asphalt modification with different polyethylene-based polymers 2005

EUROPEAN

European Polymer Journal 41 (2005) 2831–2844

www.elsevier.com/locate/europolj

POLYMERJOURNAL

Asphalt modification with differentpolyethylene-based polymers

Giovanni Polacco a,*, Stefano Berlincioni a, Dario Biondi a,Jiri Stastna b, Ludovit Zanzotto b

a Dipartimento di Ingegneria Chimica, Universita di Pisa, Via Diotisalvi 2, 56126 Pisa, Italyb Department of Civil Engineering, University of Calgary, 2500 University Drive, Calgary, Canada T2N 1N4

Received 6 May 2005; received in revised form 26 May 2005; accepted 28 May 2005Available online 22 July 2005

Abstract

Several polyethylene and polyethylene-based copolymers were used to modify a 70/100 penetration grade asphaltfrom vacuum distillation. The morphological and storage stability analyses showed that, in all cases, the obtained mate-rials were strongly biphasic and tended to separate into polymer-rich and asphalt-rich phases. However, among thetested polymers, a linear low-density polyethylene allowed for the preparation of a mix that had strongly enhancedmechanical properties, with respect to those of the base asphalt. Mixes with different percentages of this polymer were,therefore, prepared and studied from a rheological point of view, both in the range of small and large deformations. Theanalysis showed that, in spite of its insolubility, the polymer spread continuously through the asphalt matrix and thatthe obtained properties can probably be ascribed to the formation of a very low extent of crosslinking between the poly-mer chains.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Polymer modified asphalts; Polyethylene (PE); Rheology; Crosslinking

1. Introduction

Asphalts are widely employed in several applications,but the most important one is related to the pavingindustry. In consideration of increased traffic loadsand in order to improve pavement performance, poly-mer-modified asphalts (PMA) have been developed dur-ing the last few decades [1]. The added polymer (usually2–6% by weight) can strongly enhance the binder prop-erties and permit the building of safer roads and the

0014-3057/$ - see front matter � 2005 Elsevier Ltd. All rights reservdoi:10.1016/j.eurpolymj.2005.05.034

* Corresponding author. Tel.: +39 0505 11220; fax: +39 050511266.

E-mail address: [email protected] (G. Polacco).

reduction of maintenance costs. At the same time, theaddition of a polymer causes a significant increase inthe production costs and adds operative complicationsthat are mostly related to the mixing and storage. Withregard to the latter, the low compatibility between as-phalt and polymer can lead to phase separation whenthe material is stored at a high temperature (160–200 �C) in the absence of stirring. In such a case, a poly-mer-rich phase migrates to the higher part of the storagetank, while an asphalt-rich phase segregates into thelower part. This results in an inhomogeneous materialthat is useless for paving and can cause troubles dueto the extremely high viscosity of the part with very highpolymer content.

ed.

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Table 1Base asphalt

Penetration (dmm) 69.4Ring and Ball temperature (�C) 47.3

Compositiona (%)Saturates 10.6Aromatics 63.3Resins 15.2Asphaltenes 10.9

a Determined by Iatroscan.

2832 G. Polacco et al. / European Polymer Journal 41 (2005) 2831–2844

Unsaturated thermoplastic elastomers like styrene–butadiene–styrene (SBS) block copolymers are probablythe most commonly used polymers. They enhance an as-phalt�s elastic recovery capacities and, therefore, itsresistance to permanent deformations. Even so thesepolymers are not, in most cases, naturally compatiblewith majority of asphalts, addition of aromatic residues,or use of compatibilizing agents may improve their com-patibility with asphalts significantly. SBS block copoly-mers with higher molecular weight must often bedispersed in asphalt using high shear mixing. However,unsaturated elastomeric polymers are quite expensiveand subjected to degradation when exposed to atmo-spheric agents and mechanical stress. Therefore, theyhave to be added as virgin polymers and even if usedin small percentages they can double the price of the bin-der. This is the reason why many researchers have fo-cused on the use of cheaper materials for asphaltmodification. From this point of view, olefinic polymersare very good candidates. They are available in largequantities with different mechanical properties and atlow cost. Polyethylene (PE) and polypropylene (PP)are plastomers, and they can bring a high rigidity tothe materials and significantly reduce deformations un-der traffic load. Unfortunately, due to their non-polarnature, PE and PP are almost completely immisciblewith asphalt, and their use is usually limited to the pro-duction of impermeable membranes. In this case, thepolymer is added at higher quantities (6–30% by weight)with respect to those used for paving mixes. These prod-ucts are rapidly cooled to room temperature after themixing procedure, thus yielding a consistent pseudo-so-lid material that is highly unstable from a thermody-namic point of view, but whose phase separation doesnot take place for kinetic reasons.

For paving applications, some PMAs obtained witholefinic polymers have been prepared and characterized[2–12], but storage stability remains an unsolved prob-lem. A possible solution is the use of PE-based copoly-mers, where the comonomer is a polar one, either inertor reactive with respect to asphalt. Good examples ofthe former are ethylene–vinylacetate (EVA) and ethyl-ene–butyl acrylate (EBA) random copolymers[2,3,8,9,13–17], while maleic anhydride (MA) and glyc-idylmethacrylate (GMA) were added to PE for reactivefunctionality. Another possibility is the use of reactiveethylene terpolymers (RET), which are random copoly-mers containing both reactive and non-reactive polargroups [18–25].

However, when dealing with asphalts, due to the highcomposition variability depending on their source, it isalways very difficult to generalize any conclusion orestablish a priori on the behaviour of a polymer. Thisis why, in this paper, we started with a preliminary char-acterization of different PMAs obtained by modifyingan asphalt from vacuum distillation with several PE

and PE-based polymers. Low-density polyethylene(LDPE), linear low-density polyethylene (LLDPE),copolymers of polyethylene with acrylic acid (PE–AA),glycidylmethacrylate (PE–GMA) and RET copolymerswere used. Among these, one PMA showed strongly en-hanced properties with respect to the starting base as-phalt and, for that reason, it was deeply characterizedfrom a rheological point of view in order to understandthe reasons for this behaviour.

From a rheological point of view, conventional as-phalt is a viscoelastic material having high temperaturesensitivity, which usually behaves as a low molecularweight polymer [26]. Moreover, it is generally believedthat asphalt is also a rheologically simple material, butthis is not always true for PMAs whose properties canbe completely different from those of the base asphalt[27,28]. Due to the high temperature sensitivity, the mas-ter curves of the viscoelastic material functions forPMAs have to be obtained covering a very large temper-ature (frequency) range in which the behaviour of thesample changes from that of a Newtonian fluid to thatof a glassy fragile solid. Most of the available studiescover the linear viscoelastic region of small deformationsor rates of deformation [29,30]; however, it is sometimesdifficult to distinguish or to characterize different PMAsin the linear viscoelastic region, and an extension to thelarge deformations domain was performed in the presentstudy. Small amplitude oscillations, viscometry and stepstrain experiments were used.

2. Materials and methods

One asphalt from vacuum distillation of 70/100Pen grade, whose properties are reported in Table 1,was modified through the addition of 6% (by weight)of different polyethylenes that were either used as re-ceived from commercial sources or after chemical mod-ification through the addition of polar-reactive groups.The typical mixing procedure is as follows: aluminiumcans of approximately 500 cc were filled with 250–260 g of asphalt and put in a thermoelectric heater.When the asphalt temperature reached 180 �C, a high

G. Polacco et al. / European Polymer Journal 41 (2005) 2831–2844 2833

shear mixer was dipped into the can and set to 4000 rpm.The polymer was added gradually (about 5 g/min) whilekeeping the temperature within the range of 180 ± 1 �Cduring the polymer addition and the subsequent 2 hof mixing. Finally, the obtained PMA was split inappropriate amounts to prepare samples for character-ization. The samples were stored in a refrigerator at�20 �C.

The following polymers were used: Riblene�FF20(LDPE manufactured by Polimeri Europa, MFI = 0.8,density = 0.921 kg/m3, Tm = 110 �C); Riblene�FC20(LDPE manufactured by Polimeri Europa, MFI =0.25, density = 0.922 kg/m3, Tm = 111 �C); Escor�5100(PE–AA manufactured by Exxon Mobil Chemical,AA = 11% w, MFI = 0.8, Tm = 96 �C); Lotader�

AX8930 (PE–AE–GMA manufactured by Atofina,butylacrylate = 24% w, GMA = 3% w, MFI = 4,Tm = 70 �C); Lotader�AX8840 (PE–GMA manufac-tured by Atofina, GMA = 8% w, MFI = 5, Tm =105 �C); Flexirene�FF25 (LLDPE manufactured byPolimeri Europa, MFI = 0.7, density = 0.921 kg/m3,Tm = 125 �C). Other PE–GMAs were prepared in ourlaboratories by melt free radical grafting of GMA onRiblene�FF20, using a Brabender Plastograph mixer,following a procedure described in literature [31–33].Two polymers, with a GMA content of 0.7% and2.16% by weight were used, in the following they are re-ferred as PEGMA1 and PEGMA2, respectively. Theindicated melting temperatures were obtained by Dy-namic Scanning Calorimetry (DSC) measurements, car-ried out under nitrogen flow, with a scanning rate of10 �C/min, using a Pyris Perkin Elmer apparatus cali-brated with indium and tin standards.

After preparation, the mixes were characterized bythe classical Ring and Ball softening point (TR&B)(ASTM D36-76), the storage stability test (UNI EN13399) and fluorescence microscopy (UNI prEN13632). The stability test consists of keeping the PMAin a test tube stored in a vertical position at 180 �C for72 ± 1 h and then taking samples from the top and bot-tom part. The difference of TR&B between the two sam-ples indicates how much of the polymer separates andmigrates to the upper part of the tube, due to its densitybeing lower than that of asphalt. For the morphologicalanalysis, asphalt samples were taken during the PMApreparation directly from the mixed can, and pouredinto small cylindrical moulds (1 cm diameter, 2 cmheight). In order to preserve the instantaneous morphol-ogy, the moulds were preheated to the mixing tempera-ture so that the asphalt was not subjected to quenchingwhen in contact with the metal. When filled, the mouldswere put in an oven at 180 �C for 15 min, cooled toroom temperature and stored at �30 �C. After extrac-tion from the cylindrical mould, the samples were fragilefractured and the fracture surfaces examined under aLEICA, DM LB microscope.

After a preliminary characterization, the most prom-ising polymer was selected and used to produce PMAswith different polymer content to be studied from a rhe-ological point of view. In particular, three PMAs wereobtained by adding 2.0%, 4.0% and 6.0%, by weight,of polymer. Asphalt samples were poured into rubber-ized moulds before being used for rheological testing.The rheometer was a Stresstech by Rheologica Instru-ments, which operates under stress control. The testgeometry was parallel plates (diameters of 20 and8 mm, depending on the test temperature). In dynamicmeasurements, frequency sweep tests were performedin isothermal conditions. The frequency was varied from0.01 to 1 Hz at low temperatures and from 0.05 to 5 Hzat mean and high temperatures. Preliminary strainsweep tests were performed in order to be sure that allexperimental conditions remained in the linear interval.The test temperature varied from �30 �C to 110 �C inorder to construct the master curves of the dynamicmaterial functions in a wide frequency interval by usingthe time temperature superposition (TTS) principle[34,35]. Usually, frequencies in the 10�2–102 Hz intervalare associated with the normal vehicle traffic, while high-er and lower values are correlated to heavier and lightertraffic, respectively [36]. Viscosity measurements wereconducted in the 30–70 �C temperature range and10�3 � 102 s�1 shear rate range. Step strain tests wereperformed at temperatures of 10 and 35 �C, with an ini-tial step varying from 0.1% to 250%.

3. Results and discussion

A set of PMAs was prepared and characterized bythe Ring and Ball softening point, morphological analy-sis and storage stability test. The PMAs are numberedfrom M1 to M8, as reported in Table 2. The first twopolymers, used for blends M1 and M2, are low-densitypolyethylenes with different molecular weights. Aftermixing with the base asphalt, markedly biphasic materi-als were obtained in both cases. As an example, Fig. 1ashows the morphology of M1 after 30 min of high shearmixing. It can be clearly seen that a fluorescent, poly-mer-based phase is dispersed in a dark asphaltic phasein the form of almost spherical particles. Both the factsthat the dispersed phase is in spherical form and that nolinkages are visible between particles indicate that the as-phalt and polymer are strongly immiscible and very lowinterfacial adhesion is present between the two phases.This is confirmed by the Ring and Ball temperatures.TR&B for M1 is equal to 53 �C, which means an increasewith respect to the base asphalt of about 6 �C, even if aquite significant amount of polymer has been added (asimilar increase in TR&B is generally obtained with verysmall polymer percentages). This indicates that the poly-meric phase behaves as a rigid dispersed phase. Of

Table 2PMAs content and softening point

Mix Polymer (6% by weight) Temperature (�C) Mixing time (min) TR&B (�C)

After mix After cure

M1 Riblene�FF20 180 30 53.0 –M2 Riblene�FC20 180 30 53.7 –M3 Escor�5100 180 30 49.4 –M4 Lotader�AX8930 (10%) 180 30 52.6 66.0

Riblene�FC20 (90%)M5 Lotader�AX8840 (7%) 180 30 53.8 58.1

Riblene�FC20 (93%)M6 PEGMA1 180 30 59.2 73.6M7 PEGMA2 190 120 52.3 68.9M8 Flexirene�FF25 190 120 120.5 –


course, after storage at a high temperature, the poly-meric phase completely separated and the obtainedmicrographs (not reported) of the top and bottom partof the tube test appear completely white and black,respectively. Very similar results were obtained for M2(Fig. 1b).

Comparing the morphologies of Fig. 1a and b, it canbe seen that the latter shows a larger diameter of thespheres as could be expected since the molecular weightof Riblene�FC20 (used in M2) is higher than that ofRiblene�FF20 (used in M1). A lower solubility is intrin-sically related to a higher molecular weight. However, inthis case the solubility between the two phases is almostzero and the final dimension of the droplets is, therefore,predominantly determined by physical factors, e.g. den-sity and viscosity, rather than chemical factors. In fact, ahigh shear mixing needs a similar viscosity of the twocomponents to be efficient, and this is the main problemin all PMAs production where the asphalt has a very lowviscosity in comparison to the one of the polymer. Whenpolymer and asphalt are highly incompatible, the resultis a liquid–liquid dispersion where the dimension of thedroplets is determined purely by hydrodynamicconditions.

The third modifier (M3) is a copolymer polyethyl-ene–acrylic acid (PE–AA), which was supposed to bemore compatible due to both the presence of the carbox-ylic functionalities, which enhance the polarity of thepolymer with respect to PE, and the lower molecularweight. Nevertheless, the results were almost identicalto the previous ones: a polymer-rich phase dispersed inthe asphaltic one in the form of droplets, a very low in-crease in TR&B and a complete instability to high tem-perature storage were observed. For this reason, a newattempt (M4) followed, now using as the modifier amix of a low-density PE and a reactive polymer that isa random terpolymer of ethylene, butylacrylate andglycidylmethacrylate. The latter is often used as a com-patibilizer due to its epoxy ring, which can react withseveral functional groups. As an example, the three-term

ring can react with end groups of polyesters or polyam-ines; and therefore, the polymer can be useful in blendslike polyethylene and polyamide-6 [37], or polyolefinsand polyesters [38–40]. For the same reason, RET poly-mers of this kind have been proposed for asphalt modi-fication considering that carboxylic groups are presentin asphaltenic molecules, and they can react with theoxiranic ring. The reaction creates a chemical link be-tween polymer and asphalt micelles that prevents, orat least strongly limits, phase separation and can im-prove storage stability. Moreover, the high polarity ofthe polymer enhances its solubility with asphalt. Onthe other hand, the high number of functional groupspresent in a single RET macromolecule increases the riskof gel formation. For this reason, when an asphalt ismodified with RET, the polymer amount has to be cho-sen very carefully, because an excessive quantity cancause the formation of an insoluble, infusible asphaltgel. This is why RET polymers can be added only in verysmall quantities, which limit their efficacy as modifiers;and today, their use is mainly limited to a small numberof cases or to the role of compatibilizer between an as-phalt and another polymer.

Riblene�FC20 and Lotader�AX8930 were added ina weight ratio equal to 90/10, in such a quantity thatthe total amount of polymer was equal to 6% in the finalPMA. In this case, a period of curing at a high temper-ature is required in order to let the epoxy ring react;therefore, the sample was analyzed both at the end ofmixing and after 24 h of storage in an oven at 180 �C.It can be seen that after the high shear mixing(Fig. 1c), the dimensions of the particles are smaller thanthose reported in Fig. 1b (obtained with the same Rib-lene�). This is an indication that some enhancement incompatibility was probably obtained, even if withoutsignificant effect on TR&B. After the cure, TR&B was sig-nificantly higher, showing that some interaction betweenpolymer and asphalt was finally obtained. However, thecure was done in the absence of stirring and the chemicalreaction proceeded in parallel with phase separation.

Fig. 1. (a) M1 30 min, (b) M2 30 min, (c) M4 30 min, (d) M4 24 h, (e) M7 30 min mix, (f) M7 24 h curing, (g) M7 48 h curing, and(h) M8 2 h mix.


This resulted in a morphology that, in some part of theanalyzed surface, showed big spots of polymer-richphase (Fig. 1d). The morphology of the polymer ‘‘is-lands’’ was very different from the previous ones andindicates that a significant improvement of interfacialadhesion was obtained but, at the same time, the dimen-sions of the dispersion indicates that the reaction wasnot fast enough with respect to migration and aggrega-tion of the dispersed droplets and, therefore, not ableto prevent phase separation. In this case, a three-daystorage test was not necessary because the morphologycannot return to a better dispersion. A possible solution

would be continuous stirring during the cure, but this isnot convenient in industrial applications where storageis always performed in tanks, without or with very slowmixing.

A further attempt (M5) was made by using Lot-ader�AX8840, which has a higher percentage of GMAgroups, but without obtaining significant differenceswith respect to M4. Continuing with the same idea ofthe reactive polymer, another two PMAs were preparedusing two polyethylenes modified with the addition ofGMA functional groups, which were grafted directlyon Riblene�FF20. The two polymers, referred to as


PEGMA1 and PEGMA2, were used in runs M6 andM7, respectively. The advantage with respect to M4and M5 should be that, in this case, all the polymer mac-romolecules were functionalized. After mixing, thePMAs were cured for 24 h at 180 �C. Fig. 1e–g showsthe morphology of M7 after mixing and after curing.It can be noted that, at the end of the mixing, the dis-persed phase is composed of particles still well separatedbut with a quite irregular shape, indicating a bettermixing and possibly an enhanced compatibility of theasphaltic and polymeric phases. After curing, coales-cence is visible, but in a less dramatic extent than thatobserved for M4 and M5. TR&B in M6 after curingreached 73.6 �C, which was the highest TR&B yet. Again,a complete separation was observed when the high tem-perature storage was prolonged for three days.

In mix M8, the modifier was linear low-densitypolyethylene. In this case, a completely different mor-phology was obtained (Fig. 1h). As can be seen, largeirregularly shaped islands of polymer-rich phase weredispersed in the asphalt-rich phase. Of course, thismorphology was destined to separation at high temper-atures and, therefore, to storage instability, but there isan indication of intimate mixing witnessed by the pres-ence of small, irregular, dark regions inside the fluores-cent one, which allows the polymer to really ‘‘modify’’the asphalt properties. In fact, a very interesting Ringand Ball temperature of 120.5 �C was measured for thismix.

At the end of this preliminary section, we can say thatno storage stability was achieved for all the PE-basedpolymers, even for bituminous systems with added com-patibilizing groups. This confirms the well-known prob-lem of compatibility between asphalt and olefinicpolymers. Considering the poor results found withLDPE modified with polar or reactive groups and thehigh cost of this kind of polymer, we choose to studythe rheological properties of PMAs prepared with

Fig. 2. Storage compliance for BA

LLDPE. The base asphalt and PMAs with 2%, 4%and 6% LLDPE were prepared and characterized rheo-logically. In what follows, these materials will be re-ferred as BA (base asphalt) and PMA2, PMA4 andPMA6, respectively.

At first, from the dynamic data taken in isothermalfrequency sweep tests, master curves were obtained;and, it was found that the time–temperature superposi-tion principle held for all the tested materials. In Figs.2 and 3, the master curves (reference temperature equalto 0 �C) of storage (J 0) and loss (J00) compliance for thebase asphalt and PMA2, PMA4 and PMA6 are re-ported. At high frequencies, which correspond to lowtemperatures, the three PMAs showed a similar behav-iour, with jJ*j in the 1 · 10�8–1 · 10�9 Pa�1 range. Thestorage compliance of BA was in the same range of val-ues but was lower than the three PMAs. The fact thatthe different content of polymer was not strongly repre-sented in the dynamic functions at this high frequencydomain is not surprising. It is well known that varioussystems have almost universal behaviour when the glasstransition domain is approached [41,42].

More striking is the curve of BA which, starting athigher values, crossed the other curves and tended tothe lower values of the storage compliance. This is prob-ably due to the fact that the polymer, which has a verylow glass transition temperature, gives an ‘‘elastic’’ con-tribution to the PMAs when the asphalt alone is underits glass transition and is, therefore, stiffer. In fact, thiscould be interpreted as confirmation that the polymerforms a continuum inside the PMA and does not behaveas a completely separate filler. Approaching the lowerfrequencies, the storage compliance of the PMAs be-came lower than that of BA, and this difference rapidlybecame of an order of magnitude.

Unmodified and modified asphalts also differed froma qualitative point of view. J 0 of BA and PMA2increased following a power law dependence. This

, PMA2, PMA4, and PMA6.

Fig. 3. Loss compliance for BA, PMA2, PMA4, and PMA6.

Fig. 4. tand for BA, PMA2, PMA4, and PMA6.


behaviour resembled an amorphous polymer of lowmolecular weight where no equilibrium or steady statecompliance is visible. On the other hand, J 0 of PMAswith higher polymer content showed a marked plateau,corresponding to a quite low compliance modulus,which extended until the end of the investigated fre-quency region. The shape of J 0 in PMA4 and PMA6was one usually observed in very lightly crosslinkedpolymers [35] where, in some cases, both a shear compli-ance associated with entanglement network (JN) and anequilibrium compliance (Je) can be observed. In fact, thecurve of J 0 for the higher polymer content PMA showeda barely visible inflection point that could strengthenthis hypothesis (Fig. 2). The shape of J 0 was also similarto one of an amorphous polymer of high molecularweight; however in that case, the loss compliance curvewould be expected to increase while approaching lowerfrequencies. The loss compliance J 0� of PMA4 andPMA6 had a plateau (Fig. 4) and probably decreased

slightly instead of marked increasing, as would occurin the absence of crosslinking. Moreover, the loss tan-gent curves (Fig. 4), which clearly showed a maximumin all the PMAs, are further indication of the presenceof some degree of crosslinking. A possible explanationis that the thermal treatment causes the formation offree radicals, which induces a small degree of polymerdegradation. Of course, PE is expected to be stable atour operating conditions. However, it cannot be ex-cluded that the combined action of temperature andshear stress could induce the formation of free radicals,which may interact prevalently with tertiary carbonatoms on the chain and induce a polymer degradation.Once a macromolecule contains a free radical, it caneither be subjected to chain scission or react with an-other chain forming a bridge and giving rise to crosslink-ing. In any case the crosslinking, if present, must be to avery small extent as it was not revealed with the solubil-ity test.


A final consideration is that, from the presented mas-ter curves of PMA2, it seems that the tan d curves weremore sensitive to the polymer content than the compli-ance curves. In fact, the compliance of PMA2 was moresimilar qualitatively to the base asphalt than to the otherPMAs, while it was the contrary for the loss tangent.However, in this case an even more highlighted differen-tiation of the rheological curves can be seen using vari-ous modifications of the Cole–Cole plot [43], see Fig. 5.

Whatever the real internal structure of the high poly-mer content PMAs, their rheological behaviour supportsthe hypothesis that the LLDPE used has a significantinteraction with the asphalt matrix. A continuum ‘‘ma-trix’’ of the polymer chains has to be present in orderto yield the curves like the ones presented in Figs. 2–5.The peculiar characteristic of the presented mastercurves can be ascribed to an equilibrium between incom-patibility and miscibility of the two components. Theincompatibility is high so that the morphological analy-sis easily reveals distinct polymer zones where the poly-mer preserves its structure and, therefore, its mechanicalproperties. At the same time, there is enough miscibilityto have a polymer network that involves the whole mate-rial and is responsible for the strong effect on mechanicalproperties, if compared to those of the base asphalt. Thiswas not the case for the other tested polymers. The poly-mer continuum ‘‘matrix’’ allowed the material to show amarkedly ‘‘polymer-like’’ behaviour even if the polymercontent was not particularly high.

Viscosity function at different temperatures was mea-sured for all the materials. For the base asphalt, all thecurves (not reported for the sake of brevity) were similarfrom a qualitative point of view. Starting at low shearrates, there is a Newtonian plateau up until a limit shearrate, where shear thinning behaviour started. The higherthe test temperature, the later the shear thinning ap-

Fig. 5. Modified Cole–Cole plot for B

peared. For the lowest temperature, the beginning ofthe shear thinning could not be observed because, athigh shear rates, measurements are affected by macro-scopic instability of the material. It is not completelyclear how to interpret these curves. However, the sim-plest explanation is that the imposed shear stress reachesa critical value where asphaltenic micelles or micellesaggregates are broken, together with the colloidal struc-ture of the material, and an analogue of gel–sol transi-tion occurs.

The behaviour gradually changed when BA wasmodified with an increasing amount of polymer. Atlow temperatures, PMA2 showed viscosity curves quitesimilar to those of BA; however, for the highest temper-ature, there was a gradual shear thinning with a verysmall first derivative, which extended to the whole rangeof the shear rates used. The phenomenon was more pro-nounced in PMA4 and extremely evident for PMA6(Fig. 6). In the latter case, the Newtonian behaviourcould be observed only at 30 �C and, for the very leftpart of the curve, at 40 �C. The other curves showed acontinuous decrease in viscosity and, in particular at70 �C, a slope reduction appears for a shear rate around10�1 s�1.

The viscosity curves of PMAs with different polymers(styrene–butadiene–styrene block copolymers SBS, eth-ylene–vinylacetate random copolymers and RET) usedas modifiers were extensively studied in a previous work[44]. In all cases, when the polymer was able to form anetwork structure, a shear thinning behaviour wasfound, which started at shear rates lower than those ob-served for the base asphalt and with a quite complexshape to the curves. For certain materials and tempera-tures (e.g. 4% SBS modified asphalts, 110 �C), a doublestep shear thinning like the one first described by Onogiand Asada [45] was observed. Otherwise, curves similar

A, PMA2, PMA4, and PMA6.

Fig. 6. Viscosity curves for PMA6.


to those reported in Fig. 6 were found. Considering thenature of the materials and comparing similar behaviourobserved in other systems [46–50], the polymer-relatedshear thinning was interpreted as a consequence of thetemporary nature of the physically crosslinked structureof the polymer in PMA. A well-defined, double stepshear thinning means that, at first, the induced shear ratedestroys this network thus reducing the viscosity of thesystem; and then, after an intermediate Newtonian pla-teau, the second shear thinning is related to the colloidalstructure of the asphaltic matrix.

In the case of the LLDPE we used, as is shown inFig. 6, the shear thinning was more or less homoge-neously distributed along the shear rate axis and, in thisrespect, the only anomaly is the slope change observedat 70 �C. To interpret the curves, the internal structureof the PMAs has to be considered. The materials aremarkedly biphasic, as clearly evidenced by the morpho-logical analysis. However, at the same time the mechan-ical properties demonstrated that the polymer islandswere not simply dispersed in the asphaltic phase. If thiswas the case, no significant improvement in TR&B andmodification in the rheological properties would beobserved. Therefore, the PE macromolecules form acontinuum, which bridges between two or more poly-mer-rich zones. Thus, a physical network where ‘‘rigid’’PE domains are interconnected through polymer chainsis formed. Considering its high heterogeneity, the systemis composed of domains with a large distribution ofdimensions and, at the same time, probably connectedwith bridges of different length and strength. As a conse-quence, there are domains with very different ‘‘mobil-ity’’, determined by both their dimension and thenumber and strength of bridges that bond them to otherdomains. In shearing, a progressive disruption of thenetwork structure occurs, starting from the most weakly

bonded domains and gradually proceeding to those thatare ‘‘stronger’’. This explains the observed curves, wherethe above-mentioned slope changes were just a hint ofwhat could be a plateau modulus in a more homoge-neous system. With regard to the viscosity measure-ments, it must be underlined that BA satisfied theCox–Merz rule, while none of the PMAs did (curvesare not reported). In fact, this was expected and is fur-ther confirmation of the complexity and heterogeneityof the produced materials.

The last part of the rheological characterization is re-lated to step strain experiments. The stress relaxationmodulus G is defined as the stress/strain (s/c) ratio atconstant deformation. For small shear strains, the func-tion G is independent of the magnitude of shear strainsand G is a function of time only. For large strains, therelaxation modulus is a function of two variables,G = G(t, c). This function is experimentally accessiblein the step strain experiment [35].

The step strain experiments were performed at tem-peratures T = 10 �C for BA, 30 �C for PMA2, and35 �C for PMA4 and PMA6, for a set of strains thatwould be considered small in most of the dynamic testsfor bituminous materials. Interestingly enough, even atsuch small strains all the tested materials showed a cleardependence of the relaxation modulus on the strain;however, no master curves were obtainable by verticalshifting. Figs. 7–10 show 3D representation of G(t, c)for several strains. The observation, made from the dy-namic master curves, is confirmed by plotting the 3Dgraphs of G(t, c). In the ‘‘glassy’’ domain (t < 1 s), thebase asphalt reached a plateau that seems to be higherthan the values of G(t, c) measured for the tested PMAs.One has to be cautious in stating this observation, sinceonly in the base asphalt is there an indication of such aplateau, and it was quite difficult to recognize the

Fig. 7. Damping and fit of BA.

Fig. 8. Damping and fit of PMA2.


approach to the ‘‘glassy’’ plateau in the tested PMAs.What is clear, from these plots, is that G(t, c) is decreas-ing much faster in the direction of increasing c in alltested PMAs.

In studying the relaxation after large strains, it is fre-quently assumed [51] that the relaxation modulus G(t, c)can be represented by the factorized function

Gðt; cÞ ¼ GðtÞ � hðcÞ

where, G(t) is the linear viscoelastic relaxation modulus(function of time only), and h is the damping function

(function of strain only). The linear viscoelastic part isusually approximated by the superposition of Maxwellmodes [35],

GðtÞ ¼XNi¼1

gi exp½�ðt=kiÞ�

and several (simple) monotonically decreasing functionsof h(c) as used in the literature [52–54], mostly in studiesof polymeric systems. We have found that a betterdescription of G(t, c) in bituminous systems can beachieved if one assumes that




Gðt; cÞ ¼XNi¼1

GiðtÞ � hiðcÞ

i.e. each linear viscoelastic relaxation mode has its owndamping function.

In shearing, the magnitude of the shear strain is justthe time scaled by the applied shear rate, thus we assumethat h(c) has the same functional form as G(t). Gener-ally, when Maxwell modes are considered, one needsmany modes to describe the linear viscoelastic relaxationmodulus G(t). With the stretch exponential type of relax-ation modes [55] (exp(�(t/a)b)), the number of modes is

drastically reduced. Hence, for the description of G(t, c)(given by the data of the step strain experiment), wehave assumed the following form:

Gðt; cÞ ¼XNi¼1

gi exp � tai

� �bi

þ cci

� �di" # !

Note that with bi = di = 1, the Maxwell type of modesdamping is obtained.

The fit of the step strain data, for all the discussedmaterials, is given by the surface represented in Figs.7–10, with N = 2. It is seen from these figures that whenthe measured G(t, c) increased quickly (for higher c and


shorter t), the model has problems with capturing thedata. For PMA2 and PMA4, one will need additionaldata at higher strains to avoid the large gradients ofthe fit surface in the strain direction. On the other hand,the base asphalt and PMA6 seem to be reasonably wellfitted by the form of G(t, c). With the three Maxwell typemodes (accompanied by the appropriate damping), thelarge gradients in the c direction can be avoided; how-ever, the residual between the fit surface and data arelarger than for the case of the stretch exponential relax-ation modes with a similar damping. In a crude approxi-mation, one can assume that two or three mainrelaxation mechanisms are necessary for the basicdescription of nonlinear mechanical response in bitumi-nous systems.

4. Conclusions

Several polyethylenes and polyethylene-based poly-mers were used to modify an asphalt from vacuum dis-tillation. As expected, in all cases the obtainedpolymer-modified asphalt had a heterogeneous structureand was subjected to storage instability. The modifica-tion of the olefinic chain with the addition of functionalgroups allowed for an improvement of the miscibility be-tween polymer and asphalt but, in no case, was theenhancement sufficient to obtain a homogeneous andstable mix. Among the tested polymers, a linear low-density polyethylene showed a greater compatibilitywith the asphalt and, quite surprisingly, led to the prep-aration of an asphalt binder with strongly enhancedmechanical properties when compared with otherblends. The rheological properties of the base asphaltand of polymer-modified asphalts containing differentpercentages of this modifier were studied in the rangesof small and large deformations. The obtained resultssuggest two main pictures. Different from the otherpolymers used, the LLDPE was not confined in dropletsseparated from each other and dispersed in the asphalticmatrix, but probably was somehow spread continuouslythrough the domain in space. The rheological analysissuggested a possible formation, to a small extent, ofcrosslinking, which may form due to thermomechanicalstress during mixing. The presence of such crosslinkinghas to be confirmed with other experimental evidence;however it is consistent and could explain the observedmacroscopic behaviour and mechanical properties.

Step strain experiments were performed at mid-rangetemperatures where the tested materials could be classi-fied as soft solids (liquid-like character is more pro-nounced at temperatures higher than 30 �C), and thewall slip may have played a role in some experiments.Moreover, the step strain experiment performed in astress-controlled rheometer might suffer from the effectof imperfect strain history [56]. Thus, the domains of

very high gradients may not properly reflect the materialbehaviour, and the proposed simulation of G(t, c) fails insuch domains. On the other hand, the possibility of hav-ing at least a rough description of relaxation in asphalt(conventional and also PMA) with only three dampedrelaxation modes is ‘‘attractive’’. Further experimentsneed to be performed in order to improve the storagestability of these blends and to understand why differentpolyethylenes showed such marked dissimilarities intheir interactions with asphalt.

Acknowledgements

The authors express their gratitude to the NaturalSciences and Engineering Research Council of Canadaand to Husky Energy Inc. for their financial supportof this work, and to Dr. Mariano Pracella and Dr. Do-natella Chionna for kindly providing polymers PEG-MA1 and PEGMA2.

References

[1] Whiteoak CD. The Shell Bitumen Handbook. Surrey,UK: Shell Bitumen; 1990.

[2] Perez-Lepe A, Martınez FJ, Gallegos C, Gonzalez O,Munoz ME, Santamarıa A. Influence of the processingconditions on the rheological behaviour of polymer-mod-ified bitumen. Fuel 2003;82(11):1339–48.

[3] Hinishoglu S, Agar E. Use of waste high density polyeth-ylene as bitumen modifier in asphalt concrete mix. MaterLett 2003;58(3–4):267–71.

[4] Ait-Kadi A, Brahimi B, Bousmina M. Polymer blends forenhanced asphalt binders. Polym Eng Sci 1996;36(12):1724–33.

[5] Giavarini C, De Filippis P, Santarelli ML, Scarsella M.Production of stable polypropylene-modified bitumens.Fuel 1996;75(6):681–6.

[6] Gonzalez O, Pena JJ, Munoz ME, Santamarıa A, PerezLepe A, Martınez F, et al. Rheological techniques as a toolto analyze polymer–bitumen interactions: bitumen modi-fied with polyethylene and polyethylene-based blends.Energy Fuels 2002;16(5):1256–63.

[7] Yousefi AA. Polyethylene dispersions in bitumen: theeffects of the polymer structural parameters. J Appl PolymSci 2003;90(12):3183–90.

[8] Fawcett AH, McNally T, McNally GM, Andrews F,Clarke J. Blends of bitumen with polyethylenes. Polymer1999;40(23):6337–49.

[9] Fawcett AH, McNally T. Blends of bitumen with variouspolyolefins. Polymer 2000;41(14):5315–26.

[10] Panda M, Mazumdar M. Utilization of reclaimed poly-ethylene in bituminous paving mixes. J Mater Civil Eng2002;14(6):527–30.

[11] Kellomaki A, Korhonen M. Dispersion of polyolefins inbitumen by means of tall oil pitch, and microscopiccharacterization of mixes. Fuel 1996;75(7):896–8.

[12] Yousefy AA, Ait-Kadi A, Roy C. Composite asphaltbinders: effect of modified RPE on asphalt. J Mater CivilEng 2000;12(2):113–23.


[13] Airey GD. Rheological evaluation of ethylene vinyl acetatepolymer modified bitumens. Construct Building Mater2002;16(8):473–87.

[14] Garcia-Morales M, Partal P, Navarro FJ, Martinez-BozaF, Gallegos C, Gonzalez N, et al. Viscous properties andmicrostructure of recycled EVA modified bitumen. Fuel2004;83(1):31–8.

[15] Isacsson U, Lu X. Characterization of bitumens modi-fied with SEBS, EVA and EBA polymers. J Mater Sci1999;34(15):3737–45.

[16] Garcıa Morales M, Partal P, Navarro FJ, Martınez-BozaF, Mackley MR, Gallegos C. The rheology of recycledEVA/LDPE modified bitumen. Rheol Acta 2004;43(5):482–90.

[17] Gonzalez O, Munoz ME, Santamarıa A, Garcıa-MoralesM, Navarro FJ, Partal P. Rheology and stability ofbitumen/EVA blends. Eur Polym J 2004;40(10):2365–72.

[18] Dawans F. EP Patent 0448425, 1991.[19] Van Der Werff JC, Nguyen SM. US Patent 5,519,073,

1996.[20] Labib ME, Chollar BH, Mohammed GM. US Patent

6,478,951, 2002.[21] Gallagher KP, Vermilion DR. US Patent 5,604,274, 1997.[22] Engber SL, Reinke GH. WO Patent 9,628,513, 1996.[23] Van der Werff JC. US Patent 5,574,095, 1996.[24] Polacco G, Stastna S, Biondi D, Antonelli F, Vlachovicova

Z, Zanzotto L. Rheology of asphalts modified withglycidylmethacrylate functionalized polymers. J ColloidInterface Sci 2004;280(2):366–73.

[25] Da Silva LS, De Camargo Forte MM, De AlencastroVignol L, Medeiros Cardozo NS. Study of rheologicalproperties of pure and polymer-modified Brazilian asphaltbinders. J Mater Sci 2004;39(2):539–46.

[26] Lesueur D, Gerard JF. A structure-related model todescribe asphalt linear viscoelasticity. J Rheol 1996;40(5):813–36.

[27] Dickinson EJ. The visco-elastic behavior of confined thinfilms of bitumen in tension and compression. Trans SocRheol 1970;14:77–81.

[28] Attane P, Soucemarianadin A, Turrel A, Prud�homme JB.Nonlinear behavior of asphalts in steady and transientshear flow. Rheol Acta 1984;23:297–310.

[29] Stastna J, Zanzotto L. Linear response of regular asphaltsto external harmonic fields. J Rheol 1999;43(3):719–34.

[30] Cheung CY, Cebon D. Experimental study of purebitumens in tension, compression and shear. J Rheol1997;41(1):45–73.

[31] Samay G, Nagy T, White JL. Grafting maleic anhydrideand comonomers onto polyethylene. J Appl Polym Sci1995;56(11):1423–33.

[32] Clark DC, Baker WE, Whitney RA. Peroxide-initiatedcomonomer grafting of styrene and maleic anhydride ontopolyethylene: effect of polyethylene microstructure. J ApplPolym Sci 2001;79(1):96–107.

[33] Wei Q, Chionna D, Galoppini E, Pracella M. Function-alization of LDPE by melt grafting with glycidyl methac-rylate and reactive blending with polyamide-6. MacromolChem Phys 2003;204(8):1123–33.

[34] Dealy JM, Wissbrun KF. Melt rheology and its role inplastics processing. Dordrecht, Netherlands: Kluwer Aca-demic Publishers; 1999.

[35] Ferry JD. Viscoelastic properties of polymers. NewYork: Wiley; 1980.

[36] Bull AL, Vonk WC, Shell Chemical Papers, TPE 6.3.1,1994.

[37] Chiono V, Filippi S, Yordanov H, Minkova L, MagagniniP. Reactive compatibilizer precursors for LDPE/PA6blends. III: Ethylene–glycidylmethacrylate copolymer.Polymer 2003;44(8):2423–32.

[38] Pazzagli F, Pracella M. Reactive compatibilization ofpolyolefin/PET blends by melt grafting with glycidylmethacrylate. Macromol Symp 2000;149(1):225–30.

[39] Loyens W, Groeninckx G. Ultimate mechanical propertiesof rubber toughened semicrystalline PET at room temper-ature. Polymer 2002;43(20):5679–91.

[40] Holsti-Miettinen RM, Heino MT, Seppala J. Use of epoxyreactivity for compatibilization of PP/PBT and PP/LCPblends. J Appl Polym Sci 1995;57(5):573–86.

[41] Hedvig P. Dielectric spectroscopy of polymers. Bris-tol: Adam Hilger; 1977.

[42] Ngai KL. Universality of low-frequency fluctuations.Dissipation and relaxation properties of condensed matter.I. Comments Solid State Phys 1979;9(4):127–40.

[43] Cole KS, Cole RH. Dispersion and absorption in dielec-trics I. Alternating current characteristics. J Chem Phys1941;9(4):341–51.

[44] Polacco G, Stastna J, Vlachovicova Z, Biondi D, ZanzottoL. Temporary networks in polymer modified asphalts.Polym Eng Sci 2004;44(12):2185–93.

[45] Onogi S, Asada T. Rheology and rheo-optics of polymerliquid crystals. In: Astarita G, Marrucci G, Nicolais L,editors. Proceedings of the VIII international congress onrheology. New York: Plenum; 1980. p. 127–47.

[46] Cato D, Edie DD. Flow behavior of mesophase pitch.Carbon 2003;41(7):1411–7.

[47] Kiss G, Porter RS. Rheology of concentrated solutions ofpoly(c-benzyl-glutamate). J Polym Sci: Part B: Polym Phys1996;34(14):2271–89.

[48] Suto SM, Ito OR, Karasawa M. Capillary rheometry oflyotropic liquid crystals of hydroxypropyl cellulose. Poly-mer 1987;28(1):23–32.

[49] Magda JJ, Baek SG, De Vries KL. Shear flows of liquidcrystal polymers: measurements of the second normalstress difference and the Doi molecular theory. Macromol-ecules 1991;24(15):4460–8.

[50] Sigillo I, Grizzuti N. The effect of molecular weight on thesteady shear rheology of lyotropic solutions. A phenom-enological study. J Rheol 1994;38(3):589–99.

[51] Wagner MH. Prediction of primary normal stress differ-ence from shear viscosity data using a single integralconstitutive equation. Rheol Acta 1977;16:43–50.

[52] Wagner MH. Analysis of time-dependent non-linear stress-growth data for shear and elongational flow of a low-density branched polyethylene melt. Rheol Acta 1976;15:136–42.

[53] Laun HM. Description of the non-linear shear behavior ofa low density polyethylene melt by means of an experi-mentally determined strain dependent memory function.Rheol Acta 1978;17(1):1–15.

[54] Soskey PR, Winter HH. Large step shear strain experi-ments with parallel-disk rotational rheometers. J Rheol1984;28(5):625–45.


[55] Kohlrausch F. Experimental-untersuchung uber die elasti-sche Nachwirkung bei der Torsion, Ausdehnung undBiegunge. Pogg ANN Phys 1876;8:337–41.

[56] Venerus DC. A critical evaluation of step strain flows ofentangled linear polymer liquids. J Rheol 2005;49(1):277–95.

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