1048
INDUSTRIAL AND ENGINEERING CHEMISTRY
atmospheric pressure, gas bubbles form during the fusion. The greater the surface exposed during solidification, the greater is the evolution attending subsequent fusion. During this solidification a t atmospheric pressure no gas formation ITas observed. I n these experiments t'he gaq evolved during fusion is probably due to absorption from the atmosphere. It would appear that the application of a vacuum of 1 mm. of mercury to a pure sample for 15 hours, during which time the temperature v a s raised from 125' to 200" a,nd then cooled to 125' C., mould give a gas-free sample. However, this appears not to be the case, in spite of the fact that after the first half hour of the above treatment no more gas can be seen leaving the liquid. If the sulfur a t the end of this long vacuum treatment is alloived to solidify in vacuo and is then remelted under this reduced pressure, the fusion produces so much gas a t times that the liquid appears to effervesce. As soon as all the solid has melted, no more gas is evolved even when the temperature is raised. Resolidification followed by remelting while still under the reduced pressure gives another crop of bubbles and, as in the first and all succeeding cases, the evolution ceases when fusion is complete. A t o t d of ten to twelve solidifications followed by the remelting are necessary before the solid will fuse without liberating gas bubbles. From beginning to end of this experiment the pressure was kept a t 1 mm. of mercury. Other samples of pure sulfur when treated as above gave the same results. Viscosity determinations on a sample resulting from the above treatment gave essentially the same values as a pure sulfur which had not had these repeated fusions and solidifications under reduced pressure. This behavior of liquid sulfur under vacuum indicates that the character of t'he gas removed by the repeated solidifications and fusions differs from that of the gas evolved in the
Vol. 34, No. 9
preliminary treatment. The first vacuum treatment removes the common adsorbed and dissolved gases such as air, mater vapor, aiid sulfur dioxide. What the repeated solidifications and fusions remove is open to experimentation and speculation. It is not a t all unlikely that it is, a t least in part, sulfur monoxide. Schenck (17) showed that this compound is a gas a t room temperature and that it is the first product of the oxidation of sulfur by oxygen. Literature Cited (1) Aynsley, C. E., and Robinson, P. L., Chemistry & I n d u s t r y ,
1935,392. (2) Bacon and Fanelli. J . Am. Chcm. Soc., t o he published. (2h) Biltz, H . , and Preuner, G., 2.physik. Chem., 39, 331 (1902). (3) Capelle, G., Bull. soc. chim., [4]3, 764 (1908). (4: F a n , C. C., and Macleod, D. B., Proc. Roy. SOC.(London), 97, 80 (1920); 118A, 534 (1928). (5) Hasslinger, R. von, Monatsh., 24, 729 (1903). (6) Jones, A. C., Chem. N e w s , 41, 224 (1880). (7) Kellas, A. M., J . Chem. Soc., 113, 903 (1918). 18) Knapp, F., J . prakt Chem., [21 38, 48 (1888). (9) Magnus, G., Pogg. A n n . , 92, 308 (1854). (10) Malus, C., Ann. chim. phys., [7] 24, 491 (1901). (1 1) Mellor, "Comprehensive Treatise on Inorganic and TheoreticaI Chemistry", Vol. V I , p. 986 (1926). (12) Meyer, K. H., Trans. Faraday Soc., 32, 148 (1936). (13) Moissan, H., A n n . chim. phys., [SI 10, 433 (1907). (14) Neumann, B., 2 . angew. Chem., 165, 301 (1917). (15) Ramsey and Shields, J . Chcm. Soc., 83, 1089 (1893). (16) Rotinjam, L., 2.phgsik. Chem., 62, 609 (1908). (17) Schenck, P. W., Trans. Faradag Soc., 30, 31 (1934); 2.anory. allgem. Chem., 220, 268 (1934). (18) Rchwah, J. W., unpuhlished data. (19) Taylor, H. F., M e m . Proc. Manchester Lit. h Phil. Soc., 79, 99 (1935). (20) Threlfall, R., and Brearley, J. H. D.. Proc. Rov. SOC.,(London). 56, 32 (1894); Trans. Roy. SOC.(London), A187 (1896). (21) Zickendraht, H., Ann. Physik., 141 21, 141 (1906).
Oxidation of Asphalt in Thin Films A method for measuring the oxidation of
A. R. EBBERTS State of California Division of Highways, Sacramento, Calif
OR the past several years many investigations in the field of bituminous pavements have been concerned Tvith the hardening of the bitumen with age and particularly in the mixing operation (l+,6,9,11,la). There is considerable evidence that the niajor factor in this hardening is oxidation. Kicholson ( 7 ) , Skidmore ( I O ) , and Raschig and Doyle (8) have published laboratory methods for determining the effect of oxidation with air on paving asphalts. That this reaction is photocatalyzed is well known, and Thurston and Knoides reported a laboratory method for measuring this effect (14). Ultraviolet light is commonly used for such procedures, but vork in this laboratory (not yet reported) has uncovered evidence of the potency of radiant energy in the infrared. Anderson, Stross, aiid Ellings ( 1 ) gave details of a procedure for subjecting asphalt in benzene solution to the action of oxygen under pressure in a bomb. They derived what they termed the "deterioration index" from the pressure
F
asphalts in thin films by means of potassium permanganate has been devised. It is fast and precise. It requires no special apparatus. Correlation with pavement service has been established in several instances. The effect of air blowing on asphalts is still obscure. The method will differentiate between asphalts cracked at different levels and illustrates graphically the long known fact that cracking increases susceptibility to oxidation.
loss and hardening of the recovered asphalt. This method is admittedly better adapted to research than to routine testing, clue to the time required. It has the additional drawback of requiring recovery from solution, a difficult procedure. Stross in an unpublished communication described a test in d-hich he shook together a solution of asphalt in carbon tetra-
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1942
chloride with a s o l u t i o n of persulfuric acid, using potassium perm a n g a n a t e as i n d i c a t o r . Benson ( 3 ) described what he termed the “coagdation” of asphalt spread in thin films on microscope slides. The present rep o r t covers a method for t h e oxidation of asp h a l t s in t h i n f i l m s w i t h acid potassium permanganate,
J .
= 1.5 d W
4
__ TIME
FIGURE1. PERIMENTAL
- MIN.
ASPHdLTS U S E D I N
6
EX-
PAVEMENT SECTIONS, ONE
GOOD,ONE BAD,
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D6-3QTis desirable, but any well-designed oven will suffice. If a water bath is used, results are uniformly about 20 per cent higher than those reported here, possibly due to a difference in rate of heat transfer. Flasks are removed a t intervals of 10 to 20 minutes and rinsed into other containers. Small, wide-mouth extraction flasks are convenient. Five milliliters of 0.1 N sodium oxalate solution (in 5 per cent sulfuric acid) are added and, after being heated to incipient boiling, are titrated with 0.1 AT potassium permanganate from a microburet. For the less susceptible asphalts, results are
t
T W 7 0 INTERMEDIATE
Procedure One-half gram of asphalt is weighed into a 25-ml. volumetric flask. The asphalt is dissolved and the solution diluted t o the mark with carbon tetrachloride. One-milliliter aliquots are pipetted into 50-ml. Pyrex Erlenmeyer flasks. The solvent is evaporated off with a gentle air stream. When only a few drops remain, the asphalt is mainly in a thick ridge around the edge of the film. If allowed to stand a few minutes, this will dilute with solvent and a uniform concentration will result. It is then easy to obtain an even distribution of the asphalt over the bottom of the flask by tilting slightly and rotating. The film is set with the air stream. The last traces of solvent can be removed by placing the flasks for a few minutes in an oven a t 140’ F. and then blowing out with the air stream while warm. The films can be subjected to vacuum to remove the last traces of solvent, but care t o avoid the formation of bubbles in the films is necessary. Anything that results in the film coming loose from the flask tends t o high results. This is not common, and the error caused is so large that the operator is always aware of it. Five milliliters of sulfuric acid (5 per cent by volume of 1.84-sp. gr. acid) and 5 inl. of 0.1 N potassium permanganate are added to each flask, and the flasks are placed in an oven maintained as closely as practicable to 140’ F. To avoid difficulty from nonuniform temperature within the oven, one equipped with a rotating shelf such as that described in A. S. T. M. Method
1.51
-3 TIME
60
- MINUTES
90
FIGURE 2. CURVESFOR SAMPLES WHERE ASPHALT SOURCE WAS CHAXGED DURING CONSTRUCTION
FIGURE 3. EFFECTOF PROCESSING ON AsPHALTS MADEFROM THE SAME CRUDE
reproducible within 0.03 ml. Asphalts with a high oxygen demand are slightly more erratic. Where extreme accuracy with susceptible materials is desired, an inert gas such as nitrogen can be used to evaporate the solvent from the flasks with beneficial results.
Results Obtained Figure 1 shows the results obtained with four asphalts for which service records have been obtained in experimental pavements. Curve 1 represents a stable, straight-run California asphalt with an excellent service record; curve 4 shows a highly cracked material with practically no service life. Curves 2 and 3 represent intermediate materials that give satisfactory service for a few years. Properly preserved samples retained during construction of pavement many years old are unfortunately not available. Figure 2 represents retained samples from a contract where the source of asphalt was changed during construction. After three years the pavement laid with asphalt from source A was in good condition, while that laid with asphalt from source B showed marked distress. Curves 1 and 2 represent two samples from source A; 3 and 4 represent two samples from source B. Curves 3 and 4 of Figure 2 show less oxidation than curves 2 and 3 of Figure 1, but the service behavior is slightly in favor of the Figure 1 samples. These asphalts are higher in penetration (Table I), and some slight weighting of the values would be justified on that ground. Also, it must be borne in mind that oxidation is not the only factor in age hardening, so that perfect correlation of any oxidation test with service behavior at the borderline cannot be expected.
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
1050
I
I
30
60
I 90
TIME-MINUTES
FIGURE4. Two SAMPLESMADE FROM MEXICAN CRUDE,ONE STRAIGHT-RUN TO FINALCONSISTENCY, THE OTHER FINISHED BY BLOWING
Figure 3 represents samples processed by four different methods from a California crude yielding lubricating stock. Figure 4 represents two samples made from a Mexican crude. Again it is emphasized that oxidation, although probably the most important mechanism of asphalt failure, is not the only one. Figures 3 and 4 show cases where blowing decreases oxygen demand on the one hand, increases it on the other. Yet both of these blown asphalts show high losses by abrasion after weathering by a modification of the method of Lang and Thomas (6). This may be due to a marked coagulation or flocculation of the asphalt similar to that observed by Benson (3) but hard to demonstrate. All the films exposed to the action of acid permanganate closely resemble Benson’s coagulated films and like them shorn a greatly reduced solubility in the common asphalt solvents. Figure 5 shows this coagulation clearly. Oxidation is probably not the only agency that can upset the colloidal equilibrium of an asphalt, just as it is not the only factor in age hardening. The fact that every case of lack of durability cannot be attributed to oxidation susceptibility does not detract from its importance.
Vol. 34, No. 9
A direct measure of lack of durability might satisfy the engineer, but the producer must understand its causes if he desires a cure. Figure 6 represents three samples made from a Mid-Continent crude cracked at three different levels-that is, from residues of different A. P. I. gravities. Cracking increases oxygen demand but also increases adhesion to aggregates. For this reason low-level cracking may not materially harm and may even improve the over-all results in practical paving operations with asphalts of poor adhesive qualities made from certain crudes. The permanganate method will differentiate between asphalts that are irreparably damaged by cracking and those that are not. Results roughly parallel those of the heptane-xylene equivalent ( 2 ) . Figure 7 represents three straight-run asphalts from different crudes, all of which have satisfactory service records. A curve for paraffin wax is shown for comparison. I n some cases a low oxygen demand goes with a low adhesion tension for common road aggregates. I n such case failure may be due to stripping of the asphalt in the presence of moisture. Paraffin wax has a low oxygen demand with a n induction period in excess of 50 minutes but such a material would not make a good pavement. It lacks other qualities. Petrolatum is intermediate in oxygen demand between paraffin wax and the best asphalts. Asphalt technologists have lacked real criteria of quality for so many years that they have resorted to identification tests as safeguards. The above described method gives a reliable measure of quality in so far as resistance to oxidation only is concerned. Used as a routine test it would be unnecessary to run out the curves. -4single run a t a fixed time could be selected and limits of oxygen demand specified. The time should be long enough to allon a clear differentiation between samples and probably be not less than 60 minutes. I n Figure 1, curves 2 and 3 appear about to cross around 50 minutes, and this was confirmed by running out these two curves after the chart was drawn.
FIGURE 5. PHOTOMICROGRAPHS OF A STRAIGHT-RUX CALIFORNIA ASPHALT OF 119 PEKETRATION, FILMFORMED ACCORDING TO BENSON
INDUSTRIAL AND ENGINEERING CHEMISTRY
September, 1942
1 ...“*.................. -7 0.2
..*...*
*
I
. 1.0r
1051
9-1
e
)-e-
”_..
oat"" .e/
x
r
--
1
............. ***....,*..............**’.
PARAFFIN WAX-, I
-L.-.--,A---
30
60
T I M E - MINUTES
90
FIGURE7 . THREESTRAIGHT-RUN ASPHALTSFROM DIFFERENTSOURCES WITH SATISFACTORY SERVICE WITH PARAFFIN WAX RECORDS, COMPARED
TIME-MINUTES
FIGURE 6. SAMPLESFROM A MID-CONTINENT CRUDE CRACKED AT THREE DIFFERENT LEVELS
All the asphalts covered by the Figures 1-4, 6 , and 7 are paving grades of consistency usually specified by penetration.
2.0i
-
2
-
d
-
Penetrations, softening points, and heptane-xylene equivalents are given in Table I. Under the conditions prevailing in these experiments any such asphalt that consumes as much as 1.2 ml. of 0.1 N potassium permanganate.in 90 minutes may fail rapidly in service. If between 1.2 and 0.8 ml. of potassium permanganate are consumed, long-range service is doubtful. Below a consumption of 0.8 ml., failure due solely to oxidation is improbable. TABLEI. PROPERTIES OF ASPHALTS, Fig. NO. 1
Penetration
Softening Point, ’ F. 118.3 109.5 106 109
4
96 89 101 95 62 60 52 61
Straight-run Blown Cracked Cracked blown Straight-run Blown
94 10s 107 103 100
Curve 1 2 3 4
2
1
2
3 3
4 6 7
1
2 3
%%die Bottom
97
108 92 96 97 103 101
... ... ... ...
H e t s n e Xylene 8qquivient 15-20
loof
80-85
loof .,.
2lj125 20-25
110 112
... ...
110 119.5 144
...
108
109.5 112 111 110 119.5 117
TIME-MINUTES
FIGURE8. COMMERCIAL SAMPLES OF SLOW-CURIKG ROADOILSFROM A SINGLE REFINERY
... . .
... 95-100 Light spot 100 Very dark spot 100
... ...
...
Figure 8 represents slow-curing liquid asphalts of grades SC-1 t o SC-6, inclusive, all from the same refinery. The oxygen demand decreases markedly as the liquid asphalts increase in viscosity, This confirms the conclusion of Thurston and Knowles (14) that the oily constituents of an asphalt are the more readily oxidized. It is probable that the various hydrocarbons present in the oily fractions vary in suceptibility to oxidation as do those of the other fractions. Otherwise highly blown asphalts would be entirely lacking in oily constituents, and that is known not to be the case. Swanson (IS) and Lang and Thomas (6, page 58) also recognize the importance of these fractions. As data accumulate, it is hoped the method will throw some light on the curing rates of various slow-curing road oils.
Since this will require correlation with curing under field conditions, no data are offered a t this time.
Acknowledgment The author is indebted to the Shell Development Company for many of the samples of asphalt used in this work.
Literature Cited (1) Anderson, A. P., Stross, F. H., and Ellings, A., IND. ENG.CHEM., ANAL.ED., 14, 45 (1942). ( 2 ) Benediet, A. H., Assoc. Asphalt Pavine Tech., Proc. Tech. Sessions, 11, 13 (Jan., 1940); (3) Benson, J. R., Ibid., 9, 63 (Dee., 1937). (4) Hnbbard, Prevost, and Gollomb, Henry, Ibid., 9, 165 (Dee., 1937). (5) Lang, F. C., and Thomas, T. W., Univ. Minn. Ens. Expt. Sta., Bull. 15 (Nov., 1939). (6) Lewis, R. H., and Wellborn, J. W., Assoc. Asphalt Paving Tech., Proc. Tech. Sessions, 12, 14 (Deo., 1940). (7) Nicholson, Victor, Ibid., 9, 208 (Dee., 1937). (8) Raschig, F. L., and Doyle, P. C., Ibid., 9, 215 (Dec., 1937). (9) Shattuck, C. L., Ibid.,11, 186 (Jan., 1940). (10) Skidmore, H . W., Ibid., 12, 69 (Deo., 1940). (11) Skidmore, H. W., and Abson, Gene, Ibid., 9, 195 (Dee., 1937). (12) Steinbaugh, V. B., and Brown, J. D., Ibid., 9, 138 (Dec., 1937). (13) Swanson, E. G., Roads and Streets, 84, 31 (1941). (14) Thurston, R. R., and Knowles, E. C., IND. ENG.CHEM.,33, 320 (1941).
