Effect Of Spinning Variables Of Mesophase Pitch On The Structural Evolution Of Its Derived Fibers

Mesophase pitch has an optically anisotropic microstructure.Carbon fibers prepared from mesophase pitch have attracted extensive attention because of their high tensile modulus,high thermal conductivity and electric conductivity.Spinning process is a key process for obtaining carbon fibers with excellent structural properties.In this work,we studied the structure and properties of circular carbon fibers obtained from home-made mesophase pitch under different heat treatment conditions（holding temperature and time）in heating chamber.On this basis,we explored the spinning process using C-shaped spinneret hole.The influences of spinning process parameters（temperature,pressure and winding rate）on the fiber cross-section morphology was systematically investigted.By establishing the evaluation method of C-shaped fiber based on parameters including fiber outer diameter,thickness and opening angle,we obtained optimum technological conditions for spinning of C-shaped carbon fiber.The detailed results are as follows:（1）XRD,Raman and FTIR showed that with the increase of holding temperature and time,the interlayer space(d₀₀₂)of mesophase gradually decreased,the stacking height（L_c）gradually increased,the aromaticity of raw materials increased and the degree of order（R）increased.The softening point of the raw material increased from219℃to 241℃,and the viscosity increased.The mesophase pitch was spun at 320℃and 335℃for 1 h,4 h and 12 h to obtain fiber precursor.Through the statistical analysis of polarizing microscope,with the extension of holding time,the fiber diameter reduced from 15.5μm,17.9μm to 13.5μm,16.3μm.After oxidation and carbonization heat treatment,d₀₀₂ first increased and then decreased,L_c first decreased and then increased,and R also decreased and then increased.With the extension of holding time,d₀₀₂ and fiber orientation degree gradually decreased,L_c and R gradually increased.After carbonization,the fiber diameter decreased and cleavage occured.With the extension of holding time,the diameter of carbonized fiber at 1400℃at 320℃and 335℃reduced from 12.3μm,12.8μm to 10.9μm,11.7μm.The fiber splitting angle decreased from 79.2°and 105.1°to 78.4°and 94.5°respectively.Finally,with the increase of holding time,the tensile strength of the fiber increased from 1.70 and0.95 GPa to 2.72 and 1.53 GPa respectively.The tensile modulus increased from 152and 111 GPa to 246 and 190 GPa respectively.（2）The influence of spinning parameters（temperature,pressure and winding rate）on the fiber cross-section morphology was systematically investigated using optical microscopy.With the increase of temperature,the viscosity of raw material decreased,the loss of stored elastic potential energy decreased,the extrusion swell of fiber and the thickness of fiber increased.With the increase of temperature,the fiber extrusion velocity increased,resulting in the increase of fiber outer diameter and unstable fiber shear flow.With the increase of pressure,the shear action of raw material increased and the extrusion swell effect decreased.At the same time,the flow velocity distribution of fiber in the flow channel changed,resulting in uneven shear action at both ends and middle,resulting in the increase of fiber opening.At the same time,with the increase of pressure,the flow velocity of raw material increased,resulting in the increase of fiber outer diameter.With the increase of winding rate,the inner and outer diameters of the fiber were reduced,which was conducive to finer fiber.Finally,by comparing the fiber outer diameter,thickness and opening angle under different spinning conditions,it was concluded that in order to obtain C-type fiber with thin and uniform thickness and small opening angle,spinning conditions should be carried out at low temperature and medium pressure within the spinnable range.