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Rheological properties of pitches and bitumina up to temperatures of 500°C
Corbo,,Vol 18.pp. 155-161 0 PergamonPress Ltd., 19X0 Printed
m Great
Britain
RHEOLOGICAL BITUMINA
PROPERTIES
UP TO TEMPERATURES R.
Institut
OF PITCHES
fiir Chemische
BALDUHN
Technik. (Receiced
and E.
Universitlt
AND
OF 500°C
FITZER
Karlsruhe,
West Germany
30 July 1979)
Abstract-A
high-temperature rotating viscosimeter was set up for application up to 6OO’C. Temperature dependence of viscosity was measured with various pitches and bitumina. The former finding of other authors on the occurrence of an intermediate maximum and minimum of viscosity is confirmed. It is shown that the shear stresses causing these extreme points are characteristic of precursor type and can be correlated with the temperature of mesophase formation. The structure of mesophase as affected by shear is discussed in detail:
1. INTRODUCTION Since
Brooks and Taylor [l] described their observations on mesophase formation during thermal degradation of molten hydrocarbons, many efforts have been made to get more information on these phenomena. Most studies were concerned with optical and kinetic measurements and with chemical analysis of the species involved in mesophase formation and the effect of feedstock and additives [225]. Although the rheological behaviour is most important for controlled growth and coalescence of mesophase spherules [2,5] only few measurements have been published so far (Table 1). Table 1 compiles the measuring conditions, sample materials and main results of published viscosity measurements with rotational viscosimeters. Didchenko et al. [6] first described results on high-temperature viscosity measurements with a petroleum pitch up to temperatures of 500°C. They found a viscosity increase up to the temperature range of 4OO‘C, followed by an intermediate decrease and a second increase after passing through a minimum at about 420°C. Such typical viscosity/temperature-behaviour of coal tar pitches was confirmed experimentally by Collett and Rand [7] and interpreted as the effect of a sol/gel transformation of the pitch. Also Whittaker and Grindstaff [lo] had measured the plasticity of pitches up to temperatures of 480°C but they only described a steady increase of plasticity without observing an intermediate minimum. Recently published work of Bathia er al. [I 11 describes viscosity results of pitches up to 180°C only. The last mentioned authors discussed non-Newtonian behaviour in the low temperature range, whereas Collett and Rand [7] have shown Newtonian behaviour of pitches up to temperatures of 250°C and nonNewtonian behaviour only at further increased temperatures This was also confirmed by Nazem [S] who showed the non-Newtonian behaviour of several preheat treated pitches at elevated temperatures. As shown schematically in Table 1 the present pub155
lication is concerned with studies of the temperature dependence of viscosity of various pitches (compare [9]). The rotational viscosimeter, especially designed for this purpose is similar to those used by Didchenko et al. [6], Collett and Rand [7] and Nazem [8]. The viscosity of 5 different pitches was measured up to 520°C. In all cases the intermediate maximum; minimum of the viscosity temperature dependence, Newtonian behaviour at temperatures below 30°C and non-Newtonian behaviour at temperatures above was found. This viscosity behaviour will be explained on the basis of the experimental results found for rheology and for mesophase formation. 2. EXPERIME’VTAL
2.1 The viscosimrter The viscosimeter consists of a Contraves Rheomat 30 with a range of 0.055350 rev/min (same type as has been used by Collett and Rand [7]). The Searle type measuring system was developed following DIN 53 788 [12] (Fig. 1). It consists of coaxial cylindric cup and rotor, both made of stainless steel with polished surfaces. The heating is performed by a resistively heated tubular furnace. The rotor is shifted in vertical direction during the measurement in order to assure a complete covering of the rotor by the molten sample material whose volume is decreased during heating. Viscosities from 0.01 up to 570 x IO3 Pa,s can be measured with an accuracy of +50/,. The accuracy of temperature measurement is not better than + IO” at 500°C because the thermocouple was placed outside of the sample. 2.2 Sample materials The sample materials and their characteristic data are compiled in Table 2. Sample A is a commercially available binder pitch. Sample B is an European petroleum pitch, prepared in laboratory scale only. Sample C is a laboratory
156
R. BALDUHNand E. FITZER Table 1. Viscosity measurements of pitches with rotational viscosimeter
Measuring Conditions Mechanical Constant
Didch;nt~;j.a1.[61
Gap Width
VariattRo;t;f
Colleb$47and ( 1 171
yes
yes I,2 1024.103
Shear
BalduhnlFitzerI91 (1979 1
Nazem 181 (19791
yes
yes s-1
1,2.10-2-4.103s-1 1,05 - 4,89 s-1
q2 - 8Os-1
Thermal UpperTemp Heatrng
Limit Rate
Pre-Heattreatment of thePitches
Sample Material Main Results
500°C 40 “C/h
490°C 25 Y/h
25 “C/h
400°C ?
Yes
no
no
yes
1Petrol
Patch
Intermediate
5209:
2 diff. CTPttches (SP83, SPIOI) Maxrmum
and
Mrnlmum
5 diff.Pitches
of Vrscosrty
Temperature Z=fiO)
Dependence NewtonIan Behavrour Tt25O”C Non Newtonran
Behaviour
NewtonlanBehoviour T<300”C Non Newtonian
T >‘25dC
Testing Measurements
with Rotating
Viscosimeter
sample of isomeric polyphenylenes prepared by cationit polymerization of biphenyl and extraction with chlorbenzene [13]. The low softening point is caused by residual solvent only. Sample D is a high sulfur containing bitumen precipitated from a vacuum residue by treatment with specific solvents. Sample E is a low softening coal tar pitch from which the initial QI has been separated. The molecular weight was obtained by gel permeation chromatography and extraction with specific solvents.
Behavrour
T >3OO”C ‘l-f
1 Viscosity
3 diff. Pitches
Rheology
(0)
of Mesophase Formation
performed. These samples were characterized determination of QI and by optical investigations. 3. RESULTS 3.1 Viscosity and ,f!ow characteristic
All experimental results on the temperature &penof apparent viscosity of the 5 pitch grades A-E
dence
2.3 Measuring procedure The viscosity was measured in situ with three shear rates 1.05, 1.94 and 4.89 sect ’ during first heating up to 520°C and plotted as apparent viscosity in the various figures. The heating rate during all viscosity measurements was 25”C/hr. The samples were not preheat-treated before the measurements. Additionally, the flow characteristic (shear rate as function of shear stress) was measured at constant temperatures in 50°C steps from 150 up to 400°C and in 20°C steps from 400°C up to the upper temperature limit. The shear rate was varied between 0.06 and 450secY’. For these experiments the samples were heated up to the special temperature without shear. All samples were subjected to quinoline extraction rheological examination after and microscopic measurement and cooling. For comparison heat treatment of all samples with the same heating rate as described above but without shear by rotation was
by
Fig. 1. The measuring system.
Rheological properties of pitches and bitumina Table 2. Characterization
are compiled in Fig. 2. The three different shear rates are specially indicated. All samples show the typical intermediate maximum/minimum of the apparent viscosity. The position of the maximum/minimum was found to be characteristic for the pitch grade, and not to depend significantly on the shear rate in most cases. Collett and Rand [7] offer as explanation for this maximum/ minimum behaviour a sol/gel transformation. As it will be shown later on, an explanation for this behaviour by the mesophase formation seems more likely. The rheological properties of the 5 pitches at the characteristic temperatures of the observed intermediate maximum/minimum (see Fig. 3a) are shown in Fig. 3(b). Figure 3(b) shows the dependence of the viscosity on the shear rate. The open circles belong to T, the maximum and the full circles to T2 the minimum. At both of these characteristic temperatures one finds a strong decrease of viscosity with increasing shear rate which indicates non-Newtonian behaviour of the pitches. As far as the shear stress at both these temperatures is concerned, it was found to be indepen-
157
of the tested materials
dent of the shear rate as shown in Fig. 3(c). One can conclude that the shear stress at T, and T, is a characteristic number for a special pitch type. Rheological studies were also performed at temperatures below the intermediate maximum for most of the samples. The flow characteristics for samples A and D are shown in Figs. 4 and 5. As can be seen, Newtonian behaviour is confirmed up to 3OOC for the coal tar pitch SP 70 and up to 350°C for the asphaltenes. In all cases the transition of Newtonian to non-Newtonian behaviour was found to be above 3OO’C but below the characteristic temperature of the intermediate maximum. It was concluded from the results of Fig. 3 that the shear stress at both characteristic temperatures is independent on the shear rate. In the case of the asphaltenes e.g. the first characteristic temperature was 430°C and the second 435-C. But one finds for both characteristic temperatures a strong dependence of the shear stress on the shear rate. This shows one of the main problems of viscosity measurements at elevated temperatures. The results represented in Fig. 3 were obtained with pitches which have not been heattreated before the measurements and the temperature increase within the viscosimeter was performed under
t,
Fig. 2. Apparent
viscosity of the different pitches three different shear rates D.
A-E at
Shear Rate ~-
Fig. 3. (a) Schematic viscosity behaviour: (b) dependence of viscosity on shear rate: (c) dependence of shear stress on shear rate.
R. BALDUHNand E. FITZER
158
0
5
x)
io rnP0 rnj
15
25
stress-
Sheor
rY smectbc A
Fig. 4. Flow curves for coal tar pitch SP 70.
so
shear stress. In Figs. 4 and 5 the pitch has been heattreated before the measurement without rotation up to the various temperatures and was measured isothermally with increasing shear rate. If one tries to explain the viscosity/temperaturebehaviour one should be reminded of the explanation given by Collett and Rand [7]. These authors assumed that the characteristic intermediate viscosity minimum is caused by a sol/gel transformation. The sudden change in the viscosity could also be explained with a phase transformation between an isotropic matrix and the mesophase. Such abrupt changes in viscosity are known from the reversible nematic/isotropic transition with an intermediate maximum and minimum as shown in Fig. 6 [14]. Following this assumption, the further studies were directed to the formation of the mesophase within the pitch during high temperature viscosity measurements. 3.2 Injluence of shear stress on the mesophase formation 3.2.1 QI formation with and without shear stress. The QI determination of the sheared and the nonsheared pitch samples was performed. In Fig. 7 the results obtained with the coal tar pitch SP 70 are shown as example. The full lines connecting the open circles indicate the weight fraction of QI formed during the measur-
f
200:
A’
2 5
100
o,
0
./;i’ ,d,/
_./.--
&“+.-f---~_.3:.
Fig. 6. The smectic A-nematic and nematic-isotropic transitions of n-p-cyanobenzylidene-p-octyl-oxyanilene, act. to Ref. [14].
ing procedure in the viscosimeter between 400 and 500°C. The dashed line refers to the viscosity behaviour shown in Fig. 2 with the intermediate maximum and minimum. One sees that at the critical temperatures a mesophase content of about 50 wt% is present. This leads to the conclusion that the viscosity behaviour is related to mesophase formation. Analogous measurements were performed with all other pitch samples and it was found that the mesophase content at the respective critical temperatures is different for various pitch types. For example for the polyphenylenes and asphaltenes QI contents of 10 wt% are found only. But it is known that in both these cases the isotropic matrix consists of long molecule chains in contrary to the small aromatics preferably present in coal tar pitches. Figure 7 shows a second curve in full lines connecting the full circles. This curve indicates the QI content of the pitch if heated without application of shear stress. It can be seen that the mesophase formation starts earlier but also the bulk mesophase is formed earlier, or in other words, the beginning of mesophase growth under shear stresses is delayed. This means
,300’C _~_.
5
“C
AO-
/:-p.
I
110
/’
/+‘A. 7
100
Temperature,
10
-
15
sheor
strerr -
Fig. 5. Flow curves for asphaltenes.
350
-I-
cw
450
‘C
5w
HTT M
mPo103
25
Fig.
7.
Formation of QI and apparent viscosity vs HTT for coal tar pitch SP 70.
Rheological
Fig. 8. Mesophase
properties
formation
of pitches
with and without
that it starts at considerably higher temperatures but then later on with an increased growth rate if compared with that of a non-sheared matrix. The bulk mesophase is obtained at temperatures at least 30°C higher under the applied heating conditions. This corresponds to a delay of 1 hr.
Fig. 9. Bulk mesophase
and bitumina
159
shear for coal tar pitch SP 70.
3.2.2 Opticul incestigutions c$ mesophasr fhwcctiorz with and without shear stwss. In the following micrographs of the various pitches heat-treated with a heating rate of 25”C,‘hr are shown. The temperatures cover the temperature range in which the maxtmum and the minimum of the viscosity was observed. Op-
with and without
shear.
R.
160
Fig.
BALDUHN and E. FITZER
10. Influence
of shear rate on the bulk mesophase.
tical micrographs using polarized light are shown for the coal tar pitch SP 70 as example in Fig. 8. The upper row shows the pitch heated with shear, the lower row shows parallel samples heated without shear. From the amount of anisotropic domains one can see that the mesophase growth is slower if shear is applied. At 470°C when more than 5O”i, of the isotropic matrix is transformed into mesophase even under shear, one sees needle like alignment. Without shear the spherolites coalesce to globular structures as well known from a great number of publications as can be seen [l-3,5]. As shown in Fig. 9, all pitches exhibit the same influence of the shear forces on the structure of the mesophase. As mentioned before the typical temperature for the maximum/minimum and the volume of transformed matrix is different. The micrographs of Fig. 9 show the structure of various bulk mesophases, obtained with and without shear for comparison. All conclusions drawn before are confirmed with the other mesophase precursors. The degree of alignment caused by the shear forces depends on the shear rate. Figure 10 shows the results with shear rates varied between 1.05 and 4.89 set- ‘. In this connection, one should recognize the alignment if high shear rate is applied. We refer to the report by Buechler er al. [15], who have shown that preformed mesophase, that means mesophase already formed by coagulation without shear, can be deformed subsequently by shear to similar structures. The structures shown in Fig. 10 are formed under shear stresses without preformation of globular structures. 4. SUMMARIZING
DISCUSSIONS
4.1 The results described in literature viscosity/temperature-behaviour of pitches intermediate viscosity maximum/minimum temperature range of 420°C were confirmed varying and strongly differing pitches.
on the with an in the with five
4.2 The same viscosity behaviour is found for a mixture of isomeric polyphenylenes and also for high paraffinic bitumen with high sulfur content. Such behaviour seems to be typical for all mesophase forming feedstocks. 4.3 Non-Newtonian behaviour of the pitches starts before the intermediate maximum and minimum, but is dependent on the molecular structure of the pitches. 4.4 From viscosity and shear rate a critical shear stress for causing the intermediate maximum (1) and the minimum (2) of the viscosity can be derived. As shown in Table 3 these data are specific for the type of feedstock. 4.5 The discontinuities in the temperature dependence of the viscosity with the intermediate maximum and minimum can be correlated with the mesophase formation. They show specific dependence on the molecular structure of the pitches. 4.6 The nucleation and coagulation of mesophase is retarded by shear stress but the growth rate is increased. 4.7 Needle like structures are caused by shear stresses during mesophase formation. These structures are very similar to those found by mechanical deformation of spherical mesophase.
Table 3. Critical shear
stresses
and temperatures
Rheological
properties
RI I ERENCES I. J. D. Brooks and G H. Taylor, Carbon 3, 18.5 (1965). 2. H. Marsh, Carbon 8,176, Deutsche Keramische Gesellschaft. Baden-Badetl. 235 (1976). 3. J. L White, Aerospace Rep. 1974, No. TROO74 (425&407)-l. 4. E. Fitzer, K. Miiller and W. Schlfer, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), 237. Marcel Dekker, New York (1971). 5. K. J. Hiittinger, Chemie-lngenieur-Technik 43, 1145 (1971). 6. R. Didchenko, J. B. Barr, S. Chwastiak, I. C. Lewis, R. T. Lewis and L. S. Singer, Extended Abstracts, 12th Biennial Conf Carbon, 329. Pittsburgh, Pennsylvania (1975). 7. G. W. Collett and B. Rand, Extended Abstracts, 13th Biennial Conf Carbon, 27. Irvine, California (1977).
of pitches
and bitumina
161
8. F. Nazem, Extended Abstracts, 14th Biennial Con/: Carbon, 435. Penn State University, Pennsylvania (1979). 9. R. Balduhn and E. Fitzer, Extended abstracts, 14th Biennial Conf: Carbon, 437. Penn State University, Pennsylvania (1979). 10. M. P. Whittaker and L. I. Grindstaff, Carbon IO, I65 (1972). 11. G. Bathia, R. K. Aggarwal, S. S. Chari and G. C. Jain, Carbon 15, 219 (1977). 12. Deutsche Industrie Normen, 53788 (1974). 13. E. Fitzer and F. Grieser, Proc. 5th Londofl Int. Curborl and Graphite ConJ, Vol. I, 266 (1978). 14. E. Kuss, High Temp.-High Press., 9, 575 (1977). 15. M. Buechler, C. B. Ng and J. L. White, Extended Abstracts, 14th Biennial Conf Carbon, 433. Penn State University. Pennsylvania (1979).
m Great
Britain
RHEOLOGICAL BITUMINA
PROPERTIES
UP TO TEMPERATURES R.
Institut
OF PITCHES
fiir Chemische
BALDUHN
Technik. (Receiced
and E.
Universitlt
AND
OF 500°C
FITZER
Karlsruhe,
West Germany
30 July 1979)
Abstract-A
high-temperature rotating viscosimeter was set up for application up to 6OO’C. Temperature dependence of viscosity was measured with various pitches and bitumina. The former finding of other authors on the occurrence of an intermediate maximum and minimum of viscosity is confirmed. It is shown that the shear stresses causing these extreme points are characteristic of precursor type and can be correlated with the temperature of mesophase formation. The structure of mesophase as affected by shear is discussed in detail:
1. INTRODUCTION Since
Brooks and Taylor [l] described their observations on mesophase formation during thermal degradation of molten hydrocarbons, many efforts have been made to get more information on these phenomena. Most studies were concerned with optical and kinetic measurements and with chemical analysis of the species involved in mesophase formation and the effect of feedstock and additives [225]. Although the rheological behaviour is most important for controlled growth and coalescence of mesophase spherules [2,5] only few measurements have been published so far (Table 1). Table 1 compiles the measuring conditions, sample materials and main results of published viscosity measurements with rotational viscosimeters. Didchenko et al. [6] first described results on high-temperature viscosity measurements with a petroleum pitch up to temperatures of 500°C. They found a viscosity increase up to the temperature range of 4OO‘C, followed by an intermediate decrease and a second increase after passing through a minimum at about 420°C. Such typical viscosity/temperature-behaviour of coal tar pitches was confirmed experimentally by Collett and Rand [7] and interpreted as the effect of a sol/gel transformation of the pitch. Also Whittaker and Grindstaff [lo] had measured the plasticity of pitches up to temperatures of 480°C but they only described a steady increase of plasticity without observing an intermediate minimum. Recently published work of Bathia er al. [I 11 describes viscosity results of pitches up to 180°C only. The last mentioned authors discussed non-Newtonian behaviour in the low temperature range, whereas Collett and Rand [7] have shown Newtonian behaviour of pitches up to temperatures of 250°C and nonNewtonian behaviour only at further increased temperatures This was also confirmed by Nazem [S] who showed the non-Newtonian behaviour of several preheat treated pitches at elevated temperatures. As shown schematically in Table 1 the present pub155
lication is concerned with studies of the temperature dependence of viscosity of various pitches (compare [9]). The rotational viscosimeter, especially designed for this purpose is similar to those used by Didchenko et al. [6], Collett and Rand [7] and Nazem [8]. The viscosity of 5 different pitches was measured up to 520°C. In all cases the intermediate maximum; minimum of the viscosity temperature dependence, Newtonian behaviour at temperatures below 30°C and non-Newtonian behaviour at temperatures above was found. This viscosity behaviour will be explained on the basis of the experimental results found for rheology and for mesophase formation. 2. EXPERIME’VTAL
2.1 The viscosimrter The viscosimeter consists of a Contraves Rheomat 30 with a range of 0.055350 rev/min (same type as has been used by Collett and Rand [7]). The Searle type measuring system was developed following DIN 53 788 [12] (Fig. 1). It consists of coaxial cylindric cup and rotor, both made of stainless steel with polished surfaces. The heating is performed by a resistively heated tubular furnace. The rotor is shifted in vertical direction during the measurement in order to assure a complete covering of the rotor by the molten sample material whose volume is decreased during heating. Viscosities from 0.01 up to 570 x IO3 Pa,s can be measured with an accuracy of +50/,. The accuracy of temperature measurement is not better than + IO” at 500°C because the thermocouple was placed outside of the sample. 2.2 Sample materials The sample materials and their characteristic data are compiled in Table 2. Sample A is a commercially available binder pitch. Sample B is an European petroleum pitch, prepared in laboratory scale only. Sample C is a laboratory
156
R. BALDUHNand E. FITZER Table 1. Viscosity measurements of pitches with rotational viscosimeter
Measuring Conditions Mechanical Constant
Didch;nt~;j.a1.[61
Gap Width
VariattRo;t;f
Colleb$47and ( 1 171
yes
yes I,2 1024.103
Shear
BalduhnlFitzerI91 (1979 1
Nazem 181 (19791
yes
yes s-1
1,2.10-2-4.103s-1 1,05 - 4,89 s-1
q2 - 8Os-1
Thermal UpperTemp Heatrng
Limit Rate
Pre-Heattreatment of thePitches
Sample Material Main Results
500°C 40 “C/h
490°C 25 Y/h
25 “C/h
400°C ?
Yes
no
no
yes
1Petrol
Patch
Intermediate
5209:
2 diff. CTPttches (SP83, SPIOI) Maxrmum
and
Mrnlmum
5 diff.Pitches
of Vrscosrty
Temperature Z=fiO)
Dependence NewtonIan Behavrour Tt25O”C Non Newtonran
Behaviour
NewtonlanBehoviour T<300”C Non Newtonian
T >‘25dC
Testing Measurements
with Rotating
Viscosimeter
sample of isomeric polyphenylenes prepared by cationit polymerization of biphenyl and extraction with chlorbenzene [13]. The low softening point is caused by residual solvent only. Sample D is a high sulfur containing bitumen precipitated from a vacuum residue by treatment with specific solvents. Sample E is a low softening coal tar pitch from which the initial QI has been separated. The molecular weight was obtained by gel permeation chromatography and extraction with specific solvents.
Behavrour
T >3OO”C ‘l-f
1 Viscosity
3 diff. Pitches
Rheology
(0)
of Mesophase Formation
performed. These samples were characterized determination of QI and by optical investigations. 3. RESULTS 3.1 Viscosity and ,f!ow characteristic
All experimental results on the temperature &penof apparent viscosity of the 5 pitch grades A-E
dence
2.3 Measuring procedure The viscosity was measured in situ with three shear rates 1.05, 1.94 and 4.89 sect ’ during first heating up to 520°C and plotted as apparent viscosity in the various figures. The heating rate during all viscosity measurements was 25”C/hr. The samples were not preheat-treated before the measurements. Additionally, the flow characteristic (shear rate as function of shear stress) was measured at constant temperatures in 50°C steps from 150 up to 400°C and in 20°C steps from 400°C up to the upper temperature limit. The shear rate was varied between 0.06 and 450secY’. For these experiments the samples were heated up to the special temperature without shear. All samples were subjected to quinoline extraction rheological examination after and microscopic measurement and cooling. For comparison heat treatment of all samples with the same heating rate as described above but without shear by rotation was
by
Fig. 1. The measuring system.
Rheological properties of pitches and bitumina Table 2. Characterization
are compiled in Fig. 2. The three different shear rates are specially indicated. All samples show the typical intermediate maximum/minimum of the apparent viscosity. The position of the maximum/minimum was found to be characteristic for the pitch grade, and not to depend significantly on the shear rate in most cases. Collett and Rand [7] offer as explanation for this maximum/ minimum behaviour a sol/gel transformation. As it will be shown later on, an explanation for this behaviour by the mesophase formation seems more likely. The rheological properties of the 5 pitches at the characteristic temperatures of the observed intermediate maximum/minimum (see Fig. 3a) are shown in Fig. 3(b). Figure 3(b) shows the dependence of the viscosity on the shear rate. The open circles belong to T, the maximum and the full circles to T2 the minimum. At both of these characteristic temperatures one finds a strong decrease of viscosity with increasing shear rate which indicates non-Newtonian behaviour of the pitches. As far as the shear stress at both these temperatures is concerned, it was found to be indepen-
157
of the tested materials
dent of the shear rate as shown in Fig. 3(c). One can conclude that the shear stress at T, and T, is a characteristic number for a special pitch type. Rheological studies were also performed at temperatures below the intermediate maximum for most of the samples. The flow characteristics for samples A and D are shown in Figs. 4 and 5. As can be seen, Newtonian behaviour is confirmed up to 3OOC for the coal tar pitch SP 70 and up to 350°C for the asphaltenes. In all cases the transition of Newtonian to non-Newtonian behaviour was found to be above 3OO’C but below the characteristic temperature of the intermediate maximum. It was concluded from the results of Fig. 3 that the shear stress at both characteristic temperatures is independent on the shear rate. In the case of the asphaltenes e.g. the first characteristic temperature was 430°C and the second 435-C. But one finds for both characteristic temperatures a strong dependence of the shear stress on the shear rate. This shows one of the main problems of viscosity measurements at elevated temperatures. The results represented in Fig. 3 were obtained with pitches which have not been heattreated before the measurements and the temperature increase within the viscosimeter was performed under
t,
Fig. 2. Apparent
viscosity of the different pitches three different shear rates D.
A-E at
Shear Rate ~-
Fig. 3. (a) Schematic viscosity behaviour: (b) dependence of viscosity on shear rate: (c) dependence of shear stress on shear rate.
R. BALDUHNand E. FITZER
158
0
5
x)
io rnP0 rnj
15
25
stress-
Sheor
rY smectbc A
Fig. 4. Flow curves for coal tar pitch SP 70.
so
shear stress. In Figs. 4 and 5 the pitch has been heattreated before the measurement without rotation up to the various temperatures and was measured isothermally with increasing shear rate. If one tries to explain the viscosity/temperaturebehaviour one should be reminded of the explanation given by Collett and Rand [7]. These authors assumed that the characteristic intermediate viscosity minimum is caused by a sol/gel transformation. The sudden change in the viscosity could also be explained with a phase transformation between an isotropic matrix and the mesophase. Such abrupt changes in viscosity are known from the reversible nematic/isotropic transition with an intermediate maximum and minimum as shown in Fig. 6 [14]. Following this assumption, the further studies were directed to the formation of the mesophase within the pitch during high temperature viscosity measurements. 3.2 Injluence of shear stress on the mesophase formation 3.2.1 QI formation with and without shear stress. The QI determination of the sheared and the nonsheared pitch samples was performed. In Fig. 7 the results obtained with the coal tar pitch SP 70 are shown as example. The full lines connecting the open circles indicate the weight fraction of QI formed during the measur-
f
200:
A’
2 5
100
o,
0
./;i’ ,d,/
_./.--
&“+.-f---~_.3:.
Fig. 6. The smectic A-nematic and nematic-isotropic transitions of n-p-cyanobenzylidene-p-octyl-oxyanilene, act. to Ref. [14].
ing procedure in the viscosimeter between 400 and 500°C. The dashed line refers to the viscosity behaviour shown in Fig. 2 with the intermediate maximum and minimum. One sees that at the critical temperatures a mesophase content of about 50 wt% is present. This leads to the conclusion that the viscosity behaviour is related to mesophase formation. Analogous measurements were performed with all other pitch samples and it was found that the mesophase content at the respective critical temperatures is different for various pitch types. For example for the polyphenylenes and asphaltenes QI contents of 10 wt% are found only. But it is known that in both these cases the isotropic matrix consists of long molecule chains in contrary to the small aromatics preferably present in coal tar pitches. Figure 7 shows a second curve in full lines connecting the full circles. This curve indicates the QI content of the pitch if heated without application of shear stress. It can be seen that the mesophase formation starts earlier but also the bulk mesophase is formed earlier, or in other words, the beginning of mesophase growth under shear stresses is delayed. This means
,300’C _~_.
5
“C
AO-
/:-p.
I
110
/’
/+‘A. 7
100
Temperature,
10
-
15
sheor
strerr -
Fig. 5. Flow curves for asphaltenes.
350
-I-
cw
450
‘C
5w
HTT M
mPo103
25
Fig.
7.
Formation of QI and apparent viscosity vs HTT for coal tar pitch SP 70.
Rheological
Fig. 8. Mesophase
properties
formation
of pitches
with and without
that it starts at considerably higher temperatures but then later on with an increased growth rate if compared with that of a non-sheared matrix. The bulk mesophase is obtained at temperatures at least 30°C higher under the applied heating conditions. This corresponds to a delay of 1 hr.
Fig. 9. Bulk mesophase
and bitumina
159
shear for coal tar pitch SP 70.
3.2.2 Opticul incestigutions c$ mesophasr fhwcctiorz with and without shear stwss. In the following micrographs of the various pitches heat-treated with a heating rate of 25”C,‘hr are shown. The temperatures cover the temperature range in which the maxtmum and the minimum of the viscosity was observed. Op-
with and without
shear.
R.
160
Fig.
BALDUHN and E. FITZER
10. Influence
of shear rate on the bulk mesophase.
tical micrographs using polarized light are shown for the coal tar pitch SP 70 as example in Fig. 8. The upper row shows the pitch heated with shear, the lower row shows parallel samples heated without shear. From the amount of anisotropic domains one can see that the mesophase growth is slower if shear is applied. At 470°C when more than 5O”i, of the isotropic matrix is transformed into mesophase even under shear, one sees needle like alignment. Without shear the spherolites coalesce to globular structures as well known from a great number of publications as can be seen [l-3,5]. As shown in Fig. 9, all pitches exhibit the same influence of the shear forces on the structure of the mesophase. As mentioned before the typical temperature for the maximum/minimum and the volume of transformed matrix is different. The micrographs of Fig. 9 show the structure of various bulk mesophases, obtained with and without shear for comparison. All conclusions drawn before are confirmed with the other mesophase precursors. The degree of alignment caused by the shear forces depends on the shear rate. Figure 10 shows the results with shear rates varied between 1.05 and 4.89 set- ‘. In this connection, one should recognize the alignment if high shear rate is applied. We refer to the report by Buechler er al. [15], who have shown that preformed mesophase, that means mesophase already formed by coagulation without shear, can be deformed subsequently by shear to similar structures. The structures shown in Fig. 10 are formed under shear stresses without preformation of globular structures. 4. SUMMARIZING
DISCUSSIONS
4.1 The results described in literature viscosity/temperature-behaviour of pitches intermediate viscosity maximum/minimum temperature range of 420°C were confirmed varying and strongly differing pitches.
on the with an in the with five
4.2 The same viscosity behaviour is found for a mixture of isomeric polyphenylenes and also for high paraffinic bitumen with high sulfur content. Such behaviour seems to be typical for all mesophase forming feedstocks. 4.3 Non-Newtonian behaviour of the pitches starts before the intermediate maximum and minimum, but is dependent on the molecular structure of the pitches. 4.4 From viscosity and shear rate a critical shear stress for causing the intermediate maximum (1) and the minimum (2) of the viscosity can be derived. As shown in Table 3 these data are specific for the type of feedstock. 4.5 The discontinuities in the temperature dependence of the viscosity with the intermediate maximum and minimum can be correlated with the mesophase formation. They show specific dependence on the molecular structure of the pitches. 4.6 The nucleation and coagulation of mesophase is retarded by shear stress but the growth rate is increased. 4.7 Needle like structures are caused by shear stresses during mesophase formation. These structures are very similar to those found by mechanical deformation of spherical mesophase.
Table 3. Critical shear
stresses
and temperatures
Rheological
properties
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of pitches
and bitumina
161
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