January 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
would prove difficult to accomplish not only because of the d 8 cult accessibility of copper and brass parts but also because of voltage drop through the solution, within parts and across metal interfaces. The method depends on the continuity of the solution phase-in effect, immersion-across the zinc-copper interfaces. Furthermore, a free standing droplet of the washing solution on any cuprous surface can freely dissolve the metal independent of any zinc coupling action. Aside from the foregoing measures to control metallic staining of silverware there are a number of suggestions relative to practical operation which may be helpful. Make up wash solutions only immediately before use. Wash silverware before all other items. Avoid the use of old solutions. Avoid washing aluminum and silverware together. Determine the minimum time for satisfactory washing of silverware and avoid longer exposure. Wash a t the lowest suitable temperature. Renew the wash solution as frequently as practicable. Minimize the exposure of detergent solutions to cuprous metals by replacing these metals where possible. Two tank-type machines may permit more staining than single tank machines because of design for less renewal of wash solution.
IS5
zinc. Iron and Monel metal do not cause thii effect, or if so, a t a very slow rate. Machine parts are the common source of copper. Exposure of the base metal of silverware itself through imperfections in the plate also may lead to local staining. This latter defect may be overcome by replating the silverware. A general means of overcoming the plating problem is to eliminate all aluminum, zinc, or other active metals from the machine structure or load being washed with the silver. Polyphosphates are the agents in dishwashing detergents mainly responsible for solubilizing copper. Although it is impracticable to eliminate them because of their valuable water conditioning properties, their activity can be diminished by formulating with an excess of silicates. A 50% excess of sodium metasilicate pentahydrate over the polyphosphate content of dishwashing formulations affordsa practicable measure of control. Electrodeposits of copper do not appear to form except under conditions where a strongly electronegative metal is actively dissolving. Development of films or corrosion products on the surface of the electronegative metal influences the rate of sohtion and is reflected in tarnishing behavior. Tarnishing has been observed with solutions of copper content as low as 0.5 part per million in the laboratory and possibly as low as 0.25 part per million.
SUMMARY
LITERATURE CITED
Under certain conditions copper dissolved by polyphosphated detergent solutions from dishwashing machine parts may deposit on silverware causing a brassy or dark colored tarnish. This form of tarnishing occurs when silverware is in electrical contact with a more electronegative metal, such as aluminum or
(1) Bacon,
L. R., and Nutting, E. G., Jr., IND. ENO. CHEM.,44,
146 (1952). RECEIVED February 23, 1951. Presented before the Division of Water, Sewage, and Sanitation Chemistry a t the 119th Meeting of the AMERICAN CHEMICAL SOCIETY, Cleveland, Ohio.
Asphalt, a Colloidal Material R. N. TRAXLER AND J. W. ROMBERG The Texas Co., Port Neches, Tex.
T
H E physical and colloidal properties of materials as complex as asphaltic bitumen logically assume greater importance than their chemical characteristics. Unfortunately very little information is available concerning the nature of the numerous different hydrocarbons present in asphalts. Richardson (12) discussed the early work done on the identification of certain ring compounds present in asphaltic bitumen. More recently Sachahen (14) has reviewed much of the data available on the chemical constitution of asphalts, and ultimate analyses on various fractions of different asphaltic materials were published by Hillman and Barnett (3). All asphaltic materials contain some sulfur, and vanadium is to be found in most of them. Ring compounds have been found to predominate in the high boiling fractions of petroleum, and the progucts vary from naphthenes containing one or more rings with attached alkyl groups to hydrocarbons containing naphthene rings and an aromatic ring. The assumption that these types of compounds occur in asphaltic residua and in asphalts may be fallacious, but such an assumption gives a concept of the type of hydrocarbon present. This concept is confirmed, more or less directly, by certain facts presented in this paper. Empirical physical tests have been used for a long time to obtain information concerning flow characteristics, susceptibility to change in temperature, adhesiveness, and other properties of practical importance, but only in recent years have certain absolute methods of measurement been developed. The realiaation that asphalts are colloids has been a great stimulus to the understanding of their properties and, in fact, it is only when asphaltic bitumen is studied as a colloid that real progress is made
toward a comprehension of its behavior under various conditions. The idea that asphalts are colloidal systems is generally considered to have been first proposed by Nellensteyn (6) in 1923, who suggested that these materials are composed of micelles dispersed in an oily medium. He conceivedthat a patticle of carbon formed the nucleus of each micelle and that the nucleus was surrounded by layers of adsorbed asphaltenes, each successive layer being composed of hydrocarbons of lower molecular weight and a higher hydrogen to carbon ratio. Dispersion of the micelles was considered to be maintained by the presence of asphaltic “resins,” which were thought of as protective colloids or stabilizing agents. Nellensteyn’s general theory has been widely accepted but his idea of a carbon nucleus for each micelle has been the subject of considerable discussion and study. Labout in Pfeiffer’s (9)recent book on “The Properties of Asphaltic Bitumen” discusses the constitution of asphaltic bitumen, including chemical composition and colloid chemistry. He mentions that the only conclusion that can be drawn from the study of x-ray diagrams “is that in the asphaltenes there is a certain arrangement of the aromatic rings.” There is no evidence of the existence of graphitic carbon in these asphaltenes. Recently, Eilers ( 2 ) has discussed the composition and properties of asphaltenes including comments on molecular weights by cryoscopic methods, measurement of viscosities of dilute solutions, and spreading tests on the Langmuir apparatus. Pfeiffer and his associates ( I O ) have proposed explanations for the flow characteristics of various kinds of asphalt on the ’basis of their colloidal structure. These investigators believe that hydrocarbons with the greatest molecular &eight and most
INDUSTRIAL AND ENGINEERING CHEMISTRY
156
decided aromatic character are arranged closest to the nucleus of the micelle. Surrounding these are successive layers of lighter compounds of less aromatic nature, until a gradual and almost continuous transition into the intermicellar phase is formed. These investigators point out that there is no clear boundary between the oily constituents and resins or between the resins and asphaltenes. When the system contains sufficient rnsins or protective colloids t o fully peptize the heavy asphaltenes, a sol type of asphalt results. On the other hand, a gel type of bitumen occurs when, owing to a paucity of stabiliing agents, the micelles are not well dispersed and tend to form bonds because of mutual attractions. Pfeiffer and coworkers looked upon the gel structure as irregular open packing, the spaces of which are filled by the intermicellar liquid.
TABLE I. PROPERTIES OF ASPHALTS POSSESSING DIFFERENT COLLOIDAL CHARACTERISTICS
A Bo1 &:P,fE1 type Air blown Method of processing Properties 1.016 Density at 25' C. 50 ASTM softening point R&Bo C. AST.M ductility at 25O'C., 5 cm. per 200 minute, oms. ASTM penetration, 100 grams for 5 50 8ecs. at 25' C. Viscosity, megapoisee' a t 25' C. and a Dower inout of 1000 erm - sec. -1
+
cni.-a
-
3.2
B Sol-gel Air blown
C
Gel Air blown
Vol. 44, No. 1
Eilers also used the viscosity of solutions of wphalt components in pure solvents as "a simple way t o obtain an insight into their colloidal properties, especially so when the results are expressed in terms of the voluminosity of the solute, ie., the volume that s e e m to be occupied in the solution by the quantity of solute of unit volume in the dry state." The voluminosity, u, is calculated from the Einstein equation
NF = 1
+ 2.5 VC
(1)
where N , is the relative viscosity of the solution, and C the concentration of solute in parts by volume. In Table I, data are given for asphalts A , B , and C, which illustrate the different properties associated with the sol, sol-gel, and gel states, respectively. At this point attention is drawn to the fact that the amounts of high molecular weight, dark-colored materials precipitated by the nonpolar solvents are about the same for asphalts A and C , but are considerably higher for the transitional sol-gel material, asphalt B. These data indicate in that the qqantity of heavy materials-asphaltenes-present an asphalt is not a major factor in determining whether the material will show sol or gel characteristics. Asphalts A , B , and C will be used to illustrate the dependence of physical behavior upon the colloidal state of the system.
1.025
0.984
164
5.5
55
53
MICROSCOPIC EXAMlNATION
4.4
23
Schwarz (16) proposed a method for classifying bitumen, which comprised etching the surface of the solid bitumen with solvents, such as benzene, carbon tetrachloride, and carbon disulfide, for a few seconds and then photographing the surface. Traxler and Coombs (16)found that ethyl ether and light petroleum naphtha were more satisfactory solvents. Definite surface patterns were evident in photographs taken at 220X with a Leitz Ultropak microscope when a gel-type asphalt was used. When a sol-type was treated, little or no surface pattern was visible even when an aged asphalt surface was used. Although the obvious deduction is that some type of structure exists within the gel-type asphalt, the surface designs obtained cannot be assumed t o show the structure actually existing within the asphalt. The presence of appreciable amounts of solid paraffin wax may be detected in an asphalt by spreading a thin translucent film of the hot asphalt on a microscope slide and observing by transmitted light a t 450 X. If more than 1 or 2% of solid paraffin wax is present, it will appear as crystal-like masses; the use of a polariscope makes the identification more positive. High solid paraffin content, and especially over-all paraffinic content are found to be associated with the gel type of asphalt. This situation may be caused in certain instances by the formation of a structure within the asphalt by the solid paraffins, but more generally the gel state exists because the asphaltenes or micelles are flocculated by paraffinic hydrocarbons present in the continuous phase.
55
65.6
1.0 0.80 0.50 Degree of complex flow, E 0.02 0.08 0.21 Asphalt agin index b Elasticity, re%xation one-half time, 1 3 13 secs. Solubilities Insoluble in entane, % 20.6 29.7 24.2 Insoluble in $quid propane, % 37.5 44.6 38.3 a One me apoise 1 000 000 poises. b AAI is &e slope at io0 Lours of a log-log plot of viscosity versus time in hours.
-
The differences in flow and other characteristics evident in various a5phalts are caused by the presence of the sol or gel state, or of the many intermediate gradations which may exist between the two extreme states. Much of the work done in recent years confirms the concept that asphalts are colloidal systems, and that the rheological and other properties used to evaluate a material merely reflect the degree of dispersion or solvation existing within the system. Consideration is given in this paper to some of the most outstanding manifestations of the colloidal nature of asphaltic bitumens. SOLUBILITIES IN ORGANIC SOLVENTS
The high molecular weight, dark-colored asphaltenes and resins, which comprise a large portion of the micelles mentioned in the discussion above, are insoluble in solvents such as light petroleum naphtha (pentane), liquid propane, and normal butanol to mention only a few, whereas the oily malthenes or petrolenes which compose a large part of the continuous phase in the asphalt are soluble in these organic liquids. Other solvents such as acetone and furfural may be used to selectively dissolve certain of the hydrocarbons present. I n every case, the amount and, consequently, the nature of the material dissolved are dependent not only on the solvent used, but on its volume relative to that of the asphalt present, and t o the temperature maintained during the extraction. Eilers ( 2 ) reported extensive studies on the solubility of asphalts in a number of widely differentnonpolar organic solvents. He found the per cent insoluble to correlate with the internal pressure and surface tension of the solvent. Mexican asphalt of 4.0 to 50 ASTM penetration a t 25" C. was completely soluble in nonpolar solvents with an internal pressure, uV-'/8, greater than 4.5. I n this expression, u equals surface tension and V equals molecular volume of the solvent.
RHEOLOGICAL CHARACTERISTICS
Flow properties, both viscous and elastic deformations, a t service temperatures are of primary importance in the utilization of asphaltic materials. Various empirical tests, such as the ASTM penetration tests, softening point, and ductility tests, have been used by producer and consumer for many years in an attempt to evaluate these characteristics, and to determine the applicability of the asphalt for specific uses. The evaluation of the rheological properties of asphalt a t service temperatures in absolute (centimeters per gram per second) units presents many problems because of the extremely high consistencies encountered. Fortunately, rapid strides have been made during the past 15 years in the development of apparatus capable of measuring the consistency, in absolute units and a t constant rates of shear, of materials of high consistency (1). Considerable attention has also been given to the best method for
January 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
evaluating the consistency of a non-Newtonian material. Values obtained at a constant power input (1000 ergs sec.-l om.-*) have been proposed as most convenient and informative for non-Newtonian materials. A simple rBeological diagram is an arithmetic plot of shearing stress in dynes cm.-* as abscissa versus rate of shear in sec.-l as ordinate. On such a plot viscous materials give straight lines whereas non-Newtonian liquids are represented by curved lines. If the same data are plotted on log-log coordinates a more informative rheological diagram results as straight lines are obtained with most asphalts for both types of flow. On the log-log type of plot, which is illustrated in Figure 1, the slope, with respect to the rate of shear axis, of the line for simple viscous flow is always 1.0, whereas that for complex flow is usually less than 1.0. Asphalt A , which is indicated to be an essentially simple liquid in Figure 1, is the sol-type asphalt shown in Table I. Asphalts B and C are represented by lines with slopes of 0.80 and 0.50, respectively. These materials are the sol-gel- and gel-type asphalts described in Table I. It is convenient to use the slope of the line on the log-log rheological diagram as a measure of the degree of deviation from viscous flow and this value, which is listed in Table I as degree of complex flow, c, is one indication of the colloidal nature of the asphalt. Only the well-dispersed sol-type bitumens-e.g., asphalt A-show essentially viscous flow at service (atmospheric) temperatures. As the dispersion beconies less complete and gel characteristics begin to appear, the flow becomes non-Newtonian or complex in character. The value of %”-the elope of the lines in Figure 1-may drop to 0.30 but such a low value is exceptional. The chemical nature of the hydrocarbons in the petroleum from which an asphalt is prepared obviously affects the colloidal properties and, consequently, the flow characteristics of the finished asphalt. Methods and degree of processing, also, have a marked effect on the colloidal and rheological properties of the material. Air blowing, in general, results in the appearance of more gel-type characteristics (complex flow) than are obtained by vacuum or steam distillation. The air blowing process tends to convert the protective colloids or stabilizing agents present in the asphaltic residuum into asphaltenes, thus reducing the dispersing power of the continuous phase in the asphalt and increasing the amount of material to be dispersed. As the temperature of an asphaltic bitumen is raised, complex flow, if present, diminishes and may disappear entirely if the temperature is increased sufficiently. Illustrations of the effects of source, method of processing, degree of processing, and temperature on the rheological and, thus, on the colloidal properties of asphalt have been given by Traxler, Schweyer, and Romberg (17). All asphalts show some increase in consistency with time thatis not caused by loss of volatile components. Considerable experimental work involving the use of absolute viscosity data has established that the age hardening of asphalt is dependent upon the source, the method and degree of processing, and the temperature at which the bitumen is maintained during aging. A sol-type asphalt, such as asphalt A , will show a low rate of age hardening as indicated by the asphalt aging index (see AAI in Table I). As the degree of complex flow increases (value of c becomes less), that is, as the asphalt becomes more definitely gel-like in nature, the rate of age hardening (AAI) increases rapidly. This is illustrated by the data given for asphalts B and C in Table I. The appearance of thixotropy in asphalts is further evidence of their colloidal nature. Complex flow, age hardening, and thixotropy are all dependent upon structure within the colloidal system. As the micelles are better dispersed in the continuous phase, all three phenomena tend to decrease in magnitude. Many of the uses for asphalt depend on the ability of the material to deform without rupture and to recover elastically from small deformations. For viscoelastic materials, such as asphalt, this recovery is partial and frequently talres place over a considerable period of time. Also, the complications encountered rule out
157
many of the methods commonly used for determining the modulus of elasticity. Relaxation of stress has been studied recently ( l a ) as a means of evaluating the elastic properties of asphalt. If the relaxation is not too rapid a fair idea of the elasticity is obtained by taking the time required for the stress in a rotary viscometer to decrease to one half of its original value. An inspection of the data in Table I shows that as the degree of complex flow increases, the asphalt becomes more elastic. This correlation is not surprising because both phenomena Drobably have their origin in the same colloidal structures.
SHEARINO
STRESS. DYNE C W r
Figure 1. Rheological Diagrams for Asphalts on Log-Log Coordinates Change in consistency with temperature, commonly referred to as temperature susceptibility, is associated mith the colloidal nature of the bitumen. With the degree of dispersion within the asphalt changing with temperature, the same material may present an entirely different colloidal condition a t two different temperatures. Further, the measurement of temperature susceptibility ia complicated and confused by the presence of complex flow. Because of these facts most of the published data give erroneous ideas concerning the reactions of the asphalts to temperature change. Data obtained at’high temperature are inadequate for estimating the properties of the same sample a t atmospheric temperatures. DISPERSION AND FLOCCULATION OF ASPHALTENES
Mack (6), in a pioneering study of the constitution of asphaltat investigated the dispersing power of various amounts of petrolenea from different sources for asphaltenes from several asphalts. Among his conclusionshe stated: “Sols of asphaltenes from different sources in petrolenes whose oily constituents are solvents for asphaltenes are viscous liquids of high viscosity. Asphaltene sols in petrolenes whose oily constituents do not dissolve asphaltenes show plastic flow.” I n a somewhat later publication Pfeiffer and van Doormaal(f1) came to the same general conclusion and also pointed out that the aromatic nature or content of the petrolenes is responsible for any ability they may possess for dispersing the asphaltenes. T h e n asphaltenes from a particular source are blended with petrolenes from different asphalts, the mixtures have properties similar to the asphalts from which the oily fractions were obtained. For example, a blend made using petrolenea from a sol asphalt also has the properties of a sol-type material.
158
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
Vol. 44, No. 1
BETWEEN STAININQ PROPENSITY AND TABLE11. DISPERSION AND FLOCCULATION OF ASPHALTENES TABLE111. RELATION OTHERPROPERTIES OF ASPHALT Blend I Blend I1
Composition Asphaltgnes and resin, % Aromatic hydrocarbons, % Paraffinic hydrocarbons, %
Degree of complex flow,c Stain index
42.7 57.3
28.6
71.4
1.048 52.8 104 77
0.994 53.9
110
2.0 0.80 1
10
Process Asphalt Teste ASTM eofteninc Doint. R&B‘ C .
Sol-Type Asphalts Air Blown A AA 50
58.9
Gel-Type Aspaltsh Air Blown C cc 65.6
138.9
16
2.1 0.50
Thus, i t may be concluded that the chemical nature of the oily, dispersing (continuous) phase in an asphalt regulates to a great extent the colloidal and rheological properties of the bitumen. The asphaltenes from different sources appear to behave about the same, although obviously the amount of these high molecular weight materials present will affect the fundamental characteristics of the asphalt. An illustration of the dispersing and flocculating ability of the different materials present in the oily (continuous) phase of an asphalt is given in Table 11. A hard asphalt of 20 ASTM penetration a t 25” C. was fractionated by treatment with n-butanol and the insoluble material freed of solvent. After removing the solvent from the n-butanol-soluble portion the resulting heavy oil was fractionated with acetone at a low temperature to dissolve out the aromatic materials of high density and high refractive index. The material insoluble in acetone was the more paraffiic hydrocarbons of lower density and refractive index. Blends of the hard fraction (n-butanol-insolpble) were made with each of the oil fractions to give products of about 2,000,000 poises viscosity a t 25” C. It will be noted from the data, given in Table 11, that blend I, made with the high density, high refractive index (aromatic) material, is a transitional or sol-gel-type asphalt possessing only moderate complex flow, whereas blend I1 made with the paraffinic oil is a gel-type material with a high degree of complex flow, although it contains considerably less of the high molecular weight, asphaltene-resin material.
If a bitumen of the sol type is placed in contact Rith one of the gel type, a fluxing action may occur and the asphalts are said to be incompatible. This phenomenon which occasionally appears in prepared asphalt roofing under service conditions was first discussed by Oliensis (8) who developed a laboratory procedure by which two asphalts could be tested for incompatibility. More recently he has discussed ( 7 ) in considerable detail the reactions between different asphalts. Eilers (2)has explained this incompatibility between two asphalts as due to the oily phase (intermicellar liquid) sweating from the gel-type asphalt and thereby fluxing or softening the sol-type asphalt. The presence of unbound intermicellar liquid also is probably the cause of staining of paper and other materials in contact with bituminous material. Considerable amounts of asphalt are used as an adhesive for various purposes and especially for bonding together sheets of paper in the manufacture of shipping cartons. Certain asphalts when used for this purpose yield an oily exudate which stains the paper to an undesirable extent. A test has been developed ( 4 )which makes possible a quantitative comparison of the staining tendencies of different asphalts. The molten bitumen is poured into a mold and when the material has solidified, a number of disks of cigarette paper are placed in contact with the asphalt. The mold is screwed into a holder and the paper disks are held in firm contact with the asphalt by a constant air pressure. After an established number of hours, at a constant temperature, the apparatus is taken apart and the number of stained papers are counted, omitting the sheet next to the asphalt surface. The number of stained papers is called the stain index of the bitumen. This test is not only of practical value, but is of help in understanding the colloidal constitution of the asphalt. Table I11 shows the effect of the colloidal nature of
tain index
3
4
10
four asphalts on their staining propensity. The well-dispersed sol-type asphalts A and AA, which were made from the same crude source and which show little or no deviation from simple, viscous flow, have low stain indexes. Sample A A which had been processed (air blown) more than sample A shows slightly more staining propensity. This would be expected because the hard sample ( A A ) also showed slight evidence of complex flow (value of c = 0.05) indicating the appearance of some gel structure. On the other hand, the flocculated or less dispersed geltype asphalts C and CC, which were both air blown from the same residuum, showed marked deviations from viscous flow and relatively high stain indexes. Again, the more highly processed sample (CC) shows the greatest degree of complex flow, the most pronounced gel characteristics, and the highest stain index. These examples indicate clearly that staining, which results from exudation of oil from the asphalt, is a manifestation of the colloidal condition of the asphalt. LlTERATURE CITED
Alexander, J., “Colloid Chemistry,” Vol. VII, p. 501, New York, Reinhold Publishing Co., Inc., 1950. (2) Eilers, H., J . Phys. & Colloid Chem., 53, 1195 (1949). (3) Hillman, E. S., and Barnett, B., Am. SOC. Testing Materials,
(1)
Proc., 37,II, 558 (1937).
Knowles, E. C., McCoy, F. C., Schweyer, H. E., and Wilkinson, C. E., IND. ENQ.CHEM., 4 2 , 2 3 4 0 (1950). ( 5 ) Mack, C., Proc. Assoc. Asphalt Paving Technol., 5 , 4 0 (1933). (6) Sellensteyn, F. J., J.Inst. PetroZeum Technol., 10, 311 (1924). (7) Oliensis, G. L., “Exudation and Allied Reactions between Bitumens,” Forest Park, Ill., Chicago Bureau of Bituminous Technology, 1949. (8) Oliensis, G. L., IND. ENG.CHEM.,ANAL.ED.,10, 199 (1938). (9) Pfeiffer, J. P., “The Properties of Asphaltic Bitumen,” New York, Elsevier Publishing Co., Inc., 1950. (10) Pfeiffer, J. P., and Saal, R. N. J., J . Phgs. Chem., 4 4 , 139 (1940). (11) Pfeiffer, J. P., and van Doormaal, P. M., J . I n s t . (4)
Petroleum Terhnol., 2 2 , 414 (1936).
(16)
Richardson, C., “The Modern Asphalt Pavement,” New York, John Wiley & Sons, 1908. Romberg, J. W., and Traxler, R. N., J . Colloid Sci. 2 , 33 (1947). Sachanen, A. N., “The Chemical Constitution of Petroleum,” New York, Reinhold Publishing Corp., 1945. Schwarz, F., Asphalt u. Strassenbau, No. 5 , supplement of Petroleum Z.,28 (1932). Traxler, R. N., and Coombs, C. E., IND. ENG.CHEM.,30, 440
(17)
Traxler, R. N., Schweyer, H. E., and Romberg, J. W., Ibzd., 36,
(12) (13) (14) (15)
(1938). 823 (1944). RECEIVED October 3, 1950. Presented as part of the Symposium on The Nature of Bituminous RIaterials before the Divisions of Gas and Fuel, Colloid, and Petroleum Chemistry at the 118th Meeting of the AVERICAN CHESIICAL SOCIETY, Chicago, Ill.
