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 PANHoutz [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