Flow Properties of Asphalts Measured in Absolute Units R. N. TRAXLER The Barber Company, Inc., Barber, N. J.
(e. g., most air-blown asphalts) can best be evaluated with a rotating cylinder type of viscometer. Properties of asphalt that have been recognized by means of sensitive rheological methods are the existence of viscous and nonviscous asphalts, the phenomena of age hardening, thixotropy. and elasticity, the precise evaluation of change of consistency with temperature (susceptibility), and the evaluation of mineral powders as fillers by measuring the viscosities of viscous asphalt-mineral mixtures.
The asphalt technologist is confronted with the problem of measuring a wide range of consistencies. Methods are described for measuring low and high viscosities in absolute units and for evaluating the flow of nonviscous fluids. The capillary tube or rotating cylinder type of viscometer is recommended for low viscosities (less than 50,000 poises). The latter instrument, or a viscometer utilizing the principle of the falling coaxial cylinder, is suggested for measuring the viscosity of highly viscous asphalts. The flow properties of nonviscous bitumens
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N THE processing, handling, and utilization of asphalts,
modification of the Bingham-Murray capillary tube instrument (11, 97) or the convenient viscometer developed by Rhodes, Volkmann, and Barker (14) are well adapted to the measurement of viscosities up to about 1,000,000 poises. Other methods, considered below, may be more rapid than, and as accurate as capillary viscometers for viscosities above 50,000 poises.
their flow properties are very important; the usefulness of such materials in service is also dependent to a great extent upon their rheological characteristics. Although numerous empirical tests such as penetration (2) and ring and ball softening point (3) have been widely used to evaluate the consistency of asphalts, most of them measure two or more properties simultaneously. The purpose of this paper is to describe briefly the rheological methods which have been useful in investigating the flow properties of asphalts and to point out some of the advantages to be obtained with these absolute methods of measurement. Asphalt is handled and used in a molten condition, as a "cutback" with naphtha as the solvent, or as a water external-phase emulsion. These materials possess low viscosities. Under usual service conditions, however, the bitumen is seldom subjected to temperatures above 60" C. (140" F.); many asphalts undgr these conditions have a high resistance to deformation. Consequently, the asphalt technologist is confronted with the problem of measuring a wide range of consistencies. Practically, it would be difficult to develop an instrument which would evaluate rapidly and accurately all types and consistencies of asphalt over the temperature range-desired.
Measurement of High Viscosities Measurement of the %ow properties of asphalts a t atmospheric temperatures is a difficult problem because in most cases the bitumens have a high consistency. Furthermore, many asphalts, especially those employed in the manufacture of roofing and for various industrial purposes, are elastic and exhibit anomalous flow characteristics to varying extents (6, 81) ; methods for evaluating these characteristics are discussed later. Instruments and methods for measuring high viscosities (above 50,000 poises) have been described, and some of the advantages and disadvantages of the various methods considered (27). In this laboratory the alternating stress method proposed by Bingham and Stephens (4) has been successfully used (11,85,,%) as a research instrument for measuring, in absolute units, asphalts of paving consistency. However, the time and care required for preparing the sample and making the test renders it unsatisfactory as a control method. For viscous asphalts a t service temperatures, the falling coaxial cylinder viscometer suggested by Segel (18) and studied by Pochettino (12) has been found very satisfactory and can be used to measure a wide range of viscosities. A viscometer utilizing the principle of the falling coaxial cylinder has been used for obtaining data on a large number of asphalts and asphaltic mixtures (WO) possessing a wide range of viscosities. Figure 1 shows the working parts of the viscometer.
Measurement of Low Viscosities Several types of apparatus are being used to measure low viscosities (up to about 50,000 poises) in empirical units. Among these are the MacMichael, Stormer, and Saybolt instruments. Over a certain range Saybolt-Furol seconds may be converted into absolute units, or poises (8). For the direct determination of viscosity (in absolute units) of fluid asphalts, either a rotating cylinder, a falling ball of the type proposed by Broome and Thomas ( 5 ) , or a capillary tube viscometer is satisfactory. The simple 322
MARCH, 1938
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Consistencies of Nonviscous Asphalts &tends from top to bottom of inner cylinder B. Hot as6halt u testdd, is inserted into the 13akelit,e holder, E, the amalgamated base plate is removed, and the outer cylinder ia firmly clamped in
At atmospheric temperatures air-biown asphaits and certain hard vacuum or steam-refined materials are complex liquids. With increasing consisteney (decreasing penetration) the amount of nonpermanent (elastic) deformation shown by such asphalts becomes greater; in general, the deviation from true viscous flov aLsu increases as the asphalt becomes harder. Further, the measurement of flow is frequently compiicated by the appeerance o i thixotropy, by the occurrence of slippage in the viscometer at high rates of shear, or by the effect of heat generated within the sample. The complex flow properties of various hard asphalts have been discussed in considerable detail elsewhere (6, Sf). A rotating-cylinder viscometer similar in principle to that proposed and discussed by Mooney and Ewart (9) has been used in the study of asphalta that are highly eiastic, complex liquids. This instrument is illustrated diagrrarnrnatically in Figure 2. Theoutor cup, A, has an internal diemeter of 2 inches and is 1.25 with respect to the outer cylmder aGd 1s mounted on hrtrrlend
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of the outer c&dcr. 'i'tie tu& is applied in the farm Gf weights C ataached t,o cords which pass over pulleys I> and are nrapped around drum E rhich is attached to t.he inner cylinder. Kate of rotation is measured on s circular steel scale not shown in Figure 2; the viscometer is immersed in a constant-temperature water bath. also omittcd.
Asphalts of known viscosities are used to calibrate the instrument. Calibration is necessary to correct for the end effects due to the asphalt above rotor B. If a slight pressure head is not maintained on such a short annulus, most asphalts will creep away from the rotor, causing large errors in the vis-
J . The sample &d holder, E. are surrounded bv 8 constantkern-
of levers L,rocker arms K are moved so as t o hft a weliht from either rod H or M vhich results in a force acting on iMo
