30 总巳总恩总 忠总 Rc不 disappearance of 2240 cm band for the nitrile began as early and produced small fiber diameter. Sometimes a diameter s 160C under vacuum. Setnescu et al. [27] observed, with ten times lower than human hair could be produced through pyrolysis process, that two peaks almost completely [16]. The significant reduction in diameter has been observed disappeared and new peaks appeared around 800 cm and within the carbonization temperature(below 1000C)[38] 1600 cm. The change in peaks are due to the formation of Similar trend of the reduction of fiber diameter has been found C=C, C=N and =C-H and results in the formation of car- by Liu et al. [21]. In other words, the diameter diminished bon fiber structure throughout the carbonization treatment 6.2. Diameter 6.3. Density Large diameter is one of the limitations of fiber strength. As mentioned before, to give uniformity in heat treatment, fibers Various studies indicated that a significant change in the fi- must have a small diameter. Chen and Harrison [161, stated ber density occurred below carbonization temperature that small diameter can reduce any gradient temperature [31, 81, 100]. Within the carbonization temperature (300- across the fiber to form uniformity of heat treatment 1200C), the changes in the density of the fibers take place Commercial PAN fiber like Dralon T (DT)and Special up to 800"C [38]. Sometimes, it could rapidly change up to Acrylic Fiber (SAF) have diameter in the range of 8-20 um 1000C 31 The density could be changed due to the com- [23]. As stated before, plasticizer is applied in post-spinning of carbonization. It is also due to the presence of the noncar- modification to reduce fiber diameter prior to heat treatment. When heat treatment has been applied as well as the rise in bon elements in the fiber and the ladder polymer structures in- the temperature, the diameter of the fiber would shrink again terconnecting with one another [100] However, the density increase was followed by a sharp drop at 1000C which is due to the conversion of open pores to Table I closed pores [31]. As a consequence, the air would be trapped Chemical composition of some pyrolyzed PAN samples found by elemental inside the fibers and hence results in low density which could analysis[27] limit the tensile strength of the final carbon fiber [25]. How- Pyrolysis temperature (C) Element content ever, Ozbek and Isaac [79)and Sauder et al. [101] observed Carbon(%) Nitrogen(%6)Hydrogen(So) that heating temperature (HTT), which increases up to Initial 5.47 3000C, can eliminate the effect of open and closed pores 68 .51 This is because, in this region high heating rate and high tem- 7546 6.28 perature were used which made the vibrations of moleculesdisappearance of 2240 cm1 band for the nitrile began as early as 160 C under vacuum. Setnescu et al. [27] observed, through pyrolysis process, that two peaks almost completely disappeared and new peaks appeared around 800 cm1 and 1600 cm1 . The change in peaks are due to the formation of C]C, C]N and ]CeH and results in the formation of car￾bon fiber structure. 6.2. Diameter Large diameter is one of the limitations of fiber strength. As mentioned before, to give uniformity in heat treatment, fibers must have a small diameter. Chen and Harrison [16], stated that small diameter can reduce any gradient temperature across the fiber to form uniformity of heat treatment. Commercial PAN fiber like Dralon T (DT) and Special Acrylic Fiber (SAF) have diameter in the range of 8e20 mm [23]. As stated before, plasticizer is applied in post-spinning modification to reduce fiber diameter prior to heat treatment. When heat treatment has been applied as well as the rise in the temperature, the diameter of the fiber would shrink again and produced small fiber diameter. Sometimes a diameter with ten times lower than human hair could be produced [16]. The significant reduction in diameter has been observed within the carbonization temperature (below 1000 C) [38]. Similar trend of the reduction of fiber diameter has been found by Liu et al. [21]. In other words, the diameter diminished throughout the carbonization treatment. 6.3. Density Various studies indicated that a significant change in the fi- ber density occurred below carbonization temperature [31,81,100]. Within the carbonization temperature (300e 1200 C), the changes in the density of the fibers take place up to 800 C [38]. Sometimes, it could rapidly change up to 1000 C [31]. The density could be changed due to the com￾paction of the structure taking place during the early stages of carbonization. It is also due to the presence of the noncar￾bon elements in the fiber and the ladder polymer structures in￾terconnecting with one another [100]. However, the density increase was followed by a sharp drop at 1000 C which is due to the conversion of open pores to closed pores [31]. As a consequence, the air would be trapped inside the fibers and hence results in low density which could limit the tensile strength of the final carbon fiber [25]. How￾ever, Ozbek and Isaac [79] and Sauder et al. [101] observed that heating temperature (HTT), which increases up to 3000 C, can eliminate the effect of open and closed pores. This is because, in this region high heating rate and high tem￾perature were used which made the vibrations of molecules Fig. 21. Elimination of N2 and H2 at high temperature [86]. Table 1 Chemical composition of some pyrolyzed PAN samples found by elemental analysis [27] Pyrolysis temperature (C) Element content Carbon (%) Nitrogen (%) Hydrogen (%) Initial 66.33 26.00 5.47 600 68.51 11.93 3.69 900 75.46 6.28 1.46 1430 M.S.A. Rahaman et al. / Polymer Degradation and Stability 92 (2007) 1421e1432