CHARACTERIZATION OF ASPHALT BINDERS

BASED ON CHEMICAL AND PHYSICAL PROPERTIES

Jianghua Weit, Jeffrey C. Shull2,John Barak3, and Martin C. Hawley4

ABSTRACTAsphalt is a complexmixtureof manydifferenthydrocarbons.The asphalt

constituentsare classified into three categories: oils, resins, and asphaltenes.The engineeringpropertiesof asphaltbindersare directlyrelated to the quantityand qualityof asphaitenespresentand the nature of the dispersionmedium, oilsandresins. Uponadditionof a polymerto a straight asphalt, these relationshipsand thus engineering properties may be altered. It is therefore necessarytodevelopa protocolfor characterizingand fingerprintingasphaltbinders.

The chemical compositionsand physical properties of straight(AC-5 andAC-10) and polymer(SBS and SEBS) modified asphaltswere studiedusinghighperformance gel permeation chromatography (HP-GPC), Fourier transforminfrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), thermalmechanical analysis (TMA), and differentialscanning calorimetry(DSC). TheHP-GPC systemwas equipped with a photodiode array detector (PDA) and aliquid chromatography(LC) transformunit which allowed chemical analysisofeach asphalt constituentusingFTIR.

It was concluded that the combination of HP-GPC and FTIR is anexcellent tool for fingerprinting and quality control of polymers and asphaltbinders. The rheologicalpropertiesof an asphalt binder ware good indicesfordeterminingthe optimumpolymerconcentrationsfor effective modifications.TheDSC results indicatedthat differentasphalt grades have differentlevelsof polarassociationsas detected from changes in enthalpy. Polymermodificationaltersthese associations. TMA was a fast methodfor determiningthe highestpossibleservicetemperatureof the pavement.

1. Research Associate,Departmentof Chemical Engineering2. Research Assistant,Departmentof Chemical Engineering3. Engineer, Michigan Departmentof Transportation4. Professor,Departmentof ChemicalEngineering

B100A Research Complex - Engineering, Michigan State University, EastLansing,Michigan 48824-1326

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CHARACTERIZATION OF ASPHALT BINDERSBASED ON CHEMICAL AND PHYSICAL PROPERTIES

Jianghua WeiJeffrey C. Shull

Martin C. Hawley

The Department of Chemical EngineeringThe Composites Materials and Structures Center

B100A Research Complex - EngineeringMichigan State University

East Lansing, Michigan 48824-1326

and

John Barak

Michigan Department of TransportationSecondary Government Complex

8885 Ricks Road

Lansing, Michigan 48909

A Paper Prepared for TheIntemational GPC Symposium '94

June, 1994

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,f

TABLE OF CONTENTS

Abstract ............................................................................................................... 1

Introduction ......................................................................................................... 1

Background ................................................. :....................................................... 2

AsphaltCharacterization .......................................................................... 2

PolymerModifierCharacterization ........................................................... 3

PolymerModified Asphalt BinderCharacterization ..................................4

Materials and Experiments .................................................................................. 4

BasicChemical Characteristicsof Asphalt Binders :........................................... 6

High PerformanceGel PermeationChromatographyAnalysis .................6

FourierTransformInfrared SpectroscopyAnalysis ..................................7

LiquidChromatographyTransformAnalysis ............................................ 8

Basic Physical Characteristicsof Asphalts ......................................................... 9

RheometricsAnalysis ............................................................................... 9

Thermal MechanicalAnalysis ................................................................. 10

DifferentialScanningCalorimetry ........................................................... 11

Conclusions ...................................................................................................... 11

Acknowledgments ............................................................................................. 12

References ........................................................................................................ 13

Listof Tables ..................................................................................................... 14

Listof Figures ................................................................................................... 14

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ABSTRACT

Asphalt is a complexmixtureof many differenthydrocarbons. The asphaltconstituentsare classified into three categories: oils, resins, and asphaltenes.The engineering propertiesof asphaltbindersare directly related to the quantityand qualityof asphaltenespresentand the natureof the dispersionmedium,oilsand resins. Upon additionof a polymerto a straightasphalt, these relationshipsand thus engineering propertiesmay be altered. It is therefore necessarytodevelopa protocolfor characterizingandfingerprintingasphaltbinders.

The chemical compositionsand physicalproperties of straight(AC-5 antiAC-IO) and polymer(SBS and SEBS) modifiedasphaltswere studiedusinghighperformance gel permeation chromatography (HP-GPC), Fourier transforminfrared spectroscopy (FTIR), dynamic mechanical analysis (DMA), thermalmechanical analysis (TMA), and differential scanning calorimetry (DSC). TheHP-GPC system was equippedwith a photodiode array detector (PDA) and aliquid chromatography(LC) transform unit which allowed chemical analysis ofeach asphalt constituentusingFTIR.

It was concluded that the combination of HP-GPC and FTIR is anexcellent tool for fingerprinting and quality control of polymers and asphaltbinders. The theologicalpropertiesof an asphalt binder were good indicesfordeterminingthe optimumpolymerconcentrationsfor effectivemodifications.TheDSC results indicated that differentasphalt grades have different levels of polarassociationsas detected fromchanges in enthalpy. Polymermodificationaltersthese associations. TMA was a fast method for determiningthe highestpossibleservice temperatureof the pavement.

INTRODUCTION

The long term pavement performance of asphalt concrete (AC) surfacedroads is a function of traffic load and volume, material properties, constructionpractices, and environmental factors. A typical AC surfaced pavementdeterioratesover time and withincreasingnumberof load repetitions. Pavementdeterioration manifests itself in several common types of distress includingrutting, fatigue cracking, low temperature cracking, reflective cracking, aging,ravelling, and stripping13. The conventional materials used in the asphaltconcrete mixture may perform satisfactorilyrelative to one distresstype but failprematurelyrelative to the others. For example,asphalt concrete mixturesmadeby using hard binders will have low rutting but, high fatigue and temperaturecracking potentials. On the other hand, mixtures made with softasphalt binderswill have low fatigue and temperature cracking but, high rutting potentials.Hence, modification of the asphalt binder to enhance its performance at bothhigh and low temperatures and under traffic loading is essential to the successof constructing superior pavements. Such modifications includethe addition ofpolymers to enhance the binder propertiesat both low and high temperatures.

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And indeed, it has been shown that polymer modified asphalts can improvepavementperformance4_.

Michigan State University is currently conducting a study of polymermodified asphalt pavements that will address the above considerations. Thestudy is divided into three sections including, the fundamental physical,chemical, and thermodynamic properties of asphalt binders, the basicmorphologyand microstructureof polymer-fiber-asphalt-aggregatemixtures,andthe structural and engineering properties of polymer-fiber-asphalt-aggregatemixturesunder extremelow and hightemperaturesfound in Michigan.

Materials being used in the study include AC-2.5, AC.-5, AC-10, AC-20,styrene-butadiene-styrene (SBS), styrene-ethylene-butylene-styrene (SEBS),epoxy, polyethylene (PE), ethylene-vinyl-acetate (EVA), various fibers, andrecycled crumbrubbertires.

This paper focuses on the development of a protocol for fingerprinting,including characterization and identification,asphalts using high performancegel permeation chromatography(HP-GPC) and other instrumentation. Theasphalts studied were straight, styrene-butadiene-styrene(SBS) and styrene-ethylene-butylene-styrene (SEBS) modified AC-5 and AC-10 asphalts. Themolecular information of the binders was studied using HP-GPC, Fouriertransform infrared spectroscopy (FTIR), and differential scanning calorimetry(DSC). The physical properties of the binders were studied using DSC,dynamicmechanicalanalysis(DMA), and thermal mechanicalanalysis(TMA).

BACKGROUND

Asphalt Characterization

Asphalt is a complex mixture of many different hydrocarbonsconsistingprimarily of molecules that contain mainly carbon and hydrogen atoms. Thechemical compositionof an asphalt is determined by the source of crude oil,refinery process, and the grade of the asphalt. A generic asphalt elementalanalysis shows that approximately84 percent of the sample is carbon, 10percent hydrogen, 1 percentoxygen,and the remainder consistsof several traceelements includingnitrogen,sulfur, vanadium, nickel, and ironr. The averagemolecularweightof a genericasphaltmoleculeranges from 500 to 50007. TableI shows basic molecular information of three commonly used asphalts, AC-5,AC-10, and AC-207.

In general, asphalt constituentsare classified into three categories: oils,resins,and asphaltenes. Oils are the lightcompounds in asphalt whichhave thelowest molecularweights (24 - 800) and have a large numberof side chains andfew rings. Accepted criteria for the oil classification are molecules withcarbon/hydrogenatom number ratiosless than 0.6 that are soluble in hexanea.Resins are intermediate molecular weight compounds (800 - 2000). It isimportantto note that resinscan contain sulfurand nitrogen. Resins are polar,have a carbon/hydrogen ratio between 0.6 and 0.8 and are soluble in light

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petroleum naphtha8. Asphaltenes are the highestmolecular weight compounds(1800 - 8000) with aromatic ring structures, few side chains, andcarbon/hydrogen ratios greater .than 0.8a. Asphaltenes contain the traceelements mentioned earlier which may react with potential polymers and aresoluble in carbon-tetrachloride. An average asphalt sample has anasphaltenelresin/oilweight ratio of approximately231271509and the asphaltenecontentis higher for harder asphalts. HP-GPC is usuallyused to determinethemolecular weight, molecular weight distribution, and the fraction of eachconstituentin the asphalt.

Using the asphaiteneiresin/oilclassifications,a two phase asphalt modelwas developed8 and is illustrated in Figure 1. Phase 1 is an assembly phaseconsistingof asphaltenes and resins that is dispersed in phase 2, the solventphaseconsistingof the oily constituents. The resinsbehave as peptizingagentsthat stabilizethe asphaitenes inthe oily constituents.

The asphaltene core of the assembly phase can vary between 40 and 60angstroms in diameter. As the asphaltene content of an asphalt sampleincreases, the assembly phase also increases. This adds structure to theasphalt and gives it better high temperature properties, such as increased lossand storagemoduli,resultingin enhanced ruttingresistance. The percentageofthe assembly phase present is the definingfactor between different grades ofasphalt. The resins contain aromatic compoundssubstitutedwith longer alkyisand a largernumber of side chains attached to the ringsthan asphaltenes. Thecombination of the saturated and the aromatic characteristics of the resinsstabilizesthe colloidalnature of the asphaltenes in the oil medium. Asphaltenesare present as discrete or coUoidally dispersed particles in the oily phase.Colloidaily dispersed asphaltenes are not stable in the oil medium bythemselves,but can be stabilized through polar resins. Asphaltenes can existboth in a randomly oriented particle aggregate form or in an ordered micelleform. In miceUeform, the polar groups (water, silica, or metals such as V, Ni,and Fe) are either oriented toward the center to form an oil-externalmicelle fromhydrogenbonding,charge transfer, or salt formation,or oriented outwardto forman oil-internal micelle (Hartley micelle). The growth and ultimate size of themicellesare dependent on temperature,resin content,and the presence of otherchemicalssuch as polymer modifiers. The engineering properties of asphaltsare directly related to the quantity of asphaltenes, the size of the micellestructure,and the nature of the dispersionmedium,oilsand resins.

Polymer Modifier CharacterizationAccordingto the functions and behaviorsof variousmodifiers in asphalts,

modifierscan be categorized into five types: dispersed thermoplastics,networkthermoplastics, reacting polymers, fibers, and crumb rubber (CRM) particles.Dispersed thermoplastics behave like asphaltenes and normally requirepeptizing agents like resins to stabilize the modified systems. Usually, itrequires a considerable amount of material before forming a macrostructuralnetwork. Network thermoplasticsbehave like resins and will form a network of

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themselves inside asphalts. Reacting polymers bond chemically to the asphalt(normally to the asphaltenes) and will form asphalt/polymernetworks. Typicalfunctional groups that may react with polymers include: carboxylic acids,ketones, phenols, suifoxides, acid anhydrides, pyrroles, and quinones7. CRMparticlesbehave as aggregates if their sizes are large and behave as dispersedthermoplastics if their sizes are small (below l OOMm). Fibers increase theavailable wetting surface area and behave as binder thickenerswhich reduceasphalt bleeding. In all cases, the goal is to create more structure inside an

asphalt without losingits low temperatureproperties.

Polymer Modified Asphalt Binder CharacterizationAsphalt is a viscoelastic material that displays a variety of different

propertiesdepending on the temperatureof the sample. Goodrich, proposed amodel of asphalt depicted as a shock absorber and a spring_°. The shockabsorber representsthe viscous propertiesof asphalt and the springrepresentsthe elastic properties. At high temperatures,asphalt shows good viscous flowpropertiesand behaves as a shockabsorberwith littleor no elasticbehavior. Atlow temperatures, asphalt becomes a "brittle elastic solid" with little or noviscous properties. In the spring/shockabsorber analogy, the spring becomesover loaded and snapswhen low temperaturecrackingoccurs.

In order to reduce the potential of rutting at high temperatures andthermal cracking at low temperatures, various polymers/fibers/rubbershavebeen added to asphalt. The goal is to increase the temperaturerange of boththe elastic and viscous properties of the asphalt binders and mixtures.Convenientmeasuresof these propertiesare G', storage modulus(elastic), andG", loss modulus (viscous). The ideal case will be that at high temperaturesboth G' and G" increase upon addition of polymer due to a network structureformation withinthe asphalt; and at low temperatures,G' and G" decrease uponadditionof polymerdue to a decrease in the material'seffectiveglass transitiontemperature. The ideal case leads to the improvementof both viscous andelasticpropertiesovera wide range of temperatures.

MATERIALS AND EXPERIMENTS

Materials used in this studywere AC-2.5, AC-5, AC-10, AC-20, SBS, andSEBS. SBS and SEBS were used to modifyboth AC-5 and AC-10 at 0 to 10weight percent polymercontent. Aged asphaltswere also studied. Short termaging (experienced during processing and the first year of service) wassimulatedby using the Rolling Thin Film Oven Test (AASFTO T 240; ASTM D2872). Long term aging (experienced duringapproximately15 years of servicelife)was simulatedby usingthe Pressure AgingVessel Test (SHRP B-005).

A Millipore/Waters' high performance gel permeation chromatographysystem was used to analyze the molecular weight distributionsof straightasphalts,polymermodifiers,and polymermodified asphaltbinders. The system

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' Wei. ShulI. and Hawley.

was equipped with a differential refractometer, a photodiode array detector, achromatographymanager (Millenniumsoftware), a dual reciprocatingpump,anda manual injectionport. A Lab. Connections' liquid chromatographytransform(LC-Transform) unit was also installedas an additional detectorfor the system.The HP-GPC systemwas equipped with four Waters' styragelHR columnsthatencompassedan effectivemolecularweight range from 0 to 600,000. HP-GPCgrade tetrahydrofuran at a flow rate of 1.0 mL/min, was used as the mobilephase for this study.The sample concentrationwas kept constant at 10 gramsper I liter of solventaswas the injectionvolumeof 250 mL.

The differentialrefractometerwas capable of measuringmolecularweightdistributions of asphalts, polymers, and polymer modified asphalts. Thedifferential refractometerwas very reproducible. Figure 2 is an overlay of twoAC-10 chromatogramsprepared independentlyfrom each other.

The photodiodearray detector allows for the characterization of usefulchemical functionalgroupspresent in asphalt. PDA was not used extensivelyinthis study, but it will be useful in the future to discriminate between certainasphalts and polymersbased on their ability to absorb UV light. The axes ofPDA data are time (molecularweight), UV absorbance, and wavelength. Usingthe Millenniumsoftwarepackage, dissectionof this data is possible that allowsindividualspectraand chromatogramsat a fixed molecularweight or wavelengthto be examined.

The LC-Transform is an off-line unit that collects the effluent from thedifferential refractometer(separated by molecularweight) on a germaniumdiscthat can be analyzed using Fourier transform infrared spectrometryat a laterdate. The LC-transform/FTIRwill also allow us to characterize useful chemicalfunctional groupsthat absorb infraredlight.

Fourier transform infrared spectroscopywas used to examine unaged,aged, and polymer modified asphalts. The samples were prepared byevaporating a THF/binder solutionon a potassiumbromidesalt pellet leavingathin film of the materialto be examined. It was determined that the absorptionpeaks at 1375, 1450, 1600, and 1700 cm1 correspond to CH3, CH2, aromaticcarbon, and carboxyl groups in the asphalt, respectively. The heights of theabsorption peaks were assumed to be proportional to the amount of thefunctional groupspresentin the sample. Figure 3 showsthe FTIR spectrumof astraight AC-5 sample. FTIR was also used to determine the possiblecharacteristicpeaks corresponding to the functional groups of SBS and SEBSpolymers. Figure4 showsthe infrared spectrumof SBS in which a uniquepeakcan be identified at a wavelength of 965 cm"_(There is no 965 cm"1peak inFigure 3). This peak is assigned to the trans-l,4 contributionof butadiene inSBS. No characteristic peak was identified in the FTIR spectrum of SEBS.Clearly, FTIR will be a useful tool for fingerprinting and quality controlas well asdeterminationof polymercontentfor SBS modified asphalts.

Dynamic mechanical analysis experiments were performed using aRheometrics RMS-800 apparatus. All experiments utilized smooth, 25 mmdiameter plates in a parallel arrangementwith a gap width between 1.4 and 2.0

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mm. Sample handling procedures were consistent with those used for theSHRP (Strategic Highway Research Program) Bohlin instrumentin which thesample is poured hot directly onto the plates and allowed to cool to roomtemperature. Temperature sweeps were then conducted from 25 to 60°C withmeasurementstaken at five degree intervalswith an equilibrationperiod of twominutes. A frequency of 10 radianslsecondwas used (compliancewith SHRPspecification)and strain levels were controlled to insure that the testing wasconducted in the linearviscoelasticrange. The strain levelwas within0.5 to 9%

for all tests. All tests were replicated between three and six times to insureaccurate and reproducible results. For example, Figure 5 shows thereproducibilityof the storagemudulusversustemperaturedata for a triplicate.

Thermal mechanical analysis tests were performed on representativeasphalt binder samples. The samplesware placed in a 5 mmdiameter by 5 mmdeep glass boat at room temperature and packed firmly forming a fiat topsurface. The samplewas then covered with a 3 mm diameter by 1.5 mm thickglass plate upon which a one gram load was placed. The sample was thencooled at approximately 15°C per minute to a temperature of -100°C andallowed to equilibrate. The temperaturewas then raised at a rate of 5°C perminute and its height change was measured until a total melt (correspondingtoa 1 mm depressionof the sample)was achieved. After testing, the boats wereretrieved to insurethat the sample did not flow over the glass cover/plate. Alltestswere duplicatedto check the reproducibilityof the results.

Differential scanning calorimetry was used to detect glass transitiontemperatures and equilibriumtemperatures of possible molecularassociations.Asphalt sampleswere equilibratedat different temperaturesand then quenchedusing liquid nitrogento a temperatureof-100oc. The enthalpy change duringheatingwas then recordedas the samplewas heated at a rate of 10°Clmin.

BASIC CHEMICAL CHARACTERISTICS OF ASPHALT BINDERS

Basic chemicalcharacteristics of asphalt binderswere studied using HP-GPC, LC-Transform,and FTIR. Results from each study are summarized asfollows.

High Performance Gel Permeation Chromatography AnalysisAn HP-GPC chromatogramof a straightAC-5 is shownin Figure6. Three

distinct molecularweight ranges (divided by lines A and B) are present. Theseseparationsdo not necessarilycorrespond to the molecularweight ranges thatclassifythe oil, resin, and asphaltene constituents found in the literature usingsolubilitytechniques. However, the reported percentagesof the constituentsdoexist if the molecularweight ranges (divided by lines C and D) reported in theliterature are followed. Table II lists the percentages of the constituentsaccording to the chromatography separation and separation according toreported literaturevalues. From HP-GPC chromatograms,the molecularweight

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averages and polydispersities of asphalt binders were obtained. Table III liststhe averages for variousmaterials. These data may laterbe correlatedwith bothphysicaland engineeringpropertiesof asphalts.

A comparisonof HP-GPC molecularinformationof straightAC-5 and AC-10 is listed in Table Ill. As can be seen, It is difficult at this time to distinguishbetween asphalt grades at this time.

A comparison of HP-GPC chromatogramsof aged and unaged AC-5 isshown in Figure 7. It can be seen that when AC-5 is aged, there is an increasein the high molecular weight material and a decrease in the low molecularweight material. During aging there is only approximately0.2% weight losssuggestingthat the low molecularweight material is being reacted.

Gel permeation chromatography has also been used to examine thedifferences between pure SBS and SEBS. Table III indicates that the SBSpolymerbeing studied has a weight average molecularweight of approximately62,000 g/mol and a polydispersityof two. The SEBS polymerbeing studied onthe other hand has a weight average molecularweight of about 45,000 g/tooland a polydispersityof 1.35.

Because polymermoleculesare larger than asphalt moleculesas shownin Table III, gel permeationchromatographycan be used as an effectivetool fordetermining the polymer content of the asphalt/polymerblends being studied.Figure 8 is an example of this techniquefor AC-5 modifiedwith4 weight percentSEBS. When the originalAC-5 sample is subtracted from the asphalt/polymerblend, the entire chromatogramcan be divided into three regions:A, B, and Cwith corresponding areas of 2.52%, 1.49%, and 95.99 %, respectively. RegionA is the large polymer moleculesthat are totally separated from the asphalts.Region B is the small polymermoleculesthat are eluted with the asphalL Thiscan be easily seen by comparisonof Figure 8 with Figure 7. A total of 4 weightpercent SEBS can be identified from the HP-GPC data. It is importantto notethat it appears the polymer is being degraded, sheared, or reacted during themixingprocess. Similar resultswere obtainedfor AC-51SBS,AC-101SEBS,andAC-10/SBS asphalt/polymerblends.

A comparison of HP-GPC chromatograms of aged, straight AC-5 andaged, 4% SBS polymer modifiedAC-5 is shown in Figure 9. Upon additionofpolymerto asphalt, we would expect to see an increase in the amount of thehighmolecularweight asphaltconstituentfor the aged sampledue to both agingand possible polymer degradation. Instead, we see a decrease in this valuesuggestingthat the polymerhas reduced the amountof aging that has occurred.

Fourier Transform Infrared Spectroscopy AnalysisFTIR spectroscopywas used to characterize various functionalgroupsin

asphaltsand polymermodifiers. Figure 3 is a representationof these groupsforan aged AC-5 sample. Ratiosof the absorbance intensitiesof these functionalgroupscan yield useful informationwhen fingerprintingdifferentasphalt grades.Examinationof absorbance intensityratiosindicatesthat an AC-10 sample has agreater percentage of aromatic carbon than AC-5 as would be expected

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Wei. Shrill.andHawley.

suggesting a larger asphaltene content. However, AC-10 appears to have lessCH2 functionalgroups than AC-5 suggestinga lesser resin content. Peaks at1375, 1450, 1600, and 1700 cm1 correspond to CHs,CH2, aromaticcarbon,andcarboxyl groups present in the asphalt, respectively.Analysis such as this incombinationwith gel permeation chromatographywill be usefulwhen selectingmaterialsand dealingwith recyclabilityconcerns.

As the FTIR spectrumof SBS has a uniquecharacteristicadsorptionpeakat a wavelength of 965 cm1, FTIR is an excellent tool for identificationanddeterminationof the SBS content in an unknownasphalt binder. To determinethe SBS content using FTIR, the absolute ratio of the 965 cmI adsorption bandto the 1375 cm"1band can be used as a content indicator. This ratio is linearlyproportionalto the SBS content in the asphalt. However, a calibration curvemust first be constructed that is a function of asphalt and polymer type andsource. A calibration curve is constructed by preparing samples that containknown concentrationsof SBS polymer in the asphalt sample and plotting thisinformation against the respective absolute absorbance band ratios. It isimportant to note that the calibration curve is valid only for the givenasphalt/polymerblend. If different materials are used, a new calibration curvemustbe constructed. The curve mustalso be corrected for aging discrepancies.Figure 10 is an example of a calibrationcurve for an unaged AC-51SBSpolymerblend. The definingequationfor the unagedcalibrationcurve is:

Ratio = 0.064*polymercontent + 0.037

while that of an aged curve is:

Ratio = 0.051*polymercontent + 0.079.

Uquid Chromatography Transform AnalysisAs not all polymermodifiers have a characteristicabsorptionpeak that is

unique from that of the asphalt spectrum,FTIR alone may not be sufficientforfingerprinting all polymers. However, it will be much easier to identify thepolymersifwe can separate them from the asphalt/polymerblends. As polymershave much higher molecular weights than asphalts (see Table III), polymerselute from the HP-GPC column first (see Figure 8). By attaching an LC-transformunitto the end of the HP-GPC system, it is possible to collectonly thepolymerportionof a blend on a germaniumdisc that can be analyzed using anFTIR system. Figure 11 shows the FTIR spectrum of SBS collected usingthistechnique. The spectrum in Figure 11 showsthe same characteristics as thosefound in the spectrumpresented in Figure4 (originalSBS).

Bycombinationof HP-GPC, LC-Transform,and FTIR, the contentand thenature of most non-reacting polymers in an asphalt/polymer blend can beobtained.

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BASIC PHYSICAL CHARACTERISTICS OF ASPHALTS

Basic physical characteristics of asphalts were studied usingRheometrics, DSC, and TMA. Results from each study are summarizedasfollows.

Rheometrics AnalysisThe major importance of DMA testing is to show that the presence of

polymerin an asphalt binder does indeed enhance the theologicalpropertiesofthe blend over a wide temperature range. As polymerswill enhance asphaltproperties more at high temperatures than at low temperatures, it is propertofocus on the propertyenhancementat high temperatures. Figure 12 showsthesensitivityof tan8 with respect to SBS polymer content and temperature for amodifiedAC-5 asphalt binder. Clearly, SBS polymersenhance the theologicalpropertiesof AC-5, especiallyat high temperatures. Similar resultswere foundfor all blend combinationsstudied in this research includingAC-51SEBS,AC-10/SBS, and AC-101SEBS. It is importantto note that an extensive mixingstudywas first conducted to insure that optimum blend properties were beingmeasured". The mixingprocedureused called for preheatingthe raw materialsto 135°C for one hour and then mixingwith a low shear mixer (approximately1600 rpm) equippedwith a four blade, 5 cm diameter impellerfor two additionalhours at 180°C.

It is also important to determine the optimum and/or critical polymercontentfor best modification.After an extensive study, the optimumwas definedas the point at which there is no longer a significant increase in the theologicalpropertiesof the blend with increasingpolymer content". The critical content,on the other hand, was defined as polymer concentrationswhere a deviationfrom the normal trend in the theologicalpropertieswas detected. The StrategicHighway Research Program(SHRP) uses llJ" at 60°C as a theologicalpropertyindicatorfor its originalbinder specification. Figure 13 shows the inverse losscompliance as a function of SBS polymer content at 60°C for a modified AC-5asphalt binder. Based on the definitionspresentedand Figure 13, the optimumand critical polymer contentsare 5 and 4 percent for this system. Additionaltheological indicators were also investigated that verified the optimum andcritical polymer contents". The same experiments were conducted for AC-51SEBS, AC-101SBS, and AC-101SEBS systems. The optimum and criticalpolymer contentsare listed in Table IV.

The decrease in properties at the critical polymer content is a realphenomenon that has been tested multiple times using several experimentaltechniques. This phenomenonwas also observed in the TMA analysisas will bediscussed in a later section. A possibleexplanationof the observed behavioristhat as polymer is added to the asphalt, the thermo-physicalproperties of theunmodified material are altered due to solubilityof the oils in the polymer, orsomeother structuralphenomenon. At the same time, the polymeris enhancingthe overall rheologicalpropertiesof the entire blend. It could be envisionedthat

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when two rheologicalpropertyprofilesare added together they yield a resultantpropertyprofilesimilarto that observed inthisstudy.

The aging effects on the. rheological properties of AC-5/SBS polymerblends (polymer contents from zero to ten percent) were also studied usingRheometrics. The samples were aged usingthe Thin Film Oven and PressureAging Vessel Tests. These samples were then tested and the results werecomparedwith those of the unaged samples. No observable trend with respectto increasing polymercontent was observed. However, Figure 14 depicts thepercent increase in G' at 60°C of the aged versus unaged polymer modifiedbinderfor several SBS polymercontents. Increasingpolymercontentappears todecrease the magnitudeof increase in the storage modulus. This may suggestthat asphalt aging is retarded throughthe addition of a polymer modifier whichagrees with the conclusiondrawnfrom the HP-GPC analysis.

Thermal Mechanical AnalysisThermal mechanicalanalysis tests have shown that the glass transition

temperatures (Tg) of both SBS and SEBS polymers to be in the range from -60to -90°C. Upon additionof polymerto asphalt, the Tg of asphalt/polymerblendswill be lowered since the Tg's of straightasphalts are much higherthan -60"C.TMA tests were also used to look at the final melt temperatures of the fourasphalt/polymer systems: AC-5/SBS, AC-51SEBS, AC-101SBS, and AC-101SEBS. Figure 15 summarizesthe TMA resultsfor the four systemsat variouspolymer contents. The TMA resultsindicatethat:

1. All of the TMA data support the conclusionsdrawn for the criticalandoptimumpolymer contentsbased on rheological properties (see TableW).

2. The final melt temperature of AC-10 is greater than that of AC-5 asexpected.

3. At their respective optimumpolymer contents,there is littledifferencein the finalmelt temperaturesof the asphalt/polymerblends.

4. At their respectiveoptimumpolymer contents, there is an increase ofapproximately 25°C in the final melt temperature for theasphalt/polymerblendswith respectto the straightbinder.

The aged AC-51SBS polymer blends were also tested using thermalmechanical analysis to determinethe final melt temperaturesof the samples asshown in Figure 16. Examination of Figure 16 indicates that increasing thepolymercontent beyondthe optimumpercentage has an insignificanteffect onthe final melt temperature of the aged AC-51SBSblends. The difference in thefinal melt temperatures of the aged versus unaged samples for increasingpolymer content decreases. At approximately nine percent polymer content,

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there is no difference in the final melt temperatures between the aged andunaged samples. This polymer content roughlycorrespondswith the level ofmodificationthought to cause a matrix inversionof the AC-51SBS blend. TheTMA data support the HP-GPC result that addition of polymer retards agehardening. Also, TMA data show that no age hardening occurs beyond thematrixinversion(about9 wt.%).

Differential Scanning CalorimetryDifferentialscanningcalorimetrywas usedto measure the glass transition

temperatures of AC-5 and AC-10 asphalts. Figure 17 shows the DSCthermographof an AC-5 asphalt. The glass transitiontemperaturesof both AC-5 andAC-10 are withinthe range from -10 to -15°C. Upon additionof polymertoasphalt, the blend's glass transitionbecomes a broad range between those ofthe polymersand asphalts.

Differential scanning calorimetry has also been used to examine anytemperaturedependentstructurethat may exist in asphalt. StraightAC-2.5, AC-5, AC-10, and AC-20 asphalt sampleswere quenchedfrom room temperatureto-100°C using liquid nitrogen. The samples were then heated at a rate of10°C/minute. Figure 18 shows the DSC thermographsfor representative runs.it has been found that there are three distinct thermal transitions that exist instraight asphalt at approximately -12, 10, and 33°C. The first transition isbelieved to be the glass transition of the asphalt. The second and thirdtransitionsmay be due to polar associationsbetween asphalt molecules. Thetransition temperature corresponds to the breakdown temperature of theassociation. This speculationwas qualitativelyverified by the fact that if anasphalt sample was equilibratedat an elevated temperature for a period of onehour before quenching, the transitionsat 10 and 33°C could be eliminated asseen in Figure 19. The temperatures in Figure 19 represent the equilibriumtemperatureused beforethe DSC test. After ackee of the AC-5 sample at 350Cfor one hour,the transitionat 10°C was eliminatedbut the transition at 33°C stillexists. After ackee of the AC-5 sample at 135 or 180°C for one hour, bothtransitionsat 10°C and 33°C were eliminated. Also, this is a reversibleprocess.If a heated sample is allowed to equilibrateat room temperature, the transitionswill reappear as seen in Figure 20. The reversible phenomenon verifies thespeculation that these transitions are structural changes due to polarassociationsand not chemical transitions. More research is needed in this areato determine the nature of these transitionsand how they will be affected uponadditionof polymer.

CONCLUSIONS

This study of the characterization of asphalt binders based on chemicaland physicalpropertiesshows the following:

742

Wei, Shull. and Hawley.

• HP-GPC, LC-transform, FTIR, TMA, DSC, and DMA were used tomeasure chemical and physical properties of straight and polymermodifiedasphaltbinders.

• HP-GPC was an excellent tool for determiningthe polymercontent in anasphalt binder.

• A combination of HP-GPC and LC-transform/FTIR can be used forfingerprintingthe nature of asphalts and polymers as well as qualitycontrolof asphaltsand polymermodified asphaltbinders.

• DMA results showed that rheoiogical properties were enhanced uponadditionof polymersto asphalt binders,especiallyat high temperatures.

• DMA can be used to identifythe optimum and critical concentrationsofpolymers required for producing an effective enhancement of therheologicalpropertiesof an asphalt binder.

• TMA resultsshow that the melt temperature of a binder increases withpolymercontentand asphalt grade.

• DSC results suggested that different extents of structure existed fordifferent asphaltgrades. Additionof polymersaltered these structures.

• Additionof SBS and SEBS polymersto AC-5 and AC-10 asphalt bindersappears to retard asphalt age hardening.

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financialsupportprovided by theMichigan Department of Transportation (MDOT) and the technical assistancegiven by MDOT, the CompositesMaterials and StructuresCenter, and the Civiland Environmentaland Chemical Engineering Departmentsat Michigan StateUniversity.

743

Wei. Shull. and Hawley.

REFERENCES

1. Baladi, G.Y. Fatigue Life and Permanent Deformation Characteristics ofAsphalt Concrete Mixes. In Transportation Research Record 1227, TRB,NationalResearchCouncil,Washington, D.C., 1989.

2. Baladi, G.Y., R.S. Harichandran, and R.W. Lyles. Asphalt Mix Design - AnInnovative Approach. In Transportation Research Record 1171, TRB,NationalResearch Council,Washington, D.C., 1989.

3. Baladi, G.Y. /ntegrated Material and Structural Design Method for FlexiblePavements. Final Report RD-88-109 and 110. FHWA, U.S. Department ofTransportation,!987.

4. Newcomb, D.E., M. Stroup-Gardiner, and J.A. Epps. Laboratoryand FieldStudies of Polyolefinand Latex Modifiers for Asphalt Mixtures. In PolymerModified Aspha# Binders, ASTM ST/= 1108. K.R. Wardlaw and S. Shuler,Editor. AmericanSocietyFor Testing Materials, Philadelphia,1992, pp. 129-150.

5. Rogge, D.F., R.L Terrel, and A.J. George. Polymer Modified Hot MixAsphalt - Oregon Experience. In Polymer Modified Asphalt Binders, ASTMSTP 1108. K.R. Wardlaw and S. Shuler, Editor. American Society ForTestingMaterials,Philadelphia, 1992, pp. 151-172.

6. Khosla, P.N. Effect of the Use of Modifiers on Performance of AsphalticPavements. In Transportation Research Record 1317, TRB, NationalResearch Council,Washington, D.C., 1991, pp. 10-22.

7. Material Reference LJ'brary Ashalt Data, in Strategic Highway ResearchProgram, National Research Council, Washington, D.C. 1992.

8. Highway Materials Engineering - Asphalt Materials and Paving Mixtures. Vol.FHWA-I-90-008, Washington, D.C. 1990, U.S.D.O.T., Federal HighwayAdministration,NationalHighway Institute.

9. Baladi, G., PersonalCommunication,1992.

10.Goodrich, J. L., What Is The Purpose Of Modifying Asphalt With Polymers?1990, ChevronResearch And TechnologyCompany.

11.Shuil, J. C., Wei, J., Eberz, K., Hawley, M. C., Baladi, G. Y., and L. T. DrzalThe Effect of Polymer Content on the Rheological Properties of AC-51SBSBlends. Submittedto TRB, National Research Council, Washington, D.C.,for AnnualMeeting,January, 1995.

Wei. Shull. and Hawley

LIST OF TABLES

Table Caption

I AsphaltelementalanalysisII Percentageof constituentsin an AC-5 asphalt binderIII Molecularweight averagesfor variousmaterialsIV Optimumand criticalpolymercontentsfor asphaltmodification

LIST OF FIGURES

Figure Caption

1 Two phase asphaltmodel2 Reproducibilityof Refractive Index detector(AC-10)3 FTIR spectrumof an aged, unmodifiedAC-5 sample4 FTIR spectrumof SBS5 Storagemodulusas a functionof temperaturefor a triplicateat four

percentSBS polymercontent6 HP-GPC raw datafor unaged, straightAC-57 Comparisonof molecularweight distributionsbetween aged and

unagedAC-58 HP-GPC raw datafor unagedAC-5 modifiedwith 4 percentSEBS

polymer9 HP-GPC raw data comparingaged, 4 percentSBS polymer

modifiedand unmodifiedAC-5 binders10 FTIR absorbanceband ratio calibrationcurve for unagedAC-

5/SBS polymerblends11 FTIR spectrumof SBS from LC-transformseparation12 Sensitivityof tan delta with respectto SBS polymercontent13 Inverse losscompliance as a function of SBS polymercontent at

60°C14 Percentincrease inthe storage modulusat 60°C of aged versus

unaged SBS polymermodifiedAC-5 binders15 Melt temperaturesof asphalt/polymerblendsas a function of

polymercontentdeterminedby TMA16 Melt temperaturesof aged and unagedAC-5/SBS polymerblends

as a function of polymercontentdeterminedby TMA17 DSC datafor determinationof glasstransitiontemperatureof AC-518 DSC data depictingintensityof transitionsas a function of asphalt

grade19 DSC data depictingtransitionsas a function of temperaturehistory20 DSC data depictingtransitionsas a functionof repetitivecycling

745

WCLShull. andHawley'

Table I Asphalt elemental analysis

AC-5. AC-10 AC-20Cart)on,% 85.7 82.3 84.5Hydrogen,% 10.6 10.6 10.4Oxygen, % -- 0.8 1.1Nitrogen,% 0.54 0.54 0.55Sulfur, % 5.4 4.7 3.4Vanadium, ppm 163 220 87Nickel, ppm 36 56 35Iron, ppm w 16 100AromaticC, % 32.5 31.9 32.8AromaticH, % 7.24 7.12 8.66MolecularWeight 570-890 810-930 840-1300(Toluene)

Table II Pecentage of constituents in an AC-5 asphalt binder

Classification ChromatographySeparation LiteratureSeparationMolecularRange PercentArea Molecular Range PercentArea

Asphaltene 6700 + 7.45 2300 + 23Resin 2900 - 6700 11.85 950 - 2300 27Oil 0 - 2900 80.70 0 - 950 50

Table III Molecular weight averages for various materials

...................N..ame............................M....n............MP_.............................M....z............AC-5 669 862 2099 6725 3.14

AC-10 630 922 1865 5660 2.96AC-5 (aged) 792 876 3348 11121 4.23

SBS 30002 67865 61900 106729 2.06SEBS 32730 44092 44445 58327 1.36

AC-514%SBS 657 829 4320 51605 6.57AC-514%SEBS 752 871 4322 38754 5.75

AC-514%SBS (aged) 803 876 5508 48220 6.86AC-10/4% SEBS 780 917 4602 42458 5.90AC-10/4% SBS 791 907 6086 82759 7.69

Table IV Optimum and critical polymer content for asphalt modification

optimum content critical content(percentby totalweight of the asphalt/polymerblend)

AC-5/SBS 5 4AC-5/SEBS 5 3 - 4AC-10/SBS 2 3AC-101SEBS 4 3

746

We_ 5hu_ andHaSty.

phase 2

(assemblmes)

Figure I Two Phase asphalt model

0.70

Figure 2 Reproducibility of Refractive Index delmctor (AC-10)

747

Wei, Shulk and Hawley

I_ i i i i i@e

B D

9

i• •

Figure 3 FTIRspectrum of an aged, unmodifiedACE sample

,o !:.

•19 .le

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Figm 4 FTIR spectrumof SSS

748

' - w_ Shun,aadt-L_,y

1000.00

100.00

I

10.00

1.00

0.10 t I I ! I i20 30 40 SO 60 ?0 |0

Temperature(C]

Figm 5 Storage modulus as a function of temperature for a triplicate atfour pen:m SBSpolymercontent

80.00-

60 ooi

40.00

20.00,

.I

"1_ IO. 00-[ " v _ p

124:00 " 2s:oo ' 2s'oo " 30:00 ' 3_':oo ' 34:o0 _s:oo _s'oo ,io:o0 "

PU.DU_eS

Figure 6 HP-GPC raw data for unagecl,stmJghtAC-8

749

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0.80- " /'"" ,

0.40-

o.zo-_r_,_ed\

0.00,

4.'40 4.'z6 4.00 :_.eo 3.G0 3.40 ",.zo :_.00 ,.eo 2._0 2.40 z.zloq MN'

Rgure 7 Comparison of mlecular weight distributions between agedand __ AC-5

SO0.OG

400.00i

300.00.

200.00

1-00.00 !1

0.00- ,_ v

22"00 :_4'.00 26.00 28.00 30.00 " 32-'00 34.00 " 36;00 " 38'.00 " 40'.00 '

HI_tce',

Rgm 8 HP-GPC raw data for unaged AC-5 modified with 4 percentseem

750

600.0

500.00.

200.1

Figm 9 HP_I=C raw data comparing aged, 4 _ SBS polymermodified and unmodified AC-5 binders

0., : :0.$

o., • l_ m

0.3 *_ o

•| o.,01: •

0.1 •

0 l ,P l l I I l i I

I 2 3 4 S 6 7 8 9 10

Sii$ Weisbt PercoRt

Figure 10 FTIR absorbance band ratio calibration curve for unaged AC-sses bends

I

751

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Wei, ShulLand Hawi_

Figure 11 FTIR spectrum of SBS from LC-transform separation

12

10

-4--08 -4--t

:" --o,-2o=" -o..- 3

.-o--4. s --_--S

_10

2

0 | I | ! I n20 30 40 SO 60 70 80

Temperature (eC|

Rgm 12 Sens_ of tan delta with respect to SBS polymer content

752

7

100

1 I I ! I t

0 2 4 6 8 10

Percent Polymer Content

Figure 13 Inverseloss complianceas a functionof SBSpolymercontentat 60°C

0

3% 45 % S

Percent Polymer Content

Figure 14 Percentage increase in the storage modulus at SO°Cof agedversus unaged SBS polymer modified AC-5 binders

753

' w_ sb_, a_ I-I_

120

110

_-. 100

Ea.

i 9Oo

-; I -°-AC's w' 8BS 1E 80 -o--AC-$O w/SBS /u.

--o-- AC-10 w/SEBS}

7O

60

O I 2 3 4 S , ti 7 II t 10

Percent Polymer Content

i

Figure15 Melttemperaturesofasphalt/polymerMendsasa functionofpolymer content determined by TMA

120

110

gI 100

| ,om

t80 ..e.. U aalod

.S"- --a--Aged

70

S0 I _ I i2 4 6 8 le

Percen!PolymerConlen|

Figure 16 Melt temperatures of aged and unaged AC-8/SBS polymerMends as a function of polymer content determined by TMA

754

0.1

0.0

-la72-c (Ts)

•_ -0.4 -3

_ -0.2 -

-0.3- _-50 o2_ 0 25 5O "75

lemlPeratwre (*C)

Rgm 17 DSC dm for determination of glm _'wn_W_ _ ofAC_

0.41.

755

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w_, sl_u, _i l-la_.

o.3

*o.J

b_

""-o.3, _Wt_ -0

-0.5

- _0 -_5 0 _._ _o ;5

Telper e f.ur e ("C)

Figure 19 DSC data depicting transitions as a function of temperaturehistmy

|U

8'

S "_

0

-3o - io _ _ 5o 70Temlllers_r • ("C|

Figure 20 DSC data depicting Inmldtionl as a function of repelilive cycling

756