Wet Spinning of Flame-Retardant Cellulosic Fibers Supported by

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Wet Spinning of Flame-Retardant Cellulosic Fibers Supported by Interfacial Complexation of Cellulose Nanofibrils with Silica Nanoparticles Oleksandr Nechyporchuk, Romain Bordes, and Tobias Köhnke ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13466 • Publication Date (Web): 13 Oct 2017 Downloaded from http://pubs.acs.org on October 16, 2017

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ACS Applied Materials & Interfaces

Wet Spinning of Flame-Retardant Cellulosic Fibers Supported by Interfacial Complexation of Cellulose Nanofibrils with Silica Nanoparticles Oleksandr Nechyporchuk,†,* Romain Bordes,‡ Tobias Köhnke† †

Swerea IVF, Box 104, Mölndal SE-431 22, Sweden

‡

Department of Chemistry and Chemical Engineering, Applied Surface Chemistry, Chalmers University of Technology, 412 96 Gothenburg, Sweden Keywords: nanocellulose, cellulose nanofibrils, silica nanoparticles, wet spinning, flameretardant fibers Abstract. The inherent flammability of cellulosic fibers limits their use in some advanced applications. This work demonstrates for the first time the production of flame-retardant macroscopic fibers from wood-derived cellulose nanofibrils (CNF) and silica nanoparticles (SNP). The fibers are made by extrusion of aqueous suspensions of anionic CNF into a coagulation bath of cationic SNP at an acidic pH. As a result, the fibers with a CNF core and a SNP thin shell are produced through interfacial complexation. Silica-modified nanocellulose fibers with a diameter of ca. 15 µm, a titer of ca. 3 dtex and a tenacity of ca. 13 cN tex−1 are shown. The flame retardancy of the fibers is demonstrated, which is attributed to the capacity of SNP to promote char forming and heat insulation on the fiber surface.

1. Introduction Production of high-performance man-made bio-based fibers by controlling the assembly of dissolved macromolecules or colloidal particles is currently targeted by many research groups. The reason behind is that natural and wood fibers have a prebuilt structure that substantially limits their competition with petroleum-based counterparts. Petroleum-based fibers are associated with resource depletion and environmental impact, which further promotes this research in line with the development of a bio-based and circular economy. In this context, the target for new man-made bio-based fibers is to provide a sustainable alternative to petroleumbased fibers with reasonable mechanical properties and potentially additional functionalities.

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Recently, cellulose nanofibrils (CNF) have received much attention as a renewable, lightweight and strong material. CNF are the main constituents of cellulosic fibers responsible for their strength. These nanofibrils consist of highly ordered molecules assembled by nature. CNF with a diameter in the nanometer range and a length of up to several micrometers are packed together with other components, e.g., lignin and hemicelluloses, in the fiber cell wall. Various methods to extract CNF and investigation of their properties have been comprehensively discussed elsewhere.1,2 Generally, wood-based CNF exhibit a Young’s modulus of 30−40 GPa and a tensile strength of 1.6–3.0 GPa3. These values are even higher for cellulose from some special sources, such as tunicates, and for other type of cellulose nanoparticles with removed amorphous regions, called cellulose nanocrystals.4 Therefore, it is envisioned that CNF may be assembled again into microscopic fibers but in a controlled way to form high-performance man-made fibers. Different methods5 have been demonstrated previously to obtain continuous fibers from CNF, which were summarized in several reviews6,7 that provide an extensive discussion of the spinning methods used and the properties of the obtained fibers. Briefly, the fibers have been produced using wet or dry-jet-wet spinning,8–13 dry spinning,14–16 flow focusing17–19 or simple fiber drawing with tweezers from an adjacent CNF suspension and a polymer solution by interfacial complexation.20,21 The early works of Walther et al.8 and Iwamoto et al.9 demonstrated spun fibers from wood CNF with a Young’smodulus of 22.5 GPa and 23.6 GPa and a tensile strength of 275 MPa and 321 MPa, respectively. A more advanced method, flow focusing, was reported later showing improved mechanical properties of the spun fibers.17 However, different experimental conditions were reported in multiple studies, e.g., raw materials, CNF production methods, spinneret geometry, coagulation media, drawing and drying methods, as well as different measurement protocols, which makes comparison of the results difficult. Nevertheless, in all the cases some level of uniaxial orientation of CNF was targeted under shear forces and during drying, since it is known that the nanofibrils possess 2 Environment ACS Paragon Plus

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higher mechanical properties in longitudinal direction compared to the transverse direction.7 It was reported that higher spinning velocities and fiber drawing can enhance the fiber mechanical properties.9,14,22 Recently, composite CNF-based fibers have been investigated. For instance, anionic CNF were combined with cationic chitosan20 or poly(diallyldimethylammonium chloride)21 simply by pulling the two adjacent polymer-containing media with tweezers. The use of long cationic macromolecules was efficient to achieve good spinnability as well as high strength of the resulting dry fibers. The authors of this study, however, are not aware of any work that elucidates the influence of spherical colloidal particles on the processing and properties of continuous fibers made from CNF or other bio-based nanofibers. Additionally, most of the above works aimed at developing fibers with high mechanical properties. Only few of them targeted additional functionalities or advanced applications, e.g., for tissue engineering.11,12 Among advanced properties, flame retardancy is highly desired for cellulosic materials. High flammability is an inherent characteristic of cellulosic fibers that burn almost without residual char formation. Various char-forming and heat insulation agents that form a barrier on the fiber surface are used therefore as flame retardants, e.g., halogens, phosphorus, nitrogen, metal ions, or nanofillers.23–28 The halogen-based compounds are the most effective flame retardants, however, they are recognized to have a noxious effect on the environment. Silica nanoparticles (SNP) appear as one of the alternatives capable of providing flame retardancy and non-toxicity.29,30 SNP are low-cost and earth-abundant material and can be easily modified to facilitate their manipulation. This work demonstrates a facile method for the production of continuous fibers from anionic CNF and cationic silica nanoparticles (SNP), with the latter acting as a flameretardant agent and simultaneously enhancing the fiber gelation through interfacial complexation. The influence of SNP on the uniaxial alignment of CNF and on the mechanical



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and thermal properties of the spun fibers is investigated. To the best of our knowledge, this is the first study on the spinning of flame-retardant fibers based on nanocellulose.

2. Experimental Section Materials: CNF aqueous suspension with a solids content of 1 wt% was kindly provided by RISE Bioeconomy (Sweden). The CNF suspension was produced from a softwood sulphite dissolving pulp (Domsjö Dissolving plus, Domsjö Fabriker AB, Sweden) by carboxymethylation (with sodium as a counter-ion) as described previously31 followed by fibrillation using a Microfluidizer M-110EH (Microfluidics Corp., USA) at 1700 bars for 4 passes. The CNF had a charge density of −151 ± 2 µeq g−1 at pH 5.2, measured using a particle charge detector PCD-02 (Mütek Analytic GmbH, Germany) based on titration with poly(diallyldimethylammonium chloride).32 Morphology of CNF measured using atomic force microscopy is shown in Figure 1.The original CNF suspension was diluted with deionized water to 0.75 wt% and homogenized using an IKA T18 Digital Ultra-Turrax (IKA, Germany) for 30 sec. Aqueous colloidal dispersion of SNP (amorphous silicon dioxide, SiO2) Bindzil CAT80 at 44 wt% was kindly provided by Akzo Nobel Pulp and Performance Chemicals AB (Sweden). The surface of the SNP is modified with aluminum oxide to give them a positive surface charge. The mean particle diameter was ca. 75 nm measured using DLS (see Supporting Information). Hydrochloric acid (37%) was purchased from SigmaAldrich Sweden AB.

Figure 1. AFM height image of the CNF. 4 Environment ACS Paragon Plus

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Wet Spinning: The fibers were produced by extruding an aqueous suspension of CNF at 0.75 wt% using a NE-1000 syringe pump (New Era Pump Systems Inc., USA) at a flow rate of at 50 µL min−1 through a needle with an inner diameter of 180 µm, a length of 10 mm, into the coagulation bath containing 250 mL of aqueous solution of hydrochloric acid (HCl) at pH 2. To produce CNF/SNP fibers, 10 g or 20 g of SNP aqueous dispersion at 44 wt% was added to the above coagulation bath, which resulted in CNF/10SNP and CNF/20SNP fiber samples, respectively. The coagulated fibers were collected on a winder drum with a speed of 2 m min−1, thus with a draw ratio of 1. The drum had protruding bars with a thickness of 5 mm attached to the surface and alternating each 50 mm, as shown in Figure 1a. This allowed drying the fibers in air without the contact to the drum surface and to preserve their equiaxed cross sections. The fibers were dried at a temperature of 20 °C and a relative humidity of 65 % and cut afterwards from the drum in staple fibers of 40 mm. Linear Density and Tensile Testing: The linear density and tensile properties of the fibers were measured using Vibroskop and Vibrodyn (Lenzing AG, Austria), respectively. The fibers were stored and tested at a temperature of 20 °C and a relative humidity of 65 % (according to ISO 139:2005). The tensile testing was performed at a gauge length of 20 mm and a constant extension rate of 20 mm/min. The measured data represents an average of 9 separate measurements. In order to determine the Young’s modulus and tensile strength, the fiber diameter was measured using both SEM and optical microscopy as described below. Scanning Electron Microscopy (SEM): Spun fibers were examined using a field emission scanning electron microscope JSM-7800F (JEOL, Tokyo, Japan) coupled with an energydispersive X-ray spectroscopy analyzer XFlash 5010 (Bruker AXS Microanalysis, Germany). The samples were mounted on carbon conductive tabs and sputtered with a Pt layer of 1.5 nm. To prepare the cross sections, the fibers were (i) frozen in liquid nitrogen and cut with a fresh razor blade or (ii) cold mounted in an epoxy matrix by vacuum impregnation and milled



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applying a broad ion beam (BIB) using a Precision Ion Polishing System (Gatan, USA) at a beam energy of 5 keV and cooling with liquid nitrogen to avoid heat damage. Secondary electron and back-scattered electron images were taken at an acceleration voltage of 5 kV at a working distance of 4 mm. Polarized Optical Microscopy (POM): The degree of orientation in the spun cellulosic fibers was examined using birefringence measurements. The measurements were performed using a Nikon Eclipse Ci-POL polarizing optical microscope (Nikon Instruments Co., Ltd., Tokyo, Japan) equipped with a Nikon TV-lens (C-0.38x) digital camera and a Berek compensator (No. 11055, Nichika Inc., Japan). The samples were placed between a glass slide and a cover slip and measured between two cross polarizers (located at 90° to each other) at an angle of 45° to the fiber axis. The birefringence ∆n of the samples was calculated by dividing the measured retardation (Γ) of polarized light by the sample diameter (d), see Equation 1: ∆n =

Γ d

(1)

The Hermans’ orientation factor was calculated by dividing the ∆n by the maximum birefringence of cellulose ∆nmax = 0.062,33,34 see Equation 2: f (%) =

∆n × 100 ∆nmax

(2)

Atomic Force Microscopy (AFM): Morphology of CNF fibers was examined using AFM performed in a tapping mode using NTEGRA Prima equipped with an NSG01 cantilever (NTMDT, Russia). A droplet of CNF suspension at 10−2 wt% was placed on a freshly cleaned silicon wafer and dried at ambient temperature. The AFM height images were processed using Gwyddion software. Flammability tests: Simple test was developed for qualitative characterization of flame retardancy. Single fibers of CNF and CNF/SNP were fixed at both ends horizontally on steel supports using magnets, leaving the central part of the fiber open. A match was lit and placed


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at a distance of 30 mm below the fiber. Several essays were performed to verify the repeatability of the observations. The videos of fiber burning were recorded at a rate of 120 frames per second. Thermogravimetric Analysis (TGA): The manufactured fibers were examined by TGA using a Mettler Toledo TGA/DSC 1 STARe System. The samples of ca. 2 mg were placed in polycrystalline aluminum oxide crucibles and were analyzed in an air atmosphere with a flow rate of 50 mL min−1 and a temperature range from 30 °C to 700 °C at a heating rate of 10 °C min−1.

3. Results and Discussion The process for fiber wet spinning developed in this study is illustrated in Figure 2a. The main novelty of this process, compared to other available for cellulose or nanocellulose spinning, is based on uptake of one of the components, namely SNP, from the coagulation bath by the extruded CNF fiber. By extruding CNF suspension in the coagulation bath containing SNP, fibers with a CNF core and a SNP shell are produced (see Figure 2b). After being extruded, the CNF suspension is gelled by competing phenomena: (i) protonation of CNF anionic groups in the presence of acid, and (ii) interfacial complexation of carboxymethylated CNF with aluminum-modified SNP (see Figure 2c). The second phenomenon is important to enhance the wet strength of the spun fiber and is crucial to achieve flame retardancy. The ionic interactions due to opposite surface charges are aimed at providing strong interactions between CNF and SNP.35 The process is designed in a way to produce fibers with a diameter of ca. 15 µm, close to those of natural and common man-made cellulosic fibers.



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Figure 2. Schematic representation of: a) a spinning process based on wet spinning of cellulose nanofibrils (CNF) aqueous suspension into a coagulation bath of cationic silica nanoparticles (SNP) at an acidic pH; b) the spun fiber with aligned CNF in the core and SNP at the fiber surface, and c) surface functional groups of CNF and SNP.

The samples produced in this study are listed in Table 1. SEM images demonstrating the surface morphology of the reference fibers made solely from CNF are presented in Figure 3a. The produced CNF fibers have a regular diameter of 12.4 ± 1.0 µm. The fibers have a rough wrinkled surface and aggregates of aligned CNF along the fiber axis can be seen from the enlarged image. Figure 3b and Figure 3c show two fibers produced with increasing amount of SNP in the coagulation bath, samples CNF/10SNP and CNF/20SNP, respectively. The fibers exhibit similar surface morphology compared to that of neat CNF except that the surface is homogeneously covered with SNP. Inset in Figure 3b shows that the SNP have a diameter below 100 nm, which is in agreement with the data retrieved from dynamic light scattering (DLS) measurements of SNP suspension, see Figure S1 (Supporting Information). The SNPdecorated fibers possess slightly higher diameter than those composed entirely of CNF (see Table 2), which is likely due to adsorption of SNP from the coagulation bath. The more silica present in the coagulation bath, the higher the fiber diameter. This suggests that at higher concentration there is a higher probability of cationic SNP to adsorb to the carboxymethyl groups of CNF. SNP also offer groups that can be protonated by hydrochloric acid. The local


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confinement along the fiber surface combined with a low pH may induce local gelation, thus enabling the SNP to remain in place.

Table 1. List of samples produced by wet spinning from CNF and CNF/SNP. Sample name CNF CNF/10SNP CNF/20SNP

Coagulation bath composition 250 g of HCl (pH 2) 250 g of HCl (pH 2) + 10 g SNP (44 wt%) 250 g of HCl (pH 2) + 20 g SNP (44 wt%)

Figure 3. SEM images of the fiber surfaces of: a) CNF, b) CNF/10SNP, and c) CNF/20SNP samples. Figure 4a and Figure 4b show the cross sections of CNF and CNF/20SNP fiber samples made using a broad ion milling. Despite being shrunk from the wet-spun suspension having a diameter of 180 µm to ca. 15 µm upon drying, the fibers possess rather equiaxed, close to circular cross sections. Figure 4c shows the same CNF/20SNP fiber as in Figure 4b but freeze-cut in liquid nitrogen. Some parts of the fiber are cut, whereas some of them are broken off. Figure 4b and Figure 4c also show the EDS analysis of the fiber cross sections with silicon (Si) and oxygen (O) atoms highlighted in color. Despite the great extent of shrinkage, the CNF/20SNP fiber surface is homogeneously covered with Si and the concentration of O is much higher on the surface than in the core, demonstrating that the SNP (consisting mostly of SiO2) are homogeneously located at the surface of the dry fiber without substantial penetration into the fiber depth. It is assumed, therefore, that the interfacial complexation 9 Environment ACS Paragon Plus


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between two oppositely charged constituents occurs rapidly at the interface of the extruded wet CNF suspension and the coagulation bath without deep penetration of SNP into the entangled CNF network, despite as low concentration of CNF as 0.75 wt% used in this study.

Figure 4. a) CNF and b,c) CNF/20SNP sample analysis using a,b) SEM back-scattered electron images of the fiber cross sections made using a broad ion beam milling or c) SEM secondary electron images of the freeze-cut fiber. Middle and right images in (b,c) show EDS analysis with traced Si and O, respectively. d) SEM secondary electron image of the knotted CNF/20SNP fiber sample, showing its great flexibility.

The SNP are made of amorphous silica, therefore, they may alter the flexibility of the spun fibers. Figure 4d, however, shows excellent flexibility of the CNF/20SNP fiber sample, which was knotted on purpose to the size of ca. 100 µm. No visible damage of the SNP layer was observed when the fiber was inspected along with SEM at higher magnification.


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Additionally, the mechanical properties of the produced fibers are presented in Figure 5a and are summarized in Table 2. The entire set of measurements for each sample is shown in Figure S2 (see Supporting Information). The CNF samples have a tenacity of 16.7 cN tex−1, a tensile strength of 309 MPa and a Young’s modulus of 15 GPa. Comparable mechanical properties have been reported in other studies.8–10,12 These values are found to be lower compared to those reported for an individual CNF3, which is most probably due to insufficient interactions between multiple CNF that occurs due to (i) repulsive electrostatic interactions and (ii) non-uniform thickness along the axis of nanofibrils.36,37 This may lead to impeded packing of the CNF in the spun fiber. When the fibers are spun in the coagulation bath containing SNP, the tenacity reduces slightly to 13.7 cN tex−1 and 12.7 cN tex−1 for CNF/10SNP and CNF/20SNP samples, respectively. The reduction of tenacity may occur since the SNP have much higher density than CNF, 2.65 g cm−3 vs. 1.50 g cm−3, respectively. Therefore, by modifying the fiber surface with more dense material, the force per linear density decreases. The tensile strength of the spun fibers also reduces progressively by modifying the fibers with SNP, however, the Young’s modulus remained unchanged. This suggested that silica introduces rather stiffening effect when adsorbed to the fiber surface. The elongation at break reduces slightly from 6.9% to 5.4% and 4.9%, respectively. The presence of silica on cotton fibers was previously reported to reduce the ductility of the fibers.38 On the other hand, the decrease of the mechanical properties for CNF/SNP fibers is not dramatic.

Table 2. Dimensional and mechanical properties of the produced fibers. Sample name CNF CNF/10SNP CNF/20SNP

Fiber diameter (µm) 12.4 ± 1.0 13.3 ± 1.4 15.1 ± 1.4

Titer (dtex) 2.2 ± 0.2 2.7 ± 0.4 3.0 ± 0.3

Tenacity (cN tex−1) 16.7 ± 1.0 13.7 ± 1.1 12.7 ± 1.1

Young’s modulus (GPa) 15.0 ± 2.8 18.2 ± 5.7 14.0 ± 5.2

Tensile strength (MPa) 309 ± 33 270 ± 33 215 ± 35

Elongation (%) 6.9 ± 1.1 5.4 ± 1.0 4.9 ± 1.1

The CNF/SNP fibers developed in this study have similar tenacity compared to that reported for regenerated cellulose/silica flame retardant commercial fibers (Danufil BF from 11 Environment ACS Paragon Plus


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Kelheim Fibres GmbH, Germany, tenacity 11–15 cN tex−1).39 However, the fiber spinning process proposed by this study does not require cellulose dissolution and regeneration, and consequently avoids handling of hazardous carbon disulfide used in the viscose process. Fiber spinning from aqueous suspension of CNF into an aqueous dispersion of SNP shows a potential to produce man-made flame retardant fibers in a more sustainable way. The evolution of the mechanical properties of fibers while tailoring the surface with SNP may be correlated with polarized light microscopy measurements, shown in Figure 5b–d and quantified in Table 3. The fibers in Figure 5b–d were measured between two cross polarizers with an equivalent exposure time. The CNF fiber samples exhibit a strong intensity of polarized light resulting from alignment of nanocellulose along the fiber axis, which is a prerequisite for achieving high mechanical properties.40 When fibers are spun in the presence of SNP, the intensity of the polarized light remains quite high, despite some possible light scattering induced by the SNP, the diameter of which is just several times smaller than the wavelength of visible light. Therefore, the scattering introduced by the SNP can reduce the intensity of light passed through the sample. However, the light scattering by SNP is believed to be isotropic and therefore should not change the light retardation induced by the anisotropy of CNF in the fiber.

Figure 5. a) Mechanical properties of the produced CNF and CNF/SNP fibers; each curve represents an average of 9 separate measurements. Cross polarized optical microscopy images of b) CNF, c) CNF/10SNP and d) CNF/20SNP fiber samples, measured at an angle of 45° between the polarizers and the fiber axis. 12 Environment ACS Paragon Plus

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The values of retardation, however, decrease (see Table 3), which suggests that there is some reduction of the CNF orientation. This may occur due to some disorder introduced by the SNP on the fiber surface, which may hinder CNF alignment along the fiber axis. The values of birefringence (see Equation 1) reduce even more, since the fiber diameter increases with adsorption of SNP. Finally, the Hermans' orientation factor (see Equation 2) was calculated only for the CNF sample (see Table 3) as the ratio of birefringence to maximum birefringence of cellulose.

Table 3. Analysis of orientation of the produced fibers. Sample name

Retardation (nm)

Birefringence

Hermans' orientation factor (%) CNF 380 ± 29 0,0305 ± 0.0017 49.3 ± 2.7 CNF/10SNP 322 ± 49 0,0242 ± 0.0034 n.d.a) CNF/20SNP 304 ± 48 0,0213 ± 0.0031 n.d. a) n.d. – not determined, since the values are expected to be altered by the presence of SNP.

The fibers were spun with SNP with an aim of providing an additional functionality, namely flame retardancy. Since the standard tests performed to determine the extent of flammability, e.g. time to ignition (according to ISO 5660-1:2015) or limiting oxygen index (according to ISO 4589-1:2017), are designed for relatively large specimens and thus are not convenient for testing single microscopic fibers, a non-standard method was developed. The images of CNF and CNF/20SNP samples, recorded during burning with a match lit at a distance of 30 mm below the fiber, are shown in Figure 6a and Figure 6b, respectively. Slow motion movies are also available (Video S1 and Video S2, see Supporting Information). The CNF/10SNP sample had a similar behavior to that of CNF/20SNP and thus is not presented here. The CNF sample is already burned and the fiber is broken after ca. 0.30 s without noticeable char-forming. For CNF/20SNP sample, however, the burning time is significantly extended. First, the fiber chars and becomes black (1.85 s). Then, it continues to be heated and turns into red (2.96 s) and finally burns and breaks (4.21 s). The flame-retardant properties of



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SNP may be explained by enhanced char forming on the surface, which generates a barrier for the release of combustible volatiles and for heat transfer.29 Thermogravimetric analysis (TGA) was performed in air to study the influence of flame retardant on the thermal degradation properties of the produced fibers. Figure 6c shows that all the samples have some early weight loss up to ca. 120 °C, which is the result of water evaporation from the samples. At higher temperature the weight loss is associated with thermal degradation of the fibers. CNF fibers start to exhibit a considerable degradation at ca. 250 °C with a steep loss of mass followed by a more shallow loss of mass, and finally reaching a plateau at ca. 525 °C, which indicates that the degradation is finished and 5.9 wt% of ash is left at 600 °C.


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Figure 6. Qualitative tests for the flammability of a) CNF and b) CNF/20SNP single fibers; the time of burning (min:s.ms) is indicated in the right bottom corner of the images. c) TGA thermograms for CNF, SNP and CNF/SNP samples.

For CNF/SNP samples the degradation starts at a lower temperature compared to that of CNF. The higher the amount of SNP, the lower the starting temperature of degradation. The reduction of starting decomposition temperature with incorporation of flame retardants into cellulosic fibers is well in agreement with previous reports. Such behavior was demonstrated for commercial regenerated cellulose fibers with incorporated phosphorus and sulfur containing additives (Lenzing FR by Lenzing AG, Austria)41, cotton fibers treated with various phosphorous compounds42 and other systems.43,44 The low starting decomposition 15 Environment ACS Paragon Plus


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temperature of ca. 200 °C was also reported for flame retardant regenerated cellulose fibers containing polysilicic acid and aluminum (Visil AP by Kemira, Finland).45,46 The lower degradation temperature for fibers containing flame retardants is usually associated with: (i) decomposition of flame retardants to form a protective layer on the fiber surface, as well as (ii) catalyzed dehydration of cellulose by the flame retardants or their decomposition products.41,42 In the case of SNP alone, no substantial weight loss in the range of 200–300 °C was noticed (see Figure 6). The decrease of the degradation temperature for CNF/SNP fibers is likely to occur due to the second phenomena. An acidic character of aluminum oxide-modified SNP may induce an acid-catalyzed hydrolysis of cellulose. On the other hand, the presence of flame retardants, in our case SNP, enhances char forming on the surface of the fibers. The ash content of 14.0 wt% and 23.3 wt% was measured at 600 °C for CNF/10SNP and CNF/20SNP, respectively. Taking into consideration the moisture content and the ash content for all the samples, as well as the fact that SNP lost 6.0 wt% of weight while heating from 125 °C to 600 °C, it is estimated that the CNF/10SNP and CNF/20SNP samples had SNP content of 15.1 wt% and 24.7 wt%, respectively, which introduced flame retardancy to the fibers.

4. Conclusion This work focuses on the wet spinning process to produce for the first time strong continuous flame-retardant fibers based on nanocellulose. The developed fibers comprising CNF core and SNP thin shell, formed by interfacial complexation, have a uniform diameter and an equiaxed cross section along the fiber axis. A substantial level of nanocellulose alignment is achieved. Incorporation of SNP on the surface reduced slightly the tenacity, however, preserved the Young’s modulus of the fibers. Flame retardancy of the produced fibers was demonstrated, governed by the barrier layer created by SNP on the fiber surface. Further optimization can be anticipated considering the amount of SNP needed and their preferable localization either in 16 Environment ACS Paragon Plus

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the fiber bulk or on the fiber surface. These aspects should be addressed for both good mechanical properties and efficient flame retardancy. Additionally, the fiber mechanical properties may be further enhanced by applying the drawing and heating operations. Due to renewable character of CNF and earth abundance of both CNF and SiO2, the developed fibers have a great potential for use in a wide range of applications. The most reasonable areas are: reinforcement in composite materials, where there is a need for flameretardant filler, and nonwovens. In these applications the fibers are not exposed to the same wear as textile fibers, therefore, the surface modification by SNP without their deep incorporation to the fiber bulk may be sufficient for providing the flame retardancy, as well as preserving the CNF alignment. On the other hand, by developing methods to enhance interfibrillar interactions between multiple CNF, durable washable fibers can be developed for textile application. Taking into account that the research on CNF production and its industrialization is growing rapidly,47,48 it is suggested that nanocellulose-based products have a great potential to become low-cost materials readily available in the nearest future.

Associated content Supporting Information Videos of burning of nanocellulose and nanocellulose/silica nanoparticles fibers (AVI). Dynamic Light Scattering measurements of silica nanoparticles and an entire set of measurements for the mechanical properties of the spun fibers (PDF).

Author information Corresponding Authors *e-mail: [email protected]

Notes The authors declare no competing financial interest.



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Acknowledgements This work has been funded by Swerea IVF. The authors gratefully acknowledge Melina da Silva from Swerea IVF for the support with SEM measurements, Krzysztof Kolman from Chalmers University of Technology for DLS measurements and Tom Lindström and Karl Håkansson from RISE Bioeconomy for providing CNF samples.

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Title: Wet Spinning of Flame-Retardant Cellulosic Fibers Supported by Interfacial Complexation of Cellulose Nanofibrils with Silica Nanoparticles

Authors: Oleksandr Nechyporchuk, Romain Bordes, Tobias Köhnke

TOC/Abstract graphic:


Wet Spinning of Flame-Retardant Cellulosic Fibers Supported by

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