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Melt processable polyacrylonitrile copolymer precursors for carbon fibers: Rheological, thermal, and mechanical properties
Accepted Manuscript Title: Melt Processable Polyacrylonitrile Copolymer Precursors for Carbon Fibers: Rheological, Thermal, and Mechanical Properties Authors: Jae Hyeok Lee, Jeong-Un Jin, Sejoon Park, Dalsu Choi, Nam-Ho You, Yongsik Chung, Bon-Cheol Ku, Hyeonuk Yeo PII: DOI: Reference:
S1226-086X(18)30835-9 https://doi.org/10.1016/j.jiec.2018.11.012 JIEC 4249
To appear in: Received date: Revised date: Accepted date:
28 September 2018 1 November 2018 8 November 2018
Please cite this article as: Jae Hyeok Lee, Jeong-Un Jin, Sejoon Park, Dalsu Choi, Nam-Ho You, Yongsik Chung, Bon-Cheol Ku, Hyeonuk Yeo, Melt Processable Polyacrylonitrile Copolymer Precursors for Carbon Fibers: Rheological, Thermal, and Mechanical Properties, Journal of Industrial and Engineering Chemistry https://doi.org/10.1016/j.jiec.2018.11.012 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.
Melt Processable Polyacrylonitrile Copolymer Precursors for Carbon Fibers: Rheological, Thermal, and Mechanical Properties Jae Hyeok Lee,a,b Jeong-Un Jin,a Sejoon Park,a Dalsu Choi,a Nam-Ho You,a Yongsik Chung,b Bon-
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Cheol Ku,a,* Hyeonuk Yeoc,*
Carbon Composite Materials Research Center, Institute of Advanced Composites Materials, Korea
Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju, 561-
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b
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Institute of Science and Technology, Wanju 55324, Republic of Korea
756, Republic of Korea
Department of Chemistry Education, Kyungpook National University, Daegu 41566, Republic of
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Korea
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Highlights
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*Email: [email protected], [email protected]
Poly(acrylonitrile-co-methylacrylate) precursors for melt processible carbon fibers were
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synthesized by emulsion polymerization.
Thermal and rheological analysis confirmed flow characteristics suitable for melt-spinning. The precursor fibers were successfully stabilized by thermal treatment assisted by electronbeam irradiation without any re-melting and fusion. The mechanical properties of carbon fiber from the melt processable PAN precursors were 1.37 GPa in tensile strength and 110 GPa in modulus, respectively.
ABSTRACT Polyacrylonitrile (PAN) copolymers containing varying amounts of methyl acrylate (MA), P(AN-coMA), were synthesized as a melt-spinnable precursor of carbon fibers. The rheological properties of P(AN-co-MA) with MA content of 15 mol% at 190 °C proved to be suitable for melt-spinning and the PAN fiber was spun from an extruder. In order to prevent remelting and fusion of the fibers in the stabilization process, electron-beam irradiation of over 1500 kGy was used and the melt-spun PAN
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fibers were successfully converted to stabilized PAN fibers by thermal treatment up to 250 °C. Finally, carbon fibers (CFs) were produced by pyrolysis of the stabilized PAN fibers. The mechanical
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properties of the resulting-CFs were evaluated; the tensile strength, tensile modulus, and elongation at break were 1.37±0.2 GPa, 110±11.1 GPa, and 1.27±0.28%, respectively. These results suggest the
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possibility of utilizing melt-spinning as a cost-efficient method for fabrication of carbon fibers.
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KEYWORDS: Carbon fiber; Polyacrylonitrile; Melt spinning; Stabilization; Electron Beam;
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Mechanical properties.
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1. INTRODUCTION
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Carbon fibers (CF) are promising materials for fabricating light-weight composites on an industrial scale because of their outstanding functional and mechanical properties such as high electrical
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conductivity, low thermal expansion coefficient, high specific strength, and stiffness [1–3]. Owing to
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these desirable properties, CFs have been widely applied in the aerospace, automobile, sports, and energy industries, including wind turbines, fuel cells, and for gas storage. In particular, CFs have attracted significant attention from the automobile industry, presumably because of the need for energy
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conservation and other environmental concerns. Carbon fiber-reinforced plastics (CFRP) are being considered as the core materials for next-generation automotive structures to reduce CO2 emissions and enhance fuel efficiency [4,5]. However, the production cost of CFs is still high to be fully adopted by the automotive industry, and lowering the costs is a big issue for both academic and industrial researchers [6,7]. Many efforts have been made to reduce the manufacturing cost of CFs. For example, polymeric materials such as polyacrylonitrile (PAN) [8,9], rayon [10], lyocell [11], lignin [12], and polyethylene [13–16] have been utilized as precursors for the low-cost fabrication of CFs.
Unfortunately, CFs derived from precursor fibers other than PAN derivatives show significantly poorer mechanical characteristics. Thus, it is important to improve the efficiency of the fiber spinning of PAN derivatives to obtain inexpensive CFs [17]. Generally, PAN fibers for CF manufacturing are spun by the solution process because they decompose before melting owing to a high acrylonitrile (AN) content. In addition, owing to the high polarity of the AN component, solution spinning requires the use of expensive polar solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF),
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and N-methyl-2-pyrrolidone (NMP), which is a major reason for the high cost of PAN fibers [18]. As a solution to this problem, the melt-spinning process has been developed. This method is very
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attractive for obtaining low-cost CFs because it has a high production speed and does not require any solvents [19−22]. Although PAN is a melt-processable semi-crystalline polymer, it is difficult to apply melt-processing because of the high melting temperature of PAN and the decomposable cross-linking
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and cyclization reaction of the AN unit during the thermo-oxidative stabilization process that is
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essential for CF production. In addition, since the fibers fabricated by the melt process melt again in
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the thermal treatment for stabilization, CF fabrication using melt-spun fibers is exceedingly
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challenging.
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There are two main strategies for melt-processing of PAN-based polymers: introducing a soft component such as methyl methacrylate (MMA) [19], methyl acrylate (MA) [20–22], and vinyl
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imidazole (VIM) [23] for changing the polymer compositions to a lower melting temperature (Tm) and glass transition temperature (Tg) or incorporating plasticizers such as ionic liquids [24], water [25],
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and propylene carbonate [26]. Ultimately, the key point is to prevent the remelting and fusion of the melt-spun fiber, which has been attempted by UV irradiation [20] or using very sophisticated
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stabilization processes over a long time [27], but the details of the CFs produced therefrom have not been elaborated. In recent years, we have reported on our efforts to lower the stabilization temperature and reduce the stabilization time of the PAN precursor along with elaborate mechanism studies using chemically functionalized carbon nanotubes [28], graphene oxide [29], or with the assistance of an electron beam (E-beam) [30,31], and plasma irradiation [8]. As a part of our continuing efforts, in this study, we have reported the thermal and rheological properties of a PAN copolymer with MA as a soft
component, their spinning process, stabilization process by E-beam irradiation and thermal treatment, carbonization process, and mechanical properties of the resulting-CF in detail.
2. EXPERIMENTAL SECTION Materials. The monomers, AN and MA, were purchased from Aldrich (USA), and used after distillation. Ammonium persulfate (APS) as a radical initiator and dioctyl sulfosuccinate sodium salt
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as an emulsifier were also obtained from Aldrich (USA), and used after recrystallization. In addition, n-dodecyl mercaptan was purchased from TCI (Japan) as a chain transfer agent. Other commercially
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available reagents and solvents were used as received.
Characterization. 1H NMR spectra were measured by a 600 MHz Premium COMPACT NMR
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spectrometer (Agilent, USA) with tetramethylsilane (TMS) as an internal standard and DMSO-d6 as a
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solvent. The composition of the synthesized polymers was investigated by interpreting their 1H NMR
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spectra. The molecular weights of the synthesized polymers were determined by size-exclusion
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chromatography (SEC) based on polymethylmethacrylate(PMMA) standards using a high temperature gel permeation chromatograph (HT-GPC) PL-220 (Agilent, USA) at 80 °C in dimethylformamide
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with 0.05 wt% LiBr as an eluent. Glass transition temperature (Tg), melting temperature (Tm), and
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stabilization reaction were recorded by a differential scanning calorimetry (DSC) Auto-Q20 (TA Instruments, USA) at an appropriate heating rate. Thermogravimetric analysis (TGA) of the polymers
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was carried out with Q50 (TA Instruments, USA) under N2 flow. The rheological properties of the melted states were investigated by a rheometer DHR-3 (TA Instruments, USA) using pelletized specimens. The synthesized polymers were formed into films using a heating press (Ocean Science,
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Korea) and fibers using the extruder Noztek Pro (Noztek, UK). The density was calculated by placing the sample in an automatic density gradient column (Ray-Ran Test Equipment Ltd, UK) until there was no further change in position. The column was maintained at 25 °C with constant temperature water. The thermal stabilization reaction of the polymers was investigated by Fourier-transform infrared spectroscopy (FT-IR) (Nicolet iS 10, Thermo Scientific, USA). An electron-beam of 1 MeV energy and 1 mA current was used to irradiate the samples (EB Tech Co., Korea) and the electron
doses were controlled by irradiation times. The mechanical properties of the PAN fibers and CFs were recorded by an automatic fiber testing machine FAVIMAT+ (Textechno, Germany). The testing conditions were as follows: gauge length 25.4 mm, load cell power 210 cN, and speed 5 mm/min, and the results were averaged at least 15 times. The fractured surface image of the CF was observed by field emission scanning electron microscopy (FE-SEM, Helios, FEI, USA). Synthesis of polymers. The PAN copolymers with various contents of MA, P(AN-co-MA), were
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synthesized by classical emulsion polymerization. The typical method for the synthesis of the binary PAN copolymer with 15 mol% MA content is described below. For other compositions, only the
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content of MA was changed. First, distilled water (525 mL) and dioctyl sulfosuccinate sodium salt (18.2 g, 40.9 mmol) as an emulsifier were poured into a 2 L jacketed reaction vessel with a
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thermocouple and stirred until a homogeneous mixture was formed. After the mixture was degassed
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by bubbling dry argon gas, 10% of the total amounts of the monomers, AN (159.0 g, 3.00 mol and
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MA (45.5 g, 0.529 mol), were charged into the reaction vessel. The remaining amount of the
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monomers was placed in a dropping funnel and these were added dropwise to the reaction mixture in 2 h. Simultaneously, an aqueous solution of APS (1.41 g, 6.18 mmol) as a radical initiator and n-
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dodecyl mercaptan (4.54 g, 22.4 mmol) as a chain transfer agent were also added dropwise for 2 h. In the meantime, the solution was kept at 45 °C and continuously stirred with a mechanical stirrer. The
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polymerization reaction was continued for 3 h and the reaction was terminated by the addition of 2 L
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of distilled water. The obtained precipitates were filtered and washed several times with excess methanol to remove the unreacted monomers. The residual solvents were removed in a vacuum oven for 2 days, which produced the desired copolymers as a white powder in 90–95% yield. The
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compositions and molecular weights of the copolymers were determined by 1H NMR analysis and GPC measurement, respectively. Melt-spinning process. The P(AN-co-MA) copolymers with 15 mol% MA content (as calculated by NMR analysis) were used for melt-spinning. Polymers with this composition showed melting points in the range of ~180–190 °C by DSC measurement and visual observation. In addition, the rheological analysis confirmed that the viscosity remained constant at 190 °C without any viscosity change caused
by the PAN stabilization reaction, which ensured the spinnability of the P(AN-co-MA). Isothermal TGA analysis showed that no reaction, including the stabilization reaction, occurred at this same temperature. For injection molding, the polymers were uniformly ground by a mechanical mixer and then dried in a vacuum oven. Next, melt spinning was carried out using a universal extruder Noztek Pro (Noztek, UK) with a 0.2 mm injection nozzle for spinning. After the machine heated up to 190 °C, the powders (2 g) were put into the feeder and the screw was rotated at 60 rpm to melt and mix the
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polymer. During spinning, the nozzle was maintained above 185 °C and the extruded PAN fibers were collected using a winder with a diameter of 20 cm at 250 rpm. The melt-spun fibers under these
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conditions could be obtained continuously without breakage and the spinning rate was approximately 157 m/min. The obtained fibers were completely transparent with a diameter of ~15–20 μm depending
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on the winding speed.
Stabilization and carbonization process. Prior to the heat treatment, the melt-spun PAN fibers were
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pretreated with an E-beam to prevent remelting and fusion between the fibers. The E-beam was
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irradiated by placing the sample attached to the aluminum plate on the beam line to which the cooling water line was connected to prevent heating by electron beam. In addition, the E-beam equipment of
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1 MeV energy and 1 mA current was operated by the facility personnel (EB Tech Co., Korea). In order to make it convenient to understand visually, the stabilization reaction assisted by E-beam irradiation
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was first studied for the 20-μm-thick films, and it was concluded that the fusion during thermal treatment could be efficiently frustrated by E-beam irradiation at a dose of 1500 kGy or more. Then,
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after irradiating with an E-beam of 1500 kGy, stabilized fibers were prepared by a stepwise heat treatment under air using a conventional high temperature oven. The thermal treatment involved
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heating from room temperature to 160 °C at a heating rate of 5 °C/min, holding at 160 °C for 15 min, heating to 190 °C at a heating rate of 1 °C/min, holding at 190 °C for 15 min, heating to 220 °C at a heating rate of 1 °C/min, holding at 220 °C for 15 min, heating to 250 °C at a heating rate of 1 °C/min, and holding at 250 °C for 45 min. During heat treatment, the fibers were fixed to a metal frame to prevent shrinkage. After heating up to 250 °C, the density of the stabilized fibers changed from 1.18 g/cm3 for the melt-spun fiber to 1.31 g/cm3, which was judged to have stabilized successfully. The stabilized fibers were then pyrolyzed using a tube-type furnace (Korea Furnace Development Co.,
Korea) at 1200 °C at a heating rate of 5 °C/min in a nitrogen atmosphere, successfully resulting in CFs. 3. RESULTS AND DISCUSSION In essence, PAN is a semicrystalline polymer that is has a melting point when heated rapidly, but it is very difficult to utilize it in its melt state since PAN starts to react with itself prior to melting, which
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is attribute to a unique intra- and intermolecular cross-linking [32,33]. Therefore, it is necessary to introduce a soft component in order to obtain the melting property before the cross-linking reaction occurs. A promising option is addition of the MA component, which is an excellent choice for
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copolymerization with AN to form a random copolymer efficiently because of its similar reactivity to that of AN [34]. There have been a few attempts to fabricate CFs through melt-spinning of AN/MA
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copolymers [19–22], but most of them have focused only on polymer synthesis and the investigation
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of their thermal properties, remaining unclear for the resulted CFs.
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P(AN-co-MA) copolymers with different MA contents were synthesized by conventional emulsion
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polymerization. The initial feed of monomer MA was varied from 6 to 18 mol% and also included
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pure PAN, as described in Table 1. Ultimately, polymerization was performed using a total of 200 g of monomers, and the yields after washing and drying were in the range of 90–95%. The composition of
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the obtained polymers was determined by 1H NMR analysis (Figure S1). The peaks of the main chain could not be distinguished between MA and AN, but the methyl groups (−CH3) of MA observed at
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3.69 ppm as a triplet were clearly identifiable, so the MA content of the copolymer could be calculated by integration of the peak, as listed in Table 1. As the reactivity of MA is somewhat lower than that of AN, the actual MA content of the polymer was slightly lower compared to the feeding ratio [34]. In
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addition, the molecular weights and distribution of the polymers were calculated from HT-GPC analysis (Figure S2). It is known that an excessively high molecular weight increases the melt viscosity. Therefore, it was confirmed that the polymers having a number average molecular weight (Mn) in the range of 20,000–40,000 g/mol needed to be synthesized in order to control the melt viscosity adequately for melt-spinning to occur as intended. In addition, the polydispersity indices (PDIs) lay
between 2 and 3, which are expected values from typical emulsion polymerization, indicating that the polymerization proceeded well without side reactions such as crosslinking. Thermal properties of the synthesized polymers were investigated by DSC. For observation of Tg and Tm, DSC measurements were carried out at heating rates of 10 and 100 °C/min, respectively. In particular, Tm analyses had to be conducted at a rapid temperature rise because of the stabilization reactions occurring at ~260 °C. The representative values of thermal analyses, monomer composition,
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and molecular weights are listed in Table 1. The values of Tg and Tm were plotted (Figure 1) as a function of the MA content. Considering that the thermal properties vary depending on the molecular
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weight, configuration, conformation, and monomer sequence, the obtained results showed a regular trend, i.e., Tg and Tm decreased uniformly with the increasing MA content, indicating that all runs
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produced polymers of similar structure. In particular, the linear dependence of the thermal properties
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on the MA content was evident from the fact that the Tg and Tm values obtained for pure PAN on
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extrapolating to 0 mol% MA content were similar to the known values (95 and 322 °C, respectively).
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The ideal content of MA (melting point < 200 °C) for which the stabilization reaction did not initiate, was found to be ~15 mol% or more. Therefore, it was concluded that the melt-process could be
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performed at ~200 °C when the actual MA content was >15 mol%. In all further experiments, the meltprocessable polymers with an MA content of about 15 mol% were used.
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The flow characteristics of the synthesized polymer for the melting process were investigated by
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rheological analysis (Figure 2 and Figure S3). As the temperature increased and the shear rate increased, it was observed that the complex viscosity was clearly lowered. In particular, the absolute value of the complex viscosity at 190 °C and 0.1% strain, which was an indicator of the melt
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processability [35], was found to be about 5,000 and 3300 Pa∙s at 0.1 and 10 rad/s, respectively. These values were lower than those of the PAN-based polymers for melt-spinning reported in previous studies [34], which indicated sufficient feasibility of the polymers described herein for melt-spinning. In addition, in a time sweep study for 30 min, the complex viscosity increased moderately from ~3300 to 3850, but no rapid change was observed. At temperatures above 200 °C, gelation occurred because of cyclization and the cross-linking reaction, resulting in a significant increase in viscosity. However,
this phenomenon seemed to be sufficiently inhibited at 190 °C. This was also observed by isothermal TGA measurement (Figure S4). As a result of holding the temperature at 190 °C for 30 min, only 0.06% of the mass was degraded, suggesting that either there was no reaction or it occurred to an insignificant extent. Based on the insights into the melt capability, the polymer was melt-spun using a universal extruder with a 0.2 mm injection nozzle. The melt spinning was optimized under the abovementioned
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conditions and transparent fibers with a diameter of 15–20 μm were obtained (Figure S6). The mechanical properties of the obtained pristine fibers were evaluated and the tensile strength, tensile
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modulus, and elongation at break were 0.26±0.03 GPa, 6.76±1.78 GPa, and 17.6±0.84%, respectively (Table 2).
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Since the melt-spun fibers would remelt at 200 °C, a stabilization process was attempted using E-beam
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irradiation. E-beam irradiation of PAN has been reported to produce radical species in the polymer
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main chain, causing a reduction in the activation energy of the cyclization and crosslinking reactions
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[31]. Therefore, the E-beam irradiated PAN could be stabilized at a lower temperature than 200 °C compared to those without E-beam assistance. The aim of the E-beam treatment was to clearly lower
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the stabilization temperature to below the Tm. For making the monitoring of the experimental process more convenient by visual observation and automated, the film states of the samples were used to
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optimize the E-beam irradiation process. To ensure fiber-like conditions, 20-μm-thick films were
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fabricated by the hot press molding method. The experiment proceeded as follows: after the films were irradiated with electron doses of 500, 1500, and 2500 kGy, and followed up by heat treatment under various conditions, the changes in the chemical structures were analyzed by FT-IR measurements. The
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thermal treatments were performed by increasing the temperature up to 250 °C at a heating rate of 3 °C/min under air atmosphere and then varying the time at 250 °C. The retention times at 250 °C were controlled to 0, 5, 15, 30, 45, 60, and 75 min. In order to observe the fusion between the films during thermal treatment, three sheets of the films (2020 mm) were stacked vertically and heated. The FT-IR absorption spectra for samples irradiated with different E-beam doses are shown in Figure 3. The sample without E-beam irradiation showed weak peaks corresponding to νO−H and νN−H in the
range of 3100−3600 cm−1 and very weak peaks of νC=C, νC=N, and δN−H, i.e., the components that were converted by the cyclization reaction, at 1590 cm−1. This is because the cyclization rarely occurred at the end of heating up to 250 °C. In addition, the representative peaks found in a typical P(AN-co-MA) copolymer were also observed, i.e., νC−H of CH2 and CH3 groups at ~2934 cm−1, νC≡N of the AN component at 2242 cm−1, νC=O of the MA component at 1730 cm−1 and δC−H of CH2 and CH3 groups at 1451 cm−1. A strong peak was observed at 1668 cm−1 and assigned to the –HC=N−N=CH− group,
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which indicated the occurrence of intermolecular crosslinking [36]. Therefore, it was concluded that the crosslinking reaction occurred before the cyclization reaction. Similar changes in the chemical
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structures were also observed in the samples irradiated with an E-beam. However, there was a clear difference in the reaction rate of the cyclization. As the dose of E-beam was increased, the intensity of the peaks indicating the cyclization reaction such as νO−H and νN−H also increased. Especially, in the
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samples irradiated with 1500 and 2500 kGy, the difference was clear, which provided unambiguous
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evidence that E-beam irradiation promoted the stabilization reaction. Furthermore, for a longer holding
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time at 250 °C, the intensity of the peaks in the range of 3100−3600 and 1590 cm−1, which was
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evidence of cyclization, was found to become stronger, and that of the peaks at 2934, 2242, and 1730
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cm−1, originating from the functional groups such as aliphatic C−H, C≡N, and C=O converted to ring structures, became weaker. It was also found that the reaction rate increased with the increasing E-
kGy.
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beam dose, and there was no significant difference between the samples irradiated by 1500 and 2500
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To easily interpret the results of the FT-IR studies, the degrees of stabilization reaction were quantified by calculating the values of the extent of reaction (EOR) at each condition (Table S1). The values of
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EOR were calculated by the following equation [37]. Extent of reaction (EOR) (%) =
𝐴𝐶 𝐴𝐶 +𝑓∙𝐴𝑁
× 100 (%)
where AC refers to the absorption peak areas at 1590 cm−1 derived from cyclized ring structures, AN indicates the absorption peak areas at 2240 cm−1 originating from the nitrile groups as an internal standard, and f is a constant with a value of 0.29. These results were plotted against holding times and doses (Figure 4). From the plot in Figure 4(a) against holding times at 250 °C, it was evident that as
the time increased, the EOR also increased linearly. In addition, the cyclization reaction progressed 30–50% by simply heating up to 250 °C at the heating rate of 3 °C/min. From the plot against irradiated doses (Figure 4(b)), it could be confirmed that the reaction in the samples irradiated by the E-beam was almost complete in the retention time of 15 min regardless of the dose of the E-beam, whereas the sample without E-beam treatment did not successfully react even after 75 min. Thus, it was evident that E-beam treatment promoted the stabilization reaction of PAN. At first glance, only the presence
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or absence of E-beam pretreatment seemed to be related to the reaction rate, but the amount of irradiation dose actually had a substantial impact on the stabilization process. In the process of
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stabilizing melt-spun fibers, fusion between the bundles must be absolutely prevented. Therefore, we tried to stabilize P(AN-co-MA) in the film state under conditions that were similar to those used in the fiber form. In the samples irradiated by 1500 and 2500 kGy, there was no adhesion between the films,
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which meant that E-beam irradiation of 1500 kGy or more was required to efficiently prevent the
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fusion phenomenon. Therefore, we stabilized the melt-spun fibers through E-beam pretreatment using
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a dose of 1500 kGy.
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The obtained melt-spun fibers were pretreated by E-beam irradiation of 1500 and 2500 kGy and their
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stabilization reaction was investigated by DSC measurements (Figure 5 and Table 3). The fiber without E-beam treatment underwent the cyclization reaction at 264 °C, which is the same as observed
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in the powder state, and showed the maximum calorific value at 308 °C with the reaction heat of 639.6 J/g. On the other hand, the samples treated by E-beam exhibited markedly different exotherms. The
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large exothermic peaks observed near 300 °C in the neat fiber moved significantly toward lower temperature in the fibers with E-beam treatment. In addition, the values of reaction heat were also
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greatly reduced, indicating that because of E-beam irradiation, the stabilization reaction occurred at lower energy states with a different reaction mechanism. In the fibers irradiated by the 1500 kGy Ebeam, the stabilization reaction started at 203 °C and the exothermic peak maximum was at 288 °C. In the sample irradiated by 2500 kGy, the reaction initiated at 203 °C and the peak maximum was 281 °C. The heats of the reaction for the two samples were 335.9 and 311.3 J/g, respectively. There were no significant differences in the overall reaction behavior.
Thus, two types of E-beam-treated samples were stabilized with the aforementioned heat treatment profiles. In addition, to inhibit the shrinking of the bundles, the fibers were fastened to the SUS frame during thermal treatment. The stabilization reaction was terminated after being kept at 250 °C for 45 min because there was no change in the density even when it was maintained at 250 °C for more than 45 min. The density of the pristine melt-spun fibers was 1.18 g/cm3 and that after maintaining thermal stabilization was 1.30–1.31 g/cm3 regardless of the E-beam doses, which are typical values for
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stabilized PAN fibers. Therefore, E-beam irradiation of 1500 kGy was sufficient to prevent the fusion between fiber bundles and to produce stabilized PAN fibers through heat treatment.
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Finally, the stabilized PAN fibers were carbonized using a tube-type furnace under nitrogen atmosphere. The fibers were fixed to a carbon sheet by attaching a carbon tape and the carbonization of the fibers was carried out at a rate of 5 °C/min up to 1200 °C. This process successfully produced
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CFs using melt-spun fibers. The diameter of the obtained CFs was about 10–15 μm, and spherical
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fibers of the same shape as the precursor were obtained. The density of the CF was 1.70 g/cm3 which
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was slightly lower than that of commercial PAN-based CF. The fractured cross section and
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longitudinal surface of the resulting CF were observed by SEM (Figure 6). In the cross section,
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completely spherical shape was observed without any particular defect. In addition, though some defects below the micrometer order on the surface was observed, the entire surface was confirmed to
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be smooth, indicating that the difference in spinning method had no significant effect on the morphology of the resulting CF. The carbonization yield was ~40 wt%, as confirmed by TGA (Figure
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S5). The mechanical properties of the resulting CFs were evaluated; the tensile strength, tensile modulus, and elongation at break were 1.37±0.20 GPa, 110±11.1 GPa, and 1.27±0.28%, respectively
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(Table 2). A typical stress-strain curve is shown in Figure S7. These values were the highest among the reported CFs produced by melt-processing [12]. Further studies are aimed at developing a better optimized and generalized protocol, although the results described in this study show sufficient potential for inexpensive CF manufacturing through melt-spinning.
4. CONCLUSION
In this work, comprehensive studies were performed for fabricating CFs using melt-spun P(AN-coMA). First, PAN polymers containing the MA co-monomer as a soft component in different amounts were synthesized and their thermal properties were investigated in detail. As a result, when the MA content was over 15 mol%, the resulting copolymers were melt-processable above 190 °C without crosslinking and cyclization reactions, as confirmed by isothermal TGA studies. In addition, it was also confirmed by rheological analysis at 190 °C that the polymers exhibited flow characteristics
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suitable for melt-spinning. Then, melt-spinning was carried out using a conventional extruder and transparent fibers having a density of 1.18 g/cm3 and a diameter of 15–20 μm were obtained. It was
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found that E-beam pretreatment above 1500 kGy could successfully produce stabilized fibers without fusion phenomenon during thermal stabilization. After optimization of the stabilization reaction, the melt-spun fibers formed through thermal treatment up to 250 °C and assisted by E-beam irradiation
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were successfully converted to the stabilized PAN fibers without remelting and fusion. Lastly, CFs
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were prepared by carbonizing the stabilized fibers. The tensile strength, tensile modulus, and
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elongation at break of the resulting CFs were 1.37±0.2 GPa, 110±11.1 GPa, and 1.27±0.28%,
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respectively. Further research is currently underway to optimize the manufacturing process for
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improving the aforementioned mechanical properties of the product.
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5. ACKNOWLEDGEMENTS
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This work was supported by a grant from the Korea Institute of Science and Technology institutional program, and the National Research Foundation of Korea (NRF) grant funded by the Korea
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government (Ministry of Science and ICT, No. NRF-2017R1C1B5076344).
REFERENCES [1] Y. Liu, S. Kumar, Recent progress in fabrication, structure, and properties of carbon fibers, Polym. Rev. 52 (3) (2012) 234-258. [2] B. A. Newcomb, Processing, structure, and properties of carbon fibers, Composites Part A: Applied Science and Manufacturing, 91 (2016), 262-282. [3] S.-J. Park, B.-J Kim, Carbon Fibers and Their Composites, Springer series in Materials Sciences,
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(2014), 275-317.
[4] T Ishikawa, K Amaoka, Y. Masubuchi, T. Yamamoto, A. Yamanaka, M. Arai, J. Takahashi,
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Overview of automotive structural composites technology developments in Japan, Composites Science and Technology, 155 (2018), 221-246.
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Composites Science and Technology 26(4) (1986) 251−281.
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[5] P. Beardmore, C.F. Johnson, The potential for composites in structural automotive applications,
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[6] N.V. Salim, S. Blight, C. Creighton, S. Nunna, S. Atkiss, J.M. Razal, The role of tension and temperature for efficient carbonization of polyacrylonitrile fibers: toward low cost carbon fibers,
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Industrial & Engineering Chemistry Research 57(12) (2018) 4268−4276.
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[7] U. Arnold, A. De Palmenaer, T. Bruck, K. Kuse, Energy-efficient carbon fiber production with concentrated solar power: process design and techno-economic analysis, Industrial & Engineering
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Chemistry Research 57(23) (2018) 7934−7945. [8] S.-W. Lee, H.-Y. Lee, S.-Y. Jang, S. Jo, H.-S. Lee, W.-H. Choe, S. Lee, Efficient preparation of
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carbon fibers using plasma assisted stabilization, Carbon 55 (2013), 361-365. [9] S.-Y. Kim, S.Y. Kim, S. Lee, S. Jo, Y.-H. Im, H.-S. Lee, Microwave plasma carbonization for the
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fabrication of polyacrylonitrile-based carbon fiber, Polymer 56(15) (2015), 590-595. [10] R. Bacon, A. Silvaggi, Electron microscope study of the microstructure of carbon and graphite fibers from a rayon precursor, Carbon 9(3) (1971) 321−325. [11] S. Peng, H. Shao, X. Hu, Lyocell fibers as the precursor of carbon fibers, Journal of Applied Polymer Science 90(7) (2003) 1941-1947. [12] D.A. Baker, T.G. Rials, Recent advances in low‐ cost carbon fiber manufacture from lignin, Journal of Applied Polymer Science 130(2) (2013) 713−728.
[13] M.A. Hunt, T. Saito, R.H. Brown, A.S. Kumbhar, A.K. Naskar, Patterned functional carbon fibers from polyethylene, Advanced Materials 24(18) (2012) 2386−2389. [14] M.J. Behr, B.G. Landes, B.E. Barton, M.T. Bernius, G.F. Billovits, E.J. Hukkanen, J.T. Patton, W. Wang, C. Wood, D.T. Keane, Structure-property model for polyethylene-derived carbon fiber, Carbon 107 (2016) 525-535. [15] J.W Kim, J.S Lee, Preparation of carbon fibers from linear low density
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polyethylene, Carbon 94 (2015), 524-530. [16] D. Zhang, Q. Sun, Structure and properties development during the conversion of polyethylene
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precursors to carbon fibers, J. Appl. Polym. Sci., 62 (1996), 367-373.
[17] N. Yusof, A. Ismail, Post spinning and pyrolysis processes of polyacrylonitrile (PAN)-based carbon fiber and activated carbon fiber: A review, Journal of Analytical and Applied Pyrolysis 93
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(2012) 1−13.
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[18] M. MInus, S. Kumar, The processing, properties, and structure of carbon fibers, The Journal of
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The Minerals, Metals & Materials Society 57(2) (2005) 52-58.
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[19] M. Qu, F. Nilsson, Y. Qin, G. Yang, Y. Pan, X. Liu, G.H. Rodriguez, J. Chen, C. Zhang, D.W.
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Schubert, Electrical conductivity and mechanical properties of melt-spun ternary composites comprising PMMA, carbon fibers and carbon black, Composites Science and Technology 150 (2017)
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24−31.
[20] A.K. Naskar, R.A. Walker, S. Proulx, D.D. Edie, A.A. Ogale, UV assisted stabilization routes for
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carbon fiber precursors produced from melt-processible polyacrylonitrile terpolymer, Carbon 43(5) (2005) 1065−1072. MA
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[21] T. Mukundan, V. Bhanu, K. Wiles, H. Johnson, M. Bortner, D. Baird, A. Naskar, A. Ogale, D. Edie, J. McGrath, A photocrosslinkable melt processible acrylonitrile terpolymer as carbon fiber precursor, Polymer 47(11) (2006) 4163−4171. MA [22] G. Miller, J. Yu, R. Joseph, S. Choudhury, S. Mecham, D. Baird, M. Bortner, R. Norris, F. Paulauskas, J. Riffle, Melt-spinnable polyacrylonitrile copolymer precursors for carbon fibers, Polymer 126 (2017) 87−95. [23] A. Lowe, W. Deng, D.W. Smith Jr, K.J. Balkus Jr, Acrylonitrile-based nitric oxide releasing melt-
spun fibers for enhanced wound healing, Macromolecules 45(15) (2012) 5894−5900. [24] Y. Tian, K. Han, W. Zhang, J. Zhang, H. Rong, D. Wang, B. Yan, S. Liu, M. Yu, Influence of residence time on the structure of polyacrylonitrile in ionic liquids during melt spinning process, Materials Letters 92 (2013) 119−121. [25] D. Grove, P. Desai, A. Abhiraman, Exploratory experiments in the conversion of plasticized melt spun PAN-based precursors to carbon fibers, Carbon 26(3) (1988) 403−411.
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[26] S.K. Atureliya, Z. Bashir, Continuous plasticized melt-extrusion of polyacrylonitrile homopolymer, Polymer 34(24) (1993) 5116−5122.
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[27] B.L. Batchelor, S.F. Mahmood, M. Jung, H. Shin, O.V. Kulikov, W. Voit, B.M. Novak, D.J. Yang, Plasticization for melt viscosity reduction of melt processable carbon fiber precursor, Carbon 98 (2016) 681−688.
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[28] O.-K. Park, S. Lee, H.-I. Joh, J.K. Kim, P.-H. Kang, J.H. Lee, B.-C. Ku, Effect of functional
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groups of carbon nanotubes on the cyclization mechanism of polyacrylonitrile (PAN), Polymer 53(11)
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(2012) 2168-2174.
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[29] S. Lee, Y.J. Kim, D.H. Kim, B.C. Ku, H.I. Joh, Synthesis and properties of thermally reduced
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graphene oxide/polyacrylonitrile composites, Journal of Physics and Chemistry of Solids 73 (6), 741743.
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[30] H.K. Shin, M. Park, P.H. Kang, H.-S. Choi, S.-J. Park, Preparation and characterization of polyacrylonitrile-based carbon fibers produced by electron beam irradiation pretreatment, Journal of
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Industrial and Engineering Chemistry 20(5) (2014) 3789−3792. [31] S. Park, S.H. Yoo, H.R. Kang, S.M. Jo, H.-I. Joh, S. Lee, Comprehensive stabilization mechanism of electron-beam irradiated polyacrylonitrile fibers to shorten the conventional thermal treatment,
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Scientific Reports 6 (2016) 27330. [32] S. Lee, J. Kim, B.C. Ku, J. Kim, H.I. Joh, Structural evolution of polyacrylonitrile fibers in stabilization and carbonization, Advances in Chemical Engineering and Science 2 (02) (2012), 275 [33] Z. Bashir, A critical review of the stabilisation of polyacrylonitrile, Carbon 29(8) (1991) 10811090 [34] V. Bhanu, P. Rangarajan, K. Wiles, M. Bortner, M. Sankarpandian, D. Godshall, T. Glass, A.
Banthia, J. Yang, G. Wilkes, Synthesis and characterization of acrylonitrile methyl acrylate statistical copolymers as melt processable carbon fiber precursors, Polymer 43(18) (2002) 4841−4850. [35] D.G. Baird, D.I. Collias, Polymer processing principles and design. MA: Butterworth-Heinemann Newton, 1995. [36] S.M. Badawy, A.M. Dessouki, Cross-linked polyacrylonitrile prepared by radiation-induced polymerization technique, The Journal of Physical Chemistry B 107(41) (2003) 11273−11279.
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[37] G. Collins, N. Thomas, G. Williams, Kinetic relationships between heat generation and nitrile
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consumption in the reaction of poly(acrylonitrile) in air at 265 °C, Carbon 26(5) (1988) 671−679.
Figures and Tables
600 Tg Tm
500 Tm = –6.07 CMA + 555.1
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Figure 1. Thermal properties (Tg and Tm) of the synthesized P(AN-co-MA) copolymers.
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Figure 2. Rheological properties of the synthesized P(AN-co-MA) with MA content of 15 mol% at 190 °C and a constant frequency of 10 rad/s as a function of time.
(b)
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Figure 3. Tracking chemical structure changes of the E-beam irradiated PAN films by FT-IR
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spectroscopy after thermal stabilization with a heating rate of 3 °C/min followed by holding at 250 °C at several electron doses: (a) 0 kGy, (b) 500 kGy, (c) 1500 kGy, and (d) 2500 kGy. The holding time
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shown on the right of each plot is for spectra obtained at 250 °C.
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Irradiated electron dose (kGy)
Figure 4. Plot of the extent of reaction (EOR) of the E-beam irradiated PAN films after thermal stabilization with a heating rate of 3 °C/min followed by holding at 250 °C against (a) holding times
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Figure 5. DSC curves of the melt-spun PAN fibers irradiated at several electron doses with a heating rate of 10 °C/min under N2 atmosphere.
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Figure 6. SEM image of the resulting carbon fiber: (a) cross section and (b) longitudinal surface.
Table 1. Polymerization results and thermal properties of P(AN-co-MA)
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Mnb MA MA Tgc Tmd Run Feeding Contenta PDIb 10−3 [K] [K] [mol%] [mol%] [g/mol] 0 0 27 2.1 378.8 558.1 1 6 6.0 25 2.1 379.2 538.3 2 9 8.3 37 2.45 360.2 489.9 3 11.6 10.0 30 2.5 357.6 482.1 4 12 11.3 22 2.59 347.1 478.8 5 12.6 12 26 3.3 375.2 486.2 6 14.1 14.3 36 2.56 350.8 462.5 7 14.6 13.3 35 2.19 352.2 471.5 8 15 14.7 39 2.8 364.3 469.6 9 15 14.6 32 2.45 360.4 465.2 10 16.4 16 38 2.66 334.4 446.3 11 16.8 13.7 15 2.74 372.9 462.8 12 17.8 15.3 36 2.26 357.2 476.6 13 17.8 17.3 23 2.8 344.2 456.3 14 18 17.6 27 2.98 348.8 461.8 15 a b Calculated by NMR analysis. Determined by size-exclusion chromatography (SEC) based on
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100 °C/min.
Obtained by DSC measurements conducted at a heating rate of
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at a heating rate of 10 °C/min.
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PMMA standards in DMF with LiBr of 0.05 M at 80 °C. c Obtained by DSC measurements performed
Table 2. Mechanical properties of melt-spun PAN fibers and resulting-carbon fibers
Sample
Tensile modulus [GPa] 6.76±1.78 110±11.1
Elongation [%] 17.6±0.84 1.27±0.28
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PAN fiber Carbon fiber
Tensile strength [GPa] 0.26±0.03 1.37±0.20
Table 3. Kinetic results of the thermal stabilization reaction determined by DSC measurements of the melt-spun PAN fibers Tonseta Tpeaktopa Ha [°C] [°C] [J/g] 264 308 639.6 0 kGy 203 288 335.9 1500 kGy 186 281 311.3 2500 kGy a Measured by DSC conducted at a heating rate of 10 °C/min under N2 atmosphere.
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S1226-086X(18)30835-9 https://doi.org/10.1016/j.jiec.2018.11.012 JIEC 4249
To appear in: Received date: Revised date: Accepted date:
28 September 2018 1 November 2018 8 November 2018
Please cite this article as: Jae Hyeok Lee, Jeong-Un Jin, Sejoon Park, Dalsu Choi, Nam-Ho You, Yongsik Chung, Bon-Cheol Ku, Hyeonuk Yeo, Melt Processable Polyacrylonitrile Copolymer Precursors for Carbon Fibers: Rheological, Thermal, and Mechanical Properties, Journal of Industrial and Engineering Chemistry https://doi.org/10.1016/j.jiec.2018.11.012 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.
Melt Processable Polyacrylonitrile Copolymer Precursors for Carbon Fibers: Rheological, Thermal, and Mechanical Properties Jae Hyeok Lee,a,b Jeong-Un Jin,a Sejoon Park,a Dalsu Choi,a Nam-Ho You,a Yongsik Chung,b Bon-
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a
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Cheol Ku,a,* Hyeonuk Yeoc,*
Carbon Composite Materials Research Center, Institute of Advanced Composites Materials, Korea
Department of Organic Materials and Fiber Engineering, Chonbuk National University, Jeonju, 561-
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b
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Institute of Science and Technology, Wanju 55324, Republic of Korea
756, Republic of Korea
Department of Chemistry Education, Kyungpook National University, Daegu 41566, Republic of
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c
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Korea
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Highlights
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*Email: [email protected], [email protected]
Poly(acrylonitrile-co-methylacrylate) precursors for melt processible carbon fibers were
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synthesized by emulsion polymerization.
Thermal and rheological analysis confirmed flow characteristics suitable for melt-spinning. The precursor fibers were successfully stabilized by thermal treatment assisted by electronbeam irradiation without any re-melting and fusion. The mechanical properties of carbon fiber from the melt processable PAN precursors were 1.37 GPa in tensile strength and 110 GPa in modulus, respectively.
ABSTRACT Polyacrylonitrile (PAN) copolymers containing varying amounts of methyl acrylate (MA), P(AN-coMA), were synthesized as a melt-spinnable precursor of carbon fibers. The rheological properties of P(AN-co-MA) with MA content of 15 mol% at 190 °C proved to be suitable for melt-spinning and the PAN fiber was spun from an extruder. In order to prevent remelting and fusion of the fibers in the stabilization process, electron-beam irradiation of over 1500 kGy was used and the melt-spun PAN
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fibers were successfully converted to stabilized PAN fibers by thermal treatment up to 250 °C. Finally, carbon fibers (CFs) were produced by pyrolysis of the stabilized PAN fibers. The mechanical
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properties of the resulting-CFs were evaluated; the tensile strength, tensile modulus, and elongation at break were 1.37±0.2 GPa, 110±11.1 GPa, and 1.27±0.28%, respectively. These results suggest the
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possibility of utilizing melt-spinning as a cost-efficient method for fabrication of carbon fibers.
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KEYWORDS: Carbon fiber; Polyacrylonitrile; Melt spinning; Stabilization; Electron Beam;
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Mechanical properties.
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1. INTRODUCTION
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Carbon fibers (CF) are promising materials for fabricating light-weight composites on an industrial scale because of their outstanding functional and mechanical properties such as high electrical
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conductivity, low thermal expansion coefficient, high specific strength, and stiffness [1–3]. Owing to
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these desirable properties, CFs have been widely applied in the aerospace, automobile, sports, and energy industries, including wind turbines, fuel cells, and for gas storage. In particular, CFs have attracted significant attention from the automobile industry, presumably because of the need for energy
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conservation and other environmental concerns. Carbon fiber-reinforced plastics (CFRP) are being considered as the core materials for next-generation automotive structures to reduce CO2 emissions and enhance fuel efficiency [4,5]. However, the production cost of CFs is still high to be fully adopted by the automotive industry, and lowering the costs is a big issue for both academic and industrial researchers [6,7]. Many efforts have been made to reduce the manufacturing cost of CFs. For example, polymeric materials such as polyacrylonitrile (PAN) [8,9], rayon [10], lyocell [11], lignin [12], and polyethylene [13–16] have been utilized as precursors for the low-cost fabrication of CFs.
Unfortunately, CFs derived from precursor fibers other than PAN derivatives show significantly poorer mechanical characteristics. Thus, it is important to improve the efficiency of the fiber spinning of PAN derivatives to obtain inexpensive CFs [17]. Generally, PAN fibers for CF manufacturing are spun by the solution process because they decompose before melting owing to a high acrylonitrile (AN) content. In addition, owing to the high polarity of the AN component, solution spinning requires the use of expensive polar solvents such as dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF),
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and N-methyl-2-pyrrolidone (NMP), which is a major reason for the high cost of PAN fibers [18]. As a solution to this problem, the melt-spinning process has been developed. This method is very
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attractive for obtaining low-cost CFs because it has a high production speed and does not require any solvents [19−22]. Although PAN is a melt-processable semi-crystalline polymer, it is difficult to apply melt-processing because of the high melting temperature of PAN and the decomposable cross-linking
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and cyclization reaction of the AN unit during the thermo-oxidative stabilization process that is
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essential for CF production. In addition, since the fibers fabricated by the melt process melt again in
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the thermal treatment for stabilization, CF fabrication using melt-spun fibers is exceedingly
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challenging.
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There are two main strategies for melt-processing of PAN-based polymers: introducing a soft component such as methyl methacrylate (MMA) [19], methyl acrylate (MA) [20–22], and vinyl
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imidazole (VIM) [23] for changing the polymer compositions to a lower melting temperature (Tm) and glass transition temperature (Tg) or incorporating plasticizers such as ionic liquids [24], water [25],
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and propylene carbonate [26]. Ultimately, the key point is to prevent the remelting and fusion of the melt-spun fiber, which has been attempted by UV irradiation [20] or using very sophisticated
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stabilization processes over a long time [27], but the details of the CFs produced therefrom have not been elaborated. In recent years, we have reported on our efforts to lower the stabilization temperature and reduce the stabilization time of the PAN precursor along with elaborate mechanism studies using chemically functionalized carbon nanotubes [28], graphene oxide [29], or with the assistance of an electron beam (E-beam) [30,31], and plasma irradiation [8]. As a part of our continuing efforts, in this study, we have reported the thermal and rheological properties of a PAN copolymer with MA as a soft
component, their spinning process, stabilization process by E-beam irradiation and thermal treatment, carbonization process, and mechanical properties of the resulting-CF in detail.
2. EXPERIMENTAL SECTION Materials. The monomers, AN and MA, were purchased from Aldrich (USA), and used after distillation. Ammonium persulfate (APS) as a radical initiator and dioctyl sulfosuccinate sodium salt
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as an emulsifier were also obtained from Aldrich (USA), and used after recrystallization. In addition, n-dodecyl mercaptan was purchased from TCI (Japan) as a chain transfer agent. Other commercially
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available reagents and solvents were used as received.
Characterization. 1H NMR spectra were measured by a 600 MHz Premium COMPACT NMR
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spectrometer (Agilent, USA) with tetramethylsilane (TMS) as an internal standard and DMSO-d6 as a
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solvent. The composition of the synthesized polymers was investigated by interpreting their 1H NMR
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spectra. The molecular weights of the synthesized polymers were determined by size-exclusion
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chromatography (SEC) based on polymethylmethacrylate(PMMA) standards using a high temperature gel permeation chromatograph (HT-GPC) PL-220 (Agilent, USA) at 80 °C in dimethylformamide
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with 0.05 wt% LiBr as an eluent. Glass transition temperature (Tg), melting temperature (Tm), and
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stabilization reaction were recorded by a differential scanning calorimetry (DSC) Auto-Q20 (TA Instruments, USA) at an appropriate heating rate. Thermogravimetric analysis (TGA) of the polymers
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was carried out with Q50 (TA Instruments, USA) under N2 flow. The rheological properties of the melted states were investigated by a rheometer DHR-3 (TA Instruments, USA) using pelletized specimens. The synthesized polymers were formed into films using a heating press (Ocean Science,
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Korea) and fibers using the extruder Noztek Pro (Noztek, UK). The density was calculated by placing the sample in an automatic density gradient column (Ray-Ran Test Equipment Ltd, UK) until there was no further change in position. The column was maintained at 25 °C with constant temperature water. The thermal stabilization reaction of the polymers was investigated by Fourier-transform infrared spectroscopy (FT-IR) (Nicolet iS 10, Thermo Scientific, USA). An electron-beam of 1 MeV energy and 1 mA current was used to irradiate the samples (EB Tech Co., Korea) and the electron
doses were controlled by irradiation times. The mechanical properties of the PAN fibers and CFs were recorded by an automatic fiber testing machine FAVIMAT+ (Textechno, Germany). The testing conditions were as follows: gauge length 25.4 mm, load cell power 210 cN, and speed 5 mm/min, and the results were averaged at least 15 times. The fractured surface image of the CF was observed by field emission scanning electron microscopy (FE-SEM, Helios, FEI, USA). Synthesis of polymers. The PAN copolymers with various contents of MA, P(AN-co-MA), were
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synthesized by classical emulsion polymerization. The typical method for the synthesis of the binary PAN copolymer with 15 mol% MA content is described below. For other compositions, only the
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content of MA was changed. First, distilled water (525 mL) and dioctyl sulfosuccinate sodium salt (18.2 g, 40.9 mmol) as an emulsifier were poured into a 2 L jacketed reaction vessel with a
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thermocouple and stirred until a homogeneous mixture was formed. After the mixture was degassed
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by bubbling dry argon gas, 10% of the total amounts of the monomers, AN (159.0 g, 3.00 mol and
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MA (45.5 g, 0.529 mol), were charged into the reaction vessel. The remaining amount of the
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monomers was placed in a dropping funnel and these were added dropwise to the reaction mixture in 2 h. Simultaneously, an aqueous solution of APS (1.41 g, 6.18 mmol) as a radical initiator and n-
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dodecyl mercaptan (4.54 g, 22.4 mmol) as a chain transfer agent were also added dropwise for 2 h. In the meantime, the solution was kept at 45 °C and continuously stirred with a mechanical stirrer. The
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polymerization reaction was continued for 3 h and the reaction was terminated by the addition of 2 L
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of distilled water. The obtained precipitates were filtered and washed several times with excess methanol to remove the unreacted monomers. The residual solvents were removed in a vacuum oven for 2 days, which produced the desired copolymers as a white powder in 90–95% yield. The
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compositions and molecular weights of the copolymers were determined by 1H NMR analysis and GPC measurement, respectively. Melt-spinning process. The P(AN-co-MA) copolymers with 15 mol% MA content (as calculated by NMR analysis) were used for melt-spinning. Polymers with this composition showed melting points in the range of ~180–190 °C by DSC measurement and visual observation. In addition, the rheological analysis confirmed that the viscosity remained constant at 190 °C without any viscosity change caused
by the PAN stabilization reaction, which ensured the spinnability of the P(AN-co-MA). Isothermal TGA analysis showed that no reaction, including the stabilization reaction, occurred at this same temperature. For injection molding, the polymers were uniformly ground by a mechanical mixer and then dried in a vacuum oven. Next, melt spinning was carried out using a universal extruder Noztek Pro (Noztek, UK) with a 0.2 mm injection nozzle for spinning. After the machine heated up to 190 °C, the powders (2 g) were put into the feeder and the screw was rotated at 60 rpm to melt and mix the
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polymer. During spinning, the nozzle was maintained above 185 °C and the extruded PAN fibers were collected using a winder with a diameter of 20 cm at 250 rpm. The melt-spun fibers under these
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conditions could be obtained continuously without breakage and the spinning rate was approximately 157 m/min. The obtained fibers were completely transparent with a diameter of ~15–20 μm depending
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on the winding speed.
Stabilization and carbonization process. Prior to the heat treatment, the melt-spun PAN fibers were
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pretreated with an E-beam to prevent remelting and fusion between the fibers. The E-beam was
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irradiated by placing the sample attached to the aluminum plate on the beam line to which the cooling water line was connected to prevent heating by electron beam. In addition, the E-beam equipment of
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1 MeV energy and 1 mA current was operated by the facility personnel (EB Tech Co., Korea). In order to make it convenient to understand visually, the stabilization reaction assisted by E-beam irradiation
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was first studied for the 20-μm-thick films, and it was concluded that the fusion during thermal treatment could be efficiently frustrated by E-beam irradiation at a dose of 1500 kGy or more. Then,
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after irradiating with an E-beam of 1500 kGy, stabilized fibers were prepared by a stepwise heat treatment under air using a conventional high temperature oven. The thermal treatment involved
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heating from room temperature to 160 °C at a heating rate of 5 °C/min, holding at 160 °C for 15 min, heating to 190 °C at a heating rate of 1 °C/min, holding at 190 °C for 15 min, heating to 220 °C at a heating rate of 1 °C/min, holding at 220 °C for 15 min, heating to 250 °C at a heating rate of 1 °C/min, and holding at 250 °C for 45 min. During heat treatment, the fibers were fixed to a metal frame to prevent shrinkage. After heating up to 250 °C, the density of the stabilized fibers changed from 1.18 g/cm3 for the melt-spun fiber to 1.31 g/cm3, which was judged to have stabilized successfully. The stabilized fibers were then pyrolyzed using a tube-type furnace (Korea Furnace Development Co.,
Korea) at 1200 °C at a heating rate of 5 °C/min in a nitrogen atmosphere, successfully resulting in CFs. 3. RESULTS AND DISCUSSION In essence, PAN is a semicrystalline polymer that is has a melting point when heated rapidly, but it is very difficult to utilize it in its melt state since PAN starts to react with itself prior to melting, which
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is attribute to a unique intra- and intermolecular cross-linking [32,33]. Therefore, it is necessary to introduce a soft component in order to obtain the melting property before the cross-linking reaction occurs. A promising option is addition of the MA component, which is an excellent choice for
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copolymerization with AN to form a random copolymer efficiently because of its similar reactivity to that of AN [34]. There have been a few attempts to fabricate CFs through melt-spinning of AN/MA
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copolymers [19–22], but most of them have focused only on polymer synthesis and the investigation
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of their thermal properties, remaining unclear for the resulted CFs.
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P(AN-co-MA) copolymers with different MA contents were synthesized by conventional emulsion
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polymerization. The initial feed of monomer MA was varied from 6 to 18 mol% and also included
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pure PAN, as described in Table 1. Ultimately, polymerization was performed using a total of 200 g of monomers, and the yields after washing and drying were in the range of 90–95%. The composition of
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the obtained polymers was determined by 1H NMR analysis (Figure S1). The peaks of the main chain could not be distinguished between MA and AN, but the methyl groups (−CH3) of MA observed at
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3.69 ppm as a triplet were clearly identifiable, so the MA content of the copolymer could be calculated by integration of the peak, as listed in Table 1. As the reactivity of MA is somewhat lower than that of AN, the actual MA content of the polymer was slightly lower compared to the feeding ratio [34]. In
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addition, the molecular weights and distribution of the polymers were calculated from HT-GPC analysis (Figure S2). It is known that an excessively high molecular weight increases the melt viscosity. Therefore, it was confirmed that the polymers having a number average molecular weight (Mn) in the range of 20,000–40,000 g/mol needed to be synthesized in order to control the melt viscosity adequately for melt-spinning to occur as intended. In addition, the polydispersity indices (PDIs) lay
between 2 and 3, which are expected values from typical emulsion polymerization, indicating that the polymerization proceeded well without side reactions such as crosslinking. Thermal properties of the synthesized polymers were investigated by DSC. For observation of Tg and Tm, DSC measurements were carried out at heating rates of 10 and 100 °C/min, respectively. In particular, Tm analyses had to be conducted at a rapid temperature rise because of the stabilization reactions occurring at ~260 °C. The representative values of thermal analyses, monomer composition,
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and molecular weights are listed in Table 1. The values of Tg and Tm were plotted (Figure 1) as a function of the MA content. Considering that the thermal properties vary depending on the molecular
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weight, configuration, conformation, and monomer sequence, the obtained results showed a regular trend, i.e., Tg and Tm decreased uniformly with the increasing MA content, indicating that all runs
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produced polymers of similar structure. In particular, the linear dependence of the thermal properties
N
on the MA content was evident from the fact that the Tg and Tm values obtained for pure PAN on
A
extrapolating to 0 mol% MA content were similar to the known values (95 and 322 °C, respectively).
M
The ideal content of MA (melting point < 200 °C) for which the stabilization reaction did not initiate, was found to be ~15 mol% or more. Therefore, it was concluded that the melt-process could be
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performed at ~200 °C when the actual MA content was >15 mol%. In all further experiments, the meltprocessable polymers with an MA content of about 15 mol% were used.
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The flow characteristics of the synthesized polymer for the melting process were investigated by
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rheological analysis (Figure 2 and Figure S3). As the temperature increased and the shear rate increased, it was observed that the complex viscosity was clearly lowered. In particular, the absolute value of the complex viscosity at 190 °C and 0.1% strain, which was an indicator of the melt
A
processability [35], was found to be about 5,000 and 3300 Pa∙s at 0.1 and 10 rad/s, respectively. These values were lower than those of the PAN-based polymers for melt-spinning reported in previous studies [34], which indicated sufficient feasibility of the polymers described herein for melt-spinning. In addition, in a time sweep study for 30 min, the complex viscosity increased moderately from ~3300 to 3850, but no rapid change was observed. At temperatures above 200 °C, gelation occurred because of cyclization and the cross-linking reaction, resulting in a significant increase in viscosity. However,
this phenomenon seemed to be sufficiently inhibited at 190 °C. This was also observed by isothermal TGA measurement (Figure S4). As a result of holding the temperature at 190 °C for 30 min, only 0.06% of the mass was degraded, suggesting that either there was no reaction or it occurred to an insignificant extent. Based on the insights into the melt capability, the polymer was melt-spun using a universal extruder with a 0.2 mm injection nozzle. The melt spinning was optimized under the abovementioned
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conditions and transparent fibers with a diameter of 15–20 μm were obtained (Figure S6). The mechanical properties of the obtained pristine fibers were evaluated and the tensile strength, tensile
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modulus, and elongation at break were 0.26±0.03 GPa, 6.76±1.78 GPa, and 17.6±0.84%, respectively (Table 2).
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Since the melt-spun fibers would remelt at 200 °C, a stabilization process was attempted using E-beam
N
irradiation. E-beam irradiation of PAN has been reported to produce radical species in the polymer
A
main chain, causing a reduction in the activation energy of the cyclization and crosslinking reactions
M
[31]. Therefore, the E-beam irradiated PAN could be stabilized at a lower temperature than 200 °C compared to those without E-beam assistance. The aim of the E-beam treatment was to clearly lower
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the stabilization temperature to below the Tm. For making the monitoring of the experimental process more convenient by visual observation and automated, the film states of the samples were used to
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optimize the E-beam irradiation process. To ensure fiber-like conditions, 20-μm-thick films were
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fabricated by the hot press molding method. The experiment proceeded as follows: after the films were irradiated with electron doses of 500, 1500, and 2500 kGy, and followed up by heat treatment under various conditions, the changes in the chemical structures were analyzed by FT-IR measurements. The
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thermal treatments were performed by increasing the temperature up to 250 °C at a heating rate of 3 °C/min under air atmosphere and then varying the time at 250 °C. The retention times at 250 °C were controlled to 0, 5, 15, 30, 45, 60, and 75 min. In order to observe the fusion between the films during thermal treatment, three sheets of the films (2020 mm) were stacked vertically and heated. The FT-IR absorption spectra for samples irradiated with different E-beam doses are shown in Figure 3. The sample without E-beam irradiation showed weak peaks corresponding to νO−H and νN−H in the
range of 3100−3600 cm−1 and very weak peaks of νC=C, νC=N, and δN−H, i.e., the components that were converted by the cyclization reaction, at 1590 cm−1. This is because the cyclization rarely occurred at the end of heating up to 250 °C. In addition, the representative peaks found in a typical P(AN-co-MA) copolymer were also observed, i.e., νC−H of CH2 and CH3 groups at ~2934 cm−1, νC≡N of the AN component at 2242 cm−1, νC=O of the MA component at 1730 cm−1 and δC−H of CH2 and CH3 groups at 1451 cm−1. A strong peak was observed at 1668 cm−1 and assigned to the –HC=N−N=CH− group,
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which indicated the occurrence of intermolecular crosslinking [36]. Therefore, it was concluded that the crosslinking reaction occurred before the cyclization reaction. Similar changes in the chemical
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structures were also observed in the samples irradiated with an E-beam. However, there was a clear difference in the reaction rate of the cyclization. As the dose of E-beam was increased, the intensity of the peaks indicating the cyclization reaction such as νO−H and νN−H also increased. Especially, in the
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samples irradiated with 1500 and 2500 kGy, the difference was clear, which provided unambiguous
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evidence that E-beam irradiation promoted the stabilization reaction. Furthermore, for a longer holding
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time at 250 °C, the intensity of the peaks in the range of 3100−3600 and 1590 cm−1, which was
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evidence of cyclization, was found to become stronger, and that of the peaks at 2934, 2242, and 1730
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cm−1, originating from the functional groups such as aliphatic C−H, C≡N, and C=O converted to ring structures, became weaker. It was also found that the reaction rate increased with the increasing E-
kGy.
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beam dose, and there was no significant difference between the samples irradiated by 1500 and 2500
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To easily interpret the results of the FT-IR studies, the degrees of stabilization reaction were quantified by calculating the values of the extent of reaction (EOR) at each condition (Table S1). The values of
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EOR were calculated by the following equation [37]. Extent of reaction (EOR) (%) =
𝐴𝐶 𝐴𝐶 +𝑓∙𝐴𝑁
× 100 (%)
where AC refers to the absorption peak areas at 1590 cm−1 derived from cyclized ring structures, AN indicates the absorption peak areas at 2240 cm−1 originating from the nitrile groups as an internal standard, and f is a constant with a value of 0.29. These results were plotted against holding times and doses (Figure 4). From the plot in Figure 4(a) against holding times at 250 °C, it was evident that as
the time increased, the EOR also increased linearly. In addition, the cyclization reaction progressed 30–50% by simply heating up to 250 °C at the heating rate of 3 °C/min. From the plot against irradiated doses (Figure 4(b)), it could be confirmed that the reaction in the samples irradiated by the E-beam was almost complete in the retention time of 15 min regardless of the dose of the E-beam, whereas the sample without E-beam treatment did not successfully react even after 75 min. Thus, it was evident that E-beam treatment promoted the stabilization reaction of PAN. At first glance, only the presence
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or absence of E-beam pretreatment seemed to be related to the reaction rate, but the amount of irradiation dose actually had a substantial impact on the stabilization process. In the process of
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stabilizing melt-spun fibers, fusion between the bundles must be absolutely prevented. Therefore, we tried to stabilize P(AN-co-MA) in the film state under conditions that were similar to those used in the fiber form. In the samples irradiated by 1500 and 2500 kGy, there was no adhesion between the films,
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which meant that E-beam irradiation of 1500 kGy or more was required to efficiently prevent the
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fusion phenomenon. Therefore, we stabilized the melt-spun fibers through E-beam pretreatment using
A
a dose of 1500 kGy.
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The obtained melt-spun fibers were pretreated by E-beam irradiation of 1500 and 2500 kGy and their
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stabilization reaction was investigated by DSC measurements (Figure 5 and Table 3). The fiber without E-beam treatment underwent the cyclization reaction at 264 °C, which is the same as observed
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in the powder state, and showed the maximum calorific value at 308 °C with the reaction heat of 639.6 J/g. On the other hand, the samples treated by E-beam exhibited markedly different exotherms. The
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large exothermic peaks observed near 300 °C in the neat fiber moved significantly toward lower temperature in the fibers with E-beam treatment. In addition, the values of reaction heat were also
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greatly reduced, indicating that because of E-beam irradiation, the stabilization reaction occurred at lower energy states with a different reaction mechanism. In the fibers irradiated by the 1500 kGy Ebeam, the stabilization reaction started at 203 °C and the exothermic peak maximum was at 288 °C. In the sample irradiated by 2500 kGy, the reaction initiated at 203 °C and the peak maximum was 281 °C. The heats of the reaction for the two samples were 335.9 and 311.3 J/g, respectively. There were no significant differences in the overall reaction behavior.
Thus, two types of E-beam-treated samples were stabilized with the aforementioned heat treatment profiles. In addition, to inhibit the shrinking of the bundles, the fibers were fastened to the SUS frame during thermal treatment. The stabilization reaction was terminated after being kept at 250 °C for 45 min because there was no change in the density even when it was maintained at 250 °C for more than 45 min. The density of the pristine melt-spun fibers was 1.18 g/cm3 and that after maintaining thermal stabilization was 1.30–1.31 g/cm3 regardless of the E-beam doses, which are typical values for
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stabilized PAN fibers. Therefore, E-beam irradiation of 1500 kGy was sufficient to prevent the fusion between fiber bundles and to produce stabilized PAN fibers through heat treatment.
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Finally, the stabilized PAN fibers were carbonized using a tube-type furnace under nitrogen atmosphere. The fibers were fixed to a carbon sheet by attaching a carbon tape and the carbonization of the fibers was carried out at a rate of 5 °C/min up to 1200 °C. This process successfully produced
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CFs using melt-spun fibers. The diameter of the obtained CFs was about 10–15 μm, and spherical
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fibers of the same shape as the precursor were obtained. The density of the CF was 1.70 g/cm3 which
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was slightly lower than that of commercial PAN-based CF. The fractured cross section and
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longitudinal surface of the resulting CF were observed by SEM (Figure 6). In the cross section,
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completely spherical shape was observed without any particular defect. In addition, though some defects below the micrometer order on the surface was observed, the entire surface was confirmed to
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be smooth, indicating that the difference in spinning method had no significant effect on the morphology of the resulting CF. The carbonization yield was ~40 wt%, as confirmed by TGA (Figure
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S5). The mechanical properties of the resulting CFs were evaluated; the tensile strength, tensile modulus, and elongation at break were 1.37±0.20 GPa, 110±11.1 GPa, and 1.27±0.28%, respectively
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(Table 2). A typical stress-strain curve is shown in Figure S7. These values were the highest among the reported CFs produced by melt-processing [12]. Further studies are aimed at developing a better optimized and generalized protocol, although the results described in this study show sufficient potential for inexpensive CF manufacturing through melt-spinning.
4. CONCLUSION
In this work, comprehensive studies were performed for fabricating CFs using melt-spun P(AN-coMA). First, PAN polymers containing the MA co-monomer as a soft component in different amounts were synthesized and their thermal properties were investigated in detail. As a result, when the MA content was over 15 mol%, the resulting copolymers were melt-processable above 190 °C without crosslinking and cyclization reactions, as confirmed by isothermal TGA studies. In addition, it was also confirmed by rheological analysis at 190 °C that the polymers exhibited flow characteristics
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suitable for melt-spinning. Then, melt-spinning was carried out using a conventional extruder and transparent fibers having a density of 1.18 g/cm3 and a diameter of 15–20 μm were obtained. It was
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found that E-beam pretreatment above 1500 kGy could successfully produce stabilized fibers without fusion phenomenon during thermal stabilization. After optimization of the stabilization reaction, the melt-spun fibers formed through thermal treatment up to 250 °C and assisted by E-beam irradiation
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were successfully converted to the stabilized PAN fibers without remelting and fusion. Lastly, CFs
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were prepared by carbonizing the stabilized fibers. The tensile strength, tensile modulus, and
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elongation at break of the resulting CFs were 1.37±0.2 GPa, 110±11.1 GPa, and 1.27±0.28%,
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respectively. Further research is currently underway to optimize the manufacturing process for
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improving the aforementioned mechanical properties of the product.
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5. ACKNOWLEDGEMENTS
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This work was supported by a grant from the Korea Institute of Science and Technology institutional program, and the National Research Foundation of Korea (NRF) grant funded by the Korea
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government (Ministry of Science and ICT, No. NRF-2017R1C1B5076344).
REFERENCES [1] Y. Liu, S. Kumar, Recent progress in fabrication, structure, and properties of carbon fibers, Polym. Rev. 52 (3) (2012) 234-258. [2] B. A. Newcomb, Processing, structure, and properties of carbon fibers, Composites Part A: Applied Science and Manufacturing, 91 (2016), 262-282. [3] S.-J. Park, B.-J Kim, Carbon Fibers and Their Composites, Springer series in Materials Sciences,
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(2014), 275-317.
[4] T Ishikawa, K Amaoka, Y. Masubuchi, T. Yamamoto, A. Yamanaka, M. Arai, J. Takahashi,
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Overview of automotive structural composites technology developments in Japan, Composites Science and Technology, 155 (2018), 221-246.
N
Composites Science and Technology 26(4) (1986) 251−281.
U
[5] P. Beardmore, C.F. Johnson, The potential for composites in structural automotive applications,
A
[6] N.V. Salim, S. Blight, C. Creighton, S. Nunna, S. Atkiss, J.M. Razal, The role of tension and temperature for efficient carbonization of polyacrylonitrile fibers: toward low cost carbon fibers,
M
Industrial & Engineering Chemistry Research 57(12) (2018) 4268−4276.
ED
[7] U. Arnold, A. De Palmenaer, T. Bruck, K. Kuse, Energy-efficient carbon fiber production with concentrated solar power: process design and techno-economic analysis, Industrial & Engineering
PT
Chemistry Research 57(23) (2018) 7934−7945. [8] S.-W. Lee, H.-Y. Lee, S.-Y. Jang, S. Jo, H.-S. Lee, W.-H. Choe, S. Lee, Efficient preparation of
CC E
carbon fibers using plasma assisted stabilization, Carbon 55 (2013), 361-365. [9] S.-Y. Kim, S.Y. Kim, S. Lee, S. Jo, Y.-H. Im, H.-S. Lee, Microwave plasma carbonization for the
A
fabrication of polyacrylonitrile-based carbon fiber, Polymer 56(15) (2015), 590-595. [10] R. Bacon, A. Silvaggi, Electron microscope study of the microstructure of carbon and graphite fibers from a rayon precursor, Carbon 9(3) (1971) 321−325. [11] S. Peng, H. Shao, X. Hu, Lyocell fibers as the precursor of carbon fibers, Journal of Applied Polymer Science 90(7) (2003) 1941-1947. [12] D.A. Baker, T.G. Rials, Recent advances in low‐ cost carbon fiber manufacture from lignin, Journal of Applied Polymer Science 130(2) (2013) 713−728.
[13] M.A. Hunt, T. Saito, R.H. Brown, A.S. Kumbhar, A.K. Naskar, Patterned functional carbon fibers from polyethylene, Advanced Materials 24(18) (2012) 2386−2389. [14] M.J. Behr, B.G. Landes, B.E. Barton, M.T. Bernius, G.F. Billovits, E.J. Hukkanen, J.T. Patton, W. Wang, C. Wood, D.T. Keane, Structure-property model for polyethylene-derived carbon fiber, Carbon 107 (2016) 525-535. [15] J.W Kim, J.S Lee, Preparation of carbon fibers from linear low density
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polyethylene, Carbon 94 (2015), 524-530. [16] D. Zhang, Q. Sun, Structure and properties development during the conversion of polyethylene
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precursors to carbon fibers, J. Appl. Polym. Sci., 62 (1996), 367-373.
[17] N. Yusof, A. Ismail, Post spinning and pyrolysis processes of polyacrylonitrile (PAN)-based carbon fiber and activated carbon fiber: A review, Journal of Analytical and Applied Pyrolysis 93
U
(2012) 1−13.
N
[18] M. MInus, S. Kumar, The processing, properties, and structure of carbon fibers, The Journal of
A
The Minerals, Metals & Materials Society 57(2) (2005) 52-58.
M
[19] M. Qu, F. Nilsson, Y. Qin, G. Yang, Y. Pan, X. Liu, G.H. Rodriguez, J. Chen, C. Zhang, D.W.
ED
Schubert, Electrical conductivity and mechanical properties of melt-spun ternary composites comprising PMMA, carbon fibers and carbon black, Composites Science and Technology 150 (2017)
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24−31.
[20] A.K. Naskar, R.A. Walker, S. Proulx, D.D. Edie, A.A. Ogale, UV assisted stabilization routes for
CC E
carbon fiber precursors produced from melt-processible polyacrylonitrile terpolymer, Carbon 43(5) (2005) 1065−1072. MA
A
[21] T. Mukundan, V. Bhanu, K. Wiles, H. Johnson, M. Bortner, D. Baird, A. Naskar, A. Ogale, D. Edie, J. McGrath, A photocrosslinkable melt processible acrylonitrile terpolymer as carbon fiber precursor, Polymer 47(11) (2006) 4163−4171. MA [22] G. Miller, J. Yu, R. Joseph, S. Choudhury, S. Mecham, D. Baird, M. Bortner, R. Norris, F. Paulauskas, J. Riffle, Melt-spinnable polyacrylonitrile copolymer precursors for carbon fibers, Polymer 126 (2017) 87−95. [23] A. Lowe, W. Deng, D.W. Smith Jr, K.J. Balkus Jr, Acrylonitrile-based nitric oxide releasing melt-
spun fibers for enhanced wound healing, Macromolecules 45(15) (2012) 5894−5900. [24] Y. Tian, K. Han, W. Zhang, J. Zhang, H. Rong, D. Wang, B. Yan, S. Liu, M. Yu, Influence of residence time on the structure of polyacrylonitrile in ionic liquids during melt spinning process, Materials Letters 92 (2013) 119−121. [25] D. Grove, P. Desai, A. Abhiraman, Exploratory experiments in the conversion of plasticized melt spun PAN-based precursors to carbon fibers, Carbon 26(3) (1988) 403−411.
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[26] S.K. Atureliya, Z. Bashir, Continuous plasticized melt-extrusion of polyacrylonitrile homopolymer, Polymer 34(24) (1993) 5116−5122.
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[27] B.L. Batchelor, S.F. Mahmood, M. Jung, H. Shin, O.V. Kulikov, W. Voit, B.M. Novak, D.J. Yang, Plasticization for melt viscosity reduction of melt processable carbon fiber precursor, Carbon 98 (2016) 681−688.
U
[28] O.-K. Park, S. Lee, H.-I. Joh, J.K. Kim, P.-H. Kang, J.H. Lee, B.-C. Ku, Effect of functional
N
groups of carbon nanotubes on the cyclization mechanism of polyacrylonitrile (PAN), Polymer 53(11)
A
(2012) 2168-2174.
M
[29] S. Lee, Y.J. Kim, D.H. Kim, B.C. Ku, H.I. Joh, Synthesis and properties of thermally reduced
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graphene oxide/polyacrylonitrile composites, Journal of Physics and Chemistry of Solids 73 (6), 741743.
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[30] H.K. Shin, M. Park, P.H. Kang, H.-S. Choi, S.-J. Park, Preparation and characterization of polyacrylonitrile-based carbon fibers produced by electron beam irradiation pretreatment, Journal of
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Industrial and Engineering Chemistry 20(5) (2014) 3789−3792. [31] S. Park, S.H. Yoo, H.R. Kang, S.M. Jo, H.-I. Joh, S. Lee, Comprehensive stabilization mechanism of electron-beam irradiated polyacrylonitrile fibers to shorten the conventional thermal treatment,
A
Scientific Reports 6 (2016) 27330. [32] S. Lee, J. Kim, B.C. Ku, J. Kim, H.I. Joh, Structural evolution of polyacrylonitrile fibers in stabilization and carbonization, Advances in Chemical Engineering and Science 2 (02) (2012), 275 [33] Z. Bashir, A critical review of the stabilisation of polyacrylonitrile, Carbon 29(8) (1991) 10811090 [34] V. Bhanu, P. Rangarajan, K. Wiles, M. Bortner, M. Sankarpandian, D. Godshall, T. Glass, A.
Banthia, J. Yang, G. Wilkes, Synthesis and characterization of acrylonitrile methyl acrylate statistical copolymers as melt processable carbon fiber precursors, Polymer 43(18) (2002) 4841−4850. [35] D.G. Baird, D.I. Collias, Polymer processing principles and design. MA: Butterworth-Heinemann Newton, 1995. [36] S.M. Badawy, A.M. Dessouki, Cross-linked polyacrylonitrile prepared by radiation-induced polymerization technique, The Journal of Physical Chemistry B 107(41) (2003) 11273−11279.
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[37] G. Collins, N. Thomas, G. Williams, Kinetic relationships between heat generation and nitrile
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A
N
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consumption in the reaction of poly(acrylonitrile) in air at 265 °C, Carbon 26(5) (1988) 671−679.
Figures and Tables
600 Tg Tm
500 Tm = –6.07 CMA + 555.1
450 400 350
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Temperature (K)
550
Tg = –2.36 CMA + 388.9
0
5
10
15
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300 20
MA Content (mol%)
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N
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Figure 1. Thermal properties (Tg and Tm) of the synthesized P(AN-co-MA) copolymers.
5
3
5x10
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10
4
Complex viscosity ()
8x10
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Loss modulus ()
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4x10
3
3x10
3
2x10
Storage modulus ()
4
2x10
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Modulus (Pa)
4
0
300
600
Complex viscosity (Pa·s)
4
6x10
3
4x10
3
900
1200
10 1500
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Time (sec)
Figure 2. Rheological properties of the synthesized P(AN-co-MA) with MA content of 15 mol% at 190 °C and a constant frequency of 10 rad/s as a function of time.
(b)
0 kGy
Absorbance (a.u.)
75 min 60 min 45 min 30 min 15 min 5 min
500 kGy 75 min 60 min
Absorbance (a.u.)
(a)
45 min 30 min 15 min 5 min
0 min 3500
3000
2500
2000
1500
1000
4000
3500
1
1500 kGy
1000
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45 min 30 min 15 min 5 min
4000
3500
0 min 3000
2500
2000
1500
1000
1
Wavenunber (cm )
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Wavenunber (cm )
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1500 1
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2000
75 min 60 min
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15 min 5 min
2500
1000
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30 min
Absorbance (a.u.)
Absorbance (a.u.)
45 min
3000
1500
2500 kGy
(d) 75 min 60 min
3500
2000
Wavenunber (cm )
0 min 4000
2500
1
Wavenunber (cm )
(c)
3000
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4000
0 min
Figure 3. Tracking chemical structure changes of the E-beam irradiated PAN films by FT-IR
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spectroscopy after thermal stabilization with a heating rate of 3 °C/min followed by holding at 250 °C at several electron doses: (a) 0 kGy, (b) 500 kGy, (c) 1500 kGy, and (d) 2500 kGy. The holding time
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shown on the right of each plot is for spectra obtained at 250 °C.
(b) 100
100
80
90
EOR (%)
60 40 0 kGy 500 kGy 1500 kGy 2500 kGy
20
80
Holding Time at 250 °C
60
0
5 min 15 min 30 min 45 min 60 min 75 min
70
50
0
15
30
45
60
75
0
Holding Time at 250 °C (min)
500
1000
1500
2000
2500
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EOR (%)
(a)
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Irradiated electron dose (kGy)
Figure 4. Plot of the extent of reaction (EOR) of the E-beam irradiated PAN films after thermal stabilization with a heating rate of 3 °C/min followed by holding at 250 °C against (a) holding times
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N
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and (b) irradiation doses.
0 -1
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exo
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Heat Flow (W/g)
1
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0 kGy 1500 kGy 2500 kGy
2
M
3
-2
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-3 100
150
200
250
300
350
Temperature (°C)
Figure 5. DSC curves of the melt-spun PAN fibers irradiated at several electron doses with a heating rate of 10 °C/min under N2 atmosphere.
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Figure 6. SEM image of the resulting carbon fiber: (a) cross section and (b) longitudinal surface.
Table 1. Polymerization results and thermal properties of P(AN-co-MA)
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Mnb MA MA Tgc Tmd Run Feeding Contenta PDIb 10−3 [K] [K] [mol%] [mol%] [g/mol] 0 0 27 2.1 378.8 558.1 1 6 6.0 25 2.1 379.2 538.3 2 9 8.3 37 2.45 360.2 489.9 3 11.6 10.0 30 2.5 357.6 482.1 4 12 11.3 22 2.59 347.1 478.8 5 12.6 12 26 3.3 375.2 486.2 6 14.1 14.3 36 2.56 350.8 462.5 7 14.6 13.3 35 2.19 352.2 471.5 8 15 14.7 39 2.8 364.3 469.6 9 15 14.6 32 2.45 360.4 465.2 10 16.4 16 38 2.66 334.4 446.3 11 16.8 13.7 15 2.74 372.9 462.8 12 17.8 15.3 36 2.26 357.2 476.6 13 17.8 17.3 23 2.8 344.2 456.3 14 18 17.6 27 2.98 348.8 461.8 15 a b Calculated by NMR analysis. Determined by size-exclusion chromatography (SEC) based on
d
A
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100 °C/min.
Obtained by DSC measurements conducted at a heating rate of
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at a heating rate of 10 °C/min.
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PMMA standards in DMF with LiBr of 0.05 M at 80 °C. c Obtained by DSC measurements performed
Table 2. Mechanical properties of melt-spun PAN fibers and resulting-carbon fibers
Sample
Tensile modulus [GPa] 6.76±1.78 110±11.1
Elongation [%] 17.6±0.84 1.27±0.28
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A
N
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PAN fiber Carbon fiber
Tensile strength [GPa] 0.26±0.03 1.37±0.20
Table 3. Kinetic results of the thermal stabilization reaction determined by DSC measurements of the melt-spun PAN fibers Tonseta Tpeaktopa Ha [°C] [°C] [J/g] 264 308 639.6 0 kGy 203 288 335.9 1500 kGy 186 281 311.3 2500 kGy a Measured by DSC conducted at a heating rate of 10 °C/min under N2 atmosphere.
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A
N
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Sample