Study on the structure evolution and temperature zone regulation mechanism of polyacrylonitrile fibers during pre-oxidation process

By regulating the holding time of the temperature zone, the structure evolution and structural defect source of polyacrylonitrile fibers were investigated under two distinct pre-oxidation processes but the same heat treatment time. The chemical structure, crystal structure, bulk density, element content change and cross section relative O content of the pre-oxidized fiber were characterized by FTIR, XRD, sink-float method, EA and SEM techniques, respectively. It was found that as the temperature rose, the chemical structure of polyacrylonitrile fibers changed to heat-resistant trapezoidal structure faster, which improved the production efficiency of their pre-oxidation process. However, when the temperature rose to a certain value, if the polyacrylonitrile fiber contained a large amount of uncyclized C=N functional groups, some of them would be thermally decomposed and released small molecules of CH4, NH3 and HCN. The thermal decomposition of C=N groups resulted in the chain breaking of the macromolecular chain, and it was difficult to form a disordered graphite network structure with high degree of cross-linking. Meanwhile, the overflow of the small molecules gases increased the microscopic pores of the fibers. In the other hand, rapid cyclic exothermic reaction occurred, which promoted oxidation reaction. The fibers took the initiative to form a dense layer in their skin as O diffused from the fiber skin to its core, which hindered further O diffusion to the core and aggravated the non-uniformity of the transverse structure. Therefore, if the polyacrylonitrile fiber can quickly complete the cyclization reaction at a higher temperature that can withstand the cyclization reaction by regulating the holding time in the temperature zone, the production efficiency in the pre-oxidation stage and the structural defects of the polyacrylonitrile fibers can be effectively improved. [GRAPHICS] .