The introduction of a skin-core structure in mesophase pitch fibers through a successive stabilization by oxidation and solvent extraction

Two-step stabilization, air oxidation, and solvent extraction were applied to introduce a skin-core structure in a coal tar mesophase pitch based carbon fiber. Pitch fiber was first oxidized at 270°C for short periods in air or 10% O₂, successively extracted in a Soxhlet for 15 to 300 min with tetrahydrofuran (THF) or benzene, and then carbonized to 600°C at a selected heating rate (from 1 to 10dgC/min). A distinct skin-core structure was introduced in carbonized fibers of about 10 μm diameter. By adjusting the processing variables, one could sensitively control the skin-core structure without any adhesion remaining between the filaments. The extent of oxidation governed the thickness of the skin and the domain size of the core, whereas the extent of extraction prohibited the adhesion by removing the soluble or fusible fractions in the surface layers, fixing the thickness of the skin that had been oxidatively stabilized. The heating rate to carbonization should be selected according to the extent of stabilization, to prevent adhesion and deformation of the carbonized fibers.