1) The effect of processing conditions on a polyacrylonitrile fiber produced using a solvent-free free coagulation process

Materials Science and Engineering A 485 (2008) 251–257
The effect of processing conditions on a polyacrylonitrile fiber produced
using a solvent-free free coagulation process
A.F. Ismail a,∗ , M.A. Rahman a , A. Mustafa a , T. Matsuura b
a
Membrane Research Unit, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia
b Department of Chemical Engineering, Industrial Membrane Research Institute, University of Ottawa, Ottawa, Ont., Canada KIN 6N5
Received 3 April 2007; received in revised form 25 July 2007; accepted 20 August 2007
Abstract
PAN fibers were fabricated using a solvent-free coagulation process and characterized using SEM and tensile testing. Eighteen weight percent of
polymer solution was found to be the most suitable composition for fabrication process and consequently produced fibers with the best mechanical
properties. The PAN fibers fabricated using a coagulation bath temperature of 13 ◦ C exhibited the highest Young’s modulus of 2.93 GPa and the
highest tensile strength. The number of nano-pores was significantly reduced due to low inward diffusion of non-solvent.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Polyacrylonitrile fiber; Carbon fiber; Solvent-free coagulation process; Nano-pores; Young’s modulus
1. Introduction
PAN fiber is considered as a crucial precursor for the production of carbon fiber. PAN is transformed into fiber form by
melt spinning, wet spinning and dry spinning [1]. In addition to
these techniques, PAN precursor fibers are also fabricated using
dry–wet spinning to improve the mechanical properties [2]. In
this method, the dope travels through an air gap of less than 1 cm
before emerged into coagulation bath.
PAN homopolymer is rarely used as a carbon fiber precursor
since it initiates a retrograde core during the stabilization process
due to partial melting caused by exothermic nature of oxidation
process [3]. It is also generally not favorable for spinning purposes compared to its copolymers which are more soluble in
solvents and easier to handle for preparation and storage [2].
The solubility of PAN and thermal properties of PAN fibers can
be enhanced by the incorporation of either acidic, neutral or
hydrophilic moieties as comonomers during polymerization or
as spinning dope additives before the fabrication process [4].
Numerous studies have shown that the PAN fibers should
possess the following characteristics; small diameter, maximum
crystallinity, low comonomer contents and high modulus for the
∗
Corresponding author. Fax: +60 7 5581463.
E-mail address: afauzi@utm.my (A.F. Ismail).
0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.msea.2007.08.060
preparation of good quality carbon fibers [5–7]. The Young’s
modulus of PAN precursor fiber is being the best parameter representing the carbon fiber performance since there is a direct
correlation between the Young’s modulus of primary precursor
and the resulting carbon fiber [8]. Therefore, the conditions of
PAN fiber fabrication process play an important role in the production of high performance carbon fibers since the properties
of PAN precursor fibers depend much on it.
Normally, the spinning solution with polymer concentration
ranging from 10 wt.% to 30 wt.% was used for the fabrication of
PAN fibers by the wet spinning process [1,9]. By increasing the
polymer concentration in the spinning dope, the spinnability can
be improved, the tenacity increases and the porosity decreases
[4,10]. The polymer concentration in the spinning solution also
influences the fiber morphology and density. According to Knudsen [10], an increase in the dope solid in the range of 15–25%
improved the homogeneity of the fiber structure by reducing the
generation of large voids. Besides, an increase in the dope solid
slightly increased the density of PAN fibers [9]. As compared
lower polymer concentration with less viscosity, a higher concentration of polymer solution contributes to a higher packing
of the polymer molecules per unit volume inside the fiber.
Besides polymer composition, dope viscosity plays an important role in PAN fiber fabrication process. For example, Bajaj
et al. kept the spinning dope viscosity between 5700 cps and
6250 cps in order to eliminate the die-swelling effect and to
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enhance the spinning process [2]. The spinning process could
be performed neither at lower nor higher than the suggested
range since it could interrupt the fiber fabrication process and
producing low quality PAN fibers.
The coagulation bath temperature has a substantial effect
on the coagulation process. It is responsible for controlling the
mass transfer and the counter diffusion of the solvent and the
non-solvent [2]. The mass transfer between the spun fibers and
the surrounding medium influences microscopic and morphological structures as well as the mechanical properties of PAN
fibers [6]. A reduction in the number of large voids in the crosssection of PAN fibers could be observed as the coagulation bath
temperature is decreased [2,4,10]. As the coagulation temperature is reduced, the outward diffusion of solvent become greater
than inward diffusion of non-solvent resulting in the gel-likefibers with a small pore size, higher density, less skin and low
diameter of fibers [2]. This also leads to an improvement in
the mechanical properties of PAN fibers due to less quantity of
micro-pores inside the fibers [4]. However, at higher coagulation
bath temperature, the fibers exhibit a maximum number of larger
voids, an increment in diameter and reduction in the mechanical properties. The typical cross-sections of the PAN fibers are
usually are rounding, kidney-bean-shaped or dog-bone-shaped
[11]. The cross-section of the fibers could change from a beanshape to an oval-shape if the polymer solution was spun at higher
coagulation bath temperatures and a high solvent content in the
coagulation bath [2,4].
The above reports show that, in the solvent/non-solvent
exchange process, the outward diffusion of solvent from the
PAN fiber should be far greater than the inward diffusion of nonsolvent in order to produce the fiber without pores and with high
mechanical integrity. Hence the conventional strategy for spinning PAN fibers was established by optimizing several spinning
conditions. Large amount of solvent was added in the coagulation bath for decreasing the activity of non-solvent, which in turn
suppressed the inward diffusion of the non-solvent. The coagulation bath temperature was lowered in order to let the solvent
outflow dominated over the non-solvent inflow. By increasing
the solvent content in the coagulation bath and reducing the
coagulation bath temperature, a long coagulation bath is required
for compensating the decrease in the diffusion rates.
However, there are some drawbacks related to the application
of conventional method in producing PAN fibers for carbon fiber
production. The organic solvents such as dimethylformamide
(DMF) and dimethylacetamide (DMAc) used in the coagulation
bath could be hazardous since it could cause cancer for a long
period of exposure. A high amount of solvent in the coagulation
bath and low coagulation bath temperature enlarge the size of
coagulation bath and could increase the production cost in terms
of electricity consumption for cooling purposes.
In order to circumvent these drawbacks, a novel strategy of
PAN fiber spinning is proposed. The development of solvent-free
coagulation process is considered because it is the best approach
to avoid to the aforementioned difficulties. In this system, several modifications had been made to the conventional spinning
system such as reducing the residence time, introducing a high
jet stretch to the as-spun fiber and moderate coagulation bath
temperature during coagulation process. Therefore, the objective of this research is to test if the newly proposed strategy
is workable; in particular, the as-spun fiber is coagulated in a
solvent-free coagulation bath under a high stretch ratio. The
mechanical strength of the fibers is measured and the effect
of coagulation bath temperature on the mechanical strength is
examined.
2. Experimental
2.1. Spinning dope preparation
In order to facilitate uniform dissolution of PAN (Aldrich,
USA; Mw , 86,200) and acrylamide (AM) (Across Organics,
USA) without formation of gel particles, the polymer and additive were firstly dispersed in cold DMF (Merck, Germany),
which formed a fine slurry [11]. Then, heat was supplied continuously on the slurry at 80 ◦ C for 5 h to produce a highly
viscous solution. The spinning solution was transferred into a
solution bottle and then degassed in order to remove bubbles
using ultrasonic bath (Branson 3510 Ultrasonic).
2.2. Dry–wet spinning
The dry–wet spinning method was used to fabricate PAN
fibers. The schematic diagram of the spinning machine as shown
in Fig. 1 consists of a coagulation bath, spinneret, wind-up
drum, gear pump, roller and refrigerator. During the fabrication
process, the polymer solution was delivered from the storage
reservoir by gear pump to the spinneret under pressure of nitrogen gas. The storage reservoir pressure was kept at 1 atm as a
precaution against cavitation in the line to the pump. Throughout the experiment, jet stretch was maintained at a draw ratio of
4 in order to reduce fiber diameter, to improve molecular orientation as well as to remove pores. The residence time for the
as-spun fiber in the coagulation bath was fixed at 5 s. Then, the
fibers were collected onto a wind-up drum which was 17 cm in
diameter. Next, the fibers were stretched and tied at a metal net
and underwent a drying process. The air-circulated oven was set
at 50 ◦ C and the PAN fibers were dried for 3 h.
The polymer concentrations chosen as the spinning dope
were 18 wt.% and 20 wt.%. Although the solution of 16 wt.%
Fig. 1. Laboratory dry–wet spinning line [12].
A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257
polymer concentration had excellent fluidity, the as-spun fiber
could not withstand the high jet stretch. This solution was considered to be a dilute solution of poor draw-ability [13]. Moreover,
a low polymer concentration led to formation of a rigid skin
before the center of the fiber was solidified, yielding a heterogeneous structure of the PAN fiber [1]. This further led to
irregular distribution of stress during the drawing process and
consequently caused the fiber breakage [9]. The fibers spun from
the solutions of 18 wt.% to 20 wt.% polymer concentration, on
the other hand, could easily be stretched up to the draw ratio of
4.
The coagulation bath temperature chosen was in a range
of 13–19 ◦ C. At a temperature below the selected temperature range, the counter diffusion of solvent and non-solvent
was too slow. The as-spun fibers became less elastic and were
not suitable for stretching. On the other hand, at a coagulation
bath temperature above the suggested range, a skin appeared
at the fiber surface. The skin prevented the solidification of
the inner core, leading to fiber breakage by stretching. The
relatively low coagulation bath temperature was required in
this work because the coagulation bath contained no solvent.
But the temperature was still higher than the conventional
approach.
The dope temperature was maintained at 22 ◦ C in order
to reduce skin formation due to substantial temperature different during coagulation process [14]. At this temperature,
solvent evaporation can be ignored, which also reduced
the possibility of skin layer formation at the fiber surface.
The details of the spinning conditions are summarized in
Table 1.
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Table 1
The dry–wet spinning process specifications
Spinning dope
Percentage of AM to PAN
Polymer concentration
Dope temperature
Spinneret diameter
Air gap distance
Dope extrusion rate
Extrusion velocity
Wind-up drum velocity
Jet stretch
Residence time
Coagulation bath composition
Coagulation bath temperature
Recirculation rate of water to
and from coagulation bath
PAN/AM/DMF
2.5 wt.% [15]
18 wt.% and 20 wt.%
22 ◦ C
200 ␮m
1.5 cm
0.012 cm3 /min
35 cm/s
140 cm/s
4
5s
100% H2 O
13 ◦ C, 15 ◦ C, 17 ◦ C and 19 ◦ C
1 L/min
2.3. Characterization methods
2.3.1. Scanning electron microscopy
Scanning electron microscopy (SEM) was used to observe
the morphology of PAN fiber cross-section. The PAN fiber was
immersed in liquid nitrogen for 10–30 min and then fractured
carefully. For each fiber, three SEM samples were prepared.
The sample was attached to copper-double-sided-tape in order
to support the PAN fiber and then sputtered with gold by using
an ion sputtering (Biorad Polaron Division) before viewing on
the Scanning Electron Microscope (Phillips SEMEDAX; XL
40; PW6822/10) with potential of 10 kV under magnifications
ranging from 1000× to 20,000×.
Fig. 2. The cross-sections of PAN fiber fabricated at different coagulation bath temperatures (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C and using a polymer solution
of 18 wt.%.
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Fig. 3. The cross-section of PAN fibers fabricated at different coagulation bath temperature (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C and using a polymer solution
of 20 wt.%.
2.3.2. Tensile testing for a tow of fibers
Because of the lack of the sensitivity of the equipment for
measuring the mechanical properties of PAN fiber, a tensile
testing for single fiber could not be performed. Therefore, some
modifications were made in order to obtain the Young’s modulus
of the PAN fibers. A PAN fiber tow consisting of approximately
200 fibers was made and both ends were wrapped with masking
tape, leaving approximately 25 mm gauge length. The load of
the tensile testing machine was set to 1 kN and the test was carried out using a 50 mm/min crosshead speed. A high crosshead
speed was chosen in order to obtain clear ‘load versus extension
gauge length correction’ graphs instead of using the recommended crosshead speed at 5 mm/min. The strain and stress
value were calculated manually using Excel spreadsheet and the
strain versus stress were plotted.
observable on the cross-sectional surface appeared during the
SEM sample preparation due to a fracture effect. These pictures show that dense fibers could be spun under the spinning
conditions adopted in this work. This shows clearly that the
strategy to spin fibers of dense structure, i.e. immersion of fibers
into coagulation bath that contains no solvent and a short residence time in the coagulation bath with a high jet stretch was
successful.
3. Results and discussion
3.1. Effect of spinning process on the morphology of PAN
fibers
Although a single hole spinneret with a hole diameter
of 200 ␮m was used throughout this work, the diameter of
the as-spun fibers was 50–70 ␮m due to the high jet stretch
of 4. Figs. 2(a–d) and 3(a–d) show the low magnification
(1000–4000×) SEM pictures of fibers prepared from dope solutions of 18 wt.% to 20 wt.% polymer concentration, respectively,
at different coagulation bath temperatures. At this magnification
level, pores (defects) that will reduce the mechanical strength
of the fibers could not be observed. Some cracks that were
Fig. 4. The change in the Young’s modulus of PAN fibers with coagulation bath
temperatures.
A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257
255
Fig. 5. The cross-section of PAN fibers fabricated using a polymer solution of 18 wt.% at magnificent of 20,000× (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C.
3.2. Effects of coagulation bath temperature and polymer
concentration on the morphology and mechanical
properties of PAN fibers
Fig. 4 shows Young’s modulus versus coagulation bath temperature for fibers spun from spinning dopes of 18 wt.% to
20 wt.% polymer concentrations. Young’s modulus increased
with a decrease in temperature for both polymer concentrations,
although the increase was more dramatic for 18 wt.% polymer concentration. These results were in agreement with those
reported by Bahrami et al. [4]. In Fig. 4, the Young’s modulus
increased from 1.95 GPa to 2.93 GPa, with respect to the polymer
concentration of 18 wt.% when the temperature was decreased
from 19 ◦ C to 13 ◦ C. This steep increase can be explained by
Fig. 6. The cross-section of PAN fibers fabricated using a polymer solution of 20 wt.% at magnificent of 20,000× (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C.
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the SEM pictures of higher magnification (20,000×) shown in
Fig. 5. The figure shows that the structures of the fibers are
nodular and the interstitial void spaces of nano-sizes become
less as the temperature decreases from 19 ◦ C (Fig. 5(d)) to 13 ◦ C
(Fig. 5(a)). The presence of the nano-sized void spaces seems
to reduce Young’s modulus, weakening the mechanical strength
of the fiber.
The enhancement of void spaces at higher coagulation
bath temperature can be explained by the solvent/non-solvent
exchange. While most of the solvent is squeezed out of the
fiber while it is stretched, some non-solvent will enter into
the fiber by counter diffusion. As the coagulation bath temperature increased, the rate of the counter diffusion increased,
leaving more non-solvent inside the fiber. The spaces filled
with the non-solvent would eventually turn into nano-sized void
spaces.
Further looking into Fig. 4, Young’s modulus increased from
1.86 GPa to 1.97 GPa, with respect to the polymer concentration
of 20 wt.%, as the temperature decreased from 19 ◦ C to 13 ◦ C.
The reason was found in Fig. 6. These pictures also depicted
the nodular structure of the fibers. In contrast to the polymer
concentration of 18 wt.% (Fig. 5), however, the interstitial void
spaces did not increase with an increase in temperature. Instead,
the number of the cracks that were distributed uniformly across
the cross-section becomes the largest at 19 ◦ C. This probably
was the reason why the Young’s modulus was the smallest at
this temperature. The cracks did not disappear even at the lowest temperature and the Young’s modulus remained low. The
formation of crack in the matrix of the fiber was due to the concentrated polymer solution used during spinning process. This
result was in agreement with Stoyanoz [9]. The outflow of solvent for concentrated polymer solution became rapid and turned
the as-spun fiber into rigid structure during coagulation process.
Consequently, high draw ratio applied during spinning process
led to the distribution of crack in the cross-section of PAN
fiber.
Fig. 8. The relationship between Young’s modulus and polymer solution composition during fiber fabrication process in the solvent-free coagulation bath.
Fig. 7 shows tensile strength versus coagulation bath temperature for the polymer concentrations of 18 wt.% and 20 wt.%.
Similar to the Young’s modulus, tensile strength was also
increased with a decrease in coagulation bath temperature and
the increase was steeper for 18 wt.%.
Stoyanoz reported that tensile strength dry (TSD) of acrylic
fibers showed a maximum at a certain polymer concentration
[9]. The relationship between the PAN polymer concentration and Young’s modulus (mechanical strength) would
probably exhibits the same pattern as illustrated in Fig. 8.
When the polymer concentration was 16 wt.%, it was in the
dilute solution range and the as-spun fiber could not withstand any draw. The polymer concentration of 18 wt.% was
in the semi-dilute solution range and showed the highest
mechanical strength. When the polymer concentration was
20 wt.%, it was in the concentrated solution range, where the
mechanical strength of fiber started to diminish. As Stoyanoz mentioned, the solution of high polymer concentration
has a relatively high viscosity. The outflow of the solvent is
rapid, causing the formation of a skin layer. Breakage by ‘slippage’ is possible between the solid skin and the fluid core
[16].
4. Conclusion
Fig. 7. The change in the tensile strength of PAN fibers with coagulation bath
temperatures.
PAN fibers were fabricated using analytical grade PAN (Mw ,
86,200) via a solvent-free coagulation process. These studies
investigated the appropriate range of coagulation bath temperature and polymer solution composition for spinning process
in the solvent-free coagulation process and the effect of these
parameters on PAN fiber formation. The fabrications of PAN
fibers were conducted using polymer solution of 18 wt.% and
20 wt.% and in the coagulation bath temperature range of
13–19 ◦ C. Based on qualitative analysis, the polymer solution of
18 wt.% enhanced the fabrication process. This polymer solution showed a fine fluidity which allowed the spinning process
A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257
to run smoothly and the as-spun fibers were able to undergo a
stretching process without failure. The SEM images revealed
the presence of nano-sized voids and nano-sized crack for PAN
fiber fabricated using polymer solutions of 18 wt.% and 20 wt.%,
respectively and they affected the mechanical properties of PAN
fibers.
Acknowledgements
The authors would like to acknowledge the Ministry of Science, Technology and Innovation of Malaysia (MOSTI) for
financial support under the National Science Fellowship (NSF)
and for funding this research.
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