Asphalt Oxidation Studies at Elevated Temperatures

8/13/2019 Asphalt Oxidation Studies at Elevated Temperatures

1/4

ASPHALT OXIDATION STUDIES A TELEVATED TEMPERATURES

F . J . H U G H E S Research and Development Department, American Oil Go., Whiting , IndAsph alt expos ed to the weat her is af fect ed b y many var iabl es that can cause i t to lose plast ic i ty . Al thoughabsorpt ion of ox yg en b y asphalt is wel l know n, the mechanism is not fu l ly un derstood. Thin-f i lm ovenox id a t ion and a i r -b lowin g o f mid -con t inen t asphal ts were used to s tudy the ro le o f ox yg en and the e f fec tsof dept h in the asphalt , t ime, and tem peratur e on the react ion. Ox yg en from the air is required for th in-f i lm hardening; i t reacts fastest at the surface and very s lowly below 0.07 nch into the asphalt. Conversionof resins to asphaltenes and of asphaltenes to insoluble polymeric mater ia l causes loss of p las t ic i ty andfa i lu re by c rack ing . Astemp erature is increased, fa i lu re t ime ind icates three di f ferent rate-contro l l ing steps in the oxidat io n process.

Ai r -b lowing app aren t ly leads to the same reac t ion as thin -f ilm oven ox ida t ion .

SPHALTS under the influence of the weather are affectedA by temperature changes, heat. air, light, and moisture.These factors harden the asphalt in time and cause it to loseits plasticity.

Oxygen of the air appears to be one of the most importantfactors in the dete riora tion of asphalt . Asphalts in the absenceof light absorb oxygen when heated (6) and when blown withair a t high temperatures (5). A thorough study of the kineticsof asphalt blowing has clarified engineering aspects of theoxidation process but has yielded little information on themechanism (2). Changes in asphalt composition and proper-ties during blowing have been measured 3)and, based on theresults of earlier workers. a mechanism has been proposedto explain t he effects of antioxi dants on the adsorption of oxygenby asphalts 7 ) . A general mechanism of hardening of poly-mers by oxidative polymerization has been advanced (4 )andmay be applicable to asphalts.T o establish further the role of oxygen in asphalt hardening,oxidation reactions have been carried out with blown andunblown asphalts. Both were subjected to oven tests to de-termine the need for external oxygen, the effect on oxidationof depth in the asphalt, and the effects of time and temperature.The effect of time on oxidation was also investigated duringair-blowing of the unblown asphal t. All these effects weremeasured in terms of changes in asphalt composition, deter-mined by empirical analysis for oils, resins. asphaltenes. andmaterials insoluble in benzene. and by elemental analysisIn addition, the reaction time needed to cause each asphalt

I N A I R5

Iz -__N N I T R O G E N:0 I

to crack upon cooling, defined as "failure time," was measuredand treated kinetically to shed light on oxidation mechanism.xperimentalFour mid-continent asphalts were used : three blown asphaltshaving a common origin and softening points between 180'

and 200 ' F.. and an unblown material of like source with asoftening point of 90' F.For the oven tests, samples were prepared in rectangular3 X 6 inch aluminum trays with rounded corners. Th eamount of asphalt needed to give the desired depth wasweighed into the tray, which was then heated until the asphaltformed a smooth layer. For the experiment to deter mine theneed for external oxygen, similar thin films were prepared in4-ounce tins; half of the tins were left open to the air andthe other half were flooded with nitrogen, covered, and sealedwith an adhesive tape . Both the trays and the tins wereallowed to cool to room te mpera ture before use.In each oven test, all samples were placed at the same timein the oven. maintained within 2' F. of the selected tempera-ture between 200' and 450' F. The thermometer bulb wasplaced close to the asphalt surfaces. to assure that the indicatedtemperatu re was actually that of the asphalt. To observe theeffects of time, trays were withdrawn at intervals; either theywere examined for visible cracks upon cooling and replaced inthe oven if none appeared, or they were used for analysis.In each air-blowing test. 600 grams of the unblown asphaltwas blo\vn with 700 ml. of air per minute in a stirred steelreactor.For the empirical analyses, vertical cross sections from theaspha lt trays were taken up in benzene. Mater ial insoluble in

At intervals, samples were withdrawn for analysis.

- ~e o \

+zwzIu+zYwI

a

7 --\60-

5 -

40.-

30

\

1O I L S

A R E S I N S0 A S P H A L T E N E S. N S O L U B L E S

Ic 4o--c- --c ____ / ' 5 ..g 30 \fl-3-t-j , sample thickness on com- 20//' / > y A 7 4C 7TC : -A Figure 2. Effect of= z - - -5 : = -aA i , .2. A c pos i t ion o f b lown as- -* LeI O ~ , P O 12 0 r A O I L SE S I N S 1 p h a l t a f t e r o x i d a t i o n b -------.A S P H A L T E N E Sl o - I NSOLUBLE S

I7 5 100 125 . I 5 0 1750

/.-.--.-.d. /~ -~- -325 0 5 00 2 4 6 8 IO 12 I4 16

T I ME H C U R S S A M P L E T H I C K N E S S , I N C H E S

29 l E C P R O D U C T R E S E A R C H A N D D E V E L O P M E N T


2/4

benzene was dried and weighed as such. Th e benzene solu-tion was passed through an alumina-packed chromatographiccolumn, from which oils were eluted with hexane and resinswere eluted with 97y0 ethyl ether plus 3% eth >l alcohol.Asphaltenes were determined by difference. Th e elementalanalyses were carri ed out by conventional metho ds. Allresults lvere expressed in weight per ce nt.Resu l ts and Discussion

The difference in reaction in a thin film of blown asphaltwhen oxygen is present or absent is shown in Figure 1. Lt'itha film 0.025 inch thick at 375' F. in air, composition changedmarkedly; in nitrogen, little change occurred . Combinedoxygen that may have been retained in blowing during manu-facture was apparently not enough to cause appreciable re-acti on. By the time evidence of insolubles form ation innitrogen appeared, the tapes sealing the tins had begun tofail.

The depth in the asphalt to which the oxidation reactionpenetrates is indicated in Figure 2. In this experiment: trayscam)-ing different am ount s of a bl own asphalt \\-ere heated for48 hours at 375' F. Because vertical cross sections wereanalyzed, the composition at each thickness is actually theaverage composition to the depth indicated. Apparentlylittle reaction takes place at depths beyond 0.07 inch. Dif-fusion of oxygen is apparently much slo\ver at 375' F. thanreaction of aspha lt Lvith oxygen. To ensure tha t a largeportion of the asphalt Ivould react, an asphalt thickness of0.025 inch was used in subsequent oven tests.

Th e effect of time on the thin-film oxida-tion of unblown asphalt at 375 Oilsapparentl y d o not reacl., but decrease slightly by evap oration.The resins change to asphaltenes, which later change to in-solubles. M'hen the insolubles comprise 10% or more of theasphalt, the sample cracks or fails when cooled. Failure timeseems to coincide with the maximum rate of formation of in-solubles.

Air-bloiving oxidation of the unbloivn asphalt at 385" F.gave the data points plotted in Figure 4. These data showthe same decline in resins and increase in asphaltenes observed

Effects of Time.F. is shown in Figure 3.

q, 2 .O 0 5

I0

5 10 5 20 2 5AIR-BLOWING T I M E , O U R SFigure 4. Air-blowing oxidation of unblown asphalt

in thin-film oxidation. If the curves of Figure 3 are super-imposed on these data at one tenth the time scale here in-dicated, the resultant dashed curves fit the data points fairlywell. Th e similarity of patter n suggests that the same re-actions are taking place at a rate governed by the amount ofasphalt surface presented t o the air.

Th e effect of te mperature on thin-film oxidation of a sphal ts is most readily seen from theformation of insolubles in bloivn asphalt and the elementalanalysis of them.

The pattern of insolubles formation at four temperatures isshown in Figure 5. As expected, insolubles form faster at hightempera tures. Hoivever, the similarity of the curves suggeststha t the same type of reaction is occurring at all temperaturesstudied. Moreover , oxidation at the loiver temperature s seemsto result in ultimate formation of larger amounts of insolubles.

Results of elemental analyses of the insolubles formed at thefour temperatures are given in Table I . In each case? oxida-tion had been carried to the point where the percentage ofinsoluble mate rial was leveling off. Th e da ta show no sig-

Effects of Temperature.

I I IO I L SA S P H A L T E N E SR E S I N S

a I N S O L U B L E S

A6A A

WL20

t

Wc3

B E G I N N I N G OFF A I L U R E B YC R A C K I N G

laa0 2 4 6 8 I O 12 14 16

T I M E , HOURSFigure 3. Thin-film ox idation of unblown asphalt

V O L . 1 NO. 4 D E C E M B E R 1 9 6 2 291


3/4

ZWzaPVL

-

tWVanIx2W

375F.40

b / i I I I J0 10 2 32 40 50 60 7 6 90

400'F.

t

T I M E , H O U R S

nificant variations or trends except at the lowest temperature.Here, a notably higher oxygen content suggests the formationof peroxy linkages. Because the insolubles are also insolublein all common solvents, polar and nonpolar, they are probablythe result of cross linking of the resins and asphaltenes.C onc l us i ons

The combined effects of time and temperature are revealedby relating failure time to temperature. Such a relationshipcan be derived from the Arrhenius equation if failure time is

Figure 5 Effect of temperatureon formation of insolubles

taken to be equivalent to the time of a constant amount ofreaction in all samples. Th enLog (failure time) = - og k f constant

where k is the reaction rate constant.the Arrhenius equation gives

Substitution for k kom

EaLog (failure time) = constantRTwhere Ea is the Arrhenius activation energy, R is the gasconstant, and T i s the absolute temperature.

T- 103 K -Figure 6. Variation of failure time with reciprocal temperature for blown asphalt

292 I & E C P R O D U C T R E S E A R C H A N D D E V E L O P M E N T


4/4

Documents

Asphalt Oxidation Studies at Elevated Temperatures