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Author(s): Hauru, Lauri; Hummel, Michael; Michud, Anne; Sixta, Herbert
Title: Dry jet-wet spinning of strong cellulose filaments from ionic liquidsolution
Year: 2014
Version: Author accepted / Post print version
Please cite the original version:Hauru, Lauri; Hummel, Michael; Michud, Anne; Sixta, Herbert. 2014. Dry jet-wet spinningof strong cellulose filaments from ionic liquid solution. Springer Netherlands, Cellulose,volume 20, issue 6, pages 4471-4481. ISSN 1572-882X. DOI:10.1007/s10570-014-0414-0
Rights: © 2014 Springer. This is author accepted / post print version of the article published by Springer "Hauru,Lauri; Hummel, Michael; Michud, Anne; Sixta, Herbert. 2014. Dry jet-wet spinning of strong cellulosefilaments from ionic liquid solution. Springer Netherlands, Cellulose, volume 20, issue 6, pages 4471-4481".
This publication is included in the electronic version of the article dissertation:Hauru, Lauri K.J. Lignocellulose solutions in ionic liquids.Aalto University publication series DOCTORAL DISSERTATIONS, 87/2017.
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1
Dry jet-wet spinning of strong cellulosefilaments from ionic liquid solution
Lauri K. J. Hauru, Michael Hummel, Anne Michud, Herbert Sixta
Department of Forest Products Technology, Aalto University, School of ChemicalEngineering, P.O. Box 16300, 00076 Aalto, Espoo, FinlandPhone: +358-50-384 1764
Fax: +358-9-470 24259
E-mail: [email protected]
Considerable growth is expected in the production of man-made cellulose textile fibers, which are
commercially produced either via derivatization to form cellulose xanthate (viscose) or via direct
dissolution in N-methylmorpholine N-oxide (Lyocell). In the study at hand, cellulosic fibers are
spun from a solution in the ionic liquid [DBNH][OAc] into water, resulting in properties equal or
better than Lyocell (tensile strength 37 cN tex–1 or 550 MPa). Spinning stability is explored, and
the effects of extrusion velocity, draw ratio, spinneret aspect ratio and bath temperature on
mechanical properties and orientation are discussed. With the given set-up, tenacities and moduli
are improved with higher draw ratios, while elongation at break, the ratio of wet to dry strength,
modulus of resilience and birefringence depend little on draw ratio or extrusion velocity, elastic
limit not at all. We find the process robust and simple, with stretching to a draw ratio of 5 effecting
most improvement, explained by the orientation of amorphous domains along the fiber axis.
textile fibers, spinning, extrusion, regeneration, tenacity, modulus
Introduction
The global demand for textile fibers is expected to increase following population
growth and improvement in standard of living. Due to the inherent properties of
cellulosic fibers they will cover 33–37% of the total fiber market, resulting in an
increase of per-capita demand from the current 3.7 kg to 5.4 kg in 2030. Cotton
production cannot be increased significantly, creating a 1.7 kg per capita
"cellulose gap" for man-made cellulosic fibers. With the present capacities, there
will be an additional demand of 15 million t of cellulosic fibers in 2030
(Hämmerle 2011). Currently, most man-made cellulosic fibers are produced by
the viscose process, with minor shares from Lyocell (3.8% or 222 000 tons out of
a 5.8 million ton market) and cupro (special applications) (Happel 2014). In the
viscose process, cellulose is xanthogenated, dissolved in caustic soda, and wet
2
spun in an aqueous bath containing sulfuric acid, sodium sulfate and additives that
delay the regeneration. In the Lyocell process, cellulose is dissolved in a direct
solvent (N-methylmorpholine N-oxide, NMMO) to give a solution (termed dope)
and dry jet-wet spun through an air gap into an aqueous coagulation bath. The
mechanical properties of Lyocell filaments are significantly better than those of
regular viscose filaments: e.g. the tensile strengths are 630 MPa vs. 330 MPa,
respectively (Fink et al. 2001; Röder et al. 2009). The production of NMMO-
based Lyocell fibers has been thoroughly investigated with respect to the effects
of spinning variables (Mortimer and Péguy 1996a; Mortimer et al. 1996;
Mortimer and Péguy 1996b).
In filament spinning, a molten polymer or a solution of a polymer is extruded
through a die (spinneret) with a certain extrusion velocity (expressed either as a
velocity in m/min or as a volumetric flow rate in ml/min) and drawn with rotating
godets. The take-up godet is set to a higher linear velocity than the extrusion
velocity to develop the targeted tenacity by stretching the filament. The ratio of
velocities is termed the draw ratio (DR). Polymer molecules are rotated towards
the axial direction, pulled, untangled and straightened, thus producing a highly
oriented structure. In cellulose filaments, orientation promotes crystallization and
produces a dense, continuous microfibrillar structure that has a high tensile
strength. Crystallinity and crystalline orientation is measured with wide-angle X-
ray scattering, while the total orientation is observed as an increase in
birefringence. The difference of total and crystalline orientation is denoted as the
amorphous orientation, which is strongly related to the tensile strength. In
polymers, relaxation is slow, thus structure, including axial orientation of even
amorphous material, is retained after mechanical deformation of a solution or
melt, and may be set by solidification. Thus, the extruded and drawn jet is quickly
solidified by cooling (melt spinning), evaporation of the solvent (dry spinning), or
by precipitation with an antisolvent (wet spinning) (Stibal et al. 2000). Together,
crystallite size (measured with level-off degree of polymerization), molar mass,
total orientation, and crystallinity determine the strength properties in excess of
those in an unoriented, undrawn state. (Krässig 1993)
3
In the viscose process, dopes with relatively low viscosity (zero shear viscosity
about 5–20 Pas) are used, and the dope is extruded directly to the bath (wet
spinning) to immediately form a regenerated "skin" layer on the surface, after
which sulfuric acid diffuses in and solidifies the interior. In Lyocell, by contrast,
highly viscous dopes (zero shear viscosity in the range of 5 000–30 000 Pas at
spinning temperature) are produced. The dope is extruded into an ca. 1–5 cm air
gap, where drawing and cooling is accomplished, and then the filament
immediately enters a coagulation bath, where the structure is set uniformly by a
spinodal decomposition-type phase separation. Neutron diffraction studies have
shown that the fiber cross section consists of micro- and nanofibrils of uniform
dimensions surrounded by a very thin fiber skin. (Schuster et al. 2003) In this dry
jet-wet spinning process, high orientation is accomplished by stretching the
solution before regenerating it. Whereas, in the regular viscose process, relatively
amorphous, low-tenacity fibers are spun with draw ratios less than 1, and
stretching is done after the regeneration (Hearle and Schawaller 2000; Stibal et al.
2000; Fink et al. 2001; Coulsey and Smith 1996). Hence, potentially, higher
viscosities and thus lower temperatures would be preferable. However, these are
limited by the corresponding increase of viscosity of the solution and higher
extrusion pressure, and the melting points of solvents. In the coagulation bath,
higher temperatures would promote diffusion and rapid solidification, but would
allow the polymers to relax and reduce the strength of the forming filament.
There has been considerable research interest in ionic liquids (ILs) during the past
decade, with numerous applications proposed. ILs with high hydrogen bonding
basicities dissolve cellulose, which can be then regenerated by reducing the net
hydrogen bonding basicity (β–α) of the solution with water (Swatloski et al. 2002;
Doherty et al. 2010; Hauru et al. 2012a). Thus, dopes can be prepared and spun
from suitable ionic liquids such as the 1-butyl-, 1-allyl or 1-ethyl-3-
methylimidazoliums [bmim]Cl, [amim]Cl or [emim]OAc (Viswanathan et al.
2006; Sun et al. 2008; Zhang et al. 2007). We have reviewed these systems in
detail (Hummel et al. 2014). ILs have several advantages; the chemical inertness
of ILs if compared to the viscose reagents (NaOH/CS2) or the Lyocell solvent (a
N-oxide) reduces chemical hazards and allows functional additives that would
react with other solvents. However, these are not without their own problems.
4
Chloride-containing ILs such as [bmim]Cl cause corrosion problems in industrial
equipment, and can be environmentally toxic (Bernot et al. 2005). Imidazolium
ILs are quaternary ammonium compounds that, once formed, remain liquids up to
very low pressures and are exceedingly difficult to purify, as distillation requires
high vacuum (0.03–0.05 mbar) and a high temperature (175 ºC) and causes
dealkylation and other degradation of the IL (King et al. 2012). Imidazoliums are
also not chemically inert: deprotonation at the C-2 position gives a reactive N-
heterocyclic carbene. It can attack the reducing end of the polysaccharide, and as a
nucleophilic catalyst, causes depolymerization (Grasa et al. 2002; Schrems et al.
2010). Also, depolymerization occurs, as the proportion of the longest chains
(DP>2000) is significantly reduced when cellulose is dissolved in [emim]OAc.
Most importantly, however, the tensile strengths achieved in such processes are
typically 130–375 MPa (9–25 cN/tex), which is inferior to Lyocell, but similar to
viscose. In other examples with sufficiently high cellulose concentration and draw
ratio, reasonable (402–569 MPa or 27–38 cN/tex, Laus et al. (2005)) or good
tenacities (654–801 MPa or 44–53 cN/tex, Kosan et al. (2008)) were obtained
with alkylmethylimidazolium chlorides or [emim]OAc. We have also studied the
use of [emim]OAc for spinning. However, we have only attained a tenacity of 13–
16 cN/tex (200–240 MPa) with a maximum DR of 3–4 for 6–20% solutions, and
only so if spun from a large 250 µm spinneret (Hummel et al. 2011; Hauru et al.
2012b).
In this study, we present spinning from a novel ionic liquid solvent, 1,5-
diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) (Michud et al. 2014;
Hummel et al. 2014; Parviainen et al. 2013). This solvent solves many of the
aforementioned problems: it is distillable, forms dopes with a rheology practically
identical to that of NMMO, requires only a cold water bath for cellulose
regeneration, is noncorrosive and safe to handle, and most importantly, gives
exceptionally strong filaments, reaching or exceeding the level achieved in
Lyocell. Recycling of [DBNH][OAc] in the spinning process is expected to be
very similar to that in the Lyocell process: after treatment to remove impurities,
the solvent is simply reconcentrated by evaporation of water. The effects of draw
ratio, extrusion velocity, spin bath temperature and spinneret aspect ratio on
mechanical properties and orientation are explored by experiment.
5
Materials and methods
[DBNH][OAc] was synthesized by protonation of DBN (1,5-
diazabicyclo[4.3.0]non-5-ene, 99%, Fluorochem, UK) with acetic acid (99.8%).
The acid was added carefully in increments, allowing the temperature to rise to 80
ºC to keep the forming IL in liquid state. The stoichiometry was checked with 1H
NMR analysis with a Varian Unity Inova 600 spectrometer (King et al. 2011).
Prehydrolysis-kraft dissolving pulp (Eucalyptus urograndis, 93% cellulose I, Mw
= 269 kg mol–1, Mn = 79 kg mol–1 and polydispersity 3.4, Bahia Speciality
Cellulose, Brazil) was cut into powder in a Wiley mill (1 mm sieve). The dope
(13% cellulose) was prepared with a vertical kneader system with an anchor blade
as described in detail previously (Hauru et al. 2013). Air-dry pulp was added to
the molten IL in 2 m-% increments, while kneading for 3 h at 10 rpm and 80 ºC in
50–200 mbar vacuum to remove air bubbles due to entrained air from the pulp.
The solution was press-filtered with 2 MPa at 100 ºC through layered filter mesh
(GKD Ymax2, 5 µm nominal, Gebr. Kufferath AG, Germany), deaerated
overnight at 80 ºC in a vacuum oven (<50 mbar), and stored protected from
moisture in double heat-sealed polyethylene bags.
To determine the rheological properties of the dope, a frequency sweep was done
at spinning temperature (70 °C) with a Paar Physica MCR 300 rheometer, with a
plate-plate geometry (25 mm plate, 1 mm measuring gap). The zero-shear
viscosity from the Cross model (Cross 1965) was 11551 Pa·s. The cross-over
point was at the angular frequency of 2.31 s–1, where the moduli were 5315 Pa.
The dope exhibits the expected shear thinning behavior (Fig S1), and is in terms
of rheology practically identical to a corresponding 13% NMMO dope at 95 °C in
the high-frequency regime.
Filament spinning was done with a laboratory piston spinning system (Fourné
Polymertechnik, Germany) through monofilament spinnerets with a diameter of
100 µm and a length-diameter ratio of the spinneret capillary (L/D) of 0.2 or 2
(Enka Tecnica, Germany). Cylinder pressure was measured with a sensor and
limited with a torque-limiting coupling. The spinning cylinder was heated to 70 ºC
and loaded with a piece of the solidified dope (10 g), softened by slight heating,
6
by means of a syringe-type applicator. The filaments were spun over a 1 cm air
gap into a cold water bath (15 ºC), where the formed filament was led over a
teflon guide roller (at 20 cm depth) and via another guide onto a godet couple.
The extrusion velocity (ve) was varied between 0.01 and 0.045 ml min–1 (1.27–
5.73 m/min), and the draw ratio (DR) varied between 1 and 12.5 by adjusting the
take-up speed (5–38 m min–1). The minimum godet speed was 5 m min–1, thus DR
1 was outside the envelope for 0.02 ml min–1 and only DR 1.3 was attainable at
0.03 ml min–1. The filaments were collected on the godet, carefully cut with a
razor blade and removed, washed first in cold (5 ºC) and then in hot water (50 ºC,
5 min), and dried and conditioned without tension in a controlled atmosphere (23
ºC, 50% RH).
Titer determination and tensile testing in dry and wet mode were carried out with
a Vibroskop–Vibrodyn system (Lenzing Instruments GmbH & Co KG, Austria) in
23 ºC and 50% RH. 10 samples were tested, the gap length was 20 mm and speed
10 mm s–1, and a pretension of 5.9±1.2 mN tex–1 was used (BISFA 2004). Breaks
at <5% (dry) and <10% (wet) elongation were considered incidental and ignored.
The proportional limits and initial moduli were calculated from the stress-strain
curves with a MatLab script according to ASTM standard D2256/D2256M; see
supporting information for details (ASTM 2010). All specimens had clearly
distinguishable elastic and plastic regions in their stress-strain curves when dry,
and only a single region when wet. Resilience (elastic energy density before yield)
was calculated as:
(1)
as yield stress and U is defined as , where U is resilience
(MJ m–3), E is Young's modulus (in MPa), εp is strain at proportional limit
(dimensionless) and σp is stress at proportional limit (MPa).
Birefringence was determined from three selected filaments with a Zeiss Axio
Scope A1 microscope and a Leica B 5λ-Berek tilting compensator. Titer was
measured three times with the vibroscope, permitting 1.5% variation to exclude
7
artifacts, and the diameter calculated assuming a density of 1.5 g ml–1. The
filament was tensioned between two pieces of double-sided tape on a microscope
slide. The optical retardation was determined in triplicate from a selected spot.
Birefringence is the retardation divided by the diameter; a birefringence of 0.062
was assumed to be equivalent to 100% orientation (Lenz et al. 1994).
Results and discussion
A spinning envelope (Fig. 1) for this system was constructed by finding the
extrusion velocities and draw ratios where spinning is possible, in DR steps 1, 3, 5,
7.5, 10 and 12.5, using a spinneret with an length-diameter ratio of the capillary
(L/D) of 2. For Fig. 1, spinning was defined as stable if continuous take-up was
possible for 1–2 minutes without breakage. (This was a practical choice; proper
stability testing for industrial use would involve a multifilament spinneret.) Due to
limitations in take-up speed (minimum godet speed: 5 m min–1) not all draw
ratios were accessible at low extrusion velocities, which is indicated by the white
area in Fig. 1. Only DR 2 was possible at 0.02 ml min–1 and DR 1.3 at 0.03 ml min–
1. The extrusion velocities were limited to 0.045 ml min–1. At higher throughput
(purple area in Fig. 1) fluctuations of the cylinder pressure result in critical
overpressure (33–41 bar). At 0.01 ml min–1, take-up was difficult and spinning not
very stable, with breaks in the bath (yellow area). Filament breaks between the
second guide and the godet limited draw ratios in an extrusion velocity-dependent
manner, with limits of DR 7.5–12.5 (30–40 m min–1). According to the usual
classification into cohesive, capillary and telescope breaches (Gries et al. 2011),
this is a cohesive fracture. At the breakpoint, the filament has solidified, thus it
cannot transmit capillary waves, and does not have the liquid core a telescope
breach requires.
8
Fig. 1 The spinning envelope, with circles for sampled points; white dashed lines indicate
takeup velocities
Linear density (titer)
In order to determine the function describing the dependency of the titer from the
draw ratio, the titer (linear density, d) was measured as a function of the draw
ratio for L/D 2 at extrusion velocity ve = 0.02 ml min–1 and L/D 0.2 at 0.04 ml
min–1. There was no major difference between the two, and combined, they are
well described (Fig. 2) by a reciprocal function: d = (22.4±0.4 dtex) DR–1.
Permitting the negative exponent to vary, it was 0.95±0.02, i.e. consistent with 1,
which is in contrast to 0.5 found by Kong and Eichhorn and Mortimer for NMMO
(Kong and Eichhorn 2005; Mortimer and Péguy 1996a). For NMMO, the change
of extensional viscosity with cooling of the jet results in an exponent other than
unity. Instead, here, a mass transport balance for simple deformation describes the
draw:
(2)
where ρ is dope density (1100 kg m–³), w polymer concentration (13 m-%), r
spinneret radius (50 µm) and s a correction factor for contraction on solidification
and drying. ρwπr² = 11.2 dtex, thus s = 1.994±0.004. This factor has the same
value as in the Lyocell process. (See supporting information for further
discussion.)
9
Fig. 2 Titer as a function of draw ratio, with aspect ratios and guide rotation varied; gray lines:
confidence and prediction bands
Titers from varying extrusion velocity (Ve) fit to the same form of equation: d =
(23.1±0.2 dtex) DR–1 (Fig. 3), showing that titer was unaffected by extrusion
velocity and thus absolute speed. In contrast, Mortimer et al. (Figure 2, p. 254–
255) found a large effect from extrusion velocity on jet profile with NMMO. In
that system (15% solution extruded at 115 ºC, 150–200 µm spinneret, 0.041–
0.096 ml/min, DR 3.6–15.4), the filament profile was set by solidification due to
cooling. Due to this, a higher extrusion velocity produced significantly thicker jets
up to an air gap of 100 mm. Line speed itself changed the jet profile only a little
due to slightly more efficient cooling at lower speeds (Mortimer et al. 1996).
Another contrasting system was 5% cellulose in [bmim]Cl at 90 ºC (spinneret 150
µm, 0.14–0.22 ml/min or 7.7–12.3 m/min), for which Jiang et al. (2012) found a
significant change of properties with speed. However, they used a relatively
nonviscous dope (zero-shear viscosity 50 Pas) for which other forces than
resistance to deformation, such as the inertial contribution or skin drag, could be
important (see supporting information). Whereas, in our system an extruded
volume element undergoes only simple deformation in the jet.
10
Fig. 3 Titer vs. draw ratio and extrusion velocity (ml min–1)
Tenacity and modulus
As expected, tenacity (specific tensile strength, σ) increases with higher draw
(Fig. 4), reaching 36.8±2.2 cN tex–1 (552±34 MPa) at DR ≥ 5, which is similar to
Lyocell. At 18.9–19.9 cN tex–1 (310±33 MPa), the tenacity of undrawn (DR 1)
filaments is comparable to viscose (340 MPa, Adusumali et al. (2006)). A
function of the form σ = σmax(1–a/DR) was employed to describe the development
of tenacity vs. DR. If no extrusion velocity dependency is assumed, the parameters
are σmax = 39.66±0.87 cN tex–1 and a = 0.51±0.04. Indeed, there was no trend with
extrusion velocity within the tested range. Although the scatter was too high to
evaluate any correlation, a 3rd degree polynomial surface is added in Fig. 4 assist
with visualization of the data points. Elongation at break was 14.6±1.7% at DR 1,
but already at DR 3, it reduced to 9.1±1.4%, i.e. statistically consistent with its
ultimate value of 8.2±1.8% for DR ≥ 5 (Fig. S2). This is significantly smaller than
values obtained for other ILs (11.2–15.5%), but comparable to Lyocell (Kosan et
al. 2008; Adusumali et al. 2006).
11
Fig. 4 Tenacity (cN tex–1) vs. draw ratio and extrusion velocity (ml min–1); the vertical scale is
inverted for clarity
The elastic modulus (Fig. 5) was rather high, up to 15.4±2.0 cN tex–1 %–1
(23.2±3.1 GPa), which is in the range of Lyocell and rayon tirecord fibers
(Adusumali et al. 2006). The maximum was reached at DR 5. However, it returned
to 11.9±2.2 cN tex–1 %–1 (17.8±3.3 GPa) at DR 12.5. This, along with the absence
of changes in other parameters suggests that at DR 7.5 and beyond, the filament is
probably slipping on the godet, causing fluctuations of the instantaneous DR, thus
causing damage (slight lateral cracks). This is supported by the observation that
the filament must be still wet when stretched. If the stretching was done while the
filaments were dry, this would be obviously apparent in filament testing: the
elastic region would be absent and only plastic deformation would occur with a
modulus of ca. 5 cN tex–1 %–1, a behavior which is not observed (Gindl et al.
2008b).
The proportional limit was 1.05±0.09%, in a manner independent of DR and ve
(Fig. S3). A durable and processable yarn would have a high elastic resilience, i.e.
the strain energy density where the material is yet undamaged. Indeed, resilience
(Fig. S4) was found to be largely constant at 1.10±0.32 MJ m–3, which is very
similar to Lyocell (0.8–1.6 MJ m–3) (Gindl et al. 2008a). DR 12.5 filaments had a
higher limit of 1.25±0.24% and a correspondingly higher resilience of 1.39±0.78
MJ m–3, but due to the scatter—again probably because of damage—this is not
statistically significant.
12
Fig. 5 Elastic modulus vs. draw ratio and extrusion velocity (ml min–1)
Orientation
The total orientation starts at a high level (58±3%) and immediately increases to
the final value (69±5%), and is not a function of extrusion velocity: birefringence
is described by the function Δn = (0.044±0.001) – (0.0080±0.0023)/DR, with no
pattern in the residuals (see Figs. 6, S5). Qualitatively, this is in agreement with
results with NMMO by Mortimer et al., who found Δn starting at a constant value
(Δn = 0.023) and tending towards an asymptote (0.041, Mortimer et al. (1996)).
Quantitatively, our filaments start even higher (0.036±0.002) although the final
value is similar (0.043±0.003).
Northolt proposed a correlation between birefringence and compliance (E–1, the
inverse of modulus) (Northolt 1985; Kong and Eichhorn 2005). With our data, the
correlation gave the shear modulus of g = 0.60±0.06 GPa (see Fig S6), a rather
low value if compared to g = 2.5 GPa from Northolt (1985) or 3.6 GPa from Kong
and Eichhorn (2005). However, the fit was not good, because low compliances are
not reached, probably because of the fibers being damaged at high draw ratios.
Additionally, tenacity was not significantly correlated with birefringence for
DR>1.
13
Fig. 6 Birefringence vs. draw ratio and extrusion velocity (ml min–1)
Wet ultimate tensile strength was 76±28% of dry strength for DR>1, and 67±18%
for DR 1, unaffected by ve (Fig S7). Similarly, elongation at break when wet was
143±49% of dry strength for DR>1, and 137±55% for DR 1 (Fig S8). This is
concurrent with the trends in total orientation (constant at DR>1). The explanation
for the independence of the ratio from DR and ve is that the orientations of
crystalline and amorphous parts increase in lockstep, because the deformation is
so fast that there is no time for relaxation and crosstalk between the proto-
crystalline and proto-amorphous domains in the jet (Fig S9). Or, by
contraposition, a simple increase in crystallinity does not cause the increase in
tenacities, nor do the crystallites orient independently without concomitant
amorphous orientation. In contrast to Jiang et al. (2012), speed had no observable
effect, since for our very high-viscosity dope, the viscous contribution to the line
stress dominates over other contributions such as inertial and drag. The same view
is also supported by a similar argument with wet modulus. The wet modulus is
traditionally reported as the BISFA modulus, which is the stress at 5% strain
while immersed in water (BISFA 2004). Since wet fibers have a simple, almost
exactly linear stress-strain curve ending in a random break, this is a valid proxy
for the true wet modulus. In Fig. 7, it can be seen that the BISFA modulus follows
a rather different pattern from the dry modulus, with a near-linear increase to a
maximum at DR 8. The ratio of dry to wet modulus (Fig. 8) decreases from
9.2±0.5 to 4.2±0.6 from DR 1 to 10, which is in quantitative agreement with
results from NMMO (Coulsey and Smith 1996). Namely, there is a smooth
14
transition from randomly oriented amorphous "defects" aligning into an axial
orientation, transferring the load from the swollen defects into aligned crystallites
and thus significantly increasing the stiffness when wet. Thus, the key parameter
for mechanical properties is suggested to be the orientation of amorphous domains
rather than aggregate quantities such as total orientation or crystallinity. An
unbroken load path through crystallites (Fig. S9) gives the filament its high
modulus. The low absolute modulus at high DR is incidental and caused by
slipping, and can be corrected by technical improvements in the process.
Orientation inferred from Fig. 8 continues to improve to ca. DR 5, which
demonstrates that the system behaves according to the Kratky II limiting case
(linked chains drawn axially into alignment) rather than the Kratky I limiting case
(rigid rods being forced together) (Ziabicki 1976; Mortimer et al. 1996). However,
tenacities improve only slightly after DR 5 and cannot continue to improve with
draw if lateral defects remain or are created, because even a single significant
defect along the 20 mm measurement length can cause early rupture in the tensile
strength test. The damage to the filament can be inferred from the modulus data
(Fig. 5), because filaments are stretchier at weak points due to a smaller effective
area, reacting with greater deformation to the same force.
Fig. 7 BISFA modulus vs. draw ratio and extrusion velocity (ml min–1)
15
Fig. 8 Ratio of dry to wet modulus with an exponential fit
Effects of the aspect ratio of the spinneret and guide-to-godet stress
A spinneret has a convergent conical exit nozzle, terminating in a short straight
capillary, and the aspect ratio is the length-to-diameter ratio (L/D) of the latter.
For comparison, filament was spun with a L/D 0.2 spinneret and ve = 0.04 ml
min–1 and compared to L/D 2.0 results above (Figs 3, 4, 5). First, the maximum
draw ratio was 5.0 instead of 7.5, with consistent breaks at DR 5.5 and 7.5.
Furthermore, there were no statistically significant differences in titer, modulus or
tenacity (Figs 2 and 9), and thus the correction factor s remained the same at 2
(exactly, s = 1.95±0.08). For NMMO, a slight birefringence of the jet near the
nozzle has been observed (Mortimer and Péguy 1996a), and better strength
properties at a given DR can be obtained by increasing L/D (Zikeli et al. 1992),
which suggests that a longer spinneret gives a slightly better orientation. Indeed,
at DR 5, total orientation at L/D 0.2 was 0.65±0.11 or slightly lower than for L/D 2
(0.72±0.11), albeit the difference is not statistically significant.
Given that breaks occur around the second guide, the question is if reducing the
stress there would give access to higher draw ratios. Thus, the second guide was
connected to the godet with a rubber belt, giving it equal linear speed. The DR 1
filament was indeed thinner, but the DR 5 filament was unaffected (Figs 2 and 9).
The breaks occur in the bath, and the same limit remains; thus, the limit may be
16
because of stress on the partially regenerated filament in and immediately after
the bath rather than only because of the stress at the observed breakpoint.
Fig. 9 Effects of aspect ratio and guide rotation on tenacity and modulus; some tenacity points
offset on x-axis for clarity
Effect of bath temperature
Given that diffusion is faster at higher temperatures, it is natural to ask if the bath
temperature could be increased. However, relaxation of the polymer chains is also
promoted by higher temperatures (Ziabicki 1976). To resolve this question,
coagulation bath temperatures 15, 30 and 45 ºC were explored (with ve = 0.04 ml
min–1, L/D 2, Table 1). The incipient filament was weakened: maximum DR was
limited to 3 at 30 ºC and to 1 at 45 ºC. In the Lyocell process, cooling in the air
gap effectively solidifies the jet before regeneration (Mortimer et al. 1996), giving
access to high draw ratios. Reheating in the bath halts this process, allowing for
more polymer relaxation and causing a loss of orientation. As a result, with the
bath at 30–45 ºC, elongation at break was higher and modulus and resilience were
lower. For drawn (DR 3) filaments the tenacity (24 cN tex–1) was similar to
viscose. A decrease in orientation was apparent from the drop in birefringence:
total orientation decreased from 57±2% to 42±5% for DR 1 and from 70±3% to
59±3% for DR 3. Thus, proper solidification of the filament prior to regeneration
is crucial: in a warmer bath, orientation is lost, producing a relatively amorphous
structure that can deform plastically but has low tensile strength.
Table 1 Effects of increasing bath temperature on filament properties
17
DR bath
[ºC]
titer
[dtex]
tenacity
[cN tex–1]
elongation
[%]
modulus
[cN tex–1 %–1]
resilience
[MJ m–3]
birefringence
1 15 24.0±3.0 18.9±2.0 16.3±2.8 6.9±1.0 0.9±0.3 0.036±0.001
30 22.8±3.5 16.4±2.3 28.3±8.5 5.7±1.1 0.7±0.3 0.026±0.002
45 24.9±3.6 14.0±1.8 29.0±5.4 4.7±0.6 0.7±0.2 0.026±0.003
3 15 7.9±1.2 35.1±4.4 9.7±1.2 13.1±1.7 1.0±0.2 0.043±0.002
30 7.6±0.5 24.1±1.7 12.6±1.4 10.1±1.3 0.9±0.2 0.037±0.002
7.5 15 3.0±0.9 38.5±8.4 7.9±1.7 14.5±1.0 1.2±0.2 0.042±0.002
Conclusions
A dry-jet wet spinning process for spinning cellulose fibers from an ionic liquid
solution is characterized and its limitations and sensitivities discussed. A 13 wt-%
solution of dissolving pulp in an ionic liquid ([DBNH][OAc]) was extruded and
drawn in air and regenerated in water. With extrusion velocities in the range 0.02
to 0.04 ml min–1, drawing in the air gap with DR 7.5–12.5 gave filaments with
high tenacities (36.8±2.2 cN tex–1 or 552±34 MPa) and moduli (15.4±2.0 cN tex–1
%–1 or 23.2±3.1 GPa). Linear density of the finished fibers was proportional to
DR–1. This suggests a simple deformation, rather than the more complicated
deformation seen in NMMO spinning, where the viscosity increase in the air gap
due to cooling leads to a dependency on DR–0.5 instead (Kong and Eichhorn 2005).
Mechanical properties (tenacity, modulus, resilience) and birefringence were
independent of extrusion velocity, and orientation was nearly complete at DR 5.
The lack of a speed dependency suggests again a simple deformation, where
amorphous domains are stretched and thus oriented in lockstep with crystalline
regions. This is explicitly revealed by dry/wet modulus ratio changing from 9 to 4
with increasing draw ratio. Strong filaments were readily spun only with a
coagulation bath temperature of 15 °C, but not with 30 or 45 °C. A spinneret
aspect ratio (L/D) of 2.0 gave a better orientation than L/D 0.2 and allowed to
reach higher draw ratios. However, our small-scale monofilament setup seems to
slightly damage high-draw ratio filaments, because there is slipping on the takeup
godet. Thus, improvement is expected when spinning in multifilament mode,
where the fiber bundle is stronger and less likely to slip than a single filament. In
summary, the process was found to be robust and simple even with the suboptimal
setup of a small monofilament system, and in performance comparable to the
commercial NMMO-based Lyocell process.
18
Acknowledgements The authors wish to thank TEKES – the Finnish Funding Agency for
Technology and Innovation and Finnish Bioeconomy Cluster FIBIC Ltd. for funding in the
framework of the Future Biorefinery Cellulose project. Arno Parviainen synthesized the
[DBNH][OAc] with Dr. Alistair King and Prof. Ilkka Kilpeläinen (University of Helsinki). Dope
samples were kindly provided by Shirin Asaadi. Kaarlo Nieminen assisted with MatLab and GPC
was operated by Lasse Tolonen.
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