The effect of hydrogenation degree on the properties of coal-based spinnable mesophase pitch

The excellent mechanical properties and thermal conductivity of mesophase pitch-based carbon fibers are intricately linked to the rheological properties and molecular structure of mesophase pitch (MP). In this study, spinnable mesophase pitch was prepared through hydrogenation and thermal condensation, and the impact of refined coal tar pitch (RCTP) hydrogenation on the rheological properties and molecular structure of MP was investigated. Molecular structure characterization results of hydrogenated pitch (HP) showed a significant increase in aliphatic chain structure, naphthenic structure, and methylene structure, along with an elevated H/C atomic ratio ranging from 0.602 to 0.700 with an increase in hydrogenation degree. The rheological and spinning properties of MP were assessed using a capillary rheometer, high-temperature rotational rheometer, melt spinning machine and other equipment. Results indicated a lower viscosity and softening point of MP, improved fluidity, and enhanced spinning stability with increasing hydrogenation degree. Polarizing microscope, X-ray diffraction, and Raman spectroscopy were used to characterize the molecular structure of MP. Findings emphasized that an optimal degree of hydrogenation facilitated the formation of a large domain structure and orderly stacking of MP molecules, maintaining molecular planarity to a certain extent. Excessive hydrogenation of RCTP, however, led to a reduction in the degree of order in the molecular arrangement of MP. When the H/C atomic ratio of HP ranged from 0.648 to 0.669, a notable reduction in MP system viscosity, improved spinning stability, and maintained molecular planarity were observed. During the spinning process, favorable rheology and planarity promoted the orderly arrangement of MP aromatic planar molecules along the tensile direction. This holds significant implications for preparing mesophase pitch-based carbon fiber with excellent mechanical properties and thermal conductivity.