Modification of isotropic pitch by tailoring chemical structure and fraction contents for enhancing carbon fiber mechanical properties

The composition of isotropic pitch (IP) is the key factor to determine the performance of carbon fibers (CFs). It is necessary to investigate the effect of chemical structure of pitch and compositions change on the performance of CFs. An isotropic pitch was fractionated into seven fractions by solvent fractionation to characterize the molecular structural features by FT-IR, 1H NMR etc. The oxidative reactivity of pitch fibers, mechanical properties, microstructure and morphology of CFs derived from four IPs with different components were characterized and evaluated. The results revealed that with the removal of light fractions, the synergistic effect between pitches components in melting significantly decreased, leading to the decrease of spinnability of pitch and deteriorating the uniformity of carbon fiber microstructure. The oxidative reactivity of the pitch fibers gradually decreased with the removal of light fractions. The reason was that these light fractions contained more alkyl side chains, which were preferentially attacked by oxygen at the low oxidation temperature, than that of these heavy fractions. The mechanical properties of CFs showed an increase first and then decrease with the removal of these light fractions. Especially, the carbon fiber derived from the isotropic pitch (H80-T20I) exhibited an excellent high tensile strength of 1190 MPa.