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Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers
Accepted Manuscript Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers Juliane Meinl, Katrin Schönfeld, Martin Kirsten, Karsten Kittler, Alexander Michaelis, Chokri Cherif PII: DOI: Reference:
S1359-835X(17)30058-1 http://dx.doi.org/10.1016/j.compositesa.2017.02.010 JCOMA 4571
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
22 August 2016 25 January 2017 7 February 2017
Please cite this article as: Meinl, J., Schönfeld, K., Kirsten, M., Kittler, K., Michaelis, A., Cherif, C., Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa.2017.02.010
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Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers Authors: Juliane Meinla, Katrin Schönfelda, Martin Kirstenb, Karsten Kittlerc, Alexander Michaelisa, Chokri Cherifb a
Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Winterbergstraße 28, 01277 Dresden, Germany b Technische Universität Dresden / Institute of Textile Machinery and High Performance Material Technology, Breitscheidstr. 78, 01237, Dresden, Germany c P-D Glasseiden GmbH Oschatz, Wellerswalder Weg 17, 04758 Oschatz, Germany Juliane Meinl (Corresponding author) Fraunhofer Institute for Ceramic Technologies and Systems IKTS / Thermal Analysis and Thermal Physics Winterbergstraße 28 01277 Dresden, Germany Phone +49 351 2553-7236 Fax +49 351 2554-290 [email protected] Katrin Schoenfeld [email protected] Dr. Martin Kirsten [email protected] Dr. Karsten Kittler [email protected] Prof. Dr. rer. nat. habil. Alexander Michaelis [email protected] Univ.-Prof. Dr.-Ing. habil. Dipl.-Wirt. Ing. Chokri Cherif [email protected]
ABSTRACT The production of carbon fibers from polyacrylonitrile (PAN) includes a stabilization step before carbonization. Transferring this stabilization from the laboratory to a bigger scale cannot be performed without changes of the process parameters. Using a pure PAN polymer, fibers were spun and their stabilization as well as carbonization was studied in laboratory scale. The resulting materials were analyzed with FTIR, SEM and XRD. The stabilization step was then transferred to a pilot scale. Limitations given by the production line had to be taken into account to develop a suitable temperature treatment. The formation of a core/shell structure during stabilization was observed and was detrimental to the properties of the fiber after carbonization. Adjustments were made based on obtained results to balance the negative effects of high heating rates and strong temperature gradients. With the implemented changes the mechanical properties could be reproduced on the larger production scale. KEYWORDS Carbon fibers (A), Fiber conversion processes (E), Scale-up, Thermal analysis (D) INTRODUCTION Carbon fibers offer a unique combination of light weight and mechanical stability, making them an ideal material for high performance applications. They are a key factor in today’s trend in transport and construction towards light weight design, energy efficiency and environmental awareness. The majority of carbon fibers (about 90%) is produced from polyacrylonitrile (PAN) [1]. Although a rising interest in the use of alternative precursors can be observed, PAN will remain the main carbon fiber precursor in the near future. The transformation of PAN
fibers to carbon fibers is realized by a thermal treatment. It is essentially a two-step process, consisting of a stabilization step in air at temperatures up to 400°C and a subsequent carbonization step at high temperatures (1200°C and higher) in inert atmosphere. The influences on the properties of the final fibers are manifold, including material composition, spinning conditions, post-spinning treatment, stabilization and carbonization conditions and further modifications after carbonization [2–4]. Each step will have a significant impact on the product quality. A thorough comprehension of these influences facilitates the production towards tailor-made fiber materials. In recent publications, we investigated the structure-property relationships during carbon fiber production focusing on the spinning conditions [5] and the influence of stretching during the stabilization [6]. The wet-spinning process of PAN uses solvents such as dimethyl formamide, dimethyl acetamide or dimethyl sulfoxide. The polymer solution is filtered and degassed until a homogenous spin dope is obtained. The spinning solution is metered through spinning nozzles directly into a coagulation bath. Washing and drawing of the fiber follows immediately, removing the residues of solvent. The quality of the precursor and thus of the carbon fibers is highly influenced by the properties of the process parameters of spinning [3] and post-drawing [7–10]. The stabilization is crucial to the process in the way that the PAN-structure undergoes several changes that render it suitable for further carbonization [2,11]. The main reactions during the stabilization process are cyclization, dehydrogenation and oxidation. As a result, the stabilized PAN shows a complex cyclized, partially aromatic structure containing oxygen functional groups (Figure 1Error! Reference source not found.). The material becomes infusible and is an optimal precursor for the
carbonization to yield a graphitic structure. The reactions during stabilization are very sensitive to the temperature treatment and the process conditions have to be controlled carefully to prevent thermal runaway reactions. The cyclization process is exothermal and local overheating of the fibers can accelerate side reactions, which lower the carbon yield or may damage the molecular structure. The stabilization process therefore demands slow heating rates. This renders the process energy- and time-consuming. From an economical point of view, there is a strong interest to optimize the temperature treatment. Furthermore, depending on the targeted properties of the resulting carbon fiber, compromises between shorter processing time and higher fiber performance can be made. Studies dealing with different analytical aspects of the PAN stabilization are mainly performed in a laboratory scale. On the smallest scale, the stabilization and carbonization is studied via thermal analysis, using very small amounts of material. At this scale, there already is an influence of the sample preparation on the reaction process [12]. To produce larger samples in the laboratory, the stabilization is done either in a batch mode or by a continuous process, where the fiber is respectively placed in or pulled through a small furnace (e.g. a tube furnace). The conditions can be varied over a wide range in these small setups and parameters which are unlikely to be applicable at a larger scale can be used, for example very low winding speeds, constant tension or length and complex heating programs. In an industrial size process, the stabilization is realized in large furnaces, in which the fiber passes through different heating zones [4,11]. The temperature is slowly raised in each heating zone and the fibers are pulled multiple times through one temperature zone, to extend the duration of treatment at a specific temperature. The fibers are
turned over rollers outside of the oven, which is an additional possibility to control the reactions by cooling down the fiber every few minutes. The optimum temperature treatment varies depending on the material used, thus the analytical step of characterizing the behavior of the material during stabilization cannot be skipped. And with the complexity of the effects that influence the reactions, the scale-up to a pilot plant remains critical for a successful translation to the industrial processing. But the process of scaling up the stabilization from the laboratory to a bigger processing scale is not documented in the scientific literature. In this work, we will focus on translating the stabilization from a laboratory setup to a pilot scale. Continuing our previous work, the PAN used is a homopolymer. This is contrary to industrial standards, where copolymers and additives are used to improve the stabilization process by lowering the activation energy or changing the exothermal behavior [2]. By focusing on the PAN homopolymer, we aimed for an academic approach gaining a deeper understanding of the basic stabilization process and fiber formation mechanisms. The scale-up demands for modifications in the temperature program. The influence of these modifications on the reaction progress and the fiber quality is monitored and an optimization is realized. EXPERIMENTAL The fibers were spun on a wet-spinning pilot plant by Fourné Polymertechnik GmbH, Germany consisting of a coagulation-bath, washing baths, drying and sizing-unit and a wind-up unit with an overall speed up to 90 m/min. The spinning dope solution was prepared using PAN (Mw 200.000 g/mol) in dimethylformamide (DMF). Spinning nozzles with 1000 holes, each with a diameter of 120 µm were used. The post-drawing
was performed on a pilot plant scale post-drawing unit from DIENES Apparatebau GmbH, Germany, to improve the mechanical and structural properties of the PAN precursor. For detailed Information see references [13,5]. Table 1 shows parameters and results of the wet spinning and post drawing experiments. DMF was used as received from Overlack GmbH. The oxidation on a laboratory scale was performed in a Carbolite CTF tube furnace through which the fiber bundle was pulled by a Mikrosam MAW20-LS1 winding machine, allowing for a constant fiber tension during the process. The furnace was open and air passed through the tube while the fiber bundle was continuously running with a defined speed. The pilot scale oxidation was carried out on a modified plant by the Preiss-Daimler Glasseiden GmbH Oschatz (P-D GSO). The plant consisted of eight units each 2 meters long. Seven of the units were heatable, each temperature could be adjusted separately. The last unit was used as cooling zone. Through a system of external pulleys the filaments could pass the heating zones several times to enable a better oxidation. The winding speed was adjustable from 1 to 6 m/min. The carbonization was performed under static conditions with a TT2 furnace (Thermal technology GmbH) with graphite heater and crucible in nitrogen atmosphere. IR spectra of the fibers were obtained by using the Attenuated Total Reflection method on a Bruker Tensor 27 FTIR spectrometer. A total number of 32 scans was taken and averaged from 400 to 4000 cm-1 with a resolution of 4 cm-1 for each spectrum.
Thermal Gravimetric Analysis (TGA) was carried out on a STA 449C (NETZSCHGerätebau GmbH) in air or nitrogen with a flow rate of 5 l/h and different temperature programs. Tensile properties of the PAN yarn samples were measured according to DIN EN ISO 2062 on a Z2.5 tensile tester (Zwick, Germany) with a 100 N load sensor. The gauge length was 250 mm and the extension rate was 250 mm/min. The tests were performed at 20 °C, 65 % relative humidity and carried out on 20 samples of each fiber quality, the standard deviation being generally less than 5 %. Tensile properties of the carbon fiber samples were measured according to DIN EN ISO 5079 on a UPM Hegewald & Peschke tensile tester (Germany) with a 10 N load sensor. The gauge length was 45 mm and the extension rate was 10 mm/min. For this test, single filaments were used. The tensile tests were carried out on 20 samples of each fiber quality. XRD studies of the oxidized and carbonized fiber bundles were carried out on a Bruker D8 diffractometer with PSD (position sensitive detector) and Cu K-alpha radiation (wave length: 0.154 nm). The scan angle ranged from 10° to 60° in 0.02° step width. Cross Sections were prepared in order to examine the homogeneity, shape and the cross-section diameter of the prepared yarn. The cross sections were studied by scanning electron microscopy on a Leica S260 Microscope with magnifications between 1000x and 3000x. To prevent charging effects, the samples were sputtered with graphite in a high vacuum deposition system.
RESULTS AND DISCUSSION After spinning, the fibers were first characterized and analyzed to determine the basic conditions suitable for the stabilization. The size and construction of the pilot plant limited dwell time and heating rate, therefore changes in the temperature treatment and their influence on fiber behavior were studied on a laboratory scale. Afterwards first trial runs on a pilot plant scale were conducted. Combining laboratory and pilot plant experiments we were able to optimize the temperature treatment and improve the properties of the resulting fibers. 1) Wet spinning of PAN fiber A study improving the wet-spinning process itself and with it also the properties of the carbon fibers has been conducted in previous work [5]. It was shown, that a significant improvement and an adjustment of the physical and mechanical properties can be obtained by post-drawing. A change in fiber material occurred during the course of the stabilization experiments, as the pilot scale experiments used up a lot of material and the spinning and post-drawing had been further improved at the same time. Regarding the mechanical properties, small differences were observed (Table 1). While the cross-section diameter of fiber B was smaller due to an increased stretching ratio and a lower solid content, a slight increase of tenacity could be found. The Young’s modulus changed in the same way which was mainly caused by an improved intermolecular polymer chain interaction and induced crystallization. 2) Stabilization/Carbonization (Laboratory scale) During the stabilization, PAN fibers undergo a significant mass loss as a result of the ongoing reactions. Thermogravimetric analysis of the fiber A is shown in Figure 2. The analysis was performed with a constant heating rate of 2 K/min, which has proven to
be a suitable temperature treatment in preliminary experiments. Generally, slow heating rates are used, since the stabilization is exothermal and would lead to overheating and fiber damage, if performed too fast [6]. The mass loss up to 215°C was attributed to solvent residues from the spinning process, while the main mass loss due to stabilization reactions started at 215°C. The mass loss rate reached its maximum at approx. 290°C as apparent from the DTG curve. The sharp peak was accompanied by a second overlapping peak visible as a shoulder in the range of 310 to 350°C. The first peak could be assigned to the cyclization and dehydrogenation, while the latter was attributed to oxidation reactions [14], which are important in a successful stabilization. After the mass loss rate slows down at approx. 350°C, it accelerates again up to 400°C, owing to the beginning of degradation and combustion. An appropriate oxidation regime should reach the temperature range of the stabilization reactions but prevent loss of carbon and damage of the fiber structure from degradation. For the continuous stabilization in the laboratory, we established a treatment where the fiber was drawn through a tube furnace with a maximum temperature Tmax of 280°C and a fiber speed of 2.7 mm/min. Because of the open ends of the tube, a temperature gradient results over the total furnace length. The temperature distribution in the furnace was measured. In combination with the drawing speed it lead to a heating rate of approx. 1.5 K/min. The length of the heating zone with maximum temperature was 150 mm, resulting in a dwell time of 55 min at this temperature.
The laboratory oxidation treatment could not be translated directly to the pilot plant scale. Due to the constraints of winding speed and length of the heating zones, the heating rate in the pilot plant setup available was estimated to be 20K/min and higher. Thermogravimetric analysis was used to compare such different heating rates prior to the dwell time. From Figure 3 it can be seen, that at a slow heating rate (comparable to the laboratory oxidation), the mass loss from the stabilization reactions started during the linear heating, meaning that the reaction had already advanced before the maximum temperature was reached. Using a higher heating rate (comparable to the pilot plant experiments), the kinetics of the stabilization lead to an increase in reaction temperatures and the stabilization reaction took place during the dwell time at Tmax. The corresponding maximum of the mass change rate (DTG) was much higher as the reaction was accelerated, which can promote local overheating and result in fiber damage. The first peak, corresponding to the evaporation of solvent residues was also increased with the higher heating rate. As an alternative to a lower heating rate, the maximum temperature during the stabilization was lowered. Thermogravimetric experiments with a fast heating rate followed by a dwell time at different temperatures showed the strong influence of maximum temperature on the mass loss in the PAN material (Figure 4). By reducing Tmax the maximum mass change rate decreased significantly. Thus, the reaction rate was slowed down as well and the stabilization would continue more moderately. This would reduce the heat and gas release, both factors that can decrease the quality of the fiber structure if their levels are too high. The reduction of the maximum temperature from 280°C to 275°C already reduced the mass change rate to a similar
value as in the laboratory experiments. At 260°C and below no DTG peak was visible and the mass loss and ongoing reactions were very slow. The fibers treated at different temperatures were investigated with FTIR spectroscopy to compare the degree of stabilization. Some of the spectra are shown in Figure 5. In the initial state, the characteristic vibration bands of PAN at 2930 and 2860 cm-1 (CH2-stretching), 2240 cm-1 (C≡N-stretching) and 1450 cm-1 (CH2-deformation) were noticeable. During the course of the stabilization, the cyclization and dehydrogenation processes lead to the disappearance of the nitrile and methylene groups. At the same time, C=N- and C=C-structures appeared, detectable as a vibration band at 1600 cm-1 [15]. The bands at 1335 und 1223 cm-1 can be attributed to aromatic heterocycles [16] and C-O-stretching [17], respectively. The introduction of oxygen was also visible as a small band at 1735 cm-1, originating from C=O-structures. In the spectra of fibers treated at 280°C for one hour, the methylene and nitrile bands disappeared completely. At 260°C and lower treatment temperatures some residues of the vibration bands of PAN were still visible, even when the dwell time was prolonged to two hours, and the pattern in the range of 1700 to 1100 cm-1 still differed from the treatment at higher temperatures. The degree of stabilization at these temperatures was significantly lower. From electron microscopy (Figure 6) a core/shell structure was visible in the fiber cross sections after the stabilization. This effect was attributed to the diffusion of oxygen into the fiber during stabilization [18,19]. The oxygen lead to a stronger cross linking in the stabilized PAN, but the cross-linked structure inhibited the oxygen diffusion, so the fiber core was stabilized under inert conditions, where less cross linking takes place. This can weaken the fiber structure after carbonization and lower the mechanic
stability of the material. Additionally, the two-phase structure can lead to the formation of voids in the center of the fiber during the carbonization treatment, further corrupting the stability of the resulting material. While insufficient oxidation is seen as the cause of the void formation [20], no explanation of its development is given in the literature. The structural differences between the two phases might cause internal stress in the fiber structure during the carbonization or the reactions in the core result in strong gas emissions, both effects could explain the formation of voids during the subsequent carbonization. The fiber from the laboratory scale experiment with a low heating rate shows a core/shell structure with a small core. After thermoanalytical experiments with a higher heating rate the core size was much larger, probably because the fast reaction could cause more cross linking in the outer shell and inhibit further diffusion. Local overheating could further accelerate inert reactions in the core. When lowering the maximum treatment temperature, the size of the outer shell increased and the core size was reduced. The two phases became less distinguishable. The reaction rate was probably slowed down sufficiently to allow a further diffusion of oxygen into the fiber, which helped in the overall cross linking of the material to produce a more homogenous fiber. However, at the same time, a longer treatment time would be necessary to reach the desired degree of stabilization. The high heating rate experiments showed, that overheating and the formation of a core/shell structure are two critical points when switching from the laboratory to the pilot scale processing of the fibers. By prolonging the dwell time and lowering the maximum temperature, the stabilization process could be improved in the limited setup of the pilot plant.
To ensure comparability of the produced carbon fibers, the carbonization of all samples was done with the same temperature profile. The parameter choice was based on data from the literature [11,21–23], from which some essential conditions for the carbonization could be derived. The carbonization temperature should not exceed 1500°C and the heating rate should not be higher than 20 K/min, otherwise there is a decrease of fiber strength. The dwell time at maximum temperature was suggested to be between 1-2 min [22] and 10-20 min [23]. With regard to the literature suggestions, carbonization experiments were performed with a maximum temperature of 1400°C. Different heating profiles were evaluated with a test fiber in which the heating rate and dwell time were investigated, some of which are shown in Figure 7. The tensile strength of the carbonized fibers was chosen as the evaluation parameter to compare the different carbonization profiles. As visible in Figure 8, the carbonization profile P8 with slow heating (3 K/min) to 900°C, fast heating (10 K/min) to 1400°C and 5 min dwell time generated the highest tensile strength and was therefore chosen as the carbonization profile for all further experiments. 3) Stabilization/Carbonization (Pilot plant scale) Based on the laboratory investigations, pilot scale runs were conducted, where we aimed to adapt and optimize the temperature treatment. An overview of the experiments which will be discussed here is given in Table 2. The first stabilization experiment on the pilot plant (Ox1) was performed with a constant temperature of 280°C over all heating zones. Under this condition, the fibers undergo an isothermal treatment with an extremely high heating rate (approx. 100 K/min) at the beginning. As stated before this is contrary to established knowledge about optimal processing of PAN stabilization. But this way, all heating zones could be
kept at the maximum treatment temperature and contributed to the dwell time at Tmax. Some intermediate cooling effect was realized by the last cooling zone in the furnace and by the position of the pulleys for the fiber turns outside of the furnace. The fibers were inflammable after the stabilization process, indicating that a certain degree of stabilization was accomplished. After carbonization, these fibers showed a lower tensile strength than the material produced in the laboratory scale (Figure 9). The SEM cross sections of the fibers showed the formation of a core/shell structure after stabilization and large voids in the core after the carbonization (Figure 10). Similar voids did not appear in fibers from the laboratory stabilization and were probably the main cause for the lower tensile strength. In a next step (Ox2), the first four heating zones were programmed with ascending temperatures. When only three out of seven heating zones work at the maximum temperature, the dwell time at this temperature is shortened. Below 150°C, no stabilization reaction occurs and fast heating is not as critical, so the temperatures were staggered from 150°C to 280°C. Thereby we achieved a temperature gradient which is equivalent to a heating rate of approx. 20 K/min when the fiber passes these heating zones. The most significant improvement can be seen in the cross sections of the fibers (Figure 10). After switching to a staggered temperature profile in the second experiment (Ox2), the stabilized fibers still showed the core/shell structure, but after carbonizing them under identical conditions as the fibers from Ox1, no voids could be found. Although the difference in the two-phase structure was not visible in SEM, we assume there is a difference in the microstructure after stabilization, which prevents the formation of voids during the carbonization. Consequently, the introduction of a lower temperature gradient is an important aspect for creating a homogenous void-
free structure in the fibers, but the dwell time at Tmax is shorter in such a process setup and might result in an insufficient stabilization as well. Different dwell times during stabilization were tested in the next experiments (Table 2: Ox3, Ox4 and Ox5) by passing the fibers through the heating zones several times. Stabilized and carbonized fibers were investigated with XRD measurements to obtain information about the structure formation and the extent of stabilization. Characteristic features of the XRD pattern of pristine PAN are a moderately sharp reflection in the region 2θ = 16° - 17°, a broad diffuse scattering maximum around 2θ = 25° - 27°, and a small diffraction peak at 2θ = 30° [24]. The structure is described in detail in several studies [5,25,26]. After stabilization a new peak developes at 2θ = 25°, which is related to the graphite like ladder structure of stabilized PAN. The degree of stabilization, which is often named aromatization index (AI) because of the partially aromatic structure, can be calculated according to the equation
= ܫܣூ
ூೌ
ೌାூ
with Ia and Ip being the diffraction intensity values of the stabilized and pristine structure at 2θ = 25° and 17° respectively [27]. After stabilization no peak at 2θ = 17° was visible for all dwell times, resulting in a constant AI = 1 and therefore indicating that the PAN structure was completely removed already at the shortest dwell time. With increasing dwell time in the heating zone the intensity of the peak at 2θ = 25° was increased (Figure 11). The formation of a graphite-like structure therefore continued during the prolonged dwell time. The laboratory experiments showed the benefits of lower maximum temperature when working with relatively high heating rates. In the experiment Ox6 we tested a
(1)
temperature treatment with a lower Tmax in addition to the temperature gradient in the heating zones. Figure 12 compares the XRD spectra of the fibers Ox5 and Ox6 after oxidation and after carbonization. The formation of the stabilized structure was visible in both cases. But in the fiber from experiment Ox6, treated at the lower temperature of Tmax = 260°C, the remaining peaks of the pristine PAN structure were still visible, although a long dwell time of the fiber in the furnace was realized. The stabilization degree was lower after treatment under this conditions. Based on equation (1) the degree of stabilization could be calculated to be 0.55 in experiment Ox6, while it was 1 in experiment Ox5. In both experiments the FTIR spectra of the oxidized materials showed the disappearance of the nitrile group and the CH2-backbone, while the C=C- and C=Ngroups appear (Figure 13). The absorption band 800 cm-1, which can be attributed to =C-H-groups formed in the cyclized, dehydrogenated and partially aromatic structure, are less intense in the fiber from experiment Ox6 which had been treated at the lower temperature. Although the PAN structure has largely degraded, the formation of the ladder structure has not progressed as far as in the fiber Ox5. From SEM microscopy in Figure 14, the formation of a two-phase structure after oxidation was still visible in the fiber Ox5. In comparison to the cross section of the fiber Ox2 in Figure 10, which was treated with a shorter dwell time, the size of the core was reduced, indicating that by prolonging the reaction time, the cross linking will continue further to the center of the fiber. After carbonization, the formation of voids could be seen in most fibers, although we used the same temperature gradient in the
pilot plant setup that had been beneficial before. In the experiment Ox6, no crosssection structure was visible anymore and no voids were detected in the carbonized material. Zhang et al. [20] claimed insufficient oxidation to be the cause of the void formation, which is contrary to our findings. Instead, we suggest that besides the ratio of both phases, the magnitude of the gradient which is formed between them as a result of the cross-linking process in the shell, is an important factor. As observed from FTIR and XRD, the stabilization degree was lower in the fibers from Ox6 and consequently the structural gradient will therefore be less prominent. To compare the effect of differing degrees of stabilization on the subsequent carbonization in inert atmosphere, we compared thermogravimetric measurements of stabilized fibers in nitrogen (Figure 15). The stabilization had been performed with different Tmax. The results show, that with a decreased temperature during stabilization, the mass loss at the beginning of the carbonization starts earlier (resuming slightly above the maximum temperature from stabilization) and is higher. Reactions from the stabilization process therefore continue during the beginning of the carbonization. When stabilized at 280°C, the fibers showed a higher mass loss in the region of 600 to 800°C during carbonization. The tensile strength of the fibers from pilot scale experiments shows the improvement of the mechanical properties with the changes in temperature treatment (Figure 16). After the elimination of voids in Ox2, the overall tensile strength was slightly improved and especially eliminated single measurements of very low tensile strength. The prolonged stabilization time up to experiment Ox5 increased the tensile strength as well. Although the fibers from Ox6 were categorized as having a lower degree of stabilization, they showed a higher tensile strength after carbonization. Under the
conditions used to treat the fibers at the pilot plant scale, the lower temperature maximum was beneficial to the overall structure formation, since the reaction rate was reduced and the diffusion of oxygen into the fiber was improved. The insufficient stabilization was not as problematic and continued in the beginning of carbonization. Nonetheless the stabilization remains an important reaction step that cannot be skipped, as oxygen is introduced into the fiber and improves the stabilization process. The fibers with a lower degree of stabilization exhibit less cross linked regions after stabilization, but a sufficient amount of oxygen will be introduced into the material and is available for further stabilization reactions during the carbonization process. While first pilot scale experiments produced fibers with lower mechanical strength in comparison to the laboratory scale fibers, the consequent optimization of the thermal treatment resulted in an improvement of the mechanical properties. In the final experiments, the tensile strength of the laboratory experiments could be accomplished and even exceeded on a pilot plant scale. At the same time, the amount of oxidized fiber produced was raised by factor 370. CONCLUSIONS The process parameters of wet-spun PAN fibers were adjusted to the stabilization process. A translation of the PAN stabilization from a laboratory scale to a pilot plant scale was realized. The pilot scale line consisted of a modified plant which limited the accessible range of process parameters. Restrictions regarding the range of parameters during the thermal treatment were found to be a higher heating rate and shorter dwell time and their influence on the stabilization reaction and the properties of the fibers (after oxidation as well as after carbonization) was investigated. While the optimization was performed specifically for the utilized pilot plant layout, the
principles of the optimization process can be applied to the setup of other pilot lines. Critical process conditions have to be recognized and compensated, which can be can be achieved by a close combination of pilot plant runs with laboratory scale and thermoanalytical experiments. The heating rate was a critical factor, as the stabilization is an exothermal process and the structure formation is very sensitive to overheating. By lowering the dwell temperature, the reaction rate could be reduced. This facilitated the controllability of the stabilization even with high heating rates. The dwell time was controlled by multiple passing through the furnace. The formation of a core/shell structure was observed in the fibers and in some cases, lead to the development of defects and voids after the carbonization. It was influenced by oxygen diffusion processes and could be improved during the optimization process. Contrary to existing literature, the void formation is not only a result of insufficient oxidation. Evidence was found that a lower degree of stabilization with a lesser structural gradient between the two phases was found to be beneficial to the subsequent structure formation during carbonization. As a result of the optimization process, the tensile strength of the fibers from laboratory stabilization could be reached on the pilot plant scale as well, while the output of oxidized fiber was increased significantly. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support by the European Union (EFRE) and Freistaat Sachsen (SAB EFRE 100062605).
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[18] Sun T, Hou Y, Wang H. Effect of Atmospheres on Stabilization of Polyacrylonitrile Fibers. J. Macromol. Sci., Part A 2009;46(8):807–15. [19] Xue Y, Liu J, Lian F, Liang J. Effect of the oxygen-induced modification of polyacrylonitrile fibers during thermal-oxidative stabilization on the radial microcrystalline structure of the resulting carbon fibers. Polym. Degrad. Stabil. 2013;98(11):2259–67. [20] Zhang W, Liu J, Liang J. New evaluation on the preoxidation extent of different PAN precursors. J. Mater. Sci. Technol., 2004;20(4):369–72. [21] Mittal J, Konno H, Inagaki M, Bahl OP. Denitrogenation behavior and tensile strength increase during carbonization of stabilized pan fibers. Carbon 1998;36(9):1327–30. [22] Jäger H, Hauke T. Carbonfasern und ihre Verbundwerkstoffe: Herstellungsprozesse, Anwendungen und Marktentwicklung. München: Verl. Moderne Industrie; 2010. [23] Fitzer E, Frohs W, Heine M. Optimization of stabilization and carbonization treatment of PAN fibres and structural characterization of the resulting carbon fibres. Carbon 1986;24(4):387–95. [24] Gupta AK, Singhal RP. Effect of copolymerization and heat treatment on the structure and x-ray diffraction of polyacrylonitrile. J. Polym. Sci. Polym. Phys. Ed. 1983;21(11):2243–62. [25] He D, Wang C, Bai Y, Zhu B. Comparison of Structure and Properties among Various PAN Fibers for Carbon Fibers. J. Mater. Sci. Technol. 2005;21(03):376–80.
[26] Ge H, Liu H, Chen J, Wang C. The microstructure of polyacrylonitrile-stabilized fibers. J. Appl. Polym. Sci. 2009;113(4):2413–7. [27] Zhao L, Jang BZ. Fabrication, structure and properties of quasi-carbon fibres. J. Mater. Sci. 1995;30(18):4535–40.
Figure Captions: Figure 1: Molecular structure of PAN (left) and overview of constitutional properties of stabilized PAN (right) [11] Figure 2: Mass loss and mass change rate (DTG) of the PAN fiber during linear heating at 2 K/min Figure 3: Comparison of thermogravimetrical analysis representing laboratory and pilot scale experiments (a: 1 K/min heating rate to 280°C, b: 20 K/min heating rate to 280°C) Figure 4: Thermogravimetric measurements of fiber A with different maximum temperatures Figure 5: FTIR spectra of fiber A from thermogravimetric analysis with different maximum temperatures Figure 6: SEM of fiber A from thermogravimetric analysis with different treatment temperatures: laboratory scale stabilization with 280°C and 1.5 K/min (left), 280°C and 20 K/min (middle), 250°C and 20 K/min (right) Figure 7: temperature profiles for carbonization Figure 8: tensile strength of test fiber carbonized with different carbonization profiles Figure 9: tensile strength of the fibers from laboratory and pilot plant scale oxidation after carbonization Figure 10: Formation of two phase structures and voids in the fibers from Ox1 (isothermal) and Ox2 (temperature gradient), comparison of fiber cross sections in SEM after oxidation and carbonization
Figure 11: XRD pattern of pristine PAN material and stabilized fibers with different stabilization durations Figure 12: XRD spectra of fibers from experiments Ox5 and Ox6 after stabilization and carbonization Figure 13: FTIR-spectra of the stabilized fibers from experiments Ox5 and Ox6 Figure 14: Formation of two phase structures and voids in the fibers from Ox5 (Tmax = 280°C) and Ox6 (Tmax = 260°C), comparison of fiber cross sections in SEM after oxidation and carbonization Figure 15: Mass loss and mass change rate (DTG) of PAN fiber A during carbonization after stabilization treatment at different temperatures Figure 16: tensile strength of the fibers from laboratory and pilot plant scale oxidation after carbonization
Tables: Table 1: Parameters and results of the wet-spinning and post-drawing experiments
Wet spinning Fiber A
Fiber B
Number of orifices
1000
Cross section diameter of orifices
120 µm
Solid content
18 %
Composition coagulation bath
17 % 85/15 (DMF/water)
Post-Drawing Temperature
145 °C
145 °C
Drawing Ratio (overall)
1 : 8.6
1 : 9.8
Mechanical and physical properties Tenacity
37.6 cN/tex
39.1 cN/tex
Young’s modulus
13.2 GPa
14.5 GPa
Cross section diameter
13.9 µm
13.2 µm
Table 2: Parameters of the pilot scale oxidation experiments
Winding speed (m/min) 1
Number of heating cycles 4x
Temperature of heating zones 280°C
Tmax (°C)
Ox1
PAN fiber type A
Ox2
A
1
4x
150-280°C
280
56
24
Ox3
B
1
4x
150-280°C
280
56
24
Ox4
B
1
8x
150-280°C
280
112
48
Ox5
B
1
12x
150-280°C
280
168
72
Ox6
B
1
12x
150-260°C
260
168
72
280
Dwell time Dwell time in furnace at Tmax (min) (min) 56 56
S1359-835X(17)30058-1 http://dx.doi.org/10.1016/j.compositesa.2017.02.010 JCOMA 4571
To appear in:
Composites: Part A
Received Date: Revised Date: Accepted Date:
22 August 2016 25 January 2017 7 February 2017
Please cite this article as: Meinl, J., Schönfeld, K., Kirsten, M., Kittler, K., Michaelis, A., Cherif, C., Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers, Composites: Part A (2017), doi: http://dx.doi.org/10.1016/j.compositesa.2017.02.010
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Optimization of the temperature program to scale up the stabilization of polyacrylonitrile fibers Authors: Juliane Meinla, Katrin Schönfelda, Martin Kirstenb, Karsten Kittlerc, Alexander Michaelisa, Chokri Cherifb a
Fraunhofer Institute for Ceramic Technologies and Systems IKTS, Winterbergstraße 28, 01277 Dresden, Germany b Technische Universität Dresden / Institute of Textile Machinery and High Performance Material Technology, Breitscheidstr. 78, 01237, Dresden, Germany c P-D Glasseiden GmbH Oschatz, Wellerswalder Weg 17, 04758 Oschatz, Germany Juliane Meinl (Corresponding author) Fraunhofer Institute for Ceramic Technologies and Systems IKTS / Thermal Analysis and Thermal Physics Winterbergstraße 28 01277 Dresden, Germany Phone +49 351 2553-7236 Fax +49 351 2554-290 [email protected] Katrin Schoenfeld [email protected] Dr. Martin Kirsten [email protected] Dr. Karsten Kittler [email protected] Prof. Dr. rer. nat. habil. Alexander Michaelis [email protected] Univ.-Prof. Dr.-Ing. habil. Dipl.-Wirt. Ing. Chokri Cherif [email protected]
ABSTRACT The production of carbon fibers from polyacrylonitrile (PAN) includes a stabilization step before carbonization. Transferring this stabilization from the laboratory to a bigger scale cannot be performed without changes of the process parameters. Using a pure PAN polymer, fibers were spun and their stabilization as well as carbonization was studied in laboratory scale. The resulting materials were analyzed with FTIR, SEM and XRD. The stabilization step was then transferred to a pilot scale. Limitations given by the production line had to be taken into account to develop a suitable temperature treatment. The formation of a core/shell structure during stabilization was observed and was detrimental to the properties of the fiber after carbonization. Adjustments were made based on obtained results to balance the negative effects of high heating rates and strong temperature gradients. With the implemented changes the mechanical properties could be reproduced on the larger production scale. KEYWORDS Carbon fibers (A), Fiber conversion processes (E), Scale-up, Thermal analysis (D) INTRODUCTION Carbon fibers offer a unique combination of light weight and mechanical stability, making them an ideal material for high performance applications. They are a key factor in today’s trend in transport and construction towards light weight design, energy efficiency and environmental awareness. The majority of carbon fibers (about 90%) is produced from polyacrylonitrile (PAN) [1]. Although a rising interest in the use of alternative precursors can be observed, PAN will remain the main carbon fiber precursor in the near future. The transformation of PAN
fibers to carbon fibers is realized by a thermal treatment. It is essentially a two-step process, consisting of a stabilization step in air at temperatures up to 400°C and a subsequent carbonization step at high temperatures (1200°C and higher) in inert atmosphere. The influences on the properties of the final fibers are manifold, including material composition, spinning conditions, post-spinning treatment, stabilization and carbonization conditions and further modifications after carbonization [2–4]. Each step will have a significant impact on the product quality. A thorough comprehension of these influences facilitates the production towards tailor-made fiber materials. In recent publications, we investigated the structure-property relationships during carbon fiber production focusing on the spinning conditions [5] and the influence of stretching during the stabilization [6]. The wet-spinning process of PAN uses solvents such as dimethyl formamide, dimethyl acetamide or dimethyl sulfoxide. The polymer solution is filtered and degassed until a homogenous spin dope is obtained. The spinning solution is metered through spinning nozzles directly into a coagulation bath. Washing and drawing of the fiber follows immediately, removing the residues of solvent. The quality of the precursor and thus of the carbon fibers is highly influenced by the properties of the process parameters of spinning [3] and post-drawing [7–10]. The stabilization is crucial to the process in the way that the PAN-structure undergoes several changes that render it suitable for further carbonization [2,11]. The main reactions during the stabilization process are cyclization, dehydrogenation and oxidation. As a result, the stabilized PAN shows a complex cyclized, partially aromatic structure containing oxygen functional groups (Figure 1Error! Reference source not found.). The material becomes infusible and is an optimal precursor for the
carbonization to yield a graphitic structure. The reactions during stabilization are very sensitive to the temperature treatment and the process conditions have to be controlled carefully to prevent thermal runaway reactions. The cyclization process is exothermal and local overheating of the fibers can accelerate side reactions, which lower the carbon yield or may damage the molecular structure. The stabilization process therefore demands slow heating rates. This renders the process energy- and time-consuming. From an economical point of view, there is a strong interest to optimize the temperature treatment. Furthermore, depending on the targeted properties of the resulting carbon fiber, compromises between shorter processing time and higher fiber performance can be made. Studies dealing with different analytical aspects of the PAN stabilization are mainly performed in a laboratory scale. On the smallest scale, the stabilization and carbonization is studied via thermal analysis, using very small amounts of material. At this scale, there already is an influence of the sample preparation on the reaction process [12]. To produce larger samples in the laboratory, the stabilization is done either in a batch mode or by a continuous process, where the fiber is respectively placed in or pulled through a small furnace (e.g. a tube furnace). The conditions can be varied over a wide range in these small setups and parameters which are unlikely to be applicable at a larger scale can be used, for example very low winding speeds, constant tension or length and complex heating programs. In an industrial size process, the stabilization is realized in large furnaces, in which the fiber passes through different heating zones [4,11]. The temperature is slowly raised in each heating zone and the fibers are pulled multiple times through one temperature zone, to extend the duration of treatment at a specific temperature. The fibers are
turned over rollers outside of the oven, which is an additional possibility to control the reactions by cooling down the fiber every few minutes. The optimum temperature treatment varies depending on the material used, thus the analytical step of characterizing the behavior of the material during stabilization cannot be skipped. And with the complexity of the effects that influence the reactions, the scale-up to a pilot plant remains critical for a successful translation to the industrial processing. But the process of scaling up the stabilization from the laboratory to a bigger processing scale is not documented in the scientific literature. In this work, we will focus on translating the stabilization from a laboratory setup to a pilot scale. Continuing our previous work, the PAN used is a homopolymer. This is contrary to industrial standards, where copolymers and additives are used to improve the stabilization process by lowering the activation energy or changing the exothermal behavior [2]. By focusing on the PAN homopolymer, we aimed for an academic approach gaining a deeper understanding of the basic stabilization process and fiber formation mechanisms. The scale-up demands for modifications in the temperature program. The influence of these modifications on the reaction progress and the fiber quality is monitored and an optimization is realized. EXPERIMENTAL The fibers were spun on a wet-spinning pilot plant by Fourné Polymertechnik GmbH, Germany consisting of a coagulation-bath, washing baths, drying and sizing-unit and a wind-up unit with an overall speed up to 90 m/min. The spinning dope solution was prepared using PAN (Mw 200.000 g/mol) in dimethylformamide (DMF). Spinning nozzles with 1000 holes, each with a diameter of 120 µm were used. The post-drawing
was performed on a pilot plant scale post-drawing unit from DIENES Apparatebau GmbH, Germany, to improve the mechanical and structural properties of the PAN precursor. For detailed Information see references [13,5]. Table 1 shows parameters and results of the wet spinning and post drawing experiments. DMF was used as received from Overlack GmbH. The oxidation on a laboratory scale was performed in a Carbolite CTF tube furnace through which the fiber bundle was pulled by a Mikrosam MAW20-LS1 winding machine, allowing for a constant fiber tension during the process. The furnace was open and air passed through the tube while the fiber bundle was continuously running with a defined speed. The pilot scale oxidation was carried out on a modified plant by the Preiss-Daimler Glasseiden GmbH Oschatz (P-D GSO). The plant consisted of eight units each 2 meters long. Seven of the units were heatable, each temperature could be adjusted separately. The last unit was used as cooling zone. Through a system of external pulleys the filaments could pass the heating zones several times to enable a better oxidation. The winding speed was adjustable from 1 to 6 m/min. The carbonization was performed under static conditions with a TT2 furnace (Thermal technology GmbH) with graphite heater and crucible in nitrogen atmosphere. IR spectra of the fibers were obtained by using the Attenuated Total Reflection method on a Bruker Tensor 27 FTIR spectrometer. A total number of 32 scans was taken and averaged from 400 to 4000 cm-1 with a resolution of 4 cm-1 for each spectrum.
Thermal Gravimetric Analysis (TGA) was carried out on a STA 449C (NETZSCHGerätebau GmbH) in air or nitrogen with a flow rate of 5 l/h and different temperature programs. Tensile properties of the PAN yarn samples were measured according to DIN EN ISO 2062 on a Z2.5 tensile tester (Zwick, Germany) with a 100 N load sensor. The gauge length was 250 mm and the extension rate was 250 mm/min. The tests were performed at 20 °C, 65 % relative humidity and carried out on 20 samples of each fiber quality, the standard deviation being generally less than 5 %. Tensile properties of the carbon fiber samples were measured according to DIN EN ISO 5079 on a UPM Hegewald & Peschke tensile tester (Germany) with a 10 N load sensor. The gauge length was 45 mm and the extension rate was 10 mm/min. For this test, single filaments were used. The tensile tests were carried out on 20 samples of each fiber quality. XRD studies of the oxidized and carbonized fiber bundles were carried out on a Bruker D8 diffractometer with PSD (position sensitive detector) and Cu K-alpha radiation (wave length: 0.154 nm). The scan angle ranged from 10° to 60° in 0.02° step width. Cross Sections were prepared in order to examine the homogeneity, shape and the cross-section diameter of the prepared yarn. The cross sections were studied by scanning electron microscopy on a Leica S260 Microscope with magnifications between 1000x and 3000x. To prevent charging effects, the samples were sputtered with graphite in a high vacuum deposition system.
RESULTS AND DISCUSSION After spinning, the fibers were first characterized and analyzed to determine the basic conditions suitable for the stabilization. The size and construction of the pilot plant limited dwell time and heating rate, therefore changes in the temperature treatment and their influence on fiber behavior were studied on a laboratory scale. Afterwards first trial runs on a pilot plant scale were conducted. Combining laboratory and pilot plant experiments we were able to optimize the temperature treatment and improve the properties of the resulting fibers. 1) Wet spinning of PAN fiber A study improving the wet-spinning process itself and with it also the properties of the carbon fibers has been conducted in previous work [5]. It was shown, that a significant improvement and an adjustment of the physical and mechanical properties can be obtained by post-drawing. A change in fiber material occurred during the course of the stabilization experiments, as the pilot scale experiments used up a lot of material and the spinning and post-drawing had been further improved at the same time. Regarding the mechanical properties, small differences were observed (Table 1). While the cross-section diameter of fiber B was smaller due to an increased stretching ratio and a lower solid content, a slight increase of tenacity could be found. The Young’s modulus changed in the same way which was mainly caused by an improved intermolecular polymer chain interaction and induced crystallization. 2) Stabilization/Carbonization (Laboratory scale) During the stabilization, PAN fibers undergo a significant mass loss as a result of the ongoing reactions. Thermogravimetric analysis of the fiber A is shown in Figure 2. The analysis was performed with a constant heating rate of 2 K/min, which has proven to
be a suitable temperature treatment in preliminary experiments. Generally, slow heating rates are used, since the stabilization is exothermal and would lead to overheating and fiber damage, if performed too fast [6]. The mass loss up to 215°C was attributed to solvent residues from the spinning process, while the main mass loss due to stabilization reactions started at 215°C. The mass loss rate reached its maximum at approx. 290°C as apparent from the DTG curve. The sharp peak was accompanied by a second overlapping peak visible as a shoulder in the range of 310 to 350°C. The first peak could be assigned to the cyclization and dehydrogenation, while the latter was attributed to oxidation reactions [14], which are important in a successful stabilization. After the mass loss rate slows down at approx. 350°C, it accelerates again up to 400°C, owing to the beginning of degradation and combustion. An appropriate oxidation regime should reach the temperature range of the stabilization reactions but prevent loss of carbon and damage of the fiber structure from degradation. For the continuous stabilization in the laboratory, we established a treatment where the fiber was drawn through a tube furnace with a maximum temperature Tmax of 280°C and a fiber speed of 2.7 mm/min. Because of the open ends of the tube, a temperature gradient results over the total furnace length. The temperature distribution in the furnace was measured. In combination with the drawing speed it lead to a heating rate of approx. 1.5 K/min. The length of the heating zone with maximum temperature was 150 mm, resulting in a dwell time of 55 min at this temperature.
The laboratory oxidation treatment could not be translated directly to the pilot plant scale. Due to the constraints of winding speed and length of the heating zones, the heating rate in the pilot plant setup available was estimated to be 20K/min and higher. Thermogravimetric analysis was used to compare such different heating rates prior to the dwell time. From Figure 3 it can be seen, that at a slow heating rate (comparable to the laboratory oxidation), the mass loss from the stabilization reactions started during the linear heating, meaning that the reaction had already advanced before the maximum temperature was reached. Using a higher heating rate (comparable to the pilot plant experiments), the kinetics of the stabilization lead to an increase in reaction temperatures and the stabilization reaction took place during the dwell time at Tmax. The corresponding maximum of the mass change rate (DTG) was much higher as the reaction was accelerated, which can promote local overheating and result in fiber damage. The first peak, corresponding to the evaporation of solvent residues was also increased with the higher heating rate. As an alternative to a lower heating rate, the maximum temperature during the stabilization was lowered. Thermogravimetric experiments with a fast heating rate followed by a dwell time at different temperatures showed the strong influence of maximum temperature on the mass loss in the PAN material (Figure 4). By reducing Tmax the maximum mass change rate decreased significantly. Thus, the reaction rate was slowed down as well and the stabilization would continue more moderately. This would reduce the heat and gas release, both factors that can decrease the quality of the fiber structure if their levels are too high. The reduction of the maximum temperature from 280°C to 275°C already reduced the mass change rate to a similar
value as in the laboratory experiments. At 260°C and below no DTG peak was visible and the mass loss and ongoing reactions were very slow. The fibers treated at different temperatures were investigated with FTIR spectroscopy to compare the degree of stabilization. Some of the spectra are shown in Figure 5. In the initial state, the characteristic vibration bands of PAN at 2930 and 2860 cm-1 (CH2-stretching), 2240 cm-1 (C≡N-stretching) and 1450 cm-1 (CH2-deformation) were noticeable. During the course of the stabilization, the cyclization and dehydrogenation processes lead to the disappearance of the nitrile and methylene groups. At the same time, C=N- and C=C-structures appeared, detectable as a vibration band at 1600 cm-1 [15]. The bands at 1335 und 1223 cm-1 can be attributed to aromatic heterocycles [16] and C-O-stretching [17], respectively. The introduction of oxygen was also visible as a small band at 1735 cm-1, originating from C=O-structures. In the spectra of fibers treated at 280°C for one hour, the methylene and nitrile bands disappeared completely. At 260°C and lower treatment temperatures some residues of the vibration bands of PAN were still visible, even when the dwell time was prolonged to two hours, and the pattern in the range of 1700 to 1100 cm-1 still differed from the treatment at higher temperatures. The degree of stabilization at these temperatures was significantly lower. From electron microscopy (Figure 6) a core/shell structure was visible in the fiber cross sections after the stabilization. This effect was attributed to the diffusion of oxygen into the fiber during stabilization [18,19]. The oxygen lead to a stronger cross linking in the stabilized PAN, but the cross-linked structure inhibited the oxygen diffusion, so the fiber core was stabilized under inert conditions, where less cross linking takes place. This can weaken the fiber structure after carbonization and lower the mechanic
stability of the material. Additionally, the two-phase structure can lead to the formation of voids in the center of the fiber during the carbonization treatment, further corrupting the stability of the resulting material. While insufficient oxidation is seen as the cause of the void formation [20], no explanation of its development is given in the literature. The structural differences between the two phases might cause internal stress in the fiber structure during the carbonization or the reactions in the core result in strong gas emissions, both effects could explain the formation of voids during the subsequent carbonization. The fiber from the laboratory scale experiment with a low heating rate shows a core/shell structure with a small core. After thermoanalytical experiments with a higher heating rate the core size was much larger, probably because the fast reaction could cause more cross linking in the outer shell and inhibit further diffusion. Local overheating could further accelerate inert reactions in the core. When lowering the maximum treatment temperature, the size of the outer shell increased and the core size was reduced. The two phases became less distinguishable. The reaction rate was probably slowed down sufficiently to allow a further diffusion of oxygen into the fiber, which helped in the overall cross linking of the material to produce a more homogenous fiber. However, at the same time, a longer treatment time would be necessary to reach the desired degree of stabilization. The high heating rate experiments showed, that overheating and the formation of a core/shell structure are two critical points when switching from the laboratory to the pilot scale processing of the fibers. By prolonging the dwell time and lowering the maximum temperature, the stabilization process could be improved in the limited setup of the pilot plant.
To ensure comparability of the produced carbon fibers, the carbonization of all samples was done with the same temperature profile. The parameter choice was based on data from the literature [11,21–23], from which some essential conditions for the carbonization could be derived. The carbonization temperature should not exceed 1500°C and the heating rate should not be higher than 20 K/min, otherwise there is a decrease of fiber strength. The dwell time at maximum temperature was suggested to be between 1-2 min [22] and 10-20 min [23]. With regard to the literature suggestions, carbonization experiments were performed with a maximum temperature of 1400°C. Different heating profiles were evaluated with a test fiber in which the heating rate and dwell time were investigated, some of which are shown in Figure 7. The tensile strength of the carbonized fibers was chosen as the evaluation parameter to compare the different carbonization profiles. As visible in Figure 8, the carbonization profile P8 with slow heating (3 K/min) to 900°C, fast heating (10 K/min) to 1400°C and 5 min dwell time generated the highest tensile strength and was therefore chosen as the carbonization profile for all further experiments. 3) Stabilization/Carbonization (Pilot plant scale) Based on the laboratory investigations, pilot scale runs were conducted, where we aimed to adapt and optimize the temperature treatment. An overview of the experiments which will be discussed here is given in Table 2. The first stabilization experiment on the pilot plant (Ox1) was performed with a constant temperature of 280°C over all heating zones. Under this condition, the fibers undergo an isothermal treatment with an extremely high heating rate (approx. 100 K/min) at the beginning. As stated before this is contrary to established knowledge about optimal processing of PAN stabilization. But this way, all heating zones could be
kept at the maximum treatment temperature and contributed to the dwell time at Tmax. Some intermediate cooling effect was realized by the last cooling zone in the furnace and by the position of the pulleys for the fiber turns outside of the furnace. The fibers were inflammable after the stabilization process, indicating that a certain degree of stabilization was accomplished. After carbonization, these fibers showed a lower tensile strength than the material produced in the laboratory scale (Figure 9). The SEM cross sections of the fibers showed the formation of a core/shell structure after stabilization and large voids in the core after the carbonization (Figure 10). Similar voids did not appear in fibers from the laboratory stabilization and were probably the main cause for the lower tensile strength. In a next step (Ox2), the first four heating zones were programmed with ascending temperatures. When only three out of seven heating zones work at the maximum temperature, the dwell time at this temperature is shortened. Below 150°C, no stabilization reaction occurs and fast heating is not as critical, so the temperatures were staggered from 150°C to 280°C. Thereby we achieved a temperature gradient which is equivalent to a heating rate of approx. 20 K/min when the fiber passes these heating zones. The most significant improvement can be seen in the cross sections of the fibers (Figure 10). After switching to a staggered temperature profile in the second experiment (Ox2), the stabilized fibers still showed the core/shell structure, but after carbonizing them under identical conditions as the fibers from Ox1, no voids could be found. Although the difference in the two-phase structure was not visible in SEM, we assume there is a difference in the microstructure after stabilization, which prevents the formation of voids during the carbonization. Consequently, the introduction of a lower temperature gradient is an important aspect for creating a homogenous void-
free structure in the fibers, but the dwell time at Tmax is shorter in such a process setup and might result in an insufficient stabilization as well. Different dwell times during stabilization were tested in the next experiments (Table 2: Ox3, Ox4 and Ox5) by passing the fibers through the heating zones several times. Stabilized and carbonized fibers were investigated with XRD measurements to obtain information about the structure formation and the extent of stabilization. Characteristic features of the XRD pattern of pristine PAN are a moderately sharp reflection in the region 2θ = 16° - 17°, a broad diffuse scattering maximum around 2θ = 25° - 27°, and a small diffraction peak at 2θ = 30° [24]. The structure is described in detail in several studies [5,25,26]. After stabilization a new peak developes at 2θ = 25°, which is related to the graphite like ladder structure of stabilized PAN. The degree of stabilization, which is often named aromatization index (AI) because of the partially aromatic structure, can be calculated according to the equation
= ܫܣூ
ூೌ
ೌାூ
with Ia and Ip being the diffraction intensity values of the stabilized and pristine structure at 2θ = 25° and 17° respectively [27]. After stabilization no peak at 2θ = 17° was visible for all dwell times, resulting in a constant AI = 1 and therefore indicating that the PAN structure was completely removed already at the shortest dwell time. With increasing dwell time in the heating zone the intensity of the peak at 2θ = 25° was increased (Figure 11). The formation of a graphite-like structure therefore continued during the prolonged dwell time. The laboratory experiments showed the benefits of lower maximum temperature when working with relatively high heating rates. In the experiment Ox6 we tested a
(1)
temperature treatment with a lower Tmax in addition to the temperature gradient in the heating zones. Figure 12 compares the XRD spectra of the fibers Ox5 and Ox6 after oxidation and after carbonization. The formation of the stabilized structure was visible in both cases. But in the fiber from experiment Ox6, treated at the lower temperature of Tmax = 260°C, the remaining peaks of the pristine PAN structure were still visible, although a long dwell time of the fiber in the furnace was realized. The stabilization degree was lower after treatment under this conditions. Based on equation (1) the degree of stabilization could be calculated to be 0.55 in experiment Ox6, while it was 1 in experiment Ox5. In both experiments the FTIR spectra of the oxidized materials showed the disappearance of the nitrile group and the CH2-backbone, while the C=C- and C=Ngroups appear (Figure 13). The absorption band 800 cm-1, which can be attributed to =C-H-groups formed in the cyclized, dehydrogenated and partially aromatic structure, are less intense in the fiber from experiment Ox6 which had been treated at the lower temperature. Although the PAN structure has largely degraded, the formation of the ladder structure has not progressed as far as in the fiber Ox5. From SEM microscopy in Figure 14, the formation of a two-phase structure after oxidation was still visible in the fiber Ox5. In comparison to the cross section of the fiber Ox2 in Figure 10, which was treated with a shorter dwell time, the size of the core was reduced, indicating that by prolonging the reaction time, the cross linking will continue further to the center of the fiber. After carbonization, the formation of voids could be seen in most fibers, although we used the same temperature gradient in the
pilot plant setup that had been beneficial before. In the experiment Ox6, no crosssection structure was visible anymore and no voids were detected in the carbonized material. Zhang et al. [20] claimed insufficient oxidation to be the cause of the void formation, which is contrary to our findings. Instead, we suggest that besides the ratio of both phases, the magnitude of the gradient which is formed between them as a result of the cross-linking process in the shell, is an important factor. As observed from FTIR and XRD, the stabilization degree was lower in the fibers from Ox6 and consequently the structural gradient will therefore be less prominent. To compare the effect of differing degrees of stabilization on the subsequent carbonization in inert atmosphere, we compared thermogravimetric measurements of stabilized fibers in nitrogen (Figure 15). The stabilization had been performed with different Tmax. The results show, that with a decreased temperature during stabilization, the mass loss at the beginning of the carbonization starts earlier (resuming slightly above the maximum temperature from stabilization) and is higher. Reactions from the stabilization process therefore continue during the beginning of the carbonization. When stabilized at 280°C, the fibers showed a higher mass loss in the region of 600 to 800°C during carbonization. The tensile strength of the fibers from pilot scale experiments shows the improvement of the mechanical properties with the changes in temperature treatment (Figure 16). After the elimination of voids in Ox2, the overall tensile strength was slightly improved and especially eliminated single measurements of very low tensile strength. The prolonged stabilization time up to experiment Ox5 increased the tensile strength as well. Although the fibers from Ox6 were categorized as having a lower degree of stabilization, they showed a higher tensile strength after carbonization. Under the
conditions used to treat the fibers at the pilot plant scale, the lower temperature maximum was beneficial to the overall structure formation, since the reaction rate was reduced and the diffusion of oxygen into the fiber was improved. The insufficient stabilization was not as problematic and continued in the beginning of carbonization. Nonetheless the stabilization remains an important reaction step that cannot be skipped, as oxygen is introduced into the fiber and improves the stabilization process. The fibers with a lower degree of stabilization exhibit less cross linked regions after stabilization, but a sufficient amount of oxygen will be introduced into the material and is available for further stabilization reactions during the carbonization process. While first pilot scale experiments produced fibers with lower mechanical strength in comparison to the laboratory scale fibers, the consequent optimization of the thermal treatment resulted in an improvement of the mechanical properties. In the final experiments, the tensile strength of the laboratory experiments could be accomplished and even exceeded on a pilot plant scale. At the same time, the amount of oxidized fiber produced was raised by factor 370. CONCLUSIONS The process parameters of wet-spun PAN fibers were adjusted to the stabilization process. A translation of the PAN stabilization from a laboratory scale to a pilot plant scale was realized. The pilot scale line consisted of a modified plant which limited the accessible range of process parameters. Restrictions regarding the range of parameters during the thermal treatment were found to be a higher heating rate and shorter dwell time and their influence on the stabilization reaction and the properties of the fibers (after oxidation as well as after carbonization) was investigated. While the optimization was performed specifically for the utilized pilot plant layout, the
principles of the optimization process can be applied to the setup of other pilot lines. Critical process conditions have to be recognized and compensated, which can be can be achieved by a close combination of pilot plant runs with laboratory scale and thermoanalytical experiments. The heating rate was a critical factor, as the stabilization is an exothermal process and the structure formation is very sensitive to overheating. By lowering the dwell temperature, the reaction rate could be reduced. This facilitated the controllability of the stabilization even with high heating rates. The dwell time was controlled by multiple passing through the furnace. The formation of a core/shell structure was observed in the fibers and in some cases, lead to the development of defects and voids after the carbonization. It was influenced by oxygen diffusion processes and could be improved during the optimization process. Contrary to existing literature, the void formation is not only a result of insufficient oxidation. Evidence was found that a lower degree of stabilization with a lesser structural gradient between the two phases was found to be beneficial to the subsequent structure formation during carbonization. As a result of the optimization process, the tensile strength of the fibers from laboratory stabilization could be reached on the pilot plant scale as well, while the output of oxidized fiber was increased significantly. ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support by the European Union (EFRE) and Freistaat Sachsen (SAB EFRE 100062605).
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Figure Captions: Figure 1: Molecular structure of PAN (left) and overview of constitutional properties of stabilized PAN (right) [11] Figure 2: Mass loss and mass change rate (DTG) of the PAN fiber during linear heating at 2 K/min Figure 3: Comparison of thermogravimetrical analysis representing laboratory and pilot scale experiments (a: 1 K/min heating rate to 280°C, b: 20 K/min heating rate to 280°C) Figure 4: Thermogravimetric measurements of fiber A with different maximum temperatures Figure 5: FTIR spectra of fiber A from thermogravimetric analysis with different maximum temperatures Figure 6: SEM of fiber A from thermogravimetric analysis with different treatment temperatures: laboratory scale stabilization with 280°C and 1.5 K/min (left), 280°C and 20 K/min (middle), 250°C and 20 K/min (right) Figure 7: temperature profiles for carbonization Figure 8: tensile strength of test fiber carbonized with different carbonization profiles Figure 9: tensile strength of the fibers from laboratory and pilot plant scale oxidation after carbonization Figure 10: Formation of two phase structures and voids in the fibers from Ox1 (isothermal) and Ox2 (temperature gradient), comparison of fiber cross sections in SEM after oxidation and carbonization
Figure 11: XRD pattern of pristine PAN material and stabilized fibers with different stabilization durations Figure 12: XRD spectra of fibers from experiments Ox5 and Ox6 after stabilization and carbonization Figure 13: FTIR-spectra of the stabilized fibers from experiments Ox5 and Ox6 Figure 14: Formation of two phase structures and voids in the fibers from Ox5 (Tmax = 280°C) and Ox6 (Tmax = 260°C), comparison of fiber cross sections in SEM after oxidation and carbonization Figure 15: Mass loss and mass change rate (DTG) of PAN fiber A during carbonization after stabilization treatment at different temperatures Figure 16: tensile strength of the fibers from laboratory and pilot plant scale oxidation after carbonization
Tables: Table 1: Parameters and results of the wet-spinning and post-drawing experiments
Wet spinning Fiber A
Fiber B
Number of orifices
1000
Cross section diameter of orifices
120 µm
Solid content
18 %
Composition coagulation bath
17 % 85/15 (DMF/water)
Post-Drawing Temperature
145 °C
145 °C
Drawing Ratio (overall)
1 : 8.6
1 : 9.8
Mechanical and physical properties Tenacity
37.6 cN/tex
39.1 cN/tex
Young’s modulus
13.2 GPa
14.5 GPa
Cross section diameter
13.9 µm
13.2 µm
Table 2: Parameters of the pilot scale oxidation experiments
Winding speed (m/min) 1
Number of heating cycles 4x
Temperature of heating zones 280°C
Tmax (°C)
Ox1
PAN fiber type A
Ox2
A
1
4x
150-280°C
280
56
24
Ox3
B
1
4x
150-280°C
280
56
24
Ox4
B
1
8x
150-280°C
280
112
48
Ox5
B
1
12x
150-280°C
280
168
72
Ox6
B
1
12x
150-260°C
260
168
72
280
Dwell time Dwell time in furnace at Tmax (min) (min) 56 56