Cokes and graphites produced by the extrusion of pitch mesophase

• Main Author: Jenkins, J. C.
• Published: Swansea University 1983
• Subjects: 622
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.637413

aCarbonaceous mesophase derived by the pyrolysis of highly aromatic pitches low in insoluble material may be mechanically deformed to induce a preferred orientation of the flat molecules. This principle was employed to produce extruded rod mesophases with high degrees of preferred orientation in the extrusion direction. Ashland 240 mesophase (HTT = 455°C) was fluid enough to produce a large mass of extruded rod mesophase which was subject to a drawing action under its own weight as it left the extruder, thereby enhancing the anisotropy directly after extrusion. The extrusion in rod form rather than a shapeless mass was possible by removing most of the tin transformed pitch by an initial extrusion at 370°C. This enabled the more viscous residue in the extruder vessel to be reheated to 455°C and extruded as mesophase rods. Bacon anisotropy analysis was applied to X-ray diffraction patterns of radial sections of extruded rods whose maximum orientation corresponded with the extrusion direction. The Bacon analysis was modified to cater for specimens whose maximum orientation was slightly offset from the extrusion direction. This technique was also applied to rods after oxidation by slowly heating in air to 360°C and subsequent heating to produce graphite rods. These processes corresponded with decreases of 3% and 34% respectively in orientation anisotropy factor (M). Mean M = 2.73 for graphitized rods show that mesophase extrusion can be employed to produce highly anisotropic graphite particles. CTE measurements on rod graphites perpendicular and parallel to the extrusion direction supported this demonstrating the effectiveness of mesophase extrusion in producing graphite with a low volumetric CTE suitable for good resistance to thermal shock. A mathematical model of the extrusion of flat molecules through a tube was developed to match the experimental results of Ashland 240 and 170 mesophases extruded through nozzles of different lengths. The theoretical model predicted longitudinal structures gradually increasing in anisotropy from centre to edge. This matched with a gradually increasing anisotropy factor M in an Ashland 240 4700C pyrolysate extruded at 470°C. Lower temperature extrudates tended to exhibit a uniform anisotropy in the direction of extrusion throughout the rod section. Radial structures were predicted as 'random' (rather than 'onion skin' or 'radial') and this was supported by experimental observations in this work. Much lower experimentally determined anisotropy factors were obtained in comparison to those predicted by mathematical analysis. This was imputed to the inhomogeneous nature of the extruding mesophase which produced slight deviations from the laminar flow required to produce the very high theoretical anisotropy factors predicted for the nozzle dimensions employed.