Silica-rich regenerated cellulose fibers enabled by delayed dissolution of silica nanoparticles in strong alkali using zinc oxide

Carbohydrate Polymers 264 (2021) 118032

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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.

1. Introduction

chain is making viscose toxic" 2017). Alternatively, the approaches
ăinen 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 do 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: (O. Nechyporchuk).
/>Received 8 October 2020; Received in revised form 29 March 2021; Accepted 30 March 2021
Available online 3 April 2021

0144-8617/© 2021 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license ( />

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
ăhnke,
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 3 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 1
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 2 h. The slurry was transferred to a bucket and the re­
action was quenched by adding 5 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 5 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, Ham­
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 min at a temperature of − 4 ◦ C
measured directly in the dissolution reactor.

2. Materials and methods
2.1. Materials
SNPs, with a mean particle size of 7 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
5 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, 1 h each) and finally in aqueous solution of
1 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 2 L of
ethanol each) were soaked for 4 h. Each batch was dispersed for 30 000
revolutions using a pulp disintegrator (the principle and construction is
2

O. Nechyporchuk et al.

Carbohydrate Polymers 264 (2021) 118032

Fig. 1. Optical microscopy images of alkaline solutions: (a–c) in the absence of ZnO with added SNPs after (a) 0 h, (b) 1 h and (c) 3 h of stirring; and (d–f) in the
presence of ZnO with added SNPs after (d) 0 h, (e) 6 h, and (f) 21 h of stirring.

2.6. Rheometry

an acceleration voltage of 5 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 9 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
1 STARe System. The samples with a weight of ca. 5 mg were placed in
polycrystalline aluminum oxide crucibles and were analyzed in air at­
mosphere with a flow rate of 50 mL min− 1 and a temperature range from
50 ◦ C to 650 ◦ C at a heating rate of 10 ◦ C min− 1.

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 1 M
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Carbohydrate Polymers 264 (2021) 118032

Fig. 2. 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. 2. 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 7 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.
3. 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
3 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 1 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 6 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

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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
5 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 min after cellulose dissolution). When using 5 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 7 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 7 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 min 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 do not appear to be suitable for wet
spinning. In comparison, the dopes with 5 wt% silica (see Fig. 4c and d)
remain stable even for 300 min 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 min after the preparation. The dopes were extruded using a

Fig. 3. Complex viscosity of 7 wt% cellulose solutions: (a) prepared with/
without ZnO and 5 wt% SNPs; (b) in the presence of ZnO, where 5 wt% and
10 wt% silica was added to alkali and the resulting dispersion/solution was
used after 0 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 min 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 0 wt% silica (left) and 30 wt% sil­
ica (right).

still widely used.
The adapted pulp was dissolved at 7 wt% in the NaOH solutions at a
temperature of − 4 ◦ C. The solutions were centrifuged to remove air
bubbles and the rheological properties were measured (15 min after
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Carbohydrate Polymers 264 (2021) 118032

Fig. 4. Evolution of (a, c) the complex viscosity and (b, d) phase angle of the dopes with (a, b) 30 wt% silica and (c, d) 5 wt% silica in comparison with the bare
cellulose dopes.

Fig. 5. 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.

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Carbohydrate Polymers 264 (2021) 118032

Fig. 6. 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. 1. Low solution viscosity, which led to good fiber

Fig. 7. Mechanical properties of the fibers spun: (a) without siliceous additives, (b–f) with silica and (g–i)sodium metasilicate.
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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 5 % 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 5 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− 1 and 14.2 nm− 1, see Table 2. If all regenerated
cellulose in the fiber is aligned in the longitudinal direction, fc = 1, and if
it is randomly distributed, fc = 0. 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. 8
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 2
Orientation index for bare cellulose fibers and with 5 wt% non-matured and
matured silica, as estimated from WAXS measurements.
Sample
Bare cellulose

5 wt% silica
5 wt% silica matured

Peak, nm−
8.7
14.2
8.7
14.2
8.7
14.2

1

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. 6. 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. 8. TGA of the spun fibers (a–d) and visual appearance of the fibers before and after pyrolysis (e).

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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 do not influence the level of char
formation (Fig. 8d). Moreover, fibers containing non-matured silica
result in a greater extent of retained structural integrity after pyrolysis
compared to fibers with matured silica or sodium metasilicate (Fig. 8e).

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4. Conclusions
This study proposes an 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: />References
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