Chapter 8
Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential Downloaded from pubs.acs.org by UNIV OF NEW SOUTH WALES on 08/26/18. For personal use only.
Spinning of Cellulose Nanofibrils Meri J. Lundahl* Department of Bioproducts and Biosystems, Aalto University School of Chemical Engineering, P.O. Box 16300, 00076 Aalto, Finland *E-mail:
[email protected].
Textile fibers are used in numerous produts, but various commercial fibers are unsustainably produced or lack necessary properties. Sustainable textile fibers with useful properties could be produced by spinning of cellulose nanofibrils (CNF) originating from forest resources or bioresidue. CNF can be extruded into either a coagulating solvent or air to obtain solid filaments where individual CNF have preferred orientation along the filament axis. The filament properties can be influenced by the choice of the CNF precursor as well as the spinning conditions. So far, the mechanically strongest filament has attained a Young’s modulus of >50 GPa and a tensile strength of >800 MPa. In addition, several functionalities have been incorporated; e.g., conductivity, magnetic properties, bioactivity and flame retardance.
© 2018 American Chemical Society
Introduction A large variety of both natural organisms and man-made products relies on fibers as structural and functional units. Fibers typically have a high aspect ratio and anisotropy (i.e., different properties in the longitudinal and radial directions). For example, cellulose fibers in wood bear most of the load in their longitudinal direction and muscle fibers in animals have the capacity to contract longitudinally, while absorbent fibers can swell mostly in the radial direction. This anisotropy makes a fiber structure very efficient, as the desired property is concentrated in one direction. Since fibers are such common structures in nature, a variety of natural fibers are used in materials that benefit from the respective fiber properties. As a prominent example, various textile fibers are extracted from plant and animal sources. For many applications, these natural fibers have practical benefits, such as inexpensive availability and special properties like breathability or biocompatibility. However, as they are cultivated through an organic process, they lack homogeneity and controllability in structure and properties. Textile fibers with more controlled and consistent properties can be achieved through spinning of polymer solutions or melts. Both biobased and oil-based polymers can be used for this purpose, though biopolymers are becoming increasingly preferable, whenever competitive, in order to decrease the dependency on fossil resources and move towards a circular bioeconomy. Cellulose represents a common biopolymer used in man-made fibers, as it also acts as the main structural component in plant-based fibers. Conventional fiber spinning of cellulose requires harmful chemicals and alters the natural molecular arrangement of cellulose chains, which affects the properties of the ensuing textile fibers. Spinning of cellulose nanofibrils (CNF) has been proposed as an alternative approach employing milder conditions and maintaining the native elementary cellulose fibrils intact (1, 2). Instead of cellulose dissolution, cellulose fibers are broken down into water-dispersible nanofibrils, which can be spun into air or a recyclable organic solvent. This method was first reported in 2011 (3, 4) and has since then been developed towards increased scalability and strength. In this chapter, we introduce the qualities of CNF and how spinning can be applied on them in practice. Also, we discuss the characterization techniques for CNF filaments as well as the applications where the filaments can be used.
Cellulose Nanofibrils Here, CNF refers to aggregates of elementary cellulose fibrils. When producing cellulose, plants stack several cellulose molecules together in a parallel way to form the elementary fibrils. These fibrils agglomerate into cellulose microfibrils, which the plant uses to construct its cell wall in combination with hemicelluloses and lignin. From this cell wall structure, CNF can be liberated via mechanical disintegration, possibly preceded by mechanical (5), chemical (6–8) or enzymatic (9) pretreatment of the fibers used as raw material (10, 11). Morphologies of CNF with different kinds of pretreatments are presented in Figure 1. 154
Figure 1. Atomic force micrographs (height image, size 2 x 2 µm) of CNF prepared (a) without pretreatment or after (b) carboxymethylation (c) TEMPO-mediated oxidation or (d) cationization via modification with quaternized amine. Reprinted with permission from ref. (12). Copyright 2010 TAPPI. The pretreatments can decrease the energy consumption of the mechanical disintegration, add new functionalities as well as influence the morphology of the ensuing CNF. For example, carboxymethylation makes fibers easier to divide into smaller CNF (Figure 1b), owing to the increased electrostatic repulsion between the introduced carboxymethyl groups (10). The same effect can be enhanced by oxidation mediated by 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO, Figure 1c), which introduces even higher negative surface charge. In fact, TEMPO-mediated oxidation enables dividing fibers to almost completely individualized elementary fibrils with reduced energy usage (6). This pretreatment has also been coupled with benzophenone-modification of the carboxylic groups (13). The oxidized and benzophenone-modified fibers were then disintegrated into CNF which crosslink upon UV exposure. Also, TEMPO-oxidized bacterial CNF have been loaded with cadmium telluride (CdTe) to obtain fluorescent CNF (14). Carboxymethylated, TEMPO-oxidized, benzophenone-modified and CdTe-loaded CNF have all been wet-spun into fibers with properties influenced by the pretreatment, as will be discussed below. 155
Regardless of the possibly applied pretreatment, CNF contain both amorphous and crystalline cellulose. This combination makes them somewhat flexible and able to entangle while still containing strong and stiff crystallites. A single cellulose crystallite has a Young’s modulus of up to 160 GPa in its longitudinal direction (15) but only 8−57 GPa in its transverse direction (16). This kind of anisotropy signifies that the mechanical performance of cellulose can be maximized in structures where the crystallites are oriented along the load-bearing axis. Owing to the entanglement ability of CNF, this kind of orientation can be achieved by spinning, as spinning processes involve both orientation of the structural units and their entanglement into a filament. Next, we will introduce those spinning techniques that are practically applicable on CNF.
Spinning Essentially, spinning refers to extrusion of a spinning dope (usually polymer dispersion, solution, or melt) through a forming element (needle, die or spinneret), thus forcing the dope into the shape of a long filament. The filament structure can be locked by solvent exchange (wet or dry-jet wet-spinning), solvent evaporation (dry or electrospinning) or cooling (melt or electrospinning). Among these approaches, melt-spinning is most commonly used in commercial filament production, owing to its low cost, as no solvent is needed to be added or removed. CNF have been included in filaments prepared by all the techniques mentioned above (2). However, since dry-jet wet-spinning, electrospinning and melt-spinning require the CNF to be mixed with a polymer, we limit the discussion here to wet and dry-spinning, which can be performed on neat CNF hydrogels. Wet-spinning (Figure 2a) involves extrusion of the CNF hydrogel through a forming element submerged in a solvent which is miscible with the solvent in the dope (here, usually water) but a poor dispersant for the CNF. This kind of solvent acts as a coagulant, collapsing the CNF dispersion into a solid filament. The coagulants applied on CNF include organic solvents, such as acetone (3, 14, 17–20), ethanol (4, 21, 22) or isopropyl alcohol (4), as well as salt (23, 24) and acid (25, 26) solutions. Dry-spinning (Figure 2b) functions without any coagulant by extruding a concentrated CNF hydrogel into air and letting the water evaporate, leaving behind a solid filament.
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Figure 2. Schematic illustrations of simplified systems for (a) wet-spinning and (b) dry-spinning.
Spinning of CNF Wet and dry-spinning have so far been applied on CNF only on a laboratory scale, using a syringe pump (3, 17–20, 23–28), extruder (29), capillary rheometer (30) or 3D printer (21) for extrusion, and a syringe needle (3, 4, 17–20, 24, 26, 28, 29), capillary (22, 30) or flow focusing channel (23, 25, 27) as the forming element. These systems allow for variation of several parameters, such as the type and solids content of CNF, speed of extrusion, needle dimensions, coagulation and drying conditions etc. Furthermore, the CNF can be modified or blended with other components and drawing can be added to the process. The impact of all these variables and adjustments has been analyzed in more detail elsewhere (1). In general, spinning of strong, stiff and/or tough filaments is facilitated by selecting CNF with high aspect ratio or slenderness (i.e., fibril length per diameter) (1). The aspect ratio can be influenced by the selection of the raw material and pretreatment. For example, upgrading from wood-based to more slender tunicate-based CNF enabled spinning of tougher filaments (3). Also, increasing the aspect ratio of wood-based CNF by TEMPO-mediated oxidation led to stiffer filaments (18). However, thus increased aspect ratio and surface charge also made the filaments more prone to mechanical deterioration in water (18). In contrast, mechanical pretreatment by grinding was shown to both increase the water contact angle of CNF and strengthen dry-spun filaments (28). Furthermore, the first demonstration of a continuously functioning system for CNF wet-spinning was accomplished with bacterial cellulose (19), which has a higher aspect ratio than CNF from wood. This suggests that the increased aspect 157
ratio would also make spinning easier in practice, possibly owing to the increased potential for interfibrillar entanglement. The effect of CNF solid fraction has only been systematically analyzed for unmodified CNF. In the studied wet-spinning systems, 2 wt% has been concluded as the ideal solids content for attaining high CNF alignment and filament strength (18, 22). Dry-spinning requires higher solid fractions, from 3% (29) or 6.5% (30) upwards, depending on the system. In both wet and dry-spinning systems, filament performance tends to improve with decreasing solids content, until approaching suspensions that are too diluted to spin (1). TEMPO-oxidized and carboxymethylated CNF can be spun with even lower solid fractions of approximately 1 wt% (3, 4, 17, 29) or 0.3 wt% (23, 25, 27), respectively. However, the effect of concentration on modified CNF has not been studied. Regarding the spinning set-up, a high shear rate tends to improve CNF alignment and filament performance (1). The shear rate can be increased by accelerating the extrusion speed (3, 30) or by diminishing the needle diameter (22). For example, accelerating the extrusion speed from 0.1 to 100 m/min, with fixed needle size, has augmented filament Young’s modulus from 8.4 to 23.6 GPa and tensile strength from 90 to 321 MPa (3). Furthermore, the effect of shear can be intensified by lengthening the time that the CNF hydrogel spends under shear by using a long capillary. Lengthening the capillary from 2 cm to 150 cm, while keeping the shear rate high, increased the Young’s modulus from 11.77 to 20.17 GPa and tensile strength from 236 to 331 MPa (3). Especially, this approach enhanced the filament yield strength and modulus of toughness (i.e., area under the stress-strain curve), achieving ~150 MPa and ~30 MJ, respectively. The improvements achieved in filament Young’s modulus and tensile strength by increasing the shear rate and time are highlighted in Table 1. In addition to shear, CNF alignment can also be facilitated by extensional flow. This is expected to be even more effective, as extensional flow promotes fibril alignment rather exclusively, while shear flow induces both alignment and rotation. In conventional spinning of dissolved polymers, extensional flow is usually created by stretching the filament during or after the spinning process. For CNF, stretching has been reported as part of dry-spinning (29) or drying after wet-spinning (24) and as an additional, slow wet-stretching step after wet-spinning (17, 19). Also, CNF has been stretched when enclosed in a supportive shell made of an easily stretchable polymer solution (31). As an alternative to filament stretching, extensional flow has also been applied inside a flow focusing channel (23, 25, 27). Among these methods, wet-stretching has achieved the strongest improvement in Young’s modulus (17) and flow focusing in tensile strength (23). In addition, for the purposes of studying the extensional rheology of CNF, flow through a hyperbolic die has been reported as a useful method to generate controlled shear and extensional flow so that the extensional component is maximized (32, 33). However, the application of hyperbolic or any contracting dies on CNF spinning still remains to be explored.
158
Table 1. Young’s moduli and tensile strengths achieved in filaments spun at different speeds and capillary sizes (i.e., different shear rates) System Dry-spun CNF from bioresidue
Wet-spun TEMPOoxidized CNF from wood
159
Wet-spun unmodified CNF from wood
Young’s modulus (GPa)
Low speed
8.3
147
High speed
11.2
198
Improvement
35%
35%
Low speed
8.4
90
High speed
23.6
321
Improvement
181%
257%
9.35
251
10.86
263
Short capillary
11.77
236
Long capillary
20.17
331
116%
32%
Low
speeda
High speed
Thick
capillarya
Thin capllary
Improvementb a
Thick and short capillary.
b
Tensile strength (MPa)
From low speed, thick and short capillary to high speed, thin and long capillary.
Ref. (33)
(3)
(22)
Finally, coagulation and drying conditions are expected to influence CNF filament properties, as they have been shown to have a major contribution to alignment in lyocell-type of regenerated cellulose systems (34). Considering their potentially important influence, they have received surprisingly little attention. In filaments spun through a flow focusing channel, the coagulation and drying have been shown to generate even more alignment than the channel itself (23). And filaments spun coaxially with a supportive polymer shell developed more strength via thorough coagulation than via stretching (31). In the case of dry-spun filaments, the drying conditions have been systematically analyzed by varying the drying temperature (28). This study showed that the dry-spun CNF filament could be dried at room temperature, 320 or 430 °C without significant change in filament Young’s modulus. Drying at 320 °C even somewhat augmented the tensile strength. This concludes that the filament drying can be accelerated by heating without negative effects on the filament quality. To summarize, the mechanical performance of CNF can, in most cases, be enhanced by using CNF with a high aspect ratio and relatively low solid fraction, and spinning the CNF hydrogel with a high shear rate and some form of drawing. The filament also develops its properties during coagulation and drying, even though the contributions of these processes are not yet fully clarified. These conclusions have been made on the basis of characterization methods for the CNF filaments and their precursor hydrogels, which will be outlined next.
Characterization Characterization of the CNF filaments reveals how the spinning has succeeded and what properties have been achieved. In addition, characterization of the CNF hydrogel used as precursor can provide explanations for certain filament qualities. For industrial application of CNF spinning, standardized characterization methods for both hydrogels and filaments are essential in order to ensure consistent quality of both the precursor material and the product. Here, we introduce the methods that have mainly been used for both of these purposes so far. The CNF hydrogels have mainly been characterized by rheometry, atomic force microscopy (AFM, Figure 1) and transmission electron microscopy (TEM). Especially rheometry could develop into a fast and simple method to industrially monitor the quality of the hydrogels based on their viscosity profile. In wet-spinning systems with a shear rate of 100-200 s-1, the viscosity of the best-performing CNF hydrogel has been ~2 Pa s at the applied shear rate (18, 22). This finding suggests that this level of viscosity should be aimed for when designing the CNF hydrogel and its wet-spinning set-up. However, more work would be needed on defining the rheological requirements also for systems employing different shear rates, dry-spinning systems as well as more complex hydrogels with additional components to CNF. AFM and TEM have been used to visualize the morphology of the CNF used for spinning. AFM has especially been applied to study the width and/or aspect ratio of the CNF (3, 17, 18, 20, 22, 28, 30), which significantly influence the filament properties as explained above. When using additives, such as carbon 160
nanotubes or recombinant silk proteins, TEM has been employed to reveal the quality of their dispersion and interaction with CNF (25, 27). Regarding filament characterization, tensile testing has become the standard method to determine the filament quality. Indeed, many applications require certain tensile properties, which makes this test very useful in revealing how the filament would behave in practice. The discussion around tensile testing results has mainly revolved around Young’s modulus, tensile strength and breaking elongation (1, 2). Recently, though, attention has been brought also to the other useful data obtainable from the tensile test, such as yield strength and modulus of toughness (22). Indeed, further discussion would be welcome regarding all the practically important mechanical properties. So far, the strongest tensile performance for a neat CNF filament has been obtained by aligning CNF in a double flow focusing channel involving coagulation with acid (27). This filament had a Young’s modulus above 50 GPa, tensile strength above 800 MPa, breaking elongation of ~6%, yield strength of ~500 MPa and modulus of toughness of ~35 MJ/m3. Usually, the abovementioned tensile properties are derived based on a stress-strain curve, where stress is defined as engineerings stress (i.e., the applied tensile load divided by the filament cross-sectional area in the beginning of the test). However, it should be noted that the value of engineering stress is heavily influenced by the measured cross-sectional area, which depends heavily on the measurement method and conditions. In fact, up to 45% increase was found in the filament diameter when the measurement method was changed from micrometer gauge at a relative humidity of 50% to scanning electron microscope in vacuum (18). This signifies more than doubling of the cross-sectional area and thus 52% decrease in tensile strength. Moreover, the cross-section may vary along the filament length. To overcome the unreliability of a filament cross-sectional area, textile filament strength is usually reported as tenacity (i.e., tensile load divided by filament linear density). So far, only few authors have reported the tenacity of CNF filaments (26, 35). If this became more widely adopted, possibly more comparable tensile testing data could be achieved. Furthermore, tensile strength and tenacity values highlight slightly different filament qualities and can thus be valuable to report in parallel. Mostly, the tensile testing has only been performed in dry state, at a relative humidity of 50%. Even though many applications involve exposure to water, only a few studies have studied wet tensile properties (18, 19, 21, 35). Several of these authors implemented covalent (21, 35) or electrostatic (19) crosslinking in order to enhance the wet strength. The dry and wet tensile properties achieved with these methods are illustrated in Figure 3. The highest absolute wet strength could be achieved via electrostatic crosslinking (19), while the benzophenone-crosslinked filament retained the largest proportion of dry strength after wetting (35). Since the tensile properties are heavily influenced by the orientation of the CNF in the filament, measuring this orientation has become typical, especially through X-ray scattering techniques. Inside a filament, the cellulose crystallites diffract X-rays to a specific direction dictated by the diffracting crystal plane. Thus, an X-ray beam directed through a CNF filament produces a diffraction 161
pattern which reflects the positioning of the crystals inside the filament. This diffraction pattern can be detected by wide angle X-ray scattering, which has become the standard method for quantifying CNF alignment in filaments.
Figure 3. Dry and wet tensile properties attained in CNF filaments with different crosslinking methods. Data with crosslinking based on refs. (19, 21, 35) and without crosslinking on their average. For example, a vertically positioned filament scatters X-rays mostly in the horizontal direction (Figure 4a, b, insets), provided that it contains cellulose crystallites preferentially aligned along the filament axis. The scattered intensity is distributed to two horizontally intensified rings with radii depending on the scattering angles specific to the axially aligned crystal planes. In contrast, crystal plane (004) aligned perpendicular to the crystal axis produces a faint, vertically intensified ring (Figure 4a, inset). Similarly to a single crystal, a whole CNF suspension scatters X-rays, though with smaller scattering angles as the fibrils are located much further from each other than the spacing between the lattices in a cellulose crystallite. Thus, the scattering caused by a CNF suspension can be best detected by small angle X-ray scattering. The alignment forming under flow can be detected as the deformation of the scattering pattern (23). In addition to the X-ray scattering techniques, CNF orientation can be detected by polarized optical microsopy. An oriented structure makes the filament birefringent (i.e., having a different refractive index parallel and perpendicular to the filament axis direction). When polarized light is passed through a birefringent filament, its component perpendicular to the filament axis gets retarded from the parallel component. When these components are again unified through another polarization, they interfere depending on the wavelength of the light. Thus, the filament is seen in an interference color consisting of those wavelengths at which the two light components reinforce each other. This approach has mainly been used to qualitatively validate CNF alignment during flow (23) or in a filament 162
(4, 18, 19, 22, 26). Examples of CNF filaments between crossed polarizers and respective wide angle X-ray scattering diffractograms are presented in Figure 4.
Figure 4. Optical micrographs of CNF filaments imaged between crossed polarizers. Insets in (a), (b) display the corresponding wide angle X-ray scattering diffracrogram. References: (a) (22), (b) (18), (c) (19), and (d) (26). (a), (b) Reprinted with permission from refs (18, 22). Copyright 2017, 2016 Nature. (c), (d) reprinted with permission from refs (19, 26). Copyright 2017 American Chemical Society. (see color insert) As seen in Figure 4, polarized optical microscopy provides an indication not only about the filament orientation (observed as an interference color) but also about its surface morphology. Usually, filaments contain longitudinal grooves aligned along the filament axis. In more detail, filament surface morphology is typically analyzed via scanning electron microscopy (SEM). In addition to filament surface, SEM has been used to image filament cross-sections. This approach can reveal details about the filament fracture mechanism (20), porosity (3, 30), extent of flattening during drying (29, 30), development of CNF alignment upon stretching (17), distribution of additives (26) as well as the effect of hydrogel viscosity and spinning speed on the ensuing cross-sectional shape (3). The abovementioned tensile testing, X-ray scattering and imaging constitute the standard characterization methods applied typically to all CNF filaments. Naturally, when filaments are functionalized, additional methods are considered to detect the functionality, such as conductivity (25), magnetic properties (4), (bio)sensing activity (14, 35) cell adhesion (21, 27) or flame retardance (26). Owing to all this previous work, the connections between CNF filament morphology, alignment as well as mechanical and functional properties are increasingly well understood. In the future, more characterization would be welcome related to filament porosity and interaction with external conditions, such as heat, water or moisture. 163
So far, filament porosity has been studied only visually (4, 14, 19, 30) and through calculations (4, 18). BET and thermoporosimetry would be important techniques to incorporate in future work in order to more thoroughly quantify the porosity, including pore size distribution. Thermal stability has been measured through thermogravimetric analysis (4, 26) and interaction with moisture through dynamic vapor sorption (18). These types of techniques could be further capitalized on to develop a better fundamental understanding of the filament properties. This kind of knowledge could even reveal new application areas in addition to load-bearing fibers, which have been the main focus of most previous work.
Prospects The practical applicability of CNF spinning is mostly limited by the high cost of CNF as well as the difficulty in continuous filament production due to the fragility and slow coagulation of the extruded CNF hydrogel. These barriers can be lowered in the future by usage of bioresidues as a raw material (30); development of more cost-effective pretreatments for CNF preparation as well as optimization of the spinning processes. However, as the price is still unlikely to become a differentiator for CNF-based materials, their most likely applications are found in areas where their underlying fibrillar structure provides an added value. In this section, we highlight examples of these kinds of prospective applications, focusing on those that already have a proof of concept regarding suitable CNFbased filaments. The ability of CNF to provide added value to a material is mainly related to their mechanical strength, morphology, chemical versatility and biocompatibility (36–39). In practice, the cellulose crystallites in the CNF can provide structural support to the material while the CNF surfaces can contribute with a specific functionality. When surface-functionalized CNF are spun into filaments, the functionality can be obtained not only on the filament surface but also inside the pores and interfaces between adjacent CNF. This has been demonstrated by functionalizing CNF with both carboxylate and benzophenone groups (35). After wet-spinning this kind of CNF, the ensuing filaments could be crosslinked under UV light via the benzophenone groups, while carboxylate groups remained available for further modification with anti-hemoglobin. This kind of filament functioned as a hemoglobin detector (Figure 5a), enduring the wet sensing conditions owing to the benzophenone-crosslinking (Figure 3) (35). More recently, also pH and glucose sensing luminescent filaments (Figure 5b) were prepared by wet-spinning of bacterial CNF loaded with CdTe quantum dots (14). Likewise, crosslinking with glutaraldehyde was shown to increase wet strength enough to allow for stitching through tissue (Figure 3) (21). On this kind of filament, stem cells were shown to grow (Figure 5c), capitalizing on the porosity arising from the CNF morphology and arrangement in the filament (21). Filaments decorated with stem cells could be used as sutures that would not only close surgical wounds but also speed their recovery and reduce inflammation (21). Cell and protein binding to filaments was also enhanced by blending CNF with 164
recombinant silk protein (27). Curiously, these authors found some cell binding ability in CNF alone, too, provided that it was shaped as a filament instead of a film (27). This conclusion suggests that the CNF filament structure has advantages in biomedical applications compared to a film structure, possibly also compared to other cellulose structures, such as natural or regenerated cellulose fibers.
Figure 5. Examples of functional CNF filaments. (a) Filament reacting to hemoglobin concentration c(Hb) with green fluorescence. Adapted with permission from ref. (35). Copyright 2017 American Chemical Society. (b) Luminescent filament changing intensity at indicated glucose concentrations. Reprinted with permission from ref. (14). Copyright 2018 Elsevier. (c) Filament decorated with stem cells; red: cytoplasmic intermediate filaments; blue: nuclei. Reprinted from ref. (21). Copyright 2015 Elsevier. (d) LED light operated using conductive filaments as cables. Reprinted with permission from ref. (25). Copyright 2014 American Chemical Society. (e) Flame-retardant filament exposed to flame. Reprinted with permission from ref. (26). Copyright 2017 American Chemical Society. (see color insert) In addition, conductive filaments have been prepared by blending CNF with carbon nanotubes (Figure 5d) (25) as well as dipping a CNF filament in a solution of a conductive polymer (4). Conductive filaments could be used in electrical components (e.g., resistors) in applications that require flexibility, such as wearable electronics. Potentially, these filaments could enable biodegradable and biocompatible electronics, though the influence of the required additives on the degradability and safety of cellulose should still be confirmed. 165
If conductive filaments can be made sufficiently porous, they can also function in supercapacitors, which are seen as a highly potential solution for energy storage challenges (40). Both porous and conductive filaments have been demonstrated by blending CNF with reduced graphene oxide, which acted as a template for filament carbonization (41). However, carbonization involves a heavy loss of material due to the high temperature. The carbon yield could most probably be improved by replacing part of the CNF with lignin that contains more carbon (42, 43). Thus obtained biobased carbon fibers could be used instead of those prepared from fossil-based polymers (e.g., polyacrylonitrile) in order to reduce the oil dependency and possibly also costs in the carbon fiber industry. Also, flame-retardant filaments have been attained through interfacial complexation with silica nanoparticles during the coagulation (Figure 5e) (26). In the future, this type of filaments could replace synthetic filaments containing toxic flame-retardants in interior textiles or other materials in contact with humans and subjected to strict fire safety regulation. This could enable flame-retardant textiles to become safer as well as potentially biodegradable. If the high moisture sorption capacity of CNF filaments (18) can be maintained in flame-retardant filaments, too, they could provide an additional benefit of moisture-buffering interior spaces. Another possibility for utilizing functionalized CNF filaments could be found in filtering specific substances from gases or liquids. For macroscopic cellulose fibers, functionalization with cyclodextrin has already been demonstrated in combination with spinning into fiber yarn (44). This kind of yarn was able to capture synthetic estrogen hormone from water (44). Similar types of modifications on CNF could enable spinning of filaments with filtering ability. Possibly, the nanoscale size of CNF could even enable a higher specific surface area and thus higher capacity for capturing targeted molecules. If CNF production and spinning can be optimized enough to significantly lower the cost of the filaments, they could also be adopted in disposable materials. For example, disposable biodegradable clothing could become a sustainable response to the demands of the fast fashion industry. However, CNF may be too expensive of a raw material for disposable clothing. As an example of a less costly approach, filaments have been dry-spun from microfibrillated cellulose together with a thickening agent and optionally calcium carbonate (45). Sections of this kind of filament could potentially be mixed with other textile fibers in order to create new textures to fabric-like non-wovens, which represent a probable material choice for disposable clothing (46). Optimization of the spinning process could also improve the mechanical strength of CNF filaments towards taking full advantage of the native strength of the cellulose crystallites included in CNF. Thus, CNF filaments could develop an ability to compete with glass fibers, which are used as a reinforcement for plastics but suffer from a high density. Already, the specific strength and stiffness of CNF filaments have reached close to those of glass fibers (23). In the future, they could achieve even a comparable reinforcing effect while decreasing the weight of the ensuing composite. Furthermore, if this kind of reinforcement is impregnated in a biodegradable resin, the whole material can become biodegradable, provided that any applied additives (e.g., combatibilizer between the filament and resin) allow for safe degradation. 166
To summarize, CNF-based filaments are most likely to become commercialized first in applications that benefit from their morphology. Probably, this will involve surface modification of the CNF, which can provide certain functionalities on the filament surface as well as inside the pores between adjacent CNF. For these purposes, optimization of the specific surface area of the filaments may become an essential factor. Alternatively, filament strength can be optimized for use in materials requiring high strength combined with low density. In any of the given examples, CNF-based filaments can have the advantage of biodegradability, provided that the employed modifications and additional components are designed for safe degradation.
Conclusions Wet and dry-spinning of CNF can be regarded as high-potential techniques to prepare filaments from sustainably sourced cellulose, while benefiting from the native strength and morphology of the CNF. The filament properties can be influenced through the type and solids content of CNF, speed of extrusion, needle dimensions, drawing, coagulation and drying conditions as well as CNF modification or blending with other components. Rheometry, imaging techniques, tensile testing and X-ray scattering techniques have mostly been used to characterize the CNF hydrogels and filaments spun from them. In the future, more attention would be needed on additional properties, such as pore size distribution; and reporting different aspects of tensile testing data, such as toughness and tenacity. The first commercial examples of spun CNF filaments could probably be created by using functional filaments for applications with high added value.
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