- Email: [email protected]
Structural transformation of polyacrylonitrile fibers during stabilization and low temperature carbonization
Accepted Manuscript Structural Transformation of Polyacrylonitrile Fibers during Stabilization and Low Temperature Carbonization Nishar Hameed, Jordan Sharp, Srinivas Nunna, Claudia Creighton, Kevin Magniez, P. Jyotishkumar, Nisa V. Salim, Bronwyn Fox PII:
S0141-3910(16)30052-0
DOI:
10.1016/j.polymdegradstab.2016.02.029
Reference:
PDST 7884
To appear in:
Polymer Degradation and Stability
Received Date: 20 December 2015 Revised Date:
17 February 2016
Accepted Date: 27 February 2016
Please cite this article as: Hameed N, Sharp J, Nunna S, Creighton C, Magniez K, Jyotishkumar P, Salim NV, Fox B, Structural Transformation of Polyacrylonitrile Fibers during Stabilization and Low Temperature Carbonization, Polymer Degradation and Stability (2016), doi: 10.1016/ j.polymdegradstab.2016.02.029. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Structural Transformation of Polyacrylonitrile Fibers during Stabilization and Low Temperature Carbonization Nishar Hameed1*, Jordan Sharp1, Srinivas Nunna1, Claudia Creighton1, Kevin Magniez1, Jyotishkumar P.2, Nisa V. Salim1 and Bronwyn Fox3 1
Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India 3
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Carbon Nexus, Institute for Frontier Materials, Deakin University, Geelong, Australia.
Factory of the Future, Swinburne University of Technology, Hawthorn, Australia
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Abstract: The effect of oxidative stabilization and carbonization processes on the structure, mass and mechanical properties of polyacrylonitrile (PAN) precursor fibers was analyzed. A gradual densification of the fibers occurring from mass loss, decrease in fiber diameter and
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increase in density were observed after stabilization at a maximum temperature of 255oC and carbonization at a maximum temperature of 800oC. The tensile strength and modulus of the fibers were found to decrease after stabilization but then increased after low temperature carbonization. The thermal processing of the precursor fibers affected their mode of failure after tensile loading, changing from a ductile type of failure to a brittle type. The type of failure correlated well with the crystal structure changes in the fibers. Whilst the PAN
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precursor fiber started to exotherm above 225oC in air, no prominent exothermic reaction was measured in the carbonized fibers in air up to 430oC. The aromatization index of stabilized fiber was calculated to be ~ 66%, and that of carbonized fiber was ~ 99%. FTIR studies
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indicated that the variation in the chemical structure of the fibers with the stabilization of the fibers. Radial heterogeneity in the stabilized fibers was observed however it was not
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promoted to the carbonized fibers. Finally, a method to calculate mass retention of PAN precursor fiber after heat treatment was developed, and the calculated percentage mass retained of the precursor fiber after oxidation and carbonization were found to be 81% and 51%, respectively. . This study proposes an effective method to calculate the percentage of mass retained by precursor fibers after stabilization and low temperature carbonization to provide a model for evaluating carbon fiber yield from a given amount of fibers. Keywords: Carbon fibers, polyacrylonitrile, stabilization, carbonization.
Corresponding author email: [email protected]
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1. Introduction Carbon fiber is commonly utilized as a reinforcing material in composite materials because of its relatively high strength and lightweight characteristics [1]. Commercial carbon fibers are produced by heat treatment of polyacrylonitrile (PAN) precursor fibers due to its potential for the manufacturing carbon fibers with high strength and stiffness [2]. Since its inception,
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improvements on the mechanical properties and effective manufacturing costs have constantly been sought [3-6]. In order to develop cost forecasting of the carbon fiber it is imperative to understand the mass and energy changes in precursor fiber when processed in a manufacturing environment. The manufacture of carbon fiber involves the stages of
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stabilization and carbonization [2]. The PAN precursor fibers are stabilized to withstand high temperature during carbonization process. During stabilization transformation on of linear macromolecular structure into aromatic ladder structure takes place [7]. This occurs via
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cyclization reaction around 280°C, followed by dehydrogenation and oxidation reactions [8] leading to the stabilization of the fiber for high temperature carbonization [2]. Once stabilized, it is referred to as oxidized PAN fiber (OPF). Carbonization removes most noncarbon elements from the fiber by heating to extreme temperatures, violently shaking them off and reducing the fiber mass. This further compacts the aromatic ring structure,
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transforming it into turbostratic graphite structure to form carbon fibers [9]. At this point typically up to 95% or more of the atomic composition consists of carbon atoms [10].
Many scientific studies and reports have focused on improving mechanical properties of
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carbon fiber, namely modulus and strength by optimizing the process parameters [1]. Also many literatures have been written based on understanding the chemical transitions of PAN
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precursor fiber and changes in fiber structure when converted into carbon fiber [3, 9, 11]. Thus a large knowledge bank already exists on how PAN precursor fiber changes mechanically and chemically during heat treatment and variation of process factors. One of the most important parameters in the process of carbonization is temperatures which has been shown to significantly affect the mechanical properties of the fibers [12]. However, little research focuses on low temperature carbonization and its effects on the structure property relationships of the fibers. Moreover little work has been undertaken to analyze linear mass transition specifically, but the other effects of pyrolysis have been heavily analyzed [1, 2, 4-7, 10].
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The goal of this study is to gain knowledge of the chemical and physical transformations and the level of mass retained after PAN precursor fiber is stabilized and carbonized. This will provide insights into the approximation of carbon fiber yield after low temperature treatment, as the fiber changes chemically and physically at each stage. Here the whole process of stabilization and carbonization was carefully performed on a pilot scale carbon fiber
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manufacturing line. This enables a better understanding of the process parameters as well as the material properties that can be beneficial for the industrial scale production of carbon fibers.
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2. Materials and Methods
PAN precursor fibers provided from Bluestar Fibres Co. Ltd were stabilized and then carbonized using Carbon Nexus multi-tow carbon fiber production line at Deakin University
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(Figure 1). Thirty tows were processed under same operating conditions. Each tow consists of 24000 filaments (24K). The composition of the precursor is 93% acrylonitrile, 6% Methyl acrylate and 1% Itaconic acid. The details of the properties of the precursor are shown in Table 1
Table 1. Average properties of PAN fibers.
0.51±0.09
Tensile Modulus (GPa) 10.1±1.18
Diameter (µm) 13.69±1.32
Elongation Linear Density (%) (dTex) 17.3±1.45
1.75±0.37
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PAN
Tensile Strength (GPa)
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Sample
The line possesses four stabilization ovens (zones 1, 2, 3, and 4) and low temperature and
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high temperature furnaces with four temperature zones as shown in Figure 1.
Figure 1. Schematic representation of the Carbon Nexus carbon fiber manufacturing line.
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2.1. Fiber Heat Treatment The process parameters used for stabilization and carbonization of precursor fibers are shown in Table 2. The precursor fibers were stabilized in air at 225oC, 235oC, 245oC and 255oC in each oven respectively. Ovens 1 and 2 operated under a 4.7% draw, while ovens 3 and 4
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operated under 0.9% at drive three. They were then carbonized in the low temperature furnace in nitrogen atmosphere at 487oC, 650oC, 750oC and 800oC in each zone respectively under a 2.2% draw. Nitrogen flow levels were set to 75L/min at either end of the furnace. The speed of the line was set to 120m/h at the unwinding site. The resident time of fiber at this
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speed in the heated zones 1 and 3 was twenty-four minutes each, and in zones 2 and 4 was twenty minutes respectively. Residence time in the low temperature furnace was approximately four minutes. Draw is the pull experienced by the fibers by maintaining
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slightly higher speed at the output roller compared to input roller of an oven.
Table 2. Fiber production specifications. Recirculation fan speed (Hz) 33 33 33 33
Draw ratio (%) 4.7 4.7 0.9 0.9 2.2 2.2 2.2 2.2
Nitrogen injection (L/min)
75 75 75 75
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Low Temperature Carbonization
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Oxidation
zone-1 zone-2 zone-3 zone-4 zone-1 zone-2 zone-3 zone-4
Temperature (°C) 225 235 245 255 487 650 750 800
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Processing stage
2.2. Density Testing
The density of fibers was measured with an Ultrapycnometer (ultrapyc 1200e Quantachrome instruments) under Helium environment. The stabilized and carbonized fibers were dried in an oven prior to density measurement in order to reduce the effect of moisture. Density of the fiber samples was obtained by performing several iterations on each samples until the standard deviation between the values <0.005. The average density of the iterations was reported.
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2.3. Single Fiber Tensile Testing The tensile properties were measured using the FAVIMAT tensile testing machine equipped with a vibroscope and a load cell of 210cN. A gauge length of 25mm was considered. A set of 25 filaments were tested for each sample. A test speed of 12.5mm/min was used for PAN and OPF samples were as 2mm/min was used for carbon fibers. A grip force of 40cN was
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considered for this testing. Each tensile test was conducted with an application of initial pretension of 0.5cN. Initially, linear density of each filament was determined by the vibroscope before proceeding with the tensile testing. Moreover, the obtained linear density was used to calculate the percentage mass retained by the treated samples using the following equation 1.
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= 100 −
100
(1)
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Where Ldp represents linear density of the precursor and Ldt represents linear density of the treated fiber.
2.4. Differential Scanning Calorimetry
The DSC measurements were performed on Q200 (TA Instruments) under air atmosphere. About 5 mg of the fiber samples were punched inside an Aluminium pan. The DSC was set to
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equilibrate at 30oC then ramped up to 430oC at a rate of 10oC/min. The aromatization index was calculated using the following equation [13].
× 100%
(2)
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=
Where AI is the aromatization index, Hv is the enthalpy of the PAN precursor fiber in J/g, and Ho is the enthalpy of the OPF. Hv and Ho were obtained by calculating the area under the
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exothermic curves (Please see Appendix I).
2.5. FTIR
Fourier transform infrared spectroscopy was performed on the fiber samples using Bruker Lumos FTIR instrument with ATR (Attenuated Total Reflectance) mode. The instrument was equipped with germanium crystal. The absorbance spectra was collected between 600 and 4000cm-1 wavenumbers at a resolution of 4cm-1. 128 scans were performed on each sample.
2.6. XRD
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X-ray diffraction studies are conducted on PANALYTICAL XPert Powder X-ray diffraction instrument with radiation source of CuKα. The wavelength of the source is 1.5418Å. An operating voltage 40V and current 30mA is considered. The material is scanned between angles of 10° and 40°. The scanning is conducted at step size of 0.0130 and time per step of
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100secs.
2.7. Optical Microscopy
Optical microscopy studies was performed to examine the fiber cross sections using Olympus DP70 at a magnification of 100x. The samples were prepared by mounting the fibers
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vertically in epoxy and polished perpendicular to the fiber cross sections on 9µm for 4mins, 3µm for 6mins and finally 1µm polishing pad for 10minutes at an operating speed of 300rpm
3. Results and Discussion
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using Rotopol automatic polishing machine.
The various physical properties of the fibers at each stage of the heat treatment process was analyzed and the results are given in Figure 2. The density was found to increase gradually after stabilization and carbonization as shown in Figure 2b. These trends are the result of the
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formation of a ladder polymer structure during stabilization and further compaction of structure with the intermolecular cross linking between the ladder structures during carbonization [2] The fiber diameter and linear density decreased from 13.69µm to 8.45µm and from 1.75dTex to 1.03dTex, respectively after the stabilization and low temperature
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carbonization processes. (Figure 2a). The decrease in the linear density and diameter of the fibers can be ascribed to the mass loss resulting from structural and chemical transformation
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occurring within the fibers during thermal treatment. A rapid loss of heteroatoms other than carbon occurred during this carbonization process led to a more pronounced variation in linear density and diameter of the fibers after the carbonization stage [2]. Finally, using the changes in linear density, the percentage mass retention was calculated using equation 1t each stage (as shown in Figure 2(d)). From Figures 2 (a to d) the development of compact structure in association with the mass loss is evident.
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Linear Density (dTex) 1.6
1.6
1.2
1.2
0.8
0.8
0.4
0.4
0.0 PAN Precursor Fiber 16
(c)
OPF
LT Carbonized Fiber
(b)
0.0 PAN Precursor Fiber
(d) 100
Diameter (µm)
Density (g/cc)
60 8
4
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40
OPF
LT Carbonized Fiber
Percentage Mass (%)
80
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2.0
(a)
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2.0
20
0 PAN Precursor Fiber
OPF
LT Carbonized Fiber
0 PAN Precursor Fiber
OPF
LT Carbonized Fiber
Figure 2. The changes in (a) linear density, (b) density, (c) diameter and (d) mass
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transformations of PAN fiber during stabilization and carbonization.
The tensile mechanical properties of all produced fibers were monitored and the results are shown in Figure 3. We found that the maximum elongation of the PAN precursor increased from 17% to 22% after stabilization Whilst its tensile strength and modulus decreased from
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0.51GPa to 0.27GPa and from 10.11GPa to 8.97GPa respectively, indicating a transition towards increasingly ductile failure. After carbonization, the elongation at break decreased
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dramatically to 1.85% whereas the strength and modulus increased to 1.24GPa and 69.39GPa respectively, indicating that low temperature carbonized fiber fails in a brittle manner rather than ductile. The elastic modulus of ~ 70GPa highlights the potential for high modulus carbon fibers upon further carbonization and graphitization [2, 3].
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(a)
25
Tensile Strength (GPa)
(b)
Tensile Elongation (%)
1.2
20
1.0 0.8
15
0.6
10 0.4
0.0 PAN Precursor Fiber
OPF
80
LT Carbonized Fiber
(c)
0 PAN Precursor Fiber
Tensile Modulus (GPa)
60
OPF
LT Carbonized Fiber
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40
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5
0.2
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20
0 PAN Precursor Fiber
OPF
LT Carbonized Fiber
Figure 3. The tensile strength, elongation and modulus transformation of PAN fibers during stabilization and carbonization.
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The variation in the crystal structure of the fibers was examined by X-ray diffraction. The variation in the diffraction peaks of the fiber samples are shown in Figure 4. The first peak of the PAN precursor at an angle ~ 17° corresponds to 100 crystal plane whereas the peak at 29.5° corresponds to 110 crystal plane [14]. The shoulder peak at the initial ~ 17° peak of the
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precursor is assumed to be due to the sizing on the precursor fibers. With the stabilization of the fibers the peak at 17°almost disappeared and at the same time a broad peak at around
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25.5° started to appear which corresponds to the crystal plane 002 [8]. This indicates with the progress in stabilization the crystal structure ruptures and increase in the amorphous content in the fibers and thus disordered regions. This led to an increase in flexibility associated with an increase in the percentage elongation at break of the fibers with the progress in stabilization. Conversely, a drop in the tensile strength of the fibers was also observed. The appearance of the peak at 25.5° in the stabilized fiber indicates the initiation of the graphitic structures and the removal of heteroatoms assisting the development of the ordered structure [8]. After low temperature carbonization the peak at 25.5° significantly increased as the compact graphitic structure formed, and this co-occurred with a sharp enhancement in tensile strength and modulus.
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Figure 4. Intensity vs 2θ of the fiber samples.
The thermal characteristics of the fibers during treatment were investigated using DSC and the curves are shown in Figure 5. The precursor PAN fiber was found to display a prominent exothermic reaction initiating at around 225oC and reaching maximum intensity around
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280oC. This large exothermic event (∆H= 1956 J/g) has been attributed to the cyclization, dehydrogenation and oxidation reactions [8]. With the stabilization treatment of the fibers it can be seen that the heat released is largely diminished (∆H= 686 J/g) due to a reduction of unreacted nitriles. The low temperature carbonized fiber displayed a very small exothermic
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PAN.
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event (∆H~ 10 J/g) possibly related to presence of a small fraction of incompletely cyclized
The DSC data was also utilized to calculate the aromatization index of the fiber using equation 2. It is observed that the transformational changes occurring during heat treatment of the fibers, which include a cyclization of the structure (noticeable from the increase in the aromatization index) induces a decrease in the mass of precursor fiber.
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Figure 5. The thermal behaviors of fiber samples.
The changes in the chemical functional groups of PAN during heat treatment were monitored by FTIR analysis. The absorbance spectra of the samples is shown in Figure 6. The spectrum show a number of distinct bands corresponding to C=N (1582cm-1), C≡N (2243cm-1), C=O (1735cm-1), CH (1364cm-1) and CH2 (1454cm-1) functional groups [15]. The variation in the functional groups from precursor to stabilized fiber shows a clear indication of occurrence of
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cyclization, dehydrogenation and oxidation reactions. The occurrence of these reactions further lead to the formation of ladder polymer structure. For instance, it is clear that after the stabilization stage the peak corresponding to the C≡N functionality has almost vanished and
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this co-occurred with the appearance of a sharp peak at 1580cm-1 indicating the formation of the cyclic structure in the molecular chains [15]. Conversely, the peak corresponding to the CH2 groups almost disappeared after stabilization whereas a sharp peak related to CH
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functional group appeared indicating the loss of hydrogen due to dehydrogenation reaction. A sharp peak in the stabilized fiber at 806cm-1 corresponding to C=C-H functional group and after a dehydrogenation reaction was detected [16]. A shoulder like peak at 1660cm-1 corresponding to ketone functional group in a cyclic structure appears in the stabilized fiber indicating the oxidation reaction [17]. During low temperature carbonization treatment removal of elements other than carbon in the form of volatiles at higher temperatures further leads to the evolution of higher order structures with pure C=C bonds which is very infra-red inactive, As a result the resulting infra-red spectrum display very features apart from to the C=N, C=C-H bands and a broader peak corresponding to CH groups.
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Figure 6. FTIR spectra of PAN, stabilized and low temperature carbonized fibers.
Finally optical microscopy analysis (Figure 7) was conducted on the fiber samples to examine the formation of the radial heterogeneity in the fibers. The stabilized fibers displayed a very faint skin-core effect in the outer region of the fibers as indicated by yellow arrows in Figure 7b. The appearance of the skin-core effect could be an indication of heterogeneous distribution of oxygen along the fiber cross section [18] which can also be induced by
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thermal inhomogeneity across the fiber section [18]. However, in the current study radial heterogeneity from the stabilized fibers to the carbonized fibers (as shown in Figure 7c) was not observed, also suggesting that the severity of the radial heterogeneity in the fibers was not high enough to further accentuate any ski-core effects and negatively affect the properties of
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the carbon fibers. This shows that, there are possibilities to develop a uniform carbon fiber
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structure if the radial heterogeneity in the stabilized fibers is controlled.
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Figure 7. Optical microscopy of fiber samples. a) Precursor fiber b) Stabilized fiber c) Low
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temperature carbon fiber. Conclusion
A method to calculate mass retention of PAN precursor fiber after heat treatment was developed. After stabilization, PAN precursor fiber maintained approximately 81% mass, with reduction in fiber diameter. Elongation and density were increased during stabilization while tensile strength was decreased which is attributed to the structural variations in the
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fibers with thermal treatment. When subsequently carbonized to 800oC it retains approximately 59% of the precursor mass with further reduction in the fiber diameter. The aromatization index of OPF was found to be approximately 66%, and that of low temperature carbonized fiber 99%. The increase in strength, modulus and decrease in elongation signify a
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transition from ductile failure to brittle failure. This study provides a better understanding of the approximation of carbon fiber yield after low temperature treatment and analyzing
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alternate precursor fibers of differing composition could generate a yield forecasting model for those specific fibers.
Appendix I
The calculation of enthalpies for each sample is shown in Figure 8. Before proceeding with the area calculation under heat flow curve, the curves were baseline corrected as shown Figure 8(b, d). The calculated enthalpies were used in equation 2 to find out the aromatization index. =
1956 − 686.34 × 100% = 64.91% 1956
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Figure 8. Enthalpy calculation procedure a) Heat flow of precursor b) Baseline corrected
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precursor heat flow curve c) Heat flow of OPF d) Baseline corrected OPF heat flow curve.
Acknowledgements: This project was funded by the Summer Scholarship from Deakin University’s Institute of Frontier Materials. The authors would like to thank Steve Atkiss and
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the carbon fiber production line operational team for their time and support. NH would like to
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acknowledge VESKI for the Victoria Fellowship.
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8.
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Fu, Z., et al., Structure evolution and mechanism of polyacrylonitrile and related copolymers during the stabilization. Journal of Materials Science, 2014. 49(7): p. 2864-2874. Arbab, S. and A. Zeinolebadi, A procedure for precise determination of thermal stabilization reactions in carbon fiber precursors. Polymer Degradation and Stability, 2013. 98(12): p. 2537-2545. Yu, M.-J., et al., Effect of oxygen uptake and aromatization on the skin–core morphology during the oxidative stabilization of polyacrylonitrile fibers. Journal of Applied Polymer Science, 2008. 107(3): p. 1939-1945.
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16.
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S0141-3910(16)30052-0
DOI:
10.1016/j.polymdegradstab.2016.02.029
Reference:
PDST 7884
To appear in:
Polymer Degradation and Stability
Received Date: 20 December 2015 Revised Date:
17 February 2016
Accepted Date: 27 February 2016
Please cite this article as: Hameed N, Sharp J, Nunna S, Creighton C, Magniez K, Jyotishkumar P, Salim NV, Fox B, Structural Transformation of Polyacrylonitrile Fibers during Stabilization and Low Temperature Carbonization, Polymer Degradation and Stability (2016), doi: 10.1016/ j.polymdegradstab.2016.02.029. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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ACCEPTED MANUSCRIPT Structural Transformation of Polyacrylonitrile Fibers during Stabilization and Low Temperature Carbonization Nishar Hameed1*, Jordan Sharp1, Srinivas Nunna1, Claudia Creighton1, Kevin Magniez1, Jyotishkumar P.2, Nisa V. Salim1 and Bronwyn Fox3 1
Department of Polymer Science and Rubber Technology, Cochin University of Science and Technology, Cochin, India 3
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2
Carbon Nexus, Institute for Frontier Materials, Deakin University, Geelong, Australia.
Factory of the Future, Swinburne University of Technology, Hawthorn, Australia
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Abstract: The effect of oxidative stabilization and carbonization processes on the structure, mass and mechanical properties of polyacrylonitrile (PAN) precursor fibers was analyzed. A gradual densification of the fibers occurring from mass loss, decrease in fiber diameter and
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increase in density were observed after stabilization at a maximum temperature of 255oC and carbonization at a maximum temperature of 800oC. The tensile strength and modulus of the fibers were found to decrease after stabilization but then increased after low temperature carbonization. The thermal processing of the precursor fibers affected their mode of failure after tensile loading, changing from a ductile type of failure to a brittle type. The type of failure correlated well with the crystal structure changes in the fibers. Whilst the PAN
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precursor fiber started to exotherm above 225oC in air, no prominent exothermic reaction was measured in the carbonized fibers in air up to 430oC. The aromatization index of stabilized fiber was calculated to be ~ 66%, and that of carbonized fiber was ~ 99%. FTIR studies
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indicated that the variation in the chemical structure of the fibers with the stabilization of the fibers. Radial heterogeneity in the stabilized fibers was observed however it was not
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promoted to the carbonized fibers. Finally, a method to calculate mass retention of PAN precursor fiber after heat treatment was developed, and the calculated percentage mass retained of the precursor fiber after oxidation and carbonization were found to be 81% and 51%, respectively. . This study proposes an effective method to calculate the percentage of mass retained by precursor fibers after stabilization and low temperature carbonization to provide a model for evaluating carbon fiber yield from a given amount of fibers. Keywords: Carbon fibers, polyacrylonitrile, stabilization, carbonization.
Corresponding author email: [email protected]
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1. Introduction Carbon fiber is commonly utilized as a reinforcing material in composite materials because of its relatively high strength and lightweight characteristics [1]. Commercial carbon fibers are produced by heat treatment of polyacrylonitrile (PAN) precursor fibers due to its potential for the manufacturing carbon fibers with high strength and stiffness [2]. Since its inception,
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improvements on the mechanical properties and effective manufacturing costs have constantly been sought [3-6]. In order to develop cost forecasting of the carbon fiber it is imperative to understand the mass and energy changes in precursor fiber when processed in a manufacturing environment. The manufacture of carbon fiber involves the stages of
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stabilization and carbonization [2]. The PAN precursor fibers are stabilized to withstand high temperature during carbonization process. During stabilization transformation on of linear macromolecular structure into aromatic ladder structure takes place [7]. This occurs via
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cyclization reaction around 280°C, followed by dehydrogenation and oxidation reactions [8] leading to the stabilization of the fiber for high temperature carbonization [2]. Once stabilized, it is referred to as oxidized PAN fiber (OPF). Carbonization removes most noncarbon elements from the fiber by heating to extreme temperatures, violently shaking them off and reducing the fiber mass. This further compacts the aromatic ring structure,
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transforming it into turbostratic graphite structure to form carbon fibers [9]. At this point typically up to 95% or more of the atomic composition consists of carbon atoms [10].
Many scientific studies and reports have focused on improving mechanical properties of
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carbon fiber, namely modulus and strength by optimizing the process parameters [1]. Also many literatures have been written based on understanding the chemical transitions of PAN
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precursor fiber and changes in fiber structure when converted into carbon fiber [3, 9, 11]. Thus a large knowledge bank already exists on how PAN precursor fiber changes mechanically and chemically during heat treatment and variation of process factors. One of the most important parameters in the process of carbonization is temperatures which has been shown to significantly affect the mechanical properties of the fibers [12]. However, little research focuses on low temperature carbonization and its effects on the structure property relationships of the fibers. Moreover little work has been undertaken to analyze linear mass transition specifically, but the other effects of pyrolysis have been heavily analyzed [1, 2, 4-7, 10].
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The goal of this study is to gain knowledge of the chemical and physical transformations and the level of mass retained after PAN precursor fiber is stabilized and carbonized. This will provide insights into the approximation of carbon fiber yield after low temperature treatment, as the fiber changes chemically and physically at each stage. Here the whole process of stabilization and carbonization was carefully performed on a pilot scale carbon fiber
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manufacturing line. This enables a better understanding of the process parameters as well as the material properties that can be beneficial for the industrial scale production of carbon fibers.
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2. Materials and Methods
PAN precursor fibers provided from Bluestar Fibres Co. Ltd were stabilized and then carbonized using Carbon Nexus multi-tow carbon fiber production line at Deakin University
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(Figure 1). Thirty tows were processed under same operating conditions. Each tow consists of 24000 filaments (24K). The composition of the precursor is 93% acrylonitrile, 6% Methyl acrylate and 1% Itaconic acid. The details of the properties of the precursor are shown in Table 1
Table 1. Average properties of PAN fibers.
0.51±0.09
Tensile Modulus (GPa) 10.1±1.18
Diameter (µm) 13.69±1.32
Elongation Linear Density (%) (dTex) 17.3±1.45
1.75±0.37
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PAN
Tensile Strength (GPa)
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Sample
The line possesses four stabilization ovens (zones 1, 2, 3, and 4) and low temperature and
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high temperature furnaces with four temperature zones as shown in Figure 1.
Figure 1. Schematic representation of the Carbon Nexus carbon fiber manufacturing line.
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2.1. Fiber Heat Treatment The process parameters used for stabilization and carbonization of precursor fibers are shown in Table 2. The precursor fibers were stabilized in air at 225oC, 235oC, 245oC and 255oC in each oven respectively. Ovens 1 and 2 operated under a 4.7% draw, while ovens 3 and 4
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operated under 0.9% at drive three. They were then carbonized in the low temperature furnace in nitrogen atmosphere at 487oC, 650oC, 750oC and 800oC in each zone respectively under a 2.2% draw. Nitrogen flow levels were set to 75L/min at either end of the furnace. The speed of the line was set to 120m/h at the unwinding site. The resident time of fiber at this
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speed in the heated zones 1 and 3 was twenty-four minutes each, and in zones 2 and 4 was twenty minutes respectively. Residence time in the low temperature furnace was approximately four minutes. Draw is the pull experienced by the fibers by maintaining
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slightly higher speed at the output roller compared to input roller of an oven.
Table 2. Fiber production specifications. Recirculation fan speed (Hz) 33 33 33 33
Draw ratio (%) 4.7 4.7 0.9 0.9 2.2 2.2 2.2 2.2
Nitrogen injection (L/min)
75 75 75 75
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Low Temperature Carbonization
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Oxidation
zone-1 zone-2 zone-3 zone-4 zone-1 zone-2 zone-3 zone-4
Temperature (°C) 225 235 245 255 487 650 750 800
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Processing stage
2.2. Density Testing
The density of fibers was measured with an Ultrapycnometer (ultrapyc 1200e Quantachrome instruments) under Helium environment. The stabilized and carbonized fibers were dried in an oven prior to density measurement in order to reduce the effect of moisture. Density of the fiber samples was obtained by performing several iterations on each samples until the standard deviation between the values <0.005. The average density of the iterations was reported.
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2.3. Single Fiber Tensile Testing The tensile properties were measured using the FAVIMAT tensile testing machine equipped with a vibroscope and a load cell of 210cN. A gauge length of 25mm was considered. A set of 25 filaments were tested for each sample. A test speed of 12.5mm/min was used for PAN and OPF samples were as 2mm/min was used for carbon fibers. A grip force of 40cN was
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considered for this testing. Each tensile test was conducted with an application of initial pretension of 0.5cN. Initially, linear density of each filament was determined by the vibroscope before proceeding with the tensile testing. Moreover, the obtained linear density was used to calculate the percentage mass retained by the treated samples using the following equation 1.
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= 100 −
100
(1)
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Where Ldp represents linear density of the precursor and Ldt represents linear density of the treated fiber.
2.4. Differential Scanning Calorimetry
The DSC measurements were performed on Q200 (TA Instruments) under air atmosphere. About 5 mg of the fiber samples were punched inside an Aluminium pan. The DSC was set to
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equilibrate at 30oC then ramped up to 430oC at a rate of 10oC/min. The aromatization index was calculated using the following equation [13].
× 100%
(2)
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=
Where AI is the aromatization index, Hv is the enthalpy of the PAN precursor fiber in J/g, and Ho is the enthalpy of the OPF. Hv and Ho were obtained by calculating the area under the
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exothermic curves (Please see Appendix I).
2.5. FTIR
Fourier transform infrared spectroscopy was performed on the fiber samples using Bruker Lumos FTIR instrument with ATR (Attenuated Total Reflectance) mode. The instrument was equipped with germanium crystal. The absorbance spectra was collected between 600 and 4000cm-1 wavenumbers at a resolution of 4cm-1. 128 scans were performed on each sample.
2.6. XRD
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X-ray diffraction studies are conducted on PANALYTICAL XPert Powder X-ray diffraction instrument with radiation source of CuKα. The wavelength of the source is 1.5418Å. An operating voltage 40V and current 30mA is considered. The material is scanned between angles of 10° and 40°. The scanning is conducted at step size of 0.0130 and time per step of
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100secs.
2.7. Optical Microscopy
Optical microscopy studies was performed to examine the fiber cross sections using Olympus DP70 at a magnification of 100x. The samples were prepared by mounting the fibers
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vertically in epoxy and polished perpendicular to the fiber cross sections on 9µm for 4mins, 3µm for 6mins and finally 1µm polishing pad for 10minutes at an operating speed of 300rpm
3. Results and Discussion
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using Rotopol automatic polishing machine.
The various physical properties of the fibers at each stage of the heat treatment process was analyzed and the results are given in Figure 2. The density was found to increase gradually after stabilization and carbonization as shown in Figure 2b. These trends are the result of the
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formation of a ladder polymer structure during stabilization and further compaction of structure with the intermolecular cross linking between the ladder structures during carbonization [2] The fiber diameter and linear density decreased from 13.69µm to 8.45µm and from 1.75dTex to 1.03dTex, respectively after the stabilization and low temperature
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carbonization processes. (Figure 2a). The decrease in the linear density and diameter of the fibers can be ascribed to the mass loss resulting from structural and chemical transformation
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occurring within the fibers during thermal treatment. A rapid loss of heteroatoms other than carbon occurred during this carbonization process led to a more pronounced variation in linear density and diameter of the fibers after the carbonization stage [2]. Finally, using the changes in linear density, the percentage mass retention was calculated using equation 1t each stage (as shown in Figure 2(d)). From Figures 2 (a to d) the development of compact structure in association with the mass loss is evident.
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Linear Density (dTex) 1.6
1.6
1.2
1.2
0.8
0.8
0.4
0.4
0.0 PAN Precursor Fiber 16
(c)
OPF
LT Carbonized Fiber
(b)
0.0 PAN Precursor Fiber
(d) 100
Diameter (µm)
Density (g/cc)
60 8
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40
OPF
LT Carbonized Fiber
Percentage Mass (%)
80
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2.0
(a)
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2.0
20
0 PAN Precursor Fiber
OPF
LT Carbonized Fiber
0 PAN Precursor Fiber
OPF
LT Carbonized Fiber
Figure 2. The changes in (a) linear density, (b) density, (c) diameter and (d) mass
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transformations of PAN fiber during stabilization and carbonization.
The tensile mechanical properties of all produced fibers were monitored and the results are shown in Figure 3. We found that the maximum elongation of the PAN precursor increased from 17% to 22% after stabilization Whilst its tensile strength and modulus decreased from
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0.51GPa to 0.27GPa and from 10.11GPa to 8.97GPa respectively, indicating a transition towards increasingly ductile failure. After carbonization, the elongation at break decreased
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dramatically to 1.85% whereas the strength and modulus increased to 1.24GPa and 69.39GPa respectively, indicating that low temperature carbonized fiber fails in a brittle manner rather than ductile. The elastic modulus of ~ 70GPa highlights the potential for high modulus carbon fibers upon further carbonization and graphitization [2, 3].
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(a)
25
Tensile Strength (GPa)
(b)
Tensile Elongation (%)
1.2
20
1.0 0.8
15
0.6
10 0.4
0.0 PAN Precursor Fiber
OPF
80
LT Carbonized Fiber
(c)
0 PAN Precursor Fiber
Tensile Modulus (GPa)
60
OPF
LT Carbonized Fiber
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5
0.2
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20
0 PAN Precursor Fiber
OPF
LT Carbonized Fiber
Figure 3. The tensile strength, elongation and modulus transformation of PAN fibers during stabilization and carbonization.
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The variation in the crystal structure of the fibers was examined by X-ray diffraction. The variation in the diffraction peaks of the fiber samples are shown in Figure 4. The first peak of the PAN precursor at an angle ~ 17° corresponds to 100 crystal plane whereas the peak at 29.5° corresponds to 110 crystal plane [14]. The shoulder peak at the initial ~ 17° peak of the
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precursor is assumed to be due to the sizing on the precursor fibers. With the stabilization of the fibers the peak at 17°almost disappeared and at the same time a broad peak at around
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25.5° started to appear which corresponds to the crystal plane 002 [8]. This indicates with the progress in stabilization the crystal structure ruptures and increase in the amorphous content in the fibers and thus disordered regions. This led to an increase in flexibility associated with an increase in the percentage elongation at break of the fibers with the progress in stabilization. Conversely, a drop in the tensile strength of the fibers was also observed. The appearance of the peak at 25.5° in the stabilized fiber indicates the initiation of the graphitic structures and the removal of heteroatoms assisting the development of the ordered structure [8]. After low temperature carbonization the peak at 25.5° significantly increased as the compact graphitic structure formed, and this co-occurred with a sharp enhancement in tensile strength and modulus.
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Figure 4. Intensity vs 2θ of the fiber samples.
The thermal characteristics of the fibers during treatment were investigated using DSC and the curves are shown in Figure 5. The precursor PAN fiber was found to display a prominent exothermic reaction initiating at around 225oC and reaching maximum intensity around
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280oC. This large exothermic event (∆H= 1956 J/g) has been attributed to the cyclization, dehydrogenation and oxidation reactions [8]. With the stabilization treatment of the fibers it can be seen that the heat released is largely diminished (∆H= 686 J/g) due to a reduction of unreacted nitriles. The low temperature carbonized fiber displayed a very small exothermic
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PAN.
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event (∆H~ 10 J/g) possibly related to presence of a small fraction of incompletely cyclized
The DSC data was also utilized to calculate the aromatization index of the fiber using equation 2. It is observed that the transformational changes occurring during heat treatment of the fibers, which include a cyclization of the structure (noticeable from the increase in the aromatization index) induces a decrease in the mass of precursor fiber.
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Figure 5. The thermal behaviors of fiber samples.
The changes in the chemical functional groups of PAN during heat treatment were monitored by FTIR analysis. The absorbance spectra of the samples is shown in Figure 6. The spectrum show a number of distinct bands corresponding to C=N (1582cm-1), C≡N (2243cm-1), C=O (1735cm-1), CH (1364cm-1) and CH2 (1454cm-1) functional groups [15]. The variation in the functional groups from precursor to stabilized fiber shows a clear indication of occurrence of
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cyclization, dehydrogenation and oxidation reactions. The occurrence of these reactions further lead to the formation of ladder polymer structure. For instance, it is clear that after the stabilization stage the peak corresponding to the C≡N functionality has almost vanished and
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this co-occurred with the appearance of a sharp peak at 1580cm-1 indicating the formation of the cyclic structure in the molecular chains [15]. Conversely, the peak corresponding to the CH2 groups almost disappeared after stabilization whereas a sharp peak related to CH
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functional group appeared indicating the loss of hydrogen due to dehydrogenation reaction. A sharp peak in the stabilized fiber at 806cm-1 corresponding to C=C-H functional group and after a dehydrogenation reaction was detected [16]. A shoulder like peak at 1660cm-1 corresponding to ketone functional group in a cyclic structure appears in the stabilized fiber indicating the oxidation reaction [17]. During low temperature carbonization treatment removal of elements other than carbon in the form of volatiles at higher temperatures further leads to the evolution of higher order structures with pure C=C bonds which is very infra-red inactive, As a result the resulting infra-red spectrum display very features apart from to the C=N, C=C-H bands and a broader peak corresponding to CH groups.
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Figure 6. FTIR spectra of PAN, stabilized and low temperature carbonized fibers.
Finally optical microscopy analysis (Figure 7) was conducted on the fiber samples to examine the formation of the radial heterogeneity in the fibers. The stabilized fibers displayed a very faint skin-core effect in the outer region of the fibers as indicated by yellow arrows in Figure 7b. The appearance of the skin-core effect could be an indication of heterogeneous distribution of oxygen along the fiber cross section [18] which can also be induced by
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thermal inhomogeneity across the fiber section [18]. However, in the current study radial heterogeneity from the stabilized fibers to the carbonized fibers (as shown in Figure 7c) was not observed, also suggesting that the severity of the radial heterogeneity in the fibers was not high enough to further accentuate any ski-core effects and negatively affect the properties of
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the carbon fibers. This shows that, there are possibilities to develop a uniform carbon fiber
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structure if the radial heterogeneity in the stabilized fibers is controlled.
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Figure 7. Optical microscopy of fiber samples. a) Precursor fiber b) Stabilized fiber c) Low
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temperature carbon fiber. Conclusion
A method to calculate mass retention of PAN precursor fiber after heat treatment was developed. After stabilization, PAN precursor fiber maintained approximately 81% mass, with reduction in fiber diameter. Elongation and density were increased during stabilization while tensile strength was decreased which is attributed to the structural variations in the
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fibers with thermal treatment. When subsequently carbonized to 800oC it retains approximately 59% of the precursor mass with further reduction in the fiber diameter. The aromatization index of OPF was found to be approximately 66%, and that of low temperature carbonized fiber 99%. The increase in strength, modulus and decrease in elongation signify a
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transition from ductile failure to brittle failure. This study provides a better understanding of the approximation of carbon fiber yield after low temperature treatment and analyzing
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alternate precursor fibers of differing composition could generate a yield forecasting model for those specific fibers.
Appendix I
The calculation of enthalpies for each sample is shown in Figure 8. Before proceeding with the area calculation under heat flow curve, the curves were baseline corrected as shown Figure 8(b, d). The calculated enthalpies were used in equation 2 to find out the aromatization index. =
1956 − 686.34 × 100% = 64.91% 1956
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Figure 8. Enthalpy calculation procedure a) Heat flow of precursor b) Baseline corrected
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precursor heat flow curve c) Heat flow of OPF d) Baseline corrected OPF heat flow curve.
Acknowledgements: This project was funded by the Summer Scholarship from Deakin University’s Institute of Frontier Materials. The authors would like to thank Steve Atkiss and
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the carbon fiber production line operational team for their time and support. NH would like to
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acknowledge VESKI for the Victoria Fellowship.
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Jean-Baptise Donnet, T.K.W., Serge Rebouillat, Jimmy C.M. Peng, Carbon Fibers. 3rd edition ed. 1998: Marcel Dekker, Inc. 575. Ko, T.-H., The influence of pyrolysis on physical properties and microstructure of modified PAN fibers during carbonization. Journal of Applied Polymer Science, 1991. 43(3): p. 589600. Savage, G., Carbon-Carbon Composites. First edition ed. 1993: Chapman & Hall. 76. Ko, T.-H., Influence of continuous stabilization on the physical properties and microstructure of PAN-based carbon fibers. Journal of Applied Polymer Science, 1991. 42(7): p. 1949-1957. Tsai, J.-s., Measurement of the aromatization index for oxidised polyacrylonitrile fibre by the oriation distribution pattern. Journal of Materials Science Letters, 1993. 12(24): p. 19111913. Wang, B., et al., Evolution of aggregation structure of polyacrylonitrile fibers in the cyclization reaction. Journal of Applied Polymer Science, 2012. 124(4): p. 3413-3418. Ouyang, Q., et al., Mechanism and kinetics of the stabilization reactions of itaconic acidmodified polyacrylonitrile. Polymer Degradation and Stability, 2008. 93(8): p. 1415-1421.
10.
11. 12. 13.
14. 15.
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17.
Fu, Z., et al., Structure evolution and mechanism of polyacrylonitrile and related copolymers during the stabilization. Journal of Materials Science, 2014. 49(7): p. 2864-2874. Arbab, S. and A. Zeinolebadi, A procedure for precise determination of thermal stabilization reactions in carbon fiber precursors. Polymer Degradation and Stability, 2013. 98(12): p. 2537-2545. Yu, M.-J., et al., Effect of oxygen uptake and aromatization on the skin–core morphology during the oxidative stabilization of polyacrylonitrile fibers. Journal of Applied Polymer Science, 2008. 107(3): p. 1939-1945.
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