Controllable preparation of spinnable isotropic pitches for carbon fibers with high tensile strength from low-cost ethylene tar pitch by a selective photobromination–debromination method

N-bromosuccinimide (NBS) has notable selectivity for brominating aromatic compounds with alkyl side chains. This study employs NBS in lieu of liquid bromine to prepare spinnable isotropic pitch derived from ethylene tar pitch (ETP) using a selective photobromination-debromination approach. The prepared isotropic pitches were then utilized to fabricate isotropic pitch-based carbon fibers (IPCFs) through a process involving melt spinning, oxidative stabilization, and subsequent carbonization. As the amount of NBS added increases in the photobromination stage, the softening point, pitch yield, average molecular weight, and degree of polymerization of the resulting isotropic pitch gradually increase, whereas its spinnability first improves but then decreases. Compared with the isotropic pitch manufactured through thermal polymerization alone, the isotropic pitch that undergo photobromination–debromination exhibits a more linear molecular structure formed by methylene/ethylidene-bridged aromatic units. This molecular structure enhances its spinnability, significantly improving the mechanical performance of the resulting IPCFs. The isotropic pitch produced with 15 wt% NBS during photobromination demonstrates exceptional spinnability, yielding carbon fibers with excellent mechanical characteristics. These fibers exhibit a tensile strength of 1333 MPa, Young's modulus of 64 GPa, and an elongation property of 2.4 %. This work provides a new method for the high value-added utilization of ET by controlling the molecular structure of the pitch precursor.