Isotropic pitch, viscosity-temperature

At temperatures above the softening point, isotropic pitch often displays Newtonian flow characteristics (18,19), but this may well depend upon the concentration of any insoluble particles (i.e., primary QI in the case of coal tar based materials) present within the pitch. A high concentration of QI could lead to non-Newtonian character as a result of the particle-particle attractive forces. Figure 3 shows n -T curves for a variety of pitch materials and their pyrolysis products. Pyrolysis increases the Tg of the system and shifts the viscosity-temperature curve to higher temperatures. 

Figure 3. Typical viscosity-temperature curves for a variety of isotropic pitch samples and partially pyrolysed pitches.

Three points are noteworthy for the Kyukoshi method. (i) Rapid heating of the hydrogenated pitch to above 450°C produces a pitch suitable for smooth spinning. (ii) This pitch appears to be isotropic at the spinning temperature of 370°C. (iii) The viscosity-temperature relationship, plotted in Figure 8 in terms of the Andrade equation... 

Nazem [259] showed that a 100% isotropic pitch reached a steady viscosity in about 50 s, whereas a mesophase pitch prepared from it (97% anisotropic) failed to achieve a steady state even after 400 s. However, intermediate pitches prepared with about 75% anisotropy did attain a reasonably constant viscosity at high shear rates and are termed pseudo-Newtonian, which are suitable for spinning. Brooks and Taylor mesophase and neomesophase have also been shown to behave in a similar manner. Figures 4.28 and 4.29 show the dependence of viscosity on temperature for a number of pitches, emphasizing the difference between isotropic and mesophase pitches. 

Figure 4.29 The dependency of viscosity on temperature for several isotropic pitches, a mesophase pitch and a typical thermoplastic polymer Ashland 240 (isotropic petroleum pitch) Aerocarb 60 (isotropic pitch distilled from A240) Aerocarb 75 (isotropic pitch distilled from A240) Source. Sumner MB, Thermal properties of heavy isotropic petroleum pitches. Carbon 88, Proceedings of the International Conference on Carbon, University of Newcastle upon Tyne, 52-54, Sep 18-23,1988, Mesophase (produced by pyrolysis of A240) Nylon 6 (a typical melt spur synthetic polymer). Source Reprinted from Whitehouse S, Rand B, Rheology of mesophase pitch from A240. Carbon 88, Proceedings of the International Conference on Carbon, University of Newcastle upon Tyne, 175-176, Sep 18-23, 1988.

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