- Email: [email protected]
Rheological characteristics of coal tar and petroleum pitches with and without additives
Cnrbotl. 1977, liol
IS, pp. X9-223.
Pergamon Press.
Printed in Great Bntain
RHEOLOGICAL CHARACTERISTICS OF COAL TAR AND PETROLEUM PITCHES WITH AND WITHOUT ADDITIVES G. BHATIA, R. K. ACGARWAL,S. S. CHARIand G. C. JAIN DiviGon of Materials, Carbon Technology Unit, National Physical Laboratory, New Delhi 110012,India (Received 21 Janunry 1977) Abstract--An extensive study of rheological characteristics of coal tar and petroleum pitches with and without additives, namely, petroleum coke, natural graphite and carbon black has been made. It is found that all pitches, pure or mixed with a carbon additive are not Newtonian as reported in the literature, but behave rheologically as Bin&am plastics with certain yield stress and plastic viscosity at all temperatures of measurement between 85-180°C. The yield stress and plastic viscosity both decrease with increase in temperature of the pitch. A pure petroleum pitch having the same softening point as that of a coal tar pitch is found to have a lower viscosity compared to that of the latter at all temperatures of measurement. This suggests that the criterion of softening point as a measure of suitability of a coal tar pitch binder in the manufacture of arti~~~l carbon is not sufficient for petroleum pitches. Addition of ten parts of carbon black by weight of pitch results in a considerable decrease in viscosity change with temperature of the coal tar pitch compared to almost insignificant change in the case of the petroleum pitch of the same softening point. However, the addition of petroleum coke or natural graphite makes the pitch more viscous but does not change the temperature dependence of viscosity of either of the two types of pitches. The implications are discussed. _
1.~TRODUCTION A very important raw component in the manufacture of artificial carbon and graphite is the binder. The function of this material is to plasticize the coke powder mix so that it ca.n be formed into bodies of desired shape and size. Coal tar pitch has been most used as bonding materiaf because of its high specific gravity, high carbon content and good plasticity in comparison to other binders such as derived from crude petroleum, synthetic resins, etc. The binder carbon is an important factor that determines the nature of the graphite, and variations in binder are known to affect the micro-structure and the properties of the graphite[l~]. A number of investigations were reported on the carbonization of coal tar pitch but very little has appeared in the literature concerning the characterization (especially rheological) of these binder pitches[5-71. These authors[5-71 have tried to characterise the pitches from the solvent analysis and viscosity behaviour. Furthermore, it was reported[6] that the flow properties of ail types of coal tar pitches are Newtonian in character and that their rheological properties are adequately specified by one viscosity measurement at one temperature, which may be conveniently chosen to be the softening point of the pitch. Walloucb et ai.[8] have stressed the importance of the rheological properties of coal tar pitch binders and found[9] that coal tar pitches can be regarded from the viewpoint of rheology as chemically similar and that the temperature variation of their visccsity can be described by a single characteristic parameter, the softening point. In the above papers on pitch binders no mention has been made on the type of instrument used for measuring the viscosity of the pitch. It is obvious that a rotation type coaxial cylinder viscometer would be the ideal one for determining the rheological behaviour of the pitches. Such an instrument was used in the present investigation.
The present authors have undertaken an extensive study of the rheological behaviour of coal tar pitch, petroleum pitch and air blown petroleum pitch. Coal tar pitch and petroleum pitch were chosen with the same softening points. Their rheological behaviour was studied in detail at various temperatures, in the hope that this wili help in finding the suitability of the petroleum pitch as a binder in fabrication of artificial carbon. Another aim of the present investigation was to study the changes that take place in the flow behaviour of these pitches when varying amounts of carbon dust, viz. petroleum coke, natural graphite and carbon black are added to them. Such a condition can be practically realised on a plant scale production of artificial carbon where fine dust could so often be found mixed with the binder. The results of the study are described and discussed below. 2. THEORY
The expressions for the shear stress and the shear rate for Newtonian fluids can be easily written in the following form [ 10, 111 Shear stress,
Shear rate,
P =&
2wR’
i =-i-_
(R - r’)-
Where M represents the torque acting on the inner cylinder of radius r and length 1 when rotating with a constant angular velocity w and R is the radius of the fixed outer cylinder. The apparent viscosity can then be written as
219
220
G. BHATIAetal.
For a Newtonian fluid, when the viscosity at a particular temperature is constant, the shear stress-shear rate diagram will be a straight line passing through the origin. The slope of the straight line gives the viscosity of the fluid which is independent of the shear stress or shear rate applied. For a Bingham fluid, the simplest of non-Newtonian, no flow occurs until the stress exceeds a particular value called yield stress, TV The rheological equation for a Bingham body then becomes (3)
7 - Ty = Ylpr
in the temperature range 8%180°C using eqns (1) and (2) in series of steps. The results of the present study are diagrammatically shown in Figs. l-8. 4.RFSULTSANDDISCUSSION
4.1 Temperature dependence of viscosity Figure 1 gives the variation of logarithmic viscosity vs temperature for various pitches. Curves A, 0 and L are for coal tar pitch, straight petroleum pitch and air blown petroleum pitch respectively. It is seen that the curves A and 0 are almost parallel and that viscosity of the straight petroleum pitch 0 at any temperature is always below that of coal tar pitch A, though both have the
where q,,f is the plastic viscosity. When a plot of shear rate vs shear stress is constructed, the plastic viscosity is the reciprocal of the slope of the straight line and the yield stress is the intercept on the stress axis. 3.EXPERIMENTALDJCTAILS
The pitches used in the present investigation were coal tar pitch, straight petroleum pitch and air blown petroleum pitch. The characteristics of these pitches are given in detail in Table 1. These pitches were mixed with 2, 5 and 10 parts by weight of the pitch, of petroleum coke, natural graphite and carbon black additives respectively. 40g portions of the pure pitch or pitch with additives were taken each time and subjected to viscosity measurements on Rheotest II. The Rheotest II essentially consists of a coaxial cylinder system of the couette type. The material under test lay in the annular gap of the coaxial cylinder system. The outer cylinder containing the test material was surrounded by a temperature regulating bath which could be connected to liquid circulation thermostat in order to maintain the temperature at a constant level with an accuracy of +0.25”C. The inner cylinder rotating with constant angular velocity was connected through the measuring shaft to a helical spring the deflection of which gave a measure of torque M effective at the inner cylinder. The deflection of the spring was scanned by a resistance potentiometer positioned in its bridge circuit. With this viscometer, shear stresses ranging from 30 to 60OOdynl cm* and shear rates from 0.5 to 437 see-’ could be easily measured. The viscosity of various pitches was determined
00 80
90
100
110
120
130
140
150
160
170
Temperature, T “C
Fig. 1. Log 9 as a function of temperature T for various pitches.
Table 1. Characteristics of pure pitches. r CHARACTERISTIC
1. Softening Pomt IR hB1.Y
PITCH
PITCH
PITCH
70
76
94
01 I30
’
90
’ 100
3 110
’
120
’ 130
’
140
n
150
’ I60
’
170
’ 160
Temperature, T. “C
Fig. 2. Logarithmic temperature coefficient of viscosity as a function of temperature for various pitches.
221
Rheological characteristics of coal tar and petroleum pitches with and without additives
2.0 \
2000
1000
3Om
4ooo
Shear stress, T, dyu cm? Fig. 3. Shear stress-shear rate curves for various oitches at temperature of 100°C.
Shear stress, T, dyn
arm2
Fig. 6. Shear stress-shear rate curves for various pitches at temperature of 140°C.
IO.01 9.0. 60. ‘u4: 7.0. .); 6.0, g L
5.0~.
1
8 4.0 e In 3.0. 2.0.
o-If
0
3OcQ
2000
1000
4Gw
Shear stress, T. dyn cmW2 Fig. 4. Shear stress-shear rate curves for various pitches at temperature of 100°C.
Fig. 7. Shear stress-shear rate curves for various pitches at temperature of 180°C.
Ii
5QOi 8
v) 400.x g mL
b g
200-
IOO-
100
300
400
500
600
700
Shear stress, T, dyn cm-*
0
Sheaf stress, r,
dyn
cRi2
Fig. 5. Shear stress-shear rate curves for various pitches at temperature of 140°C.
Fig. 8. Shear stress-shear rate curves for various pitches at temperature of 180°C.
same Ring and Ball softening point (Table 1). The curve
to have a similar behaviour as obtained for the pure coal tar pitch but has a little higher viscosity. The curve J for a coal tar pitch containing 10 parts of carbon black as seen from Fig, 1 indicates that the addition of carbon black results in the fowering of temperature dependence of the viscosity compared to that with the addition of
L for air blown petroleum pitch is displaced higher because of its higher softening point. The curves 5 and J refer to the coal tar pitch containing 10 parts by its weight of petroleum coke (-350 B.S. mesh) and fine carbon black powder respectively. The curve D is seen
222
G.
BHATIA et
petroleum coke (curve I)). The addition of tS parts of carbon black flattens the curve further (Jr). The addition of 10 parts of carbon black to the straight petroleum pitch and air blown petroleum pitch does not show much influence on the temperature viscosity dependence though it makes the pitch a little more viscous. The flattening of the curve obtained in the case of petroleum pitch containing 15 parts of carbon black (curve P,) is similar to that obtained with coal tar pitch containing 10 parts of carbon black (curve J) though viscosity of the former is lower compared to that of the latter at all temperatures. Figure 2 represents the logarithmic temperature coefficient of viscosity for various pitches and mixes as a function of temperature of measurement which has been defined by McNeil and Wood161 as log 91 -log qz n=logT*-logT* where q1 and v2 represent the viscosities of the pitches at temperatures of T, and Tz respectively. It is easily seen that the coe~cient n decreases linearly with temperature for pure coal tar pitch (curve A), while addition of 10 parts of carbon black to the latter (curve J) rest&s in an initial steep fall which flattens with further increase in temperature. But there is a comTable 2. Rheobgical
properties
paratively slight decrease in the value of n with temperature for the petroleum pitch containing 10 parts of carbon black (curve P). This difference may probably be due to initial free carbon present in the coal tar pitch. 4.2 Shear stress-shear rate characteristics Figures 3-8 present the shear stress vs shear rate diagrams for various pitches with and without additives at temperatures of 100, 140 and 180°C. It is easily seen from the trends of the curves that all the pitches behave like Bingham plastics with certain yield stresses which are given by the intercepts on the stress axis. The plastic viscosity is determined from the inverse of the slopes of such straight lines. The plastic viscosity of the pitch remains constant at a particular temperature, while the apparent viscosity as measured from the ratio of shear stress and shear rate is found to decrease with the shear stress or shear rate. The change in viscosity with shear rate is considerable and the rate of change of viscosity with shear rate is a function of the temperature of the pitch. This observation leads to the conclusion that pitches are not Newtonian and that their non-Newtonian behaviour is enhanced by the presence of additives. The values of yield stress and plastic viscosity for various pitches are given in Table 2 for three different temperatures. It is obvious from the Table 2 that plastic viscosity and with and without additives, at different temperatures.
Yield Stress, cyLyldd(r cn?tat tempemtun, PlasticVwzocity,r+$oise1
MATERIAL
BATCH
of various pitches;
al.
100 %
l6O’C
14o’c
at temperature
IO0 “c
140 ‘c
180 ‘C
IO0
I5
IO
326
A
CTPlk=78t
665
0.56
B
CTPt2tPC
00
1520
20
421
962
I,18
C
CTPtSXPC
100
IS-20
20
435
862
I.18
D
CTPtIOKPC
80
IO
20
600
II.52
I.56
E
CTPtP%NG
80
IS
25
421
8,26
1.0:
F
CTP+S%NG
120
20-30
20
491
61.20
I.43
G
CTPtlO%NG
60
1
l-t
CTPtPXC8
I
CTPtS%CB
120
J
CTPt
500
J!
CtPtawce
K
OSCTP
L
lOICE
20
[
523
Il.60
-
20
25
444
9.60
I.28
40
40
450
10.10
I.56
100
1
160
1
1000
1
37.80
1
5.71
950
780
I75
1900
20
I5
15
211
5.09
0.71
BPPiT.=944:)
400
15-20
20
3429
40
2.25
M
BPP+IO%
400
35-40
3429
N
SPPI
0
SPPII
P
SPPIltlOXCB
PI
sPP11+15%c8
R
SPPII+ESYC8
_.
C8
20
I
/
66.7
3.40
1
6-70
i Ts.85’C i fT*.?a*C
CT P= Coal tar pitch; soluble cool tar point.
I
65
t 70
IO
100 250 PC = Petroleum
360 50 300
coke; NG= Natural
IS.0
380 1380
5.80 300
180
grophite; CB= Carbon black; OSCTP=Ouina&e
pitch; BPP = Blown petroleum pitch; SPP= Straight
petroleum
pitch; k= Ring &Ball softening
. .._
223
Rheological characteristics of coal tar and petroleum pitches with and without additives
yield stress (at 100°C) for coal tar pitch increase by factors of about three and five respectively with the addition of ten parts of carbon black. The yield stress and plastic viscosity both are found to decrease with temperature of the pitch (with and without additives). However., for the quinoline soluble coal tar pitch the yield stress remains almost constant with temperature though the plastic viscosity shows a decrease with temperature. It is also interesting to note from the Table 2 that petroleum pitch containing 10 parts of carbon black (Batch P) is almost equivalent to the pure coal tar pitch (Batch A) with regard to the rheological behaviour. It is a common observation that the softening point of a coal tar pitch is taken as a criterion for its suitability as a binder in the manufacture of carbon products. But the same criterion may not be applicable to a petroleum pitch having the same softening point as that of the coal tar pitch. ‘Thisis because of the lower viscosity observed of the petroleum pitch at all temperatures of measurement. This suggests that carbon mixes should require a lesser amount of the petroleum pitch binder; this has been experimentally confirmedt. As a result a carbon body made using such a binder is less dense compared to that obtained using coal tar pitch of the same softening point.t The work is continuing in this direction and will be reported soon.
may probably be due to the presence of free carbon component in the pure coal tar pitch. (3) Rheologically both the types of pitches (with and without additives) are found to behave as Bingham plastics with certain yield stress and plastic viscosity, and not as Newtonian, as reported in the literature. The plastic viscosity of a pitch at a particular temperature is independent of the shear rate, whereas there is a considerable decrease in the apparent viscosity of the pitch with shear rate at the same temperature observed. The yield stress and plastic viscosity decrease with increase in the temperature. (4) The addition of small amounts of a carbon powder to a pure pitch (coal tar or petroleum) results in a considerable increase of its plastic viscosity but has almost no significant effect on its yield stress at any temperature. (5) In the manufacture of carbon product when petroleum pitch is used as a binder it should be taken with higher softening point than that for the coal tar pitch. This will make the pitch to have higher density and subsequently higher coking value. The higher softening point pitch obtained by blowing air into the petroleum pitch would not be suitable for carbon products fabrication due to its lower coking value (excessive oxygen in the pitch will come out as CO or CO, during coking).
5.CONCLUSIONS
Acknowledgements-Theauthors express their thanks to Dr. A.
(1) Coal tar pitch and petroleum pitch of the same softening point have similar rheological behaviour (Bingham plastic). However, the viscosity of coal tar pitch is seen to be slightly higher than that of the petroleum pitch at all temperatures of measurement. (2) The addition of petroleum coke or natural graphite has no pronounced effect on the temperature dependence of viscosity for either the coal tar or petroleum pitch (Fig. 1). But the addition of carbon black lowers it considerably in the case of coal tar pitch although it has no significant effect up to an addition of 10 parts of carbon black in the case of petroleum pitch of the same softening point. The effect becomes pronounced with the addition of 15 parts of carbon black (curve P,). This difference in the behaviour of the two types of pitches
R. Verma, Director, National Physical Laboratory for his permission to publish the paper. One of the authors (RKA) is thankful to the Council of Scientific and Industrial Research for the award of a Junior Research Fellowship.
tG. Bhatia and R. K. Aggarwal, unpublished work.
REFERENCES 1.
J. Okada and Y. Takeuchi, Proc. 4th Carbon Conf. p. 657,
Pergamon Press, Oxford (1960). 2. J. Okada, Ibid. p. 547. 3. J. M. Hutcheon and M. S. T. Price, Ibid. p. 645. 4. G. B. Engle, Carbon 9, 383 (1971). 5.
A. Darney, Jndusttial Carbon
and
Graphite,
Society
of Chemical Industry, p. 152 (1958). 6. D. McNeil and I. J. Wood, Ibid. p, 162. 7. T. H. Blakely and F. K. Earp, Ibid. p, 173. 8. R. W. Wallouch et al. Carbon 10, 729 (1972). 9. R. W. Wallouch, Private communication. 10. J. R. Van Wazer et al. Viscosity and Flow Measurement, p. 62. Intersciences, New York (1963). 11. S. Oka, Rheology 3, Principles of Rheology (Edited by R. S. Eirich), p. 29. Academic Press, New York (1960).
IS, pp. X9-223.
Pergamon Press.
Printed in Great Bntain
RHEOLOGICAL CHARACTERISTICS OF COAL TAR AND PETROLEUM PITCHES WITH AND WITHOUT ADDITIVES G. BHATIA, R. K. ACGARWAL,S. S. CHARIand G. C. JAIN DiviGon of Materials, Carbon Technology Unit, National Physical Laboratory, New Delhi 110012,India (Received 21 Janunry 1977) Abstract--An extensive study of rheological characteristics of coal tar and petroleum pitches with and without additives, namely, petroleum coke, natural graphite and carbon black has been made. It is found that all pitches, pure or mixed with a carbon additive are not Newtonian as reported in the literature, but behave rheologically as Bin&am plastics with certain yield stress and plastic viscosity at all temperatures of measurement between 85-180°C. The yield stress and plastic viscosity both decrease with increase in temperature of the pitch. A pure petroleum pitch having the same softening point as that of a coal tar pitch is found to have a lower viscosity compared to that of the latter at all temperatures of measurement. This suggests that the criterion of softening point as a measure of suitability of a coal tar pitch binder in the manufacture of arti~~~l carbon is not sufficient for petroleum pitches. Addition of ten parts of carbon black by weight of pitch results in a considerable decrease in viscosity change with temperature of the coal tar pitch compared to almost insignificant change in the case of the petroleum pitch of the same softening point. However, the addition of petroleum coke or natural graphite makes the pitch more viscous but does not change the temperature dependence of viscosity of either of the two types of pitches. The implications are discussed. _
1.~TRODUCTION A very important raw component in the manufacture of artificial carbon and graphite is the binder. The function of this material is to plasticize the coke powder mix so that it ca.n be formed into bodies of desired shape and size. Coal tar pitch has been most used as bonding materiaf because of its high specific gravity, high carbon content and good plasticity in comparison to other binders such as derived from crude petroleum, synthetic resins, etc. The binder carbon is an important factor that determines the nature of the graphite, and variations in binder are known to affect the micro-structure and the properties of the graphite[l~]. A number of investigations were reported on the carbonization of coal tar pitch but very little has appeared in the literature concerning the characterization (especially rheological) of these binder pitches[5-71. These authors[5-71 have tried to characterise the pitches from the solvent analysis and viscosity behaviour. Furthermore, it was reported[6] that the flow properties of ail types of coal tar pitches are Newtonian in character and that their rheological properties are adequately specified by one viscosity measurement at one temperature, which may be conveniently chosen to be the softening point of the pitch. Walloucb et ai.[8] have stressed the importance of the rheological properties of coal tar pitch binders and found[9] that coal tar pitches can be regarded from the viewpoint of rheology as chemically similar and that the temperature variation of their visccsity can be described by a single characteristic parameter, the softening point. In the above papers on pitch binders no mention has been made on the type of instrument used for measuring the viscosity of the pitch. It is obvious that a rotation type coaxial cylinder viscometer would be the ideal one for determining the rheological behaviour of the pitches. Such an instrument was used in the present investigation.
The present authors have undertaken an extensive study of the rheological behaviour of coal tar pitch, petroleum pitch and air blown petroleum pitch. Coal tar pitch and petroleum pitch were chosen with the same softening points. Their rheological behaviour was studied in detail at various temperatures, in the hope that this wili help in finding the suitability of the petroleum pitch as a binder in fabrication of artificial carbon. Another aim of the present investigation was to study the changes that take place in the flow behaviour of these pitches when varying amounts of carbon dust, viz. petroleum coke, natural graphite and carbon black are added to them. Such a condition can be practically realised on a plant scale production of artificial carbon where fine dust could so often be found mixed with the binder. The results of the study are described and discussed below. 2. THEORY
The expressions for the shear stress and the shear rate for Newtonian fluids can be easily written in the following form [ 10, 111 Shear stress,
Shear rate,
P =&
2wR’
i =-i-_
(R - r’)-
Where M represents the torque acting on the inner cylinder of radius r and length 1 when rotating with a constant angular velocity w and R is the radius of the fixed outer cylinder. The apparent viscosity can then be written as
219
220
G. BHATIAetal.
For a Newtonian fluid, when the viscosity at a particular temperature is constant, the shear stress-shear rate diagram will be a straight line passing through the origin. The slope of the straight line gives the viscosity of the fluid which is independent of the shear stress or shear rate applied. For a Bingham fluid, the simplest of non-Newtonian, no flow occurs until the stress exceeds a particular value called yield stress, TV The rheological equation for a Bingham body then becomes (3)
7 - Ty = Ylpr
in the temperature range 8%180°C using eqns (1) and (2) in series of steps. The results of the present study are diagrammatically shown in Figs. l-8. 4.RFSULTSANDDISCUSSION
4.1 Temperature dependence of viscosity Figure 1 gives the variation of logarithmic viscosity vs temperature for various pitches. Curves A, 0 and L are for coal tar pitch, straight petroleum pitch and air blown petroleum pitch respectively. It is seen that the curves A and 0 are almost parallel and that viscosity of the straight petroleum pitch 0 at any temperature is always below that of coal tar pitch A, though both have the
where q,,f is the plastic viscosity. When a plot of shear rate vs shear stress is constructed, the plastic viscosity is the reciprocal of the slope of the straight line and the yield stress is the intercept on the stress axis. 3.EXPERIMENTALDJCTAILS
The pitches used in the present investigation were coal tar pitch, straight petroleum pitch and air blown petroleum pitch. The characteristics of these pitches are given in detail in Table 1. These pitches were mixed with 2, 5 and 10 parts by weight of the pitch, of petroleum coke, natural graphite and carbon black additives respectively. 40g portions of the pure pitch or pitch with additives were taken each time and subjected to viscosity measurements on Rheotest II. The Rheotest II essentially consists of a coaxial cylinder system of the couette type. The material under test lay in the annular gap of the coaxial cylinder system. The outer cylinder containing the test material was surrounded by a temperature regulating bath which could be connected to liquid circulation thermostat in order to maintain the temperature at a constant level with an accuracy of +0.25”C. The inner cylinder rotating with constant angular velocity was connected through the measuring shaft to a helical spring the deflection of which gave a measure of torque M effective at the inner cylinder. The deflection of the spring was scanned by a resistance potentiometer positioned in its bridge circuit. With this viscometer, shear stresses ranging from 30 to 60OOdynl cm* and shear rates from 0.5 to 437 see-’ could be easily measured. The viscosity of various pitches was determined
00 80
90
100
110
120
130
140
150
160
170
Temperature, T “C
Fig. 1. Log 9 as a function of temperature T for various pitches.
Table 1. Characteristics of pure pitches. r CHARACTERISTIC
1. Softening Pomt IR hB1.Y
PITCH
PITCH
PITCH
70
76
94
01 I30
’
90
’ 100
3 110
’
120
’ 130
’
140
n
150
’ I60
’
170
’ 160
Temperature, T. “C
Fig. 2. Logarithmic temperature coefficient of viscosity as a function of temperature for various pitches.
221
Rheological characteristics of coal tar and petroleum pitches with and without additives
2.0 \
2000
1000
3Om
4ooo
Shear stress, T, dyu cm? Fig. 3. Shear stress-shear rate curves for various oitches at temperature of 100°C.
Shear stress, T, dyn
arm2
Fig. 6. Shear stress-shear rate curves for various pitches at temperature of 140°C.
IO.01 9.0. 60. ‘u4: 7.0. .); 6.0, g L
5.0~.
1
8 4.0 e In 3.0. 2.0.
o-If
0
3OcQ
2000
1000
4Gw
Shear stress, T. dyn cmW2 Fig. 4. Shear stress-shear rate curves for various pitches at temperature of 100°C.
Fig. 7. Shear stress-shear rate curves for various pitches at temperature of 180°C.
Ii
5QOi 8
v) 400.x g mL
b g
200-
IOO-
100
300
400
500
600
700
Shear stress, T, dyn cm-*
0
Sheaf stress, r,
dyn
cRi2
Fig. 5. Shear stress-shear rate curves for various pitches at temperature of 140°C.
Fig. 8. Shear stress-shear rate curves for various pitches at temperature of 180°C.
same Ring and Ball softening point (Table 1). The curve
to have a similar behaviour as obtained for the pure coal tar pitch but has a little higher viscosity. The curve J for a coal tar pitch containing 10 parts of carbon black as seen from Fig, 1 indicates that the addition of carbon black results in the fowering of temperature dependence of the viscosity compared to that with the addition of
L for air blown petroleum pitch is displaced higher because of its higher softening point. The curves 5 and J refer to the coal tar pitch containing 10 parts by its weight of petroleum coke (-350 B.S. mesh) and fine carbon black powder respectively. The curve D is seen
222
G.
BHATIA et
petroleum coke (curve I)). The addition of tS parts of carbon black flattens the curve further (Jr). The addition of 10 parts of carbon black to the straight petroleum pitch and air blown petroleum pitch does not show much influence on the temperature viscosity dependence though it makes the pitch a little more viscous. The flattening of the curve obtained in the case of petroleum pitch containing 15 parts of carbon black (curve P,) is similar to that obtained with coal tar pitch containing 10 parts of carbon black (curve J) though viscosity of the former is lower compared to that of the latter at all temperatures. Figure 2 represents the logarithmic temperature coefficient of viscosity for various pitches and mixes as a function of temperature of measurement which has been defined by McNeil and Wood161 as log 91 -log qz n=logT*-logT* where q1 and v2 represent the viscosities of the pitches at temperatures of T, and Tz respectively. It is easily seen that the coe~cient n decreases linearly with temperature for pure coal tar pitch (curve A), while addition of 10 parts of carbon black to the latter (curve J) rest&s in an initial steep fall which flattens with further increase in temperature. But there is a comTable 2. Rheobgical
properties
paratively slight decrease in the value of n with temperature for the petroleum pitch containing 10 parts of carbon black (curve P). This difference may probably be due to initial free carbon present in the coal tar pitch. 4.2 Shear stress-shear rate characteristics Figures 3-8 present the shear stress vs shear rate diagrams for various pitches with and without additives at temperatures of 100, 140 and 180°C. It is easily seen from the trends of the curves that all the pitches behave like Bingham plastics with certain yield stresses which are given by the intercepts on the stress axis. The plastic viscosity is determined from the inverse of the slopes of such straight lines. The plastic viscosity of the pitch remains constant at a particular temperature, while the apparent viscosity as measured from the ratio of shear stress and shear rate is found to decrease with the shear stress or shear rate. The change in viscosity with shear rate is considerable and the rate of change of viscosity with shear rate is a function of the temperature of the pitch. This observation leads to the conclusion that pitches are not Newtonian and that their non-Newtonian behaviour is enhanced by the presence of additives. The values of yield stress and plastic viscosity for various pitches are given in Table 2 for three different temperatures. It is obvious from the Table 2 that plastic viscosity and with and without additives, at different temperatures.
Yield Stress, cyLyldd(r cn?tat tempemtun, PlasticVwzocity,r+$oise1
MATERIAL
BATCH
of various pitches;
al.
100 %
l6O’C
14o’c
at temperature
IO0 “c
140 ‘c
180 ‘C
IO0
I5
IO
326
A
CTPlk=78t
665
0.56
B
CTPt2tPC
00
1520
20
421
962
I,18
C
CTPtSXPC
100
IS-20
20
435
862
I.18
D
CTPtIOKPC
80
IO
20
600
II.52
I.56
E
CTPtP%NG
80
IS
25
421
8,26
1.0:
F
CTP+S%NG
120
20-30
20
491
61.20
I.43
G
CTPtlO%NG
60
1
l-t
CTPtPXC8
I
CTPtS%CB
120
J
CTPt
500
J!
CtPtawce
K
OSCTP
L
lOICE
20
[
523
Il.60
-
20
25
444
9.60
I.28
40
40
450
10.10
I.56
100
1
160
1
1000
1
37.80
1
5.71
950
780
I75
1900
20
I5
15
211
5.09
0.71
BPPiT.=944:)
400
15-20
20
3429
40
2.25
M
BPP+IO%
400
35-40
3429
N
SPPI
0
SPPII
P
SPPIltlOXCB
PI
sPP11+15%c8
R
SPPII+ESYC8
_.
C8
20
I
/
66.7
3.40
1
6-70
i Ts.85’C i fT*.?a*C
CT P= Coal tar pitch; soluble cool tar point.
I
65
t 70
IO
100 250 PC = Petroleum
360 50 300
coke; NG= Natural
IS.0
380 1380
5.80 300
180
grophite; CB= Carbon black; OSCTP=Ouina&e
pitch; BPP = Blown petroleum pitch; SPP= Straight
petroleum
pitch; k= Ring &Ball softening
. .._
223
Rheological characteristics of coal tar and petroleum pitches with and without additives
yield stress (at 100°C) for coal tar pitch increase by factors of about three and five respectively with the addition of ten parts of carbon black. The yield stress and plastic viscosity both are found to decrease with temperature of the pitch (with and without additives). However., for the quinoline soluble coal tar pitch the yield stress remains almost constant with temperature though the plastic viscosity shows a decrease with temperature. It is also interesting to note from the Table 2 that petroleum pitch containing 10 parts of carbon black (Batch P) is almost equivalent to the pure coal tar pitch (Batch A) with regard to the rheological behaviour. It is a common observation that the softening point of a coal tar pitch is taken as a criterion for its suitability as a binder in the manufacture of carbon products. But the same criterion may not be applicable to a petroleum pitch having the same softening point as that of the coal tar pitch. ‘Thisis because of the lower viscosity observed of the petroleum pitch at all temperatures of measurement. This suggests that carbon mixes should require a lesser amount of the petroleum pitch binder; this has been experimentally confirmedt. As a result a carbon body made using such a binder is less dense compared to that obtained using coal tar pitch of the same softening point.t The work is continuing in this direction and will be reported soon.
may probably be due to the presence of free carbon component in the pure coal tar pitch. (3) Rheologically both the types of pitches (with and without additives) are found to behave as Bingham plastics with certain yield stress and plastic viscosity, and not as Newtonian, as reported in the literature. The plastic viscosity of a pitch at a particular temperature is independent of the shear rate, whereas there is a considerable decrease in the apparent viscosity of the pitch with shear rate at the same temperature observed. The yield stress and plastic viscosity decrease with increase in the temperature. (4) The addition of small amounts of a carbon powder to a pure pitch (coal tar or petroleum) results in a considerable increase of its plastic viscosity but has almost no significant effect on its yield stress at any temperature. (5) In the manufacture of carbon product when petroleum pitch is used as a binder it should be taken with higher softening point than that for the coal tar pitch. This will make the pitch to have higher density and subsequently higher coking value. The higher softening point pitch obtained by blowing air into the petroleum pitch would not be suitable for carbon products fabrication due to its lower coking value (excessive oxygen in the pitch will come out as CO or CO, during coking).
5.CONCLUSIONS
Acknowledgements-Theauthors express their thanks to Dr. A.
(1) Coal tar pitch and petroleum pitch of the same softening point have similar rheological behaviour (Bingham plastic). However, the viscosity of coal tar pitch is seen to be slightly higher than that of the petroleum pitch at all temperatures of measurement. (2) The addition of petroleum coke or natural graphite has no pronounced effect on the temperature dependence of viscosity for either the coal tar or petroleum pitch (Fig. 1). But the addition of carbon black lowers it considerably in the case of coal tar pitch although it has no significant effect up to an addition of 10 parts of carbon black in the case of petroleum pitch of the same softening point. The effect becomes pronounced with the addition of 15 parts of carbon black (curve P,). This difference in the behaviour of the two types of pitches
R. Verma, Director, National Physical Laboratory for his permission to publish the paper. One of the authors (RKA) is thankful to the Council of Scientific and Industrial Research for the award of a Junior Research Fellowship.
tG. Bhatia and R. K. Aggarwal, unpublished work.
REFERENCES 1.
J. Okada and Y. Takeuchi, Proc. 4th Carbon Conf. p. 657,
Pergamon Press, Oxford (1960). 2. J. Okada, Ibid. p. 547. 3. J. M. Hutcheon and M. S. T. Price, Ibid. p. 645. 4. G. B. Engle, Carbon 9, 383 (1971). 5.
A. Darney, Jndusttial Carbon
and
Graphite,
Society
of Chemical Industry, p. 152 (1958). 6. D. McNeil and I. J. Wood, Ibid. p, 162. 7. T. H. Blakely and F. K. Earp, Ibid. p, 173. 8. R. W. Wallouch et al. Carbon 10, 729 (1972). 9. R. W. Wallouch, Private communication. 10. J. R. Van Wazer et al. Viscosity and Flow Measurement, p. 62. Intersciences, New York (1963). 11. S. Oka, Rheology 3, Principles of Rheology (Edited by R. S. Eirich), p. 29. Academic Press, New York (1960).