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10.1002/spepro.003968
Thermal stabilization of
polyacrylonitrile fibers
Ismail Karacan
Guanidine carbonate pretreatment is effective in accelerating thermal
stabilization of polyacrylonitrile precursor fiber prior to the carbonization stage.
For both economic and environmental reasons, cars and airplanes need
to increase their fuel efficiency. Consequently, a major incentive exists
for developing carbon-fiber-reinforced polymer-matrix composite construction materials characterized by low cost, corrosion resistance, light
weight, and high strength and modulus for use in a wide range of applications in the aerospace and automotive industries, among others.1
Lightweight polyacrylonitrile (PAN)-based carbon fiber composite materials have been shown to save up to 50% of the total weight of vehicles
and up to 25% of the fuel required. Carbon fiber is manufactured from
PAN precursor fiber in a process that involves thermal stabilization,
carbonization, and optional graphitization stages at temperatures of up
to 3000ı C based on the intended end use.2 Thermal stabilization, in
particular, is critical to obtaining high-quality carbon fiber and can take
up to several hours, depending on the temperature, precursor diameter,
and precursor fiber characteristics.3, 4
However, carbon fiber is too expensive at present to be economically
viable on a large scale. Since fluctuations in worldwide oil prices make
permanent reduction of the precursor price (about 50% of the total cost)
unlikely, the only real alternative is to cut the cost of thermal stabilization (about 20% of the total cost) by accelerating it. Previous attempts
at various chemical pretreatments resulted in weaker stabilized fibers
(up to 30–40% strength loss). Here, we describe an aqueous guanidine
carbonate treatment to increase the thermal stabilization of PAN precursor fibers prior to carbonization.5 This chemical pretreatment proved
effective in speeding stabilization with no loss of strength. The accelerated activity of guanidine carbonate is due to the presence of a carbonnitrogen double bond (C=N) containing singly charged guanidinium
cations in solution. Because the guanidinium ion is a strong base, with
a pK value of 13.6, it promotes enormous resonance stabilization when
protonated on the imine nitrogens.
We proceeded in three steps. First, we washed PAN multifilament
yarn to remove spin-finish oil. After drying in air, we immersed the
washed sample in 15% (w/v) guanidine carbonate solution for 15min at
Figure 1. Variation of density values of untreated and thermally stabilized polyacrylonitrile (PAN) fibers pretreated with 15% guanidine
carbonate aqueous solution and stabilized at 240ı C as a function of
stabilization time. The density of the untreated sample is shown with
zero stabilization time.
80ı C. Finally, we used a square steel frame to keep the chemically pretreated sample under tension and thermally stabilized it in air at 240ı C
for times ranging from 5 to 75mins.
Measuring the density of the fibers usually provides a good idea of
the progress of the process. The density of untreated and thermally
stabilized PAN fibers varies as a function of stabilization time (see
Figure 1). Density values increase with time, a result we attribute to
the formation of a ladderlike structure incorporating closer packing of
the polymer chains as a result of cyclization of nitrile groups present in
the polymer structure.6 This result confirms the lateral compaction of
the entire fiber structure due to the development of extensive cross-links
in the structure of the stabilized fiber samples.
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10.1002/spepro.003968 Page 2/3
Figure 2. Left: Differential scanning calorimetry (DSC) thermograms
of unstabilized and thermally stabilized PAN fibers pretreated with 15%
aqueous guanidine carbonate solution and stabilized at 240ı C as a
function of treatment time: (a) untreated PAN fiber; (b) 240ı C, 5mins;
(c) 240ı C, 15mins; (d) 240ı C, 30mins; (e) 240ı C, 45mins; (f) 240ı C,
60mins; (g) 240ı C, 75mins.
Differential scanning calorimetry (DSC) thermograms are useful in
analyzing unstabilized and thermally stabilized PAN fibers with respect
to changes in the nature and extent of exotherms. Figure 2 shows the
area under the exothermic peak gradually and continuously decreasing with stabilization, whereas the peak width simultaneously increases
with it. We attribute the decrease to cyclization of the nitrile (CN)
groups present in the unstabilized PAN precursor fiber.7
Complete disappearance of the peak suggests that the sample stabilized for 75min completes the cyclization reaction. Plots of thermogravimetric analysis (TGA) of stabilized samples show decreasing
weight loss with increasing stabilization time, indicating enhanced carbon yield due to greater crosslinking density (see Figure 3). Untreated
samples lose weight over a narrow temperature range and thermally stabilized samples over a wider one due to greater formation of ladderlike
structures related to oxidation-based crosslinking.
We used IR spectroscopy to follow and monitor structural transformations in terms of the chemical changes taking place during the thermal stabilization stage. Figure 4 shows two major bands of methylene
(CH2 / vibrations between 3000 and 2800cm 1 , located at 2920 and
Figure 3. Thermogravimetric analysis (TGA) plots of unstabilized and
thermally stabilized PAN fibers pretreated with 15% aqueous guanidine
carbonate solution and stabilized at 240ı C as a function of treatment
time: (a) untreated PAN fiber; (b) 240ı C, 5mins; (c) 240ı C, 15mins;
(d) 240ı C, 30mins; (e) 240ı C, 45mins; (f) 240ı C, 60mins.
2852cm 1 , respectively. In addition, the intensity of the CN groups
of the acrylonitrile monomeric units located around 2242cm 1 as part
of the polymer chain varies with stabilization time. For example, the
intensity of the methylene (CH2 / bands diminished dramatically after only 5min, indicating the loss of hydrogen atoms due to dehydrogenation reactions occurring along the polymer backbone as well as
in the crosslinked structure. It is known that incorporation of oxygen
during the stabilization stage is partly responsible for these reactions.
Figure 4 also shows that the nitrile absorption band at 2242cm 1
rapidly loses its intensity while a new band appears as a shoulder at
about 2200cm 1 , both due to CN group cyclization.
In summary, we found that guanidine carbonate pretreatment of PAN
precursor fiber results in faster formation of the cyclized structure required to withstand high carbonization temperatures while reducing
thermal stabilization time. We characterized the structure of the fibers
using density, DSC, TGA, and IR spectroscopy measurements. The information we have gained from various measurements was useful in
following the chemical and structural transformations that occurred
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10.1002/spepro.003968 Page 3/3
Author Information
Ismail Karacan
Erciyes University
Kayseri, Turkey
Ismail Karacan is an associate professor in textile and polymer
science. His research interests cover a wide range of topics, including thermotropic polyesters, carbon fibers from polymeric precursors,
high-performance fibers, x-ray diffraction, thermal analysis, and IR
spectroscopy techniques.
References
Figure 4. Infrared spectra of unstabilized (a) and thermally stabilized
PAN fibers pretreated with 15% guanidine carbonate solution and stabilized at 240ı C in the 4000–2000cm 1 region. Thermal stabilization
conditions: (b) 240ı C, 5mins; (c) 240ı C, 15mins; (d) 240ı C, 30mins;
(e) 240ı C, 45mins; (f) 240ı C, 60mins; (g) 240ı C, 75mins.
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during the accelerated thermal stabilization process. As a next step, we
will carry out carbonization at high temperatures (1000ı C) to see the
effect of accelerated thermal stabilization in the presence of guanidine
carbonate.
c 2012 Society of Plastics Engineers (SPE)