Influences of Stabilization Extent on the Properties of General Purpose Carbon Fiber and Activated Carbon Fiber from Isotropic Air-blown Coal Tar Pitch

The influences of stabilization extent were examined on the yield, strength and surface area of general purpose carbon and activated carbon fibers, respectively, prepared from isotropic air-blown coal tar pitch. The stabilization extent was described by the content of oxygen uptake after the stabilization. The amounts of oxygen uptake optimum for the high tensile strength, elongation, and surface area were found 10, 9 and 13wt%, respectively for the carbon and activated carbon fibers. Larger extent of stabilization reduced the yield of both fibers. The stabilization introduced oxygen into the pitch in forms of carbonyl, ketone, and phenol types of functional groups and accelerated the dehydrogenative condensation and aromatization as well. The volatilization also took place during the stabilization, becoming very marked above 350°C by evolving cracked gas. The cross-linkage can stabilize the pitch not to deform the fiber shape and maintain the axis-alignment of component aromatic planes induced at the spinning to give the strength of carbon fiber. An optimum oxygen uptake for the highest elongation may moderate cross-linkage to form linear chains of aromatic sheets leaving some flexibility among the chains. In contrast, optimum oxygen uptake for the largest surface area by activation was higher than that for the highest strength. Sufficient three-dimensional cross-linkage may induce the voids among the aromatic planes during the carbonization and gas evolution which contribute to form large surface area by limited burn off. Excess stabilization increases the number of oxygen functional groups, all of which leave as cracked gas at the carbonization, reducing the yield. Thus the optimum stabilization extents must be carefully achieved for the best performances of resultant pitch based fiber materials.