Ind. Eng. Chem. Prod. Res. Dev., Vol. 17, No. 3, 1978 275
LETTER T O THE EDITOR Rheological Properties of Coal-Tar Pitch Binders
Sir: I refer to the paper published in this journal by Wallouch et al. (1977) on the above subject and I am disclosing the experience we gathered during several years of measuring the rheology of hundreds of laboratory and commercial pitches (Romovacek, 1977). Our experimental results are consistent with the data published by Wallouch et al. (1977) although we used a different apparatus and a different approach in presenting our correlations. The viscosity was measured by Haake Viscobalance. This instrument measures the time necessary for a metal sphere to rise in a cylindrical container filled with the sample. The sphere is connected via a metal rod and a stirrup link to one arm of a balance; the other arm carries a balance pan. The driving force for the rising ball is provided by a weight placed on the pan. The travel distance of the ball is read on a scale along which the balance pointer moves. We found the results obtained with this instrument very reproducible and free from many disturbing influences causing dubious results when some other types of viscometers are used for the determination of the pitch viscosities. A typical viscosity temperature correlation for coal-tar pitches having different softening points is given in Figure 1. Here the logarithm of the viscosity is plotted against the temperature ("0. The softening point is essentially a temperature of equal viscosity (equiviscous temperature) determined by the softening point apparatus. Equiviscous temperatures can be read easily from the viscosity-temperature curves for the chosen levels of viscosity. We selected two viscosities (1000 and 4000 cP) and determined from the viscositytemperature curves the corresponding equiviscous temperatures, i.e., temperatures at which the pitch sample attains the viscosity of 1000 or 4000 cP, respectively (see Figure 1). These equiviscous temperatures determined for about 150 coal tar pitches were correlated with the corresponding softening points. The following equation can be used to calculate the equiviscous temperatures from the known softening point: for 1000 CP EVTlooo= 32.3 + 1.231 X S.Pt. (1) (standard deviation for 95% confidence limit is equal to f 4 "C); for 4000 CP EVT4000 = 26.2 + 1.117 X S.Pt. (2) (standard deviation for 95% confidence limit is equal to f 5 "C) The effect of temperature on liquid viscosity may be correlated within the accuracy of most experimental data with the Andrade equation u = AeBIT (3) (u = viscosity, cP, A,R = constants, and T = absolute temperature) which requires knowledge of two or more values of u for evaluation of the constants A and B. Equivalent to the use of the equation is a linear plot of log
EVTooo
1 154 I6 I 167 I74 180
EVTIOOO
~
5,000t \ \ \ \ \
s Pt
' c i -100
-
\ 2,000 CP
1,000
I/T
140
160
I80
200
C'
Figure 2. Plot of log viscosity of coal-tar pitches with variable softening point vs. reciprocal value of absolute temperature.
u vs. 1/T. The validity of this correlation (3) holds well for pitches. For the construction of lines representing statistical averages of many measurements we used equiviscous temperatures calculated from eq 1 and 2 for individual softening points, plotted the calculated temperature values against viscosities of 1000 and 4000 cP, respectively, into log u vs. 1/T correlation and connected both points by straight lines. The graph given in Figure 2 is used for plotting experimental values of viscosities. In this way excessive deviation from the statistical average is immediately apparent. Such a deviation is very rare in the case of coal tar pitches but can be found more frequently within pitches produced from feedstocks different from coal tar.
Literature Cited Andrade, E. N. da C., Nature (London), 25,309-310 (1930). Romovacek, G.R., "Proceedings of 106th AIME Annual M e e t i n w i g h t Metals", 1977 VOI. I, 257-288, 1977. Wallouch, R. W., Murty, H. N., Heintz, E. A,, Ind. Eng. Chem. Prod. Res. Dev., 16, 325-329 (1977).
Koppers Company, Inc. Research Department Monroeuille, Pennsylvania 15146
George R. Romovacek
Received for reuieu February 22, 1978 Accepted April 10, 1978
