September, 1944
INDUSTRIAL AND ENGINEERING CHEMISTRY
A silica sol was formed by neutralizing a dilute solution of sodium silicate with sulfuric acid to a pH of 8.2. Fifty-three grams of the neutralized J M brand of 42' BB. sodium silicate were diluted to 1000 ml.; the resulting soIution had a silica content of 1.5%. The electrolyte to be removed was sodium sulfate. The cation and anion exchangers used were of the synthetic resin type. Passage of this sol through the cation exchange bed resulted in the replacement of sodium ions with hydrogen ions. The effluent from this bed then contained silica sol and sulfuric acid. Passage of this effluent through the anion exchanger resulted in the replacement of sulfate ions with hydroxyl ions. The final effluent contained the silica sol. The following analysis indicates the purity of the sol obtained: SiOz, 99.3%; "azo, 0.51; SOI, 0.022; C1, 0.118; Fez03, 0.04. To prepare sols of greater purity, it is possible that redistilled water should be used; also the exchange bed would have to be washed for a longer period of time to remove the maximum amount of excess regenerant present in the bed.
823
ACKNOWLEDGMEVT
The writer wishes t o express his appreciation to P. G. Bird, J. A. Holmes, and F. K. Lindsay for their helpful suggestions. LITERATURE CITED
(1) Baylis, J., J . Am. Wuter Works Assoc., 29, 1355-92 (1937). ( 2 ) Bird, P. G., U.S. Patent 2,244,325 (June 3, 1941). (3) Bird. Kirkpatrick, and Melof, J. Am. Wuter F o r k s Assoc., 29, 1526 (1937). (4) Bradfield, R., J. Am. C h m . SOC.,44, 965 (1922). ( 5 ) Ebler and Fellner, Ber., 44, 1915 (1911). (6) Graham, T., Trans. 151, Roy. SOC.(London), 183 (1861). (7) Grimaux, Compt. r e d . , 98, 1434 (1884). (8) Jordis, E., 2. anorg. Chem.. 34, 457 (1903). (9) Lindsay, F. K., Trans. Am. Inst. Chem. Engrs., 37, 547 (1941). (10) Lottermnser and Kiehn, Kolloid-Beihefte,35, 123 (1932). (11) McElroy, K. P., U. 9. Patent 1.811.587 (June 23, 1931). (12) Myers, F. J., IND.ENQ.CHEM..35,858 (1943). (13) Radczewski and Richter, Kolloid Z., 96, 1 (1941), P R W E N Tbefore E ~ the Division of Colloid Chemistry a t the 107th Meeting of the AMERICAN CHBIXIICAL S n c r w r , Cleveland, Ohio.
heological Properties of Asphalt R. N. TRAXLER, H. E. SCHWEYER, AND J. W. ROMBERG The Texas Company, Port Neches, Texas The flow characteristics of twenty-seven asphalts from different sources and processed by various methods are evaluated in rotary viscometers of a type suitable for the measurement of high consistencies a t a constant rate of shear. Consistencies of each asphalt a t fixed temperatures were determined a t two or more mean rates of shear. If a n asphalt is a complex liquid, the measured consistency decreases a s the rate of shear is increased. The magnitude or degree of complex flow may be evaluated by c in the equalion, M = F / S c ; c i s unity for asphalts that are simple (Newtonian) liquids but varies from unity for those that are complex liquids. This equation is valid over a considerable range in rate of shear and is not limited to one
type or size of viscometer. Data are given which show that certain asphalts are simple liquids a t service (atmospheric) temperatures, while others have the characteristics of complex liquids. Evidence is given that the type of flow (and if complex, the degree) depends on the source of the asphalt, the method and degree of processing, the age of the sample, and the temperature a t which the evaluation is made. It is shown that some asphalts retain their complex flow characteristics a t temperatures as high as their ring and ball softening point. The relations between the fundamental rheological charaeteristics of asphalts and the empirical tests commonly used by bituminous technologists are illustrated.
ATERIALS of high viscosity when subjected to stress frequently exhibit many of the rheological properties of colloidal systems. The principles involved and problems met in these rheological investigations are well illustrated by studies on asphalt; in such materials the flow may vary greatly with temperature, it may be simple (Newtonian) or complex (non-Newtonian), and the flow properties may change with age because of the development or increase in amount of collojdal structure. These flow characteristics in many instances are the controlLing factor in the applicability of a given material foi a particular use. Rheological studies provide a sound basis for explaining the behavior of high-viscosity materials under service conditions as exemplified by asphalts when used in roofing and other waterproofing applications or when applied as road binders and paper plying adhesives. This paper discusses the application to asphalts of a complex flow equation, the effect of the dimensions of the viscometer and the rate of shear upon the rheoIogica1constants, the factors which affect the flow characteristics of asphalt, and the relations among consistency in absolute units and data obtained by certain empirical flow tests that are commonly used.
EVALUATION OF FLOW CHARACTERISTICS
M
The most satisfactory instrument for evaluating the flow characteristics of asphalts is the rotating viscometer; one type (16) was used to obtain most of the data discussed below. An arithmetical plot of shearing stress against rate of shear is curvilinear for many asphalts. However, such a rheological diagram plotted on logarithmic coordinates is usually a straight line (Figure 1) which may be expressed by the relation:
M where F S c M
=
F/&
(1)
= shearing stress, dynes/sq. cm. =
rate of shear, reciprocal sec.
= slope of log S us. log F plot = value of F when S = 1
Equations of this type are discussed by Barr (6). If the materid a simple (Newtonian) liquid, constant c is unity and M is the viscosity in poises. For materials possessing complex flow characteristics, constant c may be used to evaluate the degree of compIex flow quantitatively. The more this comtant varies from one, the greater is the deviation from viscous flow since its numerical value is a direct measure of the shearing stress OS. rate of shear relation. is
Vol. 36, No. 9
I N D U S T R I A L A-N D E N G I N E E R I N G C H E M I S T R Y
824
For the asphalts discussed ia this paper, c lies between 0.45 and 1.00. However, values between 0.30 and 1.00 have been obtained on certain asphalts, and it is possible that the range will be extended by further investigations. Numerical values of c are rounded to the nearest 0.05, based on an average precision of ~ 3 0 j bfor the consistency determination. It is probable that increments in the value of c of less than 0.05 are not significant for asphalts. Obviously an evaluation of c alone is not sufficient'to explain thr! flow of a material that behaves as a complex liquid; in addition a consistency value must be given. Constant M in Equation 1 cannot be used to evaluate the consistency (in absolute units) of complex liquids because of dimensional considerations resulting from taking S to the c power. However, the ratio FIX has the dimensions of poises and may be used to calculate the consistency in absolute units at any given value of the rate of shear, X. For this purpose a value of X should be selected which is approximately the mean of the range of experimental rates of shear employed in the rheological measurements. Low rates of shear (0.001 and 0.0005 sec.-l) were suggested previously (18) for consistency measurements on asphalts a t service temperatures such as 25" C. (77" F.) because the experimental rates of shear used must be in this low range to prevent slippage (16). Conversely the viscosity of many soft asphalts is measured in the rotary viscometer at rates of shear up to 10 sec.-l which necessitates considerable extrapolation in order to obtain the ratio of F/S at 0,001see.-'. Improvements in viscometers and increased knowledge of the flow characteristics of asphalts have bdicated the suitability of selecting 0.1 reciprocal second as the rate of shear at which to compare the consistencies of both hard and soft asphalts with a minimum of error introduced by extrapolation. This procedure has been followed in reporting the results in this paper. As is well known ( 5 ) ,the calculation of F/S for complex liquids necessarily requires consideration of the effect of complex flow in the differential equations for F and S for the particular instrument involved. However, complicated equations can be eliminated by judicious se!ection of the area a t which the mean rate of shear and shearing stress are measured. I n the case of the rot u y viscometer, the rate of shear is calculated at a radius which is the square root of the harmonir mean of the squares of the ra-
v)
a
z 0 w 0
v)
-I
9 0 a
5
w 0
a v)
dii of the inner and outer cylinders. At this selected radius the equations for F and S are the same for a viscous or complex liquid as stated by Mooney and Ewart (8). VALIDITY OF FLOW EQUATION
The validity of Equation 1 for a ten- to thirty-fold variation in rate of shear may be illustrated by plotting the data in Table I for any one of the twenty-five asphalts investigated. The data for two of these, A (a simple liquid) and B (a complex liquid), are plotted in Figure 1 on logarithmic coordifiates. The resulting straight lines indicate that Equation 1 is Figure2. Rotary Viscometer valid over a thirty-fold range in rate of shear. The standard size rotary viscometer (No. j),described in detail elsewhere (15 ) , was employed in obtaining the rheological data of Table I. A sketch of the instrument is shown in Figure 2. Each measurement in Table I represents the equilibrium consistency (16) at a constant rate of shear. The upper limits for the rate of shear used depended on the material under study since thixotropy and slippage had to be considered. The lower values for the rate of shear were fixed by the time alloted for each measurement since equilibrium viscosities a t low rates of shear are time consuming for Newtonian asphalts of high viscosity as weU as for those exhibiting complex flow. For soft materials the minimum values for rate of shear were selected to produce torques that could be measured with precision. Further evidence of the validity of Equation 1 is shown in Figure 3 which gives the data for three different asphalts obtained in five rotary Viscometers having different dimensions. The dimensions of these viscometers together with those for the standard size instrument (No. 5) are given in Table 11. Instruments 5A, 5B, and 5C have the same rotor size as No. 5, but the stators have different diameters which result in variable b/a ratios and annuli of different widths. Instrument G has a large-diameter rotor and a large-diameter stator but the same b/a ratio as viscometer 5. The rheological dafa obtained on the three asphalts with the five viscometers are summarized in Table 111, and indicate that the consistency (at a rate of shear of 0.1 reciprocal second) and the value of c in Equation 1 are independent of the dimensions of the viscometers.
L
I
I
K
a
W
LIMITATIONS OF THE METHOD
I
v)
8 W c 4 K
z a W
a
MEAN
SHEARING STRESS,F,
DYNES/SP..CM.
Figure 1. Rheological D i a g r a m s a t 25" C. f r o m Instrument 5
The use 03 Equation 1 as applied to asphalts has the following limitations: When data for certain asphalts exhibiting a high degree of thixotropy are plotted on logarithmic coordinates such as Figure 1, a slight curvature appears (concave toward the log S axis) at high rates of shear. It is difficult to ascertain whether the curvature of the log F us. log S plot is actual or whether the lowered equilibrium shearing stress is the result of thixotropic effects produced during operation of the viscometer. I n extreme cases this curvature or failure to obtain an equilibrium consistency may be caused by slippage at the walls of the viscometer. The experimental determination of equilibrium consistency for use in Equation 1 is time consuming for the hardest asphalts be-
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1944
x
Process Air-blown
Kesiduuni Source Gulf Coast I
Softening i'oint, Ring 6 Ball, O C. ( O F.) 47.8 (118)
P'
.lir-hloan
Northeast Texas
44.4(112)
C
Steam
Pressure still
46.1(115)
D
Air-blown
Gulf Coast I1
33.9(93)
E
Air-blown
Gulf Coast I1
50.6(123)
F
Air-blown
Mixed
39.4(103)
G
Air-blown
Mixed
43.3(110)
w
Air-blown
E a s t Texas
58.3(137)
I
Air-blown
Northeast Texas
72.2 (162)
J
Vacuum
Gulf Coast I11
48.3(119)
K
Vacuum
Gulf Coast I11
66.7(152)
I,
Air-blown
Mexican
44.4(112)
$1
Sir-blown
hlexican
67.8(154)
N
Steam
East Texas
58.3(137)
0
Steam
Gulf Coast I11
59.4(139)
P
Air-blown
California
52.8(127)
Q
Air-blown
Midcontinent
51.1 (12)
R
Steam
E a s t Texas
39.4 (103)
s
Air-blown
East Texas
T
Fluxed
Gulf Coast II
53.9(129)
bi
Air-blown
Gulf Coast I
54.4(130)
V
Air-blown
Gulf Coast I
61.7(143)
W
Steam
Venezuela
51.1(124)
x
Air-blown
Gulf Coast I1
53.9 (129)
Y
Air-blown
Mexican
57.2 (135)
Asphalt
4
41.1 (106)
825
Rheoloqioai PenetraEvaluation Durtilitv tion a t Rheolonical DQ'e ~. a t 25' 6. 2 5 0 C., Mean ConsistConsistency (77' F.), 100 Temp. of shearing ency, a t S = Degreeof 5 Cm. per Grams, measurement, Mean rate of stress P , mepa0.1 sec.,-l, complex >fin,, Cm. 5 Sec. O C. (" F.) shear, S, sec.-l dynes/cm.z poises megapolses flow, e 3,780 1.15 3.27 x 10-3 1.16 1.00 184 98 2 5 (77) 11,700 1.19 9.81 x 10-3 37,300 1.14 3.27 x 10-2 9 . 8 1 x 10-2 114,030 1.16 0.93 3,070 0 64 0.90 131 149 3.27 x 10-3 25 (77) 0.87 8,520 9 . 8 1 x 10-3 24,900 0.76 3.27 X 10-2 63,500 0.6.5 9 . 8 1 x 10-2 2.59 8,450 3.27 x 10-3 2.66 150459 25 (77) 1 .oo 25,300 2.57 9.81 x lo-; 86,500 2.64 3.27 X 105,300 0.054 0.056 25 (77) 9.81 x 10-2 n.90 300$ 15,100 3.27x i n - 1 0.046 40,200 0.041 9.81 x 10-1 7.7 7,560 0 85 9.81 x 10-4 4.0 25 (77) 86 80 21,000 6.4 3.27 x 10-3 Slippage 9 . 8 1 x 10-3 5,130 oli.47 3.27 x 10-2 225 0.142 0.98 25 (77) 126 0.144 14,100 9 . 8 1 x 10-2 40,800 0.125 3.27 X 10-1 9.81 x 10-8 6,OGO 0.90 0.49 0.62 139 25 (77) 184 18,400 0.56 3.27 x 10-2 0.49 48,N O 9.81 x 10-2 0.75 3.27 X 10-6 8,lCiO 25.0 25 (77) 64 6.8 31 9.81 x 10-4 19,200 19.6 4 7 ~ 0 14.6 3.27x 10-3 0 50 44,000 135 3.27 x 10-4 7.9 4.7 49 25 (77) 78,600 80 9 . 8 1 x 10-4 43 3.27 x 10-3 139,000 1.00 3,310 0,0102 0.327 0.0103 48.3(119) 66 200 10,300 0.0105 n.9si 37,100 o.0102 3.27 1.w 3,780 0 , 0 1 0 3 0.0104 0.327 11 11 66.7(152) 10,100 o o i n q 0.981 34,200 0.0105 3.27 n . {JO 6,380 0.0195 0,0225 44.4 (112) 0.327 140 200 O.!IRl 17,300 0.0177 49,100 0.0150 3.27 0,0216 0.0265 0.85 7,050 0.327 67.8(154) 46 32 0.0172 16,900 0.981 47,800 0.0146 3.27 17.1 16,900 14.3 0.95 25 (77) 9.81 x 10-4 200 30 53,600 10.4 3.27 x 10-3 157,000 1 5 . 6 9.81 x lo-; 1.00' 12,000 37 38 200 25 (77) 3.27X 1017 39 38,300 9 . 8 1 x 10-4 121 no0 37 3.27 x 10-3 0 . 9R 7:$90 4.6 i . x~ 10-3 3.0 150f 61 25 (77) 14,100 4.4 3 . 2 x 10-3 122,000 3.8 3 . 2 X 10-2 0.86 11,500 1 . 6 x 10-3 43 7.2 25 (77) 170 67 21,500 6.7 3.2 x 10-3 m , n )o 5.0 3.2 X 10-2 1.00 0.40 4,080 0.42 0.81 x 10-3 176 25 (77) 116 13,400 0.41 3.27X 10-2 39,600 0.40 9.81 x 10-2 0.85 4,280 0.44 0.32 9.81 x 10-3 113 196 25 (77) 12,500 0.38 3.27 X 10-2 31,800 0.32 9.81 x 10-2 0.70 5,320 16.3 3.0 3.27 x 10-4 17.5 94 25 (77) 12,200 12.4 9.81x 10-4 27,100 8.3 3.27 X 10-8 Slippage 9 . 8 1 x 10 Q.95 8,260 8.4 7.2 9.81 X 10-4 200 52 25 (77) 8.0 3.27 x 10-3 26,000 76,600 7.8 9.81x 10-3 0.85 28,100 86 36 3.27 x 10-4 32 25 (77) 38 71 69,600 9.81 x 10-4 196,000 60 3.27x 10-3 0.05 3,130 0.33 0.29 9.81x 10-3 45 (113) 3.27 x 10-2 10,300 0.31 28,100 0.29 9.81 x 10-2 0.95 2,550 0,0082 0,0078 65 (149) 0.327 7,220 0.0074 0.981 22,900 0,0070 3,27 0.90, 12,100 3.7 3.27X I O - a 2.45 57 25 (77) 150-t. 3.3 32,000 9.81x 10-3 2 . 8 93,000 3 . 2 7 X 10-2 238,000 2.4 9 . 8 1 x 10-2 4,850 0,049 9.81 x 10-2 0 ,050 45 (113) 0.98 16,300 0.047 0.327 0.044 43,400 0.081 0.981 2,100 O.OO215 0.00216 1 .oo 65 (149) 7,100 0,00217 3.27 21,100 o.onzi6 9.81 3,500 10.7 77 25 (77) 3.27 x 10-4 5.4 0.88 52 9,460 9.81 x 10-4 0.7 26,000 3.27 X 1 0 - a 8.0 3,t30 11.7 3.27 X 10-4 25 (77) 5.0 0.83 64 125 9,570 9 . 8 9.81 x lo-' 27,200 3.27 X 10-a 8.3
...
.
cause of the low rates of shear that must be used. This limits the application of the equation to research studies of a fundamental nature as described below. However, otl'er methods of analyzing the rheological data on asphalts are being studied and may provide the means for rapid absolute consistency measurements for use in control and specification work.
FqCTORS AFFECTING FLOW PROPERTIES
Experienced asphalt rheologists recognize that the difficulties in evaluating the flow characteristics of asphalt are caused by the different types of complex flow which may be encountered. Too frequently these effects have been overlooked or ignored and general conclusions have been drawn from meager data on an in-
Vol. 36, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
826
sufficient number of asphalts. The necessity for adequate information on a wide variety of asphalts is illustrated. Many of these variations in the flow properties of asphalts apparently are closely related to their colloidal nature. SOURCE.The nature of the hydrocarbons in the petroleum from which an asphalt is prepared profoundly affects the colloidal structure and thus the flow properties of the finished asphalt. This is illustrated by the data in Table I for asphalts A, B, P, and Q, all made by air blowing petroleum residua from widely different sources. Asphalt A is a simple liquid whereas B, having o is a lower consistency but air-b1own from a different complex liquid. Asphalts P and Q (complex liquids), air-blown to about the same consistency, show different degrees of complex flow. asphalt is processed OfMETHODOF PROCESSINQ. The ten has a definite effect on the fl eristics. Two commonly used processes are steam distillation and air blowing. I n general, air b l o ~ n ga residuum will result in an asphalt having complex flow characteristics than is obor vacuum reduction. This isiflmtrated by the tained by data on asphalts H, N, R, and S prepared from the same East Texas residuum (Table I). Steam-refined asphalt R is essentially a simple liquid, whereas air-blown asphalt 8, of the same consistency, exhibits complex flow with a value for of 0.85. Similar relative variations in care indicated for N and H. In asphalt manufacture many materials are prepared by blend,ing. If the components are greatly different in consistency (such
IO MEAN SHEARING STRESS, F
Figure 3.
TABLE11. DIMENSIONSAND CONSTANTS FOR ROTARY VISCOMETERS
Instrument No.5c 5 5A 1.111 1.270 1.588 1.905 1.905 1.906 2.540 2.640 2.540
-
1.397
1.433
90 0.953 2.000
90 0.794 1.714
1.588 69 90 0.636 1.500
a ~ c ~ " , ~ ~ ~ $ ~ ~ ~ e 59 e s ,6 3 , .5
rotor angfe degrees Gidth of annul&, cm. bio
1.965 80
90 0.317 1.200
6 1.906 2.868 2.640 1.077 69 90 0.953 1.600
a For each viscometer, a angle of stator cones) wan selected BO that the following equation was satis8ed; by using this angle the mean rate of shear at the ends is approximately the same as the mean rate in the cylindrical regions of the viscometer (8):
where If
-
Be + - a + sin -p a')/(bs ai) t
(b:
€8
(a/s sins eo
-
l/n
sin 80)
as when fluxing hard asphalt with residuum), the blended product may exhibit more complex flow than straight processed residuum of the same consistency. This is illustrated by the data in Table I on asphalts T and X. Sample T was prepared by fluxing a hard asphalt (softening point, 121.7' C. or 251' F.), air-blown from Gulf Coast residuum 11, with an equal weight of the parent residuum. Asphalt X was made by air blowing the same residuum to a ring and ball softening point of 53.9' C. (129' F.). DEQREEOF PROCESSWQ. It appears that the degree of proceesing required to cause the appearance of detectable complex flow varies with the method of manufacture and the source (nature) of the material. The effect of extent of air blowing on the flow characteristics of asphalts is illustrated by the rheological data in Table I for asphalts A, U, and V which were manufactured from Gulf Coast residuum I. Asphalts R and N illustrate the effect of extent of steam reduction on flow characteristics. Certain types of residua may be steam- or vacuum-refined to fairly hard asphalts (with consistencies above 2 megapoises) which are essentially simple liquids a t atmospheric temperatures. This is indicated by the data for asphalts C, 0, and BB. The transformation from Newtonian to complex flow as consistency increases is not very definite for a given series of asphalts. Previously (18) this was indicated to occur a t 3.5 to 7.0 megapoises at 25' C.; the moraextensive data given in this paper show that complex flow may be exhibited by asphalts of much lower consistency (D. F, and S, Table I). TEMPERATURE OF MEASUREMENT.As the temperature of an asphalt is raised, any comp!ex flow characteristics tend to diminish and may disappear enti'rely if the temperature is increased sufficiently. This behavior may be attributed to a gradual change in the asphalt from a gel to sol structure. Examples of the effect of temperature are given in Table I on asphalts V and W at 25', 45', and 65' C. AQEHARDENINQ. All asphalts show an increase in consistency with time not caused by loss of volatile components; the magnitude of this effect is dependent on the source of the asphalt and the method by which it was processed. The phenomenon of age hardening is probably a manifestation of the colloidal nature (solgel structure) of asphalt, since the increased consistency occurring with time may be wholly or partially destroyed by heat or mechanical working. The data on asphalts C, I, P, and Q, of widely different origins, show that increased rates of age hardening (higher values for the asphalt aging index, AAI) are usually associated with higher degrees of complex flow (lower values of c). The age hardening of
IO6
,
-
5B 0.952 1.905 2.540
c
Viscometer Dimension radius, om. rotor radius, om. 2 rotor length, cm. I , =. length of cvlindrical
DYNES / SQ. CM.
Data Obtained in Various Rotary Viscometers at 25" C.
Y W'
I~ALUATIONS AT 25' C. (77" F.) TABLU111. RHEOLOGICAL c
Instrument No. Consistency at 0.1 set.-'. megapoisea Degree of complex flow, c I
5B 2.30 1.00
6C 2.60 1.00
Asphalt C 6A 6 2.56 2.66 1.00 1.00
-
6 2.55 1.00
7
6B 0.44 0.90
6C 0.44 0.90
Asphalt G 6 5A 0 . 4 9 0.49 0.90 0.90
6
0.44 0.90
-
-Alphalt 6C 6 8.5 4.0 0.86 0.86
E
6A
8.7
0.85
6 3.8
0.85
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
September, 1944
MEAN
Figure 4.
SHEARING
STRESS
,F , QYNES/ SP.
CM.
Rheological Diagrams at 25' C . from Instrument
5B
mphalts was discussed and AAI was defined elsewhere (16). The
data were obtained in falling coaxial-cylinder viscometers (17). Five or six viscometers were filled with each asphalt and placed in a cabinet maintained a t 25" C. At intervals up to about 1000 9ours a viscometer was removed from the constant-temperature cabinet, and several consistency measurements were made at 25" C. The AAI for each asphalt was obtained by determining &heslope a t 100 hours of the log time us. log consistency curve: Asphalt AAX 0
C 0.017 1.00
P 0.023 0.95
Q
I
0.073 0.85
0.183 0.50
Because of the occurrence of age hafdening it is essential that evaluations of consistency and complex flow of asphalts be made at the same age if values are to be comparable. Consistency is usually determined one hour after the viscometer is filled with hot asphalt. During this time the asphalt is brought to the desired temperature for testing. RHEOLOGICAL ASPECTS OF TESTS
Various investigators have attempted to evaluate rheological properties by empirical flow tests such as the A.S.T.M. ring and ball softening point, A. S .T. RI. penetrometer a t 26" C., and A. S.T.M. ductility test a t 25" C. These attempts have been unsuccessful for the most part, largely because of the combination of flow and other properties measured by each empirical test. RING AND BALLSOFTENING POINT. The distinctly diflerent flow characteristics of asphalts a t their ring and ball softening points (4) are illustrated by the data in Table I for samples J and K, prepared by vacuum distillation of a Gulf Coast residuum, and 11 and M, air-blown from a Mexican residuum. These data refute the concept that all asphalts at their ring and ball softening points are simple liquids having the same viscosity. Of course, by judicious selection of the shearing stresses used in the measurements, it is possible to alter the consistency values of the complex liquids to a value similar to those for asphalts showing Newtonian Bow, as illustrated by Figure 4. At a mean shearing stress of 35,000 dynes per sq. cm., the consistencies of asphalts AA and BB are identical (250 megapoises), but at a mean shearing stress of 100,000 dynes the viscosity of BB is 250 megapoises while that of AA is only 59. The viscosity of bituminous materials at their ring and ball softening points has been determined by a number of
827
investigators (11, 18, 13), who have shown that the Viscosity a t that temperature may vary from 8000 to 30,000 poises, Fundamental reasons for this variation, with non-Newtonian materials, were given elsewhere (18). The data in Table I on asphalts H, N, 0, and Y show how the penetration and flow properties may vary for different materials of similar ring and ball softening points when the consistency is evaluated a t 0.1 reciprocal second. Although low consistency in megapoises a t 25" C. is associated with high penetration at 25' C.. attention is directed to the high degree of complex flow (low value of c) possessed by asphalt H. Asphalt 0 with a lower penetration and high consistency for the same softening point is a viscous liquid a t 25" C. PENETRATION TEST.At recurring intervals the penetrometer (3) has been proposed as an apparatus for measuring consistency in absolute units, but its use for this purpose is highly controversial. The most serious criticism of the penetrometer method can be directed a t assumptions concerning the method of shearing which provides the basis for the theoretical equations, such as those of Pendleton (9). The penetrometer needle moves to displace a relatively small volume of fluid. The flow under such conditions is not the same as in a coaxial cylinder viscometer where the displacement of the moving member results in no movement of the material at the wall unless slippage occurs. Furthermore, the theoretical equations make no correction for the conical depression around the needle. This depression was recognized by Rhodes and Volkmann (10) and was shown in photographs by Traxler and Moffatt (14). Another rheological fault of the penetrometer is the end effect of the needle point passing through the material. The use of an equivalent cylindrical surface for the surface of the needle may be a satisfactory assumption for calculating the surface, but it does not correct for the end effect. In general, the needle point correction8 are minimized by employing greater depths of penetration or other arbitrary correction factors, but these expedients do not correct for the theoretical limitations. Empirical equations for converting penetration readings to consistency in absolute units are subject to the same objections as the theoretical equations unless their validity ran be proved. Thus, the needle point effects were indicated by Fair and Volkmann (6) to be of sufficient magnitude to limit the applicability of the Saal and Koens relation to penetration depths greater than 55 for Newtonian materials, The presence of complex flow further complicates these penetrometer equations, with the result that the theoretical and empirical equations are validaked for o d y a few asphalts when ade-
TABLEIV.
Asphalt A B ' C
H
I I
0
T
U X Y
COMPLEXFLOW*MEAWURED IN DIFFERENT APPARATUS
Rotary Viscometer Mp. c
1.16
0.84
2.G5 6.8 7.9 7.9 38 3.0 7.2 5.4 5.0
1.0
0.9
1.0 0 75
0.6 0.5
Falling Coaxial-Cylinder Viscometer Alp. c 0:Sl 2.75 5.0 7.2 7.2
019 1.0 0.75 0.5 0.5
.. ..
U.YJ
I .
0.85
0.85
* Consistency in megapoises at 25' plex flow c.
-
Penetrometer (50 Grams) Mp. cn cb 1.80 1.17 1.09 0.50 1.04 1.08 7.5 1.30 1.13 3.4 0.67 0.60 5.8 0.51 0.62 4.9 0.47d 0.49d
11.d
1.75 3.1
1.028 0.56 1.29 0.67 0.77
0.896 0.52 1.01 0.60
0.70
C. and 0.1 s m - 1 , and deeree of com-
-
5 Calculated from c I/n k/(l k), where k is the slo e of log penetration plotted against log time ( s e d , as discussed by Pendcton (9). corrected penetrations employing data greater than 100 penetration onl; were used. b Calculated from Equation 1 , where F and S were computed according t o Pendleton's equations (9). d 100-gram load. 6 Impossible to obtain checking results on this material with a 50-gram load; results are for 200-pram load. Asphalt did not adhere to needle.
828
INDUSTRIAL AND ENGINEERING CHEMISTRY quate data are obtained. This conclusion is based on data obtained in the rotary viscometer compared to those by the method of successive penetrations. Certain of the available data are shown in Table IV. Although the falling coaxial-cylinder viscometer yields log 8 06. log F plots that are parallel to those for rotary viscometer data, t h e lines d o not necessarily superimpose because elastic and thixotropic effects may interfere. T h e s e complications were noted by Pendleton (9),and their presence precludes the use of dataobtained w i t h t h e falling coaxial-cylinder viscometer for confirming o t h e r methods of measurement on nonKew tonian materials. T h e apparatus is limited to Kewtonian fluids or to comparative studies on complex liquids (e.g., age hardening) where a strictly arbitrary procedure is followed. For complex liquids widely different consistency results can be obtained, depending upon the shearing stress and the distance through which the inner cylinder moves. The data in Table I for asphalts Q, S, V, X, and Y, having the same degree of complex flow, illustrate the confusion which may f-
Figure 5. Illustrations of Necking Effect
Vol. 36, No. 9
result from an attempt to explain the flow properties of asphalts by results of the standard penetration test ( 3 ) . They indicate a general trend of decrease in penetration with increase in consistency. However, there are exceptions to this trend as shown by asphalts Y and Q. Their low penetration values may be caused by their relatively high adhesion for the steel needle (as compared to asphalt X) which retards the rate of penetration and gives low results. Other asphalts (see note on asphalt 0 in Table IV) manifest little tendency to adhere to the needle, as shown by the fact that the needle can be withdrawn from the sample with a clean surface. Accordingly, the penetrometer must be considered as measuring a combination of consistency and adhesiveness of asphalt to steel. DUCTILITYTEST. Although the A.S.T.M. ductility test ( 2 ) does not evaluate consistency, it has been shown (18) that ductility values a t various temperatures and at different rates of deformation are readily explained by the rheological properties of the asphalt a t the temperature of test. The ductility test result is actually the length at which a thread of the material breaks because the shearing stress at the area of the break exceeds the cohesive strength of the rraterial when i t is deformea under perscribed conditions. For many soft materials at 25" C. this breaking point is not attained under the conditions of the test, and high ductility values result; for very hard materials at 25" C. the break point is attained soon after the start of the test because the cohesive strength is exceeded before the material can flow appreciably, with the result that zero ductility values are obtained. Between these two extremes there is a range of ductility where the results vary with different asphalts and the degree of complex flow exhibited by them. This point is illustrated by the data OD asphalts H, V, and Y, 0 (Table I) : Amhalt Diotility (25' C.), cm. C
H
V
31
38
0.75
0.85
Y 125
0.85
0 ZOO$ 1.00
I n the case of Y and H, with the same penetration and softening point, the decrease in ductility is in direct agreement with the decrease in the value of c. The importance of the degree of complex flow on ductility is also illustrated by the data for asphalts V and 0 which have about the same consistency in megapoises and about the Fame Eofteninp point. An expianation of these results lies in the fact that the materials with the greater degree of coxrplex flow (lower values of c ) decreaee more in consistency with increase in shearing stress. AS t h e cross section of tho thread decreases during the ductility test, the shearing strees increaees rapidly and induces a greater flow of =aterial. I n t h e case of complex liquids, such as V, the region of lowest consistency is localized at the point of highest shearing stress, with the result that "necking" occurs. This rapid reduction in cross-sectional area produces low ductility values because the shearing stress exceeds the cohesive strength of the material in a short time after the test is started. Where the material is a simple liquid, such as asphalt 0, the nature of the flow is independent of the shearing stress and necking does not occur. Such materials will deform readily into a long thread, under the conditions of the test, before the cohesive strength of the material is exceeded. This elongation results in high ductility values. The upper photograph of Figure 5 (taken a t an elongation of 11 cm. for asphalts H, V, and 0) illustrates the effects of complex flow.
TABLE v. COMPLEX FLOW AND DUCTILITYTEST AA Asphalt Gulf Coast I1 Residuum source Air-blown Process 6 7 . 2 (153) Ring and ball, C. (" F.) 44 Penetration at 26' C 100 g 5 sec. 5.5 Ductility at 25' C $'cm./min., om. 6.0 Consistencv at 25'"c., S = 0.1 8ec.-1, megapoises Degree of complex flow, c 0.45
BB
Gulf Coast 111 Steam 6 5 . 6 (150) 10
12 250
1.00
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
September, 1944
The necking effect for materials with a high degree of complex flow, as distinguished from the elongation of the viscous materials is accentuated in materials of high consistency; the two lower photographs of Figure 5 show asphalts AA and BB which have the characteristics given in Table V. The center picture was made at an elongation of 3.5 em., and the lower one was taken after asphalt BB failed. The high consistency of these materials made it necessary to use instrument 5B for the rheological data given in Figure 4. Since elongation under standard test conditions is the only property measured by the ductility test, it is futile to deduce other interpretations from the results. If any fundamental information concerning asphalt is to be obtained from the ductility test, it is necessary to measure the force required to deform the material as is done in tensile strength tests. This was done by Abraham (1) and by Grant and Pullar (7') with apparatus of special design. I t is obvious that extremely sensitive equipment would he necessary for materials having ductilities of 100 cm. or higher and for those soft maieiials with low ductility. I t is possible that even such special tests would be no more informative than the rheological data obtained over a range of shearing stresses in viscometers designed on scientific principles.
for Testing Materials, Standard Method of Test for Ductility. of Bituminous Materials (D113-39), Part 11, p.
(2) Am. Soo.
466 (1942). (3) Ibid., Test for Penetration of Bituminous Materials ( D 5 - 2 5 ) , Part 11, p. 483 (1942). (4) Ibid., Test for Softening Point of Bituminous Materials (Ring and Ball Method) (D36-26), Part 11, p. 488 (1942).
(5) Barr, G., "Monograph on Viscometry", London, Oxford Univ. (6)
Press, 1931. Fair, E. F., Jr., and Volkmann, E. W., IND. ENG. CHEX.,
AXAL.ED., 15, 240-2 (1943). (7) Grant, F. R., and Pullar, H. B., Proc. Assoc. Asphalt Paving Tech., Jan., 1936, 124. (8) Mooney, M., and Ewart, R. H., Phvsics, 5, 350-4 (1934). ( 9 ) Pendleton, W. W.,J . Applied Physics, 14, 170-80 (1943). (10) Rhodes, E. O., and Volkmann, E. W., Zbid., 8 , 492-5 (1937). (11) Rhodes, E. O., Volkmann, E. W., and Barker, C. T., A m . Soc. Tasting Materials, S y m p o s i u m o n Consistency, 1937, 30-46. (12) Saal, R . N. J., Pmc. W o r l d Petroleum Congr., London, 2, 515-23 (1933). (13) (14)
Traxler, R. N., IXD.ENG.CHEM.,30, 322-4 (1938). Traxler, R. N., and Moffatt, L. R., IND. ENG.CHEM.,ANAL.
(15)
Traxler, R. N., Romberg, J. W., and Schweyer, H. E., Ihid.,
(16)
Traxler, R. N ~and , Schweyer, H. E., Proc. Am. Soc. Testing
ED., 10, 188-91 (1938). 14, 340-3 (1942).
i M a t e ? i ~ l36, ~ , 544-51 (1936). (17) Ibid., 36, 518-30 (1936). (18) Traxler, R. N., Sohweyer, H. E., and Romberg. J. W.. Ibid..
LITERATURE CITE13 (1)
40, 1182-1200 (1940).
Abraham, H., "Asphalt and Allied Substances", 4th ed., New York, D. J7an Nostrand Co., 1938.
Ther
829
P m e E N r m a t the annual meeting of the Society of Rheology, in New York,
N. Y.,
1848
ynamic J
SPECIFIC HEAT AND RELATED PROPERTIES KENNETH S. PITZER University of California, Rerkrley, Calif.
F
The values for methane (4), ethane (17), and propane (18) are based on reasonably definite molecular-structure parameters (vibration frequencies, internal rotation potential barriers, etc.). For the heavier normal paraffins a structural picture was assumed which is uniform throughout the series and which, evidence now available indicates, is substantially correct (10, 12). While approximations are made both in the basic picture and in the mathematical analysis, their nature is such as to lead to small and uniform errors which can be compensated by the use of adjusted structural parameters.
When this treatment was first presented, the data necessary to fix certain parameters was inadequate. There is still much to be desired, but considerable improvement has led to the following revised values. The vibration frequencies of the CH, group are now taken as 827, 1170, 1375, 1460(2), 2950(3) em.-' (degeneracy), which are the exact ethane values and are substantially thoso used previously. Although the propane frequencies (13) cannot be definitely classified between CHa and CH2 groups, certain frequencies group closely around the CH8 values and leave 940, 1278, 1338, 1460, and 2950(2) em.-" to be ascribed statistically to the CH2 group. The use of these frequency sets gives good agreement with the experimental gaseous heat capacity data on the higher paraffins. The potential barrier and reduced moment for the end methyl groups of a lonqer chain are now assigned exactly the propane values (IS), 3400 calories per mole and 4.51 X lou4"
Available experimental gas specific-heat values for the normal paraffins are in excellent agreement with curves calculated by methods previously published by the writer. Certain parameters in these calculations are revised on the basis of recent spectroscopic studies. Calculated entropies are still in excellent agreement w-ithmeasured values. The
corresponding results for heat content and the free energy function are also presented. The data for branched paraffins are too meager to allow generalizations except that the change in specific heat with isomerization is small. Entropy differences calculated previously have been confirmed, in so far as additional data are available.
OR several years the writer has been interested in the specific heat of gaseous paraffin hydrocarbons. The accompanying figure and tables present a convenient summary of the present status of results. NORMAL PARAFFINS
