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UV stabilization route for melt-processible PAN-based carbon fibers
Carbon 41 (2003) 1399–1409
UV stabilization route for melt-processible PAN-based carbon fibers M.C. Paiva 1 , P. Kotasthane, D.D. Edie, A.A. Ogale* Department of Chemical Engineering, and Center for Advanced Engineering Fibers and Films, Clemson University, Clemson, SC 29634 -0910, USA Received 18 January 2003; accepted 29 January 2003
Abstract Ultraviolet radiation-based stabilization routes were explored to produce carbon fibers from melt-processible PAN-based copolymers. An acrylonitrile / methyl acrylate (AN / MA) copolymer was melt-spun into fibers that were crosslinked using UV radiation. The fibers could then be stabilized by oxidative heat treatment, and subsequently carbonized. Physical and mechanical testing was performed to determine the degree of stabilization and the properties of the stabilized and carbonized fibers. 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; B. Stabilization; C. Differential scanning calorimetry (DSC); Infrared spectroscopy; D. Mechanical properties
1. Introduction In contrast to wet-spinning, the melt spinning technique converts pure precursor directly into fiber form at high process speeds and without added expense of solvent recovery and recycling [1]. However, the bulk of carbon fibers are produced from polyacrylonitrile (PAN) precursors that are converted into fiber form by wet and dry spinning methods [2]. The reason behind the use of wetspinning methods is that commercial PAN copolymer precursors thermally decompose below their melting temperature, making melt spinning impossible. Recently, BP Amoco Chemicals produced a melt-spinnable PAN copolymer [3,4] containing a high amount of methyl acrylate comonomer located irregularly along the polymer chain, which most likely decreases the crystallinity of the copolymer. A stabilizing agent was also added to inhibit thermal degradation. Although this meltspinnable copolymer might appear to be attractive as a carbon fiber precursor, its thermal stability makes standard oxidative stabilization techniques [1,5–9] impractical. Since this type of PAN copolymer melts before any *Corresponding author. Fax: 11-864-656-0784. E-mail address: [email protected] (A.A. Ogale). 1 On leave of absence from the Department of Polymer Engineer˜ ing, University of Minho, 4800-058 Guimaraes, Portugal.
thermally-induced reactions occur, other approaches must be developed to crosslink the precursor fibers. Various grades of melt-spinnable PAN precursors are currently being developed and evaluated for carbon fiber production in a joint Clemson / Virginia Tech project funded by the US Department of Energy. The research team at Virginia Tech is synthesizing melt-spinnable PAN copolymers [10], and the team at Clemson is converting these into melt-spun PAN and carbon fibers. The present paper reports the stabilization procedure developed for these melt-spun PAN fibers as well as the conversion of these stabilized fibers into carbon fibers.
2. Background: reactions of polyacrylonitrile precursor fibers
2.1. Heat stabilization of PAN The stabilization of polyacrylonitrile fibers for carbon fiber production involves thermal treatment, usually in air, at temperatures ranging from 180 to 300 8C. This part of the process is intended to increase the stiffness of the PAN molecules and hold them together in such a way as to avoid extensive relaxation and chain scission during the final carbonization step. The increase in molecular stiffness is mainly achieved
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through the cyclization of PAN [1,5,6,11]. The cyclization is an exothermic reaction during which nitrile groups react, transforming part of the PAN into a ladder-type polymer. The precise reaction mechanism for cyclization can differ, depending on the experimental conditions and type of copolymer [7]. Numerous reactions can take place during heating of PAN, and many are still not well-understood, as described by Bashir [8]. Burland and Parsons [12] showed that the first step of the stabilization was the cyclization through reaction of the nitrile groups, dehydrogenation being significant only above 300 8C. Grassie and McGuchan [6] proposed that dehydrogenation and cyclization reactions take place simultaneously, the former occurring both within the non-cyclized polymer chain as well as within the condensed heterocyclic rings. Cyclization reactions are extremely exothermic, but this behavior can be considerably reduced if a co-monomer such as methyl acrylate, vinyl acetate, or itaconic acid, for example, is introduced into the polymer chain. Furthermore, the activation energy of the cyclization reaction is smaller for the copolymer, relative to the PAN homopolymer, indicating that the co-monomer acts as an alternative initiator for the cyclization reaction. When PAN fibers are thermally stabilized the amount of co-monomer in the precursor not only affects the rate of oxidative stabilization [1,7], it also affects temperature and applied tension requirements [13]. The kinetic data for the cyclization reaction can be obtained by differential thermal analysis (differential scanning calorimetry, DSC), using the Kissinger method [14]. The method is based on the observation that when the rate of reaction varies with temperature (i.e., when the reaction has an activation energy), the position of the DSC peak varies with heating rate, if all other variables are kept constant. At the molecular structure level, recent studies point out the influence of the polymer structure on the final ladderpolymer formation. Several authors have discussed the stereospecificity of the cyclization reaction [15–17]. In fact, the cyclization reaction should be stereospecific, occurring preferentially in isotactic sequences to form a straight rod-like structure. Gupta and Harrison [9,18] observed that intramolecular cyclization reactions occur at lower temperatures (175–230 8C) in the amorphous phase of the polymer, leading to a considerable decrease in intermolecular interactions due to the decrease in concentration of the highly polar nitrile groups. This would account for the macroscopic shrinkage observed at this stage. The crystalline regions would act as ‘‘bridge’’ points between the amorphous regions, holding the structure together. The authors report that, at temperatures above 320 8C, oxidation and intermolecular crosslinking take place, and that oxidative degradation reactions occur above 380 8C. To summarize, stabilization of PAN precursors is a complex process that depends both on the chemical composition of the copolymer and on its structural charac-
teristics. Molecular orientation significantly affects the properties of the polymer fibers, and orientation must be maintained as much as possible during stabilization if the final properties of the carbon fibers are to be maximized.
2.2. Crosslinking reactions of PAN When PAN is irradiated with UV light in vacuum, it evolves hydrogen, methane, acrylonitrile and hydrogen cyanide, leading to chain scission and crosslinking reactions simultaneously [19], as represented in Fig. 1. The crosslinking reactions take place preferentially at the tertiary carbon atom in the polymer backbone and, thus, does not lead to the formation of conjugated imine bonds (–C=N–) x . The photo-oxidation of this polymer, especially at elevated temperatures, is described by Ranby and Rabek [20] as resulting in the formation of a ladder structure, following a mechanism similar to that observed for thermal oxidation [5]. Other radiation sources have been used to achieve the crosslinking of PAN. Dietrich et al. [21] used electronbeam irradiation on PAN fibers. For fiber irradiation in air they observed, using electron spin resonance, the formation of an alkyl radical structure, when there was poor oxygen diffusion through the fiber, and the formation of a peroxide radical structure, for good oxygen diffusion. The authors also found that the radicals formed were extremely stable, with a lifetime of several days. Heat treatment of the fibers led to cyclization, and this process was observed to be faster for the irradiated fibers than for the non-treated fibers.
2.3. Melt-spinning of PAN precursors As BP discovered, the controlled introduction of a comonomer such as methyl acrylate (MA) into the acrylonitrile (AN) polymer backbone in adequate amounts (higher than 10%) and with an appropriate stabilizing system, decrease T g and allows the polymer to melt before exothermic cyclization reactions occur [3]. Two main problems arise when trying to produce carbon fibers from this new class of PAN copolymer precursors: one is chemical in nature, and the other is structure-related. The introduction of a significant amount of methyl acrylate as a
Fig. 1. Effect of UV irradiation on PAN [19].
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co-monomer reduces the length of the acrylonitrile sequences in the copolymer, therefore limiting the extent of cyclization that can occur during stabilization. Crosslinking can also affect the extent of cyclization at the structural level by ‘‘freezing’’ the spatial distribution, thus inhibiting molecular mobility.
3. Experimental The materials used in the current work were: (a) commercial fibers produced from a Mitsubishi copolymer by wet spinning, hereafter designated as M fibers and, (b) acrylonitrile / methyl acrylate copolymer, produced at Virginia Tech by solution polymerization and stabilized with 1% of boric acid [10], hereafter designated as VT fibers. The Mitsubishi copolymer had a nominal AN / MA ratio of 94:6 and an intrinsic viscosity (IV), obtained by dilute solution viscometry, of 1.98 dl / g. The VT copolymer had a comonomer ratio of 88:12, and an IV of 0.49 dl / g. The VT copolymer was melt spun into fibers using a rate-controlled capillary rheometer Instron 3211 and a capillary die with a diameter of 150 mm diameter and an L /D of 3. The results reported in this paper were all obtained for single-filaments. The extrusion temperature for all tests was 225 8C and the nominal shear rate was 500 s 21 . The fibers solidified as they exited the capillary and were collected on a winder for a nominal draw-down ratio of 4. The fibers were placed inside a temperature-controlled oven equipped with a window that allowed exposure to UV radiation (100 W Hg arc lamp, Oriel). The lamp was mounted in a Series Q housing equipped with a rear reflector and a condenser, to concentrate the radiation on a circle of approximately 60 mm of diameter. The distance
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between the sample and the light source was approximately 100 mm. The fibers were thermally stabilized at different conditions, as summarized in Table 1. After UV irradiation, the industrial M fibers were heated to 230 8C in air for periods of 45 min, 1 h and 2 h. One set of fibers were stabilized by thermal oxidation conducted under constant weight condition of 0.03 g / denier (approximately 4 MPa stress level). The second set was thermally stabilized under constant length condition by wrapping a continuous filament around a Grafoil sheet, exposing the sample to UV radiation, and subsequently subjecting the fibers to thermal oxidation. The final degree of stabilization was compared to that obtained for the fibers heated to 230 8C for 2 h without UV irradiation (M a ). The melt-spun VT fibers were heat stabilized in air after UV irradiation (Table 1). After several trials, a heating program was developed that rendered the fibers infusible during the final carbonization step. The present work reports results obtained from fibers heat stabilized following this four-step heating program: 2 h at 180 8C, 2 h at 200 8C, 2 h at 210 8C, and finally 1 h at 220 8C. No load was applied during the stabilization of the VT fibers. Recall that these were melt-spun fibers. Like all melt-spun materials, they are soft and tend to break easily when even a small weight is applied at a temperature close to T g . Therefore, the VT fibers were stabilized only at constant length. As noted earlier, M fibers were also stabilized at constant length for comparison. After stabilization, both sets of fibers (M and VT) were carbonized in an Astro furnace, at 1500 8C, under a constant flow of He. The thermal stability and reactivity of the precursors, as-spun and UV irradiated fibers were studied by DSC, using a Pyris 1 DSC (Perkin-Elmer). Isothermal experiments were performed in which the polymer was heated to a given temperature and was held at that temperature for
Table 1 Conditions for UV and heat stabilization of the M and VT fibers studied at constant load and constant length Sample
UV irradiation (h) (T5130 8C)
Heat oxidation
Stabilization performed at constant load Ma Mb Mc Md Me
– 2.5 2.5 2.5 2.5
2 h (230 8C) 45 min (230 8C) 1 h (230 8C) 2 h (230 8C) –
Stabilization performed at constant length* M1 2.5 M2 2.5 VT 1 1 VT 2 2 VT 3 2.5 VT 4 2.5 * UV irradiation performed at T5150 8C.
– 2 h (230 8C) – – – 2 h (180 8C), 2 h (200 8C), 2 h (210 8C), 1 h (220 8C)
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Fig. 2. Isothermal differential scanning calorimetric scans for the VT polymer (AN / MA: 88 / 12) copolymer at various temperatures.
60 min. A temperature interval ranging from 220 to 270 8C was studied, and the scans were performed at heating rates of 5, 10 and 20 8C / min. Chemical changes as measured by nitrile conversion were analyzed using Fourier transform infrared spectroscopy (FT-IR) techniques. The variation in nitrile concentration across large diameter fibers was studied by FT-IR microscopy, using a Nicolet Magna 550 with NicPlan FT-IR microscope and mapping stage. The thinner fibers were analyzed using an Endurance Foundation Diamond ATR and the Nicolet Magna 550. The tensile properties of both fiber types were measured at four different stages: as-spun fibers, after UV irradiation, after heat stabilization, and after carbonization. The single filament tensile tests were performed on approximately 20 fibers from each stage, using a computer controlled MTI tensile testing machine equipped with a 500-g load cell.
Fig. 3. DSC thermograms for VT polymer at different heating rates.
4. Results and discussion
4.1. Precursor thermal analysis Isothermal DSC experiments were conducted to evaluate the thermal stability of the VT precursor by heating the polymer samples to a set temperature, ranging from 220 to 270 8C, for 1 h. It was reasoned that if the precursor was thermally stable for approximately 1 h, it could be meltspun in a batch or a continuous extruder. As displayed in Fig. 2, the polymer is very stable up to 230 8C. At 240 8C a slow reaction is initiated, and a small amount of heat is evolved toward the end of the experiment. At 250 8C the exothermic cyclization reaction takes place after a brief delay. As the temperature is increased, the reaction begins earlier and the reaction rate increases. These isothermal tests indicated that the VT precursor can be maintained in a
Fig. 4. Evaluation of the order of reaction from the shape of the DSC curve.
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Table 2 Activation energies and rate constants determined by the Kissinger method [8] Precursor
f (K / min)
Tm (K)
Activation energy (KJ / mol)
Average reaction order
Average frequency factor (s 21 )
M
5 10 20
553.6 564.1 575.7
157
1.0
3310 12
VT
5 10 20
571.5 586 604
112
1.0
7310 7
Fig. 5. ATR-FT-IR spectra of (a) M fibers and (b) VT fibers.
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molten state for a sufficiently long duration (| 1 h) below 240 8C to allow further melt spinning. From the DSC thermograms obtained at different heating rates (5–20 8C / min), Fig. 3, it was observed that the maximum of the reaction peak was dependent on heating rate. Based on these data, the Kissinger method [14] was used to estimate reaction kinetics parameters. The activation energy was derived from the temperature dependence of the peak maximum on the heating rate, as described by Eq. (1):
S D
f d ln] T 2m E ]]] 5 2] 1 R d ] Tm
S D
(1)
The frequency factors were calculated using Eq. (2), and
the order of the reaction was obtained from Eqs. (3) and (4), where a and b were estimated as described in Fig. 4. E f E ]] ln k 5 ln ] 1 ln ] 2 1 R RT m Tm
(2)
a S5] b
(3)
] n 5 1.26ŒS
(4)
The results for kinetic parameters, summarized in Table 2, suggest that the activation energy is smaller for the VT polymer than that of M polymer. This would be expected because the VT polymer has a higher methyl acrylate content, and methyl acrylate is known to act as an initiator for the cyclization reaction [6]. However, the frequency
Fig. 6. Nitrile ratio for samples treated at various conditions, as determined by FT-IR-ATR spectroscopy.
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factor was smaller for the VT polymer, which would tend to reduce the overall reaction rate. The net result was that overall reaction rate was similar for both polymers.
4.2. UV/thermal stabilization Initial testing verified that the VT precursor fibers melted before they could be thermally stabilized. However, after they had been exposed to UV irradiation, these same fibers could be thermally stabilized with little or no melting. This indicated that UV-induced crosslinking reactions were taking place. To determine the proper stabilization procedure for the VT precursor, the melt-spun fibers were UV and / or thermally stabilized under the conditions listed in Table 1. Then, DSC analyses were used to assess the resulting crosslinking, and FT-IR analysis to estimate the extent of cyclization reactions. Since the heat released by the exothermic cyclization reaction decreases as the prior degree of crosslinking increases, DSC measurements of enthalpy may be used to detect the evolution, and eventually saturation, of crosslinking. This approach was used to estimate the degree of crosslinking in the as-spun M fibers, and for the same fibers after prior exposure to UV for periods of 1 and 2.5 h (at constant length). The heat for cyclization reactions for the as-spun fiber was the highest (600 J / g) and the value decreased with increasing prior exposure to UV radiation, reaching a value of 477 J / g for an exposure time 1 h, and 463 J / g after 2.5 h of exposure. Similar DSC measurements were performed on the VT 1 , VT 2 , and VT 3 fibers before and after exposure to UV radiation. The measured heat of reaction for the as-spun fibers (prior to UV exposure) averaged 470 J / g. After UV exposure for 1 and 2.5 h, the enthalpy values had reduced to 434 and 304 J / g, respectively. Although the small size of the DSC sample (less than 1 mg) resulted in about 15% variation in measured values, there was a clear decrease in
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enthalpy of cyclization as a result of crosslinking from prior UV irradiation, similar to the one observed for M fibers. The measured enthalpy approaches an asymptotic value as the UV exposure time approaches 2 h. To determine if cyclization reactions took place, FT-IR spectra were obtained in the ATR mode for fibers stabilized at various stages, as displayed in Fig. 5. In these tests, the intensity of the CH 2 (1450 cm 21 ) and CN (2240 cm 21 ) bands were measured. The cyclization reaction would involve reaction of the nitrile unit, but not dehydrogenation. Therefore, the nitrile ratio, I(CN) /I(CH 2 ), should provide a good estimate of the degree of cyclization since nitrile groups are consumed and the CH 2 groups are unaffected [12]. Fig. 6a displays the evolution of the nitrile ratio for M fibers stabilized at constant load and constant length conditions, and Fig. 6b the evolution of this same ratio for the VT fibers stabilized at constant length. From Fig. 6a, it can be inferred that the nitrile ratio for the as-spun M fibers is not significantly different (at 95% confidence interval) from that of the UV irradiated fibers, indicating that UV irradiation induces little or no cyclization in the solution spun precursor (M). However, the nitrile ratio decreases significantly after thermal / oxidative stabilization and continues to decrease with increasing stabilization time. This indicates that cyclization does occur during thermal stabilization and longer times enhance the degree of cyclization achieved. Similar trends were observed for the VT fibers (as those observed for M fibers). However, this copolymer precursor had a lower acrylonitrile content than the Mitsubishi copolymer precursor, and the measured nitrile ratio for VT samples, reported in Fig. 6b was correspondingly lower after each step. Cyclization was not observed during the UV irradiation for either of the two precursors, indicating potential differences from the reactions suggested by Ranby and Rabek [20]. The differences may arise from differences in resin composition and reaction conditions
Fig. 7. Optical micrograph of a fiber cross-section, the squares represent the approximate area where FT-IR spectra where obtained, and the results are presented in the graph .
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(temperature and intensity of UV radiation). It is emphasized, however, that the UV irradiation step was successful in crosslinking the polymer to an extent that further thermal stabilization was feasible. Because of the limited depth of penetration of UV radiation in majority of polymers, the directional, asymmetric extent of crosslinking in the PAN-based fibers was examined by IR microscopy. Specifically, the larger VT fibers (diameter |50 mm) were ideally suited because these could be embedded in a resin and thin cross sections (461 mm) could be obtained by microtoming. These sections were spread on a ZnSe crystal and analyzed after drying. The area of fiber to be analyzed by FT-IR was reduced to the lower limit of the equipment by using an aperture of approximately 20320 mm 2 . This way, the fiber cross-section could be divided in five different regions, as shown in Fig. 7. Measurements taken from several cross sections of two different fibers show a nitrile ratio of 0.5560.04 for the fiber center area. For the external areas, values varying from 0.39 to 0.58 where measured, showing regions where the nitrile consumption was reasonably higher relative to the sample center, and regions with results similar to the center area. These results suggest a variation in nitrile ratio not only in the radial direction but also across the fiber diameter, possibly as a consequence of the decrease of UV intensity accessible to the fiber along its diameter. The results suggest that the depth of penetration of UV for these PAN-based fiber precursors is |15 mm. Therefore, in a scaled-up process the precursor fiber diameter should be held below 15 mm, a value not different from that used in current commercial processes.
4.3. Evolution of properties Scanning electron microscopy (SEM) images of the fracture surfaces (after tensile testing) of the VT fibers that are displayed in Fig. 8. The as-spun fibers display a ductile response in Fig. 8a. With crosslinking and thermal stabilization, the fiber ductility decreases, as illustrated by the relatively smoother fractured surfaces of Fig. 8b and c. In contrast, the rather brittle nature of carbonized fibers is confirmed by micrographs presented in Fig. 9 for both VTand M-based carbon fibers. It is also noted that the microstructure of the PAN-based fibers is fairly featureless. This is in contrast to the radially oriented graphene-layer arrangement observed in mesophase pitch-based carbon fibers at similar (or even lower) carbonization temperatures. The lateral surfaces of the two fibers display a significant difference. The carbon fibers produced from M fibers by continuous spinning process display far fewer flaws than do carbon fibers produced from batch VTpolymer. Single filament tensile tests were performed on as-spun, M 1 , M 2 , VT 3 and VT 4 fibers. The effect of UV irradiation and thermal oxidation on the tensile properties of the M
Fig. 8. FESEM micrographs of VT fibers (a) as spun (b) UV irradiated for 2.5 h, and (c) UV irradiated and heat stabilized.
and VT fibers is displayed in Fig. 10a–d. The effect of UV irradiation on properties is similar for both types of fibers, although in absolute terms the M fibers possess better properties at all stages. M and VT fibers show a considerable reduction in strain-to-failure after UV irradiation. This reduction likely results from crosslinking of the polymer, which was observed earlier by solubility tests and enthalpy measurements. The effect of thermal oxidation is also similar for both types of fibers, although the yield strength slightly decreased for the UV irradiated fibers, and slightly increased for the thermally oxidized fibers. The decrease in
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Fig. 9. SEM micrographs of carbonized fibers obtained from (a) M 2 and (b) VT 4 .
tensile strength results primarily from the critical flaws present in these fiber samples prepared by a batch process. In contrast, the flaw-insensitive property, tensile modulus, is not very different for the two types of fibers that have undergone stabilization. Fig. 10a also clearly depicts the importance of the properties of the as-spun fibers as related to those of the final fibers. The higher molecular weight and degree of orientation in M fibers that went through a post-drawing step (whereas the VT fibers did not) result in better properties for M fibers. Also, as illustrated in micrographs of Fig. 9, the larger number and size of flaws in the VT fibers manifested into low strength and strain-to-failure. It
seems reasonable to anticipate better mechanical properties for the VT fibers if the molecular weight and orientation can be improved, and the polymer and fibers are produced by a continuous process. It is noted that fibers that were only UV-treated did not survive the carbonization step. However, fibers that received thermal oxidation treatment after UV irradiation could be successfully carbonized, i.e., thermal oxidation is necessary for fiber stabilization. Table 3 summarizes the tensile properties of the M fibers that were stabilized using the dual UV-thermal treatment and subsequently carbonized (M a to M d ). The results indicate that the introduction of a UV irradiation step does not decrease the final
Table 3 Tensile properties of the M fibers UV irradiated, heat oxidized and carbonized at 1500 8C Sample
No. tests
Diameter (mm)
Max. strength (MPa)
Modulus (GPa)
Strain-to-failure (%)
Ma Mb Mc Md M2 VT 4 M *commercial
10 9 16 18 22 18 –
9.260.3 8.860.3 8.560.3 8.660.2 7.160.3 15.760.7 7
10306260 6746250 6906230 10206320 12636309 334673 4410–4900
188626 136617 143628 206630 181620 5866 235–295
1.060.4 1.060.3 1.160.3 1.160.2 0.760.2 0.660.1 1.5–2.0
* Range of values from the technical data sheet of Mitsubishi Rayon, for a standard PAN-based carbon fiber.
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Fig. 10. The effect of UV irradiation, thermal oxidation, and carbonization on the tensile properties of M and VT fibers.
properties of the carbon fibers obtained. Comparison of the mechanical properties of M a, M d and M 2 shows that this conclusion is valid for fibers stabilized at constant load as well as constant length.
Acknowledgements The authors gratefully acknowledge the financial support from the Department of Energy, through grant No. 4500011036. This work made use of ERC Shared Facilities supported by the National Science Foundation under Award No. EEC-9731680.
5. Conclusions This study establishes the feasibility of producing carbon fibers from melt-spinnable polyacrylonitrile copolymers. DSC and solubility tests performed on PANbased wet-spun fibers and on experimental melt-spun fibers showed that UV irradiation effectively crosslinked the copolymers to an extent that enabled subsequent thermal oxidation. The experimental fibers could also be successfully carbonized. Although the carbon fibers exhibited low mechanical properties, there is a potential to substantially improve these properties by optimizing polymer structure (copolymer composition and molecular weight) and melt spinning conditions to form smaller diameter precursor fibers with higher degrees of molecular orientation.
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UV stabilization route for melt-processible PAN-based carbon fibers M.C. Paiva 1 , P. Kotasthane, D.D. Edie, A.A. Ogale* Department of Chemical Engineering, and Center for Advanced Engineering Fibers and Films, Clemson University, Clemson, SC 29634 -0910, USA Received 18 January 2003; accepted 29 January 2003
Abstract Ultraviolet radiation-based stabilization routes were explored to produce carbon fibers from melt-processible PAN-based copolymers. An acrylonitrile / methyl acrylate (AN / MA) copolymer was melt-spun into fibers that were crosslinked using UV radiation. The fibers could then be stabilized by oxidative heat treatment, and subsequently carbonized. Physical and mechanical testing was performed to determine the degree of stabilization and the properties of the stabilized and carbonized fibers. 2003 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon fibers; B. Stabilization; C. Differential scanning calorimetry (DSC); Infrared spectroscopy; D. Mechanical properties
1. Introduction In contrast to wet-spinning, the melt spinning technique converts pure precursor directly into fiber form at high process speeds and without added expense of solvent recovery and recycling [1]. However, the bulk of carbon fibers are produced from polyacrylonitrile (PAN) precursors that are converted into fiber form by wet and dry spinning methods [2]. The reason behind the use of wetspinning methods is that commercial PAN copolymer precursors thermally decompose below their melting temperature, making melt spinning impossible. Recently, BP Amoco Chemicals produced a melt-spinnable PAN copolymer [3,4] containing a high amount of methyl acrylate comonomer located irregularly along the polymer chain, which most likely decreases the crystallinity of the copolymer. A stabilizing agent was also added to inhibit thermal degradation. Although this meltspinnable copolymer might appear to be attractive as a carbon fiber precursor, its thermal stability makes standard oxidative stabilization techniques [1,5–9] impractical. Since this type of PAN copolymer melts before any *Corresponding author. Fax: 11-864-656-0784. E-mail address: [email protected] (A.A. Ogale). 1 On leave of absence from the Department of Polymer Engineer˜ ing, University of Minho, 4800-058 Guimaraes, Portugal.
thermally-induced reactions occur, other approaches must be developed to crosslink the precursor fibers. Various grades of melt-spinnable PAN precursors are currently being developed and evaluated for carbon fiber production in a joint Clemson / Virginia Tech project funded by the US Department of Energy. The research team at Virginia Tech is synthesizing melt-spinnable PAN copolymers [10], and the team at Clemson is converting these into melt-spun PAN and carbon fibers. The present paper reports the stabilization procedure developed for these melt-spun PAN fibers as well as the conversion of these stabilized fibers into carbon fibers.
2. Background: reactions of polyacrylonitrile precursor fibers
2.1. Heat stabilization of PAN The stabilization of polyacrylonitrile fibers for carbon fiber production involves thermal treatment, usually in air, at temperatures ranging from 180 to 300 8C. This part of the process is intended to increase the stiffness of the PAN molecules and hold them together in such a way as to avoid extensive relaxation and chain scission during the final carbonization step. The increase in molecular stiffness is mainly achieved
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through the cyclization of PAN [1,5,6,11]. The cyclization is an exothermic reaction during which nitrile groups react, transforming part of the PAN into a ladder-type polymer. The precise reaction mechanism for cyclization can differ, depending on the experimental conditions and type of copolymer [7]. Numerous reactions can take place during heating of PAN, and many are still not well-understood, as described by Bashir [8]. Burland and Parsons [12] showed that the first step of the stabilization was the cyclization through reaction of the nitrile groups, dehydrogenation being significant only above 300 8C. Grassie and McGuchan [6] proposed that dehydrogenation and cyclization reactions take place simultaneously, the former occurring both within the non-cyclized polymer chain as well as within the condensed heterocyclic rings. Cyclization reactions are extremely exothermic, but this behavior can be considerably reduced if a co-monomer such as methyl acrylate, vinyl acetate, or itaconic acid, for example, is introduced into the polymer chain. Furthermore, the activation energy of the cyclization reaction is smaller for the copolymer, relative to the PAN homopolymer, indicating that the co-monomer acts as an alternative initiator for the cyclization reaction. When PAN fibers are thermally stabilized the amount of co-monomer in the precursor not only affects the rate of oxidative stabilization [1,7], it also affects temperature and applied tension requirements [13]. The kinetic data for the cyclization reaction can be obtained by differential thermal analysis (differential scanning calorimetry, DSC), using the Kissinger method [14]. The method is based on the observation that when the rate of reaction varies with temperature (i.e., when the reaction has an activation energy), the position of the DSC peak varies with heating rate, if all other variables are kept constant. At the molecular structure level, recent studies point out the influence of the polymer structure on the final ladderpolymer formation. Several authors have discussed the stereospecificity of the cyclization reaction [15–17]. In fact, the cyclization reaction should be stereospecific, occurring preferentially in isotactic sequences to form a straight rod-like structure. Gupta and Harrison [9,18] observed that intramolecular cyclization reactions occur at lower temperatures (175–230 8C) in the amorphous phase of the polymer, leading to a considerable decrease in intermolecular interactions due to the decrease in concentration of the highly polar nitrile groups. This would account for the macroscopic shrinkage observed at this stage. The crystalline regions would act as ‘‘bridge’’ points between the amorphous regions, holding the structure together. The authors report that, at temperatures above 320 8C, oxidation and intermolecular crosslinking take place, and that oxidative degradation reactions occur above 380 8C. To summarize, stabilization of PAN precursors is a complex process that depends both on the chemical composition of the copolymer and on its structural charac-
teristics. Molecular orientation significantly affects the properties of the polymer fibers, and orientation must be maintained as much as possible during stabilization if the final properties of the carbon fibers are to be maximized.
2.2. Crosslinking reactions of PAN When PAN is irradiated with UV light in vacuum, it evolves hydrogen, methane, acrylonitrile and hydrogen cyanide, leading to chain scission and crosslinking reactions simultaneously [19], as represented in Fig. 1. The crosslinking reactions take place preferentially at the tertiary carbon atom in the polymer backbone and, thus, does not lead to the formation of conjugated imine bonds (–C=N–) x . The photo-oxidation of this polymer, especially at elevated temperatures, is described by Ranby and Rabek [20] as resulting in the formation of a ladder structure, following a mechanism similar to that observed for thermal oxidation [5]. Other radiation sources have been used to achieve the crosslinking of PAN. Dietrich et al. [21] used electronbeam irradiation on PAN fibers. For fiber irradiation in air they observed, using electron spin resonance, the formation of an alkyl radical structure, when there was poor oxygen diffusion through the fiber, and the formation of a peroxide radical structure, for good oxygen diffusion. The authors also found that the radicals formed were extremely stable, with a lifetime of several days. Heat treatment of the fibers led to cyclization, and this process was observed to be faster for the irradiated fibers than for the non-treated fibers.
2.3. Melt-spinning of PAN precursors As BP discovered, the controlled introduction of a comonomer such as methyl acrylate (MA) into the acrylonitrile (AN) polymer backbone in adequate amounts (higher than 10%) and with an appropriate stabilizing system, decrease T g and allows the polymer to melt before exothermic cyclization reactions occur [3]. Two main problems arise when trying to produce carbon fibers from this new class of PAN copolymer precursors: one is chemical in nature, and the other is structure-related. The introduction of a significant amount of methyl acrylate as a
Fig. 1. Effect of UV irradiation on PAN [19].
M.C. Paiva et al. / Carbon 41 (2003) 1399–1409
co-monomer reduces the length of the acrylonitrile sequences in the copolymer, therefore limiting the extent of cyclization that can occur during stabilization. Crosslinking can also affect the extent of cyclization at the structural level by ‘‘freezing’’ the spatial distribution, thus inhibiting molecular mobility.
3. Experimental The materials used in the current work were: (a) commercial fibers produced from a Mitsubishi copolymer by wet spinning, hereafter designated as M fibers and, (b) acrylonitrile / methyl acrylate copolymer, produced at Virginia Tech by solution polymerization and stabilized with 1% of boric acid [10], hereafter designated as VT fibers. The Mitsubishi copolymer had a nominal AN / MA ratio of 94:6 and an intrinsic viscosity (IV), obtained by dilute solution viscometry, of 1.98 dl / g. The VT copolymer had a comonomer ratio of 88:12, and an IV of 0.49 dl / g. The VT copolymer was melt spun into fibers using a rate-controlled capillary rheometer Instron 3211 and a capillary die with a diameter of 150 mm diameter and an L /D of 3. The results reported in this paper were all obtained for single-filaments. The extrusion temperature for all tests was 225 8C and the nominal shear rate was 500 s 21 . The fibers solidified as they exited the capillary and were collected on a winder for a nominal draw-down ratio of 4. The fibers were placed inside a temperature-controlled oven equipped with a window that allowed exposure to UV radiation (100 W Hg arc lamp, Oriel). The lamp was mounted in a Series Q housing equipped with a rear reflector and a condenser, to concentrate the radiation on a circle of approximately 60 mm of diameter. The distance
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between the sample and the light source was approximately 100 mm. The fibers were thermally stabilized at different conditions, as summarized in Table 1. After UV irradiation, the industrial M fibers were heated to 230 8C in air for periods of 45 min, 1 h and 2 h. One set of fibers were stabilized by thermal oxidation conducted under constant weight condition of 0.03 g / denier (approximately 4 MPa stress level). The second set was thermally stabilized under constant length condition by wrapping a continuous filament around a Grafoil sheet, exposing the sample to UV radiation, and subsequently subjecting the fibers to thermal oxidation. The final degree of stabilization was compared to that obtained for the fibers heated to 230 8C for 2 h without UV irradiation (M a ). The melt-spun VT fibers were heat stabilized in air after UV irradiation (Table 1). After several trials, a heating program was developed that rendered the fibers infusible during the final carbonization step. The present work reports results obtained from fibers heat stabilized following this four-step heating program: 2 h at 180 8C, 2 h at 200 8C, 2 h at 210 8C, and finally 1 h at 220 8C. No load was applied during the stabilization of the VT fibers. Recall that these were melt-spun fibers. Like all melt-spun materials, they are soft and tend to break easily when even a small weight is applied at a temperature close to T g . Therefore, the VT fibers were stabilized only at constant length. As noted earlier, M fibers were also stabilized at constant length for comparison. After stabilization, both sets of fibers (M and VT) were carbonized in an Astro furnace, at 1500 8C, under a constant flow of He. The thermal stability and reactivity of the precursors, as-spun and UV irradiated fibers were studied by DSC, using a Pyris 1 DSC (Perkin-Elmer). Isothermal experiments were performed in which the polymer was heated to a given temperature and was held at that temperature for
Table 1 Conditions for UV and heat stabilization of the M and VT fibers studied at constant load and constant length Sample
UV irradiation (h) (T5130 8C)
Heat oxidation
Stabilization performed at constant load Ma Mb Mc Md Me
– 2.5 2.5 2.5 2.5
2 h (230 8C) 45 min (230 8C) 1 h (230 8C) 2 h (230 8C) –
Stabilization performed at constant length* M1 2.5 M2 2.5 VT 1 1 VT 2 2 VT 3 2.5 VT 4 2.5 * UV irradiation performed at T5150 8C.
– 2 h (230 8C) – – – 2 h (180 8C), 2 h (200 8C), 2 h (210 8C), 1 h (220 8C)
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Fig. 2. Isothermal differential scanning calorimetric scans for the VT polymer (AN / MA: 88 / 12) copolymer at various temperatures.
60 min. A temperature interval ranging from 220 to 270 8C was studied, and the scans were performed at heating rates of 5, 10 and 20 8C / min. Chemical changes as measured by nitrile conversion were analyzed using Fourier transform infrared spectroscopy (FT-IR) techniques. The variation in nitrile concentration across large diameter fibers was studied by FT-IR microscopy, using a Nicolet Magna 550 with NicPlan FT-IR microscope and mapping stage. The thinner fibers were analyzed using an Endurance Foundation Diamond ATR and the Nicolet Magna 550. The tensile properties of both fiber types were measured at four different stages: as-spun fibers, after UV irradiation, after heat stabilization, and after carbonization. The single filament tensile tests were performed on approximately 20 fibers from each stage, using a computer controlled MTI tensile testing machine equipped with a 500-g load cell.
Fig. 3. DSC thermograms for VT polymer at different heating rates.
4. Results and discussion
4.1. Precursor thermal analysis Isothermal DSC experiments were conducted to evaluate the thermal stability of the VT precursor by heating the polymer samples to a set temperature, ranging from 220 to 270 8C, for 1 h. It was reasoned that if the precursor was thermally stable for approximately 1 h, it could be meltspun in a batch or a continuous extruder. As displayed in Fig. 2, the polymer is very stable up to 230 8C. At 240 8C a slow reaction is initiated, and a small amount of heat is evolved toward the end of the experiment. At 250 8C the exothermic cyclization reaction takes place after a brief delay. As the temperature is increased, the reaction begins earlier and the reaction rate increases. These isothermal tests indicated that the VT precursor can be maintained in a
Fig. 4. Evaluation of the order of reaction from the shape of the DSC curve.
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Table 2 Activation energies and rate constants determined by the Kissinger method [8] Precursor
f (K / min)
Tm (K)
Activation energy (KJ / mol)
Average reaction order
Average frequency factor (s 21 )
M
5 10 20
553.6 564.1 575.7
157
1.0
3310 12
VT
5 10 20
571.5 586 604
112
1.0
7310 7
Fig. 5. ATR-FT-IR spectra of (a) M fibers and (b) VT fibers.
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molten state for a sufficiently long duration (| 1 h) below 240 8C to allow further melt spinning. From the DSC thermograms obtained at different heating rates (5–20 8C / min), Fig. 3, it was observed that the maximum of the reaction peak was dependent on heating rate. Based on these data, the Kissinger method [14] was used to estimate reaction kinetics parameters. The activation energy was derived from the temperature dependence of the peak maximum on the heating rate, as described by Eq. (1):
S D
f d ln] T 2m E ]]] 5 2] 1 R d ] Tm
S D
(1)
The frequency factors were calculated using Eq. (2), and
the order of the reaction was obtained from Eqs. (3) and (4), where a and b were estimated as described in Fig. 4. E f E ]] ln k 5 ln ] 1 ln ] 2 1 R RT m Tm
(2)
a S5] b
(3)
] n 5 1.26ŒS
(4)
The results for kinetic parameters, summarized in Table 2, suggest that the activation energy is smaller for the VT polymer than that of M polymer. This would be expected because the VT polymer has a higher methyl acrylate content, and methyl acrylate is known to act as an initiator for the cyclization reaction [6]. However, the frequency
Fig. 6. Nitrile ratio for samples treated at various conditions, as determined by FT-IR-ATR spectroscopy.
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factor was smaller for the VT polymer, which would tend to reduce the overall reaction rate. The net result was that overall reaction rate was similar for both polymers.
4.2. UV/thermal stabilization Initial testing verified that the VT precursor fibers melted before they could be thermally stabilized. However, after they had been exposed to UV irradiation, these same fibers could be thermally stabilized with little or no melting. This indicated that UV-induced crosslinking reactions were taking place. To determine the proper stabilization procedure for the VT precursor, the melt-spun fibers were UV and / or thermally stabilized under the conditions listed in Table 1. Then, DSC analyses were used to assess the resulting crosslinking, and FT-IR analysis to estimate the extent of cyclization reactions. Since the heat released by the exothermic cyclization reaction decreases as the prior degree of crosslinking increases, DSC measurements of enthalpy may be used to detect the evolution, and eventually saturation, of crosslinking. This approach was used to estimate the degree of crosslinking in the as-spun M fibers, and for the same fibers after prior exposure to UV for periods of 1 and 2.5 h (at constant length). The heat for cyclization reactions for the as-spun fiber was the highest (600 J / g) and the value decreased with increasing prior exposure to UV radiation, reaching a value of 477 J / g for an exposure time 1 h, and 463 J / g after 2.5 h of exposure. Similar DSC measurements were performed on the VT 1 , VT 2 , and VT 3 fibers before and after exposure to UV radiation. The measured heat of reaction for the as-spun fibers (prior to UV exposure) averaged 470 J / g. After UV exposure for 1 and 2.5 h, the enthalpy values had reduced to 434 and 304 J / g, respectively. Although the small size of the DSC sample (less than 1 mg) resulted in about 15% variation in measured values, there was a clear decrease in
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enthalpy of cyclization as a result of crosslinking from prior UV irradiation, similar to the one observed for M fibers. The measured enthalpy approaches an asymptotic value as the UV exposure time approaches 2 h. To determine if cyclization reactions took place, FT-IR spectra were obtained in the ATR mode for fibers stabilized at various stages, as displayed in Fig. 5. In these tests, the intensity of the CH 2 (1450 cm 21 ) and CN (2240 cm 21 ) bands were measured. The cyclization reaction would involve reaction of the nitrile unit, but not dehydrogenation. Therefore, the nitrile ratio, I(CN) /I(CH 2 ), should provide a good estimate of the degree of cyclization since nitrile groups are consumed and the CH 2 groups are unaffected [12]. Fig. 6a displays the evolution of the nitrile ratio for M fibers stabilized at constant load and constant length conditions, and Fig. 6b the evolution of this same ratio for the VT fibers stabilized at constant length. From Fig. 6a, it can be inferred that the nitrile ratio for the as-spun M fibers is not significantly different (at 95% confidence interval) from that of the UV irradiated fibers, indicating that UV irradiation induces little or no cyclization in the solution spun precursor (M). However, the nitrile ratio decreases significantly after thermal / oxidative stabilization and continues to decrease with increasing stabilization time. This indicates that cyclization does occur during thermal stabilization and longer times enhance the degree of cyclization achieved. Similar trends were observed for the VT fibers (as those observed for M fibers). However, this copolymer precursor had a lower acrylonitrile content than the Mitsubishi copolymer precursor, and the measured nitrile ratio for VT samples, reported in Fig. 6b was correspondingly lower after each step. Cyclization was not observed during the UV irradiation for either of the two precursors, indicating potential differences from the reactions suggested by Ranby and Rabek [20]. The differences may arise from differences in resin composition and reaction conditions
Fig. 7. Optical micrograph of a fiber cross-section, the squares represent the approximate area where FT-IR spectra where obtained, and the results are presented in the graph .
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(temperature and intensity of UV radiation). It is emphasized, however, that the UV irradiation step was successful in crosslinking the polymer to an extent that further thermal stabilization was feasible. Because of the limited depth of penetration of UV radiation in majority of polymers, the directional, asymmetric extent of crosslinking in the PAN-based fibers was examined by IR microscopy. Specifically, the larger VT fibers (diameter |50 mm) were ideally suited because these could be embedded in a resin and thin cross sections (461 mm) could be obtained by microtoming. These sections were spread on a ZnSe crystal and analyzed after drying. The area of fiber to be analyzed by FT-IR was reduced to the lower limit of the equipment by using an aperture of approximately 20320 mm 2 . This way, the fiber cross-section could be divided in five different regions, as shown in Fig. 7. Measurements taken from several cross sections of two different fibers show a nitrile ratio of 0.5560.04 for the fiber center area. For the external areas, values varying from 0.39 to 0.58 where measured, showing regions where the nitrile consumption was reasonably higher relative to the sample center, and regions with results similar to the center area. These results suggest a variation in nitrile ratio not only in the radial direction but also across the fiber diameter, possibly as a consequence of the decrease of UV intensity accessible to the fiber along its diameter. The results suggest that the depth of penetration of UV for these PAN-based fiber precursors is |15 mm. Therefore, in a scaled-up process the precursor fiber diameter should be held below 15 mm, a value not different from that used in current commercial processes.
4.3. Evolution of properties Scanning electron microscopy (SEM) images of the fracture surfaces (after tensile testing) of the VT fibers that are displayed in Fig. 8. The as-spun fibers display a ductile response in Fig. 8a. With crosslinking and thermal stabilization, the fiber ductility decreases, as illustrated by the relatively smoother fractured surfaces of Fig. 8b and c. In contrast, the rather brittle nature of carbonized fibers is confirmed by micrographs presented in Fig. 9 for both VTand M-based carbon fibers. It is also noted that the microstructure of the PAN-based fibers is fairly featureless. This is in contrast to the radially oriented graphene-layer arrangement observed in mesophase pitch-based carbon fibers at similar (or even lower) carbonization temperatures. The lateral surfaces of the two fibers display a significant difference. The carbon fibers produced from M fibers by continuous spinning process display far fewer flaws than do carbon fibers produced from batch VTpolymer. Single filament tensile tests were performed on as-spun, M 1 , M 2 , VT 3 and VT 4 fibers. The effect of UV irradiation and thermal oxidation on the tensile properties of the M
Fig. 8. FESEM micrographs of VT fibers (a) as spun (b) UV irradiated for 2.5 h, and (c) UV irradiated and heat stabilized.
and VT fibers is displayed in Fig. 10a–d. The effect of UV irradiation on properties is similar for both types of fibers, although in absolute terms the M fibers possess better properties at all stages. M and VT fibers show a considerable reduction in strain-to-failure after UV irradiation. This reduction likely results from crosslinking of the polymer, which was observed earlier by solubility tests and enthalpy measurements. The effect of thermal oxidation is also similar for both types of fibers, although the yield strength slightly decreased for the UV irradiated fibers, and slightly increased for the thermally oxidized fibers. The decrease in
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Fig. 9. SEM micrographs of carbonized fibers obtained from (a) M 2 and (b) VT 4 .
tensile strength results primarily from the critical flaws present in these fiber samples prepared by a batch process. In contrast, the flaw-insensitive property, tensile modulus, is not very different for the two types of fibers that have undergone stabilization. Fig. 10a also clearly depicts the importance of the properties of the as-spun fibers as related to those of the final fibers. The higher molecular weight and degree of orientation in M fibers that went through a post-drawing step (whereas the VT fibers did not) result in better properties for M fibers. Also, as illustrated in micrographs of Fig. 9, the larger number and size of flaws in the VT fibers manifested into low strength and strain-to-failure. It
seems reasonable to anticipate better mechanical properties for the VT fibers if the molecular weight and orientation can be improved, and the polymer and fibers are produced by a continuous process. It is noted that fibers that were only UV-treated did not survive the carbonization step. However, fibers that received thermal oxidation treatment after UV irradiation could be successfully carbonized, i.e., thermal oxidation is necessary for fiber stabilization. Table 3 summarizes the tensile properties of the M fibers that were stabilized using the dual UV-thermal treatment and subsequently carbonized (M a to M d ). The results indicate that the introduction of a UV irradiation step does not decrease the final
Table 3 Tensile properties of the M fibers UV irradiated, heat oxidized and carbonized at 1500 8C Sample
No. tests
Diameter (mm)
Max. strength (MPa)
Modulus (GPa)
Strain-to-failure (%)
Ma Mb Mc Md M2 VT 4 M *commercial
10 9 16 18 22 18 –
9.260.3 8.860.3 8.560.3 8.660.2 7.160.3 15.760.7 7
10306260 6746250 6906230 10206320 12636309 334673 4410–4900
188626 136617 143628 206630 181620 5866 235–295
1.060.4 1.060.3 1.160.3 1.160.2 0.760.2 0.660.1 1.5–2.0
* Range of values from the technical data sheet of Mitsubishi Rayon, for a standard PAN-based carbon fiber.
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Fig. 10. The effect of UV irradiation, thermal oxidation, and carbonization on the tensile properties of M and VT fibers.
properties of the carbon fibers obtained. Comparison of the mechanical properties of M a, M d and M 2 shows that this conclusion is valid for fibers stabilized at constant load as well as constant length.
Acknowledgements The authors gratefully acknowledge the financial support from the Department of Energy, through grant No. 4500011036. This work made use of ERC Shared Facilities supported by the National Science Foundation under Award No. EEC-9731680.
5. Conclusions This study establishes the feasibility of producing carbon fibers from melt-spinnable polyacrylonitrile copolymers. DSC and solubility tests performed on PANbased wet-spun fibers and on experimental melt-spun fibers showed that UV irradiation effectively crosslinked the copolymers to an extent that enabled subsequent thermal oxidation. The experimental fibers could also be successfully carbonized. Although the carbon fibers exhibited low mechanical properties, there is a potential to substantially improve these properties by optimizing polymer structure (copolymer composition and molecular weight) and melt spinning conditions to form smaller diameter precursor fibers with higher degrees of molecular orientation.
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