Asphalt modification with different polyethylene-based polymers

EUROPEAN POLYMER JOURNAL

European Polymer Journal 41 (2005) 2831–2844

www.elsevier.com/locate/europolj

Asphalt modification with different polyethylene-based polymers Giovanni Polacco a,*, Stefano Berlincioni a, Dario Biondi a, Jiri Stastna b, Ludovit Zanzotto b b

a Dipartimento di Ingegneria Chimica, Universita` di Pisa, Via Diotisalvi 2, 56126 Pisa, Italy 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 2005 Available online 22 July 2005

Abstract Several polyethylene and polyethylene-based copolymers were used to modify a 70/100 penetration grade asphalt from vacuum distillation. The morphological and storage stability analyses showed that, in all cases, the obtained materials were strongly biphasic and tended to separate into polymer-rich and asphalt-rich phases. However, among the tested polymers, a linear low-density polyethylene allowed for the preparation of a mix that had strongly enhanced mechanical 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. The analysis showed that, in spite of its insolubility, the polymer spread continuously through the asphalt matrix and that the obtained properties can probably be ascribed to the formation of a very low extent of crosslinking between the polymer 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 paving industry. In consideration of increased traffic loads and in order to improve pavement performance, polymer-modified asphalts (PMA) have been developed during the last few decades [1]. The added polymer (usually 2–6% by weight) can strongly enhance the binder properties and permit the building of safer roads and the * Corresponding author. Tel.: +39 0505 11220; fax: +39 0505 11266. E-mail address: [email protected] (G. Polacco).

reduction of maintenance costs. At the same time, the addition of a polymer causes a significant increase in the production costs and adds operative complications that are mostly related to the mixing and storage. With regard to the latter, the low compatibility between asphalt and polymer can lead to phase separation when the material is stored at a high temperature (160– 200 C) in the absence of stirring. In such a case, a polymer-rich phase migrates to the higher part of the storage tank, while an asphalt-rich phase segregates into the lower part. This results in an inhomogeneous material that is useless for paving and can cause troubles due to the extremely high viscosity of the part with very high polymer content.

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

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Unsaturated thermoplastic elastomers like styrene– butadiene–styrene (SBS) block copolymers are probably the most commonly used polymers. They enhance an asphaltÕs elastic recovery capacities and, therefore, its resistance to permanent deformations. Even so these polymers are not, in most cases, naturally compatible with majority of asphalts, addition of aromatic residues, or use of compatibilizing agents may improve their compatibility with asphalts significantly. SBS block copolymers with higher molecular weight must often be dispersed in asphalt using high shear mixing. However, unsaturated elastomeric polymers are quite expensive and subjected to degradation when exposed to atmospheric agents and mechanical stress. Therefore, they have to be added as virgin polymers and even if used in small percentages they can double the price of the binder. This is the reason why many researchers have focused on the use of cheaper materials for asphalt modification. From this point of view, olefinic polymers are very good candidates. They are available in large quantities with different mechanical properties and at low cost. Polyethylene (PE) and polypropylene (PP) are plastomers, and they can bring a high rigidity to the materials and significantly reduce deformations under traffic load. Unfortunately, due to their non-polar nature, PE and PP are almost completely immiscible with asphalt, and their use is usually limited to the production of impermeable membranes. In this case, the polymer is added at higher quantities (6–30% by weight) with respect to those used for paving mixes. These products are rapidly cooled to room temperature after the mixing procedure, thus yielding a consistent pseudo-solid material that is highly unstable from a thermodynamic point of view, but whose phase separation does not take place for kinetic reasons. For paving applications, some PMAs obtained with olefinic polymers have been prepared and characterized [2–12], but storage stability remains an unsolved problem. A possible solution is the use of PE-based copolymers, where the comonomer is a polar one, either inert or reactive with respect to asphalt. Good examples of the former are ethylene–vinylacetate (EVA) and ethylene–butyl acrylate (EBA) random copolymers [2,3,8,9,13–17], while maleic anhydride (MA) and glycidylmethacrylate (GMA) were added to PE for reactive functionality. Another possibility is the use of reactive ethylene terpolymers (RET), which are random copolymers containing both reactive and non-reactive polar groups [18–25]. However, when dealing with asphalts, due to the high composition variability depending on their source, it is always very difficult to generalize any conclusion or establish a priori on the behaviour of a polymer. This is why, in this paper, we started with a preliminary characterization of different PMAs obtained by modifying an 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 copolymers were used. Among these, one PMA showed strongly enhanced properties with respect to the starting base asphalt and, for that reason, it was deeply characterized from a rheological point of view in order to understand the reasons for this behaviour. From a rheological point of view, conventional asphalt is a viscoelastic material having high temperature sensitivity, which usually behaves as a low molecular weight polymer [26]. Moreover, it is generally believed that asphalt is also a rheologically simple material, but this is not always true for PMAs whose properties can be completely different from those of the base asphalt [27,28]. Due to the high temperature sensitivity, the master curves of the viscoelastic material functions for PMAs have to be obtained covering a very large temperature (frequency) range in which the behaviour of the sample changes from that of a Newtonian fluid to that of a glassy fragile solid. Most of the available studies cover the linear viscoelastic region of small deformations or rates of deformation [29,30]; however, it is sometimes difficult to distinguish or to characterize different PMAs in the linear viscoelastic region, and an extension to the large deformations domain was performed in the present study. Small amplitude oscillations, viscometry and step strain experiments were used.

2. Materials and methods One asphalt from vacuum distillation of 70/100 Pen 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 received from commercial sources or after chemical modification through the addition of polar-reactive groups. The typical mixing procedure is as follows: aluminium cans 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

Table 1 Base asphalt Penetration (dmm) Ring and Ball temperature (C)

69.4 47.3

Compositiona (%) Saturates Aromatics Resins Asphaltenes

10.6 63.3 15.2 10.9

a

Determined by Iatroscan.

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

shear mixer was dipped into the can and set to 4000 rpm. The polymer was added gradually (about 5 g/min) while keeping the temperature within the range of 180 ± 1 C during the polymer addition and the subsequent 2 h of mixing. Finally, the obtained PMA was split in appropriate amounts to prepare samples for characterization. The samples were stored in a refrigerator at
20 C. The following polymers were used: RibleneFF20 (LDPE manufactured by Polimeri Europa, MFI = 0.8, density = 0.921 kg/m3, Tm = 110 C); RibleneFC20 (LDPE manufactured by Polimeri Europa, MFI = 0.25, density = 0.922 kg/m3, Tm = 111 C); Escor5100 (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); LotaderAX8840 (PE–GMA manufactured by Atofina, GMA = 8% w, MFI = 5, Tm = 105 C); FlexireneFF25 (LLDPE manufactured by Polimeri Europa, MFI = 0.7, density = 0.921 kg/m3, Tm = 125 C). Other PE–GMAs were prepared in our laboratories by melt free radical grafting of GMA on RibleneFF20, using a Brabender Plastograph mixer, following a procedure described in literature [31–33]. Two polymers, with a GMA content of 0.7% and 2.16% by weight were used, in the following they are referred as PEGMA1 and PEGMA2, respectively. The indicated melting temperatures were obtained by Dynamic Scanning Calorimetry (DSC) measurements, carried out under nitrogen flow, with a scanning rate of 10 C/min, using a Pyris Perkin Elmer apparatus calibrated with indium and tin standards. After preparation, the mixes were characterized by the classical Ring and Ball softening point (TR&B) (ASTM D36-76), the storage stability test (UNI EN 13399) and fluorescence microscopy (UNI prEN 13632). The stability test consists of keeping the PMA in a test tube stored in a vertical position at 180 C for 72 ± 1 h and then taking samples from the top and bottom part. The difference of TR&B between the two samples indicates how much of the polymer separates and migrates to the upper part of the tube, due to its density being lower than that of asphalt. For the morphological analysis, asphalt samples were taken during the PMA preparation directly from the mixed can, and poured into small cylindrical moulds (1 cm diameter, 2 cm height). In order to preserve the instantaneous morphology, the moulds were preheated to the mixing temperature so that the asphalt was not subjected to quenching when in contact with the metal. When filled, the moulds were put in an oven at 180 C for 15 min, cooled to room temperature and stored at
30 C. After extraction from the cylindrical mould, the samples were fragile fractured and the fracture surfaces examined under a LEICA, DM LB microscope.

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After a preliminary characterization, the most promising polymer was selected and used to produce PMAs with different polymer content to be studied from a rheological point of view. In particular, three PMAs were obtained by adding 2.0%, 4.0% and 6.0%, by weight, of polymer. Asphalt samples were poured into rubberized moulds before being used for rheological testing. The rheometer was a Stresstech by Rheologica Instruments, which operates under stress control. The test geometry was parallel plates (diameters of 20 and 8 mm, depending on the test temperature). In dynamic measurements, frequency sweep tests were performed in isothermal conditions. The frequency was varied from 0.01 to 1 Hz at low temperatures and from 0.05 to 5 Hz at mean and high temperatures. Preliminary strain sweep tests were performed in order to be sure that all experimental conditions remained in the linear interval. The test temperature varied from
30 C to 110 C in order to construct the master curves of the dynamic material functions in a wide frequency interval by using the time temperature superposition (TTS) principle [34,35]. Usually, frequencies in the 10
2–102 Hz interval are associated with the normal vehicle traffic, while higher and lower values are correlated to heavier and lighter traffic, respectively [36]. Viscosity measurements were conducted in the 30–70 C temperature range and 10
3
102 s
1 shear rate range. Step strain tests were performed at temperatures of 10 and 35 C, with an initial step varying from 0.1% to 250%.

3. Results and discussion A set of PMAs was prepared and characterized by the Ring and Ball softening point, morphological analysis and storage stability test. The PMAs are numbered from M1 to M8, as reported in Table 2. The first two polymers, used for blends M1 and M2, are low-density polyethylenes with different molecular weights. After mixing with the base asphalt, markedly biphasic materials were obtained in both cases. As an example, Fig. 1a shows the morphology of M1 after 30 min of high shear mixing. It can be clearly seen that a fluorescent, polymer-based phase is dispersed in a dark asphaltic phase in the form of almost spherical particles. Both the facts that the dispersed phase is in spherical form and that no linkages are visible between particles indicate that the asphalt and polymer are strongly immiscible and very low interfacial 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 increase with respect to the base asphalt of about 6 C, even if a quite significant amount of polymer has been added (a similar increase in TR&B is generally obtained with very small polymer percentages). This indicates that the polymeric phase behaves as a rigid dispersed phase. Of

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Table 2 PMAs content and softening point Mix

Polymer (6% by weight)

Temperature (C)

Mixing time (min)

TR&B (C)

M1 M2 M3 M4

RibleneFF20 RibleneFC20 Escor5100 LotaderAX8930 (10%) RibleneFC20 (90%) LotaderAX8840 (7%) RibleneFC20 (93%) PEGMA1 PEGMA2 FlexireneFF25

180 180 180 180

30 30 30 30

53.0 53.7 49.4 52.6

– – – 66.0

180

30

53.8

58.1

180 190 190

30 120 120

59.2 52.3 120.5

73.6 68.9 –

After mix

M5 M6 M7 M8

course, after storage at a high temperature, the polymeric phase completely separated and the obtained micrographs (not reported) of the top and bottom part of 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 can be seen that the latter shows a larger diameter of the spheres as could be expected since the molecular weight of RibleneFC20 (used in M2) is higher than that of RibleneFF20 (used in M1). A lower solubility is intrinsically related to a higher molecular weight. However, in this case the solubility between the two phases is almost zero and the final dimension of the droplets is, therefore, predominantly determined by physical factors, e.g. density and viscosity, rather than chemical factors. In fact, a high shear mixing needs a similar viscosity of the two components to be efficient, and this is the main problem in all PMAs production where the asphalt has a very low viscosity in comparison to the one of the polymer. When polymer and asphalt are highly incompatible, the result is a liquid–liquid dispersion where the dimension of the droplets is determined purely by hydrodynamic conditions. The third modifier (M3) is a copolymer polyethylene–acrylic acid (PE–AA), which was supposed to be more compatible due to both the presence of the carboxylic functionalities, which enhance the polarity of the polymer with respect to PE, and the lower molecular weight. Nevertheless, the results were almost identical to the previous ones: a polymer-rich phase dispersed in the asphaltic one in the form of droplets, a very low increase in TR&B and a complete instability to high temperature storage were observed. For this reason, a new attempt (M4) followed, now using as the modifier a mix of a low-density PE and a reactive polymer that is a random terpolymer of ethylene, butylacrylate and glycidylmethacrylate. The latter is often used as a compatibilizer due to its epoxy ring, which can react with several functional groups. As an example, the three-term

After cure

ring can react with end groups of polyesters or polyamines; and therefore, the polymer can be useful in blends like polyethylene and polyamide-6 [37], or polyolefins and polyesters [38–40]. For the same reason, RET polymers of this kind have been proposed for asphalt modification considering that carboxylic groups are present in asphaltenic molecules, and they can react with the oxiranic ring. The reaction creates a chemical link between polymer and asphalt micelles that prevents, or at least strongly limits, phase separation and can improve storage stability. Moreover, the high polarity of the polymer enhances its solubility with asphalt. On the other hand, the high number of functional groups present in a single RET macromolecule increases the risk of gel formation. For this reason, when an asphalt is modified with RET, the polymer amount has to be chosen very carefully, because an excessive quantity can cause the formation of an insoluble, infusible asphalt gel. This is why RET polymers can be added only in very small quantities, which limit their efficacy as modifiers; and today, their use is mainly limited to a small number of cases or to the role of compatibilizer between an asphalt and another polymer. RibleneFC20 and LotaderAX8930 were added in a weight ratio equal to 90/10, in such a quantity that the total amount of polymer was equal to 6% in the final PMA. In this case, a period of curing at a high temperature is required in order to let the epoxy ring react; therefore, the sample was analyzed both at the end of mixing 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 than those reported in Fig. 1b (obtained with the same Riblene). This is an indication that some enhancement in compatibility was probably obtained, even if without significant effect on TR&B. After the cure, TR&B was significantly higher, showing that some interaction between polymer and asphalt was finally obtained. However, the cure was done in the absence of stirring and the chemical reaction proceeded in parallel with phase separation.

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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 the analyzed surface, showed big spots of polymer-rich phase (Fig. 1d). The morphology of the polymer ‘‘islands’’ was very different from the previous ones and indicates that a significant improvement of interfacial adhesion was obtained but, at the same time, the dimensions of the dispersion indicates that the reaction was not fast enough with respect to migration and aggregation of the dispersed droplets and, therefore, not able to prevent phase separation. In this case, a three-day storage test was not necessary because the morphology cannot return to a better dispersion. A possible solution

would be continuous stirring during the cure, but this is not convenient in industrial applications where storage is always performed in tanks, without or with very slow mixing. A further attempt (M5) was made by using LotaderAX8840, which has a higher percentage of GMA groups, but without obtaining significant differences with respect to M4. Continuing with the same idea of the reactive polymer, another two PMAs were prepared using two polyethylenes modified with the addition of GMA functional groups, which were grafted directly on RibleneFF20. The two polymers, referred to as

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PEGMA1 and PEGMA2, were used in runs M6 and M7, respectively. The advantage with respect to M4 and M5 should be that, in this case, all the polymer macromolecules were functionalized. After mixing, the PMAs were cured for 24 h at 180 C. Fig. 1e–g shows the morphology of M7 after mixing and after curing. It can be noted that, at the end of the mixing, the dispersed phase is composed of particles still well separated but with a quite irregular shape, indicating a better mixing and possibly an enhanced compatibility of the asphaltic and polymeric phases. After curing, coalescence is visible, but in a less dramatic extent than that observed for M4 and M5. TR&B in M6 after curing reached 73.6 C, which was the highest TR&B yet. Again, a complete separation was observed when the high temperature storage was prolonged for three days. In mix M8, the modifier was linear low-density polyethylene. In this case, a completely different morphology was obtained (Fig. 1h). As can be seen, large irregularly shaped islands of polymer-rich phase were dispersed in the asphalt-rich phase. Of course, this morphology was destined to separation at high temperatures and, therefore, to storage instability, but there is an indication of intimate mixing witnessed by the presence of small, irregular, dark regions inside the fluorescent one, which allows the polymer to really ‘‘modify’’ the asphalt properties. In fact, a very interesting Ring and Ball temperature of 120.5 C was measured for this mix. At the end of this preliminary section, we can say that no storage stability was achieved for all the PE-based polymers, even for bituminous systems with added compatibilizing groups. This confirms the well-known problem of compatibility between asphalt and olefinic polymers. Considering the poor results found with LDPE modified with polar or reactive groups and the high cost of this kind of polymer, we choose to study the rheological properties of PMAs prepared with

LLDPE. The base asphalt and PMAs with 2%, 4% and 6% LLDPE were prepared and characterized rheologically. In what follows, these materials will be referred as BA (base asphalt) and PMA2, PMA4 and PMA6, respectively. At first, from the dynamic data taken in isothermal frequency sweep tests, master curves were obtained; and, it was found that the time–temperature superposition principle held for all the tested materials. In Figs. 2 and 3, the master curves (reference temperature equal to 0 C) of storage (J 0 ) and loss (J00 ) compliance for the base asphalt and PMA2, PMA4 and PMA6 are reported. At high frequencies, which correspond to low temperatures, the three PMAs showed a similar behaviour, with jJ*j in the 1 · 10
8–1 · 10
9 Pa
1 range. The storage compliance of BA was in the same range of values but was lower than the three PMAs. The fact that the different content of polymer was not strongly represented in the dynamic functions at this high frequency domain is not surprising. It is well known that various systems have almost universal behaviour when the glass transition domain is approached [41,42]. More striking is the curve of BA which, starting at higher values, crossed the other curves and tended to the lower values of the storage compliance. This is probably due to the fact that the polymer, which has a very low glass transition temperature, gives an ‘‘elastic’’ contribution to the PMAs when the asphalt alone is under its glass transition and is, therefore, stiffer. In fact, this could be interpreted as confirmation that the polymer forms a continuum inside the PMA and does not behave as a completely separate filler. Approaching the lower frequencies, the storage compliance of the PMAs became lower than that of BA, and this difference rapidly became of an order of magnitude. Unmodified and modified asphalts also differed from a qualitative point of view. J 0 of BA and PMA2 increased following a power law dependence. This

Fig. 2. Storage compliance for BA, PMA2, PMA4, and PMA6.

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

Fig. 4. tan d for BA, PMA2, PMA4, and PMA6.

behaviour resembled an amorphous polymer of low molecular weight where no equilibrium or steady state compliance is visible. On the other hand, J 0 of PMAs with higher polymer content showed a marked plateau, corresponding to a quite low compliance modulus, which extended until the end of the investigated frequency region. The shape of J 0 in PMA4 and PMA6 was one usually observed in very lightly crosslinked polymers [35] where, in some cases, both a shear compliance associated with entanglement network (JN) and an equilibrium compliance (Je) can be observed. In fact, the curve of J 0 for the higher polymer content PMA showed a barely visible inflection point that could strengthen this hypothesis (Fig. 2). The shape of J 0 was also similar to one of an amorphous polymer of high molecular weight; however in that case, the loss compliance curve would be expected to increase while approaching lower frequencies. The loss compliance J 0 Õ of PMA4 and PMA6 had a plateau (Fig. 4) and probably decreased

slightly instead of marked increasing, as would occur in the absence of crosslinking. Moreover, the loss tangent curves (Fig. 4), which clearly showed a maximum in all the PMAs, are further indication of the presence of some degree of crosslinking. A possible explanation is that the thermal treatment causes the formation of free radicals, which induces a small degree of polymer degradation. Of course, PE is expected to be stable at our operating conditions. However, it cannot be excluded that the combined action of temperature and shear stress could induce the formation of free radicals, which may interact prevalently with tertiary carbon atoms on the chain and induce a polymer degradation. Once a macromolecule contains a free radical, it can either be subjected to chain scission or react with another chain forming a bridge and giving rise to crosslinking. In any case the crosslinking, if present, must be to a very small extent as it was not revealed with the solubility test.

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A final consideration is that, from the presented master curves of PMA2, it seems that the tan d curves were more sensitive to the polymer content than the compliance curves. In fact, the compliance of PMA2 was more similar qualitatively to the base asphalt than to the other PMAs, while it was the contrary for the loss tangent. However, in this case an even more highlighted differentiation of the rheological curves can be seen using various modifications of the Cole–Cole plot [43], see Fig. 5. Whatever the real internal structure of the high polymer content PMAs, their rheological behaviour supports the hypothesis that the LLDPE used has a significant interaction with the asphalt matrix. A continuum ‘‘matrix’’ of the polymer chains has to be present in order to yield the curves like the ones presented in Figs. 2–5. The peculiar characteristic of the presented master curves can be ascribed to an equilibrium between incompatibility and miscibility of the two components. The incompatibility is high so that the morphological analysis easily reveals distinct polymer zones where the polymer preserves its structure and, therefore, its mechanical properties. At the same time, there is enough miscibility to have a polymer network that involves the whole material and is responsible for the strong effect on mechanical properties, if compared to those of the base asphalt. This was not the case for the other tested polymers. The polymer continuum ‘‘matrix’’ allowed the material to show a markedly ‘‘polymer-like’’ behaviour even if the polymer content was not particularly high. Viscosity function at different temperatures was measured for all the materials. For the base asphalt, all the curves (not reported for the sake of brevity) were similar from a qualitative point of view. Starting at low shear rates, there is a Newtonian plateau up until a limit shear rate, where shear thinning behaviour started. The higher the test temperature, the later the shear thinning ap-

peared. For the lowest temperature, the beginning of the shear thinning could not be observed because, at high shear rates, measurements are affected by macroscopic instability of the material. It is not completely clear how to interpret these curves. However, the simplest explanation is that the imposed shear stress reaches a critical value where asphaltenic micelles or micelles aggregates are broken, together with the colloidal structure of the material, and an analogue of gel–sol transition occurs. The behaviour gradually changed when BA was modified with an increasing amount of polymer. At low temperatures, PMA2 showed viscosity curves quite similar to those of BA; however, for the highest temperature, there was a gradual shear thinning with a very small first derivative, which extended to the whole range of the shear rates used. The phenomenon was more pronounced in PMA4 and extremely evident for PMA6 (Fig. 6). In the latter case, the Newtonian behaviour could be observed only at 30 C and, for the very left part of the curve, at 40 C. The other curves showed a continuous decrease in viscosity and, in particular at 70 C, a slope reduction appears for a shear rate around 10
1 s
1. The viscosity curves of PMAs with different polymers (styrene–butadiene–styrene block copolymers SBS, ethylene–vinylacetate random copolymers and RET) used as modifiers were extensively studied in a previous work [44]. In all cases, when the polymer was able to form a network structure, a shear thinning behaviour was found, which started at shear rates lower than those observed for the base asphalt and with a quite complex shape to the curves. For certain materials and temperatures (e.g. 4% SBS modified asphalts, 110 C), a double step shear thinning like the one first described by Onogi and Asada [45] was observed. Otherwise, curves similar

Fig. 5. Modified Cole–Cole plot for BA, PMA2, PMA4, and PMA6.

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

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Fig. 6. Viscosity curves for PMA6.

to those reported in Fig. 6 were found. Considering the nature of the materials and comparing similar behaviour observed in other systems [46–50], the polymer-related shear thinning was interpreted as a consequence of the temporary nature of the physically crosslinked structure of the polymer in PMA. A well-defined, double step shear thinning means that, at first, the induced shear rate destroys this network thus reducing the viscosity of the system; and then, after an intermediate Newtonian plateau, the second shear thinning is related to the colloidal structure of the asphaltic matrix. In the case of the LLDPE we used, as is shown in Fig. 6, the shear thinning was more or less homogeneously distributed along the shear rate axis and, in this respect, the only anomaly is the slope change observed at 70 C. To interpret the curves, the internal structure of the PMAs has to be considered. The materials are markedly biphasic, as clearly evidenced by the morphological analysis. However, at the same time the mechanical properties demonstrated that the polymer islands were not simply dispersed in the asphaltic phase. If this was the case, no significant improvement in TR&B and modification in the rheological properties would be observed. Therefore, the PE macromolecules form a continuum, which bridges between two or more polymer-rich zones. Thus, a physical network where ‘‘rigid’’ PE domains are interconnected through polymer chains is formed. Considering its high heterogeneity, the system is composed of domains with a large distribution of dimensions and, at the same time, probably connected with bridges of different length and strength. As a consequence, there are domains with very different ‘‘mobility’’, determined by both their dimension and the number and strength of bridges that bond them to other domains. In shearing, a progressive disruption of the network structure occurs, starting from the most weakly

bonded domains and gradually proceeding to those that are ‘‘stronger’’. This explains the observed curves, where the above-mentioned slope changes were just a hint of what could be a plateau modulus in a more homogeneous system. With regard to the viscosity measurements, it must be underlined that BA satisfied the Cox–Merz rule, while none of the PMAs did (curves are not reported). In fact, this was expected and is further confirmation of the complexity and heterogeneity of the produced materials. The last part of the rheological characterization is related to step strain experiments. The stress relaxation modulus G is defined as the stress/strain (s/c) ratio at constant deformation. For small shear strains, the function G is independent of the magnitude of shear strains and G is a function of time only. For large strains, the relaxation modulus is a function of two variables, G = G(t, c). This function is experimentally accessible in the step strain experiment [35]. The step strain experiments were performed at temperatures T = 10 C for BA, 30 C for PMA2, and 35 C for PMA4 and PMA6, for a set of strains that would be considered small in most of the dynamic tests for bituminous materials. Interestingly enough, even at such small strains all the tested materials showed a clear dependence of the relaxation modulus on the strain; however, no master curves were obtainable by vertical shifting. Figs. 7–10 show 3D representation of G(t, c) for several strains. The observation, made from the dynamic master curves, is confirmed by plotting the 3D graphs of G(t, c). In the ‘‘glassy’’ domain (t < 1 s), the base asphalt reached a plateau that seems to be higher than the values of G(t, c) measured for the tested PMAs. One has to be cautious in stating this observation, since only in the base asphalt is there an indication of such a plateau, and it was quite difficult to recognize the

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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 decreasing much faster in the direction of increasing c in all tested PMAs. In studying the relaxation after large strains, it is frequently 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 is usually approximated by the superposition of Maxwell modes [35], GðtÞ ¼

N X

gi exp½
ðt=ki Þ

i¼1

and several (simple) monotonically decreasing functions of h(c) as used in the literature [52–54], mostly in studies of polymeric systems. We have found that a better description of G(t, c) in bituminous systems can be achieved if one assumes that

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Fig. 9. Damping and fit of PMA4.

Fig. 10. Damping and fit of PMA6.

Gðt; cÞ ¼

N X

Gi ðtÞ  hi ðcÞ

i¼1

i.e. each linear viscoelastic relaxation mode has its own damping function. In shearing, the magnitude of the shear strain is just the time scaled by the applied shear rate, thus we assume that h(c) has the same functional form as G(t). Generally, when Maxwell modes are considered, one needs many modes to describe the linear viscoelastic relaxation modulus G(t). With the stretch exponential type of relaxation 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), we have assumed the following form: "

d i #! b N X t i c Gðt; cÞ ¼ gi exp
þ a c i i i¼1 Note that with bi = di = 1, the Maxwell type of modes damping is obtained. The fit of the step strain data, for all the discussed materials, is given by the surface represented in Figs. 7–10, with N = 2. It is seen from these figures that when the measured G(t, c) increased quickly (for higher c and

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shorter t), the model has problems with capturing the data. For PMA2 and PMA4, one will need additional data at higher strains to avoid the large gradients of the fit surface in the strain direction. On the other hand, the base asphalt and PMA6 seem to be reasonably well fitted by the form of G(t, c). With the three Maxwell type modes (accompanied by the appropriate damping), the large gradients in the c direction can be avoided; however, the residual between the fit surface and data are larger than for the case of the stretch exponential relaxation modes with a similar damping. In a crude approximation, one can assume that two or three main relaxation mechanisms are necessary for the basic description of nonlinear mechanical response in bituminous systems.

4. Conclusions Several polyethylenes and polyethylene-based polymers were used to modify an asphalt from vacuum distillation. As expected, in all cases the obtained polymer-modified asphalt had a heterogeneous structure and was subjected to storage instability. The modification of the olefinic chain with the addition of functional groups allowed for an improvement of the miscibility between polymer and asphalt but, in no case, was the enhancement sufficient to obtain a homogeneous and stable mix. Among the tested polymers, a linear lowdensity polyethylene showed a greater compatibility with the asphalt and, quite surprisingly, led to the preparation of an asphalt binder with strongly enhanced mechanical properties when compared with other blends. The rheological properties of the base asphalt and of polymer-modified asphalts containing different percentages of this modifier were studied in the ranges of small and large deformations. The obtained results suggest two main pictures. Different from the other polymers used, the LLDPE was not confined in droplets separated from each other and dispersed in the asphaltic matrix, but probably was somehow spread continuously through the domain in space. The rheological analysis suggested a possible formation, to a small extent, of crosslinking, which may form due to thermomechanical stress during mixing. The presence of such crosslinking has to be confirmed with other experimental evidence; however it is consistent and could explain the observed macroscopic behaviour and mechanical properties. Step strain experiments were performed at mid-range temperatures where the tested materials could be classified as soft solids (liquid-like character is more pronounced at temperatures higher than 30 C), and the wall slip may have played a role in some experiments. Moreover, the step strain experiment performed in a stress-controlled rheometer might suffer from the effect of imperfect strain history [56]. Thus, the domains of

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