In the thermal environment of pre-oxidation, the highly oriented polyacrylonitrile precursor will undergo physical contraction and conformation increase, which will make it in a thermodynamic stable state. This accords with the principle of entropy increase. But it can't be allowed to shrink freely, otherwise the fiber will be loose and pulverized, and there will be no strength. Therefore, stretching is an important technical measure to prevent direction loss. Chemical reaction will inevitably lead to chemical shrinkage, and reasonable and moderate chemical shrinkage is allowed, but tension drawing should also be used to control the degree of shrinkage. Therefore, applying tension drafting is an effective measure to inhibit physical shrinkage and control chemical shrinkage, and it is also an outstanding contribution of Watt and others to the preparation of high-performance carbon fibers. In other words, the drawings should run through the pre-oxidation process from beginning to end. Even after stretching, the degree of orientation of the precursor will be reduced from about 90% of the pre-oxidized yarn to about 75%, and the de-orientation still exists.
Second, chemical reaction.
Various chemical reactions occurred in the process of pre-oxidation, and many researchers have made fruitful research on them, and calculated the chemical reaction formula through various advanced characterization methods. Among these numerous chemical reactions, cyclization, oxidation and dehydrogenation are recognized as the main reactions, which belong to exothermic reactions, and the heat release is as high as 3000~4000kJ/kg. The composition of precursors is different, the determination methods and conditions are different, and the reported heat release is also very different. How to discharge these released heat instantly is the key to the design of pre-oxidation furnace. If the reaction heat can't be eliminated instantly, it will lead to overheating of heat storage in the fiber, leading to thermal fusion between monofilaments, black smoke and even fire.
Third, structural transformation.
After intramolecular cyclization and intermolecular crosslinking, the linear macromolecular chain of PAN was transformed into a heat-resistant trapezoidal structure. After several generations of unremitting research, dozens of trapezoidal chemical structures have been proposed based on experimental data by means of TG, DTG, DSC, XRD and XPS. In the process of pre-oxidation, the fiber structure has been changing, which is intuitively reflected in the fiber color changing from white to black and from shallow to deep. Structural changes are synchronized with color changes.