Applied Clay Science 70 (2012) 37–44

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Applied Clay Science

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Conventional asphalt modified by physical mixtures of linear SBSand montmorillonite

Martin Jasso a,⁎, Dusan Bakos a, Jiri Stastna b, Ludovit Zanzotto b

a Department of Plastics and Rubber, Institute of Polymer Materials, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinského 9, 812 37 Bratislava,Slovak Republicb Department of Civil Engineering, Schulich School of Engineering, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4

⁎ Corresponding author at: Department of Civil Engingineering, University of Calgary, ENF 262, 2500 UniverCanada T2N 1N4. Tel.: +1 403 220 4484; fax: +1 403 2

E-mail address: martin.jasso@stuba.sk (M. Jasso).

0169-1317/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.clay.2012.09.004

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 March 2012Received in revised form 29 August 2012Accepted 7 September 2012Available online 17 October 2012

Keywords:AsphaltMontmorilloniteOrgano-montmorilloniteCloisitePolymer modified asphalt

The impact of addition of montmorillonite Cloisite (Ca++) and organo-montmorillonite (Cloisite 20A) onthe high temperature properties of conventional asphalt modified by linear SBS block copolymer was studied.Linear viscoelastic properties of these materials were probed by small amplitude oscillatory shear flows.Nonlinear properties of prepared materials were studied in start-up shear flows. In the linear viscoelastic do-main the presence of organo-montmorillonite was clearly manifested in the dynamic material functions. Sim-ilarly, in the nonlinear domain the effect of organo-montmorillonite was seen in the stress response uponstart-up of steady shear stress. The first normal stress difference N1 was quite large in these materials. Thestrong disturbance generated in transient experiments did not destroy the internal structure of materials,and after a sufficiently long resting period the materials returned to their initial structural state.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Asphalt is an old construction material (Read and Whiteoak, 2002)with a complicated internal structure. Basically, it is a complex mixtureof molecules with different molar masses and polarities arranged in aloosely organized micellar system held together by relatively weakbonds and interactions (chemical and physical) between its constitu-ents. As such, asphalt is in pseudo-equilibrium, i.e. its structure dependsstrongly on temperature and the stresses and strains applied to it. This isreflected in the rheological behavior of asphalt as a consequence ofchanges in the internal structure of the material.

Asphalt is used as waterproofing and protective coating. However, avast amount of asphalt production is used for the preparation of bindersin road construction. In order to improve the thermo-mechanical prop-erties of asphalt, it is frequently modified by various polymers (Airey,2003, 2004; Airey et al., 2002; Becker et al., 2003; Chen et al., 2002;Elseifi et al., 2003; Fawcett and McNally, 2001; Kumar et al., 2004; Luand Redelius, 2002; Polacco et al., 2006; Wloczysiak et al., 1997a,1997b). Polymer nanocomposites (PNCs) with improved thermo-mechanical properties, reduced flammability and enhanced barrierproperties were known for almost two decades but the use of PNCs asasphalt modifiers was not widely studied. Exceptions were described

eering, Schulich School of En-sity Dr. NW, Calgary, Alberta,82 7026.

rights reserved.

by (Galooyak et al., 2010; Ouyang et al., 2005; Yu et al., 2007; Zhanget al., 2012).

The addition of inorganic particles to polymers is a well-knownmethod of generating new materials with targeted properties. Specif-ically, layered silicate nanocomposites have attracted the attention ofengineers and technologists. When new nanoparticle-based materialsare designed, great care has to be taken to avoid particle aggregationthat can lead to inferior material properties for such nanocomposites.The nature of the silicate filler determines the morphology of the finalproduct‐layered silicate polymer nanocomposites. In intercalatednanocomposites (Ouyang et al., 2006; Wang et al., 2006) a regularalternation of the layered silicates and polymer monolayers can beobserved and the original crystallographic structure is not destroyed.In exfoliated nanocomposites, the layered silicates are randomly andhomogeneously distributed throughout the polymer matrix. Amixed intercalated–exfoliated structure can often be observed(Polacco et al., 2008; Ray and Okamoto, 2003). Conventional inorgan-ic fillers are usually added to polymers in high mass contents (20% to40%). However, in nanocomposites, these contents can be as low as2% to 5%.

Presently, a large number of polymers are used for successful fab-rication of nanocomposites. Montmorillonite seems to be a preferableinorganic filler due to its availability. To improve the chance that themontmorillonite particles are incorporated into a polymer, the hydro-philic silicate is often modified by an ion exchange reaction betweenthe interlayer inorganic and organic cations. After such modification,the surface of the the montmorillonite becomes hydrophobic, andthe interlayer spaces are expanded, thus allowing for the insertion

38 M. Jasso et al. / Applied Clay Science 70 (2012) 37–44

of polymer chains (Zhong et al., 2005). Similarly, the introduction ofpolar groups into the polymeric part can help to disperse the claymineral particles in the polymeric matrix.

In this article, we investigate asphalt modification with Mtnanocomposites. Various preparation techniques for clay mineral–polymer nanocomposites were used (Polacco et al., 2008; Zhong etal., 2005). One such technique uses the solution-intercalation method(Jordan, 1949; Okada et al., 1987), where the solvent dissolves thepolymer and delaminates the clay mineral particles. For our purposes(asphalt modification), this method seems to be useful. In a three-component system, asphalt may play the role of solvent (due to itspolarity and low molar mass). For asphalt modification by a polymernanocomposite, there is an important difference from the solution-intercalation method, where the removal of solvent is an importantstep of this method. However, asphalt remains as the main part ofthe blend, and it is not removed from the system. Thus, the prepara-tion of asphalt modified by clay mineral–polymer nanocomposite canbe done in the same way as with the conventional modification of as-phalt by a polymer (Polacco et al., 2008). In this method, one can ei-ther gradually add polymer and clay mineral into a high shear mixercontaining asphalt directly or first prepare the clay mineral–polymernanocomposites by melt compounding and then mixing thenanocomposite with asphalt. The latter method is, of course, moretime-consuming andmore expensive and, thus, is not so suitable for tech-nological utilization.

It is known (Polacco et al., 2006) that styrene–butadiene–styrene(SBS) block copolymer is a good modifier for asphalt, improving boththe low- and high-temperature properties of the resulting polymermodified asphalt (PMA). These enhancements are quite important inreducing the rutting in paving mixes prepared with such modifiedbinders. A small amount of sulfur is often added when asphalt is modi-fied by SBS block copolymer. It is believed that sulfur improves the stor-age stability, i.e. decreases the risk of phase separation in PMAsprepared with SBS.

Before turning our attention to PMAs, at least some of thework doneon the rheology of nanocomposites should be briefly mentioned. Rheo-logical properties of clay mineral–polymer nanocomposites were stud-ied since the early 1990s (Usuki et al., 1995; Yano et al., 1993). For ourpurposes, we consider the studies of Krishnamoorti and Yurekli(2001) and Solomon et al. (2001) as seminal. These authors clearly for-mulated the role of rheology in the study of the structure of clayminer-al–polymer nanocomposites. They showed that rheologicalexperiments on nanocomposites are complementary to the traditionalmethods of material characterization, such as electron microscopy andX-ray diffraction. One has to be aware that rheological methods canonly indirectly probe the structure of materials.

The studies of the linear viscoelastic properties of nanocompositescan show the effect of adding clay mineral particles to the studiedpolymer. Nonlinear properties of nanocomposites, which are studiedin the startup of steady shear flow, have been used in many papers, inorder to explore the effect of particle loading, methods of preparationof nanocomposites and the role of modifiers of inorganic material(e.g. clay mineral particles). Many authors have tried to demonstratethat rheological measurements are sensitive to the internal structureof nanocomposites. Vermant et al. (2007) studied polypropylene/montmorillonite nanocomposites prepared by melt mixing in steadyand transient experiments. The contributions of the flow-induced ori-entation of the clay mineral particles and particle network buildupwere determined via rheological investigations. Similarly, polypro-pylene nanocomposites prepared in an internal mixer and a twin-screw extruder were investigated by Letwimolnun et al. (2007).

Forward and reverse startup experiments pioneered by Solomon etal. (2001) produced stress overshoots, and the amplitudes of thesewererelated to the degree of exfoliation in the studied nanocomposites. Onemodel for colloidal dispersions and another model for fiber dispersionswere used for the description offlow reversal experiments. Bothmodels

were not quite successful in the complete characterization of the ob-served phenomena. Mobuchon et al. (2009a) studied clay mineral dis-persions in a non-polar Newtonian blend of polybutenes. Usingtwo-dimensional small-amplitude shear oscillations, they found thatboth the flow-induced orientation and large-scale microstructuralrearrangements contributed to the transient response of the studiednon-aqueous clay mineral dispersions. The same authors claimed(Mobuchon et al., 2009b) that the microstructure characteristic lengthscale (again in non-aqueous clay mineral dispersions) was dependenton the shear rate due to the reversible shear-induced aggregationprocess.

Ternary blends of polyethylene/polyamide/organoclay were stud-ied by Huitric et al. (2009). In polyethylene (PE) matrix blends, theclay mineral particles were essentially located at the interface of thetwo polymers. The thickness of the interface increased with the claymineral fraction. Oscillatory and steady shear experiments showedthat PE matrix ternary blends behaved as polymer blends. On theother hand, sufficiently filled polyamide (PA) matrix blends exhibitedyield behavior, i.e. their behavior was dominated by the dispersedorgano-Mt.

Linear and nonlinear rheological properties of claymineral–polymernanocomposites prepared by melt mixing of biodegradable polymers(poly-butylene succinate-co-adipate) were studied by Eslami et al.(2010). As in many nanocomposites, a low-frequency plateau in thestorage modulus (G′) was observed and attributed to the structure net-work observed at large time scales. At higher clay mineral loadings, thesteady-state viscosity showed a significant increase at low shear rates.At higher shear rates, the viscosity converged to that of the polymerma-trix. A peculiar behavior of the normal stress differences was detected.At low shear rates, their values were larger than those in pure polymer.An inverse relation was observed at higher shear rates. An attempt tomodel the observed behavior was only partially successful.

In this contribution, the rheological properties of conventionalasphalt (200/300 Penetration grade) modified by physical mixing ofasphalt, SBS block copolymer, a small amount of sulfur, montmoril-lonite (Cloisite CA++) and organo-montmorillonite (Cloisite CA20)– both from Southern Clay Products, Inc. – were studied.

2. Materials and methods

2.1. Materials

Conventional asphalt (a commercial product of Husky Energy, Inc.)prepared by vacuum distillation of crude oil to 200/300 Penetrationgrade with a ring and ball softening point (RB) of 37.4 °C was used asthe base for the preparation of polymer/organo-Mt modified asphalts(PMAs). This base asphalt was modified by medium molar mass linearSBS block copolymer with a styrene content of 31 mass%. The secondmodifier was either montmorillonite Cloisite CA++ or organo-montmorillonite Cloisite CA20 from Southern Clay Products, Inc. CA20was prepared from a sodium montmorillonite having a content of0.93 meq/g of Me2(HT)2NCl (dimethyl dihydrogenated-tallow ammo-nium chloride), where the hydrogenated tallow was a composition ofalkyl chains as C18 65%, C16 30% and C14 5%.

2.2. Methods

The PMAs were prepared by using a high shear mixer. The compo-nents were mixed until no separate parts of polymer were observed.The parts of mixtures were gradually dispersed into asphalt at tem-peratures above 180 °C. After mixing, the prepared binders wereleft to stabilize and were finally stored in a freezer before further rhe-ological testing.

Small-amplitude oscillatory shear, start-up of steady shear andrepeated-interrupted start-up tests were conducted in a TA-ARES-33Arheometer. This rotational rheometer operates under a strain control

Fig. 1. Dynamic material functions of PMA sample A. T=58 °C. o – G″, ● — G′, tan δ. Fig. 3. Dynamic viscosity for PMA samples A, A3, A1o, A2o, and A3o.T=58 °C. — A,◊ — A3, □ — A1o, △ — A2o, ▽ — A3o.

39M. Jasso et al. / Applied Clay Science 70 (2012) 37–44

regime. All the tests were performed at a temperature (T) of 58 °C. Indynamic tests, the frequency was varied from 0.01 to 100 rad/s, andthe geometry was 25 mm plate-plate with a gap of 1 mm. Smallamounts of each PMA were slowly heated to the testing temperatureand then placed into the rheometer, where they were left at the testingtemperature for 30 min before the appropriate test was started. Thelimits of linear viscoelasticity were determined for each sample at theappropriate temperature by running strain sweeps. The dynamic exper-iments were run at strain well inside the linear viscoelastic domain. Intransient experiments, the shear rate was generally varied from0.5 s−1 to 5 s−1, and a fresh sample was used for each of the start-uptests. In the repeated-interrupted start-up experiments, the same

Fig. 2. Dynamic material functions of PMA samples A3 (a), A1o

sample was used for each test. A shear rate of 2 s−1 was used in allthese tests. Samples were sheared for 100 s, generally followed byrest times from 60 to 3600 s.

3. Results and discussion

The conventional asphalt (denoted as B) was modified by SBS. Asmall amount of sulfur (S) was also used in the preparation of all PMAsamples. Sample A contained 3 mass% of SBS. Asphalt B was modifiedby polymer and montmorillonite. Three PMAs were prepared by the

(b), A2o (c) and A3o (d). T=58 °C. o — G″, ● — G′, tan δ.

Fig. 4. Storage compliance of PMA samples A, A3, A1o, A2o, and A3o. T=58 °C. — A,◊ — A3, □ — A1o, △ — A2o, ▽ — A3o.

Fig. 6. Creep compliance of PMA samples A, A3, A1o, A2o, and A3o. T=58 °C. — A,◊ — A3, □ — A1o, △ — A2o, ▽ — A3o.

40 M. Jasso et al. / Applied Clay Science 70 (2012) 37–44

physical mixing of SBS and Mt with asphalt B: samples A1 (3 mass% ofSBS, 1 mass% of Mt), A2 (3 mass% of SBS, 2 mass% of Mt) and A3(3 mass% of SBS, 3 mass% of Mt). Another three PMAs were also pre-pared with the same procedure; however, instead of Mt, theorgano-Mt was used. These samples are denoted as A1o (3 mass% ofSBS, 1 mass% of organo-Mt), A2o (3 mass% of SBS, 2 mass% oforgano-Mt), and A3o (3 mass% of SBS, 3 mass% of organo-Mt).

All the samples were tested in small-amplitude oscillation tests atT=58 °C with a strain of 5% (in order to ensure that the testing wasin the linear viscoelastic domain — based on the strain sweep at highfrequency). Thus, the role Mt and organo-Mt in the prepared PMAscould be assessed. The transient behavior of the prepared PMAs isdiscussed below.

When comparing the dynamic material functions (G′ — storagemodulus, G″ — loss modulus and tanδ — loss tangent) of samples A,A1, A2 and A3 at T=58 °C, quite similar behavior was observed in therange of frequencies from 0.01 to 100rad/s, e.g. Fig. 1 (sample A —

PMA with no clay mineral) with Fig. 2a (sample A3 — PMA with Mt).In further discussions, we restrict our attention to sample A3 as repre-sentative of the PMAs prepared with Mt.

A different behavior was observed in the PMA samples containingthe organo-Mt (A1o, A2o and A3o). Dynamic material functions ofthese samples are portrayed in Fig. 2b, c and d.

The linear viscoelastic properties of all the prepared PMA samplescan be described by discrete relaxation (Maxwellian) spectra {λi, Gi}

Fig. 5. Loss compliance of PMA samples A, A, A1o, A2o, and A3o. T=58 °C. — A, ◊ —

A3, □ — A1o, △ — A2o, ▽ — A3o.

(Figs. 1, 2). These spectra were calculated from the experimentalvalues of G′ and G″ (at T=58 °C) with the help of IRIS software(Mours and Winter, 2004) by fitting the data to (Ferry, 2000):

G′ ωð Þ ¼ ∑ Gi22i

1þ22i

� � ; G″ ¼ ∑ Gii

1þ22i

� �;wherei ¼ 1;2 ;…;N ð1Þ

where N is the number of relaxation modes.When the discrete relaxation spectra (i.e. also retardation spectra)

of the prepared PMAs are known, all the other linear viscoelastic ma-terial functions of the studied materials can be calculated. For exam-ple, Fig. 3 displays the dynamic viscosity, η′(ω), for PMA samples A,A3, A1o, A2o and A3o.

None of the samples (at T=58 °C) attained the domain where thedynamic viscosity approaches the steady flow viscosity, ηo. Thehighest values of η′ were attained by sample A3o, and the lowestvalues were observed in sample A (no clay mineral). In the samplewith 3 mass% of Mt (A3o), the values of η′ were close, but stilllower than in sample A1o (containing 1 mass% organo-Mt). With in-creasing content of organo-Mt, the dynamic viscosity also increased.

The storage compliance, J′(ω), which is defined as the strain in thesinusoidal deformation in phase with the stress divided by the stress(Ferry, 2000), is a measure of energy stored and recovered per cycle,when different systems are compared at the same stress amplitude

Fig. 7. Stress relaxation modulus of PMA samples A, A3, A1o, A2o, and A3o. T=58 °C.— A, ◊ — A3, □ — A1o, △ — A2o, ▽ — A3o.

Fig. 8. Normalized stress growth in start-up experiment for PMA samples A, A3 and A3o. T=58 °C, shear rates of 5, 2 and 0.5 s−1 ↓.◊ — A3o, △ — A3, ● — A.

41M. Jasso et al. / Applied Clay Science 70 (2012) 37–44

and identical temperature. The storage compliance for the discussedPMAs is shown in Fig. 4.

Here, the steady-state compliance (a measure of the elastic defor-mation in steady flow), Jeo, was not attained by any of the describedsamples (A, A3, A1o, A2o, A3o). Only two of the samples – A3o andA2o –approached the frequency domain where the values of Jeo areprobably reached. PMAs with small amounts (or none) of both clayminerals had higher elastic deformation in steady flows. The differ-ence between the discussed PMAs is a little more pronounced onthe graph of the loss compliance, J″(ω) (Fig. 5).

As J″(ω) is a measure of the energy dissipated or lost as heat percycle, PMAs with higher contents of organo-Mt are less dissipativethan PMAs with smaller amounts of organo-Mt. PMAs without Mthad the highest dissipative property from all the discussed modifiedasphalt samples. This is also reflected in the shear compliance func-tion, J(t) (Fig. 6) PMAs with higher contents of organo-Mt exhibitedsmaller shear deformations (when the same shear stress was usedin creep) than the PMAs with Mt or PMA modified only by SBSblock copolymer.

By plotting the stress relaxation modulus, G(t), a clear distinctionbetween the studied PMAs can be seen at longer times (after 10 s),(Fig. 7). G(t) is a decreasing function of time, and the rate of decreasewas smallest in sample A3o where the content of organo-Mt was thehighest.

Fig. 9. Normalized stress growth in the startup experiment as a function of strain for PMA

The role of particle loading, especially with organo-Mt, was dem-onstrated in the linear viscoelastic domain. However, the question ifthe flow-induced orientation of clay mineral particles dominates theresponse of PMAs remains. The nonlinear material functions may reg-ister such a behavior. The dependence of the dynamic viscosity on fre-quency points to the flow modification of the structure of PMAs withorgano-Mt CA20. The transient response to large deformations in thestart-up of steady shear flows can effectively probe the evolution andrelaxation of the internal structure of the studied materials.

The stress growth experiments and associated overshoots can alsoserve as crucial tests of various rheological models (Lockyer andWalters, 1976). In these tests, the equilibrium state is that of a steadysimple shear flow, and the interest is concentrated on the evolution ofthe transient shear stress and the buildup of the first normal stressdifference (N1) from the rest state to their equilibrium values.

Only the transient experiments at T=58 °C with the PMA samplewithout clay mineral (A), the PMA samples with 3 mass% of Mt (A3)and 3 mass% of organo-Mt (A3o) are discussed here. This choice wasbased on the dynamic behavior of the tested samples in the linear vis-coelastic domain. The stress growth upon the start-up of steady shearwas measured at several shear rates. The response of these PMAs isshown in Figs. 8 to 11, where the shear rates were 0.5, 2 and 5 s−1.

The magnitudes of overshoots depended strongly on the shearrate and also on the presence and type of the Mt. PMA without clay

samples A, A3 and A3o. T=58 °C, shear rates 5, 2, 0.5 s−1 ↓. ◊ — A3o, △ — A3, ● — A.

Fig. 10. Stress growth in start-up experiment as a function of strain for PMA samples A, A3 and A3o. T=58 °C, shear rates of 5, 2 and 0.5 s−1 ↓. ◊ — A3o, △ — A3, ● — A.

42 M. Jasso et al. / Applied Clay Science 70 (2012) 37–44

mineral also exhibited stress overshoots; however, they were gener-ally smaller (for higher shear rates) than the ones in PMAs with3 mass% of Mt or organo-Mt. The PMA with Mt (sample A3) reachedalmost the same overshoots as the PMAwithout any Mt (sample A) athigher shear rates. The positions of overshoots on the strain axis wereconcentrated on the interval between 10 and 30 with γmaxN1~2γmaxτ.Sample A3o had overshoots of shear stress positioned at γ~13 for allthe shear rates (5, 2, 0.5 s−1). The time dependence of the normal-ized stress, τ/τs (where τs is the estimated steady-state value ofshear stress) is shown, (Fig. 8). The stress overshoots of the normalizedstresses for samples A, A3 and A3o are also shown, (Fig. 9).

The magnitudes of shear stress and N1 can be appreciated fromFigs. 10 and 11, where these are plotted with respect to strain. As withpolymeric materials (Islam, 2006; Osaki et al., 2000a, 2000b) the stressovershoots preceded the overshoots of N1. The magnitudes of the N1overshoots were generally ten times larger than the magnitudes ofthe τ overshoots. For a shear rate of 5 s−1, overshoots were highest insample A3o (3 mass% of SBS, 3 mass% of organo-Mt).

The overshoots in N1 were rarely discussed in connection with as-phalt binders. Such large normal stress differences will, of course,play an important role in the characterization of PMAs (Wekumburaet al., 2007). No overshoots in τ or N1 were observed in the startupexperiments with conventional asphalt.

Fig. 11. First normal stress difference as a function of strain for PMA samples of A,

It is usually assumed that, when a polymer and compatible con-ventional asphalt are mixed, the polymer absorbs a part of the lowmolecular mass of the oil fraction of asphalt and becomes swollen(Lu and Isacsson, 2001; Uddin, 2003; Wekumbura et al., 2007).After some time, the swollen strands of polymer connect togetherand form a three-dimensional network. This network then givesstrong linear viscoelastic properties to the formed PMA. This is whyone can observe the stress overshoots in PMA, which have similarcharacteristics as overshoots in polymers (Wekumbura et al., 2007).The presence of montmorillonite in PMAs seems to enhance theabove-mentioned network, especially when organo-Mt was used.

During the shear start-up experiment, the network is disturbed.The important question is: how fast, if ever, such a disturbance“dies” out and the material recovers? The interrupted (shear)start-up test can elucidate the answer to the posed question. Thus,samples A, A3 and A3o were tested (at T=58 °C) in the followingprogram: first, each sample was sheared in the start-up experimentfor 100 s with a shear rate of 2 s−1. Then, the shear was suddenlystopped, and the relaxation of the shear stress was followed. Afterrest times of 60, 600, 1200, 2400 and 3600 s, each sample was sud-denly sheared up again and the stress relaxation was followed. Theresults of this test are shown in Fig. 12. The ratios of the maximumshear stress (τmw) to the steady stress (τs) versus waiting times

A3 and A3o. T=58 °C, shear rates of 5, 2, 0.5 s−1 ↓. ◊ — A3o, △ — A3, ● — A.

Fig. 12. Stress growth in the interrupted shear start-up experiment for PMA samples A, A3 and A3o. T=58 °C, rest times of 60, 600, 1200, 2400 and 3600 s, shear rate of 2 s−1.□ — A3o, ○ — A3, △ — A.

43M. Jasso et al. / Applied Clay Science 70 (2012) 37–44

(τw) are shown in Fig. 13. The full recovery for sample A3o took 1 h;sample A3 recovered after 2700 s. The stress recovery in sample A(without Mt) took 2400 s. Again, the sample with organo-Mt standsout from all the tested samples. The performance of the PMA withMt (A3) was quite close to the one of PMA without Mt at all (A).

3.1. Brief summary of the experimental observations

Small-amplitude oscillatory shear experiments at T=58 °C showedthat PMAs prepared by physical mixing of the soft conventional asphaltwith linear SBS block copolymer (3 mass%), Mt (Cloisite CA++, up to3 mass%) and a small amount of sulfur behaved similarly to the PMAwithout Mt. PMAs with organo-Mt (Cloisite CA20, up to 3 mass%)exhibited different behavior for the dynamic material functions. Atlow concentrations of organo-Mt (b3 mass%), the loss modulus waslarger than the storage modulus (G″>G′) in the range of frequenciesbetween 0.01 and 100 rad/s. When the amount of organo-Mt was in-creased to 3 mass%, a cross-over frequency at ω~0.4 rad/s wasobserved.

Fig. 13. Normalized peak stress growth as a function of rest time for PMA samples A, A3and A3o. Shear rate of 2 s−1.● — A3o, ◊ — A3, ○ — A.

The dynamic behavior of all the tested PMAswas easily described bythe discrete relaxation/retardation spectra, i.e. their linear viscoelasticbehavior was confirmed at T=58 °C and at a frequency range between0.01 and 100 rad/s. PMA sample A3o stood out from all the preparedand tested materials. From the point of technological applications, thelow values of shear compliance function in A3o are interesting, becausethey indicate the low rutting potential of this binder at highertemperatures.

Nonlinear shear start-up experiments again pointed out that sampleA3o had the most interesting properties, particularly the large magni-tudes of overshoots in shear stress, τ, and first normal stress differences,N1. Similar overshoots, albeit much smaller, were observed in otherPMA samples. These overshoots strongly depended on the shear rateand also on the type and amount of the used claymineral. The transientshear stress and the first normal stress difference were scaled withstrain over a range of higher shear rates.

In the interrupted start-up tests, a gradual reconstruction of the ini-tially disturbed internal structure of the studied PMAs was observed.Due to the different magnitudes of the shear disturbance (the highestone in A3o and the lowest one in sample A), the recovery of the internalnetwork took different rest times for different samples before the orig-inal state was restored. One cannot claim that the observed nonlinearbehavior of the studied PMAs was caused mainly by the presence ofclay mineral particles, as similar but weaker effects were observed inPMAwithout Mt. Based on the reported observations, one can concludethat the presence of organo-Mt strengthened the network of the inter-nal structure formed in PMAs (modified by linear SBS block copolymer).Strengthening of such a network was strongest with 3 mass% oforgano-Mt, although the base asphalt was modified by the physicalmixing of SBS and organo-Mt. It has to be stressed that onlyhigh-temperature properties of the tested PMAs were studied.

4. Conclusion

The impact of the addition of Mt and organo-Mt on the high-temperature properties of conventional asphalt modified by linear SBSblock copolymer was studied. The linear viscoelastic properties ofthese PMAs were probed by small-amplitude oscillatory shear flows,and the nonlinear properties of the prepared PMAs were studied in

44 M. Jasso et al. / Applied Clay Science 70 (2012) 37–44

start-up shear flows. Although one cannot expect that asphalt modifiedby a physical mixture of polymer and claymineral contains a highly ho-mogeneous dispersion of the claymineral particles, the obtained resultsshowed that PMA with 3 mass% of organo-Mt had a stronger internalnetwork than the corresponding PMA without Mt. In the linear visco-elastic domain, the presence of organo-Mt was clearly manifested inthe dynamic material functions. Similarly, in the nonlinear domain,the effect of 3 mass% of organo-Mt was seen in the stress responseupon start-up of the steady shear stress. The first normal stress differ-ence, N1, was quite large in this PMA. The strong disturbance generatedin the transient experiments did not destroy the internal structure ofPMAs; and, after a sufficiently long resting period, the materialsreturned to their initial structural state.

Acknowledgment

The authors would like to acknowledge Husky Energy Inc. for itsfinancial support of this work. We would also like to thank Mr. R.Wirth and Mrs. E. Vargova for their help with the preparation of thePMAs.

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