T H E COLLOIDAL NATURE O F ASPHALT AS SHOWN BY ITS FLOW PROPERTIES1
R. N. TRAXLER AND C . E. COOMBS The Barber Asphalt Company, Maurer, New Jersey Received June 11, 1936
Although our knowledge of the physic&hemical state of asphalt is quite incomplete, the notion of the colloidal nature of bituminous materials has been advanced by various authors. Richardson (21) did considerable pioneer work at a time when the modern conception of the colloidal state was just beginning to take form. Recently Nellensteyn (16, 17) and his coworkers have described asphalts as protected lyophobe sols in which the micelles, forming the dispersed phase, are composed of high molecular weight hydrocarbons of high carbon content, presumably enveloping nuclei of free carbon. It is generally assumed that the dispersed phase is composed of those substances, called asphaltenes, which are insoluble in 86' Baume naphtha. The continuous or dispersing phase contains low molecular weight hydrocarbons which are soluble in 86' Baumt? naphtha and are called petrolenes. Mack (14) considered asphalts as sols of asphaltenes in a mixture of asphaltic resins and oily constituents (petrolenes), and investigated the viscosity of various combinations of these phases. From his data he drew conclusions concerning the causes for the high viscosities shown by asphaltic materials. Although there is some information in the literature regarding the viscositg of asphalt, there has been little work done in the directioh of determining the presence and degree of the abnormal flow properties such as thixotropy, quasi-viscousness, and elastic effects which frequently prevail in, and are characteristic of, colloidal systems. On the basis of limited data some workers have asserted that asphaltic bitumen is truly viscous, while others have stated that it exhibits non-Newtonian flow characteristics. It is the purpose of the present paper to discuss the flow properties of asphalts of widely different source and type of processing, and to show how a knowledge of these properties substantiates the theory that asphalts are colloidal. The authors intend to show that the existence and magnitude of the anomalous flow characteristics depend not only on the type of bitumen, but also on the temperature a t which the measurements are made, 1 Presented at the Thirteenth Colloid Symposium, held a t St. Louis, Missouri, June 11-13,1936. 1133
1134
R . hT. TRASLER AKD
C.
E. COONBS
and on the method and degree of processing to n-hich the asphalt haq been subjected. VISCOUS ASPHALTS
In our laboratory a number of asphalts have been studied in order to ascertain the effect of temperature and shearing stress on the experinientally determined viscosities. Measurements have been made from 15" to 130°C. by means of the modified Bingham-Murray plastometer (6, 19), the Bingham-Stephens alternating-stress method ( 7 ) , the falling coaxial cylinder yiscometer (20), and the conicylindrical rotation viscometer (15). All of these instruments as used in our laboratory have been described and discussed elsewhere (28). To complete the study of temperature effects, viscosities a t temperatures as high as 190°C. have been measured by means of a Saybolt-Furol viscometer and the values in Furol seconds converted to poises (12). TABLE 1 Identification of asphalts DESIGNATION
1
SOURCE (TYPE)
1
1
PENETRATION
PROCESS ~
,o;t'G?i",C.
I
Californian j Mexican I Mexican Refined Trinidad native 1 lake asphalt Venezuelan Venezuelan Venezuelan Venezuelan I Venezuelan ' ~
~
I
~
--
Vacuum Vacuum Batch steam Batch steam Batch steam Air-blown rlir-blown Air-blown
00
55 62 4
1
RING A X D B h L L IOFTENI N G POINT
"C.
47 2 52 2
52.2 96 7
53
53 3
41 65
68 9
94
132
61.9
57.2 51.1
The viscosities of many steam- and vacuum-refined asphalts of paving consistency prepared from Californian, 1-enezuelan, Nid-Continent, and Mexican petroleums showed no dependence on the shearing stress employed over the large range of stresses studied at low (atmospheric) temperatures. However, n ith certain types of these asphalts deviations from viscous flow appear in the region of 40 penetration ( 3 ) a t 25"C., 100 g., 5 sec., and are more exaggerated as the asphalt becomes harder. Tacuumrefined Californian asphalt exhibited Newtonian flow a t atmospheric temperature even when it had been processed t o a4 low as 35 penetration a t 25"C., 100 g., 5 sec. (viscosity = 25 X lo6 poises a t 25OC.). Table 1 serres to identify the various asphalts discussed in this paper with regard to source, method of processing, penetration, and Ring and Ball softening points.
1135
COLLOIDAL NATURE OF ASPHALT
In table 2 data on asphalts A and B, both of 55 penetration at 25OC., 100 g., 5 see., are presented as typical examples of asphalts exhibiting truly viscous flow properties at both high and relatively low (atmospheric) temperatures. The viscosities at the higher temperatures were measured in the Bingham-Murray apparatus, while the Bingham-Stephens alternating-stress method was used for the determination of the viscosities at low temperatures. The reproducibility of the above viscosity values is within &5 per cent. Within this limit the data in table 2 are typical of numerous asphalts whose viscosities are independent of the shearing stress employed. TABLE 2 Data showing Newtonian flow for asphalts A and B SHEARING STRESS
I
VISCOSITY
I/
I/
Asphalt A at 100°C.
SEEARINQ STRESS
1
VIBCOSITY
Asphalt B a t 110°C.
dynes per Cm.l
poises
dynes per cm.2
poises
367.8 306,7 263.0 233.8 211.0 194.8 181 .O
42.6 41.6 42.0 42.0 41.6 41.5 41.8
426.9 371.0 332.3 300.9 274.9 253.1 234.5 218.4
56.9 57.0 56.9 57.4 56.5 57.2 56.7 57.2
11
Asphalt A at 20°C. 149,800 135,700 123,600 110,000 93,500 80,300
11.5 x 11.6 X 11.9 x 11.7 X 11.8 X 11.8 X
106 IO6 108 lo6 106
lo6
Asphalt B at 30°C. 17,000 15,200 13,300 11,500 9,900 8,400
2.16 2.29 2.35 2.40 2.38 2.34
X lo6
X X X X X
lo6 lo6 lo6 106 lo8
EFFECT OF TEMPERATURE ON VISCOUS ASPHALTS
It is generally assumed that asphalt is a system in which the solubility of the dispersed phase (asphaltenes) in the oily continuous phase (petrolenes) is influenced by temperature. At high temperatures the solubility is increased, except for free carbon if it is present. As the temperature is progressively lowered the two phases become more distinct and the colloidal properties of the system become more exaggerated. However, even at low temperatures the system usually is stable, the asphaltenes remaining dispersed owing to the protective action of the asphaltic resins. Various investigators are divided in their opinions as to whether this system has
1136
R. N. TRAXLER AND C. E. COOMBS
the form of a suspensoid or of an emulsoid. The opinion has been expressed that the temperature-viscosity curves for asphaltic bitumen should have two marked points of transition, one where the system passes from the liquid to the semiliquid state, and the other where a transition occurs from the semiliquid to the solid state. From the rheological standpoint, this latter statement is ill-defined for, as far as flow properties are concerned, a transition from the true liquid state to anything approximating the solid state means a change from purely viscous properties to definitely plastic behavior, involving yield values and mobilities rather than viscosities. From the numerous data collected in our laboratory no asphalt has been found to possess plastic properties as defined above. Nellensteyn and Roodenburg (18) have shown points of inflection in the relation between surface tension and temperature, while Klinkmann (13) and Spiers ( 2 6 ) have shown Such transition points in their temperature-
FIG.1. Variation of viscosity of asphalt C \\ith temperature
viscosity relationships for bituinen and tar. On the other hand, there is an abundance of data which shows that there is a distinct continuity throughout the ikcosity-temperature curyes. Evans and Pickard (9), Eymann (lo), Pochettino (20), and Rodiger ( 2 2 ) have all given data supporting the latter statement. Saal (24) has recently shoxn that there is no break in the relation between temperature and surface tension for asphaltic bitumen. In our laboratory the viscosities of fourteen asphalts of widely different origins, processed differently and to various degrees, have been measured froin 15" to 190°C. Some of these data have been reported elsewhere (19, 2 5 ) . The viscosities from 15" to 35°C. were measured by the alternating-stress method (7), from 40" to 130°C. by the modified BinghamMurray apparatus (19), and above 130°C. with a Saybolt-Furol viscometer (2). The Furol seconds were converted to absolute C.G.S. units
COLLOIDAL NATURE OF ASPHALT
1137
(poises) (12). The type of data obtained is illustrated by figure 1, which shows the logarithm of the viscosity for asphalt C plotted against the temperature in degrees Centigrade. In none of the fourteen asphalts is there any indication of a sudden break in the viscosity-temperature relationship. The fact that no sudden changes occur in the log viscosity versus temperature (“C.) relation does not discount the notion that as the temperature is lowered, the asphaltenes gradually separate from true solution with the petrolenes to give rise to a stable system whose colloidal properties become more pronounced as the temperature decreases. The facts indicate that the transition from a condition approximating true solution to a distinctly colloidal state is so gradual that no sudden change in physical properties occurs. From a plot such as that shown in figure 1 it is evident that the so-called “softening point”, which is of value to bituminous technologists, does not represent a temperature at which the viscosity changes sharply. It has already been pointed out (1) that the Ring and Ball softening point (4) is merely the temperature at which any asphalt attains a particular viscosity (approximately 12,000 poises). Saal (23) states that the Ring and Ball softening point may correspond to a viscosity value somewhere between 10,000 and 20,000 poises. At atmospheric temperatures (15” to 40”C.), a range to which paving asphalts are subjected in use, a plot of the logarithm of viscosity against temperature in degrees Centigrade is a straight line within the limits of experimental error. Therefore, for this range, viscosity and temperature may be related by the expression log Q = mt
+b
(1)
where q = viscosity in poises, t = temperature (“C.), and m and b are constants. For the higher temperatures where the viscosities vary from 1 to 5 poises, a consistency range in which asphalts are usually processed, the same type of equation applies. An asphalt viscosity index, which evaluates the susceptibility of asphalts to temperature changes in terms of percentage decrease of viscosity (in poises) for a 1°C. rise in temperature, has been derived (27), using the slope m of equation 1. This index is expressed as A.V.I. = 100 (10“
- 1)
Experimentally, it is only necessary to measure the viscosities qo and o b at two temperatures t, and t b (“C.) within the range where equation 1 is valid. Then,
A.V.I.
= 100
[e)
l’(L
-
tb)
- lI*
* In actual practice the A.V.I. is most readily calculated from the expression 100 (antilogarithm of the slope “m”
- 1).
‘(3)
1138
R. N. TRAXLER AND C . E. COOMBS
Since the viscosity of asphalt diminishes with increasing temperature the values of the il.V.1. are negative, but from the definition given above the sign may be ignored. For asphalt C, shown in figure 1, the A.V.I. changes from 20.5 at the low service temperatures to 4.4at the high processing temperatures. Thus, a great decrease in both viscosity and susceptibility to temperature change occurs with a large rise in temperature, THIXOTROPY I N ASPHALTS
Numerous cases have been found where an initial working of the asphalt sample at a high stress was required before a constant viscoaity value was TABLE 3 Viscosity data showing presence of thixotropy in asphalt D ut 60°C. DIRECTION OF NOVEMENT*
In out In out In out In out In
out In out In out In out In
out In
SREARING STRESS
VISCOSITY
dynes per cm.2
poises X 10-6
145,000 144,000 144,000 144,000 144,000 143,000 143,000 143,000 143,000 143,000 143 000 124,000 124,000 113,000 113,000 92,800 92,800 82,300 82,300 ~
23.6 21.0 18.9 18.2 17.4 17.1 16.8 16.0 16.0 16.7 15.9 16.4 15.6 16.7 15.7 16.5 15.9 16.3 16.0
* The first “out” movement is never recorded because of difficulties in focusing, etc. obtained. After a variable amount of structure, depending on the nature of the asphalt, was removed from the sample by the application of mechanical shear, the viscosity values for all stresses less than that initially used were constant. This phenomenon of thixotropy or breakdown of structure under the influence of mechanical working has becn observed in many colloidal systems, especially when the concentration of the dispersed phase is appreciable. A typical case of thixotropy is found in the viscosity data for asphalt D given in table 3. The measurements on this asphalt were made using the alternating-stress method.
1139
COLLOIDAL NATURE O F ASPHALT
It will be noted that after the initial “working” period, the viscosity of this asphalt decreases to a value which remains essentially constant over a range of decreasing shearing stresses. The viscosity value corresponding to the ‘‘in” movement, for a given shearing stress, is always lower than that corresponding to the “out” movement. This behavior probably is caused by the elasticity of the material. AGE-HARDENING OF ASPHALTS
An interesting and important manifestation of thixotropy is the phenomenon of age- or time-hardening of asphalts. Asphalt technologists know that if the penetration of a sample of asphalt is obtained immediately after cooling and again on the same undisturbed sample a t intervals of several days, the asphalt will be found to have become harder with the passage of time. Remelting the sample causes the asphalt to return to its original consistency. TABLE 4 Increase in viscosity of asphalt E with time TIME
1
YIBCOSITY
hours
poises X 10-0
4 25 52 77 148
7.54 7.80 8.21 8.56 8.89
* Remelted
I/
TIME
hours
317 820 2232 4455 9100 5*
1
VISCOSITY
poises
x
10-0
9.38 11 .o 12.1 12.7 14.2 8.3
sample.
Measurement of viscosity in absolute units offered a sensitive method of evaluating this age-hardening phenomenon (29). The falling coaxial cylinder viscometer (20, 28) was particularly well adapted to such an investigation, because several instruments could be filled simultaneously with the same asphalt and stored a t a constant temperature, and determinations of the viscosity could be made a t any desired intervals of time. Ten or twelve instruments were usually filled with a particular asphalt. The viscometers and contents were stored in a cabinet maintained a t 25’ &0.5”C. At increasing intervals of time samples were removed from the cabinet and the viscosities measured a t 25”C., using a rather low shearing stress (7200 dynes per cm.2). After each measurement was completed the sample was discarded. Table 4 gives the data obtained using asphalt E. The first value obtained (at the end of four hours) is unaccountably high, but the remaining nine values fall very close to a straight line when log viscosity is plotted against log time. The sample which had aged for 9100 hours was remelted in the viscometer, cooled, and the viscosity de-
1140
R. N . TRAXLER AND C. E. COOMBS
termined at the end of five hours, with the result shown at the bottom of table 4. It is evident that after aging for over one year a large percentage of the structure which had developed could be destroyed by heat. The phenomenon of age-hardening is apparently thixotropic in itsqnature, because by the application of a high shearing stress in a rotatingcylinder viscometer the viscosity of an aged asphalt can be reduced just as is done by heating. TABLE 5 “Equilibrium viscosities” of quasi-viscous asphalt F at 35°C. APPROXIMATE TIME OF WORKINQ
MEAN SHEARINQ STRESS
minutes
dynes per cm.2
35 30
11,600 34,800 69,600 138,700
GO 100
“EQUILIBRIUX MSCOSITY”
RATE OF SHEAR
reciprocal seconds X
IO4
5.72 17.6 40.6 114.
poises X 10-6
20.3 19.9 17.3
12.2
FIG.2. Non-Newtonian flow of asphalt F at 25°C. NON-NEWTONIAN FLOW
When a system possesses sufficient internal structure to cause the appearance of marked thixotropic effects, other anomalous flow characteristics often become apparent. At a particular shearing stress such a system must be worked in a given direction until a constant rate of movement is obtained. From this an “equilibrium viscosity” may be calculated.
1141
COLLOIDAL NATURE OF ASPHALT
However, the “equilibrium viscosity” decreases with increasing shearing stress, and a plot of the rate of shear versus shearing stress yields a curvilinear relationship passing through the origin. This phenomenon has been called non-Newtonian flow, and the materials are said to be quasi-viscous.
FIG.3. Elastic effects in asphalt F a t 25°C. Mean shearing stress = 34,800 dynes per square centimeter.
I
FIG.4. Elastic and thixotropic effects in asphalt F a t 25°C. Mean shearing stress = 69,600 dynes per square centimeter.
This type of flow indicates that the system possesses some rigidity, but the material cansot be called a plastic solid in the sense in which Bingham (5) uses the term. A “yield value” would have little physical significance since it would depend completely on the shearing stress a t which it would
1142
R. N. TRAXLER APiD C. E . COOMBS
be determined. Further, the “yield value” would be zero when the rate of shear is zero. Table 5 and figure 2 show data for asphalt F, a steam-refined asphalt of paving consistency, which exhibits a marked quasi-viscous nature. The measurements were made in the conicylindrjcal rotatioil viscometer. (15, 28) a t 25’C. Only the final “equilibrium viscosities” are recorded.
learing stress
FIG.5. = 138,700 dynes per square centimeter. ELllSTIC EFFECTS
Figures 3, 4, and 5 give some additional inforniation concerning the flow properties of asphalt F. The rate of movement of the inner cylinder is plotted against time in minutes for the last (radians per second X three shearing stresses given in table 6. If the material mere purely viscous, the rate of movement would be independent of the period of working. However, it is observed (figure 3) that at the low stress (34,800 dynes per crn.2)ithe rate of movement of the inlier cylinder is initially high and with
COLLOIDAL NATURE OF ASPHALT
1143
time decreases asymptotically to a constant value. The same behavior was noted for a shearing stress of 11,600 dynes per cm.2 Such a phenomenon has been recognized in other materials as an elastic fore-effect by Braunbek (8) and by Ferry and Parks ( l l ) ,et ai. The initial elastic displacement is gradually obscured by the viscous deformation, and the rate of movement eventually becomes constant. The “equilibrium viscosity” given in table 5 is calculated from this constant rate. At point C in figure 3 the stress is suddenly removed from the sample and an elastic return is effected, the inner cylinder of the viscometer rotating in the reverse direction of its own accord. The rate of reverse movement is shown by curves DE and EF. At a higher stress (69,600 dynes per cm.2)the same elastic fore- and aftereffects are noted (figure 4). However, this stress is high enough so that the initial elastic fore-effect is partially masked by the breaking-down of structure within the sample. As the internal structure is destroyed by the mechanical shearing the rate of movement of the inner cylinder increases and finally becomes essentially constant. This constant rate is again used to calculate the “equilibrium viscosity’’ recorded in table 5. When the high stress (138,700 dynes per cm.2)is used (figure 5), the initial elastic effect is entirely obscured by the viscous deformation and breakdown of structure within the asphalt. However, it is evident that the system still possesses some structure, even after being subjected to a high stress for 100 minutes, because an elastic return is noted when the shearing stress is removed from the sample. DEPENDENCE OF ANOMALOUS FLOW PROPERTIES ON THE DEGREE OF PROCESSING
It is known that as the processing of an asphalt continues the material becomes harder, resulting in a lower penetration and an increased percentage of asphaltenes (dispersed phase). In conjunction with this fact we have found that in air-blown asphalts of the type represented by asphalts G (65 penetration), H (94 penetration), and I (132 penetration), the harder the asphalt, the greater the deviation from Newtonian Aow. Comparisons of these asphalts were made at approximately equal ranges of rate of shear, for it seems reasonable that the magnitude of thixotropic and quasi-viscous effects should depend on the rate at which the samples are sheared. If, on the other hand, we had compared these asphalts of widely different “viscosities” a t a given range of shearing stresses, the low viscosity asphalt (132 penetration) would have been subjected to much higher rates of shear than the asphalt of highest viscosity (65 penetration). Thixotropic and quasi-viscous effects in the higher viscosity asphalts, as evidenced from a comparison of the final “equilibrium viscosities” of these three different asphalts, would therefore have been underestimated.
1144
R. N. TRAXLER AKD C. E. COOMBS
For a complete study of elastic effects, however, all samples should be run a t exactly the same shearing stresses. Since we are interested primarily in the thixotropic and quasi-viscous characteristics of these materials in the present work, we have used approximately equal ranges of rates of shear. In all of the determinations low shearing stresses were employed so as to insure the absence of slippage of the samples a t the walls of the viscometer. In table 6 are given the ‘[equilibrium viscosities” of the air-blown asTABLE 6 “Equilibrium viscosities” of air-blown asphalts G. H. and I at 25°C. ___ ASPHhlT STRESS
dunes
TIME FOR ATTAINMENT OF EQUILIBR!UM “q“
per cm.3
minutes
minutes
21,900 43,800 76,800 76,800
1,260 805 10 545
1,060 2,150 4,340 8,730 32,900 [ 32,900
G (65 penetration)
I
H (94 penetration).
TIME OF NORKING
I
RATE OF SHEAR
EQUILIB; RIUM “ n
BREAKDOWN OF STRUCTURE B Y CONTINUED
WORKING
reCipr0CUl seconds
pmes
x 104
x 10-
100 615
1.16 5.21 13.9* 30.0*
88 84 0 55 2* 25 7*
None Slight Considerable* Considerable*
400 770 445 690 10 645
50-400t 40-700t 35425t 30-630t
0.434 0.870 1.74 3.77 20.3* 38.6*
24 4 24 7 249 23 1 16 2* 8 52*
None None None None Considerable* Considerable*
260 790
50-240t 50-740t
0.231 1 .oo 27.8* 35,2*
4 58 434 3 92* 3 09*
10
440
* *
* *
*
*
None None Some* Some*
* An equilibrium velocity was not attained during the time allotted to the experiment; the viscosity continued to decrease on further working. t The time could not be ascertained more accurately because the equilibrium state was attained in the sample during the absence of the observer. phalts G, H, and I, as measured in the conicylindrical rotation viscometer a t 25’C. At low shearing stresses the samples were worked for long periods of time to make certain that “equilibrium viscosities” had really been attained. It is evident from table 6 that a t the lowest stresses none of these asphalts shows thixotropic behavior in this instrument, although they are quasiviscous (Le., the “equilibrium viscosity” is dependent on the shearing stress employed). At the highest stress employed in each case, however,
1145
COLLOIDAL NATURE OF ASPHALT
the breakdown of structure is very evident, since the “viscosities” diminish with time at constant shearing stress. These experiments substantiate the results shown in figures 3 , 4 , and 5 for asphalt F in indicating that there is a critical shearing stress at which the structure of the asphalt #ample is broken down to some extent in this instrument with continued working in a given direction. At all stresses abov,e this threshold value thixotropy is manifested. TABLE 7 “Equilibrium viscosities” of air-blown asphalt G (66 penetration) TEMPERA-
TURE
‘C.
25.0
35.0
45.0
55.0
MEAN BEEARINQ BTRE88
dune8 p e r Cflt.2
TIME OF WORKINQ
‘PIME FOR LTTNNMENT OF EOUILIBRlUM “7”
minufes
minufea
21,900 43,800 76,800 76,800
1,260 805 10 545
100 615
1,060 6,530 10,900 32,900 32,900
360 170 970 10 420
300 70 100-820t
511 1,170 3,240 8,730 8,730
840 540 260 10 235
20 320 30
504 1,050 2,490
240 130 130
*
*
RATE OF BBEAR
reciwooal
leconds
x 104
1.16 5.21 13.9* 30.0*
EQUILIBR I U Y ‘‘1’’
BREAKDOWN OF BTRUCTURE BY CONTINUED WORKINQ
&ea X 10-6
188 84 0 55 2* 25 7*
None Slight Considerable* Considerable*
0.434 2.61 4.19 19.7* 51.4*
24 4 25 0 26 0 16 7* 6 40*
*
1.00 2.60 6.65 22, o* 29.5*
5 07 4 50 4 87 3 96* 2 95*
None None None Some* Some*
90 20 30
6.96 15.0 32.5
0 724 0 696 0.766
None None None
*
*
*
None None Slight Considerable’ Considerable*
* An equilibrium velocity was not attained during the time allotted to the experiment; the viscosity continued to decrease on further working. t The time could not be ascertained more accurately because the equilibrium state was attained in the sample during the absence of the observer. If the “equilibrium viscosities” of asphalts G , H, and I are compared at equal ranges of rate of shear, it is noticed that the harder the asphalt (i.e., the more it has been processed), the greater is the deviation from Newtonian flow. This is true even if we,omit from this comparison the “equilibrium viscosity” values at the highest stresses, where breakdown of structure causes marked decreases in the “viscosities.” The anomalous flow properties of the blown asphalts G, H, and I were similar to those of the batch steam-refined asphalt F, but more pronounced.
I146
R. N. TRAXLER AND C. E. COOMBS
KOmeans has been found for expressing the “degree of deviation” from Newtonian flow as a function of the amount of dispersed phase. In the first place, there is no niethod of exprepsing the amount of divergence from purely viscous flow. Secondly, the pcrcentage of asphaltenes is not an absolute inpasure of the colloidality of the asphalt. Although ahphaltenes are defined by a soluhility test, they m r y appreciably in molecular weight and probably in composition for different types of asphalt. Also, the continuous phase (petrolenes) may differ greatly in various asphalts. Thr natures as well as the amounts of the continuous and dispersed phases are undoubtedly responsible for the colloidal characteristics of asphalts. INFLUEXCE O F TEMPERATURE OK ANOMALOUS FLOW PROPERTIES
As the temperature increases, the high molecular weight constituents of asphalt (asphaltenes) become more soluble in the continuous oily phase (petrolenes), and a gradual transition occurs in the nature of the flow exhibited by the system. With increasing temperature, the elastic and thixotropic properties of the asphalt system, as well as its quasi-viscous character, are less pronounced; a t a high enough temperature the system exhibits truly viscous flow. Table 7 presents the “viqcosity” data of the air-blown asphalt G at various temperatures. At 55°C. this sample exhibited viscous flow within the range of shear rates used in these experiments. As the temperature was lowered, the anomalous flow properties became more exaggerated. Determinations mere made a t approximately equal ranges of rate of shear for the reason stated in the previous section. CONCLUSION
The flow characteristics of asphalts are frequently complex, indicating that their physical structure and chemical composition are exceedingly involved. Some asphalts appear to be truly viscous, whereas others exhibit varying degreea of anomalous flow (thixotropy, age-hardening, quasi-viscousness, and elasticity). The magnitude of these characteristics depends largely on the source of the asphalt, the degree of processing, and the temperatures and rates of shear a t which the measurements of flow are made. It is generally believed that the source, processing, and temperature of test influence the nature and aniount of dispersed material present (i.e., the colloidality of the asphalt). Of course, the rate of shear haq a profound influence on the physical structure of the system a t the time of measurement. The presence of flow properties which are generally characteristic of colloidal systems lends strong support to the concept that asphalts are distinctly colloidal. I n the case of truly viscous asphalts, the change of viscosity with decreasing temperature is continuous and there is no evidence of a sudden change from a sol to a gel condition, or from a semisolid to a solid state, as some worl-erq have erroneously conchided from insufficient data.
COLLOIDAL NATURE O F ASPHALT
1147
The gradual formation of an internal secondary structure unstable to heat and mechanical working has been noted in all types of asphalt, whether viscous or not. This occurrence of age-hardening is another c,onclusive proof that asphalts are colloidal in nature. The authors are indebted to Mr. H. E. Schweyer for suggestions and the drawings. REFERENCES (1) Academy of Sciences a t Amsterdam. Uitgave van de N.V. Noord Hollandsche Uitgevers-Maatschappij, Amsterdam (1935). “First Report on Viscosity and Plasticity prepared by the Committee for the Study of Viscosity.” (2) A.S.T.M. Designation D8&33. Am. Soc. Testing Materials Standards Part 11, 880 (1933). (3) A.S.T.M. Designation D5-25. Am. Soc. Testing Materials Standards Part 11, 971 (1933). (4) A.S.T.M. Designation D36-26. Am. Soc. Testing Materials Standards Part 11, 984 (1933). (5) BINGHAM, E . C.: Fluidity and Plasticity. McGraw-Hill Book Co., Inc., New York (1922). (6) BINGHAM, E . C., AND MURRAY, H . A,: Proc. Am. SOC.Testing Materials 23, 11, 655 (1923). (7) BINGHAM, E. C., AND STEPHENS,R . A , : Physics 6, 217 (1934). (8) BRAUNBEK, W.: Z. Physik 67, 501 (1921). (9) EVANS, E. V., AND PICKARD, H. : An Investigation into the Nature and Properties of Coal Tar. South Metropolitan Gas Co., London (1931). (10) EYMANN, W.: Teer u. Bitumen 31, 165 (1933). J. D., AND PARKS, G. S.: Physics 6, 356 (1935). (11) FERRY, (12) International Critical Tables, Vol. I, p. 32. McGraw-Hill Book Co., Inc., New York (1926). (13) KLINKMANN, G. H.: Asphalt Teer Strassenbautech. 31, 942 (1931). (14) MACK,C . : J. Phys. Chem. 36, 2901 (1932). (15) MOONEY, M., A N D EWART,R. H.: Physics 6, 350 (1934). (16) NELLENSTEYN, F. J . : J. Inst. Petroleum Tech. 10, 311 (1924). (17) NELLENSTEYN, F. .J.: J. Inst. Petroleum Tech. 14, 134 (1928). I”. J., AND ROODENBURG, N. M.: Kolloid-Beihefte 31,434 (1930). (18) NELLENSTEYN, (19) PITTMAN, C. U., AND TRAXLER, R. N.: Physics 6,221 (1934). (20) POCHETTINO, A,: Nuovo cimento 8, 77 (1914). (21) RICHARDSON, C.: J . Ind. Eng. Chem. 7,463 (1915). (22) RODIGER, W.: Kolloid-Z. 66,351 (1934). (23) SAAL,R. N. J.: Chem. Weekblad 32,435 (1935). (24) SAAL,R. N. J.: Chem. Weekblad 32, 486 (1935). (25) SCHWEYER, H . F.,COOMBS, C. E., AND TRAXLER, R . N.: To appear in Proc. Am. SOC.Testing Materials 36, I1 (1936). (26) SPIERS,€1. M.: Brcnnstoff-Chem. 9, 77 (1928). (27) TRAXLER, R. N., AND SCHWEYER, H. E . : Physics 7 , 6 7 (1936). (28) TRAXLER, R. N . , AND SCHWEYER, H. E.: To appear in Proc. Am. Soc. Testing Materials 36, I1 (1936). (29) ‘I’RAXLER, R . N., AND SCIIWEYER, H. E . : To appear in Am. Soc. Testing Materials 36, I1 (1936).
