Stabilization of Commercial Polyacrylonitrile Fibres for Fabrication of Low-Cost Medium-Strength Carbon Fibres

http://www.e-polymers.org e-Polymers 2006, no. 001. ISSN 1618-7229

Stabilization of commercial polyacrylonitrile fibres for fabrication of low-cost medium-strength carbon fibres

Reza Eslami Farsani 1 *, Ali Shokuhfar 2, Arman Sedghi 3

1 Faculty of Mechanical Engineering, K. N. Toosi University of Technology & Islamic Azad University-South Tehran Branch, Tehran, Iran; Fax +98-21-22051368; [email protected] 2 Faculty of Mechanical Engineering, K. N. Toosi University of Technology, Tehran, Iran; Fax +98-21-22051368; [email protected] 3 Faculty of Engineering, Imam Khomeini International University, Qazvin, Iran; Fax +98-21-66520069; [email protected]

(Received: August 17, 2005; published: January 27, 2006)

Abstract: The process of fabrication of carbon fibres from polyacrylonitrile (PAN) fibres is composed of two steps including oxidative stabilization at low temperature and carbonization at high temperature in an inert atmosphere. Due to the high price of the raw material (special PAN fibres), carbon fibres are still expensive. It is shown that it is possible to produce desirable carbon fibres from (cheaper) commercial PAN fibres. In order to achieve this, it is necessary to make some changes in the conventional stabilization procedure to reach complete stabilization.

Introduction Carbon fibres are a new kind of high-performance materials, which have attracted worldwide attention. After a 40-year period of development and use in specialized applications such as aerospace industry and military aircraft, they are now on the brink of broad commercialization. They are finding wide applications in commercial and civilian aircraft, construction, transportation, sporting goods, and a variety of commercial/industrial applications. They have a low density and thus posses the highest specific strength and stiffness compared to all currently available engineering materials. They are elastic to failure at normal temperatures, which renders them creep resistant and non-susceptible to fatigue. They are chemically inert except in strongly oxidizing environments or in contact with some molten metals. Carbon fibres exhibit exceptional thermophysical properties and excellent damping characteristics [1-3]. Carbon fibres are used in composites with polymer, metal and ceramic matrices. Among the various composites, carbon fibre reinforced plastics are particularly widely used as high-performance materials in view of their low weight and special properties of the reinforcing carbon fibres. Carbon fibres are mainly used in different forms to reinforce lightweight polymer materials such as epoxy resins, polyesters or polyamides. For example, short or continuous yarns, fabrics, etc. can be used to

1 contribute increased stiffness and strength, and reduce the thermal expansion coefficient in the polymer matrix composites. ‘Stronger than steel, stiffer than titanium, and lighter than aluminium’ has become a cliché for carbon fibre composites and is now being realized in practice [2-4]. Carbon fibres have been made inadvertently from natural cellulosic fibres such as cotton or linen for thousands of years. However, it was Edison who, in 1878, purposely took cotton fibres and later, bamboo, and converted them into carbon in his quest for incandescent lamp filaments. Interest in carbon fibres was renewed in the late 1950s when synthetic rayon in textile form was carbonized to produce carbon fibres for high-temperature applications in missiles [5]. At the end of the 1960s, high-strength high-modulus carbon fibres based on polyacrylonitrile (PAN) were developed in Japan and appeared on the commercial market. Research aimed at obtaining partially carbonized fibres and carbon fibres based on PAN started for the first time (1959 - 60) in the USSR, before the Japanese investigation. However, at that time, the results were not brought into practical application [6]. The first pitch-based carbon fibre was developed by Otani in Japan using poly(vinyl chloride) (PVC) pitch as the raw material [7]. In recent years, this fibre has been produced from vapour-grown material, but it is still in the experimental stage [3]. At present, three precursors including PAN-based, rayon-based, and pitch-based fibres are mainly used for the production of carbon fibres. The majority of all carbon fibres used today are made of PAN precursor, which is a form of acrylic fibre [2,6]. Acrylic fibres manufactured presently are composed of at least 85 wt.-% of acrylonitrile (AN) units. The remaining 15% consists of neutral and/or ionic comonomers, which are added to improve the properties of the fibres. Neutral comonomers like methyl acrylate (MA), vinyl acetate (VA), or methyl methacrylate (MMA) are used to modify the solubility of the acrylic copolymers in spinning solvents, to modify the acrylic fibre morphology, and to improve the rate of diffusion of dyes into the acrylic fibre. Ionic and acidic comonomers including sulfonate groups like sodium methallyl sulfonate (SMS), sodium 2-methyl-2-acrylamidopropanesulfonate (SAMPS), sodium p-styrenesulfonate (SSS), sodium p-sulfophenyl methallyl ether (SMPE), and itaconic acid (IA) also can be used to provide dye sites apart from end groups and to increase hydrophilicity. Acrylic fibres used for obtaining precursor fibres usually contain 5 - 10% neutral comonomers, 0 - 5% acidic and ionic comonomers, and the remaining acrylonitrile units [8-10]. This organic material has an open chain structure with carbon as its backbone (Fig. 1). The molecular structure of this fibre is composed of a set of long chain molecules [7,11]. The manufacture of carbon fibres from PAN-based precursors is mainly conducted in two steps including thermal stabilization and carbonization. The first step (stabilization) involves heating the PAN fibres to approximately 180 to 300°C in an oxygen- containing atmosphere to further orient and then crosslink the molecules, so that they can survive higher-temperature pyrolysis without decomposing. The chemistry of the stabilization process is complex, but consists of cyclization of the nitrile groups (C≡N) and cross-linking of the chain molecules followed by dehydrogenation and oxidative reactions. This process transforms the linear polymer (or laterally ordered polymer) into a ladder structure (Fig. 1), which renders the polymer thermally stable and prevents melting during the subsequent carbonization process [2,11,12]. The second step involves a carbonizing heat treatment of the stabilized PAN fibres to remove the non-carbon elements in the form of different gases like H2O, NH3, CO, HCN, CO2 and N2. Carbonization is carried out at temperatures ranging from 1000 to 1500°C in an 2 inert atmosphere. During this process, the fibres shrink in diameter and lose c. 50% in weight. An additional step after carbonization is further heat treatment of the fibre at a higher graphitization temperature in order to improve the mechanical properties [2,13].

Fig. 1. Structural change in PAN fibres from open chain polymers to ladder polymers in the stabilization stage

Carbon fibres are expensive due to 2 reasons: 1. The high price of raw materials (PAN fibres). 2. The high cost of production. PAN fibres used in production of carbon fibres are a special type of these fibres, which are different from those used in textile industry in terms of chemical composition, type and amount of comonomers, cross- section area, linear density and tensile strength. Textile PAN fibres (with applications like production of blankets, carpets and clothes) have higher cross-section area and linear density and lower primary tensile strength than the special PAN fibres. It is not easy to change and modify some of the parameters in textile PAN fibres’ production (such as the type and amount of comonomers and linear density) and, if applicable, it increases the price. Meanwhile it is not possible to produce suitable carbon fibres from this type of PAN fibres and the final product has very low quality [1]. However, in recent years some studies have been conducted to use low-price textile PAN fibres, which leads to reduction of the price of carbon fibres and these studies almost succeeded in this regard [14,15]. In previous studies by using some chemical and mechanical treatments before and after stabilization, carbon fibres with suitable mechanical properties were produced. The aim of this article is to examine the possi- bility of carbon fibres fabrication from textile PAN fibres with changes in stabilization parameters. To this matter, thermal characteristics of commercial PAN fibres were investigated and based upon the obtained results, with some changes in the conventional procedure of stabilization in terms of temperature and time variables, the desirable conditions of complete stabilization are achieved.

Experimental part

Materials PAN fibres that were used in this study were produced by the dry spinning method and have a dog-bone-like cross-section. Tab. 1 shows the chemical analysis of these fibres. 3 Tab. 1. Chemical analysis of commercial PAN fibres used

Material Weight fraction in % acrylonitrile (AN) 93 methyl acrylate (MA) 6 sodium methallyl sulfonate (SMS) 1

Methods The PAN fibres were converted into carbon fibres during two stages as follows: 1 - Stabilization in a chamber furnace with the air circulation system at temperatures ranging from 180 to 330°C in discontinuous working conditions. 2 - Carbonization of the stabilized PAN fibres in a horizontal tubular furnace with a ceramic tube under a high-purity nitrogen atmosphere (99.9999%) at temperatures ranging from 1200 to 1450°C for a period of 10 min. To determine the properties of PAN precursor fibres, stabilized PAN fibres and carbon fibres, the following tests and analyses were done: 1 - Thermal analysis including differential scanning calorimetry (DSC) and thermo- gravimetrical analysis (TGA) were carried out by using a STA device (STA-625 from Rheometric Scientific). Samples were heated to 400°C under an air atmosphere at a heating rate of 2°C/min. 2 - Tensile strength test conducted on single fibre samples by the ISO 11566 standard procedure. The test apparatus consisted of an Instron 5565 tensile tester equipped with a 2.5 N load cell and a cross-head speed of 2 mm/min. The gauge length was kept at 25 mm. At least 25 tensile tests were performed on each fibre type and the average of the test results is reported here. 3 - Density was determined on short lengths of the fibre bundles in density gradient columns prepared from ZnCl2 and H2O by the ISO 10119 standard procedure. The average density of three tests was taken as the density of each sample. 4 – The cross-sectional area of fibres was determined using a Nikon YS100 optical microscope. The cross-section of a PAN fibre is dog-bone shaped. Therefore, in order to determine the tensile strength, the cross-section had to be calculated. Thus precursors of PAN fibres, stabilized PAN fibres and carbon fibres were embedded in an epoxy resin and their cross-section images were taken with an optical microscope. Their correct cross-section area was calculated by image analysis software connect- ed to this instrument. 5 - Scanning electron microscopy was carried out using a CAMSCAN MV2300 microscope.

Results and discussion According to Tab. 1, commercial PAN fibres in this study contain AN, the neutral comonomer MA and the ionic comonomer SMS. In special PAN fibres, in addition to AN, there are the comonomers MA, carboxylic acid, vinyl bromide, acrylic acid, methacrylic acid and IA. But in textile PAN fibres, there are usually MA, VA, SAMPS and SMS comonomers. Some of the comonomers (such as IA) act as initiator of 4 stabilization and cause stabilization to perform at low temperature and high speed and the time of stabilization is reduced [13,16]. But commercial PAN fibres selected for this study contain SMS comonomer instead of IA. This comonomer was added to provide dye sites for linking basic dyes. In Tab. 2, the properties of studied commercial PAN fibres are given.

Tab. 2. Properties of commercial PAN fibres used

Tensile strength of fibre 244 MPa Elongation-at-break 41% Linear density 0.5 tex

Special PAN fibres, which are commonly used to produce carbon fibres, have a circular section (with diameter up to 15 µm and cross section up to 180 µm2) and low linear density (up to 0.17 tex) but commercial PAN fibres (mentioned above) have a dog-bone shaped section with a cross-section of 491 µm2 and linear density of 0.5 tex. High cross-section area and linear density of PAN fibres cause incomplete stabilization of fibres in ordinary stabilization time-temperature cycles and only surface and middle layers become stabilized. So it is necessary to change the conventional procedure of stabilization, by selecting different time and temperature stabilization cycles. Therefore, thermal characteristics of tested commercial PAN fibres were investigated. In Fig. 2, DSC and TGA curves of commercial PAN fibres are shown.

100 269 286 1.8 TGA 244 1.6 95 307 1.4 1.2 90 1 0.8 85 0.6 Weight (%) 0.4 221 DSC 80 0.2 Heat Flow (mCal/Sec) Flow Heat 0 75 -0.2 50 100 150 200 250 300 350 400 450

Temperature (C) Fig. 2. DSC and TGA curves of commercial PAN fibres used

According to the DSC curve, two exothermic processes were observed in commercial PAN fibres. The first exothermic process occurs in a wide temperature range. This exothermic process is related to thermal stabilizing processes. The evolution of a large amount of heat in this case has been attributed to the cyclization of nitrile groups [16]. The temperature of the initiating exothermic cyclization reaction is 221°C and the range of peak temperature is 269 - 286°C. Also, a second exothermic peak

5 can be seen at 307°C. This small hump probably resulted from undesirable oxidation reactions [17] or additional intermolecular cross-linking reactions [18,19].

Tab. 3. Stabilization cycles of PAN fibres Cycle code Time-temperature cycle

S1 25 - 220°C: 60 min and hold in 220°C for 1 h

220 - 245°C: 30 min and hold in 245°C for 1 h 245 - 270°C: 30 min and hold in 270°C for 1 h

S2 25 - 240°C: 60 min and hold in 240°C for 1 h

240 - 265°C: 30 min and hold in 265°C for 1 h

265 - 275°C: 30 min and hold in 275°C for 1 h

S3 25 - 220°C: 60 min and hold in 220°C for 1 h

220 - 245°C: 30 min and hold in 245°C for 1 h

245 - 270°C: 30 min and hold in 270°C for 1 h

270 - 285°C: 30 min and hold in 285°C for 1 h

S4 25 - 240°C: 60 min and hold in 240°C for 1 h

240 - 265°C: 30 min and hold in 265°C for 1 h

265 - 275°C: 30 min and hold in 275°C for 1 h

275 - 285°C: 30 min and hold in 285°C for 1 h

S5 25 - 200°C: 60 min and hold in 200°C for 1 h

200 - 220°C: 30 min and hold in 220°C for 1 h

220 - 240°C: 30 min and hold in 240°C for 1 h

240 - 280°C: 30 min and hold in 280°C for 1 h

S6 25 - 240°C: 60 min and hold in 240°C for 1 h

240 - 275°C: 30 min and hold in 275°C for 1 h

275 - 285°C: 30 min and hold in 285°C for 1 h 285 - 300°C: 30 min and hold in 300°C for 1 h

S7 25 - 200°C: 60 min and hold in 200°C for 1 h

200 - 210°C: 30 min and hold in 210°C for 1 h

210 - 220°C: 30 min and hold in 220°C for 1 h 220 - 245°C: 30 min and hold in 245°C for 1 h

S8 25 - 180°C: 60 min and hold in 180°C for 1 h

180 - 200°C: 30 min and hold in 200°C for 1 h

200 - 220°C: 30 min and hold in 220°C for 1 h

220 - 245°C: 30 min and hold in 245°C for 1 h

S9 25 - 180°C: 60 min and hold in 180°C for 1 h 180 - 245°C: 30 min and hold in 245°C for 1 h 245 - 300°C: 30 min and hold in 300°C for 1 h 300 - 330°C: 30 min and hold in 330°C for 1 h

6 Based on the TGA curve, PAN fibre reactions are carried out with weight loss and the temperature of the initial high weight loss is 244°C. This behaviour was confirmed by DSC experiments. Comparing these results, it is revealed that weight loss at this stage is a result of oxidative stabilization, changes in polymer bonds, and also removal of some volatile materials (in the form of H2O, HCN and CO2). Of course, this weight loss is much lower compared with that during of carbonization (as a result of non-carbon elements elimination in the form of various gases). In DSC curves, the temperature of the initial reaction as well as the peak temperature depend largely on the experimental conditions, and especially on the rate of heating [20]. Therefore, the temperatures resulting from the above curve are not the exact temperatures of the exothermic reactions and can vary within a thermal range. Based on our DSC and TGA curves and publications, experiences and experimental methods of Russian scientists [21-23] multiple heat treatment cycles were designed and applied for the stabilization of PAN fibres. In Tab. 3 various types of stabilization process cycles are presented. Many investigations were concentrated on the stabilization of PAN fibres but the mechanism of this process is still not well understood and many mechanisms were proposed for the stabilization reaction. Nevertheless, it is well known that this process has a very important effect on the final properties of fabricated carbon fibres and well- stabilized PAN fibres have very good mechanical and physical properties [21,24]. Tarakanova et al. [22] found that the stabilization process must be performed step by step and in different temperature and time regimes. In the stabilization process, relaxation and thermomechanical properties of the polymer are not the same at each temperature. Thus this process needs to be divided into different steps, each step having a different temperature range, holding time and deformation amount. By this method, the molecular orientation in the structure reaches a maximum and the structural defects reach a minimum. Savchenko et al. [23] also found that performing stabilization in a continuous regime will destroy the fibre structure but dividing it into separate steps prevents structural damages, increases fibre linear density and improves mechanical properties of fabricated carbon fibres. In Tab. 4 the results of density and tensile strength measurements of the stabilized PAN fibres are presented.

Tab. 4. Density and tensile strength of stabilized PAN fibres Cycle code Density Tensile strength Fibre tensile strength reduction in g/cm3 in MPa during stabilization in %

S1 1.36 164 33

S2 1.34 161 34.2

S3 1.35 162 33.7

S4 1.33 142 42

S5 1.36 168 31.1

S6 1.34 158 35.2

S7 1.34 152 37.9

S8 1.33 142 41.7

S9 1.34 151 38.2 7 Tab. 5. Tensile strength of carbon fibres stabilized by different cycles Code of Code of Temperature of Tensile strength of stabilization cycle carbonisation cycle carbonisation carbon fibres in in °C MPa

S1 C1 1200 1641

S2 C1 1200 1422

S3 C1 1200 1532

S4 C1 1200 1326

S5 C1 1200 1742

S5 C2 1250 1757

S5 C3 1300 1854

S5 C4 1350 1902

S5 C5 1400 1965

S5 C6 1450 1898

S6 C1 1200 1280

S7 C1 1200 1162

S8 C1 1200 1129

S9 C1 1200 1025

As it was stated earlier, the major requirement for producing carbon fibres with desirable mechanical properties from PAN fibres is the fact that PAN fibres become completely stabilized. Different sources expressed different criteria for complete stabilization. Gaining density of at least 1.35 g/cm3 for stabilized PAN fibres [25], about 30% reduction in tensile strength for stabilized PAN fibres in comparison with PAN fibres [26], and 8 - 12% oxygen content in stabilized PAN fibres [3] are among those mentioned criteria. On the basis of the stabilized PAN fibres’ densities shown in Tab. 4, stabilization procedure under cycles S1 and S5 in comparison with other cycles are more complete because the stabilized PAN fibres have high density (1.36 g/cm3) in comparison with stabilized PAN fibres produced from other cycles. Also the results of Tab. 4 regarding the ratio of fibre tensile strength reduction during stabilization confirm that stabilization was completed by cycles S1 and S5, because the relative reductions under cycles S1 and S5 are 33% and 31.1%, respectively. Certainly, all the stabilized sample fibres (under cycles S1 to S9) were carbonized. Tab. 5 shows the tensile strength of different types of carbon fibres.

Tab. 5 shows that the carbon fibres produced from samples S1 and S5 in comparison with other samples have higher tensile strength. This issue confirms the proper selection of S1 and S5 as the most complete cycles of stabilization. Because of the fact that carbon fibres produced from sample S5 in comparison with sample S1 have higher tensile strength, it is concluded that cycle S5 is the best cycle of stabilization for tested commercial PAN precursor fibres.

Stabilized PAN fibres under cycle S5 were carbonized at higher temperatures from 1200°C to 1450°C, the results of which are shown in Tab. 5. The tensile strength of carbon fibres begins to increase with increasing temperature of carbonization up to 8 1400°C and then begins to decrease. This fact complies with the results of studies on special PAN fibres. Fitzer states that an increase in the final heat treatment temperature for producing carbon fibres up to 1600°C comes along with an increase of tensile strength and after that temperature there is a sudden reduction of tensile strength [27]. He claims that this reduction is related to nitrogen release from the fibre structure [28]. This finding is 200°C above our results and it is concluded that comonomers as well as fibres’ fabrication histories change the variation of tensile strength with heat treatment temperature, too. According to the results of Tab. 5, the highest tensile strength of carbon fibres fabricated from dry spinning commercial PAN fibres is 1965 MPa and comes with stabilization cycle S5 and carbonization cycle C5. Fig. 3 shows SEM images of PAN precursor fibres and fabricated carbon fibres. This images show that the section of carbon fibres is similar to PAN precursor fibres and has a dog-bone-like form. Therefore, one can say that the section of fibres during the procedure of stabilization and carbonization has been unchanged and only the cross section area has been reduced. This reduction comes along with the increase in tensile strength of fibres after carbonization.

a) Commercial PAN fibres b) Carbon fibres Fig. 3. SEM images of dog-bone-shaped sections of PAN precursor fibres and carbon fibres made from them

Conclusion Through application of stabilization and carbonization processes, it is possible to produce desirable carbon fibres from commercial PAN fibres. In order to achieve this, it is necessary to make some changes in the conventional stabilization procedure to make sure that stabilization is completed. The best tensile strength of carbon fibres is achieved with the stabilization cycle S5 and the carbonization cycle C5 and reaches 1965 MPa.

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