Silica nanoparticles (SNPs) dissolve in alkaline media, which limits their use in certain applications. Here, we report a delayed dissolution of SNPs in strong alkali induced by zinc oxide (ZnO), an additive which also limits gelation of alkaline cellulose solutions.
Carbohydrate Polymers 264 (2021) 118032 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol Silica-rich regenerated cellulose fibers enabled by delayed dissolution of silica nanoparticles in strong alkali using zinc oxide Oleksandr Nechyporchuk a, *, Hanna Ulmefors a, Anita Teleman b a b RISE Research Institutes of Sweden, P.O Box 104, SE-431 22, Mă olndal, Sweden RISE Research Institutes of Sweden, P.O Box 5604, SE-114 86, Stockholm, Sweden A R T I C L E I N F O A B S T R A C T Keywords: Regenerated cellulose Cold alkali Dissolution Wet spinning Silica nanoparticles Zinc oxide Silica nanoparticles (SNPs) dissolve in alkaline media, which limits their use in certain applications Here, we report a delayed dissolution of SNPs in strong alkali induced by zinc oxide (ZnO), an additive which also limits gelation of alkaline cellulose solutions This allows incorporating high solid content of silica (30 wt%) in cel lulose solutions with retention of their predominant viscous behavior long enough (ca 180 min) to enable fiber wet spinning We show that without addition of ZnO, silica dissolves completely, resulting in strong gelation of cellulose solutions that become unsuitable for wet spinning With an increase of silica concentration, gelation of the solutions occurs faster Employing ZnO, silica-rich regenerated cellulose fibers were successfully spun, possessing uniform cross sections and smooth surface structure without defects These findings are useful in advancing the development of functional man-made cellulose fibers with incorporated silica, e.g., fibers with flame retardant or self-cleaning properties Introduction chain is making viscose toxic" 2017) Alternatively, the approaches ¨inen et al., 2008) or cold alkali (sodium hy using cold alkali (Vehvila droxide (NaOH)) and urea (Qi, Chang, & Zhang, 2008) were found to provide a number of advantages These avoid the environmental issues inherent to the use of carbon disulphide in the viscose process and not involve the risks of using a potentially explosive solvent, which is the case in N-Methylmorpholine N-oxide (NMMO) system Moreover, the NaOH cosolvent is very attractive because of its low price and avail ability (Budtova & Navard, 2015) However, the window of cellulose solubility in cold alkali solvent is quite narrow in terms of alkalinity, temperature, cellulose molecular weight and the possible additives (Budtova & Navard, 2015; Qi et al., 2008) Therefore, incorporation of functional additives in cellulose solutions (dopes) produced using cold alkali approaches is not straightforward and should be particularly addressed Silica-modified cellulose fibers have often yielded materials with interesting properties due to their resembling hydroxylated surfaces Silica is a cheap, earth-abundant and non-toxic material (Almutary & Sanderson, 2017; Kashiwagi et al., 2003; Laoutid, Bonnaud, Alexandre, Lopez-Cuesta, & Dubois, 2009), and silica nanoparticles (SNPs) are widely used by industries in various applications A number of studies have been performed on silica-based coatings of cotton fibers, providing Man-made cellulose fibers with tunable properties have been gaining increased attention lately (Ray et al., 2020; Wang, Lu, & Zhang, 2016), since they can be seen as a sustainable alternative to conventional synthetic and cotton fibers Synthetic fibers, such as polyester, are pre dominantly fossil-based and their production and utilization largely contribute to global warming and microplastic pollution (Browne et al., 2011; Galloway, Cole, & Lewis, 2017) On the other hand, cotton is an enormously water- and pesticide-intensive crop that causes water scar city, habitat loss and soil degradation (Soth, Grasser, Salerno, & Thal mann, 1999) Regenerated cellulose produced by dissolution of cellulose, commonly wood pulp, and its coagulation into fibers provides a material with controlled morphology, composition and functionalities Additionally, it is generally perceived as a future sustainable fiber from renewable resources (Wang et al., 2016) Viscose has been historically the most widespread type of regener ated cellulose (Manian, Pham, & Bechtold, 2018; Wang et al., 2016) However, the viscose process is based on cellulose dissolution through derivatization with carbon disulphide, which is a highly toxic compound that causes huge environmental issues due to problems with wastewater treatment ("Dirty fashion: how pollution in the global textiles supply * Corresponding author E-mail address: oleksandr.nechyporchuk@ri.se (O Nechyporchuk) https://doi.org/10.1016/j.carbpol.2021.118032 Received October 2020; Received in revised form 29 March 2021; Accepted 30 March 2021 Available online April 2021 0144-8617/© 2021 The Author(s) Published by Elsevier Ltd This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 materials with superhydrophobic and self-cleaning surfaces (Anjum, Sun, Ali, Riaz, & Jeong, 2020; Pereira et al., 2011; Zhao, Xu, Wang, & Lin, 2012) In this regard, silica is a very attractive material, since its surface can be modified in a facile manner to provide a variety of functionalities (Carlsson et al., 2014; de Juan & Ruiz-Hitzky, 2000) Silica has been used as a coating for regenerated cellulose (Hribernik ¨hnke, et al., 2007) or nanocellulose fibers (Nechyporchuk, Bordes, & Ko 2017) to achieve flame retardancy The use of additives in the spinning process often leads to reduced mechanical properties of the fibers (Sain, Park, Suhara, & Law, 2004), since the strength originates from long polymeric chains, e.g., of cellulose, aligned in the fiber longitudinal di rection, which is not the case for low-aspect-ratio additive particles However, it has recently been shown that silica can be used to enhance both stiffness and strength of natural fibers upon coating (Kolman, Nechyporchuk, Persson, Holmberg, & Bordes, 2017; Kolman, Nechy porchuk, Persson, Holmberg, & Bordes, 2018), as well as spun cellulose fibers in ionic liquids (Andersson Trojer, Olsson, Bengtsson, Hedlund, & Bordes, 2019), thus making it particularly attractive for use in fibers Some studies are also available reporting the use of silica in regenerated cellulose films produced from ionic liquids and NMMO (Li, Wang, Xiao, & Liu, 2013; Wang, Wang, & Xu, 2011) While the introduction of silica in some spinning processes may be rather straightforward, like the case of ionic liquids (Andersson Trojer et al., 2019), the system of cold alkali poses a new challenge as all the solid phase of amorphous silica dissolves above pH 10.7 (Iler, 1979) It has been known that the solubility of silica at pH 8–9 can be reduced in the presence of some metals, particularly aluminum oxide, upon for mation of hardly soluble aluminum silicates on the surface (Iler, 1973, 1979; Jephcott & Johnston, 1950) However, aluminum oxide hinders cellulose solubility in cold alkali (Davidson, 1937) and therefore cannot be used in this system to stabilize silica Moreover, aluminum has been reported to be inefficient in wt% NaOH (pH 13.9) for preventing glass dissolution, whereas beryllium and zinc demonstrated high efficiency (Hudson & Bacon, 1958; Iler, 1979) Zinc oxide (ZnO) is known to have beneficial effect for cellulose dissolution (Davidson, 1937) that is commonly performed at a pH > 14 Considering the above, different metal oxides may be useful both for hindering silica dissolution and for enabling good cellulose solubility and stability of the dopes However, the other aspect of cellulose-silica interaction should also be taken into account to achieve dope stability (determined in terms of gelation) and suitability for fiber spinning Due to the good potential of ZnO both for cellulose solubility and silica delayed dissolution, we will investigate its usefulness in cold alkali process for cellulose-silica fiber spinning Table Dope composition Component Weight, % Cellulose NaOH ZnO Water 7.00 7.80 0.84 84.36 described in ISO 5263-1) The resulting slurry (0.8 kg of pulp in 20 L ethanol) was placed into a 60 L reactor equipped with mechanical stir ring and a temperature control The slurry was heated to 60 ◦ C followed by adding 0.8 L of HCl (37 %), which caused the further temperature increase The hydrolysis was then carried out at a controlled tempera ture of 70 ◦ C for h The slurry was transferred to a bucket and the re action was quenched by adding L of water at 0.5 ◦ C and additionally 15 L of water at 15 ◦ C The resulting pulp suspension was washed using first tap water followed by deionized water using vacuum filtration over a Nylon woven mesh of 100 μm until the conductivity of the supernatant was less than μS/cm Vacuum-filtered pulp “cakes” were mixed by hand into a homogeneous slurry The pulp was stored in a sealed LDPE bag at 0.5 ◦ C The dry content of the pulp of 31.15 wt% was measured using an HR73 Halogen Moisture Analyzer (Mettler-Toledo AB, Hamư marby Sjoăstad, SE) 2.3 Pulp limiting viscosity number and degree of polymerization (DP) The limiting viscosity number (ηv) of the raw and hydrolyzed pulp dissolved in 0.5 M cupriethylenediamine solution was determined using a capillary-tube viscometer according to ISO 5351 The viscometric average degree of polymerization (DPv) was estimated from the MarkHouwink-Sakurada equation using the parameters proposed by (Evans & Wallis, 1989): DP0.9 v = 1.65 × ηv , (1) and (Sihtola, Kyrklund, Laamanen, & Palenius, 1963): DP0.905 v = 0.75 × ηv (2) 2.4 Dope preparation The composition of the dope prepared in the absence of silica is ăinen shown in Table 1, based on the recipe described previously (Vehvila et al., 2008) When the required amount of SNP suspension was used (5 wt%, 10 wt%, 20 wt% or 30 wt%, based on cellulose content), the water content was adjusted accordingly The adapted pulp was dissolved in alkali using mechanical stirring for 15 at a temperature of − ◦ C measured directly in the dissolution reactor Materials and methods 2.1 Materials SNPs, with a mean particle size of nm and a specific surface area of 360 m2/g, were in the form of aqueous colloidal dispersion supplied by Akzo Nobel Pulp and Performance Chemicals AB (Sweden) under the product name Levasil CS30-236 ZnO and NaOH were analytical grade and were purchased from VWR International AB, Sweden Sodium metasilicate pentahydrate was a product of Fisher Scientific, Sweden Dissolving pulp was grade V-67 from GP Cellulose LLC, US (see Table S1, Supplementary data, for specification) It was adapted (acid-hydro lyzed) before dissolution as described below 2.5 Wet spinning The dopes were extruded through a spinneret (178 capillaries, diameter of 60 μm each, l/d of 1, Sossna GmbH, Germany) into a coagulation bath containing aqueous solution of 10 wt% sulfuric acid and 15 wt% sodium sulfate The fibers were then continuously trans ferred to the washing/stretching bath containing tap water (conduc tivity of ca 210 μS/cm) at 70 ◦ C A schematic of the wet spinning setup can be found elsewhere (Nechyporchuk, Yang Nilsson, Ulmefors, & ăhnke, 2020) Different draw ratios were applied by increasing the Ko speed of the pick-up roller (v2) after the washing bath, while keeping the extrusion speed and the take-up roller before the washing bath (v1) at m/min The draw ratio (DR) was determined as v2/v1 The fibers were then collected on stainless steel rollers, additionally washed by immer sion in tap water (two times, h each) and finally in aqueous solution of wt% fabric softener (Neutral, Unilever, Denmark) The fibers were then removed and air-dried on the rollers 2.2 Pulp hydrolysis Pulp adaptation for dissolution in cold alkali was performed through acid hydrolysis using a protocol adapted from the study of Trygg and Fardim (Trygg & Fardim, 2011) 10 batches of pulp (80 g of pulp in L of ethanol each) were soaked for h Each batch was dispersed for 30 000 revolutions using a pulp disintegrator (the principle and construction is O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 Fig Optical microscopy images of alkaline solutions: (a–c) in the absence of ZnO with added SNPs after (a) h, (b) h and (c) h of stirring; and (d–f) in the presence of ZnO with added SNPs after (d) h, (e) h, and (f) 21 h of stirring 2.6 Rheometry an acceleration voltage of kV The samples were diluted 100 times with deionized water, poured on carbon conductive tabs, heat dried, and sputtered with a Pt layer of 1.5 nm using a Cressington 208HR sputter coater (Cressington Scientific Instrument Ltd., U.K.) To analyze the cross sections, the fibers were cut with a razor blade The dopes were examined using a stress-controlled rheometer Nova (Rheologica Instruments AB, Sweden), equipped with a concentric cyl inder geometry with a bob diameter of 25 mm and a cup diameter of 27 mm The dopes were first centrifuged to remove air bubbles Stress sweeps were first performed at a fixed frequency to determine the linear viscoelastic regimes Then, frequency sweeps in the range of 10− 2–101 Hz were carried out in the linear viscoelastic regimes and the values of the complex viscosity and phase angle were determined The measure ments were performed at a controlled temperature of 23 ◦ C 2.9 Yarn count and tensile testing The yarn count and tensile properties were measured using Vibros kop 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 carried out at a gauge length of 20 mm and a constant extension rate of 20 mm/min The measured data represents an average of separate measurements 2.7 Optical microscopy Silica dispersion/solutions were measured using a Nikon Eclipse CiPOL optical microscope (Nikon Instruments Co., Ltd., Tokyo, Japan) equipped with a Nikon TV lens (C-0.38x) digital camera The samples were placed between a glass plate and a coverslip for the measurements The fibers were measured under bright-field and cross-polarized light (between two polarizers crossed at 90◦ to each other and at an angle of 45◦ to the fiber longitudinal direction) 2.10 Thermogravimetric analysis (TGA) The produced fibers were examined using a Mettler Toledo TGA/DSC STARe System The samples with a weight of ca mg were placed in polycrystalline aluminum oxide crucibles and were analyzed in air at mosphere with a flow rate of 50 mL min− and a temperature range from 50 ◦ C to 650 ◦ C at a heating rate of 10 ◦ C min− 2.8 Scanning electron microscopy (SEM) 2.11 Wide-angle X-ray scattering (WAXS) Freshly dispersed and matured silica samples were examined using a JSM-7800 F (JEOL, Tokyo, Japan) SEM in a high-vacuum mode For determining elemental composition, the SEM was coupled with an energy-dispersive X-ray spectroscopy (EDS) analyzer XFlash 5010 (Bruker AXS Microanalysis, Germany) The microscope was operated at The alignment of cellulose in the spun fibers was studied with WAXS analysis on an Anton Paar SAXSpoint 2.0 system (Anton Paar, Graz, Austria) equipped with a Microsource X-ray source (Cu K-alpha radia tion, wavelength 0.15418 nm) and a Dectris 2D CMOS Eiger R M O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 Fig SEM images of (a) bare SNPs dried from a suspension on carbon tape, (b) silica dried from the aqueous solution of NaOH and ZnO right after preparation and (c) silica dried from the solution of NaOH and ZnO 21 h after preparation detector with a 75 × 75 μm2 pixel size All measurements were per formed with a beam size of approximately 500 μm diameter and a beam path pressure of about 1–2 mbar The sample to detector distance was 111 mm during the measurements All samples were mounted on a Sampler for Solids 10 × 10 mm2 (Anton Paar, Graz, Austria) holder Fifteen frames of 30-min duration were read from the detector, giving a total measurement time of 7.5 h per sample The transmittance was determined and used for scaling of intensities The software used for instrument control was SAXSdrive version 2.01.224 (Anton Paar, Graz, Austria), and post-acquisition data processing was performed using the SAXSanalysis version 4.01.047 (Anton Paar, Graz, Austria) The orientation index (fc) of the regenerated cellulose was calculated according to the intensity distributions of the azimuthal angle using the following equation: fc = 180∘ − FWHM 180∘ zinc silicate on the surface of silica The effect of ZnO at a concentration of 0.84 wt% was examined in this study It is known that ZnO at higher concentrations precipitates in NaOH (Martin-Bertelsen et al., 2020), therefore, higher concentration of ZnO for potentially better stabiliza tion of silica has not been investigated Since optical microscopy does not elucidate the structure at the nanoscale level, the dried samples were observed at higher magnifica tion using scanning electron microscopy (SEM), see Fig The insets to the left show the visual appearance of the samples in suspensions/so lutions Fig 2a demonstrates the appearance of bare SNPs, which sup plements the data provided by the manufacturer, reporting a mean particle diameter of nm The silica dispersion becomes opaque in al kali, as seen from the left inset in Fig 2b, confirming the formation of condensed aggregates On the other hand, the SEM images on the main panel and in the inset both demonstrate a wrinkled surface, but no discrete SNPs Energy-dispersive X-ray spectroscopy (EDS) coupled with SEM confirmed that this surface contains a homogenous distribution of silicon, thus proving that this is a layer of silica Two samples were prepared by heat or vacuum evaporation and both showed similar morphology under SEM This suggests that besides condensation reac tion, dissolution of silica occurs After 21 h of magnetic stirring, the solution becomes transparent, as seen from the inset in Fig 2c, and the evaporated solution does not demonstrate any structural features observed using SEM, confirming complete dissolution of condensed aggregates While achieving delayed dissolution of silica in strong alkali, it is important to preserve good cellulose dissolution and obtain suitable dopes for wet spinning, i.e., with the appropriate rheological behavior In contrast to aluminum oxide, mentioned above, ZnO is known to facilitate cellulose dissolution in cold alkali (Davidson, 1937) Notably, ZnO also delays gelation of cellulose solutions in alkali (Liu, Budtova, & Navard, 2011) Without using ZnO, the dopes should be handled at lower temperature and lower cellulose solid contents, which both results in increased time needed for solution gelation (Liu et al., 2011) A question that remains to be answered is how the solubility of cellulose in cold alkali, as well as the stability of the obtained dopes, are influenced by silica The solutions/suspensions of silica in cold alkali with/without ZnO were used as a solvent for cellulose (dissolving pulp) Cellulose was first adapted (see Materials and methods section) to facilitate its dissolution in cold alkali by means of hydrolysis This is needed to reduce the degree of polymerization (DP) and crystallinity, which is commonly performed using acid, enzymatic and/or mechanical treatments (Isogai & Atalla, ˜ ska, Wawro, Nousiainen, & Matero, 1995) 1998; Struszczyk, Ciechan For the hydrolysis, a method adapted from the work of (Trygg & Fardim, 2011) was used This procedure allowed to reduce the limiting viscosity number of the pulp in cupriethylenediamine solution from 535 mL/g to 180 mL/g (according to ISO 5351) This corresponds to the decrease of the estimated DP from 1875 to 560, calculated according to (Evans & Wallis, 1989), or from 750 to 225, calculated according to the earlier equation of (Sihtola et al., 1963), which seems to be less correct but is (3) where FWHM is the full width at the half-maximum of the azimuthal angle distribution Results and discussion In this study, we show that ZnO can be used to delay silica dissolution in strong alkali, which allows increasing solid content of siliceous ma terial in cellulose solutions, and hence spun fibers, when keeping silica in semi-dissolved state This is demonstrated by dispersing SNPs in NaOH at a pH of 14.4 in the presence or absence of ZnO, which is further to be used as a solvent for cellulose The concentrations of NaOH and ZnO are similar to those reported for dissolution of cellulose (Vehư ăinen et al., 2008) A detailed recipe is presented in Materials and vila methods section The optical microscopy image in Fig 1a shows that after pouring an aqueous dispersion of SNPs (0.7 wt% of dry silica based on the total solution, or 10 wt% based on cellulose after it will be sub sequently dissolved) into alkali in absence of ZnO, some silica aggre gates are formed It is likely that interparticle siloxane bonds are formed when silica is added to alkali, i.e., Si− O− and Si− OH groups on the surface of SNPs condense to form Si− O− Si linkages, catalyzed by OH− (Iler, 1979) The same behavior is also observed in the presence of ZnO, see Fig 1d In the absence of ZnO, the condensed silica dissolves to the state where no visible aggregates are seen under the optical microscope after h of stirring, see Fig 1c Different silicate species are expected to be formed upon dissolution, such as monomers, dimers, linear and cyclic tetramers etc (Tanakaa & Takahashib, 2001) An intermediate state, after h, where some aggregates are still present, is shown in Fig 1b On the contrary, with ZnO present in the solution, such condensed struc tures remain for a longer time Fig 1e demonstrates that after h, silica aggregates are still present when ZnO is used After 21 h of stirring, all aggreagtes disappear completely This confirms a delaying effect of ZnO on dissolution of silica in strong alkali Delay in dissolution of silica in the presence of ZnO may be explained by formation of scarcely soluble O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 dissolution) using a rheometer In Fig 3a we demonstrate that the complex viscosity of the solution increases dramatically when no ZnO is used When cellulose is dissolved without ZnO but in the presence of wt% silica (based on cellulose content), even higher values of the complex viscosity are observed at the same frequencies The insets demonstrate visually the effect of the gelation Thus, these results confirm the ability of ZnO to hinder gelation of cellulose solutions SNPs were dispersed in NaOH solution comprising ZnO, which was further used either directly for cellulose dissolution, i.e., using in a semidissolved state, or after 21 h of maturation, allowing silica to be completely dissolved Fig 3b shows that maturation of silica results in dopes with higher complex viscosity, compared to non-matured ones (measured 15 after cellulose dissolution) When using wt% silica, the increase of viscosity is not considerable, however, in the case of 10 wt% silica, the viscosity increases dramatically, reaching a level unsuitable for wet spinning An attempt to wet spin this dope was un successful, since the extruded solution was slightly gel-like and did not form continuous fibers in the coagulation bath It is believed that upon dissolution of silica, siliceous species are released from the nanoparticles and become available to interact with cellulose polymer chains, result ing in unwanted gelation This suggests that retaining silica in a colloidal state, i.e., non-dissolved, is crucial to obtain dopes with high silica content that are suitable for wet spinning When using “freshly” added silica into alkaline solutions containing ZnO, it is possible to prepare wt% cellulose solutions with 30 wt% silica, see Fig 3c, while maintaning an acceptable level of complex viscosity It should be emphasized that upon incorporation of silica in the dopes, no noticeable effect on cellulose solubility was observed with optical microscopy (see insets in Fig 3c) When cellulose solubility is hindered, non-dissolved fibers or fiber fragments with nonhomogeneous swelling or “ballooning” are detected (Cuissinat & Nav ard, 2008) Another siliceous material dissolved in alkali that may be of interest for incorporation into the fibers is sodium metasilicate (water glass) which is a cheaper silica source Fig 3d shows complex viscosity of wt % cellulose solutions in the presence of sodium metasilicate It is seen that sodium metasilicate can be present in cellulose solutions at lower solid contents compared to silica to result in similar viscosity We have previously shown that it is crucial to maintain silica in a non-dissolved state in order to hinder dope gelation Using sodium metasilicate may be considered the same as using predissolved SNPs, where all the sili ceous material is available to interact with cellulose Therefore, complex viscosity of cellulose solutions in the presence of sodium metasilicate is higher at equivalent solid content compared to those with SNPs In order to allow wet spinning it is essential to obtain dopes that are stable over time Fig 4a demonstrates that the complex viscosity of the dopes with 30 wt% silica comprising ZnO increase with time and gela tion occurs earlier than in the case of bare cellulose dopes Fig 4b provides the values of a phase angle for the same samples A phase angle tending towards 90◦ reflects a predominant viscous behavior of the dopes, while a phase angle approaching 0◦ reflects elastic behavior All the curves lay in between these values, indicating a viscoelastic behavior However, the dopes with 30 wt% silica measured at a maturation time of 240 and longer have a phase angle lower than 45◦ at low frequency range, indicating predominant elastic, and thus gel-like, behavior We have not determined the influence of rheological proper ties of these dopes, coupled to the storage time, on the filament spinn ability, yet these gelled dopes not appear to be suitable for wet spinning In comparison, the dopes with wt% silica (see Fig 4c and d) remain stable even for 300 without detected gelation Fig S1 (see the Supplementary data) demonstrates the complex viscosity and phase angle for cellulose solutions with 20 wt% silica As silica content in fi bers increase, gelation occurs faster All the dopes prepared with a 0–30 wt% silica in alkali and ZnO were spinnable 60 after the preparation The dopes were extruded using a Fig Complex viscosity of wt% cellulose solutions: (a) prepared with/ without ZnO and wt% SNPs; (b) in the presence of ZnO, where wt% and 10 wt% silica was added to alkali and the resulting dispersion/solution was used after h (directly) or 21 h (matured) for cellulose dissolution; (c) with ZnO and freshly dispersed 0–30 wt% of silica in alkali; (d) with ZnO and 0–10 wt% sodium metasilicate All measured 15 after cellulose dissolution Insets in (a) show visual appearance of the samples in jars Insets in (c) show optical microscopy images of the dopes with wt% silica (left) and 30 wt% sil ica (right) still widely used The adapted pulp was dissolved at wt% in the NaOH solutions at a temperature of − ◦ C The solutions were centrifuged to remove air bubbles and the rheological properties were measured (15 after O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 Fig Evolution of (a, c) the complex viscosity and (b, d) phase angle of the dopes with (a, b) 30 wt% silica and (c, d) wt% silica in comparison with the bare cellulose dopes Fig Photo of the process of dope extrusion into the coagulation bath (a), SEM images of the dry wet-spun fibers with 20 wt% of silica: (b) cross-sections of fibers cut with a razor blade, (c) bird’s-eye view, (d) a single freeze-fractured fiber and (e) its EDS tracing of silicon; close-up SEM images of the cross sections of (f) pure cellulose fibers, (g) with 20 wt% silica and (h) 10 wt% sodium metasilicate O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 Fig Bright-field (top) and cross-polarized (bottom) optical microscopy images of: (a) bare cellulose fibers, and (b) 10 wt%, (c) 20 wt%, and (d) 30 wt% silica incorporation The arrows indicate the direction of the polarization filters spinneret having multiple orifices into a coagulation bath of sulfuric acid and sodium sulfate (see Fig 5a), followed by simultaneous washing and stretching (drawing) of the taw in the second water bath After addi tional washing and application of fabric softener (see Materials and methods section), the fibers were dried in a taw, i.e., in contact with each other As a result, silica-rich regenerated cellulose fibers were produced Fig 5b shows an SEM image of the fibers spun from the dopes containing 20 wt% silica at a draw ratio of 1.2 and cut in the traverse direction with a razor blade Surface topography of the same fibers is shown Fig 5c in longitudinal view The obtained fibers are well sepa rated from each other They demonstrate uniform cross-sections and smooth surfaces without noticeable defects Notably, no silica aggre gates were detected that may originate from condensed silica, observed previously in Fig Low solution viscosity, which led to good fiber Fig Mechanical properties of the fibers spun: (a) without siliceous additives, (b–f) with silica and (g–i)sodium metasilicate O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 incorporated (Fig 6d), the fiber structure became fairly rough, which may considerably affect the mechanical properties of the fiber Fig 7a shows the increase of breaking tenacity, increase of the slope of the curve at low elongation (indicating increase of the Young’s modulus) and decrease of the elongation at break as a result of increased draw ratio This is explained by the fact that drawing of fibers results in higher level of cellulose longitudinal alignment within the fibers (Tu et al., 2020) Above the draw ratio of 1.6 the fibers broke in the bath, resulting in discontinuity of the fiber spinning process Therefore, higher draw ratios were not tested Detailed results of the mechanical testing are summarized in Table S2, see Supplementary data It should be also noted that in this study the optimization of pulp adaptation has not been performed for cold alkali dissolution; therefore, even higher mechanical properties are expected when the optimized procedure is developed Incorporation of % of non-matured and matured silica into the fi bers (Fig 7b and c, respectively) results in a stiffening effect and reduction of elongation at break A slight increase in breaking tenacity was also observed for all the draw ratios This shows the synergistic effect of cellulose and silica, which has also been reported in several previous publications (Andersson Trojer et al., 2019; Kolman et al., 2017, 2018) The further increase of silica content results in reduction of the elongation at break Similar tendencies are observed when using sodium metasilicate An observed general trend is that less sodium metasilicate is required for fiber reinforcement compared to non-dissolved (or partially dissolved) silica To understand how incorporation of silica, and its state of dissolu tion, affect cellulose orientation in the fibers, WAXS was carried out (see Fig S3, Supplementary data) The fibers of bare cellulose and with wt % of non-matured and matured silica were investigated The cellulose orientation was estimated from the orientation index, fc, calculated at two peaks: 8.7 nm− and 14.2 nm− 1, see Table If all regenerated cellulose in the fiber is aligned in the longitudinal direction, fc = 1, and if it is randomly distributed, fc = The fc in the range of 0.80–0.84 was calculated for all the samples, without noticeable tendencies between different fibers Therefore, it is believed that there is no effect of silica, both non-matured and matured, on cellulose orientation Finally, thermal properties of the fibers were investigated Fig shows results of TGA for the fibers Incorporation of additives does not influence the onset of fiber degradation With introduced silica (Fig 8a), the residual char content of pyrolyzed fibers increases progressively Table Orientation index for bare cellulose fibers and with wt% non-matured and matured silica, as estimated from WAXS measurements Sample Bare cellulose wt% silica wt% silica matured Peak, nm− 8.7 14.2 8.7 14.2 8.7 14.2 Orientation index (fc) 0.82 0.80 0.84 0.81 0.83 0.80 spinnability, was the proof that silica remained undissolved in the dope Fig 5d shows a cross-section of a single fiber fractured in liquid ni trogen that does not reveal considerable inhomogeneities The corre sponding EDS analysis with traces of silicon (Si) is shown in Fig 5e, demonstrating a rather homogeneous distribution of silicon in the plane parallel to the fiber transverse direction It shows that silicon is present both in the bulk and on the surface The corresponding EDS spectra of the fiber cross-section is shown in Fig S2 EDS tracing proves that the condensed silica structures are homogeneously distributed within the fiber bulk, suggesting that the coagulation process does not induce any segregation or localization of silica in the precipitated cellulose matrix, as reported for spinning from an ionic liquid-based solvent (Andersson Trojer et al., 2019) Therefore, our approach may be of interest for the development of homogeneous silica/cellulose composite fibers The morphology of cross-sections of fractured fibers spun with and without SNPs and sodium metasilicate was also analyzed, see Fig 5f–h Detailed observation of the cross-sections reveals the presence of a rough surface for the sample with 20 wt% silica (Fig 5g), compared with pure cellulose (Fig 5f) and 10 wt% sodium metasilicate (Fig 5h) The surface roughness is likely caused by condensed structures of SNPs in the fiber, which was anticipated To better understand the microstructure of the fibers spun with sil ica, transmitted light microscopy was performed, see Fig The fiber structure was observed in bright-field and cross-polarized illumination The bare cellulose fiber (Fig 6a) exhibits a smooth and homogeneous structure without noticeable defects Incorporation of 10 wt% silica (Fig 6b) resulted in a minor structural change in the longitudinal di rection A slightly further increased heterogeneity was observed for the fiber containing 20 wt% of silica (Fig 6c) When 30 wt% of silica was Fig TGA of the spun fibers (a–d) and visual appearance of the fibers before and after pyrolysis (e) O Nechyporchuk et al Carbohydrate Polymers 264 (2021) 118032 with an increase of silica content, which is not observed for sodium metasilicate (Fig 8b) It should be noted that maturation of silica results in lower char formation (Fig 8c), the origin of which is not fully un derstood, whereas different draw ratios not influence the level of char 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environmentally benign method to produce silica-rich cellulose fibers employing ZnO in a cold alkali cellulose dissolution process By introducing ZnO in the dope the dissolution of dispersed SNPs can be efficiently delayed, thereby avoiding unwanted gelation prior to spinning Although silica-containing dopes are less stable than dopes with only cellulose, the ZnO provides a time window sufficient to allow wet spinning, even for high silica contents We show that both morphology and mechanical properties of regenerated cellu lose fibers can be tailored by the type of siliceous material, namely SNPs and sodium silicate, as well as the degree of silica dissolution in alkali We believe that this method provides a suitable background for further development of silica-based functional regenerated cellulose fi bers employing a cold alkali dissolution process In this regard, silica may provide a range of functional features to enable fibers, for instance, with flame retardant or self-cleaning properties A further development of the wet spinning process and investigation of the resulting fiber properties are expected to elucidate possible applications of such fibers in the future CRediT authorship contribution statement Oleksandr Nechyporchuk: Conceptualization, Funding acquisition, Methodology, Investigation, Writing - original draft, Project adminis tration Hanna Ulmefors: Investigation, Writing - original draft Anita Teleman: Investigation, Writing - original draft Acknowledgment We are grateful to the ÅForsk Foundation for financial support to this study (grant number 19-523) Appendix A Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2021.118032 References Almutary, A., & Sanderson, B (2017) Toxicity of four novel Polyhedral Oligomeric Silsesquioxane (POSS) particles used in anti-cancer drug delivery Journal of Applied Pharmaceutical Sciences, 7, 101–105 Andersson Trojer, M., 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After 21 h of stirring, all aggreagtes disappear completely This confirms a delaying effect of ZnO on dissolution of silica in strong alkali Delay in dissolution of silica in the presence of ZnO... observed using SEM, confirming complete dissolution of condensed aggregates While achieving delayed dissolution of silica in strong alkali, it is important to preserve good cellulose dissolution. .. remains to be answered is how the solubility of cellulose in cold alkali, as well as the stability of the obtained dopes, are influenced by silica The solutions/suspensions of silica in cold alkali