Comparative Analysis of Kevlar, PBO and Derived Carbon Fibers

Carbon fiber (CF) reinforced carbon/carbon composites (CCCs) are in increasing demand in the automotive, aerospace, and defense industries due to their robust mechanical and thermal properties. Widespread application of these advanced materials is limited because of excessive production costs which are driven up due to long, expensive densification processes that are needed to achieve these properties. After the initial pyrolysis, CCCs are exceedingly porous due to the cracks that form as a result of the rigid CFs that inhibit in-plane shrinkage of the matrix during pyrolysis. This porous structure requires densification to increase strength and prevent thermo-mechanical degradation. Cocarbonization of polymer fiber reinforced polymer composites has the potential to offer CCCs with an optimized pore morphology. Currently the CF market is dominated by PAN- and Pitch-based CFs because of their strength and economic viability. These fibers are not suited to test this hypothesis because they require a lengthy oxidative stabilization step in air which cannot occur in a polymer matrix with nearly zero permeability. Poly(p-phenylene terephthalamide) (Kevlar) and poly(p-phenylene-2,6-benzobisoxazole) (PBO) are high-performance polymer fibers that can be converted to CFs via rapid carbonization processes without oxidative stabilization. While these precursor fibers initially cost more than PAN- and pitch-based CF, the pore structure generated from simultaneous carbonization of fiber and matrix has the potential to offset those costs via facile densification. This study investigates Kevlar and PBO fibers that were treated under different thermal schedules up to 1200 °C. Raman spectroscopy was used to compare the generated carbon structures. For both Kevlar and PBO, 1000 °C was found to produce the most ordered carbon fibers. X-ray diffraction (XRD) was used to quantify the carbon nanostructure which revealed PBO possessed superior crystalline order. Mechanical testing is consistent with XRD results displaying carbonized PBO achieves higher tensile modulus and compressive strength, however carbonized Kevlar unexpectedly out performed carbonized PBO in tensile strength. Comparing these fibers under SEM imaging revealed that the lower tensile strength of carbonized PBO is due to the unique microstructure that forms. This microstructure is shown to be influenced by processing conditions during pyrolysis. The results of this study demonstrate PBO's unique characteristics that distinguish itself as the primary candidate to explore this hypothesis going forward.