J O U R N A L O F M A T E R I A L S S C I E N C E 3 9 (2 0 0 4 ) 951 – 959

Styrene butadiene styrene polymer modification

of road bitumens

G. D. AIREYNottingham Centre for Pavement Engineering, School of Civil Engineering,University of Nottingham, University Park, Nottingham, NG7 2RD, UKE-mail: gordon.airey@nottingham.ac.uk

This paper describes the polymer modification of road bitumens with SBS. Six polymermodified bitumens (PMBs) were produced by mixing bitumen from two crude oil sourceswith an SBS copolymer at three polymer contents. The rheological characteristics of theSBS PMBs were analysed by means of dynamic mechanical analysis using a dynamic shearrheometer (DSR). The results of the investigation indicate that the degree of SBSmodification is a function of bitumen source, bitumen-polymer compatibility and polymerconcentration. When the polymer concentration and bitumen-polymer compatibility allowa continuous polymer network to be established, modification is provided by a highlyelastic network which increases the viscosity, stiffness and elastic response of the PMB,particularly at high service temperatures. However, ageing of the SBS PMBs tends to resultin a reduction of the molecular size of the SBS copolymer with a decrease in the elasticresponse of the modified road bitumen. C© 2004 Kluwer Academic Publishers

1. IntroductionBitumen is a thermoplastic, viscoelastic liquid that be-haves as a glass-like elastic solid at low temperaturesand/or during rapid loading (short loading times—highloading frequencies) and as a viscous fluid at hightemperatures and/or during slow loading (long load-ing times—low loading frequencies). As a viscoelas-tic material, bitumen exhibits both elastic and viscouscomponents of response and displays both a tempera-ture and time dependent relationship between appliedstresses and resultant strains [1]. In addition, as bitu-men is responsible for the viscoelastic behaviour of allbituminous materials, it plays a dominant part in deter-mining many of the aspects of road performance, suchas resistance to permanent deformation and cracking.

Polymer modification of bitumen in road paving ap-plications has been growing rapidly over the last decadeas government authorities and paving contractors seekto improve road life in the face of increasing traffic.Currently, the most commonly used polymer for bi-tumen modification is the elastomer styrene butadi-ene styrene (SBS) followed by other polymers suchas styrene butadiene rubber, ethylene vinyl acetate andpolyethylene. The use of synthetic polymers to modifythe performance of conventional bituminous bindersdates back to the early 1970s [2], with the bindershaving decreased temperature susceptibility, increasedcohesion and modified rheological characteristics[3–12]. Globally, the distribution of the various types ofmodified binders can be estimated as 75 percent elas-tomeric, 15 percent plastomeric and 10 percent recycledcrumb rubber and miscellaneous modifiers (e.g., sul-phur) [13, 14]. Within the elastomeric group, styrenic

block copolymers have shown the greatest potentialwhen blended with bitumen [9, 10, 15]. Other exam-ples of elastomers used in bitumen modification includenatural rubber, polybutadiene, polyisoprene, isobuteneisoprene copolymer, polychloroprene and styrene bu-tadiene rubber.

Styrenic tri-block copolymers, commonly termedthermoplastic rubbers due to their ability to combineboth elastic and thermoplastic properties, can be pro-duced by a sequential operation of successive poly-merisation of styrene butadiene styrene (SBS) [14].Alternatively, a di-block precursor can be producedby successive polymerisation of styrene and the mid-block monomer butadiene, followed by a reactionwith a coupling agent [16]. Therefore, not only linearcopolymers but also multi-armed copolymers (knownas star-shaped, radial or branched copolymers) can beproduced. The structure of a SBS copolymer there-fore consists of styrene butadiene styrene tri-blockchains, having a two-phase morphology of sphericalpolystyrene block domains within a matrix of polybu-tadiene [5, 16].

SBS copolymers derive their strength and elastic-ity from physical cross-linking of the molecules intoa three-dimensional network. The polystyrene end-blocks impart the strength to the polymer while thepolybutadiene rubbery matrix mid-blocks give the ma-terial its exceptional elasticity. The effectiveness ofthese cross-links diminishes rapidly above the glasstransition temperature of polystyrene (approximately100◦C), although the polystyrene domains will reformon cooling restoring the strength and elasticity of thecopolymer [5, 9, 16].

0022–2461 C© 2004 Kluwer Academic Publishers 951

In terms of its chemical composition, bitumen is acomplex mixture of organic molecules. Both the chem-ical (constituent) and the physical (structural) part ofbitumen comprise mainly hydrocarbons with minoramounts of functional groups such as oxygen, nitro-gen and sulphur. As bitumen is extracted from crude oilwith its variable composition according to its origin, theprecise breakdown of hydrocarbon groups is difficultto determine. However, it is possible to separate bitu-men into four main fractional groups, namely; saturates,aromatics, resins and asphaltenes (SARA) [5, 16]. As-phaltenes are considered as highly polar, complex aro-matic materials, having the highest molecular weight ofall the other groups present in the bitumen. They formmicelles that are peptised by the polar resins and dis-persed in a medium consisting primarily of aromaticsand saturates, representing the fractions of the bitumenwith the lowest molecular weight. Based on this struc-ture, bitumen can be divided into two broad chemicalgroups, namely asphaltenes and a second group con-sisting of the resins, aromatics and saturates termed themaltenes. It is generally these low molecular weightmaltenes (aromatic oils) that are absorbed by the syn-thetic or natural rubbers found in polymer modified andcrumb rubber modified bitumens [17].

When SBS is blended with bitumen, the elastomericphase of the SBS copolymer absorbs the maltenes (oilfractions) from the bitumen and swells up to nine timesits initial volume [5, 14, 18]. At suitable SBS con-centrations, a continuous polymer network (phase) isformed throughout the PMB, significantly modifyingthe bitumen properties. As thermoplastic rubbers havemolecular weights similar to or higher than that of theasphaltenes, they compete for the solvency power of themaltene phase and phase separation can occur if insuf-ficient maltenes are available. This phase separation isan indication of the incompatibility of the base bitumenand polymer and care should be taken when blendingthermoplastic rubber PMBs. The compatibility of theSBS—bitumen blend can be improved through the ad-dition of aromatic oils. However, too high an aromaticcontent in the blend will dissolve the polystyrene blocksand destroy the benefits of the SBS copolymer.

Ageing of conventional bitumen, as well as PMBs, isinduced by chemical and/or physical changes that occurduring the production of the pavement and throughoutits service life. Ageing (hardening) is primarily asso-ciated with the loss of volatile components and oxida-tion of the bitumen during asphalt mixture construction(short-term ageing) and progressive oxidation of thein-place material in the field (long-term ageing). Bothprocesses are usually accompanied by hardening of thebinder, which general influences the deterioration of theasphalt pavement. Other factors may also contribute toageing, such as molecular structuring over time (sterichardening) and actinic light (primarily ultraviolet radi-ation, particularly in desert conditions) [19].

Although considerable research has been undertakenin this area, SBS PMBs have still to be comprehensivelycharacterised, due to the complex nature and interactionof the bitumen and polymer system [3, 20]. This pa-per presents a laboratory evaluation of the fundamental

rheological characteristics of a number of unaged andlaboratory aged base bitumens and SBS PMBs. TheSBS PMBs have been produced by laboratory mixingtwo different base bitumens with a SBS copolymer atthree polymer contents. The effects of bitumen source,polymer content, bitumen-polymer compatibility andageing on the fundamental rheological (viscoelastic)properties of the PMBs have been determined using dy-namic (oscillatory) mechanical analysis and presentedin the form of temperature and frequency dependentrheological parameters.

2. Experimental2.1. MaterialsA paraffinic (Russian crude source) bitumen, “BitumenA”, and a naphthenic (non-paraffinic) (Venezuelancrude source) bitumen, “Bitumen B”, were used to pro-duce a number of laboratory blended, block copolymerSBS PMBs. The conventional bitumen properties ofpenetration (25◦C, BS 2000: Part 49), softening point(BS 2000: Part 58), Fraass breaking point (IP 80), androtational viscosity (ASTM D4402) were used to char-acterise the two bitumens. Although both base bitu-mens have similar consistencies (penetrations of 73 and81 dmm and softening points of 47.0 and 46.8◦C), theydiffer in terms of their chemical composition (saturates,aromatics, resins and asphaltenes (SARA) fractions), asshown in Table I, and high and low temperature prop-erties (viscosity and Fraass breaking point), as shownin Table II.

The Colloidal Indices (CIs) of the two bitumens werecalculated in order to determine the potential compat-ibility of the base bitumens to polymer modification.Serfass et al. [21] are of the opinion that no precise CIborderline exists between what will be a “compatible”and what will be an “incompatible” bitumen. However,it is clear that the different percentages of the SARAfractions allow the CIs of the two base bitumens to differconsiderably. Inevitably this will result in differencesin the compatibility and rheological performance of thetwo sets of SBS PMBs.

TABLE I SARA analysis of base bitumens

Saturates Aromatics Resins Asphaltenes ColloidalBinder (%)a (%)a (%)a (%)a indexb

Bitumen A 4 68 19 9 0.149Bitumen B 11 58 17 14 0.333

aIatroscan thin film chromatography SARA analysis.bColloidal Index (Ic) = (asphaltenes + saturates)/(resins + aromatics).

TABLE I I Conventional physical properties of the base bitumens

Pen @ Softening Penetration Viscosity Viscosity25◦C point index Fraass @ 60◦C @ 135◦C

Binder (dmm) (◦C) (PI)a (◦C) (Pa.s) (Pa.s)

Bitumen A 73 47.0 −1.08 −12 165 0.370Bitumen B 81 46.8 −0.86 −28 213 0.380

aPI = (1952 − 500 log Pen25◦C − 20 SP)/(50 log Pen25◦C − SP − 120).

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The chemical dissimilarity between the two compo-nents (bitumen and SBS polymer) will generally resultin two distinct phases following mechanical mixing[22]. This two-phase structure of the SBS PMB con-sists of a polymer-rich phase and an asphaltene-richphase, also known as the polymer and bitumen phases.The polymer phase consists of the polymer swollenby the light, compatible fractions of the bitumen (e.g.,aromatic oils), while the heavy fractions (mainly as-phaltenes) are concentrated to form the bitumen phase.

Six SBS PMBs were produced by mixing a linearSBS copolymer (31% styrene content with a molecularweight of 120,000 and a toluene solution viscosity of11.0 cSt (11 × 10−3 Pa·s)) with both base bitumens atthree polymer contents by mass. The polymer contentsranged from low polymer modification at 3% to higherdegrees of modification at 5 and 7%. All the PMBs wereprepared with a Silverson high shear laboratory mill attemperatures between 170 and 185◦C (depending onthe viscosity of the PMB) until steady state conditionswere achieved.

The different SBS PMBs were coded as “base bi-tumen,” “polymer content” such as “A5” for the SBSPMB produced with Bitumen A and 5% polymer con-tent or “B7” for the SBS PMB produced with BitumenB and 7% polymer content.

2.2. Test methods2.2.1. Conventional binder testsThe base bitumens and SBS PMBs were subjected tothe following conventional binder tests: penetration,softening point, Fraass breaking point, ductility (10◦C,ASTM D113), elastic recovery (10◦C, modified ASTMD113) and rotational viscosity. The elastic recoverytest was performed on the SBS PMBs to measure thebinders’ elasticity (the ability of the binder to stretchand recover elastically after 1 h). In addition, the pene-tration and softening point values were used to calculatethe temperature susceptibility of the binders in terms oftheir penetration index (PI) [23].

2.2.2. Dynamic mechanical analysisDynamic mechanical analysis (DMA) was performedon the base bitumens and SBS PMBs using a BohlinDSR50 controlled stress rheometer [24–27]. The testprocedure and sample preparation method that wereused with the DSR have been described in detail previ-ously [12, 28, 29]. The DSR tests reported in this paperwere performed under controlled-strain loading condi-tions using frequency sweeps between 0.01 to 15 Hz attemperatures between 10 and 75◦C. The tests between10 and 35◦C were undertaken with a 8 mm diameter—2mm gap, parallel plate testing geometry and from 25 to75◦C with a 25 mm diameter—1 mm gap geometry. Thestrain amplitude for all the tests was confined within thelinear viscoelastic (LVE) response of the binder [28].

The principal viscoelastic parameters obtained fromthe DSR were the complex shear modulus (G∗) and thephase angle (δ). G∗ is defined as the ratio of maximum(shear) stress to maximum strain and provides a mea-

sure of the total resistance to deformation when the bi-tumen is subjected to shear loading. It is a combinationof the (shear) storage modulus (G ′) and the (shear) lossmodulus (G ′′) in the complex plane: G∗ = G ′+ iG′′.These two components are related to each other throughthe phase angle (δ), which is the phase shift between theapplied shear stress and shear strain responses duringa test. The phase angle is a measure of the viscoelasticbalance of the material behaviour as tan δ = G ′′/G ′.If δ equals 90◦ then the bituminous material can beconsidered to be purely viscous in nature, whereas δ

of 0◦ corresponds to purely elastic behaviour. Betweenthese two extremes the material behaviour can be con-sidered to be viscoelastic in nature with a combinationof viscous and elastic responses.

2.2.3. Ageing proceduresShort and long-term laboratory ageing of the base bitu-mens and SBS PMBs were performed using the RollingThin Film Oven Test (RTFOT, ASTM D 2872) and thepressure ageing vessel (PAV, AASHTO PP1), respec-tively. The standard ageing procedures of 163◦C and75 min for the RTFOT and 100◦C, 2.1 MPa and 20 h forthe PAV were used and the aged binders then subjectedto dynamic mechanical analysis to evaluate changes intheir rheological properties.

3. Results and discussion3.1. Conventional binder propertiesThe effect of SBS polymer modification on the conven-tional binder properties of the two PMB groups can beseen in Table III as a decrease in penetration and an in-crease in softening point with increasing polymer con-tent. Although the decrease in penetration is relativelyuniform with increasing polymer content, there is a sig-nificantly large increase in softening point temperatureat the high polymer contents of 5 and 7%. In additionto the increase in stiffness, the increased penetration in-dices (PIs) of the PMBs indicate a significant reductionin temperature susceptibility with polymer modifica-tion, particularly at the higher polymer contents.

In terms of low temperature performance, SBS mod-ification of base Bitumen A has almost no effect on

TABLE I I I Changes in conventional binder properties following SBSmodification

Pen @ Ductility Elastic25◦C Soft. Pt Penetration Fraass @ 10◦C recovery @

Binder (dmm) (◦C) index (PI) (◦C) (cm)a 10◦C (%)b

Bitumen A 73 47.0 −1.08 −12 63 —PMB-AS3 63 52.4 −0.05 −16 95 68PMB-AS5 57 78.0 4.41 −15 99 76PMB-AS7 50 95.0 6.13 −14 101 81Bitumen B 81 46.8 −0.86 −28 130 —PMB-BS3 63 52.2 −0.09 −18 81 71PMB-BS5 54 74.0 3.67 −16 90 78PMB-BS7 49 88.0 5.29 −14 81 80

aASTM D113.bModified ASTM D113.

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Figure 1 Rotational viscosities of SBS PMBs.

low temperature flexibility. However, for the BitumenB PMBs there is a significant increase in Fraass temper-ature with modification. Although the behaviour of theSBS PMBs relative to their respective base bitumens isdifferent, the actual Fraass breaking point temperaturesfor each polymer content PMB pair are almost identical.With regard to ductility and elastic recovery at 10◦C,the results are again similar for the SBS PMB pairs,although the ductilities of the Group A SBS PMBs areapproximately 10 to 25 percent greater than those ofGroup B. Elastic recovery differs from the elastic re-sponse of the binder, which is a dynamic rheologicalproperty of the binder quantified by means of the ma-terial’s phase angle. Whereas elasticity is usually asso-ciated solely with elastomeric PMBs, an improvementin the elastic response of a binder is usually evident inmost PMBs (elastomeric and plastomeric).

The Fraass and ductility results show the consider-able influence of the base bitumen, the nature of thepolymer and the base bitumen-polymer compatibilityon the intermediate and low temperature physical prop-erties of PMBs. Although there are slight differencesin the values of the conventional binder properties ofthe two PMB groups, overall these differences cannotbe considered to be significant.

Rotational viscosities for the two PMB groups (100to 160◦C) are shown in Fig. 1. The results show a con-sistent increase in viscosity with polymer modificationfor both PMB groups. In addition, rotational viscosi-ties (η) at 100 and 160◦C for the base bitumens andSBS PMBs are presented in Table IV together withmodification indices (η for PMB divided by η for thebase bitumen) at these two temperatures. As with the

T ABL E IV Rotational viscosities following SBS modification

Rotational Rotationalviscosity @ viscosity @100◦C 160◦C ηPMB/ηBitumen ηPMB/ηBitumen

Binder (mPa · s) (mPa · s) @ 100◦C @ 160◦C

Bitumen A 3,170 140 1.00 1.00PMB-AS3 7,000 280 2.21 2.00PMB-AS5 11,700 480 3.69 3.43PMB-AS7 24,000 740 7.57 5.29Bitumen B 3,600 140 1.00 1.00PMB-BS3 7,950 310 2.21 2.21PMB-BS5 14,000 470 3.89 3.36PMB-BS7 30,000 750 8.33 5.36

penetration and softening point tests, the viscositiesgive a clear indication of the stiffening effect of SBSmodification. The modification indices between the twogroups are relatively similar but generally higher at100◦C than at 160◦C as well as slightly higher for Bi-tumen B. However, other than requiring higher mixingand compaction temperatures, the results do not indi-cate any significant rheological differences between thebase and modified bitumens or between the two SBSPMB groups at these elevated temperatures.

3.2. Dynamic viscoelastic parametersIsochronal plots of complex modulus (G∗) versus tem-perature at 0.02 and 1 Hz for both SBS PMB groups areshown in Figs 2 and 3. Unlike the conventional binderproperties, the isochronal plots show a difference inthe degree of modification between Bitumen A and B,particularly at the upper and lower ends of the temper-ature domain at the lower frequency of 0.02 Hz. ForBitumen A, although there are only minor increases inG∗ at low temperatures due to SBS modification, thereis considerable evidence of extreme polymeric mod-ification at high temperatures with the approach of aplateau region indicative of a dominant polymer net-work. Compared to Bitumen A, the increase in G∗ forthe Group B SBS PMBs is not as marked at high tem-peratures. However, there is a more uniform increasein G∗ over the entire temperature domain indicating adifference in the compatibility of the two PMB basebitumen-polymer systems and as a consequence a dif-ference in their rheological characteristics. In addition,the differences between Figs 2 and 3 highlight the in-creased frequency-dependence of Group A comparedto Group B. Whereas the isochronal plots for GroupB are similar at 0.02 and 1 Hz, the isochronal plotsfor the Group A PMBs at 1 Hz show no evidence of ahigh temperature plateau region compared to that seenat 0.02 Hz.

Phase angle isochrones at 0.02 and 1 Hz for the SBSPMBs are presented in Figs 4 and 5. Measurementsof δ are generally considered to be more sensitive tothe chemical structure and therefore the modificationof bitumen than complex modulus. The phase angleisochrones clearly illustrate the improved elastic re-sponse (reduced phase angles) of the modified binderscompared to their respective base bitumens. What isnoticeable is the considerable difference in the extent

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Figure 4 Isochronal plots of phase angle at 0.02 Hz for SBS modified bitumens.

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Figure 5 Isochronal plots of phase angle at 1 Hz for SBS modified bitumens.

of the modification for the two groups particularly athigh temperatures (>35◦C) at the lower frequency of0.02 Hz. Whereas the phase angles of the two base bi-tumens approach 90◦ and therefore predominantly vis-cous behaviour with increasing temperatures, the SBSpolymer significantly improves the elastic response of

the modified binders. This increase in elastic responseat high temperatures can be attributed to the viscosity ofthe base bitumens being low enough to allow the elas-tic network of the polymer to influence the mechanicalproperties of the modified binders [7]. At the higherfrequency of 1 Hz (Fig. 5), the increased viscosity of

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the base bitumens reduces the effect of the polymernetwork.

The influence of the SBS polymer network is morenoticeable for the Bitumen A PMBs where the elasticresponse (decreased phase angle) is greater, particularlyat the lower frequency of 0.02 Hz. The decreased phaseangles demonstrate the tendency of the polymer to forma continuous elastic network when dissolved/dispersedin the bitumen [30]. The presence and nature of theplateau and the polymer network is a function of thechemical and physical properties of the SBS polymerand base bitumen and the compatibility of these twocomponents.

3.3. Rheological master curvesand Black diagrams

The frequency dependence of complex modulus andphase angle for the SBS PMBs has been assessed inFigs 6 and 7 by producing rheological master curvesat a reference temperature of 25◦C using the time-temperature superposition principle (TTSP) [30] andshift factors determined for the G∗ master curves. Thecomplex modulus master curves in Fig. 6 show slightlydifferent trends between the two PMB groups. For thebase bitumen A PMBs there is a significant increasein G∗ at low frequencies where the SBS polymer net-work is particularly dominant with a plateau being ap-proached for the 5 and 7% polymer content PMBs. Al-though the Group B PMBs show a similar increase incomplex modulus with increasing polymer content, noplateau can be recognised.

The phase angle master curves for the SBS PMBs(Fig. 7) show a reduction in phase angle with modifica-

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Figure 6 Master curves of complex modulus at 25◦C for SBS PMBs.

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Figure 7 Master curves of phase angle at 25◦C for SBS PMBs.

tion and the presence of a phase angle plateau at inter-mediate loading frequencies. The phase angle plateauis an indication of the presence of polymer elastic net-works or entanglements in the modified binders. In thecase of SBS PMBs, this polymer network is formed bythe physical cross-linking of polystyrene blocks. Thesignificant difference between the two bitumen groupsoccurs at low frequencies where the nature of the poly-mer network is dependent on the properties of the basebitumen (maltenes composition) and the compatibilityof the bitumen-polymer system. Whereas the polymernetwork for the Group A PMBs results in a continuedreduction in phase angle (increased elastic response) atlow frequencies, the opposite is true for the Group BPMBs where there is an increase in phase angle (de-creased elastic response). The differences in the plotsof phase angle versus frequency may be related to dif-ferences in the molecular interaction (e.g., dispersion,swelling and compatibility) between the two base bitu-mens and the SBS polymer.

The effect of SBS modification on the rheologicalparameters (complex modulus and phase angle) hasbeen combined in the form of Black diagrams (com-plex modulus versus phase angle [31]) in Fig. 8. Theenhanced polymer modification for the Group A PMBscompared to the Group B PMBs can be clearly seen inthe figure and is more significant at the higher polymercontents. The morphology and therefore the rheologicalcharacteristics of the PMBs are functions of the mutualinteraction of polymer and bitumen and consequentlyare influenced by bitumen composition and polymernature and content. At the lower polymer content (3%)the behaviour of the modified binder remains closerto that of the base bitumen. For optimum performance

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Figure 8 Black diagrams of SBS modified bitumens.

at elevated temperatures and low frequencies, the SBSpolymer needs to form a continuous rubber-elastic net-work when dissolved/dispersed in the bitumen, as dis-cussed above.

3.4. Changes in rheological properties afterlaboratory ageing

3.4.1. Dynamic viscoelastic parametersIsochronal plots of complex modulus and phase angleat 0.02 Hz for A7 and B7 in their unaged, RTFOT andPAV aged conditions are shown in Fig. 9. As with theSBS modication of the different base bitumen groups,there are distinct differences in the rheological charac-teristics of the two PMBs after ageing. Although bothbinders show an increase in G∗ between 10 and 55◦C,identical to that seen for penetration grade bitumens[27, 32, 33], the behaviour at temperatures greater than55◦C differs between the two PMB groups. For A7instead of an increase there is a decrease in G∗ afterageing, which might be attributed to be a rearrange-ment (degradation) of the SBS copolymer into lowermolecular weight fragments after ageing leading to a“softening” of the PMB [32–35]. This phenomenon isnot evident for B7, as the rubber-elastic network doesnot dominate the rheological behaviour of the PMB tothe same degree as that seen for A7.

The changes in δ after RTFOT and PAV ageing arethe same as those shown for an unmodified bitumen(decrease in δ) within the temperature domain of 10◦Cto approximately 35◦C. This region corresponds toconditions where the base bitumen is dominant and,therefore, this correlation with the behaviour shown

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Figure 9 Isochronal plots at 0.02 Hz for unaged, RTFOT and PAV aged A7 and B7 (complex modulus: solid lines; phase angle: dashed lines).

for unmodified bitumens is expected. In the tempera-ture domain greater than 40◦C, where the SBS polymernetwork is rheologically dominant, the changes afterRTFOT and PAV ageing are different from those expe-rienced for unmodified bitumens. Again the rheologi-cal behaviour for the Group A PMBs differs from thatof Group B particularly above 45◦C. At these elevatedservice temperatures there is an increase in phase angleafter RTFOT and PAV ageing indicating a more viscousresponse for A7. This increase in viscous response afterageing (where the polymer network is dominant) is notevident for B7, although δ does increase slightly afterRTFOT at temperatures greater than 65◦C.

3.4.2. Rheological Black diagramsThe changes in the rheological characteristics of A7 andB7 after ageing are also shown in the form of Black di-agrams in Fig. 10. For A7, the rheological behaviourcan be divided into two areas above and below a com-plex modulus value of 104 Pa. At high stiffness values,corresponding to low temperature and high frequencytests, the Black diagram curves show a shift towardslower phase angles indicating the hardening (ageing)of the PMB. The phenomenon is similar to the hard-ening effect seen for penetration grade bitumens [27].The second area occurs below the complex modulusvalue of 104 Pa and shows an opposite shift of thecurve towards higher phase angles rather than lowerphase angles, thereby indicating a change towards amore viscous rather than more elastic response afterageing. This change is a further illustration of the find-ings of 3.4.1. Once again the changes in the rheological

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Figure 10 Black diagrams for unaged, RTFOT and PAV aged A7 and B7.

characteristics of B7, as depicted in the Black diagramin Fig. 10, are not as marked as those seen for the GroupA PMB. However, there is a shift towards higher phaseangles, as seen for the A7, at G∗ values below 104 Pa.

4. ConclusionsThe rheological properties of road bitumens are im-proved by means of SBS polymer modification as iden-tified by both conventional and more fundamental rhe-ological parameters. Tentatively one may suggest thatthe mechanism associated with SBS polymer modifi-cation consists of a swelling of the polymer throughthe absorption of the light fractions of the base bitumenand the establishing of a rubber-elastic network withinthe modified binder. The nature of the network and itsinfluence on polymer modification is a function of thenature of the base bitumen, the nature and content ofthe polymer and the bitumen-polymer compatibility. Inaddition to conventional binder properties, such as soft-ening point, penetration index and Fraass temperature,which are generally unable to quantify the unique rhe-ological characteristics of different PMB groups, dy-namic mechanical analysis has been used to determinethe rheological parameters of complex modulus andphase angle. SBS polymer modification has increasedthe complex modulus of the two base bitumens, partic-ularly at high temperatures and low frequencies. Theextent of polymer modification has also differed de-pending on the nature of the base bitumen and subse-quently the compatibility of the bitumen-polymer sys-tem. In general, the paraffinic bitumen has shown agreater degree of polymer modification compared to thenaphthenic bitumen PMBs with the differences beingmore pronounced at higher polymer contents, highertemperatures and lower frequencies. It is under theseconditions that the lower viscosity of the base bitu-men allows the polymeric nature of the SBS polymernetwork to dominate the rheological properties of themodified binder.

Laboratory simulative short and long-term ageing ofthe SBS PMBs has shown differences in the rheologi-cal characteristics of the modified binders after ageingcompared to those experienced for penetration gradebitumens. The high polymer content modified bindershave shown some shift towards more viscous behaviourafter ageing.

AcknowledgementsSome of the research reported in this paper formed partof a Brite Euram research project entitled: “QualityAnalysis of Polymer Modified Bitumen Products byMicroscopic Image Analysis with Fluorescent Light”.The Author acknowledges the work of the Danish RoadInstitute, Dansk Vejteknologi Ramboll, Jean Lefebvreand Ooms Avenhorn Holdings, partners in this project,in producing the chemical and conventional binder testdata presented in this paper.

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Received 26 Februaryand accepted 9 October 2003

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