Spinning Mesophase Precursor Fibers

When producing carbon fibers from pitch, a critical processing parameter is the viscosity of the pitch, which is extremely dependent on the spinning temperature. If pitch was a truly Newtonian fluid, the viscosity would be independent of shear rate, attaining its value almost instantaneously. It would be expected that the ratio of the hot filament diameter to the orifice diameter (die swell ratio) would be less than 1.1 and the molten fluid would not climb the stirring rod (the so called Weissenberg effect) [228]. 

In general, Lewis and Lewis [262] showed that these pseudo-Newtonian pitches follow the Williams, Landel, Ferry (LDF) equation [263]  

Cl and C2 are constants showing significant dependence on the composition of the pitch. Although the LDF equation gives a better fit, Otani and Oya [264] used the Andrade equation (rj = to plot rj Vs IjT for two different pitches and when two straight lines 

Source Reprinted with permission from Otani S, Oya A, Progress of pitch based carbon fiber in Japan. Bacha JD, Newman JW, White JL, eds., ACS Symp Ser No. 303, American Chemicai Society, Washington, DC, 322, 1986. Copyright 1986, American Chemicai Society. 

Even with good control of the temperature, the tensile stress in the pitch filament will be about 20% of the ultimate strength, whereas this figure is only about 1% of the tensile strength when spinning nylon fiber [267]. 

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Figure 4.31 Schematic of process for melt spinning mesophase precursor fibers. Source Reprinted with permission from Edie DD, Diefendorf RJ, Carbon fibre manufacturing, Buckley JD, Edie DD eds., Carbon-Carbon Materials and Composites, Noyes Publications, Park Ridge, NJ, 1993. Copyright 1993, William Andrew Publishing.

Mesopitch based carbon fibers exhibit a variety of microtextures depending on the structure of the specific precursor fiber and the spinning conditions. Mesopitch based fibers prepared from 100% mesophase pitch have a more homogeneous microtexture than those prepared from mixtures of anisotropic and isotropic pitches. Further, disturbing the flow pattern of the... 

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