A multi-step oxidation procedure was developed to stabilize an isotropic pitch fiber prior to carbonization.
In the first step, the temperature was raised to 170-220Β°C in 1 minute and kept isothermally for 4 minutes. The temperature was then increased in the same manner to 250, 300 and 350Β°C. Stabilized fiber precursors
were finally carbonized at 1000Β°C. The three-step oxidation of 220-250-300Β°C required only 15 minutes and stabilized the pitch fiber without fusion. However, the tensile strength of the resultant carbon fiber was 70% of that of the carbon fiber stabilized by a conventional slow heating method (2.5"C/min to 360Β°C). The four-step oxidation, taking 20 minutes in total, was more effective. The tensile strength of the carbon fiber was 82 and 88% by procedures of 220-250-3Of-300Β°C and 220-250-300-350Β°C respectively. Oxidized fibers were characterized by differential scanning calorimetry (DSC) in a nitrogen atmosphere.
The three-step oxidation of 220-250~300Β°C decreased the heat capacity of the fiber but did not completely stabilize the microstructure of the fiber. The four-step oxidation up to 350Β°C however, diminished the endothermic peak in the DSC profile seen with fibers oxidized by the three-step procedure. The DSC profile of the fiber prepared by the four-step oxidation was very close to that prepared by the slow heating method.