Factors limiting the tensile strength of PBO-based carbon fibers

Poly p-phenylene benzobisoxazole (PBO) converts directly to carbon fiber without stabilization. However, the applicability of PBO-based carbon fibers is limited by their low tensile strength. This paper represents the first study of the high-temperature conversion of PBO to carbon fiber and examines some factors leading to this low tensile strength. A mixed-mode Weibull analysis implies that the flaws present in the precursor fiber persist throughout carbonization and cause the tensile failure of the carbonized fiber. Improvements in the PBO spinning process resulted in increased tensile strengths for the carbonized fibers. Further, the tensile strengths of the carbonized fibers appear to be related to the release of nitrogen during the onset of crystallite growth. The rapid heating rates associated with continuous carbonization were shown to minimize the negative effects of this nitrogen release. Fibers heated to 1600°C in a continuous operation displayed nearly double the tensile strength of those produced in a batch operation. Ultimately, this study showed that, while the tensile strengths of PBO-based carbon fibers can be increased by using a continuous process, enhancements in the solution spinning process of the polymer will be required for PBO to become a commercially viable carbon fiber precursor.