E. K. Ensley, H. Plancher, R. E. Robertson, and J. C. Petersen Laramie Energy Research Center Laramie. Wyoming 82071
II
Asphalt is one of man's oldest engineering materials, with a history going hack to Biblical times and before (I, 2). Barth ( 1 ) notes that the Adam and Eve earden area. between the Euphrates and Tigris rivers, contiins some df the earliest known deposits of asphalt. Perhaps the adhesion characteristics of asphalt werenoted and applied by our two early ancestors. Documented historical uses of asphalt include mortar in stone walls, mummification by the Egyptians, and shipbuilding by early civilizations. The great Ark of Noah was christened by "scattering several baskets of asphalt over her" ( I ) (loading area exempt). Last hut not least, civilizations such as the Romans and Incas had paved asphalt highways (3). Asphalt is derived from two principal sources-naturally occurring deposits and the nonvolatile portion in crude oil that is left over after distillation. Natural asohalt deoosits. in addition to those in areas of ancient civilization mentioned above, include the huge deposit on the island of Trinidad, the La Brea Tar Pits in Los Angeles, a gilsonite deposit in Utah, the Bermudex Lake asphalt in Venezuela. and the tar sands deposit near Lake ~ t h a h a s c aCanada , (2,4). Although past civilizations obtained their asphalt from natural deposits. today the major source of asphilt comes from the refining of crud(. nil. Cwitinr and ruofmg. asphalt are also derived from . the coking of coai. Crude oil refining produces two types of asphalt: (1)petroleum asphalt, or what gets left behind in distilling off the light materials such as gasoline, kerosene, and lubricating oils, (2) of further refinine bv" . . cracked asnhalt., a bvnroduct ". cracking. Asnhalt mav be imnroved or altered bv blendine. air hlowhg, or the incorporation of additives. over 70%ldf all asphalt in the US. is used for hiehwavs - . (.1. ).. 20%for roofs. and thk rest for coatings, adhesives, etc.
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Chemistry of Asphalts Althoueh asnhalt has a lone utilitarian historv. we know little ahoit thidremarkable makrial. Asphalt has been tested bv heine poked. stretched. flowed. saueezed. chewed. trod asphalt ubon, and'aged; hut the factors that constitutes and thus mnke pnssiblt. predictable nerfurmance is still not settled. A problem inherent in studying asphalt is its chemical complexity-the "no-two-molecules-in-a-barrel-are-alike" principle. Consequently, it is practically impossible to separate asphalt into chemically pure components. Therefore, the chemical studies have been limited to separation into fractions and identification of functional groups or types of compounds. Separation into Fractions and Characterization of Compound Types Separation of asphalt components usually involves solvent separation counled with chromatoeraohv. . - . either solid-linuid oigel-permeation. One of the earliest chromatographic ieparations on a oetroleum material was oerformed hv D. T. Dav a few years p;ior to Tswett's classic studies ( 5 6 ) :Day separated a crude oil using "a long glass tnhe filled with fuller's earth." Later, Day identified fractions in the column as "saturated hydrocarbms, aromatic substances, and finally nitrogen and sulfur compounds."
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656 1 Journal of Chemical Education
Hardly Know You
Asphalt-We
ASPHALT PARTIALLY DlSSOLVED IN PENTANE
I
ASPHALTENES INSOLUBLE PORTION 118 - 26901 Mol. Wf. 2.000
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20.000
1
I I
RESINS ADSORBED PORTION ~~
Mol.
~
MALTENES
SOLUBLE PORTION
I
1
( 3 0 . 42%) W f , b5O - 2 . 0 0 0
ELUTED WITH POLAR
EVAPORATE PEN1ANE:DISSOLVE MALTENES IN BENZENE; PASS THROUGH ALUMINA COLUMN
FILTRATE PORTION Mol. Wf. 3 0 0
- 700
Figure 1. Typical separation of asphalt into asphaltenes, resins, and oils (Hubbard-Stanfield method. ref. (7).p. 194). A separation procedure that separates asphalts into components (asphaltenes, resins, and oils) hased on solubility and adsorptionihromatography support properties is sho& in Figure 1(7,8). (Separation and characterization of asphalt fractions based on reactivity with sulfuric acid at various acid concentrations has been used more recently (8)).In the separation shown, the pentane-insoluble fraction contains the most polar (asphaltenes) or those with the highest apparent average molecular weieht. Actual molecular weiehts of this polar~ractionare undoubtedly lower than this gecause the polar asphaltene molecules associate to varvine deerees. dependingon the solvent used in the molecufar-keiiht dddtermination (usually determined by vapor-phase osmometry). The resin fraction is less polar than the asphaltene fraction, with the nil fraction the least polar fraction. Molecular weights are also in this order as shown in Figure 1. Polarity in asphalt molecules is due to (1)presence of heteroatoms-nitrogen, sulfur, and oxygen; (2) large atomic distances between regions of differing electronegativities; and (3) conjugated condensed aromatic ring systems that are easily polarized because of the mobility of the electron clouds (9, 10). Heteroatoms appear in many molecular arrangements in asphalts. Nltxgen appears in asphalts as a heteroatom in pyrrole-type (I1,12) and 2-quinolone-type (12) compounds, and undoubtedly the majority of the basic nitrogen present is of the pyridine type. The many ways that sulfur may he bonded in asphalt molecules have not been established, althoueh work on the more volatile comnonents of netrolenm ~woul'd suggest that sulfur occurs in both ring and open-chain form. Mild oxidation leads to the conversion of a significant amount of the sulfur to sulfoxides (13). Infrared techniques ~
~
.~ ~
~~
~~~~
and selective chemical reactions have shown that oxygen occurs in asphalts in carboxylic acid, 2-quinolone, and phenols (11, 14). Oxidation produces ketones, dicarhoxylicanhydrides other carhonyl- and ether-type struc(14), and tures. Why Asphalt Works Asphalt is a unique building material. It has a low molecular weight compared to polymers; typical apparent molecular weights for an asphalt used in road construction are from 700 to 1000. In spite of its low molecular weight, asphalt behaves physically i n a manner similar to that of polymeric materials. Why? In light of the above discussion, asphalt properties are probably a result of molecular association of the asphaltene and the polar molecules of the resin fraction. This mixture is nlasticized hv less oolar resins and oils. The association prohuces micellks all&edly spherical in shape (15, 16) o;, as proposed more recently (17), having a more linear configuration. The function of the more polar asphalt molecules is to keep the "stuff' t,ogether hy molecular association, while the less polar molecules allow the asphalt to flow under stress. Asphalt, Aggregate, and Surlace Chemistry An asnhalt navement is more than iust asphalt. Pavement also includes aggregate icommunly ralled rrxk). Cmswuen~ly. the science d'awhalt hirhwav constructinn includes the srientific disripllne of surface chrmiscry. Uwally. surface chcmistrv as a science (and not witrhrraft) requlres verv pure Ilquids and ultraclt~nnsurfxces. However, conditions of this t s p r art! impossible in pragmatic asphalt-aggregatv studies &ere "pure~asphalt"is c&lex mixture and a clean surface is the outside of a rock that has been washed with water just t o remove the dust. In soite of these nrohlems. asohalt-aagregate bonding has been examined h i such methods as film rupture (181, peel tests (19), contact angle measurements @0), and more recently heat-of-immersion tests (21,22). Why do some aggregates form durable bonds with asphalts while other asphalts are easily stripped away from the aggregate causing premature failure of the road? A good hond between asphalt and aggregate depends upon a rough aggregate surface with strong adsorption sites. For this reason, limestones are usually considered better aggregates for roads than quartzite materials. Dolomitic limestones and granites interact particularly well with asphalts; again an abundance of strong adsorption sites is probably present. Functional chemical tvoes in asnhalt and stronelv ,.. nresent . - .adsorbed on the aggregate surface include carboxylic acids, dicarboxylic
a
Figure 2. interphase region in an asphalt that formed when asphan had been in contact wim a substrate (mica)at 130'C for 14 days. The interphase or region in the asphalt adjacent to the mica surface is 0.05 mm thick.
anhydrides, 2-quinolones, sulfoxides, and basic nitrogen types 13RI \--,. What type of hond is formed between the asphalt and aggregate surface? The microphotograph in Figure 2 shows an interphase region (24) similar to an interphase region reported for polymer-substrate systems (26). An asphalt interphase region, or the structured region in asphalt adjacent to the aggregate, was prohahly the region observed by Mack (18) when he referred to an "alienment of molecules within the liquid, which often extends to a distance of thousands of molecules from the surface." Heat-of-immersion (22) and other studies (24) support the "alignment" or "interphase" concept. A typical heat-of-immersion curve, obtained with a microcalorimeter, is shown in Figure 3 and shows the flow of energy as asphalt interacts with the aggregate. The tail portion of the immersion curve is unique to asphalt-aggregate interactions. This section of the curve is perhaps a manifestation of continuous adsorntion of asnhalt molecules that is responsible for the formation of a n interphase region observed in Figure 2. The proposed continuous or "multilayer" adsorption mechanism (24) suggests that large conjugated polar or polarizahle molecules are adsorhed by an interaction site on the aggregate surface; further polarization in the adsorbed molecules is induced, which in turn attracts other polar or polarizahle molecules, perpetuating a multilayer adsorption process: The photoaranh of the interphase region (Fie. 2) is bf an asphaltdopedwkh an antistripping agent that was allowed to huild up on a mica surface for 14 days a t 130°C. The region is 0.06 mm thick. The striations indicate a graphite-like structure that had formed perpendicular to the interface. Again the large conjugated polar molecules are proposed as hring responsible for keeping the asphalt stuck to the rock just as they allegedly keep asphalt together. Now and Future Highways Why don't highways last forever? After all, the asphalt in natural deposits (gilsonite) or dissolved in crude oil has been around for millions of years hut no asphalt highway has lasted that long. A paved highway will disintegrate with little or no traffic, and, as drivers know, highways fall apart when used continuously. Of course, the asphalt's environment in the highway differs from its native environment. In a hiahwav - . asphalt. . hardening from exposure to an oxidative environment is one of the most serious prohlems. Asphalt also hardens by loss of volatile low-molecular-weight compounds (plasticizers) and by increased intermolecular interactions. The latter cause is a function of the first t,wo because oxidation produces a more polar molecule and because loss of nonpolar volatiles increases the concentration of polar molecules.~Therefore,we see that asphalt hardening, which leads to highway failure, is a comnlicated mechanism. Hizhwayi cnn also t'ail prematurely due to hrrakin:: of thc asphalt-aggregate hond ahlch is uc.;~kcnt!dvither hy conrin-
Figure 3. Heat-of-immersioncurve far an aggregate intoan asphalt. Someasphalt-aggregate systems have much higher curves and with some even zero tail heights after 2 or 3 hr are noted. Volume 55. Number 10, October 1978 / 657
uous traffic stress or by water interceding a t the asphaltaggregate bond. Neither cause can he eliminated, hut the effects can he minimized perhaps if the honding mechanism is better known. Anti-stripping agents have mitigated the harmful effects of water. These agents are principally longchained primary and secondary amines. Heat-of-immersion work (21) showed that these agents increased the asphaltaggregate honding energy. Asphalt hardening leading to asphalt pavement failure is difficult to control although some attempts have heen made to "doctor" failing asphalt roads hy in-place addition of softening agents. Recently what has gained more publicity than in-place rejuvenation is t,he recycling of asphalt. Recycling of an asphalt pavement requires removing the pavement from the road hed and transporting it to a hot-mix plant where the recycling agent is added. Heating and sometimes the addition of more asphalt, in addition to the recycling agent, aid in rejuvenating an asphalt. Any extension of a highway's life or a portion recycled will mean a savings in cost and resources. As with cheap energy, so it is with asphalt, we are running out. Perhaps a hetter understanding of the mechanisms t,hat keep our roads together will lead to optimum utilization of t,his important material. Literature Cited (11 h r t h . E. .I. "Arnhalt-Sei~ncesndTerhnolsay." Gordon and Rresch Puhlishers. N.Y.. 1962.
658 / Journal of Chemical Education
Wallacc. H. A., and Manin. .I. R., "Asphalt Pawment Engineering," MeCrsw~Hill. N.Y., 1917. Spenser. H.. "History o/ Arphol! Pouiny" in "Bituminous Materials: Asphalt%Tars and Pitrher."lMitnr Hoihere, J.),Vsl2..pnrt 1 , J o h n Wiley sndL,n%Ine..N.Y..
....., .... , . .. Heriman. E.."Chmmrtography,"Reinhold Puhlirhin~Co..N.Y.. 1961.p. 8. Mark, C.. "Physicnl Chamurry." in"Bitumin""l Materials: Asphalt.Tamand Pitcher." (Editor: Hsiherg.A..I.l,Voll., lntcmcience Puhlishers. N.Y., 1 9 1 4 , ~27. . F. S.. "hactionalCompmitim Analytical and Functional Significance: in "Rituminonr Materials: AsphslW, Tars and Pitchar." IEdilor: Hoiherg. A. 1.1. Yo1 2.,partI.,dohn Wileyend Suns. lnc.,N.Y., 1985. Smylh.C.P.."Dielectrie Behavior and Structure." McGraw-Hill, N.Y., 1 9 5 1 , ~:347. . Rrown,C. H.,Arnrr. Sci., 6064 (19721. Pcteraen,J. C.. h d 46.295 119671. Petersen, J. C.,Rarhaur,R. V., 1lorr~nee.S.M.. Rarbuur,F.A..and Holm R. Y..Anol i'hrm.. 43.149, 119711. Plancher. H., Green. R. L.,snd P e t m e n . J . C..Proc. Assac Arpholt Pouine Tcchnal. I" pns3. Peterren,J.C., Anal. Chem., 47,112119751. Laraon, 0. A , and Reuther. H.. Amer r h e m .Sot.. Div. Peiml. Chem., Prepr., l l 121, R-95 (1967). Ray. R. R., Wifherrpoan. P. A., and Grim, R. E.. J. Phys Chnm., 61.1296 (19571. Ens1ey.E. K.. J. Colloidlnier/nreSri., 53,462 (1976). Mack. C..lnd. Rni. C h r m . 49,422 (19571. Rikerms~,..l. d.. J . Motriiols. 1.34 119661. Dmn. R. I'..in "Contact Angle Wefiahihty and Adhesiun."in "Adv. in Chem. Serien." No. 43. Amer.Chpm.Soc.. Wash.. D. C., 1964.p.?lJ. Enrley. E. K.,nnd Srho1r.H. A . L I n s t Pelml., 58.9511972). Ensley. E.K . J . rnsr PeIn~l..59.279 (19731. Planchar. H.. Dnrrence,S. M..and Peterren..l. C., h e . A s w . Aspholl Pouing Techno!. I" pre,,. Enslay. E. ti.. J , A p p l C h e m Riolrrhnnl.. 25,671 (19751. Sehunhnrn. H., Moeromol~cul~s. I, I45 119681.
