Flow in Asphalts Shown by the Method of Successive Penetrations R. N. TRAXLER'
4YD
L. R . >IOFFATT, T h e Barber Company, Inc., Barber, N. J .
Thelen describes the method as follows: "Without touching the needle or the sample, the needle is released for a series of successive time intervals and the resulting penetration noted after each interval. As the needle sinks deeper into the sample, the time intervals will have to be increased in order to get a measurable increment of penetration for the interval." Several measurements are thus obtained, with each successive one at a lower rate of shear and smaller shearing stress because, although the applied load remains constant throughout the series of tests, with each successive penetration the needle becomes embedded deeper in the asphalt.
The method of successive penetrations proposed by Thelen and modified by Rhodes and Volkmann for the evaluation of flow properties has been applied to various kinds of asphalt. With most of the essentially viscous asphalts studied, the plot of the rate of shear versus shearing stress showed a negative intercept on the shearing stress axis; thus, the test as used is not a sound rheological method. Changes in technic were found to influence the rheological diagrams obtained with non-Newtonian but not with viscous asphalts. It was found that the flow was not laminar and that only a portion of the penetrated length of the needle was wetted by the liquid. Fundamental errors and difficulties, advantages, and precautions which must be observed are summarized.
I
Rhodes and Volkmann suggested that, if the above conditions are fulfilled, and if the resistance caused by friction between the blunt end of the needle and the asphalt is neglected, the test conditions might be comparable to those prevailing in the falling coaxial cylinder viscometer. This treatment is valid only if the needle does not approach too close to the bottom of the container. They also mentioned that the depression of the asphalt surface around the embedded needle may introduce an error because the theory assumes that the needle is wetted by the asphalt over the entire length recorded by the penetrometer. The equation given by Rhodes and Volkmann for the calculation of viscosity can be expressed as follows :
M x ~t x 1-25 x 107 (1) (Pj Pi - 46)(P/ - Pi) where 7 = viscosity in poises P, = final penetration in decimillimeters Pi = initial penetration in decimillimeters Ai = time interval in seconds M = mass in grams of the descending part' of the penetrometer
'
N RECEKT years methods and apparatus have been developed by which the flow characteristics of materials of
high consistency can be evaluated in absolute units; a review
of the methods used with asphalts has been given (5). Since the apparatus required for such measurements is not available in most laboratories, the adaptation of equipment already in common use to the estimation of flow characteristics in absolute units should result in the more extensive use of rheological terms and methods. Thelen (4) suggested that the penetrometer ( I ) , an instrument universally used by bituminous technologists, could be utilized for evaluating the flow properties of asphaIts and other materials of high consistency in absolute units. He proposed to do this by obtaining a series of successive penetrations without touching either the sample or needle. Rhodes and Volkmann (2) discussed the method critically, pointed out errors in Thelen's theory, and suggested that the test conditions could be compared with those prevailing in the falling coaxial cylinder viscometer. They also tested a viscous tar by the penetrometer method and their capillary tube viscometer ( 3 ) ; the results obtained by the two methods checked closely. I n the present paper the typical data obtained by applying the method of successive penetrations to viscous and nonviscous asphalts are reported, and conclusions are drawn concerning the merits of the test as a rheological method for the evaluation of flow characteristics.
=
+
From this equation the average shearing stress (at the surface of the needle) and the corresponding average rate of shear for any time interval At may be evaluated as follows: Shearing stress, F = dv Rate of shear, - = dr
0.6243 M X lo6 (PI Pi - 46) 0 0499(P, - P,)
+
At
(3)
The type of flow occurring in the asphalt should be deterdV mined from a plot of rate of shear! - , against shearing stress, dr F , provided Equation 1 is theoretically sound.
Experimental Data From determinations made on over one hundred different asphalts, the three following types of rheological diagram (rates of shear versus shearing stresses) have been obtained: 1. An occasional asphalt known, from measurements made in a rotating cylinder viscometer, to be essentially viscous gave
Description and Theory of Method
a straight line which could be extrapolated through the origin. This case is illustrated by asphalt C which was produced from Trinidad petroleum by vacuum-steam distillation. The penetration at 25" C., 100 grams, 5 seconds, was 53, the ring and ball softening point was 120' F. (48.89" C.). Data obtained by the method of successive penetrations are given in Table I and Figure 1. 2. Most asphalts which were known to be essentially viscous liquids gave lines which, when extrapolated, indicated a negative yield value. Obviously, the slope of such a line is not a true
The sam le is prepared and brought to temperature as described ine!t commonly used A. s.T. M. penetration test The needle is brought into contact with the surface of the asphalt at a location close t o the axis of the sample container. Since the sample frequently must be kept on the penetrometer table for some time, it should be surrounded during the test by a water bath maintained at the desired temperature (25' C., 77" F.). Present address. The Texas Company, Port Neches, Texas.
188
ASALYTICAL EDITION
APRIL 15, 1938
measure of the viscosity of the asphalt. The data for asphalt A , also included in Table I and Figure 1, gives this type of diagram. Asphalt A with a penetration of 35 at 25" C., 100 grams, 5 seconds, and a ring and ball softening point of 122" F. (50' C.), was obtained from Callfornian petroleum by vacuum-steam distillation. Because the preponderance of the data obtained on viscous asphalts by the method of successive penetrations was of this character, any comparisons of the consistencies with those obtained on the same asphalts by means of absolute viscometers are of little value. 3. Asphalts known to be non-Newtonian liquids from measurements made in a rotating cylinder viscometer gave lines which could be extrapolated to intersect the shearing stress axis. In some cases curves were obtained which were concave t o the shearing stress axis. The shape of the curve seems to be determined to a considerable extent by the rates of shear employed, which with the A. S. T. M. penetrometer are chiefly dependent upon the consistency of the material. With hard materials curved lines were usually obtained because high rates of shear could not be secured. The rate of shear-shearing stress plots a t the higher rates attained with the softer non-Xewtonian asphalts were in most cases straight lines. Venezuelan airblown asphalt L (47 penetration a t 25' C., 100 grams, 5 seconds, and 169" F., 76.11" C., ring and ball softening point), the data for which are recorded in Table I and Figure 1, illustrates one type of rheological diagram obtained for non-Newtonian asphalts by the method of successive penetrations.
Discussion of Method The fact that rheology diagrams are frequently obtained with negative yield values shows that the method of successive penetrations as now used is not a sound rheological method. The probable sources of the difficulties encountered are the technic or procedure followed, and, t o a greater extent, the theory which postulates laminar flow, as in the falling coaxial cylinder viscometer. Numerous experiments have been conducted in a n effort to determine the source of error in the method. Penetrations were made on samples of the same viscous asphalt using (1) the procedure described b y Thelen, and (2) the same procedure except that the relaxation period between
SHUiRlNG
STRESS
(dynes / s m ? ) x C3
FIQURE 1. EXPERIMENTAL DATA
189
TABLE I. FLOWDATAOBTAINED BY METHOD O F SUCCESSIVE PENETRATIONS Total Time of Penetration
Total Penetration
Sec.
.4sphalt C. 30 45 60 90 120 180 240 300 Asphalt d , b 5 15 25 35 45 55
65 85
105 125 145 165 185 205 Asphalt L . d
5 10 20 30 45 60 90 120 180 300 600
Dm.
Shearing Stress, F D y n e s / s q . cm. x 10-5
R a t e of Shear,
Fluidity
da/dr
.Mobility
100/sec.
or
Rhes X 10'
Weight of moving part of penetrometer = 50 grams 88" 108 125 152 175 211 242 268
2;07 1.67 1.35 1.11 0.918 0.767 0.673
6: 49 5.65 4.49 3.83 2.99 2.58 2.16
3122 3.48 3.45 3.61 3.45 3.60 3.47
Weight of moving part of penetronieter = 100 grams 350 .. 61a .. 79a 93 4195 i:i4 1: 3 7 e 106 4.07 6.49 1.50" 118 3 . B1 5 69 1.51C 128 3.13 514 1.52e 2.74 146 4.52 l.51C 2.39 162 3.91 1.49C 2.14 176 3.57 1.490 190 1.96 3.42 1.55C 202 1.81 3.12 1.510 214 1.68 2.92 1.510 225 1.59 2.72 1.48C Weight of moving part of penetrometer = 200 grams
70a 82 95 104 115 123 136' 1466 160' 181'
2108
11:so 9.53 8.14 7.20 6.50 5.86 5.29 4.79 4.23 3.62
12: 00 6.49 4.74 3.49 2.66 2.16 1.66 1.21 0,8.53 0.483
1:64 1.28 1.29 1.27 1.30
..
.. .. ..
..
a These determinations are discarded because of the error introduced by
the conical tip of t h e needle. b Average yield stress, Fo = -25,000 dynes per sp. c m . C These values are fictitious because of the negative yield stress. d Average yield stress, Fo = 445,000 dynes per sq. cm. a These experimental values are located i n the curved region of the rate of shearshearing stress plot a n d thus were not used t o calculate the yield stres- or mobility of the asphalt.
successive penetrations was increased six- to tenfold. Identical rheological diagrams were obtained by the two methods. This was also true when the results of a few penetrations of long duration were compared with those obtained by a larger number of penetrations of short duration. Thus, for the essentially viscous asphalts these modifications of technic have little effect on the results obtained. When a n air-blown asphalt (known to be distinctly nonKewtonian) was subjected to the two procedures described above, different rheology diagrams were obtained with each. This phenomenon occurs also in absolute riscometers b e cause the data obtained for highly elastic, nonviscous materials are influenced by the rate of shear and procedure followed. Consequently, for the nonviscous materials any modifications of procedure may have a definite influence on the results obtained. As mentioned above, Rhodes and Volkmann commented on the effect of the depression around the needle. This hollow is large enough to be seen b y the unaided eye. However, with an opaque material like asphalt, a record of its depth and shape is very difficult to obtain. T o acquire some direct evidence concerning the length of the penetrated needle wetted by the liquid material, a transparent Bakelite resin was placed in a square glass cell about 4.5 em. on a side. The viscosity of this resin a t 25" C. was 69,000 poises. The material was tested a t about 25" C. with the A. S. T. M. penetrometer and the progress of the needle into the material photographed. Figure 2 shows the extent and shape of the surface created by the penetration of the needle. Using a cathetometer to measure the wetted portion of the needle, i t was found that this varied from 79 to 82 per cent of the
INDUSTRIAL A I D EKGINEERIUG CHE31ISTRY
190 1
I-OL. 10, NO. 4
3
2
4
w
I
FIGURE 2.
PENETROMETER XEEDLE
7
6
5
EXTERIXG .4 BAKELITE RESIXPOSSESSING
L'IsCOsITY
OF
69,000
POISES .4T 25'
c.
1, 2, 3.
Entering 4. Completion of penetration 5 , 6. Receding cone 7. Receding cone, 5 minutea after 4 5 , 6, a n d 7 show flow of resin into conical depression caused b y penetration of needle
penetration recorded by the instrument. It'ith material as fluid as this resin the depression around the needle is rapidly filled after the movement of the needle has been stopped. From experiments made on a sample of resin containing air bubbles it appeared that the flow occurring in a sample of viscous material while being penetrated by a needle is not laminar, as has been assumed in the development of this test. Moreover, the flow appears to be so complex t.hat it will be difficult to develop a theoretical treatment simple enough to be useful. From a practical point of view the method has some value, although in its present form it should not be classed as a sound rheological method. Although the results are not accurate, complex liquids-e. g., air-blown asphalts-can be investigated over a range of rates of shear not easily studied in the absolute risconieters now available. The consistencies of all asphalts increase with time; the rate and extent of age-hardening of any asphalt vary wit11 its source and niethod of processing. Studies (6, 7 ) made with the falling coaxial cylinder viscometer have shown that the consistency increases a t first because of the derelopment of a reversible (thixotropic) structure which is coniplirated later by the appearance of permanent hardening. Since the niethod of successive penetrations subjects the asphalt, t o higher rates of shear and shearing stresses than does the abore-mentioned yisconieter, it was thought that the unstable reversible portion of the tin?e-hartlening phenonienon might not lie detected 117 tlie penetration method. -4 number of different asplialtb h a w becn investigated by the method of succeviT-e penetrntions. The comistencies were found to increase with time a t tlie liiglier rate5 of shear just as TT-ith the vixonieteri in 17-liiclithe rates of sllear n-ere much lov-er. Tli~is,the nietliotl seeiiis to Ire nscful i i i stiiclie.. n-liich require that a large iiumLer oi saiiipl
