Carbonization Stages of PAN Carbon Fibers

Very little is known about the stages of carbonization, with H20, NH3 and HCN being evolved. Bailey and Clarke (1970) [227] proposed an idealized concept of reactions occurring 

The carbonized fiber known as LHT (low heat treated) can have a residual nitrogen content of up to 7% and even when treated at about 1400°C, can still be about 4%. 

Watt (1970) [228] undertook the pyrolysis of stabilized (220°C) and unstabilized homopolymer PAN fibers in vacuo and analyzed the evolved gases. The HCN evolution suggested two different stages in pyrolysis, the first at about 450 C from the unladdered polymer and a further region of 450 900°C from the laddered structure and resulting in cross-linking. 

But Watt believed that this reaction could not account for the evolution of all the N2 as it would leave a twin ladder ribbon unless, fortuitously, there were favorable steric influences. He was fairly certain that the nitrogen was incorporated in the structure as a substitutional atom such as  

At 400°C, C3 hydrocarbons (propylene, propadiene and propane) were evolved, possibly from C3 links in the ring structure  

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