Materials Science and Engineering A 485 (2008) 251–257 The effect of processing conditions on a polyacrylonitrile fiber produced using a solvent-free free coagulation process A.F. Ismail a,∗ , M.A. Rahman a , A. Mustafa a , T. Matsuura b a Membrane Research Unit, Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia b Department of Chemical Engineering, Industrial Membrane Research Institute, University of Ottawa, Ottawa, Ont., Canada KIN 6N5 Received 3 April 2007; received in revised form 25 July 2007; accepted 20 August 2007 Abstract PAN fibers were fabricated using a solvent-free coagulation process and characterized using SEM and tensile testing. Eighteen weight percent of polymer solution was found to be the most suitable composition for fabrication process and consequently produced fibers with the best mechanical properties. The PAN fibers fabricated using a coagulation bath temperature of 13 ◦ C exhibited the highest Young’s modulus of 2.93 GPa and the highest tensile strength. The number of nano-pores was significantly reduced due to low inward diffusion of non-solvent. © 2007 Elsevier B.V. All rights reserved. Keywords: Polyacrylonitrile fiber; Carbon fiber; Solvent-free coagulation process; Nano-pores; Young’s modulus 1. Introduction PAN fiber is considered as a crucial precursor for the production of carbon fiber. PAN is transformed into fiber form by melt spinning, wet spinning and dry spinning [1]. In addition to these techniques, PAN precursor fibers are also fabricated using dry–wet spinning to improve the mechanical properties [2]. In this method, the dope travels through an air gap of less than 1 cm before emerged into coagulation bath. PAN homopolymer is rarely used as a carbon fiber precursor since it initiates a retrograde core during the stabilization process due to partial melting caused by exothermic nature of oxidation process [3]. It is also generally not favorable for spinning purposes compared to its copolymers which are more soluble in solvents and easier to handle for preparation and storage [2]. The solubility of PAN and thermal properties of PAN fibers can be enhanced by the incorporation of either acidic, neutral or hydrophilic moieties as comonomers during polymerization or as spinning dope additives before the fabrication process [4]. Numerous studies have shown that the PAN fibers should possess the following characteristics; small diameter, maximum crystallinity, low comonomer contents and high modulus for the ∗ Corresponding author. Fax: +60 7 5581463. E-mail address: afauzi@utm.my (A.F. Ismail). 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.08.060 preparation of good quality carbon fibers [5–7]. The Young’s modulus of PAN precursor fiber is being the best parameter representing the carbon fiber performance since there is a direct correlation between the Young’s modulus of primary precursor and the resulting carbon fiber [8]. Therefore, the conditions of PAN fiber fabrication process play an important role in the production of high performance carbon fibers since the properties of PAN precursor fibers depend much on it. Normally, the spinning solution with polymer concentration ranging from 10 wt.% to 30 wt.% was used for the fabrication of PAN fibers by the wet spinning process [1,9]. By increasing the polymer concentration in the spinning dope, the spinnability can be improved, the tenacity increases and the porosity decreases [4,10]. The polymer concentration in the spinning solution also influences the fiber morphology and density. According to Knudsen [10], an increase in the dope solid in the range of 15–25% improved the homogeneity of the fiber structure by reducing the generation of large voids. Besides, an increase in the dope solid slightly increased the density of PAN fibers [9]. As compared lower polymer concentration with less viscosity, a higher concentration of polymer solution contributes to a higher packing of the polymer molecules per unit volume inside the fiber. Besides polymer composition, dope viscosity plays an important role in PAN fiber fabrication process. For example, Bajaj et al. kept the spinning dope viscosity between 5700 cps and 6250 cps in order to eliminate the die-swelling effect and to 252 A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257 enhance the spinning process [2]. The spinning process could be performed neither at lower nor higher than the suggested range since it could interrupt the fiber fabrication process and producing low quality PAN fibers. The coagulation bath temperature has a substantial effect on the coagulation process. It is responsible for controlling the mass transfer and the counter diffusion of the solvent and the non-solvent [2]. The mass transfer between the spun fibers and the surrounding medium influences microscopic and morphological structures as well as the mechanical properties of PAN fibers [6]. A reduction in the number of large voids in the crosssection of PAN fibers could be observed as the coagulation bath temperature is decreased [2,4,10]. As the coagulation temperature is reduced, the outward diffusion of solvent become greater than inward diffusion of non-solvent resulting in the gel-likefibers with a small pore size, higher density, less skin and low diameter of fibers [2]. This also leads to an improvement in the mechanical properties of PAN fibers due to less quantity of micro-pores inside the fibers [4]. However, at higher coagulation bath temperature, the fibers exhibit a maximum number of larger voids, an increment in diameter and reduction in the mechanical properties. The typical cross-sections of the PAN fibers are usually are rounding, kidney-bean-shaped or dog-bone-shaped [11]. The cross-section of the fibers could change from a beanshape to an oval-shape if the polymer solution was spun at higher coagulation bath temperatures and a high solvent content in the coagulation bath [2,4]. The above reports show that, in the solvent/non-solvent exchange process, the outward diffusion of solvent from the PAN fiber should be far greater than the inward diffusion of nonsolvent in order to produce the fiber without pores and with high mechanical integrity. Hence the conventional strategy for spinning PAN fibers was established by optimizing several spinning conditions. Large amount of solvent was added in the coagulation bath for decreasing the activity of non-solvent, which in turn suppressed the inward diffusion of the non-solvent. The coagulation bath temperature was lowered in order to let the solvent outflow dominated over the non-solvent inflow. By increasing the solvent content in the coagulation bath and reducing the coagulation bath temperature, a long coagulation bath is required for compensating the decrease in the diffusion rates. However, there are some drawbacks related to the application of conventional method in producing PAN fibers for carbon fiber production. The organic solvents such as dimethylformamide (DMF) and dimethylacetamide (DMAc) used in the coagulation bath could be hazardous since it could cause cancer for a long period of exposure. A high amount of solvent in the coagulation bath and low coagulation bath temperature enlarge the size of coagulation bath and could increase the production cost in terms of electricity consumption for cooling purposes. In order to circumvent these drawbacks, a novel strategy of PAN fiber spinning is proposed. The development of solvent-free coagulation process is considered because it is the best approach to avoid to the aforementioned difficulties. In this system, several modifications had been made to the conventional spinning system such as reducing the residence time, introducing a high jet stretch to the as-spun fiber and moderate coagulation bath temperature during coagulation process. Therefore, the objective of this research is to test if the newly proposed strategy is workable; in particular, the as-spun fiber is coagulated in a solvent-free coagulation bath under a high stretch ratio. The mechanical strength of the fibers is measured and the effect of coagulation bath temperature on the mechanical strength is examined. 2. Experimental 2.1. Spinning dope preparation In order to facilitate uniform dissolution of PAN (Aldrich, USA; Mw , 86,200) and acrylamide (AM) (Across Organics, USA) without formation of gel particles, the polymer and additive were firstly dispersed in cold DMF (Merck, Germany), which formed a fine slurry [11]. Then, heat was supplied continuously on the slurry at 80 ◦ C for 5 h to produce a highly viscous solution. The spinning solution was transferred into a solution bottle and then degassed in order to remove bubbles using ultrasonic bath (Branson 3510 Ultrasonic). 2.2. Dry–wet spinning The dry–wet spinning method was used to fabricate PAN fibers. The schematic diagram of the spinning machine as shown in Fig. 1 consists of a coagulation bath, spinneret, wind-up drum, gear pump, roller and refrigerator. During the fabrication process, the polymer solution was delivered from the storage reservoir by gear pump to the spinneret under pressure of nitrogen gas. The storage reservoir pressure was kept at 1 atm as a precaution against cavitation in the line to the pump. Throughout the experiment, jet stretch was maintained at a draw ratio of 4 in order to reduce fiber diameter, to improve molecular orientation as well as to remove pores. The residence time for the as-spun fiber in the coagulation bath was fixed at 5 s. Then, the fibers were collected onto a wind-up drum which was 17 cm in diameter. Next, the fibers were stretched and tied at a metal net and underwent a drying process. The air-circulated oven was set at 50 ◦ C and the PAN fibers were dried for 3 h. The polymer concentrations chosen as the spinning dope were 18 wt.% and 20 wt.%. Although the solution of 16 wt.% Fig. 1. Laboratory dry–wet spinning line [12]. A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257 polymer concentration had excellent fluidity, the as-spun fiber could not withstand the high jet stretch. This solution was considered to be a dilute solution of poor draw-ability [13]. Moreover, a low polymer concentration led to formation of a rigid skin before the center of the fiber was solidified, yielding a heterogeneous structure of the PAN fiber [1]. This further led to irregular distribution of stress during the drawing process and consequently caused the fiber breakage [9]. The fibers spun from the solutions of 18 wt.% to 20 wt.% polymer concentration, on the other hand, could easily be stretched up to the draw ratio of 4. The coagulation bath temperature chosen was in a range of 13–19 ◦ C. At a temperature below the selected temperature range, the counter diffusion of solvent and non-solvent was too slow. The as-spun fibers became less elastic and were not suitable for stretching. On the other hand, at a coagulation bath temperature above the suggested range, a skin appeared at the fiber surface. The skin prevented the solidification of the inner core, leading to fiber breakage by stretching. The relatively low coagulation bath temperature was required in this work because the coagulation bath contained no solvent. But the temperature was still higher than the conventional approach. The dope temperature was maintained at 22 ◦ C in order to reduce skin formation due to substantial temperature different during coagulation process [14]. At this temperature, solvent evaporation can be ignored, which also reduced the possibility of skin layer formation at the fiber surface. The details of the spinning conditions are summarized in Table 1. 253 Table 1 The dry–wet spinning process specifications Spinning dope Percentage of AM to PAN Polymer concentration Dope temperature Spinneret diameter Air gap distance Dope extrusion rate Extrusion velocity Wind-up drum velocity Jet stretch Residence time Coagulation bath composition Coagulation bath temperature Recirculation rate of water to and from coagulation bath PAN/AM/DMF 2.5 wt.% [15] 18 wt.% and 20 wt.% 22 ◦ C 200 m 1.5 cm 0.012 cm3 /min 35 cm/s 140 cm/s 4 5s 100% H2 O 13 ◦ C, 15 ◦ C, 17 ◦ C and 19 ◦ C 1 L/min 2.3. Characterization methods 2.3.1. Scanning electron microscopy Scanning electron microscopy (SEM) was used to observe the morphology of PAN fiber cross-section. The PAN fiber was immersed in liquid nitrogen for 10–30 min and then fractured carefully. For each fiber, three SEM samples were prepared. The sample was attached to copper-double-sided-tape in order to support the PAN fiber and then sputtered with gold by using an ion sputtering (Biorad Polaron Division) before viewing on the Scanning Electron Microscope (Phillips SEMEDAX; XL 40; PW6822/10) with potential of 10 kV under magnifications ranging from 1000× to 20,000×. Fig. 2. The cross-sections of PAN fiber fabricated at different coagulation bath temperatures (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C and using a polymer solution of 18 wt.%. 254 A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257 Fig. 3. The cross-section of PAN fibers fabricated at different coagulation bath temperature (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C and using a polymer solution of 20 wt.%. 2.3.2. Tensile testing for a tow of fibers Because of the lack of the sensitivity of the equipment for measuring the mechanical properties of PAN fiber, a tensile testing for single fiber could not be performed. Therefore, some modifications were made in order to obtain the Young’s modulus of the PAN fibers. A PAN fiber tow consisting of approximately 200 fibers was made and both ends were wrapped with masking tape, leaving approximately 25 mm gauge length. The load of the tensile testing machine was set to 1 kN and the test was carried out using a 50 mm/min crosshead speed. A high crosshead speed was chosen in order to obtain clear ‘load versus extension gauge length correction’ graphs instead of using the recommended crosshead speed at 5 mm/min. The strain and stress value were calculated manually using Excel spreadsheet and the strain versus stress were plotted. observable on the cross-sectional surface appeared during the SEM sample preparation due to a fracture effect. These pictures show that dense fibers could be spun under the spinning conditions adopted in this work. This shows clearly that the strategy to spin fibers of dense structure, i.e. immersion of fibers into coagulation bath that contains no solvent and a short residence time in the coagulation bath with a high jet stretch was successful. 3. Results and discussion 3.1. Effect of spinning process on the morphology of PAN fibers Although a single hole spinneret with a hole diameter of 200 m was used throughout this work, the diameter of the as-spun fibers was 50–70 m due to the high jet stretch of 4. Figs. 2(a–d) and 3(a–d) show the low magnification (1000–4000×) SEM pictures of fibers prepared from dope solutions of 18 wt.% to 20 wt.% polymer concentration, respectively, at different coagulation bath temperatures. At this magnification level, pores (defects) that will reduce the mechanical strength of the fibers could not be observed. Some cracks that were Fig. 4. The change in the Young’s modulus of PAN fibers with coagulation bath temperatures. A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257 255 Fig. 5. The cross-section of PAN fibers fabricated using a polymer solution of 18 wt.% at magnificent of 20,000× (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C. 3.2. Effects of coagulation bath temperature and polymer concentration on the morphology and mechanical properties of PAN fibers Fig. 4 shows Young’s modulus versus coagulation bath temperature for fibers spun from spinning dopes of 18 wt.% to 20 wt.% polymer concentrations. Young’s modulus increased with a decrease in temperature for both polymer concentrations, although the increase was more dramatic for 18 wt.% polymer concentration. These results were in agreement with those reported by Bahrami et al. [4]. In Fig. 4, the Young’s modulus increased from 1.95 GPa to 2.93 GPa, with respect to the polymer concentration of 18 wt.% when the temperature was decreased from 19 ◦ C to 13 ◦ C. This steep increase can be explained by Fig. 6. The cross-section of PAN fibers fabricated using a polymer solution of 20 wt.% at magnificent of 20,000× (a) 13 ◦ C, (b) 15 ◦ C, (c) 17 ◦ C and (d) 19 ◦ C. 256 A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257 the SEM pictures of higher magnification (20,000×) shown in Fig. 5. The figure shows that the structures of the fibers are nodular and the interstitial void spaces of nano-sizes become less as the temperature decreases from 19 ◦ C (Fig. 5(d)) to 13 ◦ C (Fig. 5(a)). The presence of the nano-sized void spaces seems to reduce Young’s modulus, weakening the mechanical strength of the fiber. The enhancement of void spaces at higher coagulation bath temperature can be explained by the solvent/non-solvent exchange. While most of the solvent is squeezed out of the fiber while it is stretched, some non-solvent will enter into the fiber by counter diffusion. As the coagulation bath temperature increased, the rate of the counter diffusion increased, leaving more non-solvent inside the fiber. The spaces filled with the non-solvent would eventually turn into nano-sized void spaces. Further looking into Fig. 4, Young’s modulus increased from 1.86 GPa to 1.97 GPa, with respect to the polymer concentration of 20 wt.%, as the temperature decreased from 19 ◦ C to 13 ◦ C. The reason was found in Fig. 6. These pictures also depicted the nodular structure of the fibers. In contrast to the polymer concentration of 18 wt.% (Fig. 5), however, the interstitial void spaces did not increase with an increase in temperature. Instead, the number of the cracks that were distributed uniformly across the cross-section becomes the largest at 19 ◦ C. This probably was the reason why the Young’s modulus was the smallest at this temperature. The cracks did not disappear even at the lowest temperature and the Young’s modulus remained low. The formation of crack in the matrix of the fiber was due to the concentrated polymer solution used during spinning process. This result was in agreement with Stoyanoz [9]. The outflow of solvent for concentrated polymer solution became rapid and turned the as-spun fiber into rigid structure during coagulation process. Consequently, high draw ratio applied during spinning process led to the distribution of crack in the cross-section of PAN fiber. Fig. 8. The relationship between Young’s modulus and polymer solution composition during fiber fabrication process in the solvent-free coagulation bath. Fig. 7 shows tensile strength versus coagulation bath temperature for the polymer concentrations of 18 wt.% and 20 wt.%. Similar to the Young’s modulus, tensile strength was also increased with a decrease in coagulation bath temperature and the increase was steeper for 18 wt.%. Stoyanoz reported that tensile strength dry (TSD) of acrylic fibers showed a maximum at a certain polymer concentration [9]. The relationship between the PAN polymer concentration and Young’s modulus (mechanical strength) would probably exhibits the same pattern as illustrated in Fig. 8. When the polymer concentration was 16 wt.%, it was in the dilute solution range and the as-spun fiber could not withstand any draw. The polymer concentration of 18 wt.% was in the semi-dilute solution range and showed the highest mechanical strength. When the polymer concentration was 20 wt.%, it was in the concentrated solution range, where the mechanical strength of fiber started to diminish. As Stoyanoz mentioned, the solution of high polymer concentration has a relatively high viscosity. The outflow of the solvent is rapid, causing the formation of a skin layer. Breakage by ‘slippage’ is possible between the solid skin and the fluid core [16]. 4. Conclusion Fig. 7. The change in the tensile strength of PAN fibers with coagulation bath temperatures. PAN fibers were fabricated using analytical grade PAN (Mw , 86,200) via a solvent-free coagulation process. These studies investigated the appropriate range of coagulation bath temperature and polymer solution composition for spinning process in the solvent-free coagulation process and the effect of these parameters on PAN fiber formation. The fabrications of PAN fibers were conducted using polymer solution of 18 wt.% and 20 wt.% and in the coagulation bath temperature range of 13–19 ◦ C. Based on qualitative analysis, the polymer solution of 18 wt.% enhanced the fabrication process. This polymer solution showed a fine fluidity which allowed the spinning process A.F. Ismail et al. / Materials Science and Engineering A 485 (2008) 251–257 to run smoothly and the as-spun fibers were able to undergo a stretching process without failure. 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