《复合材料 Composites》课程教学资源（学习资料）第二章 增强体_carbon fiber_A review of heat treatment on polyacrylonitrile fiber

Availableonlineatwww.sciencedirect.col Polvmer ScienceDirect Degradation Stability ELSEVIEI Polymer Degradation and Stability 92(2007)1421-1432 www.elsevier.com/locate/polydegstab Review article A review of heat treatment on polyacrylonitrile fiber M.S.A. Rahaman. A.F. Ismail.A Mustafa Membrane Research Unit(MRU), Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia cdai 81310. Johor. Malaysia Received 23 January 2007: accepted 23 March 2007 Available online 14 April 2007 Abstract Developing carbon fiber from polyacrylonitrile(PAn) based fiber is generally subjected to three processes namely sta arboniza ion, and graphitization under controlled conditions. The Pan fiber is first stretched and simultaneously oxidized in a 200-300C. This treatment converts thermoplastic PAn to a non-plastic cyclic or a ladder compound. After oxidation, the f arbonized at about 1000C in inert atmosphere which is usually nitrogen. Then, in order to improve the ordering and orientation of the crystallites in the direction of the fiber axis, the fiber must be heated at about 1500-3000C until the polymer contains 92-100%o. High temperature process the timing of nitrogen. With better-controlled condition, the strength of the fiber can achieve up to 400 GPa after this pyrolysis process generally leads to higher modulus fibers which expel impurities in the chain as volatile by-products During heating treatment, the fiber shrinks in diameter, builds the structure into a large structure and upgrades the strength by removing the initial nitrogen content of pan precurso c 2007 Published by Elsevier Ltd Keywords: Polyacrylonitrile; Heat treatment; Stabilization; Carbonization; Carbon fiber 1. Introduction a thermally stable, extremely oriented molecular structure when subjected to a low temperature treatment [9]. PAn fiber It has been documented that the majority of all carbon fi- was also preferred to be the precursor because of its fast rate in bers used today are made from PAN precursor, which is pyrolysis without changing its basic structure [9). Optimizing a form of acrylic fiber. Pan which is a polymer with a chain the pyrolysis of pan precursor fiber would ideally result in en- of carbon connected to one another(Fig. 1)is hard, horny, rel- hanced performance of the resulting carbon fiber atively insoluble, and a high-melting material [1]. It has been Recent study has established that pan fibers were used on established that PAN-based carbon fiber is stronger than other a large scale in textile industry and one of the most suitable type of precursor-based carbon fiber [2]. PAN-based fibers also and widely applied for making high performance carbon fibers have been found to be the most suitable precursors for produc- [10-13]. Most PAN-based carbon fibers extensively applied in ing high performance carbon fibers(compared to pitch, rayon, last two decades were used in the composite technology [14] etc. )generally because of its higher melting point and greater They are highly desirable for high performance composites for carbon yield(>50% of the original precursor mass)[3-7]. automotive and aerospace technologies due to their enhanced Although carbon fiber can be from pitch precursor, the pro- physical and mechanical characteristics [9]. Fitzer [15] and cessing and purifying it to the fiber form is very expensive Chen and Harrison [16] believed that the optimization of and generally, they are more expensive than PAN-based fibers PAN fiber would ideally result in high performance for use 8. PAN with molecular formula [C3H3NIn can produce in aerospace application. Hence PAN-based fiber that leads bon fiber of relatively high carbon yield giving rise to to a good balance in properties can be used in structural ap cations and provide high strength [2] ponding author.Tel:+6075535592;fax:+6075581463 Year by year there will be an improvement on performance ail address: afauzi@ utm. my(A F. Ismail as well as strength and modulus of paN-based carbon fiber 0141-3910S. see front matter o 2007 Published by Elsevier Ltd doi: 10. 1016/j-polymdegradstab. 2007.03.023

Review article A review of heat treatment on polyacrylonitrile fiber M.S.A. Rahaman, A.F. Ismail*, A. Mustafa Membrane Research Unit (MRU), Faculty of Chemical and Natural Resources Engineering, Universiti Teknologi Malaysia, Skudai 81310, Johor, Malaysia Received 23 January 2007; accepted 23 March 2007 Available online 14 April 2007 Abstract Developing carbon fiber from polyacrylonitrile (PAN) based fiber is generally subjected to three processes namely stabilization, carboniza￾tion, and graphitization under controlled conditions. The PAN fiber is first stretched and simultaneously oxidized in a temperature range of 200e300 C. This treatment converts thermoplastic PAN to a non-plastic cyclic or a ladder compound. After oxidation, the fibers are carbonized at about 1000 C in inert atmosphere which is usually nitrogen. Then, in order to improve the ordering and orientation of the crystallites in the direction of the fiber axis, the fiber must be heated at about 1500e3000 C until the polymer contains 92e100%. High temperature process generally leads to higher modulus fibers which expel impurities in the chain as volatile by-products. During heating treatment, the fiber shrinks in diameter, builds the structure into a large structure and upgrades the strength by removing the initial nitrogen content of PAN precursor and the timing of nitrogen. With better-controlled condition, the strength of the fiber can achieve up to 400 GPa after this pyrolysis process. 2007 Published by Elsevier Ltd. Keywords: Polyacrylonitrile; Heat treatment; Stabilization; Carbonization; Carbon fiber 1. Introduction It has been documented that the majority of all carbon fi- bers used today are made from PAN precursor, which is a form of acrylic fiber. PAN which is a polymer with a chain of carbon connected to one another (Fig. 1) is hard, horny, rel￾atively insoluble, and a high-melting material [1]. It has been established that PAN-based carbon fiber is stronger than other type of precursor-based carbon fiber [2]. PAN-based fibers also have been found to be the most suitable precursors for produc￾ing high performance carbon fibers (compared to pitch, rayon, etc.) generally because of its higher melting point and greater carbon yield (>50% of the original precursor mass) [3e7]. Although carbon fiber can be from pitch precursor, the pro￾cessing and purifying it to the fiber form is very expensive and generally, they are more expensive than PAN-based fibers [8]. PAN with molecular formula [C3H3N]n can produce car￾bon fiber of relatively high carbon yield giving rise to a thermally stable, extremely oriented molecular structure when subjected to a low temperature treatment [9]. PAN fiber was also preferred to be the precursor because of its fast rate in pyrolysis without changing its basic structure [9]. Optimizing the pyrolysis of PAN precursor fiber would ideally result in en￾hanced performance of the resulting carbon fiber. Recent study has established that PAN fibers were used on a large scale in textile industry and one of the most suitable and widely applied for making high performance carbon fibers [10e13]. Most PAN-based carbon fibers extensively applied in last two decades were used in the composite technology [14]. They are highly desirable for high performance composites for automotive and aerospace technologies due to their enhanced physical and mechanical characteristics [9]. Fitzer [15] and Chen and Harrison [16] believed that the optimization of PAN fiber would ideally result in high performance for use in aerospace application. Hence PAN-based fiber that leads to a good balance in properties can be used in structural appli￾cations and provide high strength [2]. Year by year there will be an improvement on performance as well as strength and modulus of PAN-based carbon fiber * Corresponding author. Tel.: þ607 5535592; fax: þ607 5581463. E-mail address: afauzi@utm.my (A.F. Ismail). 0141-3910/$ - see front matter 2007 Published by Elsevier Ltd. doi:10.1016/j.polymdegradstab.2007.03.023 Polymer Degradation and Stability 92 (2007) 1421e1432 www.elsevier.com/locate/polydegstab

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

M.S.A. Rahaman er al. Polymer Degradation and Stability 92 (2007)1421-1432 1423 Previously two important reactions occur during stabilize- tion process which can change the chemistry of PAn structure 40]. They are dehydrogenation and cyclization reactions as illustrated in Fig 3. Both are important to form ladder polymer structure which was thermally stable and might be able to withstand high temperature during pyrolysis process. In addi tion, stabilization process also could be present in oxidation reaction which gives an insight about diffusion of oxygen through the reacting polymer [41] Fig 4. Ladder PAN structure [26]- 2 Oxidation reaction he rings are formed. The dehydrogenation reactions have at ast two elementary steps, with oxidation in the first step The oxidation reaction during PAN-based precursor stabili- and elimination of water in the second. Studies have shown zation is the least reaction and is the step which most precur- that either the original pan polymer or cyclized ladder poly sors depend. Commercially, stabilization of PAn fiber is done mer can undergo dehydrogenation [43]. As a conclusion from in an"oxidizing medium which is typically air. The reaction Fig 3, the reactions are usually written in the form of Fig. 6 exotherm when PAN is stabilized in air is partly due to reac- Since oxygen is required for the reaction to proceed, dehydro- tion with oxygen. Although stabilization could be done in an genation does not occur in inert atmosphere. This is different inert atmosphere, a polymer back-bone containing oxygen- from the cyclization reaction. The double bond or unsaturated bearing groups that evolves in PAN ladder structure(Fig. 4) bond that formed in the reaction improves the polymers ther provides greater stability to sustain high temperature carbon- mal stability and reduces chain scission during carbonization ization treatment [42 Fitzer and Muller [43 have concluded that the activation energy and the frequency factor were greater in air than in ni- trogen(inert gas). This indicates that oxygen is an initiator for 23. Cyclization reaction the formation of activated center for cyclization because of the ncrease in the activation energy. Consequently, various struc The last reaction that would be discussed is cyclization tures of oxidized pAN that account for the presence of oxygen which is the most important reaction in the stabilization of have been proposed including those containing bridging ether PAN fiber. Cyclization is the reaction of the nitrile groups in links, those containing carbonyl groups, and those in which the precursor polymer with adjacent groups to form a stable, each nitrogen atom donates its lone pair of electron to an ladder polymer and could be described by first order kinetic oxygen(as shown in Fig. 5)5,] equation [43]. Cyclization is the most important reaction in stabilization process. The cyclization of the nitrile groups is 2. 2. Dehydrogenation process an exothermic reaction and that the evolution of gaseous prod ucts accompanies this reaction [46]. The reaction is necessary Dehydrogenation is the formation of double bonds that sta- to hold molecules in fiber together and increases the stiffness bilizes carbon chain and cyclization is the process by which [47-50]. In addition, the idea of cyclization was conceived by Cyclization CEN C=N CEN H2O Dehydrogenation Dehydrogenation Cyclization Fig. 3. Proposed chemistry of PAN stabilization 19, 40

Previously two important reactions occur during stabiliza￾tion process which can change the chemistry of PAN structure [40]. They are dehydrogenation and cyclization reactions as illustrated in Fig. 3. Both are important to form ladder polymer structure which was thermally stable and might be able to withstand high temperature during pyrolysis process. In addi￾tion, stabilization process also could be present in oxidation reaction which gives an insight about diffusion of oxygen through the reacting polymer [41]. 2.1. Oxidation reaction The oxidation reaction during PAN-based precursor stabili￾zation is the least reaction and is the step which most precur￾sors depend. Commercially, stabilization of PAN fiber is done in an ‘oxidizing’ medium which is typically air. The reaction exotherm when PAN is stabilized in air is partly due to reac￾tion with oxygen. Although stabilization could be done in an inert atmosphere, a polymer back-bone containing oxygen￾bearing groups that evolves in PAN ladder structure (Fig. 4) provides greater stability to sustain high temperature carbon￾ization treatment [42]. Fitzer and Muller [43] have concluded that the activation energy and the frequency factor were greater in air than in ni￾trogen (inert gas). This indicates that oxygen is an initiator for the formation of activated center for cyclization because of the increase in the activation energy. Consequently, various struc￾tures of oxidized PAN that account for the presence of oxygen have been proposed including those containing bridging ether links, those containing carbonyl groups, and those in which each nitrogen atom donates its lone pair of electron to an oxygen (as shown in Fig. 5) [5,44]. 2.2. Dehydrogenation process Dehydrogenation is the formation of double bonds that sta￾bilizes carbon chain and cyclization is the process by which the rings are formed. The dehydrogenation reactions have at least two elementary steps, with oxidation in the first step and elimination of water in the second. Studies have shown that either the original PAN polymer or cyclized ladder poly￾mer can undergo dehydrogenation [43]. As a conclusion from Fig. 3, the reactions are usually written in the form of Fig. 6. Since oxygen is required for the reaction to proceed, dehydro￾genation does not occur in inert atmosphere. This is different from the cyclization reaction. The double bond or unsaturated bond that formed in the reaction improves the polymer’s ther￾mal stability and reduces chain scission during carbonization [45]. 2.3. Cyclization reaction The last reaction that would be discussed is cyclization which is the most important reaction in the stabilization of PAN fiber. Cyclization is the reaction of the nitrile groups in the precursor polymer with adjacent groups to form a stable, ladder polymer and could be described by first order kinetic equation [43]. Cyclization is the most important reaction in stabilization process. The cyclization of the nitrile groups is an exothermic reaction and that the evolution of gaseous prod￾ucts accompanies this reaction [46]. The reaction is necessary to hold molecules in fiber together and increases the stiffness [47e50]. In addition, the idea of cyclization was conceived by Fig. 3. Proposed chemistry of PAN stabilization [9,40]. Fig. 4. Ladder PAN structure [26]. M.S.A. Rahaman et al. / Polymer Degradation and Stability 92 (2007) 1421e1432 1423

1424 MS-A Rahaman et al. Polymer Degradation and Stability 92 (2007)1421-1432 H Fig. 7. Fully aromatic cyclized structure proposed by Houtz [51] layer or ribbon structure(shown in Fig 8)consisting of three hexagons in the lateral direction and bounded by nitrogen The initiation of the cyclization reaction has been attributed such as catalyst fragments, re- sidual polymerization products, inhibitors, etc. [53](2)the chain end groups: [54](3)random initiation by hydrogen atoms a to the nitrile; 55](4) transformation of a nitrile to an azomethine [56: 5)the presence of a ketonitrile formed by hydrolysis dur- ing polymerization; [28] and(6) hydrolysis of nitriles to acid during polymerization [57]. In addition, due to their reaction cyclization reactions can proceed in either an inert or in the presence of oxygen. In other words, oxygen is not in- volved in the reaction mechanism of cyclization. 2.4. Miscellaneous types of stabilization process Although a wide variety of stabilization processes are Fig5 Proposed structures of oxidized PAN:(a)bridging ether links;(b)car- described, they have several design objectives in common nyl groups;(c)donation of lone pair electron to oxygen atom; (d) hydroxyl and carbonyl groups [44, 45]- 1. Runaway reactions from heat must be prevented. 2. Stabilization must be completed throughout the fiber. Houtz [ 51]in 1950 from his observation that PAN stabilization 3. The shrinkage must be completed throughout the fibers led to change in colouration. 4. The reactions are slow and accelerations are helpful During the stabilization process, the PAn structure un- dergoes cyclization reaction and converts the triple bond struc- When the production volume increased specific methods of ture (e.g. CEN) to double bond structure (e.g. C=N), stabilizing the fiber were patented. The patents deal with three resulting in a six-membered cyclic pyridine ring proposed major areas: batch process, continuous process, and accelera- by Houtz [51] as illustrated in Fig. 7 and changes the aliphatic tion of stabilization reactions. This section provides general to cyclic structure prior to the formation of ladder polymer. example from each of these areas that illustrates common de- Referring to this figure(Fig. 7), cyclization reactions can pro- sign objectives described above ceed in either an inert atmosphere or in the presence of oxy- gen. In other words, oxygen is not involved in the reaction 2.4.1. Batch process c batch processes are shown in Figs.9-1 mechanism of cyclization. When the temperature rises up to Three examples 600C, the cyclized structure undergoes dehydrogenation The first process blows hot air through a spool precursor and links up in lateral direction, producing a graphite-like loosely wound on a porous core. The air permits heat removal 义人人义 Fig. 6. The dehydrogenation reaction during stabilization process: (a) PAN polymer; (b) cyclized PAN

Houtz [51] in 1950 from his observation that PAN stabilization led to change in colouration. During the stabilization process, the PAN structure un￾dergoes cyclization reaction and converts the triple bond struc￾ture (e.g. C^N) to double bond structure (e.g. C]N), resulting in a six-membered cyclic pyridine ring proposed by Houtz [51] as illustrated in Fig. 7 and changes the aliphatic to cyclic structure prior to the formation of ladder polymer. Referring to this figure (Fig. 7), cyclization reactions can pro￾ceed in either an inert atmosphere or in the presence of oxy￾gen. In other words, oxygen is not involved in the reaction mechanism of cyclization. When the temperature rises up to 600 C, the cyclized structure undergoes dehydrogenation and links up in lateral direction, producing a graphite-like layer or ribbon structure (shown in Fig. 8) consisting of three hexagons in the lateral direction and bounded by nitrogen atom [52]. The initiation of the cyclization reaction has been attributed to several sources: (1) impurities such as catalyst fragments, re￾sidual polymerization products, inhibitors, etc.[53](2) the chain end groups; [54] (3) random initiation by hydrogen atoms a to the nitrile; [55] (4) transformation of a nitrile to an azomethine; [56];(5) the presence of a ketonitrile formed by hydrolysis dur￾ing polymerization; [28] and (6) hydrolysis of nitriles to acids during polymerization [57]. In addition, due to their reaction, cyclization reactions can proceed in either an inert atmosphere or in the presence of oxygen. In other words, oxygen is not in￾volved in the reaction mechanism of cyclization. 2.4. Miscellaneous types of stabilization process Although a wide variety of stabilization processes are described, they have several design objectives in common. 1. Runaway reactions from heat must be prevented. 2. Stabilization must be completed throughout the fiber. 3. The shrinkage must be completed throughout the fibers. 4. The reactions are slow and accelerations are helpful. When the production volume increased specific methods of stabilizing the fiber were patented. The patents deal with three major areas: batch process, continuous process, and accelera￾tion of stabilization reactions. This section provides general example from each of these areas that illustrates common de￾sign objectives described above. 2.4.1. Batch process Three examples of batch processes are shown in Figs. 9e11. The first process blows hot air through a spool precursor loosely wound on a porous core. The air permits heat removal Fig. 5. Proposed structures of oxidized PAN: (a) bridging ether links; (b) car￾bonyl groups; (c) donation of lone pair electron to oxygen atom; (d) hydroxyl and carbonyl groups [44,45]. Fig. 6. The dehydrogenation reaction during stabilization process: (a) PAN polymer; (b) cyclized PAN. Fig. 7. Fully aromatic cyclized structure proposed by Houtz [51]. 1424 M.S.A. Rahaman et al. / Polymer Degradation and Stability 92 (2007) 1421e1432

M.S.A. Rahaman er al. Polymer Degradation and Stability 92 (2007)1421-1432 ∴∵:∷∴∷ ·布·· 26HHIIL LH 15 :: ;…∵∷ 15 wIll ::∵∴…∷∷ Fig. 10. Moving rack process by atomic energy authority [59] …∷∵"∷ contact of the yarn with the rollers. And the shrinkage is con- trolled by adjusting the tension applied to the rack. The final Fig 8 Schematic of graphite ribbon [52] process in Fig. Il is a step toward continuous process and probably is more expensive to operate than the two processes ( Figs. 9 and 10) described before. The initial stages of stabili- and provides a source of oxygen. Shrinkage is controlled by zation are performed continuously in a multiphase oven with the fiber itself as it is wound and spool. However, since the the fiber restrained from shrinkage by the oven roller. The air flow is not uniform and the fibers are in contact with one more stable final stages are completed in batch oven where another, a batch process with a method to move the yarn the yarn is wrapped into lo and improve the uniformity was developed as in Fig. 10 limited in its ability to produce since the yarn in contact The ends are tied and the rollers turned to minimize the with the support will differ from that surrounded by air. an he tension is not uniform in the skein l49 2. 4.2. Continuous process The continuous processes for stabilizing PAN are all based on the idea of pulling tows through heated boxes. The first sketch in Fig. 12 illustrates the basic heated box with multiple passes. The tow may be oriented horizontally or vertically in he oven and the air in the oven is circulated to control heat and mass tra answeR It also patented by Toho Company [61], where the fiber passes through the oven, turns on a roller, and re-enters the oven. In addition, the heat is controlled by the yam moving outside the hot oven every few minutes. Meanwhile Cour taulds(Fig. 13) has patented a stabilization oven which con- tains a number of different temperature zones in a single oven [62]. The yarn is wound on long rollers which pass through a series of buffled oven zones. This concept of multi- ple zones with a stage temperature is probably used in all com- mercial processes. An interesting continuous process is shown Fig. 9. Batch stabilization of polyacrylonitrile yarn on the tube [581 by the fluidized bed process(Fig. 14)[63]. Here the fibers are

and provides a source of oxygen. Shrinkage is controlled by the fiber itself as it is wound and spool. However, since the air flow is not uniform and the fibers are in contact with one another, a batch process with a method to move the yarn and improve the uniformity was developed as in Fig. 10. The ends are tied and the rollers turned to minimize the contact of the yarn with the rollers. And the shrinkage is con￾trolled by adjusting the tension applied to the rack. The final process in Fig. 11 is a step toward continuous process and probably is more expensive to operate than the two processes (Figs. 9 and 10) described before. The initial stages of stabili￾zation are performed continuously in a multiphase oven with the fiber restrained from shrinkage by the oven roller. The more stable final stages are completed in batch oven where the yarn is wrapped into loose skeins. However, the process is limited in its ability to produce since the yarn in contact with the support will differ from that surrounded by air, and the tension is not uniform in the skein. 2.4.2. Continuous process The continuous processes for stabilizing PAN are all based on the idea of pulling tows through heated boxes. The first sketch in Fig. 12 illustrates the basic heated box with multiple passes. The tow may be oriented horizontally or vertically in the oven and the air in the oven is circulated to control heat and mass transfers. It also patented by Toho Company [61], where the fiber passes through the oven, turns on a roller, and re-enters the oven. In addition, the heat is controlled by the yarn moving outside the hot oven every few minutes. Meanwhile Cour￾taulds (Fig. 13) has patented a stabilization oven which con￾tains a number of different temperature zones in a single oven [62]. The yarn is wound on long rollers which pass through a series of buffled oven zones. This concept of multi￾ple zones with a stage temperature is probably used in all com￾mercial processes. An interesting continuous process is shown by the fluidized bed process (Fig. 14) [63]. Here the fibers are Fig. 8. Schematic of graphite ribbon [52]. Fig. 9. Batch stabilization of polyacrylonitrile yarn on the tube [58]. Fig. 10. Moving rack process by atomic energy authority [59]. M.S.A. Rahaman et al. / Polymer Degradation and Stability 92 (2007) 1421e1432 1425