Significantly reduced pre-oxidation period of PAN fibers by continuous electron beam irradiation: Optimization by monitoring radical variation

Abstract

A self-shielded electron beam (EB) accelerator was connected to the front of a pre-oxidation furnace to evaluate the effect of radiation on the pre-oxidation of polyacrylonitrile (PAN) fibers; with this setup, PAN fibers could be irradiated and pre-oxidized in sequence to obtain stabilized fibers. Radiation effects during different temperature stages of the pre-oxidation process were analyzed by differential scanning calorimetry, Fourier-transform infrared spectroscopy, and electron spin resonance spectroscopy. The results indicated that radiation exerted significant but different effects at various stages of the pre-oxidation process. The alkane radicals induced by radiation are rapidly transformed into polyene structures and produced polyenyl radicals after a short heat treatment period. The polyenyl radicals promoted dehydrogenation and cyclization reactions and polyene structures improved the thermal stability of the PAN fibers. Therefore, radiation can greatly shorten the time required for the stabilization of PAN fibers, owing to which the time needed for each temperature stage can be reduced by a large extent. The effects of radiation on various temperature stages of the pre-oxidation process were analyzed accurately by studying the changes in the concentration of polyenyl radicals. An optimized pre-oxidation process for irradiated PAN fibers was designed, which reduced the time period required for stabilization by 75%. The optimized pre-oxidation plus continuous EB irradiation process induced good properties in the stabilized PAN fibers, in terms of fiber density, oxygen content, and degree of cyclization. Radical concentration was found to be an appropriate indicator to express the degree of pre-oxidation.