CARBON PERGAMON Carbon4l(2003)2805-2812 Evolution of structure and properties of pan precursors during their conversion to carbon fibers Zhang Wangxi", Liu Jie, Wu gang Department of Carbon Fibers and Composites, Bejing University of Chemical Technology, Beijing 100029, China Received 5 July 2003; accepted 13 August 2003 Abstract The formation and evolution of structure, and the changes of properties during the preoxidation, precarbonization, and carbonization of different pan precursors were studied by the combination of DSC, FT-IR, SEM and some traditional measurements, such as density and mechanical properties of various fibers. The exothermic regime of polyacrylonitrile-based precursors made of acrylonitrile/itaconic acid(AN/IA) copolymers or acrylonitrile/acrylamide(AN/AM) copolymers is much broader and the cyclization reaction starts at lower temperature, compared to that of pan homopolymer precursors, but M appears to be more effective in separating the exothermic reactions corresponding to preoxidation stages in dSC curves as compared to IA. If AN/IA(97.5/2.5 w/w) precursors and AN/AM(97.5/2.5 w/w) precursors are designated as PI and of P3 is higher than that of PI or P2. This may result from the difference of aggregation morphology among bo2 P2, respectively, the AM-containing commercial precursors(P3)are thermally more stable than the P2 ones, and the density precursors, since it is dense for P3 precursors, whereas P2 and PI precursors have some voids. The tensile strength of resultant carbon fibers from P3 precursors was better than that of carbon fibers from P2 or PI after identical conditions of preoxidation are employed. C 2003 Elsevier ltd all rights reserved Keywords: A. Carbon fibers; Carbon precursor; B. Stabilization; Carbonization; C. Mechanical properties formance carbon fibers depends mainly on the com- Carbon fibers produced from a variety of position and quality of the precursor fibers.However,the physical and chemical structural transformations that take acrylonitrile(PAN), mesophase place during heat treatments are extremely complicated pitch, rayon, etc, which PAN-based carbon fibers However, it is difficult to predict how to effectively are the preferred reinforcement for structural composites improve the performances of the resultant carbon fibers. with the result of their excellent specific strength and and it is not fully clear which one comonomer is the stiffness combined with their light weight as well as lower optimum selection and which one precursor index has the cost. In order to meet expanded use in some high-tech most influence on their structure and properties because of sectors, many novel approaches, such as dry-wet spinning commercial secrets or other reasons. One way to address [l, steam drawing [21, increasing the molecular weight of these issues is to take different pan comparatively study their different evolution of structure stabilization [4], etc, have been performed to increase the tensile strength of pan-based carbon fibers now it has features and properties of Pan precursor fibers during the thermal stabilization and carbonization process. In this study, some factors limiting the mechanical properties of *Corresponding author. Tel : +86-10-6443-8724; fax: +86- carbon fibers are investigated, with the aim of 0-6443-8724 guidelines to improve the quality of the resultant carbon E-mailaddress.zhgwgxi@sdu.edu.cn(Z.Wangxi) fibers 0008-6223/03/S-see front matter 2003 Elsevier Ltd. All rights reserved doi:1o.1016/0008-6223(03)00391-9
Carbon 41 (2003) 2805–2812 E volution of structure and properties of PAN precursors during their conversion to carbon fibers Zhang Wangxi , Liu Jie, Wu Gang * Department of Carbon Fibers and Composites, Beijing University of Chemical Technology, Beijing 100029, China Received 5 July 2003; accepted 13 August 2003 Abstract The formation and evolution of structure, and the changes of properties during the preoxidation, precarbonization, and carbonization of different PAN precursors were studied by the combination of DSC, FT-IR, SEM and some traditional measurements, such as density and mechanical properties of various fibers. The exothermic regime of polyacrylonitrile-based precursors made of acrylonitrile/itaconic acid (AN/IA) copolymers or acrylonitrile/acrylamide (AN/AM) copolymers is much broader and the cyclization reaction starts at lower temperature, compared to that of PAN homopolymer precursors, but AM appears to be more effective in separating the exothermic reactions corresponding to preoxidation stages in DSC curves as compared to IA. If AN/IA (97.5/2.5 w/w) precursors and AN/AM (97.5/2.5 w/w) precursors are designated as P1 and P2, respectively, the AM-containing commercial precursors (P3) are thermally more stable than the P2 ones, and the density of P3 is higher than that of P1 or P2. This may result from the difference of aggregation morphology among the original precursors, since it is dense for P3 precursors, whereas P2 and P1 precursors have some voids. The tensile strength of resultant carbon fibers from P3 precursors was better than that of carbon fibers from P2 or P1 after identical conditions of preoxidation are employed. 2003 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fibers; Carbon precursor; B. Stabilization; Carbonization; C. Mechanical properties 1. Introduction been popularly accepted that the quality of the high performance carbon fibers depends mainly on the composition and quality of the precursor fibers. However, the Carbon fibers can be produced from a variety of physical and chemical structural transformations that take precursors, such as polyacrylonitrile (PAN), mesophase place during heat treatments are extremely complicated. pitch, rayon, etc., among which PAN-based carbon fibers However, it is difficult to predict how to effectively are the preferred reinforcement for structural composites improve the performances of the resultant carbon fibers, with the result of their excellent specific strength and and it is not fully clear which one comonomer is the stiffness combined with their light weight as well as lower optimum selection and which one precursor index has the cost. In order to meet expanded use in some high-tech most influence on their structure and properties because of sectors, many novel approaches, such as dry-wet spinning commercial secrets or other reasons. One way to address [1], steam drawing [2], increasing the molecular weight of these issues is to take different PAN precursors and precursors polymer [3], modifying the precursors prior to comparatively study their different evolution of structure stabilization [4], etc., have been performed to increase the features and properties of PAN precursor fibers during the tensile strength of PAN-based carbon fibers. Now it has thermal stabilization and carbonization process. In this study, some factors limiting the mechanical properties of *Corresponding author. Tel.: 186-10-6443-8724; fax: 186- carbon fibers are investigated, with the aim of generating 10-6443-8724. guidelines to improve the quality of the resultant carbon E-mail address: zhgwgxi@sdu.edu.cn (Z. Wangxi). fibers. 0008-6223/03/$ – see front matter 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0008-6223(03)00391-9
2806 Z. anxi et al. / Carbon 41(2003)2805-2812 2. Experimental about 50 m/h was maintained in all the parallel experi 2. Materials Different PAn precursors(named as P0, Pl, P2 and P3, 2.3. Measurements respectively ) were selected in this study. For wet spinning PI fibers, a 20 wt. solution was prepared in Density of various fibers was obtained at 25C by the dimethylsulfoxide(DMso) by using a copolymer of use of density gradient column method. Two columns were crylonitrile/itaconic acid(AN/IA 97.5/2.5 w/w) with used: one comprising a mixture of n-heptane and carbon average molecular weight of 150 000 g mol. For pre- tetrachloride with a gradient from 1.00 to 1.60 g/cm, and cursor P2 fibers, which were wet-spun from a 22 wt. the other comprising carbon tetrachloride and 1, 3-dib- lution in dmso of a copolymer of acrylonitrile/ romopropane with a gradient from 1.55 to 1.90 g/cm. For amide(AN/AM 97.5/2.5 w/w) with average molecular a given fiber sample, dissolved in DMSO at 80C for 6 h, weight of 110 000 g mol. In comparison, a pan then filtered, washed using water then washed again with precursor, designated as PO, was also wet-spun from a 22 acetone, dried in vacuum oven to a constant weight. The wt% solution in DMsO using a homopolymer of weight of insoluble residues was divided by the total acrylonitrile with average molecular weight of ca. 120 000 weight of its before dissolving to obtain the solubility. The g mol. The precursor P3 fibers, however, were supplied exothermic reaction of Pan precursors was determined by by Mitsubishi Rayon (Japan). All these mentioned pre- the use of differential scanning calorimetry(DSC) which cursor fibers contain 3000 filaments each single tow their was carried out on a dsc-7 module of a Perkin-Elmer uality indices are listed in Table I Delta series thermal analyzer. These dsc studies were d ating rate of1o°C phere with sample weights of about 2.5 mg. A Netzsch 2.2. Preoxidation and carbonisation SC 404 was also used to obtain the high-temperature DSC curves of the P2 or P3 precursors, and the fibers from a self-designed pilot carbon fibers production line, original P2 after preoxidation or precarbonization investi- which is composed of two oxidizing furnaces and two gated in argon with a heating rate of 20C/min in the orizontal furnaces, respectively, is used for the preoxida- range of 25-1400C using about 9-mg samples. Fourier tion and closely linked carbonization of pan precursors transform infrared (FT-IR)measurements were made b Each furnace has some separated temperature zones. The loading samples on KBr disks(0.5 mg sample with 200 whole temperature zones were programmed as the follows: mg KBr) for the specimens which were selected from 125-192-203-214-231-243-253-268-283-428-603 fibers after different heat temperatures, by the use of a 803-1003-1350C. The first oxidation oven has four Nicolet750 Magna-IR. The titre (the linear density of different temperature zones, and the second oxidation oven fibers, its unit has denier, tex, and dtex, e. g, I dtex equal has five different temperature zones. In the first two hot-air to the grams of a 10 000-m long filament) of a filament circulation preoxidation furnaces, a precursor fiber was was measured by a XD-1 fiber fineness machine, and thermal stabilized in a purified air atmosphere at 125- mechanical properties of pan precursors, preoxidized 283C under a 10% stretching ratio, the total preoxidation fibers and carbon fibers were measured by a xQ-l tensile- time was about 50 min before forming a preoxidized fiber. testing machine(both XD-1 and xQ-1 were made in Subsequently, this preoxidized fiber was subjected to a Donghua University, Shanghai, China) at a crosshead pre-carbonization in an atmosphere of pure nitrogen from speed of 0.5 mm/min with a testing length of 20 mm and 428 to 803C with a fixed length, then a carbonization also load cell of 10 g In each case, at least 30 sample filaments in oxygen-free nitrogen from 1003 to 1350C under a 1% were tested, and the average of 30 filaments was taken for shrinkage to get carbon fibers. So the processing speed of each experiment Some quality indices of selected PAn precursor Precursor Density Crystallinity (dtex) strength (%) (wt%) 33.6 AN/IA(975/2.5 533.5 ANAM97.5/25) 578.5 10.6 Deduced from the result of this study
2806 Z. Wangxi et al. / Carbon 41 (2003) 2805–2812 2. Experimental about 50 m/h was maintained in all the parallel experiments. 2 .1. Materials Different PAN precursors (named as P0, P1, P2 and P3, 2 .3. Measurements respectively) were selected in this study. For wet spinning precursor P1 fibers, a 20 wt.% solution was prepared in Density of various fibers was obtained at 25 8C by the dimethylsulfoxide (DMSO) by using a copolymer of use of density gradient column method. Two columns were acrylonitrile/itaconic acid (AN/IA 97.5/2.5 w/w) with used: one comprising a mixture of n-heptane and carbon 21 3 average molecular weight of 150 000 g mol . For pre- tetrachloride with a gradient from 1.00 to 1.60 g/cm , and cursor P2 fibers, which were wet-spun from a 22 wt.% the other comprising carbon tetrachloride and 1,3-dib- 3 solution in DMSO of a copolymer of acrylonitrile/acryl- romopropane with a gradient from 1.55 to 1.90 g/cm . For amide (AN/AM 97.5/2.5 w/w) with average molecular a given fiber sample, dissolved in DMSO at 80 8C for 6 h, 21 weight of 110 000 g mol . In comparison, a PAN then filtered, washed using water then washed again with precursor, designated as P0, was also wet-spun from a 22 acetone, dried in vacuum oven to a constant weight. The wt.% solution in DMSO using a homopolymer of weight of insoluble residues was divided by the total acrylonitrile with average molecular weight of ca. 120 000 weight of its before dissolving to obtain the solubility. The 21 g mol . The precursor P3 fibers, however, were supplied exothermic reaction of PAN precursors was determined by by Mitsubishi Rayon (Japan). All these mentioned pre- the use of differential scanning calorimetry (DSC) which cursor fibers contain 3000 filaments each single tow, their was carried out on a DSC-7 module of a Perkin-Elmer quality indices are listed in Table 1. Delta series thermal analyzer. These DSC studies were performed at a heating rate of 10 8C in nitrogen atmosphere with sample weights of about 2.5 mg. A Netzsch 2 .2. Preoxidation and carbonization DSC 404 was also used to obtain the high-temperature DSC curves of the P2 or P3 precursors, and the fibers from A self-designed pilot carbon fibers production line, original P2 after preoxidation or precarbonization investiwhich is composed of two oxidizing furnaces and two gated in argon with a heating rate of 20 8C/min in the horizontal furnaces, respectively, is used for the preoxida- range of 25–1400 8C using about 9-mg samples. Fourier tion and closely linked carbonization of PAN precursors. transform infrared (FT-IR) measurements were made by Each furnace has some separated temperature zones. The loading samples on KBr disks (0.5 mg sample with 200 whole temperature zones were programmed as the follows: mg KBr) for the specimens which were selected from 125–192–203–214–231–243–253–268–283–428–603– fibers after different heat temperatures, by the use of a 803–1003–1350 8C. The first oxidation oven has four Nicolet750 Magna-IR. The titre (the linear density of different temperature zones, and the second oxidation oven fibers, its unit has denier, tex, and dtex, e.g., 1 dtex equals has five different temperature zones. In the first two hot-air to the grams of a 10 000-m long filament) of a filament circulation preoxidation furnaces, a precursor fiber was was measured by a XD-1 fiber fineness machine, and thermal stabilized in a purified air atmosphere at 125– mechanical properties of PAN precursors, preoxidized 283 8C under a 10% stretching ratio, the total preoxidation fibers and carbon fibers were measured by a XQ-1 tensiletime was about 50 min before forming a preoxidized fiber. testing machine (both XD-1 and XQ-1 were made in Subsequently, this preoxidized fiber was subjected to a Donghua University, Shanghai, China) at a crosshead pre-carbonization in an atmosphere of pure nitrogen from speed of 0.5 mm/min with a testing length of 20 mm and 428 to 803 8C with a fixed length, then a carbonization also load cell of 10 g. In each case, at least 30 sample filaments in oxygen-free nitrogen from 1003 to 1350 8C under a 1% were tested, and the average of 30 filaments was taken for shrinkage to get carbon fibers. So the processing speed of each experiment. T able 1 Some quality indices of selected PAN precursors Precursor Titre Density Tensile Elongation Crystallinity Composition 23 (dtex) (g cm ) strength (%) (%) (wt.%) (MPa) P1 1.14 1.07 633.6 10.8 52 AN/IA(97.5/2.5) P2 1.63 1.08 533.5 11.7 55 AN/AM(97.5/2.5) a P3 1.24 1.17 578.5 10.6 87 AM-containing a Deduced from the result of this study
Z. anxi et al. / Carbon 41(2003)2805-2812 2807 3. Results and discussion exothermic regime of both lA-containing and AM-con- taining pan precursors is much broader and the cycliza- 3. I. Effect of comonomers tion reaction starts at lower temperature for PI and p2 at 193.8 and 2029C, respectively, compared to that of pan The effect of copolymer composition has been studied homopolymer precursors(PO)at 2331C. It has also been by using different comonomers for PI and P2, which allow found that with the increase of content of not only IA but comparison of cyclization data for different precursors also AM in the pan precursors, the initiation temperature long as the reaction environment and temperature remain of cyclization reaction decreases, so the exothermic regime the same. It has been clearly recognized that the cycliza- becomes broader and broader. Although it is useful to tion of pan homopolymer initiates through a radical alleviate the abrupt exothermic reaction rate, more struc mechanism [5 which is faster than the cyclization of an ture flaws will be caused in the resultant carbon fibers and polymers with an ionic mechanism [5], so some will reduce their mechanical properties, which is a defect comonomers, such as IA, MA, AM, etc, have been for forming preferred sheet-like graphite structure in the copolymerized with an in order that the highly exother process of preoxidation and carbonization. As a result, a mic cyclization could be slowed which would avoid to lower and optimal amount of comonomers [6] should be some extent fusion and breaking of fibers owing to used for increasing the quality of the resultant carbon overheating due to the sharply exothermic reaction. But fibers different types of comonomers have notably different An interesting discovery is that both P2 and P3 have effects on the exothermic patterms of pan precursors. double separated DSC peaks, which makes us ask whether For comparison, the dsc is the cause of AM, and whether P3 contains AM, as does exothermic curve of PAN homopolymer precursors(named P2. However, the difference of DSC curves between P2 as PO) is also illustrated in Fig. la Fig. I also shows the and P3 is that p2 has a lower starting exothermal reaction DSC curves of the different Pan precursors tested. The temperature and a wider DSC exothermal peak from the 2807 2832 2331 100150200250300350400 50300350400 Temperaturejae) Temperaturejae 200250300350400 Fig. 1. The DSC curves of (a) P0, (b)Pl,(c) P2 and (d)P3 precursors
Z. Wangxi et al. / Carbon 41 (2003) 2805–2812 2807 3. Results and discussion exothermic regime of both IA-containing and AM-containing PAN precursors is much broader and the cycliza- 3 .1. Effect of comonomers tion reaction starts at lower temperature for P1 and P2 at 193.8 and 202.9 8C, respectively, compared to that of PAN The effect of copolymer composition has been studied homopolymer precursors (P0) at 233.1 8C. It has also been by using different comonomers for P1 and P2, which allow found that with the increase of content of not only IA but comparison of cyclization data for different precursors as also AM in the PAN precursors, the initiation temperature long as the reaction environment and temperature remain of cyclization reaction decreases, so the exothermic regime the same. It has been clearly recognized that the cycliza- becomes broader and broader. Although it is useful to tion of PAN homopolymer initiates through a radical alleviate the abrupt exothermic reaction rate, more strucmechanism [5] which is faster than the cyclization of AN ture flaws will be caused in the resultant carbon fibers and copolymers with an ionic mechanism [5], so some will reduce their mechanical properties, which is a defect comonomers, such as IA, MA, AM, etc., have been for forming preferred sheet-like graphite structure in the copolymerized with AN in order that the highly exother- process of preoxidation and carbonization. As a result, a mic cyclization could be slowed which would avoid to lower and optimal amount of comonomers [6] should be some extent fusion and breaking of fibers owing to used for increasing the quality of the resultant carbon overheating due to the sharply exothermic reaction. But fibers. different types of comonomers have notably different An interesting discovery is that both P2 and P3 have effects on the exothermic patterns of PAN precursors. double separated DSC peaks, which makes us ask whether These can be seen in Fig. 1. For comparison, the DSC is the cause of AM, and whether P3 contains AM, as does exothermic curve of PAN homopolymer precursors (named P2. However, the difference of DSC curves between P2 as P0) is also illustrated in Fig. 1a. Fig. 1 also shows the and P3 is that P2 has a lower starting exothermal reaction DSC curves of the different PAN precursors tested. The temperature and a wider DSC exothermal peak from the Fig. 1. The DSC curves of (a) P0, (b) P1, (c) P2 and (d) P3 precursors
2808 Z Wangxi et al. /Carbon 41(2003)2805-2812 start at 2029C to the end at 3019C with two peak values at 238.8 and 2651C, respectively, compared to P3 in the range of 231. 1-2940"C with two peak values at 256.9 and 271. 1C, respective herefore, assumin their comonomer contents and distributions are similar and even the presence of a little of other comonomer, for g example, methyl acrylate, in P3, the content of comonom 8104 ers in P3 may be less than that in p2 80 For IA-containing PI precursor, there is only one exothermic peak from 193.8 to 3213C with a single peak value at 283. 2C, but for AM-containing p2, there are two separated peaks, which is similar to the exothermic peak of the commercial one(P3) from Japan. Fig. 2 is the typical FT-IR spectra of three different PAn precursors. The 4000350030002500200015001000500 vibrations characteristic of pan structure are those of cN nitrile group at ca. 2243-2241 cm, and the bands in the regions2931-2870,1460-1450,1380-1350,and1270 Fig. 3. The FT-IR spectra of p2 precursors after different heat 20 cm are assigned to the aliphatic CH group treatment temperature at(1)125.C,(2)214oC,(3)231C,(4) vibrations of different modes in CH, CH2, and CH: [7 253°C,and(5)283℃ In aIr,and(6)1003℃and(7)1350℃ The strong band at 1732 cm as shown in Fig. 2a is presented in the Pl pan precursors, and is attributed to the C=O stretching due to the presence of ester or AM-containing P2 PAN precursors, as has been shown in been reported by Bajaj et al. [7 and by Gupta et al. [5] Fig. 2b, a characteristic absorbance peak in 1684 cm is This appearance may also be assigned to the formation of displayed due to amide group, which corresponds to the amino-substituted unstaturated nitrile [9]and iminonitriles peak at 1685 cm for the P3 precursor. This can further [10 as a result of partial cyclization or thermal degra- illustrate our previous deduction that P3 is an AM-con- dation. Certainly, there are other subtle differences be- taining precursor. In the end, P3 precursors are AM- tween the FT-IR spectra of samples P2 and P3, containing ones which can be deduced according to the example, a shift of some similar bands, which could be following three points: (1)a two separated DSC peak attributed to some differences of contents, compositions similar to P2; (2)both of p2 and P3 have amide group by and distributions between P2 and P3 the measurement of FT-IR; (3)some public examples in the patents of Mitsubishi Rayon [2, 8]. But, these three 3. 2. Structural changes conclusions are highly speculative. In addition, a shoulder like appearance at 2191 cm near the characteristic The FT-IR spectra were measured at several points absorption for CN (at 2243 cm )in P2 precursors, as the following different heat treatment temperature for P2 and arrow indicated in Fig. 2b, may be attributed to the P3. The data were plotted in Figs. 3 and 4 to get presence of enaminonitrile as a molecular defect formed information for the structural changes related to the during polymerization which should be avoided, as has thermal history of the process of preoxidation and carboni- 4000350030002500200015001000500 4000350030002500200015001000500 Wavenumber(cm) Wavenumber(cm) Fig. 2. The FT-IR spectra of (a)PI and P3, and(b)P2 PAN precursors
2808 Z. Wangxi et al. / Carbon 41 (2003) 2805–2812 start at 202.9 8C to the end at 301.9 8C with two peak values at 238.8 and 265.1 8C, respectively, compared to P3 in the range of 231.1–294.0 8C with two peak values at 256.9 and 271.1 8C, respectively. Therefore, assuming that their comonomer contents and distributions are similar, and even the presence of a little of other comonomer, for example, methyl acrylate, in P3, the content of comonomers in P3 may be less than that in P2. For IA-containing P1 precursor, there is only one exothermic peak from 193.8 to 321.3 8C with a single peak value at 283.2 8C, but for AM-containing P2, there are two separated peaks, which is similar to the exothermic peak of the commercial one (P3) from Japan. Fig. 2 is the typical FT-IR spectra of three different PAN precursors. The vibrations characteristic of PAN structure are those of CN 21 nitrile group at ca. 2243–2241 cm , and the bands in the regions 2931–2870, 1460–1450, 1380–1350, and 1270– 21 Fig. 3. The FT-IR spectra of P2 precursors after different heat 1220 cm are assigned to the aliphatic CH group treatment temperature at (1) 125 8C, (2) 214 8C, (3) 231 8C, (4) vibrations of different modes in CH, CH , and CH [7]. 2 3 253 8C, and (5) 283 8C in air, and (6) 1003 8C and (7) 1350 8C in 21 The strong band at 1732 cm as shown in Fig. 2a is N . 2 presented in the P1 PAN precursors, and is attributed to the C=O stretching due to the presence of ester or acid. For AM-containing P2 PAN precursors, as has been shown in been reported by Bajaj et al. [7] and by Gupta et al. [5]. 21 Fig. 2b, a characteristic absorbance peak in 1684 cm is This appearance may also be assigned to the formation of displayed due to amide group, which corresponds to the amino-substituted unstaturated nitrile [9]and iminonitriles 21 peak at 1685 cm for the P3 precursor. This can further [10] as a result of partial cyclization or thermal degraillustrate our previous deduction that P3 is an AM-con- dation. Certainly, there are other subtle differences betaining precursor. In the end, P3 precursors are AM- tween the FT-IR spectra of samples P2 and P3, for containing ones which can be deduced according to the example, a shift of some similar bands, which could be following three points: (1) a two separated DSC peak attributed to some differences of contents, compositions similar to P2; (2) both of P2 and P3 have amide group by and distributions between P2 and P3. the measurement of FT-IR; (3) some public examples in the patents of Mitsubishi Rayon [2,8]. But, these three 3 .2. Structural changes conclusions are highly speculative. In addition, a shoulder- 21 like appearance at 2191 cm near the characteristic The FT-IR spectra were measured at several points 21 absorption for CN (at 2243 cm ) in P2 precursors, as the following different heat treatment temperature for P2 and arrow indicated in Fig. 2b, may be attributed to the P3. The data were plotted in Figs. 3 and 4 to get presence of enaminonitrile as a molecular defect formed information for the structural changes related to the during polymerization which should be avoided, as has thermal history of the process of preoxidation and carboniFig. 2. The FT-IR spectra of (a) P1 and P3, and (b) P2 PAN precursors
Z. anxi et al. / Carbon 41(2003)2805-2812 2809 to the highly absorbing nature of black carbon fibers 3.3. Changes in precursor density and solubility The variation of densities and solubility of pan pre- 810 cursors after preoxidation and carbonization was shown Fig. 5. During the preoxidation treatment, PAN precursors underwent a series of physical and chemical changes which transformed the original linear polymer structure to a partially cyclized ladder structure, as was demonstrated in Fig 3 and Fig. 4. The results make the abrupt decrease, 4000 3000 2500 20001500 1000 500 especially at the range of 200-280C, of solubility of soluble pan precursors. These become insoluble in DMSO at 80C after heat treatment at 283C. which is an Wavenumber(cm) indication of the extent of preoxidation reaction The increase of density in the beginning induction 25℃m(22(1r()四 lou is very gradual, when PAN precursors mainly Fig. 4. The FT-IR spectra of P3 precursors after different heat undergo physical transformation, e.g., the morphological N structure rearrangements. High and adequate stretching is usually imposed in this period in the preoxidation proce so as to induce additional orientation and order in the fibers. For any one of Pl, P2 and P3 precursors, as a result zation. The most prominent structural changes were the of consolidation and densification occurring within the decrease in the intensities of the 2243-2241 cm, attribu- fibers, the density increases rapidly in the range of 210- ted to C=N band, and the decrease of those for aliphatic 285C in air, then increases monotonously to an extreme C-H ones and the decrease of the 1684 or 1685 cm value. then after the maximum there slight drop amide band for P2 and P3 precursors, respectively, con- because of the conversion of open pores to closed pore omitant with the advent and increase of a shoulder-like [13 when temperature rises over 1000C peak in 1700 cm(due to cyclic C=O), the band at 1590 Comparing the data on the density changes of different cm(due to C=N,C=C, N-H mixed), and the band in 810 Pl, P2 and P3 precursors after heat treatment in Fig. 5,it cm(due to C=C-H)[11]. These spectroscopic results can be observed that the density of P3 is higher than that have shown that some chemical processes occurred in the of PI or P2. This may be result from the difference of tages of preoxidation. Firstly, reaction of nitriles results in aggregation morphology between their original precursors, conjugated C=N containing structures which result from as have been listed in Table 1. The original P3 precursors intramolecular cyclization or intermolecular crosslinking. have the highest tensile strength and elongation possibly Secondly, the generation of conjugated C=C structures esults from dehydrogenation or from imine-enamine tautomerization and subsequent isomerization [12]. Third- ly, oxidation gives rise to carbonyl groups. It has also been noted that an unwanted peak at ca. 2330 cm is present 18 because of the effect of CO, in some instrumental con- ditions s. From the spectral changes, it was also shown that the bers p2 compared to P3. In the process of preoxidation, the color of fibers P2 also became yellow at a lower temperature and P3 started to become from white to yellow at a higher temperature. These were in agreement with DSC analysis results, as demonstrated in Fig. 1. As a result, the P3 precursors were thermally more stable than the p2 ones 0012001400 Ithough FT-IR showed direct evidence of the changes T ature(iae) lized. not mud the carbon fibers due mainly to the difficulty to record a to 283C is in air, the pre-carbonization and carbonization from good quality spectrum with conventional techniques owing 428 to 1350C are in N,)
Z. Wangxi et al. / Carbon 41 (2003) 2805–2812 2809 to the highly absorbing nature of black carbon fibers samples. 3 .3. Changes in precursor density and solubility The variation of densities and solubility of PAN precursors after preoxidation and carbonization was shown in Fig. 5. During the preoxidation treatment, PAN precursors underwent a series of physical and chemical changes which transformed the original linear polymer structure to a partially cyclized ladder structure, as was demonstrated in Fig. 3 and Fig. 4. The results make the abrupt decrease, especially at the range of 200–280 8C, of solubility of soluble PAN precursors. These become insoluble in DMSO at 80 8C after heat treatment at 283 8C, which is an indication of the extent of preoxidation reaction. The increase of density in the beginning induction Fig. 4. The FT-IR spectra of P3 precursors after different heat period is very gradual, when PAN precursors mainly treatment temperature at (1) 125 8C, (2) 214 8C, (3) 231 8C, (4) undergo physical transformation, e.g., the morphological 253 8C and (5) 283 8C in air, and (6)1003 8C and (7) 1350 8C in N . structure rearrangements. High and adequate stretching is 2 usually imposed in this period in the preoxidation process so as to induce additional orientation and order in the fibers. For any one of P1, P2 and P3 precursors, as a result zation. The most prominent structural changes were the of consolidation and densification occurring within the 21 decrease in the intensities of the 2243–2241 cm , attribu- fibers, the density increases rapidly in the range of 210– ted to C;N band, and the decrease of those for aliphatic 285 8C in air, then increases monotonously to an extreme 21 C–H ones and the decrease of the 1684 or 1685 cm value, then after the maximum there is a slight drop amide band for P2 and P3 precursors, respectively, con- because of the conversion of open pores to closed pores comitant with the advent and increase of a shoulder-like [13] when temperature rises over 1000 8C. 21 peak in 1700 cm (due to cyclic C=O), the band at 1590 Comparing the data on the density changes of different 21 cm (due to C=N,C=C,N–H mixed), and the band in 810 P1, P2 and P3 precursors after heat treatment in Fig. 5, it 21 cm (due to C=C–H) [11]. These spectroscopic results can be observed that the density of P3 is higher than that have shown that some chemical processes occurred in the of P1 or P2. This may be result from the difference of stages of preoxidation. Firstly, reaction of nitriles results in aggregation morphology between their original precursors, conjugated C=N containing structures which result from as have been listed in Table 1. The original P3 precursors intramolecular cyclization or intermolecular crosslinking. have the highest tensile strength and elongation possibly as Secondly, the generation of conjugated C=C structures results from dehydrogenation or from imine-enamine tautomerization and subsequent isomerization [12]. Thirdly, oxidation gives rise to carbonyl groups. It has also been 21 noted that an unwanted peak at ca. 2330 cm is present because of the effect of CO in some instrumental con- 2 ditions. From the spectral changes, it was also shown that the fibers P2 started cyclization at a lower temperature of compared to P3. In the process of preoxidation, the color of fibers P2 also became yellow at a lower temperature and P3 started to become from white to yellow at a higher temperature. These were in agreement with DSC analysis results, as demonstrated in Fig. 1. As a result, the P3 precursors were thermally more stable than the P2 ones. Although FT-IR showed direct evidence of the changes taking place in the chemistry of preoxidized fibers, once Fig. 5. The changes of density and solubility versus heat treatcarbonized, not much structural information is available on ment temperature for PAN precursors (The preoxidation from 125 the carbon fibers due mainly to the difficulty to record a to 283 8C is in air, the pre-carbonization and carbonization from good quality spectrum with conventional techniques owing 428 to 1350 8C are in N .) 2
Z. anxi et al. / Carbon 41(2003)2805-2812 39 Fig. 6. The crosswise SEM photos of (a) P3 and(b) P2 precursors 113440c toure (a) (c) Fig. 7. The DSC curves in argon of (a) P2 precursors, (b) P3 precursors, (c) preoxidized fibers from P2 after 283C, (d) precarbonized fibers mP2 after803°C
2810 Z. Wangxi et al. / Carbon 41 (2003) 2805–2812 Fig. 6. The crosswise SEM photos of (a) P3 and (b) P2 precursors. Fig. 7. The DSC curves in argon of (a) P2 precursors, (b) P3 precursors, (c) preoxidized fibers from P2 after 283 8C, (d) precarbonized fibers from P2 after 803 8C
Z. anxi et al. / Carbon 41(2003)2805-2812 2811 a result of their higher density and higher crystallinity. The with a peak at 337.2C, which indicates that the interlink cross-sectional SEM photographs of P3 and P2 are illus- ing molecular cyclization of preoxidized fibers is not fully trated in Fig. 6. It can be clearly seen that it is dense for P3 accomplished. Hence, a precarbonization process is re- precursors, whereas P2 precursors have some voids arising quired to further perform enough intermolecular cycliza- from coagulation process of spinning which cause a tion, so the precarbonized fibers do not show the same decreased density. Furthermore, the strength, modulus and exothermic reaction as that the corresponding preoxidized elongation at break of fibers depend largely on the number fibers from identical original PAN precursors and size of present voids which have substantial influence In Fig. 7, it can also be seen that there is a common on the development of the structure, through the whole endothermic reaction at ca. 1344-1350C for all pre process from original precurso on fibers. cursors, preoxidized fibers, and precarbonized fibers How ever, there are some differences between the plots of p2 3.4. Changes in precursor thermal properties and P3 precursors, as shown in Fig. 7a, b. Only the P precursors have a weak exotherm at 454.6"C, which is not The DSC curves obtained upon heating the p2 or P3 clearly discernible in P2 precursors. P2 precursors have a and the fibers from original P2 after preoxida- stronger exothermic reaction at 10519C compared that of tion or precarbonization investigated in argon with P3 ones at 1052.7C. The characteristic difference is that heating rate of 20C/min in the range of 25-1400C are P3 precursors have a strong endotherm at 1093.9C which given in Fig. 7. When the pan precursors are preoxidized may be an important factor to facilitate dealing with the in air, more functional groups (.g, C=O) form as a result arborization process. a possible reason is that this of the incorporation of oxygen and penetration from the endothermic reaction may alleviate the breakage of a surface to inner part of the fibers with increasing tempera- filament due to denitrogenation during the final carboniza- ture. This will improve the hygroscopicity of preoxidized tion stage. This can also be regarded as a new discovery, the temperature of 80-120.C is related to the release of temperatures up to -500C in the previous public litera water as shown in Fig. 7c, d. The preoxidized fibers have ure. Although the absence of experimental artifacts has still an exotherm in the temperature regime of 300-400C bee I by running blank analyses Table 2 The properties of PAN precursors, preoxidized and carbon fibers with the change of temperature P2 Tensile Elongation Titre Titre Tensile dex) strength (dtex) strength rength (% (MPa) ( MPa) 633.6 10.8 11.7 78.5 633.7 33.611.7 578.5 1.1 19 84.9 122 596.4 11.3 544.1 413.7 11.6 11.8 500.2 12.1 l12 491.6 419.0 11.3 13.3 1.24 484.6 116 13.0 1.24 525.8 11.8 355.7 13. 87.7 12 486.1 11.9 13.0 316 18 428-603 1.01 0.831189.11.2 0.85 1373.0 803-1003 0.773507.1 39894 Carbon ccording to a patent [2], the tensile strength of a carbon fiber is in the range of 4040-4850 MPa, but its titre and elongation are not
Z. Wangxi et al. / Carbon 41 (2003) 2805–2812 2811 a result of their higher density and higher crystallinity. The with a peak at 337.2 8C, which indicates that the interlinkcross-sectional SEM photographs of P3 and P2 are illus- ing molecular cyclization of preoxidized fibers is not fully trated in Fig. 6. It can be clearly seen that it is dense for P3 accomplished. Hence, a precarbonization process is reprecursors, whereas P2 precursors have some voids arising quired to further perform enough intermolecular cyclizafrom coagulation process of spinning which cause a tion, so the precarbonized fibers do not show the same decreased density. Furthermore, the strength, modulus and exothermic reaction as that the corresponding preoxidized elongation at break of fibers depend largely on the number fibers from identical original PAN precursors. and size of present voids which have substantial influence In Fig. 7, it can also be seen that there is a common on the development of the structure, through the whole endothermic reaction at ca. 1344–1350 8C for all preprocess from original precursors to resultant carbon fibers. cursors, preoxidized fibers, and precarbonized fibers. However, there are some differences between the plots of P2 3 .4. Changes in precursor thermal properties and P3 precursors, as shown in Fig. 7a,b. Only the P3 precursors have a weak exotherm at 454.6 8C, which is not The DSC curves obtained upon heating the P2 or P3 clearly discernible in P2 precursors. P2 precursors have a precursors, and the fibers from original P2 after preoxida- stronger exothermic reaction at 1051.9 8C compared that of tion or precarbonization investigated in argon with a P3 ones at 1052.7 8C. The characteristic difference is that heating rate of 20 8C/min in the range of 25–1400 8C are P3 precursors have a strong endotherm at 1093.9 8C which given in Fig. 7. When the PAN precursors are preoxidized may be an important factor to facilitate dealing with the in air, more functional groups (e.g., C=O) form as a result carbonization process. A possible reason is that this of the incorporation of oxygen and penetration from the endothermic reaction may alleviate the breakage of a surface to inner part of the fibers with increasing tempera- filament due to denitrogenation during the final carbonizature. This will improve the hygroscopicity of preoxidized tion stage. This can also be regarded as a new discovery, and/or precarbonized fibers. Therefore, the endotherm at because DSC curves have been generally reported at the temperature of 80–120 8C is related to the release of temperatures up to –500 8C in the previous public literawater as shown in Fig. 7c,d. The preoxidized fibers have ture. Although the absence of experimental artifacts has still an exotherm in the temperature regime of 300–400 8C been confirmed by running blank analyses for comparison T able 2 The properties of PAN precursors, preoxidized and carbon fibers with the change of temperature Temperature P1 P2 P3 (8C) Titre Tensile Elongation Titre Tensile Titre Tensile Elongation (dex) strength (%) (dtex) strength Elongation (dtex) strength (%) (MPa) (MPa) (%) (MPa) 25 1.14 633.6 10.8 1.63 533.5 11.7 1.24 578.5 10.6 (Precursors) 125 1.17 633.7 10.8 1.61 533.6 11.7 1.24 578.5 10.6 192 1.13 561.9 10.8 1.59 584.9 12.2 1.24 596.4 11.2 202 1.14 567.8 11.0 1.42 565.5 11.9 1.25 581.3 11.3 214 1.09 544.1 11.4 1.61 413.7 11.6 1.26 682.2 11.8 222 1.09 500.2 10.7 1.60 409.7 12.1 1.26 573.8 11.2 231 1.07 491.6 11.4 1.48 419.0 11.3 1.19 602.9 11.4 240 1.18 431.4 11.6 1.52 379.9 13.3 1.24 484.6 11.6 253 1.11 400.1 12.1 1.46 323.0 13.0 1.24 525.8 11.8 268 1.06 355.7 13.5 1.39 287.7 12.6 1.20 486.1 11.9 277 1.02 342.3 15.7 1.35 247.6 10.7 1.18 410.4 13.0 283 1.06 231.6 13.6 1.39 246.5 12.0 1.18 316.4 11.5 (Preoxidized fibers) 428–603– 1.01 769.2 0.7 0.83 1189.1 1.2 0.85 1373.0 1.5 803–1003 a 1350 0.87 2557.5 0.5 0.77 3507.1 0.8 0.83 3989.4 1.1 (Carbon fibers) a According to a patent [2], the tensile strength of a carbon fiber is in the range of 4040–4850 MPa, but its titre and elongation are not disclosed
using a Z. anxi et al. / Carbon 41(2003)2805-2812 a special grade of 99.9999 wt. argon as purge gas, and adequate strength, higher crystallinity and pre- whether some other experimental factors may complicate ferred morphology with as few voids and flaws as the high temperature dsc data need to be further studied possible 3.5. Evolution of mechanical properties Acknowledgements The change of properties of pan precursors, preoxid- ized fibers and carbon fibers with heat treatment tempera- We acknowledge the National 863 Project and the ture was tabulated in table 2. It was shown that the tensile National Natural Science Commission for providing finan- strength decreased with the increase of temperature in the cial support preoxidizing process. However, once carbonized, the ten- sile strength of carbon fibers had an abrupt increase and the elongation had an abrupt decrease. Carbon fibers P3 had the best tensile strength, P2 was the better, and the p1 References lad the lowest tensile strength. In case of directing fibers [1] Bajaj P, Streekumar TV, Sen K. Structure developmen PI to pass through the final high carbonizing furnace from during dry-jet-wet spinning of acrylonitrile/vinyl acids and 1003 to 1350C, it was very difficult to deal with because the fibers tow was broken continually. On the contrary, acrylonitrilemethyl acrylate copolymers. J Appl Polym Sci fibers P3 and P2 were easily passed through the final higl 2] Mitsubishi Rayon Co, Ltd. Acrylonitrile-based precursor carbonizing furnace. In addition, the fact that the tensile fiber for carbon fiber and method for production thereof. EP strength of carbon fibers p3 was better than that of carbon l130140A1,2001-05-09 fibers P2, could be the result of different preparation 3] wilkinson K Process and product of acrylonitrile copolymer process of the precursors, such as spinning, drawing, and so on, which cause differences in structure and propertie [4] Wilkinson K. Process for the preparation of carbon fiber. US such as differences in density, morphology, and porosity 6054214,2000-4-25 5] Gupta AK, Paliwal DK, Bajaj P. Acrylic precursors for carbon fibers (MS Review). Macromol Chem Phys 4. Conclusion [6]Chand S Review carbon fibers for composites. J Mater Sci 2000;35:1303-13 In order to obtain high performance PAN-based carbon [7 Bajaj P, Paliwal DK, Gupta AK. Acrylonitrile-acrylic acids fibers, the combination of both physical mechanical prop- copolymers, synthesis and characterization. J Appl Polym erties and chemical composition should be optimized sci1993:49823-33 Modifying a given property of a precursor at the expense [8]Mitsubishi Rayon Co, Ltd. Acrylonitrile-based precursor of other property indexes is not an optimal way to prepare fiber for carbon fiber and method for production thereof, CN carbon fibers. The following conclusion can be drawn 1271396A,2000-10-25. [9]Boccara AC, Fouruier D, Kumar A, Pandey GC. Nondestruc tive evaluation of carbon fiber by mirage- FTIR spectroscopy (1)It is uncertain that high strength PAn precursors are J Appl Polym Sci 1997: 63: 1785-91 essential in order to obtain high performance carbon [10] Usami T, Itoh T, Ohtani H, Tsuge S. Structural study of fibers, because the composition and morphology of polyacrylonitrile fibers during oxidative thermal degradation the precursors also play a very important role in the by pyrolysis-gas chromatography, solid-state C NMR and processes of preoxidation, precarbonization and car- FT-IR. Macromolecules 1990- 23- 2460-1 conization [1] Hideto K, Kohji T. Mechanism and kinetics of stabilization (2) For AM-containing precursors, AM appears to be reactions of PAn and related copolymers. Polym J more effective in separating the exothe 199729(7:557-62 corresponding to preoxidation stages in DSC curves, [12] Dalton S, Heatley F, Budd PM. Thermal stabilization of polyacrylonitrile fibers. Polymer 1999, 40: 5531-4 compared to IA, for lA-containing precursors. [13]Ko T-H. The influence of pyrolysis on physical properties (3)Except for ideal chemical composition, the optim and microstructure of modified PAN fibers during carbonize Pan precursors should have higher density, higher tion. J Appl Polym Sci 1991: 43: 589-600
2812 Z. Wangxi et al. / Carbon 41 (2003) 2805–2812 using a special grade of 99.9999 wt.% argon as purge gas, and adequate strength, higher crystallinity and prewhether some other experimental factors may complicate ferred morphology with as few voids and flaws as the high temperature DSC data need to be further studied. possible. 3 .5. Evolution of mechanical properties Acknowledgements The change of properties of PAN precursors, preoxidized fibers and carbon fibers with heat treatment tempera- We acknowledge the National 863 Project and the ture was tabulated in Table 2. It was shown that the tensile National Natural Science Commission for providing finanstrength decreased with the increase of temperature in the cial support. preoxidizing process. However, once carbonized, the tensile strength of carbon fibers had an abrupt increase and the elongation had an abrupt decrease. Carbon fibers P3 References had the best tensile strength, P2 was the better, and the P1 had the lowest tensile strength. In case of directing fibers [1] B ajaj P, Streekumar TV, Sen K. Structure development P1 to pass through the final high carbonizing furnace from during dry-jet-wet spinning of acrylonitrile/vinyl acids and 1003 to 1350 8C, it was very difficult to deal with because acrylonitrile/methyl acrylate copolymers. J Appl Polym Sci the fibers tow was broken continually. On the contrary, 2002;86:773–87. fibers P3 and P2 were easily passed through the final high [2] M itsubishi Rayon Co., Ltd. Acrylonitrile-based precursor carbonizing furnace. In addition, the fact that the tensile fiber for carbon fiber and method for production thereof. EP 1130140A1,2001-05-09. strength of carbon fibers P3 was better than that of carbon [3] W ilkinson K. Process and product of acrylonitrile copolymer. fibers P2, could be the result of different preparation WO 96/39552,1996-12-12. process of the precursors, such as spinning, drawing, and [4] W ilkinson K. Process for the preparation of carbon fiber. US so on, which cause differences in structure and properties, 6054214,2000-4-25. such as differences in density, morphology, and porosity. [5] G upta AK, Paliwal DK, Bajaj P. Acrylic precursors for carbon fibers (JMS Review). Macromol Chem Phys 1991;C31(1):1–89. 4. Conclusion [6] C hand S. Review carbon fibers for composites. J Mater Sci 2000;35:1303–13. In order to obtain high performance PAN-based carbon [7] B ajaj P, Paliwal DK, Gupta AK. Acrylonitrile-acrylic acids fibers, the combination of both physical mechanical prop- copolymers, synthesis and characterization. J Appl Polym Sci 1993;49:823–33. erties and chemical composition should be optimized. [8] M itsubishi Rayon Co., Ltd. Acrylonitrile-based precursor Modifying a given property of a precursor at the expense fiber for carbon fiber and method for production thereof. CN of other property indexes is not an optimal way to prepare 1271396A,2000-10-25. carbon fibers. The following conclusion can be drawn: [9] B occara AC, Fouruier D, Kumar A, Pandey GC. Nondestructive evaluation of carbon fiber by mirage-FTIR spectroscopy. (1) It is uncertain that high strength PAN precursors are J Appl Polym Sci 1997;63:1785–91. essential in order to obtain high performance carbon [10] U sami T, Itoh T, Ohtani H, Tsuge S. Structural study of fibers, because the composition and morphology of polyacrylonitrile fibers during oxidative thermal degradation 13 by pyrolysis-gas chromatography, solid-state C NMR and the precursors also play a very important role in the FT-IR. Macromolecules 1990;23:2460–5. processes of preoxidation, precarbonization and car- [11] H ideto K, Kohji T. Mechanism and kinetics of stabilization bonization. reactions of PAN and related copolymers. Polym J (2) For AM-containing precursors, AM appears to be 1997;29(7):557–62. more effective in separating the exothermic reactions [12] D alton S, Heatley F, Budd PM. Thermal stabilization of corresponding to preoxidation stages in DSC curves, polyacrylonitrile fibers. Polymer 1999;40:5531–43. compared to IA, for IA-containing precursors. [13] K o T-H. The influence of pyrolysis on physical properties (3) Except for ideal chemical composition, the optimal and microstructure of modified PAN fibers during carbonizaPAN precursors should have higher density, higher tion. J Appl Polym Sci 1991;43:589–600