MS-A Rahaman et al. Polymer Degradation and Stability 92 (2007)1421-1432 PAN fiber c三NC三N Fig. 1. Molecular structure of polyacrylonitrile. STABIL忆zA Oxidation in air [17]. Traceski [18] stated that the total worldwide production of PAN-based carbon fiber was 19 million lbs per year for Step 2: 1989 and increased up to 26 million lbs per year. In addition CARBONIZATION the worldwide outlook for the demand of pan carbon fibers is Heat treatment in nitrogen currently amounting to a nearly S6 billion pound per year worldwide effort [ 19, 20]. So, the wide availability of pan pre- cursor had triggered the production of carbon fiber. Step 3: GRAPHITIZATION Heat treatment in argon L. Heat treatment Heat treatment is a process that converts the PAn fiber pre DATA ANALYSIS cursor to carbon fiber. Currently 90% of all commercial carbon or graphite fibers are produced by the thermal conversion of a Pan precursor, which is a form of acrylic fiber. The successful HIGH PERFORFMANCE conversion of PAn to high strength, high modulus fibers depend CARBON FIBER in part upon the understanding of the oxidative and thermal treatment. Liu et al. [21]listed the three steps for the conversion Fig. 2. PAN precursor carbon fiber conversion process. of precursor of PAN-based fiber to carbon, which are as follows. cyclization of the nitrile groups in acrylic molecule. Setnescu 1. Oxidative stabilization, which forms ladder structure to et al. [27]observed that CH2 and Cn groups disappeared com enable them to undergo processing at higher temperatures. pletely due to elimination, cyclization and aromatization re High temperature carbonization, (<1600C)to keep out tions and formed C=C, C=N and=C-H groups. Typically, noncarbon atoms and yield a turbostatic structure during the course of stabilization, the PAN-based precursor fiber iii. Further heat up to 2000C to improve the orientation of undergoes a change in colour from white through shades of yel- he basal planes and the stiffness of fibers, which is low and browns to ultimately a black stabilized fiber. The mech called graphitization anism for colouration is not fully understood. However, the appearance of black colour is believed to be due to the formation 2. Precursor stabilization of ladder ring structure [28, 291 In this process, the required temperature is the important Among the conversion processes shown in Fig. 2, an essen- factor that would affect the heating treatment of PAn fiber tial and time-consuming step in the conversion of PAn fibers Heat treatment involved in stabilization of PAn fiber is carried to high performance carbon fiber is the oxidative stabilization out usually at the region of 180-300C [24, 30]. When tem- step [7]. This can be explained by chemical reactions that are perature exceeds 180C, the molecular chains will unfold involved in this process, which are cyclization, dehydrogena- and move around. But some researchers found that heating tion, aromatization, oxidation and crosslinking which can re- temperature within 200-300C are usually used to stabilize sult in the formation of the conjugated ladder structure the fiber [7, 23, 25, 31-34]. Fitzer et al. [35] suggested that in [22, 23]. The oxidative stabilization stage is one of the most producing best performance carbon fiber, the best stabilized complicated stages, since different chemical reactions take temperature is 270 C. However, other researchers [36-381 place and the structure of the carbon fiber is set in this stage. found that heating treatment needs higher than 300C to com- Stabilization process, which is done in atmosphere can plete the stabilization. Mathur et al. [39] also proposed that change chemical structure of the fiber and cause them to become PAN fiber does not get preferred stability at 270C but needs thermally stable and so melting will not reoccur[24]. Recently, higher temperature up to 400C. It was known that PAN fiber the stabilization process is found to play an important role in with optimum stabilization condition can produce higher mod converting PAN fiber to an infusible stable ladder polymer ulus carbon fiber than unstablized fiber or than fiber which is that converts CEN bonds to C=N bonds [25]and to develop prepared at high temperature stabilization process [31]. If the crosslink between molecules of Pan [26] which tend to operate temperature is too high, the fibers can overheat and fuse or at high temperatures, with minimum volatilization of carbona- even burn. However, if the temperature is too low, the reac- ceous material. The thermal stability of the stabilized fiber is at- tions are slow and incomplete stabilization can be resulted, tributed to the formation of the ladder structure due to yielding poor carbon fiber properties[17]. Traceski [18] stated that the total worldwide production of PAN-based carbon fiber was 19 million lbs per year for 1989 and increased up to 26 million lbs per year. In addition, the worldwide outlook for the demand of PAN carbon fibers is currently amounting to a nearly $6 billion pound per year worldwide effort [19,20]. So, the wide availability of PAN pre￾cursor had triggered the production of carbon fiber. 1.1. Heat treatment Heat treatment is a process that converts the PAN fiber pre￾cursor to carbon fiber. Currently 90% of all commercial carbon or graphite fibers are produced by the thermal conversion of a PAN precursor, which is a form of acrylic fiber. The successful conversion of PAN to high strength, high modulus fibers depend in part upon the understanding of the oxidative and thermal treatment. Liu et al. [21] listed the three steps for the conversion of precursor of PAN-based fiber to carbon, which are as follows. i. Oxidative stabilization, which forms ladder structure to enable them to undergo processing at higher temperatures. ii. High temperature carbonization, (1600 C) to keep out noncarbon atoms and yield a turbostatic structure. iii. Further heat up to 2000 C to improve the orientation of the basal planes and the stiffness of fibers, which is called graphitization. 2. Precursor stabilization Among the conversion processes shown in Fig. 2, an essen￾tial and time-consuming step in the conversion of PAN fibers to high performance carbon fiber is the oxidative stabilization step [7]. This can be explained by chemical reactions that are involved in this process, which are cyclization, dehydrogena￾tion, aromatization, oxidation and crosslinking which can re￾sult in the formation of the conjugated ladder structure [22,23]. The oxidative stabilization stage is one of the most complicated stages, since different chemical reactions take place and the structure of the carbon fiber is set in this stage. Stabilization process, which is done in atmosphere can change chemical structure of the fiber and cause them to become thermally stable and so melting will not reoccur [24]. Recently, the stabilization process is found to play an important role in converting PAN fiber to an infusible stable ladder polymer that converts C^N bonds to C]N bonds [25] and to develop crosslink between molecules of PAN [26] which tend to operate at high temperatures, with minimum volatilization of carbona￾ceous material. The thermal stability of the stabilized fiber is at￾tributed to the formation of the ladder structure due to cyclization of the nitrile groups in acrylic molecule. Setnescu et al. [27] observed that CH2 and CN groups disappeared com￾pletely due to elimination, cyclization and aromatization reac￾tions and formed C]C, C]N and ]CeH groups. Typically, during the course of stabilization, the PAN-based precursor fiber undergoes a change in colour from white through shades of yel￾low and browns to ultimately a black stabilized fiber. The mech￾anism for colouration is not fully understood. However, the appearance of black colour is believed to be due to the formation of ladder ring structure [28,29]. In this process, the required temperature is the important factor that would affect the heating treatment of PAN fiber. Heat treatment involved in stabilization of PAN fiber is carried out usually at the region of 180e300 C [24,30]. When tem￾perature exceeds 180 C, the molecular chains will unfold and move around. But some researchers found that heating temperature within 200e300 C are usually used to stabilize the fiber [7,23,25,31e34]. Fitzer et al. [35] suggested that in producing best performance carbon fiber, the best stabilized temperature is 270 C. However, other researchers [36e38] found that heating treatment needs higher than 300 C to com￾plete the stabilization. Mathur et al. [39] also proposed that PAN fiber does not get preferred stability at 270 C but needs higher temperature up to 400 C. It was known that PAN fiber with optimum stabilization condition can produce higher mod￾ulus carbon fiber than unstablized fiber or than fiber which is prepared at high temperature stabilization process [31]. If the temperature is too high, the fibers can overheat and fuse or even burn. However, if the temperature is too low, the reac￾tions are slow and incomplete stabilization can be resulted, yielding poor carbon fiber properties. Fig. 2. PAN precursor carbon fiber conversion process. Fig. 1. Molecular structure of polyacrylonitrile. 1422 M.S.A. Rahaman et al. / Polymer Degradation and Stability 92 (2007) 1421e1432