Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
Ioncell-F: A High-strength regenerated cellulose fibre
Herbert Sixta, Anne Michud, Lauri Hauru, Shirin Asaadi, Yibo Ma, Alistair W.T. King, Ilkka Kilpeläinen, and Michael Hummel
unique fibre quality and the broad variety of different
fibre types ranging from standard fibers to cotton-like
Modal and Polynosic fibers as well as very strong
technical fibers such as tyre cord. In the beginning of the
20th century, the viscose fibre production grew very
slowly; in 1930, only 3.000 t of fibres were produced.
However, the viscose boom began in the 1930s with
many new installations, triggered by the continuously
improving technology and the starting preparations for
war in Germany to become independent in cotton
imports. In 1940, the global viscose production reached
almost 600.000 t during World War II. Parallel to the
upturn of viscose fibres, nylon, the first synthetic fibre,
was invented by Wallace Carothers from DuPont. Its
production began in 1935 (Carothers 1938). The
thermoplastic polymer was first used as a material for
manufacturing women’s stockings, parachutes, ropes, etc.
before it entered the textile market. Although the viscose
fibre market continued to grow until the late 1970s, the
amount of synthetic fibers, complemented by polyester,
polypropylene and acrylic fibres, developed significantly
faster. In the late 1960s, the global production of
synthetic fibres equalled that of viscose fibre production
(FiberYear 2013). Synthetic fibre production continued to
grow by 5-10% on average per year, while that of viscose
fibres peaked at the end of the 1970s, before it lost more
than 40% of its production at the end of the 1990s due to
stricter environmental regulations mainly in Europe and
the United States. The high costs associated with the
improvement of the environmental impact of the viscose
process were the major driver for the development of
environmentally friendly man-made cellulose fibre
processes.
Two processing routes have been pursued: The aqueous
route via sulphur-free derivatization or direct dissolution
in aqueous sodium hydroxide and the direct dissolution in
organic solvents. The cellulose carbamate process, first
suggested by Neste Oy, gained a lot of attention because
cellulose carbamate can be processed like cellulose
xanthate, but without the release of toxic sulfurous
compounds (Ekman et al. 1984; Huttunen et al. 1982).
The resulting fibre properties were, however, not better
than those of regular viscose and could not reach the
tenacities of Modal or other modified viscose fibres. This
is probably the main reason why the cellulose carbamate
process, despite of several improvements (Ebeling, Fink
2008), has not gained commercialization so far. The
dissolution of steam pretreated pulp in 9 wt% aqueous
sodium hydroxide solution and subsequent wet spinning
to cellulose fibres in an aqueous sulphuric acid containing
coagulation bath was demonstrated by Kamide et al.
(Kamide et al. 1984; Yamashiki et al. 1992). However,
the fibre properties did not even reach those of regular
viscose fibres. Better fibre properties, but not yet
sufficient, could be achieved when zinc oxide was added
to the 7.8 wt% sodium hydroxide solution and the pulp
was activated by an enzyme pretreatment (Vehvilaeinen
KEYWORDS: Regenerated cellulose fibre, Ioncell-F
fibre, Ionic liquids, Structure-property relationship
SUMMARY: In this paper, we report the development of
a novel regenerated cellulose fibre process of the Lyocell
type, denoted Ioncell-F. The process is characterized by
the use of a powerful direct cellulose solvent, 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH][OAc]) a
superbase-based ionic liquid. Compared with the
commercial NMMO-based Lyocell fibre process, airgap
spinning can be conducted at higher cellulose
concentration in the dope, while temperature during
dissolution and spinning can be maintained at a lower
level. Owing to the generally milder process conditions,
the cellulose is less degraded which contributes to both
higher fibre yield and better strength properties. In this
study we demonstrated the effect of different cellulose
concentrations and draw ratios on the fibre properties.
The highest tenacities, consistently above 50 cN/tex,
were achieved by spinning from 15 and 17 wt% cellulose
solutions. A very high initial modulus of up to 34 GPa
makes the Ioncell-F fibres very interesting for technical
applications such as a reinforcing fiber in composites.
The chain orientation in the fibre direction, particularly in
the amorphous regions, revealed the best correlation with
the elastic modulus and the tensile strength of the IoncellF fibres, in agreement with other high-tenacity
regenerated cellulose fibres as reported in the literature.
ADDRESSES OF THE AUTHORS: Herbert Sixta
([email protected]),
Anne
Michud
([email protected]),
Lauri
Hauru
([email protected]),
Shirin
Asaadi
([email protected]), Yibo Ma ([email protected]),
Michael
Hummel
([email protected]),
Department of Forest Products Technology, School of
Chemical Technology, Aalto University, P.O. Box
16300, FI-00076 Aalto, Finland. Alistair W.T. King
([email protected]) and Ilkka Kilpeläinen,
([email protected]) Department of Chemistry,
University of Helsinki A.I. Virtasen Aukio 1, 00014, P.O.
Box 55, Helsinki, Finland
Corresponding author: Herbert Sixta
The first artificial fibres were introduced by the French
scientist Hilaire de Chardonnet at the World Exhibition in
Paris in 1889 (Kotek 2006). The material, consisting of
nitrocellulose, was extremely flammable and thus could
not be of significant use in the textile industry. It was
only a few years later, in 1892, that the first patent on the
viscose process was granted to the British scientists
Charles Cross, Edward John Bevan, and Clayton Beadle
(Cross et al. 1892). The first commercial viscose plant
was built by the British company Courtaulds Fibres in
1905 before the first industrial plants came on stream in
Central Europe and the United States. Although the use
of toxic carbon disulphide posed significant health and
safety risks, the viscose technology succeeded against
other fibre production technologies mainly because of the
43
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
low amount of pulp losses through degradation ensures
the high cost- and eco-competitiveness of the NMMObased Lyocell process (Firgo et al. 1994; Marini et al.
1994; Eichinger, Eibl 1995; Firgo et al. 1995). In
addition, NMMO-based fibres, e.g. Tencel®, reveal an
excellent wearing comfort owing to their superior
moisture absorption/desorption behaviour and the much
better dimension stability under wet conditions than
viscose fibres (Firgo et al. 2006).
However, NMMO, as a cellulose solvent, has some
intrinsic shortcomings such as its chemical and thermal
instability which affords the addition of stabilizers, but
are no guarantees of avoiding dangerous runaway
reactions (Rosenau et al. 2001). The high viscosity of the
cellulose solution in NMMO monohydrate already at
moderate polymer concentrations of e.g. 13 wt% sets
limits in both the process economy and the strength
properties of the fibres.
The extension of the Lyocell spinning technology to
direct cellulose solvents of high thermal and chemical
stability comprising lower solution viscosity is very
attractive from a safety, environmental and economic
point of view.
With the rediscovery of ionic liquids (ILs) as powerful
cellulose solvents in 2002 by Rogers and his co-workers
new research efforts were initiated to design task specific
ILs aiming at the substitution of NMMO as the only
commercial direct cellulose solvent (Swatloski et al.
2002). The ionic liquids, which proved to be effective
cellulose solvents and thus have been used for the
preparation of spinning dopes, were all imidazoliumbased. It was shown that, compared with the NMMO
process, the direct dissolution of cellulose is more easily
controlled, the process is inherently safer, and fibres with
properties equal to those produced from NMMO solution
were obtained (Bentivoglio et al. 2006; Laus et al. 2005).
However, the imidazolium-based ionic liquids have
shown to be not inert towards cellulose (Ebner et al.
2008). Depending on the substituents on the imidazolium
ring and the chemical nature of the anion, cellulose
underwent severe degradation, especially at higher
temperatures (>90°C) which again afforded the addition
of stabilizers. The anions of the ILs of the first generation
were typically halides which have a high corrosive
potential toward metal processing equipment (Cai et al.
2010). One of the best cellulose solvents is certainly
[emim][OAc], also due to the low viscosity of the
resulting cellulose solution. The disadvantage, however,
is that the viscoelastic properties, appropriate for a stable
spinning behavior, are only achieved at a very narrow
concentration range. Kosan et al. succeeded in successful
spinning only after increasing the cellulose concentration
to almost 20 wt% (Kosan et al. 2008).
In the search for non-imidazolium-based, efficient
cellulose solvents, researchers at Helsinki University and
Aalto University were successful in developing a novel
cellulose spinning solvent consisting of a superbase / acid
ion pair, which revealed excellent spin stability resulting
in outstanding fibre properties (Michud et al. 2014). 1,5diazabicyclo[4.3.0]non-5-enium acetate, [DBNH][OAc],
turned out to be the most suitable among the investigated
superbase-based ILs for airgap spinning and the
et al. 2008). The development of this Biocelsol fibre
process is continuing and further progress also in other
application areas has been reported (Rosenberg et al.
2008). Another sulphur-free approach constitutes the
cellulose carbonate process (Oh et al. 2002). There,
sodium cellulose carbonate is produced by reacting
carbon dioxide with alkali-cellulose in ethyl acetate at 5°C. The cellulose carbonate is dissolved in an 8.5 wt%
NaOH/ZnO solution at 0°C. The resulting fibre properties
were, however, weaker than the reference viscose fibres
(Oh et al. 2005). Lina Zhang has shown that urea can
considerably improve the solubility of cellulose in
aqueous alkaline solution. It was speculated that the role
of urea in cellulose dissolution may be explained by its
hydrophobic interactions with the hydrophobic side of
cellulose (Xiong et al. 2014). Even though higher molar
mass cellulose can be dissolved and wet spun, the
resulting tensile strength reached only 19 cN/tex, which
is clearly lower than the 25 cN/tex typical for regular
viscose fibres (Cai et al. 2007).
All aqueous regenerated cellulose fibre processes suffer
from a relatively low polymer concentration of a low
cellulose-DP in the spinning dope owing to limited
solubility power of the solvent system. This leads to
moderate or even poor strength properties of the fibres
and the need of additives (ZnO, urea, etc.) which in turn
complicates the recovery system and thus increases the
recovery costs of the solvent.
The discovery of N-methylmorpholine N-oxide
(NMMO) monohydrate as a very powerful cellulose
solvent was the starting point of the development of a
new category of a regenerated cellulose fibre process, the
Lyocell process (Johnson 1969). Lyocell was recognized
as the generic name by BISFA (International Bureau for
the Standardization of Rayon and Synthetic Fibres,
Brussels) and the Federal Trade Commission (USA) for
solvent-spun fibres (BISFA 2009). The Lyocell process
was developed in different phases. The first serious fiber
process development was done by American
Enka/Akzona Inc. from 1969 to 1979, but it was decided
not to scale up. Courtaulds started the research on the
Lyocell process in 1979 and only slightly later Lenzing
AG followed. The first full-scale production plants were
installed in Mobile (USA) in 1992 by Courtauld and 1997
in Heiligenkreuz (Austria) by Lenzing AG. In the
meanwhile, Lenzing is the only producer of Lyocell
fibres in large scale with a total annual production
capacity of 220 kt on its production sites in Mobile
(USA), Grimsby (UK), both former Courtaulds mills, and
Heiligenkreuz and Lenzing in Austria. The Lyocell
process starts with a thorough mixing of a disintegrated
bleached chemical pulp with a pre-adjusted DP (either
paper or dissolving pulp depending on the fibre use) in an
aqueous NMMO solution (Fink et al. 2001). Then water
is evaporated in a filmtruder until cellulose is dissolved
(monohydrate). The resulting dope is degassed, filtered
and extruded via an airgap, which is climatized, and
where the filaments are drawn. The liquid filaments then
enter a coagulation bath consisting of a highly diluted
aqueous NMMO solution. Staple fibre production is
completed after washing, cutting, finishing and drying.
The almost quantitative solvent recovery and the very
44
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
Rheological measurements
formation of regenerated cellulose fibres. This new dryjet wet spinning process was denoted as the Ioncell-F
process and the fibres produced therefrom, the Ioncell-F
fibres.
In this study we report about the cellulose dissolving
power of [DBNH][OAc], the rheology of 10-17 wt%
cellulose solutions, the regeneration behavior in the
presence of different amounts of water and the
performance during airgap spinning where draw ratios
(DRs) of 0.9 -17.7 have been applied. The structure and
the mechanical properties of the Ioncell-F fibres are
discussed and compared with those of existing
regenerated cellulose fibres from commercial and noncommercial sources. The ultimate strength potential of
Ioncell-F fibres is roughly estimated based on a
simplified cellulose model and the obtained structural
parameters.
Materials and Methods
Shear rheology of all solutions was measured on an
Anton Paar MCR 300 rheometer with a plate and plate
geometry (25 mm plate diameter, 1 mm gap size). The
viscoelastic domain was determined by performing a
dynamic strain sweep test and a strain, which fell well
within the linear viscoelastic regime, was chosen for the
frequency sweep measurements. A time experiment at
constant frequency showed no significant water uptake
from the laboratory atmosphere at the plate edges within
the required testing time. Thus, it was not necessary to
seal the edges with paraffin oil as previously suggested
(Gericke et al. 2009; Sammons et al. 2008). Each sample
was subjected to a dynamic frequency sweep at 25°C
over an angular velocity range of 0.1–100 s-1. In order to
analyse the structure formation upon addition of water,
values at an angular frequency of 0.85 s-1 were taken to
plot dynamic moduli and the damping factor versus the
water content.
Materials
Spinning trials
Eucalyptus urograndis prehydrolysis kraft (E-PHK) pulp,
Bahia rayon pulp, Brazil (intrinsic viscosity, Cuen, ISO
5351-1, of 468 ml/g, a Mn=79.8 kg/mol and a Mw=268.7
kg/mol, determined by GPC-MALLS) was received in
the form of sheets and ground by means of a Wiley mill.
1,5-diazabicyclo[4.3.0]non-5-ene (99%, Fluorochem,
UK) and acetic acid (glacial, 100%, Merck, Germany)
were used as received. [DBNH][OAc] was prepared by
slowly adding equimolar amounts of acetic acid to DBN
while diverting the exothermic reaction enthalpy by
active cooling.
Cellulosic fibres were spun using a customized laboratory
piston spinning unit (Fourné Polymertechnik, Germany)
depicted in Fig 1. The cylinder was loaded with solid,
shaped [DBNH][OAc]-pulp solution which was then
heated to 70 °C to form a homogeneous, air bubble free
highly viscous spin dope. The solution was then extruded
at 65–80°C through a multi-hole spinneret (36 holes,
capillary diameter 100 µm, L/D 0.2) via a 1 cm airgap
into a water coagulation bath. Airgap distance, immersion
depth to the first deflection roller, deflection angle, and
the retention distance of the filament bundle in the
coagulation bath were kept constant throughout all
spinning trials. The temperature of the coagulation bath
was kept constant at 15°C during all trials. The extrusion
rate (ve) was fixed at 1.6 ml/min (5.7 m/min) while the
take-up velocity (vtu) of the godet couple was varied from
5–100 m/min, resulting in draw ratios Dr =vtu/ve of 0.88
to 17.7. The fibres were washed offline with hot water
(60°C) and air dried.
Preparation of spinning dope
[DBNH][OAc] was first liquefied at 70°C by means of a
water bath and then mixed with air dried pulp to a final
consistency of 13 wt-% unless otherwise stated. The
pulp-IL suspension was then transferred to a vertical
kneader system which was described in detail earlier
(Hauru et al. 2012). The mixture was stirred at 80°C and
10 rpm under reduced pressure (50 mbar) and complete
dissolution was observed in less than 90 min. The
solution was then filtrated by means of a hydraulic press
filtration device (1–2 MPa, metal filter fleece, 5–6 µm
absolute fineness, Gebr. Kufferath AG, Germany) to
assure constant solution quality throughout the spinning
trials. The solution was shaped to the dimensions of the
spinning cylinder and solidified within 1 to 3 days after
preparation.
Mechanical properties
Linear density (titer), conditioned and wet tenacity were
measured at 23ºC and 50% relative humidity using
a Vibroskop 400 and Vibrodyn 400 (Lenzing
Instrument), respectively. The settings for the Vibrodyn
400 were as follows: gap length: 20 mm; pretension:
5.9±1.2 mN tex-1; filament stretching velocity: 10 mm s-1.
Regeneration studies
For detailed regeneration studies, 1 wt% stock solutions
of pulp in [DBNH][OAc] were prepared in a round
bottom flask by addition of milled pulp to [DBNH][OAc]
at ca. 80°C and subsequent stirring for 1h. Due to the low
viscosity, agitation was possible with a magnetic stir bar.
Subsequently, ca. 20 ml batches were taken from the
stock solution and water was added gradually. After each
addition of water, an aliquot was taken for rheological
characterization. Per batch, water was added no more
than 4 times to avoid error accumulation.
Fig 1 - Schematic illustration of the spinning unit. The filaments
are extruded from the piston spinning unit with a velocity ve
(extrusion velocity), pass through the air gap and immerse into
the coagulation bath. The filaments are stretched in the air gap
by increasing vtu (take-up velocity) of the godet couple.
45
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
Table 1 - Effect of DR on crystallinity and orientation
parameters of Ioncell-F fibres spun from 17 wt% E-PHK pulp in
[DBNH][OAc].
In agreement with the findings of Hauru et al. from
monofilament spinning (Hauru et al. 2014), the linear
density (titer) of fibres is inversely related to DR
⁄ , where d is the
according to the relationship
linear density in dtex (g/104 m), C is a constant and DR is
the draw ratio. For 10, 13, 15 and 17 wt% cellulose
concentrations, C values of 15.55±0.37, 19.39±0.33,
23.06±0.33 and 22.52±0.40, respectively have been
calculated.
DR
CrI,%
fcr
n
ft
fam
Kamlet–Taft parameters.
The Kamlet–Taft parameters were determined from the
absorption peaks of the three dyes Reichardt's dye (RD,
range 518–585 nm), N,N-diethyl-4-nitro-aniline (DENA,
402–414 nm) and 4-nitroaniline (NA, 406–398 nm). The
dyes were weighed as is and mixed with the ILs to a peak
absorbance of 0.2–2.5 AU (i.e. concentrations were RD
1.1, DENA 0.24 and NA 0.27 mmol/g), such that the
absorbance curve remained smooth. A Varian UV-VIS
spectrometer equipped with a thermostat (precision
±0.1°C) was used. Deionized water was measured for
background subtraction. Spectra were collected at a
resolution of 1 nm and 10–30 nm of absorbance data
around the peak was fitted with a Gaussian function in
order to precisely locate the maxima (νmax). The result
was a resolution exceeding that of the instrument (1 nm).
From the collected peak data, a linear least-square fit
against temperature was done for absorption maxima at
20–100°C (typically R² > 0.998 for all dyes, standard
deviation 0.05–0.09 nm). From these functions νmax(T),
the ET(30), π*, α and β parameters were calculated
according to Doherty et al (Doherty et al. 2010). The
resulting uncertainty from νmax(T) was ±0.008, ±0.001,
±0.001 and ±0.003 for ET(30), π*, α and β, respectively,
which is below the influence of other sources of error.
0.9
28
0.933
0.0404
0.652
0.596
2.7
33
0.960
0.0435
0.702
0.631
5.3
33
0.960
0.0487
0.785
0.769
10.6
n.d.
n.d.
0.0503
12.4
35
0.964
n.d.
0.811
14.1
36
0.966
0.0506
0.816
0.804
The average orientation of both amorphous and
crystalline cellulose in each fiber sample was determined
using a polarized light microscope (Zeiss Axio Scope)
equipped with a 5 Berek compensator. The
birefringence n of the specimen was obtained by
dividing the retardation of the polarized light by the
thickness of the fibre which was calculated from the
linear density (titer) using a cellulose density value of 1.5
g/cm3 (Maenner et al. 2011). The total orientation factor ft
was then derived by dividing n by the maximum
birefringence of cellulose 0.062 (Lenz et al. 1994). This
value was preferred over the value (0.057) which has
been obtained by the evaluation of the birefringencecompliance relationship from Ioncell-F fibres and other
regenerated cellulose fibres from the literature (see later).
A factor ft = 1 means perfect orientation parallel to the
fibre axis, ft = 0 for random orientation, and ft = -1 for
perfect transverse orientation. Subsequently, the
amorphous orientation factor fa was calculated by Eq 1
∗
∗ .
[1]
where 0.91 is the ratio of the density of amorphous and
crystalline cellulose (Fink, Walenta 1994; Gindl et al.
2008b; Hermans, Weidinger 1949). The influence of
draw ratio on the crystallinity and the orientation
parameters from both WAXS and birefringence
measurements is summarized in Table 1.
WAXS and birefringence measurements
Wide angle X-ray scattering measurements were
performed at the IAP Fraunhofer Institute, Golm
Germany, by means of a two-circle diffractometer D5000
(Fa. Bruker-AXS, Germany) using monochromatic CuK radiation in symmetric transmission (with Ge(111) as
monochromator, =0.15406 nm; at 30 mA and 40 kV).
The step width  was 0.2° with a measurement time of
180 s/. The scattering curves were corrected
concerning absorption, polarization, Compton, and parasitic scattering (Röder et al. 2006; Fink et al. 1985). From
the corrected and normalized WAXS curves, the degree
of crystallinity xc and the lattice disorder parameter k
according to the Ruland/Vonk method, and the crystallite
dimensions D(hkl) from the width of the reflections via the
Scherrer equation were determined. The crystallite chain
orientation OG was determined by an azimuthal scan of
the meridional main interference taken from well-aligned
fibre samples in the longitudinal direction. It is defined
by (180° - FWHM)/180°, where FWHM is the full width
at half maximum of the (004) reflection plane. The
Herman crystalline orientation factor was determined
with the IAP software WAXS7 (Fink et al. 2004). The
average crystallite dimension was calculated from the
FWHM of the lateral main peaks according to the
Scherrer formula: D(hkl) = 0.9 / (FWHM*cos ).
Results and Discussion
In the following section, we report about the Ioncell-F
spinning process, including the E-PHK pulp dissolution,
the dope characterization, the regeneration behaviour in
water, the spinnability of different dope concentrations
and the mechanical properties of the resulting fibres.
Further, we relate the properties to structural parameters
and compare the results with those from other
regenerated cellulose fibres reported in the literature.
Dissolution and regeneration
The empirical Kamlet-Taft solubility parameters
characterize the H-bond acceptor and H-bond donor
capabilities of a solvent, represented by  and 
respectively (Kamlet et al. 1983; Taft, Kamlet 1976;
Kamlet, Taft 1976). Quite recently it was found that the
net-basicity concept, expressed as -as a function of,
was a reasonable empirical descriptor for the cellulose
dissolving capability of direct solvents in general and
ionic liquids in particular (Parviainen et al. 2013; Hauru
et al. 2012). In the search for cellulose-dissolving nonimidazolium-based ionic liquids a couple of acid-base
conjugates revealed net-basicity values which were in the
46
30000
[TMGH][OAc]
NMMOH2O
20
NMMO2H2O
100
LiCl/DMAc
[Rmim][MeP]
[eimH][EtCO2]
-0,5
[DBNH][OAc]
[HOC2mim]
[emim]
[bmim]
-1,0
0,0
0,3
0,6

0,9
13 wt% Cellulose
1,2
1,5
Fig 2 - Net-basicity plot of selected ionic liquids and known
cellulose solvents; full symbols are cellulose solvents, empty
symbols are non-solvents. Figure adopted and supplemented
from Hauru et al. (2012)
2


6000
1
4000
COP
COP
2000
*G'=G''
3
[]*0
[]*0
15000
[Pnnnn]
[Rmim][MeOHPO2]
0,0
NMMOxH2O
[emim][OAc]
[TMGH][EtCO2]
(s-1)at COP
0,5
75000
60000
45000
COP (Pa)*
1,0
[]*0 (Pa.s)

[DBNH][OAc]
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
0
0
70
75
80
85
90
95
100
Temperature (°C)
Fig 3 - Complex viscosity ([]*0), dynamic moduli at COP and
angular frequency () of the COP recorded via oscillatory
rheology measurements (frequency sweep in the linear
viscoelastic range) from a 13 wt% solution of an E-PHK pulp in
[DBNH][OAc] and NMMO monohydrate. The shaded areas
represent the spinnability range.
obtain the complex viscosity and the dynamic moduli as a
function of the angular frequency. Further, the angular
frequency of the crossover point (COP) at which the
storage and loss moduli intersect is recorded because it
provides valuable information on the molecular mass
distribution of the cellulose and the effect of temperature
on the elastic properties of the dope. In a recent
publication, a master curve of a 13 wt% E-PHK solution
in [DBNH][OAc] is shown for a typical spinning
temperature of 75°C (Hummel et al. 2014). Fig 3
illustrates the profile of the complex viscosity, the
dynamic moduli and the angular frequency at the COP as
a function of the dope temperature for both the 13 wt%
solution of E-PHK in [DBNH][OAc] and NMMO.
The shaded areas in Fig 3 represent the temperature
range within which good spinnability is ensured. Thus,
stable spinning behaviour is observed for both solvents
where the angular frequency of the COP was
approximately at 1 s-1 or slightly higher, the crossover
modulus at approximately 4000 Pa and the zero-shear
viscosity, calculated by the Cross model assuming the
validity of the Cox-Merz rule, between 25 000 and
35 000 Pa.s. Surprisingly, the desired viscoelastic
properties of the [DBNH][OAc]-based spinning dope
were obtained at a temperature that was 20 to 25°C lower
than that of the NMMO solution at the same
concentration of the same pulp. Thus, the substantially
lower thermal stress during dissolution and spinning
enables not only considerable energy savings but leads
also to lower thermal-induced degradation reactions of
both the solvent and the polymer.
While even small amounts of water are critical for the
dissolution of cellulose in most of the ionic liquids, they
tolerate much more water when regenerating the cellulose
from solution. It was shown that the regeneration
behaviour as a function of the water content depends on
the chemical nature of the ionic liquid (Hauru et al.
2012). The onset of the storage modulus G’ increase,
indicating the formation of a gel-like structure, was
observed at 1.6 nH2O/nIL (mole/mole) for [emim][OAc]
and at relatively high 3.03 nH2O/nIL for [TMGH][OAc]
range for those of excellent cellulose solvents (Parviainen
et al. 2013). This is visualized by the net-basicity plot in
Fig 2.
Among acid-base conjugates, [DBNH][OAc] attracted
our special attention as it is solid at room temperature
with a melting point lower than that of Nmethylmorpholine-N-oxide monohydrate (NMMO.H2O).
Upon liquefaction, the viscosity of the neat solvent is
very low – 23 mPa.s at 65°C – and is comparable to that
of [emim][OAc], which is known for its low viscosity
(Hummel et al. 2014). This is of significant practical
importance since the speed at which cellulose dissolves
accelerates with decreasing solvent viscosity. In fact,
complete dissolution of a 13 wt% E-PHK pulp was
accomplished at 80°C in less than 60 min. The mild
dissolution conditions prevent the degradation of both the
solvent and the polymer. With regard to the cellulose, this
was proven in a comparison of the molar mass
distributions of the substrate, regenerate and resulting
fibre, where only a slight degradation of the high molar
mass fraction was observed (Hummel et al. 2014). The
high polymer stability in [DBNH][OAc] is a clear
advantage over other cellulose solvents such as NMMO
monohydrate or imidazolium-based ILs, which require
stabilizers to ensure that degradation is not too severe
(Bentivoglio et al. 2006). The solvation of a cellulose
substrate by direct solvents such as NMMO monohydrate
or ionic liquids results in the formation of aggregate
solutions comprising 130-660 cellulose chains even at
relatively low concentrations (Röder, Morgenstern 1999)
(Kuzmina et al. 2010). The quality of the solution, which
depends on cellulose activation, cellulose concentration
and the presence of water, is comparable for solutions in
NMMO and ionic liquids, as revealed by static and
dynamic light scattering measurements (Kuzmina et al.
2010). The viscoelastic properties, characterized by the
elastic (G’) and loss (G’’) moduli, are crucial for the
spinnability of the cellulose solution. These parameters
can be assessed using oscillatory rheology measurements
(Collier et al. 2009). Every freshly prepared and filtered
spinning dope is subjected to these measurements to
47
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
[DBNH][OAc]
''
20
15
1
0.1
0
10
20
15
10
10
G'
G'' 5
tan ()
1
0,1
10
[emim][OAc] 20
100
0
30
10
20
30
40
40
0
50
5
Damping factor (G''/G')
G
'
G
10
Dynamic moduli (Pa)
Dynamic moduli (Pa)
100
tan ()
using 1 wt% cellulose solutions. Further, the second
regeneration stage, the particle formation, was shifted to a
significantly higher water content for the cellulose
solution in [TMGH][OAc] compared to that in
[emim][OAc]. The high demand of water for the full
cellulose regeneration was explained by the hydrotropic
nature of the bulky [TMG] cation (Hauru et al. 2012).
The rapid regeneration upon the addition of water is an
important criterion for fibre spinning to ensure the
formation of stable filaments at the moment of the
immersion into the spinnbath. The regeneration
behaviour of a 1 wt% solution of an E-PHK pulp in
[DBNH][OAc] as a function of the water content was
investigated via oscillatory rheology measurement. As
shown in Fig 4, the elastic modulus G’ starts to increase
at a water content of approximately 15-17 wt% (1.80 2.09 nH2O/nIL). However, the tan() already peaks at a
water content of 10 wt% (1.14 nH2O/nIL), thereby
indicating a clear change in the solution state. The COP
appears at a water content of 18.6 wt% (2.34 nH2O/nIL
while in the [emim][OAc] solution the COP already
appears at
1.80 nH2O/nIL) which signals the formation
of gel-like structures. As supported by microscopic
images, particle formation begins at a water content of
15-18 wt% (corresponding to the onset of the G’
increase), accelerates substantially at a water content of
25 wt% and reaches completion at a water content of 4050 wt%.
In conclusion, the regeneration behaviour of cellulose
dissolved in [DBNH][OAc] is shifted to a slightly higher
water content compared to that of cellulose dissolved in
[emim][OAc] or NMMO as exemplified in the paper of
Hauru et al. (Hauru et al. 2012). However, no dramatic
change is expected in the regeneration behaviour of
cellulose in a solution of [DBNH][OAc] as compared to
that in NMMO.
0
50
Water content (w/w %)
Fig 4 - Regeneration of a 1 wt% E-PHK solution as a function
of the water content in the solvent [DBNH][OAc], demonstrated
by the profile of the dynamic moduli, G’ and G’’ and the tan()
determined thereof. Insert: Regeneration curve from 1 wt% EPHK solution in [emim][OAc] adopted from (Hauru et al. 2012)
conditions of which the draw ratio is the most important
parameter. Lowering the spin bath temperature
diminishes the relaxation of the fibre orientation during
coagulation and, thus, may also contribute to a higher
elastic modulus (Northolt et al. 2005).
In a comprehensive study, the effect of four different
cellulose concentrations, 10, 13, 15 and 17 wt% in
[DBNH][OAc] on the mechanical properties of the
regenerated cellulose Ioncell-F fibres was investigated.
The only variable of the spinning conditions was the
stepwise increase of the take-up velocity (vtu) of the godet
couple which was adjusted to different levels,
occasionally from 5–100 m/min, thereby resulting in
draw ratios (DR) of 0.88 to 17.7.
On average the temperature of the dopes was adjusted to
66°C for the 10 wt%, 73°C for the 13 wt% and 77°C for
the 15 wt% cellulose solutions to achieve the desired
viscoelastic properties as shown in Fig 3. Surprisingly,
the temperature of the 17 wt% cellulose solution had to
be reduced from the originally 82°C, when the dope was
freshly prepared, to 75°C when spinning entered a stable
phase, while simultaneously the zero shear viscosity
remained at the desired level. The decrease in the
viscosity as a function of temperature could not be
associated with the formation of an ordered phase in the
17 wt% cellulose solution. However, more research is
needed to understand the observed viscosity anomaly.
In a dry-jet wet spinning process the formation of
oriented fibres involves a solution-state draw followed by
desolvation and structure formation in a cooled
coagulation bath (water bath). The solution-state draw is
supported by the highly viscous characteristics of the
solution. The solution experiences a pre-draw in the
spinneret through shear-induced orientation. However,
the anisotropy in the final fibres is mostly induced in the
air-gap. The airgap environment is characterized by an
elongational flow where, with increasing draw, the
solvated cellulose molecules are progressively stretched,
thereby resulting in an orientation along the draw axis. In
NMMO-based Lyocell spinning trials, it was shown that
the elongational viscosity increases exponentially with
the distance from the spinneret (Cousley, Smith 1995).
Fibre spinning
According to Krässig, the fibre tenacity in the
conditioned state is related to the polymer chain length
expressed as the DPn of the cellulose in relation to the
length of the crystallites (LODP), the degree of
crystallinity (CrI) and the square of the degree of
orientation (ft) (Krässig 1993). A linear relationship was
found between the tenacity in the conditioned state of
regenerated cellulose fibres of different origin and the
products
of
the
parameters
according
to 1⁄
1⁄
∙
∙ . Picken et al. derived a
relationship between the fibre modulus and the
concentration of the lyotropic spinning solutions of
synthetic polymers (Picken et al. 1992). The dependency
of the fibre tenacity on the dope concentration was
confirmed for regenerated cellulose fibres prepared from
airgap spinning of ionic liquid solutions (Hong et al.
2013) (Zhang et al. 2008). Likewise, the amount and
nature of non-cellulosic impurities will undoubtedly alter
the perfection of the crystalline structure of the
regenerated cellulose fibre and thus impair the fibre
properties. For a given cellulose substrate and cellulose
solvent, the strength properties of regenerated cellulose
fibres are mainly influenced by the cellulose
concentration in the spinning dope and the spinning
48
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
Simultaneously, the extruded and stretched solution
experienced a decrease in the temperature and an increase
in velocity and birefringence (Mortimer et al. 1996;
Mortimer, Peguy 1996a). This was also observed in the
dry-jet wet spinning of a liquid crystalline solution of
cellulose in superphosphoric acid (Boerstoel 1998). It is
assumed that before entering the coagulation bath, the
cellulose solution is effectively a persistent, oriented
monodomain (Cousley, Smith 1995). In the coagulation
bath, the desolvation and thus structure formation begins
instantaneously, and even though no further draw is
expected, a significant increase in birefringence is
observed. This increase in birefringence in the spinbath
and during washing and drying is due to changes in the
fine structure, i.e. the formation of crystalline cellulose.
All draw-related increase in birefringence is occurring in
the spinneret and the airgap (Mortimer, Peguy 1996a).
The precipitation of cellulose in the coagulation bath is
diffusion driven and most likely follows a spinodal
decomposition mechanism (Eckelt et al. 2009). The
formation of a finely dispersed microstructure as
evidenced by small-angle X-ray scattering (SAXS)
revealing a lateral periodicity is a strong indicator of a
spinodal decomposition.
The increase of the cellulose concentration in the dope
leads to increased interactions / frictions between the
cellulose molecules in the flow through the airgap which
results in an increase in the elongational viscosity. This
was shown for the dry-jet wet spinning of a cellulose
solution in superphosphoric acid (Boerstoel 1998).
Our results clearly support the effect of polymer
concentration on the strength properties of the final
fibres. Fig 5 illustrates the tensile strength of the fibres as
a function of the draw ratios.
A similar behaviour was also observed for the elastic
modulus (E) in the conditioned state and the tensile
strength under wet conditions, FW (not shown). In
agreement with results from NMMO-based Lyocell
spinning, the tenacity increases very fast already at
relatively low DRs (Mortimer, Peguy 1996b). Mortimer
and Peguy report that the maximum birefringence of
1000
F(MPa)
800
800
600
F
40
CrI
E
35
400
600
30
crystalline orientation
200
Orientation, f
Tensile strength (MPa)*
1000
400
10 wt%
13 wt%
15 wt%
17 wt%
200
0
0
5
10
15
25
20
0,6
0,4
0
20
Draw ratio (DR)
Fig 5 - Tensile strength, FC, in the conditioned state of the
Ioncell-F fibres as a function of the draw ratio (DR) spun from
four different cellulose concentration in the [DBNH][OAc]
solution.
amorphous orientation
0,8
5
10
Youngs Modulus, E (GPa)
Crystallinity, CrI (%)
0.041 was achieved at a DR of approximately 10, which
coincides with the tenacity maximum of 600 MPa, while
the maximum elastic modulus was already obtained at a
DR of 4. According to Fig 5, the development of the
tensile strength depends on the polymer concentration.
While the maximum tensile stress, F_max, of fibres spun
from a 17 wt% cellulose concentration was reached
already at a DR of 5-8, the tensile strength of fibres spun
from a 13 wt% solution continuously increased until a
DR of 17.7. Thereby, F increased almost linearly by
approximately 100 MPa in the DR range of 8 to 17.7. The
fibres spun from a 15 wt% solution showed a very high
strength level already at a DR of 8–10, which, however,
further increased to the maximum tenacity (thus far
observed for an Ioncell-F fibre) of almost 870 MPa (58
cN/tex) in the conditioned state at the maximum DR of
17.7. The development of the tensile strength of fibres
spun from a 10% cellulose solution cannot be assessed
conclusively since only three DR levels in the range of
3.5–7.1 have been investigated. However, the level of the
achieved tensile stress is clearly lower than that from
fibres spun from higher polymer concentrations. The
overall strength level of the Ioncell-F fibres is considered
to be comparable or higher than that of both commercial
and lab-produced NMMO-based regenerated cellulose
fibres (Röder et al. 2013; Röder et al. 2009; Gindl et al.
2008a). Typically, NMMO-based Lyocell fibres reveal
tenacities of 38–44 cN/tex (values of up to 50 cN/tex
have been reported in rare cases). To the best of our
knowledge the highest published strength properties of an
“experimental” NMMO-based Lyocell fibre were a dry
tenacity of 61 cN/tex (915 MPa) and an elastic modulus
(conditioned state) of 29.3 GPa, respectively (Cousley,
Smith 1995). However, while this tensile strength was not
achieved with the Ioncell-F process yet, the reported
elastic modulus, E, of 29.3 GPa was clearly exceeded by
Ioncell-F fibres spun from 17 wt% cellulose solution. At
a DR of approximately 5, the elastic modulus already
exceeded 30 GPa. As visualized in Fig 6, the maximum E
of 34.4 GPa was reached at the highest DR applied.
15
15
Draw ratio [-]
* in the conditioned state
Fig 6 - Tensile strength, F, and Young’s modulus, E, both in
the conditioned state and the structural parameters, amorphous
and crystalline orientation and the degree of crystallinity as a
function of the draw ratio (DR) of Ioncell-F fibres spun from a
17 wt% [DBNH][OAc] solution.
49
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
Elongation at break* (%)
Tensile strength, F (MPa)
1000
10 wt%
13 wt%
15 wt%
17 wt%
800
600
400
200
Ioncell-F fibers, spun from 17 wt% E-PHK
25
20
15
10
fam, amorphous
fcr, crystalline
5
0
0
10
20
30
40
0,6
50
Elastic modulus, E (GPa)
Fig 7 - Relationship between the tensile strength, FC, and
Young’s modulus, E, both in the conditioned state using the
results from Ioncell-F fibres produced from all dope
concentrations.
*conditioned state
0,7
0,8
0,93
0,96
Orientation factor
Fig 8 - Correlation between the elongation at break and the
amorphous (fam) and crystalline (fcr) orientation factor derived
from Ioncell-F fibres spun from a 17 wt% E-PHK solution.
independently from their production conditions (Fig 7).
As predicted from theoretical considerations, the curve
has a moderate concave shape and the slope of the linear
part of the function is comparable to that of cellulose
filaments spun from a liquid crystalline solution (Northolt
et al. 2005; Northolt et al. 2001).
For textile applications, the elongation of regenerated
cellulose fibres is an important mechanical property. As a
general rule, the fibre strain is caused by the contraction
of the chain orientation distribution, which increases with
decreasing orientation. Studying 15 different NMMObased Lyocell fibres, Lenz et al. showed that the
elongation mechanism is not only associated with the
orientation of the crystalline regions, but also with the
straightening of the less ordered cellulose segments in the
amorphous hinges between the crystallites, assuming the
classical two-phase model (Lenz et al. 1994). This
conclusion was based on the observation of linear
correlations between the elongation at break in the
conditioned state and both the amorphous and crystalline
orientations. It must be noted that the high scatter of
elongation data makes it difficult to obtain reliable
conclusions. The limited data set of our results from
Ioncell-F fibres spun from 17 wt% E-PHK
[DBNH][OAc] solution suggests that the elongation at
break is mainly associated with the crystalline orientation
(Fig 8).
The overall relationship between the elongation at break
and the tensile strength in the conditioned state of
Ioncell-F fibres is highly scattered as shown in Fig 9. The
elongation at break for fibres with a tensile strength > 500
MPa appears to be largely independent of the tensile
strength. However, the level of the elongation at break
undoubtedly increases with increasing polymer
concentration in the dope at a given tensile strength. This
dependency of the elongation at break on the polymer
concentration in the spinning dope has been recently
confirmed for fibres spun from only 5 wt% polymer,
where the elongation at break in the conditioned state was
only 5-6% (unpublished). Whether this phenomenon can
be fully explained by structural fibre properties such as
the chain orientation distribution or others has to be left
open at this point.
In agreement with results from NMMO-based fibres, the
strength properties F and E of the Ioncell-F fibres
correspond well with the development of the amorphous
orientation, calculated from the total (birefringence) and
the crystalline (WAXS) orientation (Cousley, Smith
1995; Lenz et al. 1994). Parallel to F and E, the highest
incremental increase occurs in the DR range of 0.9 to 5.
More measurement points would be needed to study the
development of the orientation at higher DR values.
Interestingly, the amorphous orientation lags behind the
crystalline orientation. Obviously, a high crystalline
orientation of the crystallites is already created at low
DRs, whilst the amorphous orientation is still low.
Compared to NMMO-based Lyocell fibres, the degree of
crystallinity of the Ioncell-F fibres is generally lower,
while simultaneously the total orientation is considerably
higher (Fink et al. 2004; Fink et al. 2001). The
elucidation of whether this difference in the structural
properties between NMMO- and [DBNH][OAc]-based
Lyocell fibres is related to the high polymer
concentration in the ionic liquid needs to be confirmed by
further studies. However, the profile of the crystallinity as
a function of DR is comparable for both fibre categories
(Cousley, Smith 1995). Parallel to the crystalline
orientation, the degree of crystallinity shows the steepest
increase when increasing the DR from 0.9 to moderate
2.7, which implies that further orientation operates on the
material as a single entity (see Fig 6).
In accordance with the literature, an increase of the
modulus is associated with an increase of the tensile
strength of the fibre as was shown in Fig 6 for the fibres
spun from the 17 wt% solution. The relationship between
tensile stress, F and modulus, E, has been derived by
Northolt et al. with the chain modulus, ec, and the shear
strength, b, as independent parameters (Northolt et al.
2005). Fig 7 depicts the relation between F and E for all
produced fibres spun from different dope concentration
and DR.
Based on the data in the conditioned state derived from
the Ioncell-F fibres spun from all dope concentrations, it
is evident that there is a highly significant relationship
between E and F, which may be interpreted such that
Ioncell-F fibres show a high structural conformity
50
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
1,2
wet-to-dry tenacity
Elongation at break* (%)
25
20
15
10
5
0
200
10 wt%
13 wt%
15 wt%
17 wt%
1,0
0,8
0,6
0,4
10 wt%
13 wt%
15 wt%
17 wt%
0,2
0,0
400
400
600
800
1000
Tensile strength* (MPa)
Fig 9 - Elongation at break vs. tensile strength, F, in the
conditioned state using the results from Ioncell-F fibres
produced from all dope concentrations.
*conditioned state
600
800
1000
Tensile strength* (MPa)
*conditioned state
Fig 10 - Ratio of wet-to-dry tenacity as a function of the tensile
strength in the conditioned state of the Ioncell-F fibres spun from
four different cellulose concentration in the [DBNH][OAc]
solution.
wet-to-dry strength behaviour than the commercialized
It is well known that the strength properties of
NMMO-based Lyocell fibres.
regenerated cellulose fibres weaken with increasing
Another important characteristic of regenerated cellulose
moisture content. In the wet state, the hydrogen bonds in
fibres structure is the cross-sectional shape. The fibrillar
the accessible amorphous regions are broken; thus, the
structure of the cross-section of Ioncell-F fibres closely
amorphous matrix behaves like a rubber with a low initial
resembles that of the NMMO-based Lyocell fibres (Fink
modulus. The strength contribution of the hydrogen
et al. 2001). Recent SEM fracture micrographs prove that
bonds in accessible regions can be described by the
⁄ which showed a linear relationship
the nanofibrillar structure at the fracture surface becomes
expression 1
more pronounced with an increasing degree of orientation
with the birefringence (total orientation) and the fibre
(Hummel et al. 2014). In analogy to the NMMO fibres, it
accessibility determined by D2O exchange (Bingham
may be assumed that the tendency of fibrillation upon
1964). The structural homogeneity within the fibre crosswet abrasion increases with an increasing total orientation
section also determines the effect of humidity on the fibre
of the cellulose chains due to the weakening of the
properties. In the wet state, the cleavage of hydrogen
intercrystalline bonding.
bonds at the surface of the ordered regions causes chain
Table 2 summarizes the important average properties of
ends to become detached, thereby reducing the ties
fibres with a titer close to that of typical textile fibres
between the crystallites. Since in a pronounced skin-core
(1.7±0.2) spun from different cellulose concentrations. As
structure, as occurring in a regular viscose fibre, the core
already mentioned earlier, the tensile strength and the
swells more, the loss of tenacity is much greater for core
modulus substantially increase with increasing dope
than for skin. It is well known that the wet-to-dry tenacity
concentration. The fibres, which were prepared from 15
of regular viscose fibres is very low and values of 0.50wt% and 17wt% cellulose solutions, had tensile strengths
0.52 have been reported. Owing to the high crystalline
that were consistently higher than 50 cN/tex (750 MPa)
and amorphous orientation of the cellulose chains, this
under the given conditions. This strong improvement of
ratio is substantially higher for Lyocell fibres and reaches
the mechanical properties compared to fibres spun from
values of 0.80 to almost 0.90. In this manner, Lyocell
lower cellulose concentration can be explained by the
fibres are more cotton like, which is the only cellulose
steep increase in the orientation. Surprisingly, the
textile fibres whose strength increases with humidity
tenacity remained almost at the same level under wet
(Wakelyn et al. 2006). The dry-to-wet tenacity of the
conditions. As mentioned earlier, the high tenacities are
Ioncell-F fibres ranges from 0.6 to close to 1.0 and
associated with higher elongation at break values which
occasionally even higher (Fig 10). The proportion of wet
indicates that the degree of orientation is not the only
tenacity clearly increases with increasing initial modulus.
parameter that affects fibre extensibility (Fig 9).
In conclusion, the Ioncell-F fibres show a comparable
Table 2 - Mechanical and structural properties of Ioncell-F fibres spun from different E-PHK solutions in [DBNH][OAc] with a titer
close to commercial values for textiles.
Tw
E
Birefringence
Dope DR
Titer
Tc
w
c
av
±
av
±
conc
av
±
av
±
av
±
av
±
av
±
wt%
dtex
cN/tex
%
cN/tex
%
GPa
10
7.1
2.0 0.2
36.1 2.7
8.9 1.4
26.8 2,7 10.0 1.5 21.3
1.3 0.039 0.006
13 13.0
1.7 0.2
46.8 4.0
9.1 1.3
42.6 5.2 10.9 1.4 27.1
2.0 0.043 0.003
15 14.1
1.7 0.2
55.9 3.3 10.2 0.9
52.2 2.7 12.1 0.8 32.5
3.1 0.045 0.002
Max
15 17.7
1.4 0.2
57.6 3.4
9.5 0.7
56.7 3.7 10.7 1.1 33.9
3.8
n.d.
17 13.2
1.8 0.3
53.5 3,4 10.0 1.1
54.0 3.4 12.4 0.8 33.4
4.2 0.051 0.001
51
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Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
Structure-property relationship
formation of very strong fibers as evidenced by an initial
modulus of 50 GPa and a tensile strength of 65 cN/tex.
Polarized optical microscopic images indicated a liquid
crystalline behaviour of the solution (Fink et al. 2014)
However, none of these fibre processes has been
commercialized, most probably because of the high
production costs, and also because of the high brittleness
of the fibre which prevents the coverage of a broad
market portfolio.
The only regenerated cellulose fibre process with the
potential of producing high tenacity fibre, which has
attained commercial importance, is the Lyocell process.
In the following account, an attempt is made to revisit
the micromechanics of cellulose II fibres using the
presented results obtained from airgap spinning according
to the Ioncell-F process. In addition, suitable published
data from regenerated cellulose fibres of different origins
are used for the purpose of comparison. The discussion
and interpretation of the results follows largely from the
model considerations suggested by Northolt (Northolt et
al. 2005; Northolt et al. 2001; Northolt, Hout 1985;
Northolt 1985).
In highly oriented cellulose fibres, the cellulose
molecules are considered to be organized in a singlephase structure comprising a series arrangement of
crystallites. This assumption is controversial in the
scientific community, since many researchers believe in
the classical two-phase model, consisting of a series of
crystalline and amorphous cellulose domains. An
interesting discussion on this topic can be read in the
annex of the paper of Northolt (Northolt 1985). However,
the existence of the so-called continuous chain model has
been justified by the absence of the meridional reflection
in the SAXS pattern of some regenerated fibres (Heikens
et al. 1952). Even though SAXS measurements have not
been conducted (yet), it can be assumed that this model
assumption also applies to the highly oriented Ioncell-F
fibres. Thus, in a paracrystalline fibre the chains are
oriented parallel to the symmetry axis and the orientation
angle of this axis  with the fibre axis follows a
distribution p(). Further, the elastic tensile deformation
is due to the combined effect of the elongation of the
cellulose chain and the shear deformation of a small
chain segment. Thus, the compliance (inverse elastic
modulus E) is related to the chain modulus (ec), the
orientation distribution of the chains in the fibril
〈
〉and the shear modulus (g) in the following
manner:
Since the invention of regenerated cellulose fibres in the
end of the 19th century, researchers continued to develop
novel fibre processes with the intention of improving the
mechanical properties, particularly under wet conditions.
Surprisingly, already in 1925 Leon Lilienfeld invented
one of the strongest rayon fibers ever made. Conditioned
tenacities of up to 63 cN/tex have been reported
(Lilienfeld 1928). The necessary high stretch has been
achieved through a spin-bath containing 64% sulphuric
acid. The process did not obtain much commercial
attention because of the hazards involved in using such
strong acids. However, Lilienfeld’s process was the guide
for the development of the high-strength polynosic
rayons developed much later by the Japanese. During the
1940s, a saponified cellulose acetate fibre, the Fortisan®
fibre, was developed and produced by Celanese, which
was used for the manufacture of parachutes and for other
industrial uses where strength was important. The
tenacity of the Fortisan® fibre also reached 60 cN/tex and
even higher values. Unfortunately, the fibre was brittle
and was not well suited for textile application because of
low knot and loop strength. Therefore, and because of the
high production costs, its commercial production ceased
in the early 1960s. Very high modulus viscose fibres
were produced by the addition of small amounts of
formaldehyde to the spinning dope or to the spin-bath. In
this manner much higher stretch values could be reached
through the intermediate formation of S-methylol
derivatives and their slow decomposition. Further,
commercial products included Cordenka®EHM or
Courtauld’s Tenex fibre. They showed excellent strength
properties up to 70 cN/tex (Northolt 1985). However,
these fibres suffered from unsolvable problems associated
with the spin-bath recovery and the high brittleness of the
fibres. Currently, the only commercial high-wet modulus
viscose fibre available for textile application is the Modal
fibre where a higher stretch ratio is achieved through a
modified composition of the viscose and the additions of
modifiers that retard the regeneration process. The higher
orientation of the cellulose chains improves dimensional
stability and reduces water retention. A wet-modulus ≥4.5
cN/tex (BISFA) ensures good dimension stability and
thus meets the criterion for being legally labelled a Modal
fibre. Under certain conditions, solutions of cellulose or
cellulose derivatives show liquid crystalline behaviour,
from which very high modulus and high tenacity fibres
can be spun by dry-jet wet spinning. The following three
companies have demonstrated that liquid crystalline
solutions of cellulose (and their derivatives) are good
precursors for the manufacture of high tenacity yarns:
DuPont (O'Brien 1983), with cellulose acetate dissolved
in TFA and formic acid as cosolvent and methanol as
spin-bath: Michelin (Villaine 1985), with cellulose
dissolved in a mixture of formic acid and phosphoric acid
with acetone as spin-bath: and Akzo, with cellulose
dissolved in superphoshoric acid and spun into an acetone
bath (Boerstoel 1998; Boerstoel et al. 1996). Tenacities of
120 cN/tex (DuPont) and 85-113 cN/tex (Akzo, fibre B)
have been reported. Quite recently, it was reported that
dry-jet wet spinning of a 30 wt% cellulose carbamate
solution in NMMO monohydrate resulted in the
〈
〉
[2]
A decrease in the degree of orientation in non-flexible
chain molecules such as cellulose results in a decrease in
lateral order and, hence, a lower crystallinity.
Consequently, the average distance between the chains
increases, which in turn leads to a reduced interchain
bonding and hence a lower shear modulus g. However,
for highly oriented fibres, which show a Gaussian
distribution of chain orientation, a constant value for g
can be assumed. According to Hermans et al., the
〉, can be related to the
orientation in a fibre, 〈
birefringence (Hermans et al. 1948):
52
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
1
〈
〉
hydrogen bonded to its neighbours in the different sheets.
On the other hand, the chain modulus of cellulose I (137
GPa) is considerably higher than that of cellulose II (88
GPa). The higher stiffness of cellulose I fibres may be
attributed to the presence of two intramolecular hydrogen
bonds between successive anhydroglucose (AHG) units,
O(2)…O(6) and O(3)…O(5), while in the cellulose II
lattice only one intramolecular, bifurcated hydrogen
bond, O(3)…O(5),O(6), stiffens the glycosidic bond
between two AHG units.
A value of 0.057 for the birefringence of a perfectly
oriented fibre, nmax, is calculated from data in Fig 11
and Eq 3. This value fits very well with literature data.
Novikova and Ivanova (Novikova, Ivanova 1970)
reported a value of 0.058, Gindl et al. of 0.062 (Gindl et
al. 2008a), Okajima et al. of 0.062 (Northolt, Hout 1985)
and Northolt of 0.055 (Northolt 1985).
The strength and structural properties of Ioncell-F fibres
fit very well with the single-phase structural model, as
suggested by Northolt (Northolt et al. 2001; Northolt
1985). This implies that a further increase in tenacity and
elastic modulus of Ioncell-F fibres is achieved by an
increase in the total orientation and a decrease in
chemical (non-cellulosic compounds) and physical
(structural defects) impurities. Based on dynamic
modulus studies of fibre B, it was estimated that the
maximum achievable modulus of a cellulose II fibre is
approximately 70% of the chain modulus - that is, 61
GPa (Northolt et al. 2001; Northolt 1985).
The ultimate strength, L, of a conditioned fibre is
related to the chain modulus, ec, and the observed strain
energy, Wb, as shown in Eq 6. The relation is based on
Frenkel’s model (Kelly, Macmillan 1987) on a shear
plane system with periodic force centres:
[3]
Combining Eq 2 and Eq 3 yields Eq 4:
Δ
Δ
1
[4]
where nmax is the value for the birefringence of a
perfectly oriented fibre for which E = ec. The ratio of the
slope and the intercept of the linear function, Eq 4, yields
the following simplified relationship, Eq 5:
[5]
from which the shear modulus g can be easily calculated.
In Fig 11, the n values of the Ioncell-F fibres, NMMO
fibres from the literature (Gindl et al. 2008a; Lenz et al.
1994), some regular and high wet modulus viscose fibres
(Northolt et al. 2001) and the highly oriented fibres, Fibre
B, Cordenka® EHM and Fortisan® (Northolt 1985) are
related to the compliance (1/E) as determined from the
stress-strain curves.
A linear relation between n and 1/E exists for highly
oriented fibres with a Young’s modulus E≥20 GPa, which
is in agreement with the literature (Northolt, Hout 1985).
All the highly oriented fibres included in Fig 11 reveal
the same slope and intercept in the linear relationship
between birefringence and compliance, which means that
not only their fibre structure but also the deformation
mechanism is comparable to each other. With a
calculated slope of
0.460, an intercept 0.0623
and a chain modulus of 88 GPa (Northolt et al. 2001), a
shear modulus g of 2.69 GPa can be computed. This
value for g is very close to published values of highly
oriented regenerated cellulose fibres. Northolt (Northolt
1985) has reported a value of 2.5 GPa, while Kong and
Eichhorn reported a g value of 3.6 GPa (Kong, Eichhorn
2005). For the tested Ioncell-F fibres, a ratio E/g of 7-13
(20-34/2.69) is indicative of a highly anisotropic fibre
when compared to a value of 4 (Eiso /g ≈ 5/1.25) for a
fully isotropic fibre (Northolt et al. 2001).
The shear modulus g, which is largely determined by
the intermolecular hydrogen bonding, is higher in
cellulose II than in cellulose I fibres (g~1.5 GPa). This
can be explained by the absence of intersheet hydrogen
bonds in cellulose I, whereas in cellulose II each chain is
1.14 ∙
[6]
The tensile strain energy, which is composed of the strain
energy from the chain extension and the strain energy due
to shear deformation, can be estimated from the fracture
envelope of a series of stress-strain curves. Fig 12
illustrates the stress-strain curves of five different IoncellF fibres spun from a 17 wt% E-PHK solution where only
the DRs were varied from 0.9 to 14.4.
1000
Birefringence, n
0.06 nmax
0.05
Fiber B
Cordenka EHM
Fortisan
Ioncell, E>20 GPa
Ioncell, E<20 GPa
NMMO, E>20 GPa (lit)
NMMO, E<20 GPa (lit)
0.04
Viscose
0.03
DR=14.1
800
Stress (MPa)
0.07
DR=7.1
600
DR=3.5
DR=1.8
DR=0.9
400
0.02
200
0.01
1/ec
0.00
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
0
0
Compliance (1/GPa)
Fig 11 - Birefringence n versus compliance for different
regenerated cellulose fibres including Ioncell-F fibres.
5
10
15
20
25
Strain (%)
Fig 12 - Stress-strain curves of Ioncell-F fibres spun from a 17
wt% solution in [DBNH][OAc]. The dashed line is the hyperbola
fitted to the end points of the tensile curves.
53
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
reactions cannot be completely excluded. Another
drawback of NMMO is the resulting high viscosity of
cellulose solutions already at a relatively low
concentration, which demands high dissolution and
spinning temperatures. As shown in Fig 3, cellulose
solutions in [DBNH][OAc] achieve the same viscoelastic
properties at a temperature that is 20-25°C lower than
solutions of the same cellulose at the same concentration
in NMMO monohydrate. The lower thermal impact leads
to less degradation of the dissolved substrate and also
contributes to energy savings. [DBNH][OAc] has proven
to be an excellent solvent for homogenous derivatization
reactions of both cellulose and hemicellulose, which
could be an advantage over NMMO in the long run
(Stepan et al. 2013). Thus far, no attempts have been
made to develop a recycling concept for [DBNH][OAc].
However, ionic liquids have an intrinsic disadvantage
over NMMO since the water has to be quantitatively
removed, while the aqueous NMMO needs to be
concentrated only to its monohydrate with a residual
water content of 13.3 wt%. In contrast to the general
opinion that ionic liquids are more expensive than
NMMO, it can be highlighted that the situation at the
beginning of the development of the NMMO process was
exactly the same. In the process of commercialization, the
solvent costs came down to a reasonable level; thus,
currently, the specific solvent costs are not a major cost
factor because of the high recovery rate and reasonable
solvent costs.
Therefore, the Ioncell-F process can be economically
successful only if the solvent can be recovered at least to
the same extent, or rather to a greater extent, as that in the
NMMO process. However, this requirement is not
sufficient, since the additional costs for evaporation of
the higher water content must be overcompensated by
economically quantifiable advantages. These advantages
could be achieved by better fibre properties for both
textile and technical applications, superior ability to use
low-cost cellulose wastes such as waste paper or waste
textiles as raw material, which is currently a hot topic in
the textile industry, and/or has an even lower
environmental impact. Further, the ability to increase the
cellulose concentration in the dope due to a lower
solution viscosity (Fig 3) and lack of stabilizers could
also compensate for some of the higher evaporation costs.
Thus, the questions asked whether the Ioncell-F process
represents a serious alternative to the NMMO process
cannot be answered at this stage. Undoubtedly, the next
step in the development of the Ioncell-F process must
focus on the development of a solvent recycling system.
The work of fracture or the strain energy can be
calculated according to Eq 7, assuming a linear stressstrain curve with a modulus E, a tensile strength, b, and
an elongation at break, b.
[7]
For the endpoints of the tensile curves the work of
fracture is constant. With this assumption the endpoints
can be
connected using a hyperbolic function,
2 ⁄ .
In this manner a value for Wb of 0.039 GJ/m3 was
calculated which, deviates from the value of 0.058 GJ/m3
reported by Northolt et al. However, in their calculation
the tensile curves of the high tenacity fibres B and
Fortisan® were included (Northolt et al. 2005). Inserting
the calculated value for Wb into Eq 6, an ultimate strength
1.14 ∙ √88 ∙ 0.039
value of Ioncell-F fibres of
2.1
may be estimated. These calculated strength
properties, b = 2.1 GPa and E = 61 GPa, based on
simplified model assumptions, are of course not realistic
properties for an Ioncell-F fibre. It can be speculated that
the maximum achievable tensile strength of Ioncell-F
fibres spun from isotropic cellulose solutions could be in
the range of 60-70 cN/tex (0.9-1.05 GPa) provided that
the total orientation can be further enhanced through both
optimized spinning conditions and the use of pulp with
higher purity. A regenerated cellulose fibre with these
properties would be highly desirable for the replacement
of synthetic fibres or glass fibres in the production of biocomposites.
Critical outlook on the future of Ioncell-F
The Ioncell-F process with its solvent [DBNH][OAc] is
the first ionic liquid-based Lyocell process which
matches or even exceeds the produced fibre properties of
the commercial NMMO-based process. This conclusion
is not only based on the staple fibre characteristics but
also on two demonstration runs where the staple fibres
were converted to yarn and further to a knitted scarf and
to a knitted dress in collaboration with the Swedish
School of Textiles in Borås and the famous Finnish
textile and clothing design company Marimekko®. The
dress was presented on the occasion of the fashion show
in Helsinki Central Railway Station’s ticket hall on
March 13, 2014. The fibre’s processability to yarn was
considered to be comparable with the best commercial
regenerated cellulose fibres.
However, is there any need to replace NMMO with an
alternative direct cellulose solvent like [DBNH][OAc]?
What would be the potential advantages of a new
cellulose solvent?
NMMO is an excellent cellulose solvent and is
environmentally friendly; moreover, an economically
viable recycling concept for it has been developed and
successfully installed. However, NMMO is an oxidant
and an unstable molecule which undergoes both
homolytic and heterolytic degradation reactions (Rosenau
et al. 2001). The addition of an antioxidant, particularly
n-propyl gallate, successfully prevents severe degradation
of both the dissolved cellulose and the solvent.
Nevertheless, the danger of uncontrollable runaway
Conclusions
In this study, we presented a novel regenerated cellulose
fibre process of the Lyocell type, the Ioncell-F process.
The main characteristics of this process are the cellulose
solvent [DBNH][OAc], a superbase-based ionic liquid,
and a dry-jet wet spinning process with conditions
adapted to the viscoelastic properties of the cellulose
solution. The solvent reveals that the resultant cellulose
solution has a high dissolution power and a low viscosity.
Thus, a low spinning temperature can be applied, which
ensures high process and product stability and the
54
Special Issue: CELLULOSE DISSOLUTION AND REGENERATION: SYSTEMS AND INTERACTIONS
Nordic Pulp & Paper Research Journal Vol 30 no (1) 2015
Collier, J. R., Watson, J. L., Collier, B. J. and Petrovan, S.
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spinning solution, Forschung, F.-G. z. F. d. a. (ed.), WO 06/0 00
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7322-7324.
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possibility of preparing dopes of high cellulose
concentration. We demonstrated the production of high
tenacity fibres, spun from a 17 wt% cellulose solution at
a temperature below 80°C. The strength properties can be
controlled by the cellulose concentration in the dope and
the draw ratio. The increase of the cellulose concentration
from 13 wt% to 15 wt% caused a strong pre-orientation
of the cellulose chains, which resulted in fibre tenacities
of over 50 cN/tex (conditioned and wet) and an initial
modulus of over 30 GPa already at moderate draw ratios.
Without any special optimization of the spinning
conditions (better pulp quality, spinneret geometry,
airgap conditioning and distance, etc.), fibres with a
tensile strength of 58 cN/tex in the conditioned and 57
cN/tex in the wet state and an elastic modulus of 34 GPa
were prepared. These excellent properties make the
Ioncell-F fibres suitable as a reinforcing material in
composite structure and other technical applications. In
addition, their suitability as excellent textile fibres has
been demonstrated in its use in the manufacture of a
knitted scarf and a knitted dress. A structure-property
relationship was established based on a simplified singlephase model for cellulose, which predicts that the
orientation of the cellulose chains is the major driver of
its strength properties.
Acknowledgements
The authors are grateful to the Finnish Bioeconomy Cluster
(FIBIC) and the Finnish Funding Agency for Technology and
Innovation (TEKES). We also would like to thank Marjaana
Tanttu from the School of Arts, Design and Architecture, Aalto
University and Anders Persson and Anders Berntsson from
Swedish School of Textiles (University of Borås, Sweden) for
their kind assistance in the yarn and fabric preparation.
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Manuscript accepted December 1, 2014
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