FACTORS LIMITING THE TENSILE-STRENGTH OF PBO-BASED CARBON-FIBERS

Poly p-phenplene benzobisoxazole (PBO) converts directly to carbon fib er without stabilization. However, the applicability of PBO-based carb on fibers is limited by their low tensile strength. This paper represe nts the first study of the high-temperature conversion of PBO to carbo n 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 pro cess 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 gr owth. The rapid heating rates associated with continuous carbonization were shown to minimize the negative effects of this nitrogen release. Fibers heated to 1600 degrees C in a continuous operation displayed n early double the tensile strength of those produced in a batch operati on. Ultimately, this study showed that, while the tensile strengths of PBO-based carbon fibers can be increased by using a continuous proces s, enhancements in the solution spinning process of the polymer will b e required for PBO to become a commercially viable carbon fiber precur sor. Copyright (C) 1996 Elsevier Science Ltd