The Effect Of Atomosphere Medium On Polyacrylonitrile Fiber's Chemical Structure During Thermal Stabilization

Thermal stabilization of Polyacrylonitrile fibers is one of the key processes in the preparation of carbon fiber, and the chemical structures generated in this process will affect the final properties of carbon fiber. Therefore, further optimizing the process conditions and producing high-performance PAN-based carbon fiber could be directed significantly by deeper researches on changes of the molecular structure caused by heat treatment under low temperature, as well as finding out the correlation between the generation of chemical structure and time, temperature, environment, etc. in the process.By the methods of differential scanning calorimetry (DSC), Fourier transform infrared (FT-IR) and elemental analysis (EA), the influences of cyclization on oxidation reactions were investigated during the process of thermal stabilization, and cyclization and oxidation kinetics were studied by Kissinger method. The results show that: 1. In the process of low-temperature heat treatment, environmental atomosphere have a great influence on the chemical reactions of PAN precursor. DSC exothermic curve within inert gas only shows one exothermic peak (Peak a'), while DSC exothermic curve within oxygen atmosphere shows two exothermic peak. With the increasing of oxygen content in the environment, the temperature of the first exothermic peak (Peak a) almost remains the same, but the intensity increases gradually; the temperature of the second exothermic peak (Peak b) decreases gradually, but the intensity increases gradually.2. The chemical significance represented by Peak a in the DSC exothermic curve within inert gas is cross-linked cyclization reactions in PAN macromolecules. The chemical significance represented by Peak a in the DSC exothermic curve within oxygen atmosphere is cyclization reactions of PAN macromolecules and primary oxidation reactions of cyclic structure, while Peak b is the oxidation and pyrolysis of cyclic structure at higher temperatures.3. Cyclization reactions are the prerequisite of primary oxidation. The apparent activation energy of cyclization is 161.3KJ/mol; the apparent activation energy of primary oxidation is 96.3KJ/mol. With the increasing of oxygen in the environment, Peak a represents that the apparent activation energy of chemical reactions almost remains the same; while Peak b decreases gradually. 4. Oxygen can promote cyclization and dehydrogenation reactions in the low-temperature heat treatment of PAN precursor.5. Compared to the cyclization reactions, oxidation occurs under lower temperature conditions; after heat treatment in the air under the same condition, the higher the cyclization degree of PAN fibers, the higher the oxygen content to generate more cyclic ketone structure, that is, the higher the degree of oxidation. Therefore, cyclization can promote oxidation reactions.Through this study, the effects of the environmental atomosphere in low temperature heat treatment process on the chemical structure of PAN fibers were revealed; the chemical reaction types of the PAN fiber in air and nitrogen were identified; the influence of cyclic structure in low temperature heat treatment process on the following chemical reactions was revealed; at the same time, the above phenomenon were explained via kinetic analysis theoretically.