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Abstract
This study investigates the swelling and liquid retention properties of cellulosic pulp from cotton waste, cotton linters and conventional dissolving wood pulp in both neutral (water) and alkaline (sodium hydroxide) conditions in regard to the first phase of the viscose process (mercerization). The swelling of single fibers is investigated by microscopic observation of the diameter increase during immersion in the liquids, which resulted in a logarithmic trend over time. The retention properties are investigated by water and lye retention values, and the latter was coupled to the pressability of mercerized pulp through observation of the trend in press factor with increasing pressing times. The different materials behaved similarly in neutral conditions regarding single fiber swelling and retention properties. Alkaline conditions, on the other hand, resulted in increased swelling and retention properties for all materials compared to neutral conditions, and the cotton-based pulps showed higher single fiber swelling and retention of lye, accompanied by impeded pressability. Thereafter, several material properties were investigated; morphological fiber properties (fiber width, cell wall thickness and fiber coarseness), fines content, carbohydrate monomer composition, and charge density. The results indicate that a thin cell wall and large lumen of the cotton waste fibers affect their higher swelling and retention properties, but further investigation of other morphological, chemical and physical properties of the fibers and fiber networks in pulp sheets is necessary. However, these insights on the behavior of different pulps can already help industries with the optimization of implementation of cotton waste pulps for viscose production.
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Introduction
Cellulose is the most abundant biopolymer on earth, consisting of a linear unbranched polysaccharide of β-d-glucose residues connected through 1\(\to\)4 β-glycosidic bonds, and mostly occurring in plants as a structural constituent. This cellulose is of interest in many applications, such as in the textile and paper industry, in its initial form or derivatized/regenerated. Cotton production–being one of the purest cellulose sources–is currently contributing 24.4 million tons to the global textile fiber market. However, only 1% of the current cotton-fiber production is made from waste cotton (Textile Exchange 2024), which further encourages the need for cultivation of virgin cotton, requiring immense amounts of resources such as water, land, and chemicals for cultivation and processing. At the same time, the extensive production compared to the reuse of the materials leads to immense waste streams, which mostly end up being incinerated or as landfill. Present-day, the undeniable environmental impact of the textile industry gives rise to a growing urge to tackle various aspects of its pollution, for example, by the reuse of textile waste through different pathways (mechanical, thermal and/or chemical processes).
For example, the option of reusing waste cotton as a cellulose source instead of conventional virgin fibers as input for creating regenerated cellulose filaments is gaining industrial and academic interest (Palme et al. 2019; Bågenholm-Ruuth et al. 2024). This regeneration can be done through derivatization prior to dissolution to make the cellulose more soluble (such as is done in the well-known and large-scale viscose process). However, the current viscose industry has been optimized for the common industrial pulp sources such as wood and cotton linter (a by-product from the cotton industry) (Felgueiras et al. 2021), while industry experiences and results from previous research indicate cellulosic pulp from waste cotton to perform differently throughout the viscose process (Wisniewski et al. 2018; Palme et al. 2019; You et al. 2021), which has an impact on both the processability of the pulps and the properties of the final product.
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During the well-known viscose process, cellulose from a highly pure source is regenerated into reassembled fibers (viscose) through derivatization and dissolution of the cellulose. The principles are based on an alkali-catalyzed mechanism to derivatize the cellulose into a water-soluble intermediate (cellulose xanthate), in order to dissolve and re-spin the cellulosic dope in an acidic bath. Hereby, the xanthate regenerates back into a cellulose structure in the form of new viscose filaments. The first step of the viscose process (‘mercerization’) is performed to activate the cellulose chains, which are generally densely packed within the crystalline formation stabilized by strong intra- and inter-molecular hydrogen-bonds between the hydroxy groups, van der Waals forces, and hydrophobic interactions, causing the cellulose to be rather water-insoluble and inaccessible to chemicals (Pérez and Samain 2010). During this first step, cellulose pulp in the form of slurry or sheets is steeped in concentrated lye solution (usually NaOH 17–19 wt%) for 20–30 min at elevated temperature (45–55 °C), resulting in a swollen activated alkoxide derivative (alkali cellulose (Cell-O−Na+)) (Wilkes 2001).
The swelling is caused by an uptake of fluid into the fibers. Literature describes several contributing mechanisms to the physicochemical swelling phenomenon of cellulosic fibers, and a reviewing description is given in Hubbe et al. (2024) and Ibbett et al. (2008). An immediate uptake of water occurs due to capillary forces by the pore structure of the fibers, mainly leading to intercrystalline swelling. Furthermore, diffusion of water (and possibly other molecules in the solution) into the polymer network will lead to intracrystalline swelling, which is the main cause for the activation of the cellulose during mercerization (Fechter et al. 2020). The diffusion is caused by an osmotic force due to the presence of weakly acidic functional groups, such as hydroxy and carboxyl groups causing negative charges in the fibers in dissociated form (Neale 1929). In neutral pH, the charges originate from deprotonated carboxyl groups, which are mainly present in hemicellulose but also to a smaller extent in cellulose due to oxidation of hydroxy groups of the glucopyranose units (Herrington and Petzold 1992). This dissociation of acidic groups leads to an imbalance of ions, which causes an osmotic force of water into the fibers (Scallan 1983). In general, the presence of charges is known to increase swelling behavior in water (Barzyk et al. 1997).
However, in the case of swelling in sodium hydroxide as is the case for mercerization, the high pH will cause deprotonation of the hydroxy groups in the cellulose and thus increased presence of negative charges in the fibers, increasing the osmotic effect. Furthermore, these charges will also cause a diffusion of Na-ions into the fibers, which will create an even greater imbalance in ion-concentration and which will further drive the osmotic pressure, with increased uptake of water as a consequence (Neale 1929).
In short, swelling of cellulosic fibers often occurs via different mechanisms: fast fluid uptake due to capillary forces and osmotic swelling through the pore structures of the fibers mostly leading to intercrystalline swelling, and–in case of strong swelling agents–a slower diffusion through the polymer structure mostly driven by osmotic forces leading to intracrystalline swelling (Ibbett et al. 2008). The latter will cause a disruption of the forces keeping the cellulose chains together, which will result in increased accessibility of the chains in the cellulose fibrils and thus increased reactivity for further chemical reactions along the viscose process (Ferro et al. 2020). Swelling of the fibers has been analyzed in previous literature by following the increase in fiber diameter and circular cross-section, and a decrease in crystallinity index (Choi et al. 2016; Heinze et al. 2018; You et al. 2021).
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The diffusion of both water and Na-ions into the intracrystalline spaces during immersion of cellulose in alkaline conditions also causes changes regarding the polarity and unit-cell conformation of the cellulosic arrangements; the secondary structure of the cellulose changes from Cellulose I with parallel chains to an intermediate alkali–cellulose, whereafter the removal of lye can cause an irreversible switch to an antiparallel Cellulose II structure (with every second chain having opposite polarity to the previous, leading to a monoclinic unit cell) (You et al. 2021). Furthermore, the concentrated alkali solution should dissolve undesirable short-chain material present in the pulp (such as residual hemicellulose and low-molecular-weight cellulose) to avoid problems related to reduced purity of the cellulose content, such as undesirable consumption of chemicals later in the process or gel-formation disrupting the filtration and spinning of the dope, potentially deteriorating the viscose fiber quality (Wilkes 2001).
Therefore, the conditions of the mercerization step need to facilitate the swelling and activation of the cellulose and dissolution of the short-chain material simultaneously. Lower temperatures are generally optimal for swelling and activation in terms of crystalline rearrangements (Fechter et al. 2020; Richter & Glidden 1940; Sisson & Saner, 1941). Lower NaOH-concentrations promote swelling in terms of fluid uptake (measured in weight and dimension increase) with optimal values between 9–12 wt%, while the intracrystalline swelling with accompanying crystalline rearrangement only occurs at a certain temperature-dependent NaOH-concentration (in case of 50 °C, starting from 14–15 wt%) (Fechter et al. 2020; Nishiyama et al. 2000; Okano and Sarko 1985). Industrial processes aim for conditions to balance the mechanisms during mercerization, and often occur in 18−20 wt% NaOH and temperatures of 45–55 °C.
The properties of the input material play a role in performance during mercerization. Considering the chemical differences, dissolving wood pulp consists of 90–95% cellulose and still some remaining hemicellulose (Mendes et al. 2021), while cotton typically results in pulp with cellulose contents of around 99% after removal of pectins, proteins, waxes and minerals (Candido 2021; Mendes et al. 2021). The non-cellulosic carbohydrates play a role in the reactivity of the fibers, for example due to more accessible functional groups present in the hemicellulose compared to cellulose (Stagno et al. 2022), and the tendency of fibers to have a more porous structure when higher hemicellulose contents are present (Duchesne et al. 2001).
Also, the fiber morphology will influence swelling behavior, for example when it comes to pore volume and size distribution, and related fiber wall density and surface area (You et al. 2021). The morphology of cotton fibers depends on their maturity, with more mature fibers leading to thicker cell walls, reduced lumen cavity and consequently reduced porosity of the fibers, which can affect the interactions with different liquids (Venkatesh and Dweltz 1975). Furthermore, the processing during the lifespan of the material can cause differences in properties and behavior. For example, during pulping, different processing parameters during cooking and drying can influence both the structure and porosity of the sheets and the cellulose fibers, which can influence rewetting properties (Stone and Scallan 1965). When considering industrial or post-consumer waste cotton, the fibers might have undergone structural changes through processing–with many chemical and mechanical treatments with accompanying rewetting and drying steps–and usage during the lifespan of the end-product. For example, laundering or processing treatments can on the one hand result in reduced molecular weight of the cellulose in the cotton, which can be desired for the viscose process (Palme et al. 2014), but it can also lead to fibril aggregation and reduction in specific surface area on the other hand, which worsens the swellability (Palme et al. 2014, 2019).
In essence, the swelling and activation during mercerization depend on many factors. On the one hand, the mercerization step is critical for sufficient reactivity in further process steps and removal of undesirable components in the pulp for improved processability. On the other hand, excessive swelling can also complicate further process steps, such as the removal of excess lye during pressing after mercerization (Wilkes 2001) aiming to obtain an optimal alkali/cellulose ratio for further process steps. This ratio is often indicated as the ‘press factor’ (PF)–a ratio of the pressed alkali-cellulose cake weight relative to the starting material–and is often aimed at 2.8–3.0 in industrial viscose processes, corresponding to a dryness of the alkali cellulose of 30–35% (Wilkes 2001; Kvarnlöf 2007). The pressing step is critical for the further viscose process steps, seeing that too much remaining alkali can lead to undesirable side-reactions in later stages, such as alkaline degradation of the cellulose or consumption of valuable chemicals in the formation of by-products with the excess NaOH (Wilkes 2001; Kvarnlöf 2007). Therefore, a balance between swelling behavior for improved chemical accessibility and achieving acceptable liquid-retention properties for manageable pressing throughput is important.
For that reason, this study aims to understand the swelling and liquid retention behavior of different sources of cellulosic pulp material in relation to mercerization during viscose production, aiming to increase understanding for the implementation of cotton waste pulps in the viscose process. Dissolving pulps from wood, cotton linters and cotton waste were used for this comparative investigation. The insights from this study will be of value for industries aiming to reduce cotton waste by reusing it as input for creating regenerated viscose filaments.
Experimental
Materials
4 different pulps were used; one sodium sulfite softwood dissolving pulp from Northern Europe (further referred to as dissolving wood pulp ‘DWP’), one cotton linter pulp from Europe (further referred to as ‘CLP’) and 2 different batches of cotton waste pulp from industrial waste, provided by Circulose AB, Sweden (further referred to as ‘CWP1’ from Ortviken and CWP2’ from Kristinehamn). All materials have undergone pulping and bleaching processes in order to reach target properties–such as pulp viscosity–for viscose manufacturing. The two different cotton waste batches are produced from similar raw material, but are produced on an industrial scale in Ortviken (CWP1) versus pilot scale in Kristinehamn (CWP2).
Analytical-grade sodium hydroxide pellets (NaOH, 99% purity) and hydrochloric acid (HCl, 37%) were obtained from Merck (Darmstadt, Germany). Sodium chloride (NaCl, > 99.8% purity) was obtained from Sigma-Aldrich (St. Louis, Missouri, US). Aqueous solutions were made with deionized water throughout all experiments.
Analysis of pulp material properties
Average fiber length, fiber width and coarseness of starting material
Fiber morphology was investigated with the Valmet Fiber Image Analyzer (Valmet FS5), following the standard SS-ISO 16065–2:2015 (Pulps–Determination of fiber length by automated optical analysis–Part 2: Unpolarized light method) for the length-weighted mean length and average fiber width, cell wall thickness, fiber coarseness and fines content. This method, based on flow cytometry and optical analysis through pixel calculations and estimations performed by ultra-high definition (UHD) technology, results in a distribution of around 80,000 measured values. From the obtained morphological parameters, the theoretical fiber wall density and lumen diameter were calculated when the fibers are assumed to be hollow cylinders.
Carbohydrate analysis
The relative carbohydrate monomer composition of the starting material was analyzed by acid hydrolysis of the samples described in SCAN CM 71:09 (Pulps–Carbohydrate composition) and detection of the monomer composition with HPLC (Thermo Scientific Dionex). For each type of pulp, 2 samples of 100 mg ground pulp were randomly selected.
Charge density
Negative charges of the pulp fibers are caused by the presence of dissociated acidic groups, such as carboxyl groups. The total charge of the pulp fibers was therefore measured through the total acidic group content (in μeq/g) obtained by conductometric titration according to the SCAN-CM 65:02 standard.
Microscopic observation of single fiber swelling in neutral and alkaline conditions
The method for investigating single fibers during swelling under a microscope was inspired by You et al. (2021). Single fibers of the pulp were isolated from the sheets and taped to an object glass plate, after which a capillary suction ‘channel’ was built over the fiber by two cover glasses (borosilicate glass, 18 × 18 mm, 0.13–0.16 mm thickness, obtained from Avantor by VWR) glued on both sides of the fiber with nail-polish (quick dry, from Nail Kind), which was then covered with one more cover glass. A graphical representation and an image of the actual single fiber sample construction are given in Fig. 1.
Fig. 1
Graphical representation (left) and image of actual sample (right) for the observation of single fiber swelling under a microscope
The samples were observed with an Olympus BX51-microscope with a 10 × binocular set and 50 × magnification objective (and thus a total magnification of 500 ×). A drop of liquid (water or 18 wt% aqueous NaOH solution, respectively, at room temperature (21 °C)) was then added to the entrance of the capillary ‘channel’, whereafter the liquid was sucked through the channel and reached the taped fiber, which started changing in diameter. A snapshot was captured at certain times (0 s (dry), 10 s, 1 min, 2 min, 3 min, 10 min, 20 min, 30 min), from which the diameters were measured by comparing to a 1 mm scale with 0.01 mm subdivision. Per pulp type and swelling-liquid, 10 fibers were randomly selected for microscopic analysis.
Laboratory-scale mercerization of pulp
An industrial-scale mercerization process was imitated by a mercerization-slurry with 3 wt% pulp consistency in an 18 wt% NaOH solution, prepared by adding 25 g of a dissolving pulp sheet–cut into pieces of around 1 × 1 cm2–to 800 g of the NaOH solution prepared with deionized water, into a stainless-steel vessel. The mercerization vessel was then lowered into a water bath at 50 °C and left for 20 min under continuous stirring with a rotating system. For the four different types of pulp, this treatment was repeated 3 times.
Pressability measured as press factor during increasing pressing times
The laboratory-scale pressing was performed by means of a pneumatic pressure vessel according to Östberg et al. (2012), with a diameter of 7.5 cm and vacuum suctioning at the top and bottom of the vessel to remove the pressed-out lye after applying a pressure load of 2.8–3.0 tons (corresponding to a pressure between 63.4 and 67.9 kg/cm2). The pressability of the alkali-cellulose slurry was investigated by observing the trend in press factor (PF) during pressing time (after 50 s, 75 s and 100 s), by comparing the weight of the remaining pressed alkali-cellulose cake to the initial weight of the pulp (around 25 g). After pressing, the pulp was thoroughly washed with deionized water to remove the remaining lye, after which it was air-dried and saved for further analysis.
Water-retention value (WRV) and Lye-retention value (LRV)
The water-retention value of the different kinds of starting material was measured according to SS-ISO 23714:2015, with 4 test-pads generated per pulp type.
For an alternative retention value of other liquids such as NaOH solution, there is, to the author's knowledge, no commonly accepted standard method. However, methods for alkali centrifugal value (ACV) and liquor retention value (LRV) are described in literature. The ACV is measured by leaving cotton fibers (not in sheet form) to swell in NaOH solution (15–18 wt%) for 15 min at 21 °C under shaking, which is followed by centrifugation (Marsh et al. 1953; Venkatesh and Dweltz 1975). Other sources describe the use of sodium hydroxide liquor retention value (‘NaOH LRV’ or ‘LRV’) of pulp, which also consists of swelling of the pulp in sodium hydroxide solution of mercerizing strength at 20 °C, followed by centrifugation (Abou‐State and Helmy 1972). However, these sources use a lower relative centrifugal force compared to the standard method for WRV, which makes comparison of the retention values difficult.
Therefore, to introduce an alternative value for retention of alkali-solution (or ‘lye’) comparable to WRV, the standard WRV method was adapted to obtain a ‘lye-retention value’ (further referred to as LRV). Hereby, the retention of lye after industrial mercerization was simulated by prior mercerization in 18 wt% aqueous NaOH solution (at 50 °C for 20 min), whereafter samples of pulp slurry were taken from the vessel and diluted with 18 wt% NaOH solution at room temperature (21 °C) to a pulp consistency of 2 g/L to obtain a slurry before test-pad formation and centrifugation according to the same method as for WRV (SS-ISO 23714:2015; at 23 °C, for 30 min at a centrifugal force of 3000*g). The centrifuged samples were then weighed, whereafter they were extensively washed with de-ionized water to remove the remaining lye and oven-dried overnight at 105 °C. The LRV was then calculated from the centrifuged mercerized wet test-pad and the final oven-dried test-pad. For each pulp type, 4 test-pads were generated and used for measurements.
Method of statistical analysis and graphical representation
All figures were created with GraphPad Prism, and SPSS was used for the statistical analysis. For comparison of two groups of measurements, two-sided t-tests with CI = 95% (or α = 0,05) were performed. For comparing multiple groups of measurements, one-way ANOVA was performed, possibly with transformed parameters to obtain normality and a correction of the p-values for multiple comparisons (LSD). For obtained p-values ≤ 0,05, a significant difference in results can be assumed. The effect size (\({\eta }^{2}\)) was also investigated as an indication of the effect of an independent variable (such as pulp type) on a dependent variable, with a strong relationship indicated by a threshold value of \({\eta }^{2}\approx\) 0.14.
Correlations between parameters were investigated with Spearman's rank correlation coefficient (ρ), with values close to 1 (either positive or negative) indicating a strong monotonic relationship between parameters (indicating that the parameters tend to change–increase or decrease–consistently and similarly).
Results and discussion
Several material properties on the fiber level were analyzed to understand the differences in swelling and liquid retention behavior during mercerization. The swelling properties were investigated by observing the diameter increase of single fibers versus time in alkaline solution or water. The retention properties of the swollen pulp were investigated using alkaline and water retention values. To give these retention properties a practical meaning coupled to the industrial viscose process, the required pressing time with mechanical press after mercerization was measured.
Analysis of pulp material properties
The question of which properties of the fibers and pulp material influence the swelling and liquid-retention behavior requires a broad analysis of material properties as a first step. The swelling of a single fiber will solely depend on the fiber properties, while retention of liquid and pressability of the pulps will also be influenced by fiber network interactions and properties, such as fines content, pulp sheet density and surface area. This study primarily focuses on fiber properties and some pulp sheet related properties, such as fines content. However, further analysis of fiber network properties is not within the scope of this study and requires additional investigation. The investigated average fiber-morphology properties such as fiber length, width, coarseness and cell wall thickness–obtained from a distribution of around 80,000 measured values–of the pulps are presented in Table 1.
Table 1
Average fiber length, width, coarseness and cell wall thickness obtained by optical analysis
Pulp source
Fiber length (mm)
Fiber width (μm)
Fiber coarseness (mg/m)
Cell wall thickness (μm)
Dissolving wood pulp
1.50
24.3
0.223
9.7
Cotton linter pulp
1.76
22.8
0.298
1)
Cotton waste pulp 1
1.58
23.4
0.134
4.5
Cotton waste pulp 2
1.47
21.9
0.152
6.1
1) Due to the very high cell wall thickness of cotton linters, it was not possible to obtain a measurement of cell wall thickness for CLP through the optical analysis.
Wood fibers and cotton fibers differ in morphology, but also within these groups, large variations can occur depending on the source, maturity and time of sourcing, processing and other factors. Cotton maturity, for example, plays an important role in the thickening and shape of the fibers. In general, cotton staple fibers are rather thin-walled (3–6 μm), bean-shaped, collapsed and convoluted (Sczostak 2009), with growing cell wall thickness with higher maturity (Kljun et al. 2014). Cotton linters, on the other hand–being the short fibrous fuzz on the cotton seed in between the long cotton staple fibers, are generally more cylindrical (with a typical fiber diameter of 17–27 μm), with a thicker cell wall (6–12 μm) and narrower lumen (Sczostak 2009). Based on these typical fiber parameters for cotton linters, a theoretical average lumen diameter can be as low as 3 μm. On the other hand, typical Norway spruce wood fibers from parts of the tree with mature wood (non-pulped) have a common diameter of 29–40 μm and cell wall thickness of 2–8 μm, while parts with juvenile wood have lower fiber diameter with thinner cell walls (Tyrväinen 1995). Furthermore, a distinction between earlywood and latewood also causes differences in fiber and cell wall morphology, and pulping of the fibers can also influence the properties, resulting in a wide range of common values for fiber morphology properties.
In case the fibers in Table 1 are assumed to be hollow cylinders and the pulp only consists of cellulose with a density of 1.5 g/cm3, it is possible to calculate a theoretical lumen diameter (dlumen, in μm), fiber wall density (ρ, in mg/cm3) and accompanying fiber wall porosity (ϕ). However, in reality, the fibers are rather flat and collapsed, and bean-shaped and convoluted in the case of cotton, and do not consist solely of cellulose. The calculated dimensions given in Table 2 are therefore a simplified estimation. Furthermore, it needs to be emphasized that the calculations for CLP are based on estimates from literature, and therefore the calculated morphological properties for CLP in Table 2 should not be considered too literally.
Table 2
Theoretical morphology of cellulosic fibers, with lumen diameter (dlumen), fiber wall density (ρ) and fiber wall porosity (ϕ) calculated from measured properties in Table 1, and assuming the fibers to be hollow cylinders and the pulp only to consist of cellulose with a density of 1.5 g/cm3
Dissolving wood pulp
Cotton linter pulp
Cotton waste pulp 1
Cotton waste pulp 2
dfiber = 24.3 µm
dlumen = 4.9 µm
ρ = 501.2 mg/cm3
ϕ = 0.67
dfiber = 22.8 µm
1)dlumen = 3 µm
1)ρ = 742.8 mg/cm3
1)ϕ = 0.5
dfiber = 23.4 µm
dlumen = 14.4 µm
ρ = 501.5 mg/cm3
ϕ = 0.67
dfiber = 21.9 µm
dlumen = 9.7 µm
ρ = 502.0 mg/cm3
ϕ = 0.67
1) Based on the average morphology of cotton linters from literature (Sczostak 2009), a large cell wall thickness and small lumen can be assumed, and the lumen diameter is estimated to have a minimal value of 3 μm.
From the measurements and calculations of morphological properties in Tables 1 and 2, the obtained fiber morphology of the cotton waste pulps and the cotton linter pulp seems to resonate with the common values found in literature as mentioned above (Sczostak 2009), while the obtained fiber morphology for the softwood dissolving pulp seems to result in rather thin fiber width and rather thick cell walls compared to literature (Tyrväinen 1995), although these values in literature are from un-pulped softwood fibers.
The different materials seem to have similar average fiber length, width and density, with the latter also implying similar porosity, although the estimated values of density and porosity for CLP are respectively slightly higher and lower. However, noticeable differences in cell wall thickness and lumen diameter are obtained, with the cotton waste pulps showing thinner cell walls, also implying larger lumen cavities compared to the softwood fibers and cotton linters. Also, between the two cotton waste pulps, a difference in cell wall thickness and lumen cavity is observed, which could be caused by differences in maturity of the cotton, with a thicker cell wall for more mature cotton fibers (Kljun et al. 2014).
The Fiber Image Analyzer also measured the fines content of the different pulps, as presented in Table 3. Two categories of fines were identified, with fines A defined as flake-like solids shorter than 0.2 mm and fines B as lamella-shaped particles with a width less than 10 µm and length over 0.2 mm (Ahadian et al. 2023). Literature indicates higher fines content to impede drainage (Wisur et al. 1993) and increase water retention values (Mayr et al. 2017).
Table 3
Average total fines content (%) with fraction of primary (A) and secondary (B) fines, obtained by optical analysis for the different pulps
Pulp source
Total fines (%)
Fines A (%)
Fines B (%)
Dissolving wood pulp
25.1
23.7
1.4
Cotton linter pulp
17.6
13.6
4.0
Cotton waste pulp 1
24.7
18.3
6.4
Cotton waste pulp 2
18.1
16.1
2.0
Also, other aspects such as other morphological properties, physical properties (such as crystallinity) and chemical composition should be considered to obtain a full picture. For this study, a first step towards analysis of chemical properties is taken by analysis of carbohydrate composition in Table 4 and total charges in Table 5.
Table 4
Average relative mass percentage of monosaccharides present in the starting material for the different kinds of pulp, with n = 2
Pulp source
Glucose (%)
Xylose (%)
Arabinose (%)
Galactose (%)
Mannose (%)
Dissolving wood pulp
95.81
1.94
0.01
0.02
2.24
Cotton linter pulp
98.64
0.98
0.04
0.03
0.32
Cotton waste pulp 1
98.91
0.61
0.03
0.06
0.39
Cotton waste pulp 2
98.82
0.63
0.05
0.08
0.43
Table 5
Average total acidic group content (TAC) of the starting material for the different kinds of pulp, with n = 6
Pulp source
Average TAC (μmol/g)
Dissolving wood pulp
13.85 ± 1.81
Cotton linter pulp
5.97 ± 0.56
Cotton waste pulp 1
12.03 ± 1.90
Cotton waste pulp 2
8.16 ± 1.33
Although a higher glucose percentage can indicate higher cellulose amounts, the glucose percentage is not directly transferable to cellulose due to the presence of glucose in hemicellulose and other polysaccharides with various compositions. However, it can be observed that the relative monomer compositions for the different cotton fibers show a higher glucose content compared to the softwood pulp, first of all indicating similar chemical compositions for the cotton fibers, but also indicating lower amounts of matrix polysaccharides than the softwood fibers. This was to be expected, seeing as pulped wood fibres usually contain around 90–95% cellulose with some remaining hemicellulose (and possibly traces of lignin), while cotton fibres typically result in cellulose contents of around 99% after pulping (You et al. 2021).
The obtained total charges show significant differences between all pulp types, with a large and significant effect size of material type on the total acidic group content (p < 0.001, \({\eta }^{2}\) = 0.837), meaning that the material type and the total charges of the material are strongly related. However, as expected due to the high cellulose content of the dissolving grade pulps, all materials show a relatively low number of charges compared to for example bleached standard paper grade pulps–with higher hemicellulose content compared to dissolving grade pulp–with average total charge amount of 30–70 µeq/g at neutral pH (Herrington and Midmore 1984; Budd and Herrington 1989). The higher value for the wood pulp in Table 5 is likely due to the traces of hemicellulose, i.e., arabinoxylan, which contains more carboxylic groups. The lower total charge of the other samples is presumably connected to the higher cellulose purity of these samples. However, both of the cotton waste pulps seem to obtain higher charges compared to the cotton linters, likely introduced due to oxidation during certain processing steps during the lifespan of the fibres (Barzyk et al. 1997).
The influence of charges on the swelling has been investigated in literature. For example, Zanuttini and Marzocchi (2003) and Zhao et al. (2017) saw an increase in WRV with an increase in acidic groups in the pulp. However, their change in WRV was measured over a range of acidic group content from 100–800 µeq/g TAC. Therefore, although the obtained charges in Table 5 show some differences between the pulps, the total charge densities are still low compared to studies in literature and are not likely to cause noticeable differences in liquid retention behaviour.
Microscopic observation of single fiber swelling in neutral and alkaline conditions
The trends in diameter increase of single fibers swollen in water and NaOH solution are given in Fig. 2, from which a logarithmic trend of single fiber diameter increase during swelling is observed, whereby most of the diameter increase tends to occur in the first couple of minutes, after which the swelling rate slows down. This observed logarithmic trend resembles the trend of diameter increase obtained in You et al. (2021), and corresponds to the description of the phases of swelling by Ibbett et al. (2008), with a quick diameter increase dominated by capillary forces (and osmotic forces to a lesser degree), whereafter the swelling-rate is mostly dominated by slower diffusion into the polymer network driven by osmosis, which is associated with the activation of the cellulose chains. The latter only occurs with strong swelling agents, as can be seen by the nearly stagnated trendlines of swelling in neutral conditions, while the diameter of the fibers in alkaline conditions continues to increase slowly over time, implying that further diffusion of water and Na-ions into the polymer network continues, leading to intracrystalline swelling.
Fig. 2
Trend in average diameter increase (%) of single fibers of dissolving wood pulp (DWP), cotton linter pulp (CLP) and two different batches of cotton waste pulp (CWP1 and CWP2) during time (s) in water and alkaline solution (18 wt% NaOH) at room temperature (with n = 10). The average diameter increase Δd (%) with standard deviation after 30 min of swelling is presented next to the trendlines (top). The linear trendlines with equations (y = A*x + B) for diameter increase (‘y’) as a function of ln(time) (‘x’) are also given (bottom), with R2\(\ge\) 0.97 for all linear trendlines
The upwards shifting of the logarithmic trend in Fig. 2 (top) shows greater swelling of all fibers in alkaline conditions–as can also be seen as the increase in intercept and the steeper slopes of the linear trendlines of diameter increase against logarithmic timescale in Fig. 2 (bottom)–which could be caused by the increased presence of charges, increasing the osmotic force and causing diffusion of Na-ions and water into the crystalline structure.
Despite the relatively large variations accompanying microscopic observations for a limited sample size, comparison of the maximum swelling obtained after 30 min of the pulps in water versus NaOH solution (averages and standard deviations are presented in Fig. 2 (top)) results in significant differences (p < 0.001) for all materials. This also corresponds to literature on swelling, saying that immersion in NaOH solution will result in increased swelling compared to water due to several mechanisms involving the high pH leading to a high number of charges in the fibers (Herrington and Petzold 1992), increasing an osmotic force and attracting Na-ions (Scallan 1983).
When comparing the swelling of the different materials in one type of swelling liquid, swelling in water does not lead to any significant differences between the materials, also accompanied by a non-significant effect size of material type on diameter increase in water (p = 0.459, \({\eta }^{2}\) = 0.069), implying a weak relationship between material type and swelling in water. Swelling in NaOH solution, on the other hand, does show differing observations for the different materials, with a large and significant effect size of material type on swelling in NaOH solution (p < 0.001, \({\eta }^{2}\) = 0.381), implying that the material type has an important influence on swelling in alkaline conditions. Moreover, the softwood fibers show significantly lower swelling compared to the cotton-based fibers (39% average diameter increase for the softwood fibers versus around 50–60% for cotton-based fibers, with p = 0.037 in comparison with CLP, and p < 0.001 for both CWP1 and CWP2). This also corresponds to observations by You et al. (2021), who found higher swelling of cotton fibers compared to dissolving wood pulp fibers in mercerizing conditions. It should also be noted that the two cotton waste batches (CWP1 and CWP2) do not result in significantly differing swelling behavior measured by single fiber diameter increase (p = 0.565). The cotton linter pulp on the other hand differs significantly from CWP2 (p = 0.040), but not from CWP1 (p = 0.129).
Comparing these swelling properties to the analyzed material properties, the thin cell wall and large lumen of the cotton waste batches seem to have an impact on their higher alkaline swelling ability. Furthermore, the higher glucose content for the cotton-based pulps also seems to correlate with the trends in alkaline swelling, although literature suggested an opposite trend; non-cellulosic carbohydrates have more accessible functional groups present in the hemicellulose compared to cellulose (Stagno et al. 2022) and higher hemicellulose content in the pulp tends to result in a more porous structure (Duchesne et al. 2001). However, removal of non-cellulosic polysaccharides during pulping can also lead to fibril aggregation, with formation of a more compact structure with reduced accessibility of the fibers (Wan et al. 2010). Since the pulping of the softwood needs to remove more polysaccharides compared to the more purely cellulosic cotton, harsher pulping conditions are required, which might cause differences in the extent of fibril aggregation. On the other hand, the cotton waste pulps have been through many rewetting and drying steps during processing, which might also cause differences in the extent of fibril aggregation with the less frequently rewetted dissolving wood pulp. Therefore, the obtained data needs to be complemented with additional analysis of fiber properties for drawing concise conclusions about their influence on the alkaline swelling behavior. However, this preliminary unclear relationship between the analyzed morphological properties and swelling behavior so far does not rule out that they have an impact.
The Spearman’s rank correlation between swelling in water and NaOH solution is non-significant and weak (with p = 0.6 and ρ = -0.4), implying that the measurements of single fiber swelling in water are not relevant for the prediction of swelling in NaOH solution during mercerization.
Water-retention value (WRV) and lye-retention value (LRV)
A comparison of the retention values in water (WRV) versus lye (LRV) in Fig. 3 results in significantly increased LRV compared to WRV for all materials (p < 0.001), which could be caused by the more extensive swelling in NaOH solution resulting in increased interaction of the swelling agent with the cellulosic material. Furthermore, according to Berthold et al. (1996), the presence of ions such as Na+ causes more water molecules to adsorb per hydrophilic adsorption-site, increasing the ‘adsorbed water’ compared to ‘free water’ (not interacting with adsorption sites), which might also contribute to the increased LRV compared to WRV.
Fig. 3
Water retention value (WRV) and lye-retention value (LRV) for different pulp materials, with n = 4
The inconsistent relative increase of LRV compared to WRV (with a ratio of 2.7 for DWP, 4.8 for CLP, 2.4 for CWP1 and 4.2 for CWP2) and the non-existent Spearman’s rank correlation between WRV and LRV of a pulp (with p = 1 and ρ = 0) imply that also here, the measurements of WRV are not relevant for the prediction of retention properties of NaOH solution during mercerization.
The differences in WRV for the different materials are small, however, some still significant–likely partly due to the low deviations in the measurements–with a large and significant effect size of material type on retention of water (p < 0.001, \({\eta }^{2}\) = 0.994). For the LRV, the material type has a large and significant effect size (p < 0.001, \({\eta }^{2}\) = 0.933) as well, implying that material type has a strong influence on liquid retention in both water and alkaline conditions.
The softwood fibers (DWP) show a significantly lower LRV compared to all the cotton-based fibers (p < 0.001), which resonates with the trend in diameter increase during single fiber swelling in alkaline medium in Fig. 2. Also, some differences can be observed within the cotton-based materials, such as a difference between the cotton waste pulp batches (CWP1 and CWP2, with p < 0.001), which was not the case for the observed fiber diameter increase in Fig. 2. Yet, the trends in swelling (diameter increase after 30 min) and retention of the liquid after swelling shows a significant and large positive Spearman’s rank correlation (with p = 0.015 and ρ = 0.81), indicating that fibers which swell more also retain that liquid more after swelling.
The resulting liquid retention behavior of the pulps will be influenced by fiber properties as well as properties of the fiber network. Therefore, further investigation of fiber network properties in the pulp sheets is required for decisive conclusions on the cause for the differences in liquid retention behavior noticed in the WRV and LRV measurements displayed in Fig. 3.
However, seeing that the single fiber swelling and liquid retention properties show a high correlation, it seems logical that the liquid retention properties are also influenced by fiber morphology properties such as the cell wall thickness of the fibers. Marsh et al. (1953) investigated the ACV of cotton fibers with differing maturity level (and thus different cell wall thickness) and found that fibers with similar diameter but thinner cell walls (and thus larger lumen cavity) showed higher retention of the alkali during centrifugation, which they reasoned to be due to increased internal swelling into the lumen because of the restrictive force of the (undamaged) outer cell wall.
Although comparison of the obtained LRV to ACV found in literature is difficult due to differences in method (such as centrifugation parameters) and starting material (such as cotton lint fibers instead of pulped cotton fibers or wood fibers), this observation by Marsh et al. (1953) also corresponds to the higher LRV value for the cotton waste batches (CWP1 and CWP2) in this study. However, some aspects need to be kept in mind in this comparison; Marsh et al. (1953) used undamaged cotton fibers (with the outer cell wall still intact), while this study uses pulped fibers whereby the cell wall has been (partly) deteriorated during the process. Therefore, the restrictive effect of the outer cell wall is not as present as for Marsh et al. (1953), and thus cell wall thickness and lumen cavity will influence but not fully correlate with the retention properties. Furthermore, the observed difference in diameter increase also implies that not only internal swelling into the lumen but also external swelling occurs. In addition, the inconsistency of the cotton linter pulp with–according to the estimation–similar cell wall thickness and lumen cavity as the softwood, but higher swelling and retention properties (similar to the cotton waste batches), makes it difficult for this study to observe a clear trend between the analyzed aspects of fiber morphology and the differences in swelling and retention properties in alkaline-solution.
Furthermore, another observation by Marsh et al. (1953) indicates that deteriorative treatments (chemical, heat and weathering) result in higher ACV. During the chemical pulping processes, deterioration of the fibrous material occurs by removal of fiber wall components, with increased fiber wall porosity but also fibril aggregation as a result (Brännvall et al. 2021). The difference in extent of deterioration due to differences in pulping processes is unknown, although the fiber wall density calculations in Table 2 indicate that they have similar fiber wall porosity after pulping. However, the pulping processes can also lead to fibril aggregation, and although morphological properties such as fibril aggregation and accompanying reduction in specific surface area but also total pore volume and pore size distribution were not considered in this study, this aspect could be of value for future research.
When considering the fines content of the pulps, literature indicates higher fines content to impede drainage (Wisur et al. 1993) and increase water retention values (Mayr et al. 2017), although the obtained data does not show as clear of a trend as described in literature; the pulp with slightly elevated WRV (CWP1) also has an elevated fines content, however with similar fines content as DWP which has lower WRV. When it comes to LRV, the pulps with higher fines content result in lower LRV. Therefore, a clear trend between fines content and retention properties cannot be obtained from this study due to the rather small differences in fines content between the pulps and likely also the effect of other fiber and/or pulp properties influencing the retention properties which require further investigation.
Lastly, the total charges in pulp material are said to influence swelling and liquid retention behaviour in literature, such as increased WRV by Zanuttini and Marzocchi (2003) and Zhao et al. (2017), who measured differences in WRV over a range of acidic group content from 100–800 µeq/g. However, since the measured total charges and the differences between the materials in Table 5 are much lower–likely due to the high cellulose-purity of the materials used in this study–a clear correlation with the differences in swelling and retention properties is not visible. Furthermore, charges already present in the pulp due to the deprotonation of carboxylic groups mostly influence the interaction with neutral liquids, while alkaline liquids also cause additional deprotonation of far more abundant hydroxy groups.
Yet again, the preliminary indistinct relationships between the analyzed morphological properties and swelling and retention behavior so far do not rule out that they have an impact, although further investigation of fiber and fiber network properties is required for more decisive conclusions.
Pressability measured as press factor during increasing pressing times
The laboratory-scale testing of pressability performed in this study mimics the pressing in industrial viscose processes, where the ease of pressing out the excess NaOH solution after mercerization is of importance. Lower pressability of the pulps can, in practice, imply a reduced production rate and volume. However, with the knowledge of the pressability of different types of pulps, a change in pressing equipment in the viscose plant can overcome these issues.
Figure 4 shows the average trends in press factor over time, with an accurate data fit given by the power trend-lines (y = A*xB). These trendlines can be used to estimate the pressing time (tpress) needed to obtain a target PF of 2.8, corresponding to a dryness of around 35%. As discussed previously, this target value has been industrially determined as a target alkali content for further process steps in order to avoid side reactions and alkaline degradation of the cellulose. The estimation of the required pressing time was made for the 3 different batches of mercerized slurry for each material type (with all trendlines with R2\(\ge\) 0.94).
Fig. 4
Trend of average press factor as the ratio of the pressed alkali-cellulose cake weight relative to the starting material with increasing pressing times for the different pulps (with n = 3) and the industrial target PF of 2.8 indicated as a horizontal line. The pressing time (tpress) required to obtain a press factor of 2.8 is estimated from the obtained power trendlines
A lower required pressing time can be observed for the softwood fibers (~ 46 s) compared to the cotton-based fibers with pressing times ranging from around 70 to 90 s, which is also in accordance with the observed differences in diameter increase during swelling in NaOH solution and the lye-retention properties obtained in the previous sections. Therefore, the time that is needed to press the alkali pulp slurry seems to be correlated to the degree of swelling in NaOH in terms of diameter increase of the single fibers at 30 min (with perfect positive Spearman’s rank correlation, with p < 0.001 and ρ = 1.00)) and to the retention properties in NaOH (with large positive–however statistically non-significant–Spearman’s rank correlation, with p = 0.20 and ρ = 0.80). This non-significant yet large correlation could be due to the small sample size, making it difficult to detect significant correlations, which does not necessarily mean that the correlation is non-relevant.
Therefore, in short, more swelling of the fibers in NaOH during mercerization also seems to be accompanied by increased retention behavior of the alkaline solution and more difficult pressing after mercerization.
In literature, a strong relation between the dewatering under centrifugation in lab conditions (WRV) and the dewatering performance during pressing in paper mills was found by Lebel et al. (1979), with a higher WRV leading to reduced water removal during pressing, which is important for the papermaking process. Therefore, Lebel et al. (1979) saw WRV as a predictive variable for the runnability of the paper machine in industrial settings. In the same trend, the LRV in this study also shows some correlation with the required pressing time after mercerization. Efforts to develop a standardized LRV-method and further investigation of the correlation between LRV and pressability therefore shows potential of leading to a laboratory-scale predictive tool for the pressability of a pulp batch.
Conclusion
There is a logarithmic trend in diameter increase during time in both alkaline and neutral medium, with a quick initial increase in diameter likely dominated by capillary forces through the pore structure. In case of alkaline medium, a slower swelling continues at longer immersion times, which is likely driven by osmosis through the polymer network and is related to activation of the cellulose. Greater swelling is observed for alkaline conditions due to increased charges through deprotonation of hydroxy groups, causing an increase in osmotic forces. There seems to be no relation between the observed performances in neutral conditions versus alkaline conditions, implying that behavior in neutral conditions is not relevant for predicting the behavior in NaOH solution during mercerization.
The swelling behavior of the pulps only showed significant differences in alkaline conditions, with all cotton-based fibers resulting in higher single fiber swelling compared to the softwood fibers. Additionally, the lye-retention properties and required pressing times after mercerization are also higher for cotton-based fibers compared to softwood pulp fibers. Consequently, more swelling of the fibers in NaOH solution during mercerization is accompanied by higher retention behavior and lower pressability after mercerization. However, whether this increased swelling of the cotton fibers is also accompanied by a difference in activation of the cellulose (with conversion of Cellulose I to Cellulose II as a measure) and reactivity in further processes along the viscose production remains to be investigated.
Which fiber properties influence the differences in swelling and liquid-retention behavior remains difficult to answer, although the results seem to indicate that the thin cell wall and large lumen of the cotton waste batches affect their higher swelling (diameter increase) and retention properties.
However, this comparative study of swelling and liquid retention behavior in relation to mercerization already brings industries one step closer to successful implementation of viscose production from cotton waste. Building further on these observations, further investigations of material properties, other mechanisms during the viscose process, and performance during different steps will lead to the fundamentals for optimization of viscose production from cotton waste pulp.
Acknowledgement
The authors would like to express their gratitude to Anna Borgström, Anne Christoffersen and Britt-Mari Norberg from Valmet AB for their help with performing measurements of carbohydrate composition and fiber dimensions. The study is part of Pro2BE–a research environment for processes and products for a circular biobased economy. The financial support by the Swedish Knowledge Foundation via the KKS Industrial Research School program project “EXACT-Excellence in Advancing for a Circular Transition” (grant number 20220134) is gratefully acknowledged.
Declarations
Competing interests
S.A. is employed by Circulose AB. E.M. was also employed by Renewcell AB (now Circulose AB) until March 2024. All other authors have no relevant financial or non-financial interests to disclose.
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