Viscosity, temperature dependence pitches

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. 

Although a number of equations have been used to describe the temperature dependence of viscosity of pitch systems(15), the Williams, Landel, Ferry equation (WLF) has received relatively little attention. 

The precursor is melted in an extruder which pumps the melt into a die head equipped with a filter and a multihole spinneret [5-6] [24]. As the precursor fibers exit the spinneret holes they cool and solidify, and are drawn before windup. The window for achieving successful and continuous fiber formations is small. The temperature dependence of the viscosity is large and the failure strength of the mesophase pitch fibers is low (30-40 MPa). Thus, the extrusion temperature must be precisely controlled. 

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]. 

Although experimentally the pitch dependence is difficult to measure, the temperature dependence of the apparent viscosity indeed was foimd to show oscillating behavior, which is due to the temperature dependence of the pitch. Details of the Leslie and other theories and of the relevant experiments are discussed in the book of Chandrasekhar. ... 

In other words, molecular design and the synthesis were carried to express various characteristics for the required purposes that depend on the specific application of the ferroelectric liquid crystal. For example, ferroelectric liquid crystals with large spontaneous polarization, a large tilt angle with small temperature dependence, a specific viscosity, a small rotational viscosity of the molecules along their long axis, a chiral smectic C phase over a wide temperature range, a characteristic anisotropy of the dielectric constant, a suitable phase sequence, a suitable helical pitch, photochemical stability, and so on have been newly synthesized [3-5]. 

A very unusual characteristic of mesophase pitch is the extreme dependency of its viscosity on temperature [19,34,35]. This factor has a profound influence on the melt-spinning process (described above), as a mesophase pitch fiber will achieve its final diameter within several millimeters of the face of the spinnerette, in sharp contrast to most polymeric fibers. 

Heat treatment of the S-type fluoride in a fluorine atmosphere Based on the results above mentioned, Fujimoto et al. developed a new fluorination procedure in order to prepare the perfluorinated pitch, and obtained two types of other fluorinated pitches [23,24], The new process is by the heat treatment at 200-400°C of S-type of fluorinated pitch prepared at relatively low temperature in a fluorine atmosphere. They firstly fluorinated the mesophase pitch at 70°C for 10 h (first step for the preparation of S-type fluorinated pitch) and then heated up to a selected temperature between 200°C and 400°C, and maintained this temperature for 12 h (second step for the heat-treatment of fluorinated pitch). Thus, they obtained two kinds of fluorocarbons, a transparent resin (R-type) and a liquid (L-type). L-type is a viscous fluid containing some volatile materials and the viscosity gradually becomes higher when it is kept for a few weeks in an air atmosphere even at ambient temperature. They reported that the R-type was obtained in the nickel boat in the heating zone and L-type at the bottom of the vertical reaction vessel which was cooled down by the water. Therefore, it is likely that the liquid fluorocarbon is formed by the vaporization of some component contained in the S-type fluoride or decomposition reaction during the heat treatment of the S-type fluoride. The yields of these compounds depends on the heat treatment temperature. In Fig. 3, the yields of the R-type and L-type fluorocarbons are plotted as a function of the heat treatment temperature of the S-type fluoride. The yield of the former decreases with increase of the heat treatment temperature and finally, at 400 C, it can not be obtained at all. On the other hand, the yield of the latter increases with increase of temperature and it is selectively obtained at 400°C. 

Tar (i.e., pitch) is formed during combustion and contains a vast assortment of PAHs. The exact distribution depends on the variability of the feedstock and processing temperature. In the past, tar was used an asphalt additive and even as a roof sealer due to its viscosity. However, this and other PAH uses have diminished with time. The purification and distribution of most of the 16 PAHs on the EPA Priority... 

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 4.28 The dependence of the viscosity on temperature for various pitches. Source. Reprinted with permission from Edie DD, Dunhan MG. Meltspinning pitch based carbon fibers Carbon 27, 647, 1989. Copyright 1989, Elsevier.

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.

Research in ACF has attracted increasing attention in the last few years in terms of their synthesis, and their suitability in different applications that include solvent recovery, molecular sieving, gas storage and catalysis. Activated carbon fibres are usually prepared from precursors of low or intermediate crystallinity such raw materials include polyacrylonitrile (PAN) fibres, cellulose fibres, phenolic resin fibres, pitch fibres, cloth or felts made from them, and viscose rayon cloth. They are first pyrolysed and then activated at a temperature of 700-1000 C in an atmosphere of steam or carbon dioxide. Both the processing costs and the properties of the fibre products are dependent on the nature of the starting material. 

---