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International Conference on Chemical, Civil and Environment (ICCEE'2012) March 24-25, 2012 Dubai

Production of from Acrylic Fibers

R. Eslami Farsani

matrix composites. ‘Stronger than , stiffer than titanium, Abstract— are fabricated from different materials and lighter than aluminum’ has become a cliché for carbon such as acrylic fibers, cellulose fibers and . But acrylic fibers are composites and is now being realized in practice [4-7]. recognized as the most widely used precursor for the present-day At present, three precursors including acrylic-based, - manufacture of carbon fibers. The process of fabrication carbon based, and pitch-based fibers are mainly used for the fibers from acrylic fibers is composed of two steps including oxidative stabilization at low temperature and carbonization at high production of carbon fibers. The majority of all carbon fibers temperatures in an inert atmosphere. Today carbon fibers are still used today are made from acrylic precursor [1,6]. Acrylic expensive because of the high price of their raw material. This study fibers manufactured presently are composed of at least 85% focuses on making carbon fibers from acrylic fibers (low price by weight of (AN) units. The remaining 15% acrylic fibers used in ). The results shows that in case consists of neutral and/or ionic comonomers which are added of conducting complete stabilization process, it is possible to produce to improve the properties of the fibers. Neutral comonomers desirable carbon fibers from commercial acrylic fibers. With some changes in conventional procedure of stabilization in terms of like methyl acrylate (MA), vinyl acetate (VA), or methyl temperature and time of operation, the desirable conditions of methacrylate (MMA) are used to modify the solubility of the complete stabilization are achieved. acrylic copolymers in solvents, to modify the morphology, and to improve the rate of diffusion of Keywords—Acrylic Fibers, Stabilization, Carbon Fibers. into the acrylic fiber. Ionic and acidic comonomers including the sulfonate groups like sodium methallyl sulfonate (SMS), I. INTRODUCTION sodium 2-methyl-2-acrylamidopropane sulfonate (SAMPS), ARBON is not found naturally in a fibrous form. It exists sodium p-styrene sulfonate (SSS), sodium p-sulfophenyl C as , various amorphous (in e.g. coals) and methallyl ether (SMPE), and itaconic acid (IA) also can be . Thus carbon fibers are artificial products. Carbon used to provide sites apart from end groups and to fibers are fibers made by human containing minimum 92% increase hydrophilicity. The composition of acrylic fiber were carbon with diameter 6-10 μ (thinner than human hair) and used for obtaining precursor fibers usually contains 5-10 % with arrangement completely directional towards carbon neutral comonomers, 0-5 % acidic and ionic comonomers and . Carbon fiber was manufactured for the first time in the remaining acrylonitrile units [8-10]. The molecular 1880 by Thomas Edison by carbonization of . It was structure of this fiber is composed of a set of long chain intended for use as a filament in electric lamps. But carbon molecules [3,11]. fibers did not find application in electric lamps because of the The manufacture of carbon fibers from acrylic-based advent of tungsten filaments. Interest in carbon fibers was precursor is composed of two steps including thermal renewed in the late 1950s when synthetic in textile stabilization and carbonization. The first step (stabilization) form were carbonized to produce carbon fibers for high- involves heating the acrylic fibers to approximately 180 to temperature missile applications [1-3]. 300 ºC in an oxygen-containing atmosphere to further orient Carbon fibers are used in composites with , metal and then crosslink the molecules, such that they can survive and ceramic matrices. Among the various composites, carbon higher temperature without decomposing. The fiber reinforced have been particularly widely used as chemistry of the stabilization process is complex, but consists high materials in view of their light weight and of cyclization of the nitrile groups (C≡N) and cross-linking of special properties of the reinforcing carbon fibers. Carbon the chain molecules followed by dehydrogenation and fibers are mainly used in different forms to reinforce oxidative reactions. This process transforms the linear lightweight polymer materials such as epoxy resin, polymer (or laterally ordered polymer) into a ladder structure or . For example, short or continuous , which renders the polymer thermally stable and prevents fabrics, etc. can be used to contribute stiffness, strength and melting during the subsequent carbonization process [6,11]. reduce the thermal expansion coefficient in the polymer The second step involves a carbonizing heat treatment of the stabilized acrylic fibers to remove the non-carbon elements in the form of different gases like H2O, NH3, CO, HCN, CO2 R. Eslami Farsani is with South Tehran Branch, Islamic Azad University, Tehran, Iran (corresponding author to provide phone: 98-21-88303278; fax: and N2. Carbonization is carried out at temperatures ranging 98-21-88830012; e-mail: [email protected]). from 1000 to 1500 ºC in an inert atmosphere. During this

310 International Conference on Chemical, Civil and Environment engineering (ICCEE'2012) March 24-25, 2012 Dubai

process, the fibers shrink in diameter and lose approximately III. R2B ESULTS AND DISCUSSION 50% in weight [6,12]. In table 2, the properties of tested acrylic fibers are given. Acrylic fibers used in production of carbon fibers are special type of these fibers which are expensive and different TABLE II from what are used in . In recent years some THE SPECIFICATION OF ACRYLIC FIBERS studies have been done to use textile acrylic fibers with low Property Amount price which led to reduction of price of carbon fibers and Tensile Strength of Fiber 226 MPa these studies almost succeeded in this regard [13,14]. In previous studies by using some chemical and mechanical Elongation-at-Break 43.9% treatments before and after stabilization, carbon fibers with Linear Density 0.56 Tex suitable mechanical properties were produced. The aim of this Cross Section 530 μ² article is to examine possibility of carbon fibers fabrication from textile acrylic fibers with changes in fabrication parameters. Special acrylic fibers which are commonly used to produce carbon fibers have the circular section and low linear density II. EXPERIMENTAL PROCEDURE but commercial acrylic fibers have the bean shaped section Acrylic fibers are used in this study were produced by wet and high linear density. spinning method and have the bean like cross section. Table 1 High cross section and linear density of commercial acrylic shows the chemical analysis of these fibers. Heat treatment on fibers cause incomplete stabilization of fibers in ordinary acrylic fibers includes two phases: stabilization and stabilization time-temperature cycles and only surface and carbonization. In stabilization phase, acrylic fibers were put in middle layers become stabilized. So it is necessary to change the furnace with air circulating system and were heated in air time and temperature of stabilization cycles in order to find environment under different time-temperature cycles up to when stabilization becomes complete. By selecting different 330 ºC. In carbonization phase, stabilized acrylic fibers were stabilization cycles, some cycles performed and after each put in the horizontal muffle furnace and were heated in purity procedure, density and tensile strength of stabilized nitrogen (99.99%) up to 1350 ºC and hold for 5 minutes. commercial fibers were measured and results were compared. TABLE I Table 3 shows procedures of stabilization and table 4 shows THE CHEMICAL ANALYSIS OF ACRYLIC FIBERS the results of density and tensile strength of stabilized commercial fibers. Name of Material Weight (%) TABLE III AN 93 STABILIZATION CYCLES OF ACRYLIC FIBERS

MA 6 Cycle Time-Temperature Cycle Code SAMPS 1 25-200 (ºC): 60 min. and hold in 200 ºC for 1 hour To examine the properties of acrylic precursor fibers, 200-240 (ºC): 30 min. and hold in 240 ºC for 1 hour stabilized acrylic fibers and carbon fibers, the following S1 240-280 (ºC): 30 min. and hold in 280 ºC for 1 hour instrument were used: - Tensile strength testing was done on single fiber samples. 280-300 (ºC): 30 min. and hold in 300 ºC for 1 hour The test apparatus consisted of an Instron 5565 tensile tester 25-200 (ºC): 60 min. and hold in 200 ºC for 1 hour 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 25 200-240 (ºC): 30 min. and hold in 240 ºC for 1 hour S2 tensile tests were performed on each fiber types and average 240-280 (ºC): 30 min. and hold in 280 ºC for 1 hour of test results were reported here. 280-330 (ºC): 30 min. and hold in 330 ºC for 1 hour - Fibers density was determined on by density gradient columns prepared from ZnCl2 and H2O. The average 25-185 (ºC): 60 min. and hold in 185 ºC for 1 hour density computed from three tests and was taken as the 185-200 (ºC): 30 min. and hold in 200 ºC for 1 hour density of each sample. S3 200-215 (ºC): 30 min. and hold in 215 ºC for 1 hour - Nikon YS100 optical was applied to determine the cross section of acrylic fibers, stabilized acrylic fibers 215-245 (ºC): 30 min. and hold in 245 ºC for 1 hour and carbon fibers. Because the cross section of acrylic fibers 25-185 (ºC): 60 min. and hold in 185 ºC for 1 hour is not totally round and it is bean shaped, to calculate correct 185-200 (ºC): 30 min. and hold in 200 ºC for 1 hour cross sectional area fibers were embedded in epoxy resin S4 and their cross sections images were taken by optical 200-215 (ºC): 30 min. and hold in 215 ºC for 1 hour microscope. Their correct cross sectional area calculated by 215-270 (ºC): 30 min. and hold in 270 ºC for 1 hour image analysis software connected to this instrument.

311 International Conference on Chemical, Civil and Environment engineering (ICCEE'2012) March 24-25, 2012 Dubai

TABLE IV IV. C3B ONCLUSION DENSITY & TENSILE STRENGTH OF STABILIZED ACRYLIC FIBERS It is possible to produce carbon fibers from commercial Tensile Ratio of Fiber Tensile acrylic fibers. But, it is necessary to make some changes in Cycle Density Strength Strength Reduction Code (g/cm³) conventional stabilization procedure to make sure that (MPa) During Stabilization (%) stabilization procedure is completely done. The best tensile strength of carbon fibers is gained with stabilization cycle S4 S1 1.35 150 32.4 and reaches to 1439 MPa. S2 1.35 144 33.9 S3 1.36 155 30.8 S4 1.38 162 29.8 REFERENCES [1] V. I. Kostikov, “Fibre science & technology”, Chapman & Hall, 1995. [2] T. J. Reinhart, et al., “Engineered materials handbook, Vol. 1, As it was stated earlier, the major requirement for Composites, ASM International, 1988. [3] J. B. Donnet and R. C. Bansal, “Carbon fibers”, Marcel Dekker, 1990. producing carbon fibers with desirable mechanical properties [4] R. C. Bansal and J. B. Donnet, “Comprehensive Polymer Science, from acrylic fibers is the fact that acrylic fibers become Polymer Reactions”, Pergamon, 1990. completely stabilized. Different sources expressed different [5] O. P. Bahl and L. M. Manocha, “Development of High Performance criteria for complete stabilization. Gaining density in ranges Carbon Fibers from PAN Fibers”, Chemical Age of India, vol. 38, no. 5, pp. 181-188, 1987. between 1.35-1.40 gr/cm³ [15], reduction about 30 % in [6] P. J. Walsh, ASM handbook- composites, vol. 21, ASM International, tensile strength for stabilized acrylic fibers in comparison with USA, 2001. untreated fibers [16] and 8-12 % oxygen content in stabilized [7] S. M. Lee, “International encyclopedia of composites”, vol. 1, VCH, 1991. acrylic fibers [4] are among those criteria. [8] D. C. Gupta, “Acrylic fibers-polymerization”, Synthetic fibers, pp. 14- On the basis of the stabilized acrylic fibers densities were 20, Oct. / Dec., 1984. shown in table 4, stabilization procedure under cycles S3 and [9] K. K. Garg, “Poly-acrylonitrile and copolymers”, Synthetic fibers, pp. 29-35, Apr. /Jun., 1985. S4 in comparison with other cycles are more complete [10] J. C. Masson, “Acrylic fiber technology and applications”, Marcel because the stabilization acrylic fibers have high density (1.36 Dekker, 1995. and 1.38 gr/cm³ respectively) in comparison with other [11] S. K. Mukhopadhyay and Y. Zhu, “Structure - property relationships of stabilized acrylic fibers produced from other cycles. pan precursor fibers during thermo-oxidative stabilization”, Textile Res. J., vol. 65, no. 1, pp. 25-31, 1995. Also the results of table 4 regarding the ratio of fiber tensile [12] P. Rajalingam and G. Radhakrishnan, “ precursor for strength reduction during stabilization confirms this fact that carbon fibers”, JMS-REV. Macromol. Chem. Phys., vol. C31, no. 283, stabilization was completed by cycles S3 and S4, because the pp. 301-310, 1991. [13] H. M. Ezekiel, “Formation of very high modulus graphite fibers from a percentage of the reduction under cycles S3 and S4 are 30.8 % commercial polyacrylonitrile ”, Composite & Fibrous Materials and 29.8 % respectively. But for more certainly, all the Branch, Nonmetallic Materials Divisions, Air Force Materials stabilized sample fibers were carbonized. Table 5 shows the Laboratory, Wright Patterson Air Force Base ,Ohio, pp. 184-201. [14] D. D. Edie, “The effect of processing on the structure and properties of tensile strength of different types of carbon fibers. carbon fibers”, Carbon, vol. 36, no. 4, pp. 345-362, 1998. [15] W. Sweeny, “Cyclization of acrylic fiber”, EP. No. 0169690 B1, 1989. TABLE V [16] M. T. Azarova, et al., “Study of thermal conversions of polyacrylonitrile TENSILE STRENGTH OF CARBON FIBERS STABILIZED BY fibers”, in 1974 Proc. International Symposium on Chemical Fibers, DIFFERENT CYCLES Kalinin, pp. 56-60. [17] R. Eslami Farsani, et al., “Fabrication of carbon fibers from wet-spun Code of Tensile Strength of commercial polyacrylonitrile fibers”, Fibre Chemistry, vol. 38, no. 5, pp. Stabilization Cycle Carbon Fibers (MPa) 383-386, 2006.

S1 920 S2 1128 S3 1360 S4 1469

The results of table 5 shows that the carbon fibers produced from samples S4 in comparison with other samples have higher tensile strength. This issue emphasizes on the proper selection of sample of S4 as the most complete cycle of stabilization. The tensile strength of carbon fibers fabricated from commercial acrylic fibers under cycle S4 is 1469 MPa. In previous studies [17], tensile strength of carbon fibers fabricated from commercial acrylic fibers (which mentioned above) was 1401 MPa.

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