Adverse effect of mesophase pitch draw-down ratio on carbon fiber strength

The adverse effect of draw-down ratio (DDR) during melt-spinning of mesophase pitch on resulting properties of carbon fibers (CFs) is reported. Precursor fibers were obtained using spinneret capillaries of various diameters. For CFs with equivalent diameters (ranging between 8 and 13 μm), the tensile strength of CFs increased from 1.4 to 2.3 GPa as the DDR of precursor fibers decreased from 189 to 14. This is contrary to that observed for CFs derived from polyacrylonitrile precursor, where strength increases with increasing precursor fiber DDR. Raman spectroscopy revealed an inverse relationship between pitch fiber DDR and CF coherence length (increase from 58 to 85 nm when DDR decreased from 189 to 14). Further, x-ray diffraction results confirm a larger CF crystalline orientation with decreasing DDR. Thus, a smaller DDR during mesophase spinning could facilitate intra-crystalline graphitic-plane orientation and decrease defects in the inter-crystalline regions along fiber axis in resulting CFs, leading to increased strength (a defect-controlled property). Thus, while it is generally known that tensile strength increases with decreasing CF diameter, for mesophase pitch-based carbon fibers this relationship is not fully valid if the diameter reduction is obtained by merely increasing the draw-down ratio of precursor fibers during melt-spinning.