Effect of waste polymer addition on the rheology of modified bitumen

Fuel 85 (2006) 936–943 www.fuelfirst.com

Effect of waste polymer addition on the rheology of modified bitumen M. Garcı´a-Morales, P. Partal *, F.J. Navarro, C. Gallegos Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias Experimentales, Campus El Carmen, Universidad de Huelva, 21071 Huelva, Spain Received 23 May 2005; received in revised form 23 September 2005; accepted 27 September 2005 Available online 25 October 2005

Abstract This paper deals with the modification of petroleum bitumen with four different types of waste polymers. EVA, EVA/LDPE blend, crumb tire rubber and ABS, all of them coming from recycling plants of waste plastic materials, were used as modifying agents of the bitumen employed in the pavement building. Optical microscopy, modulated calorimetry and a set of different rheological tests were developed in order to characterise the modified bitumens. The results obtained reveal that tire rubber as well as its blends with other polymers can be considered as an interesting modifier of the bitumen in a wide range of temperatures. As an elastomer, it endows the pavement a higher flexibility, which makes it more resistant to the traffic loading. The blend composed of EVA and LDPE displays quite promising results at high in-service temperatures, due to the development of a polymer network throughout the modified bitumen. Furthermore, the calorimetry tests carried out demonstrate different degrees of compatibility between the bitumen and the polymers used. q 2005 Elsevier Ltd. All rights reserved. Keywords: Modified bitumen; Waste polymer; Rheology

1. Introduction Bitumen, the black-coloured substance coming from the bottom of the vacuum distillation columns in the crude oil refineries [1], has been employed in the road building for ages. It has been reported that bitumen owes many interesting properties and characteristics [2,3], such as impermeability, adhesivity, elasticity and cost, which make it the most suitable material as a binder of mineral aggregates in paving applications. Bitumen composition depends on the crude source and the refining process. A large number of different separation procedures, most often defined in terms of fractions obtained by chromatography, have been described [4–6]. The most common is probably that which divides bitumen into four generic groups (SARAs): saturates, aromatics, resins (which make up the maltene fraction) and asphaltenes. Each SARAs fraction is a mixture with a complexity, aromaticity, heteroatom content, and molecular weight that increase in the order S!A!R!As [7,8]. According to the colloidal model, bitumen is composed of solid black particles, the asphaltenes, dispersed into a liquid oily matrix of saturates, aromatics and resins. A shell of resins, whose thickness depends on * Corresponding author. Tel.: C34 959 21 99 89; fax: C34 959 21 99 83. E-mail address: [email protected] (P. Partal).

0016-2361/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2005.09.015

temperature, is set around the asphaltene particles, and an equilibrium is established between the amount of resins around the asphaltenes and those dissolved into the liquid [9–11]. Bitumen composition and temperature strongly influence the mechanical properties and microstructure of bitumen. In this sense, rheology has become a useful tool in the characterisation of the bitumen performance on the pavement [1,12,13], above all since the SHRP (Strategic Highway Research Program) protocol brought up a set of tests and methods in a very wide range of temperatures which allowed to obtain information about the suitability of a given bitumen in a further application. In addition, suitable properties of the final blend at high temperatures, related to the mixing, pumping and lay-down operations, should be accomplished, in accordance with the SHRP protocol [14], for a modified bitumen to be used in paving applications. Low viscosities at high temperature reduce the power-consumption related costs, as well as allow the bitumen to be handled at safe conditions [14]. As a conclusion, an enhanced behaviour of the modified bitumen is commonly demanded in a wide range of temperatures [1]. Unfortunately, neat bitumen no longer results suitable for paving applications, due to the characteristics of the current traffic. Hence, different blends of bitumen with a large variety of materials have been studied. Among them, synthetic polymers seem to be the ones that have worked out better [15–18], due to the fact that polymer addition may result in both a more flexible bituminous binder at low in-service

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temperature and enhanced properties, which significantly prevent the pavement from being deformed, at high in-service temperature. In spite of the small proportion of synthetic polymer added to the binder, the high cost of those may cut down the use of modified bitumen in the road building. For that reason, and as a quite effective way of disposing of the increasing volume of residues which each year are unavoidably generated in different sectors of the human activity, waste plastics [19–22] and rubbers [23] have been applied to the bitumen modification. The purpose of the present work is to study the benefit of the waste polymers addition on the bitumen performance. In that sense, blends of bitumen and several waste polymers were prepared, and a further rheological and microstructural characterisation, mainly, was carried out. Four types of polymers were used in the work: waste EVA and EVA/LDPE blend, sourced from the agriculture, scrap tire rubber and waste ABS. 2. Method Bitumen of penetration grade 60/70, provided by Construcciones Morales S.A. (Spain), was used as base material for polymer modification. Four waste polymers were used as modifying agents: EVA and EVA/LDPE blend from agriculture, crumb scrap computer shell (which was shown to be and referred to as ABS terpolymer hereafter), provided by Egmasa (Spain), and crumb scrap tire rubber, provided by Alfredo Mesalles S.A. (Spain). Asphaltene content, determined by the procedure outlined in ASTM D3279 [24], and penetration grade of the base bitumen, as well as some physico-chemical characteristics of the polymers are shown in Table 1. Blends of bitumen and polymers, at different concentrations, were prepared in an open mixer, using an IKA RW-20 stirring device (Germany). Samples were processed for 6 h, at 180 8C and agitation speed of 1200 rpm, using silicone oil as heating fluid. Modified bitumen was compared with neat bitumen and bitumen processed under the above described conditions, which will be referred to as ‘processed bitumen’ hereafter. The compositions of the modified bitumens studied are shown in Table 2. The rheological characterisation of the neat and modified bitumens was carried out using three different rheometers, namely, a controlled-strain Rheometrics Scientific ARES rheometer (USA) and two controlled-stress Haake RS150 and RS100 rheometers (Germany). Several studies on the wall effects of the larger particles were previously performed. Thus, different gaps with parallel plates geometries (1–3 mm) and no gap-dependent geometries (vane geometries and helical ribbon) were used to examine wall effects on the plate surface and sample spilling on parallel plates configurations. At temperatures below the settling point of the particles, steady flow tests conducted with wide enough gaps showed viscosity values similar to those obtained with vane geometries, where the interactions of particles with the surfaces of plates the are not significant.

937

Table 1 Physico-chemical characteristics of the bitumen and waste polymers used Bitumen Asphaltene content (wt%) Penetration grade (1/10 mm)

20.0 60–70

Waste polymers

Vinyl acetate (wt%) Black carbon (wt%) Tm (8C) DHm (J gK1)

EVA/LDPE (2:1)

EVA

5 1 109/122 77.19

2 – 112.5 62.37 Tire rubber

D32 (mm) Specific surface area (m2 gK1) Total rubber hydrocarbon (wt%) Carbon black (wt%) THF extractable (wt%) Ash (wt%)

211 0.0284 50G5 32G3 11G3 4G2 ABS

D32 (mm) Specific surface area (m2 gK1) Tg (8C)

407 0.0147 95

Viscous flow tests, at 50 8C, were carried out with the RS100 rheometer, coupled with the heating system Haake TC501, and using a serrated plate-and-plate geometry (20 mm diameter, 1–3 mm gap). Frequency sweep tests, at different temperatures, within the linear viscoelasticity region were performed with the RS150 rheometer, using different serrated plate-and-plate geometries (10, 20 and 35 mm diameter, 1– 3 mm gap). Viscous flow curves, at 135 8C, were obtained with the ARES rheometer, using a Couette geometry (ASTM D4402 [25]), having the inner and outer cylinders 32 and 34 mm diameter, respectively. Dynamic temperature sweep tests, at 1% strain, a heating rate of 1 8C minK1 and 10 rad sK1 (AASHTO TP5 [26]) were carried out with the RS150 rheometer, in the range of temperature 20–80 8C. Modulated DSC measurements were carried out with a TA Instruments Q100 (USA), using 5–10 mg of sample in hermetic aluminium pans, an oscillation period of 60 s, amplitude of G 0.50 8C and a heating rate of 5 8C minK1. The sample was purged with nitrogen at a flow rate of 50 cm3 minK1. Optical microscopy was used to study the morphology of the modified bitumens. A LTS-350 Heating-Freezing Stage, manufactured by Linkam Scientific Instruments (UK), coupled with a standard Olympus BX51 microscope, by Olympus Optical (Japan), was employed with that purpose. Samples Table 2 Compositions of the modified bitumens studied

EVA/LDPE (wt%) EVA (wt%) Rubber (wt%) ABS (wt%)

A5

A9

B5

B9

AR5 AR7 AR9 R9

S9

5

9

0

0

2.5

3.5

4.5

0

0

0 0 0

0 0 0

5 0 0

9 0 0

0 2.5 0

0 3.5 0

0 4.5 0

0 9 0

0 0 9

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were prepared by using standard microscope slides (76! 26 mm). Very thin slides of the modified bitumens were set on them and heated up to 75 8C, that is, below the polymer melting point.

3. Results and discussion 3.1. Recycled-polyolefin modified bitumen The effect of polymer addition on the mechanical behaviour of bitumen has been evaluated at low temperature, K20 8C, by means of oscillatory shear measurements (Fig. 1). Processed bitumen shows a significant increase in the elastic modulus, what has been previously related to a bitumen oxidation during its processing, called primary ageing [27–30]. As a result, the resistance to thermal fracture, at low in-service temperatures, of processed bitumen decreases [28–30]. Similar or even higher values are obtained with the polymer–bitumen blends studied. A continuous increase of G 0 in the whole frequency range tested is observed in all the cases, and a trend towards the glassy region is found at high frequencies, with values close to 109 Pa. As can be seen in Fig. 1(A), a bitumen containing 9 wt% polymer shows similar values of G 0 to those corresponding to the processed bitumen in the high frequency region. However, they present a significant decrease in the slope of the elastic modulus versus frequency, which hints that the glassy region, where the binder presents a very high brittleness, may be shifted to higher frequencies. In the same way, the slope of loss tangent decreases as polymer concentration increases (Fig. 1(B)), which suggests that polymer modified bitumens will exhibit higher values of tan d than neat bitumen at higher frequencies (or lower temperatures). This fact would imply interesting effects on the bitumen performance at very low temperature. It is known that the lower the elastic modulus and the higher tan d are, the higher the energy release results, and thus, thermal cracking is less likely to occur as a consequence of the traffic loadings. It is well known, that one of the effects of the addition of a small amount of polymer to bitumen is to increase the viscosity

[15,16,19,27,31–39], beyond the viscosity enhancement due exclusively to the oxidation of the bitumen during the mixing process. Fig. 2 displays the viscous flow curves, at 50 and 135 8C, for the modified bitumens A5, A9, B5 and B9, as well as for the neat and processed bitumens. At 50 8C, the modified bitumens present a clear shear-thinning behaviour. As it has been reported in the literature, the higher the polymer content is, the more evident the pseudoplastic characteristics of the modified binder appear [40]. These flow curves have been fitted to the Ostwald–De Waele model: h Z kg_ nK1

(1)

where k and n are the consistency and flow index, respectively. As can be seen in Table 3, much lower values of the flow index, that is more remarkable shear-thinning properties, are obtained for the bitumen with the highest polymer content, as well as for the binders containing EVA/LDPE. On the contrary, nearly Newtonian behaviour is exhibited by both neat and processed bitumen, at 50 8C, in the range of shear rates considered, although an important increase in viscosity is noticed for the processed bitumen. On the other hand, a significant increase in the consistency index and, therefore, in the binder viscosity, happens as the amount of polymer in the blend reaches 9 wt%, mainly for the sample A9, which suggests better modifying characteristics for this polymer at high in-service temperature (Table 3). Measurement of the binder viscosity at 135 8C (see Fig. 2) is of a particular interest to have some insight in the modified bitumen behaviour at handling, lay-down and compactation temperatures [41]. A polymer concentration of 5 wt% gave rise to systems whose viscosity keeps below the limit (3 Pa s), which, according to AASHTO MP1 [14], should not be exceeded for the blend to be considered as a suitable binder in the pavements building. Through the observation of Fig. 2, it can be concluded that the shear-thinning behaviour of the modified bitumens softens at high temperature. 3.2. Bitumen modification by polymeric fillers Fig. 3 shows the frequency dependence of the linear viscoelasticity functions, at K20 8C, for bitumens modified by

100

109

A

102

B

T=50°C

107

T=135°C

T = –20°C Neat bitumen Processed bitumen A5 B5 A9 B9

107

10–1

100

101

ω [rad·s–1]

102 10–1

100

101

101

η [Pa·s]

108

η [Pa·s]

tan δ

G' [Pa]

106 3 Pa·s 105

100 104

102

ω [rad·s–1]

Fig. 1. Frequency dependence of the linear viscoelasticity functions, at K20 8C, for neat, processed and modified bitumens A5, A9, B5 and B9.

103 10–4

Neat bitumen Processed bitumen A5 B5 A9 B9 Ostwald–De Waele model

10–3

10–2 10–1 . –1 γ [s ]

100

10–2

10–1

100 101 . –1 γ [s ]

10–1

102

Fig. 2. Viscous flow curves, at 50 and 135 8C, for neat, processed and modified bitumens A5, A9, B5 and B9.

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Table 3 Fitting parameters of the Ostwald–De Waele model, at 50 8C, for the different binders studied

k n

A5

A9

B5

B9

AR5

AR7

AR9

R9

S9

19,656 0.856

92,608 0.490

13,965 0.904

51,343 0.767

9034 0.895

15,747 0.815

26,320 0.704

12,603 0.836

7715 0.945

crumb tire rubber, ABS and blends of rubber and recycled polyolefin. Both crumb rubber and ABS additives do not show any melting process during the mixing operations of the modified bitumens. Sample R9 seems to exhibit the most suitable rheological response at low temperature, because the values of G 0 are lower (and tan d are higher) than those found for the neat bitumen in a wide range of the frequency window studied, a result that makes tire rubber an interesting additive for bitumen modification in cold climates. In fact, tire rubber, as an elastomer endows the binder a larger flexibility and prevents it from thermal cracking [23]. On the contrary, binder S9 exhibits a response which would give rise to a non-desirable behaviour as subjected to loading at low temperatures. The blend was prepared by mixing the bitumen and a rigid organic filler, ABS, which seems to be unable to improve the mechanical characteristics of the binder, because the blend becomes extremely brittle as temperature decreases. This kind of filler, as pointed out by some authors [42], may be considered as a Griffith cracking initiator, because cracking is likely to initiate in the points where the filler particles are located. Viscous flow curves for different blends of bitumen and waste polymers, at 50 8C, are displayed in Fig. 4. The Ostwald– De Waele model fits fairly well the experimental results obtained. The values of the consistency and flow indexes are presented in Table 3. As can be observed, a remarkable increase in viscosity occurs as bitumen is mixed with 9 wt% tire rubber (sample R9). The modifying ability of the ABS is revealed by the mere observation of the flow curve for binder S9. As can be seen, the viscosity values of binder S9 are quite similar to those obtained for the processed bitumen. For this reason, the viscosity increase should be mainly attributed to an 100 A

106

B

108 T = –20°C Neat bitumen Processed bitumen AR5 AR7 AR9 R9 S9

107 10–1

100

101

ω [rad·s–1]

102 10–1

100

101

105

Neat bitumen Processed bitumen AR5 AR7 AR9 R9 S9 Ostwald-De Waele model

η [Pa·s]

tan δ

G' [Pa]

T=50˚C

T=135˚C

102 Neat bitumen Processed bitumen AR5 AR7 AR9 R9

101

η [Pa·s]

109

oxidation process in the bitumen and not to polymer modification [21]. From previous results, it can be sustained that polyolefins mainly improve the asphalt performance at high in-service temperature. Thus, 5 wt% EVA/LDPE modified bitumen shows higher viscosity than a binder containing 9 wt% crumb tire rubber at 50 8C (see Figs. 2 and 4, and Table 3). Moreover, 5 wt% polyolefin modified bitumens show a lower viscosity dependence with shear rate, as may be deduced from the values of flow index, n, shown in Table 3 (nZ0.856 for A5 and nZ 0.836 for R9). On the contrary, the addition of an elastic polymeric filler, such as crumb rubber, provides the best results in the low temperature regime. As can be observed in Figs. 1 and 3, the values of the linear viscoelasticity functions, at K20 8C, for binder R9 are much lower than those shown by binder A5. In order to obtain enhanced mechanical characteristics at both low and high in-service temperatures, bitumens modified with different concentrations of polyolefin/crumb rubber blends were prepared (see Figs. 3 and 4). Thus, Fig. 3 shows that the values of the linear viscoelasticity functions, at K20 8C, decrease as the total concentration of the polymeric blend in the bitumen increases (from AR5 to AR9). On the contrary, viscosity, at 50 8C, increases as the total polymer concentration in the bitumen is raised (see Fig. 4). On the other hand, the flow curves obtained at 135 8C show that binder AR5 is the only system with a lower viscosity than the limit stated by AASHTO MP1 [14]. Binder R9 shows values of viscosity lower than 3 Pa s at shear rates higher than 2 sK1, whereas AR7 presents values of viscosity slightly above the limit. Therefore, these systems should be also taken into account since, as the specifications make clear, they can be used if the supplier warrants safe pumping conditions. In this sense, binder AR7 may be considered as an interesting

3 Pa·s

104

102

ω [rad·s–1]

Fig. 3. Frequency dependence of the linear viscoelasticity functions, at K20 8C, for neat and processed bitumen, and different blends of bitumen and waste polymers.

103 10–3

100

10–2

10–1

. γ [s–1]

100

10–2 10–1

100

101

. γ [s–1]

102

10–1 103

Fig. 4. Viscous flow curves, at 50 and 135 8C, for neat and processed bitumen, and different blends of bitumen and waste polymers.

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formulation with improved rheological characteristics at low (values of the linear viscoelasticity functions lower than those shown by binder A5 and processed bitumen) and high (values of viscosity quite similar to those shown by binder A5) inservice temperature. The remaining binder (AR9) owns values of viscosity that significantly exceed the above-mentioned specifications. 3.3. Rheology and bitumen-polymer compatibility Previous results have pointed out the different modification ability of the recycled polymers tested. In this sense, the differences found between EVA and EVA/LDPE modified bitumen and crumb rubber and ABS modified bitumens should be understood. Temperature sweep tests, at 10 rad sK1, in the linear viscoelasticity region, ranging from 20 to 80 8C, at the conditions outlined in AASHTO TP5 [26], were carried out on samples A5, A9, B5 and B9. The results obtained are portrayed in Fig. 5. In all cases, the values of G 0 and G 00 decrease as temperature increases. Binders containing 5 wt% polyolefin exhibit a similar behaviour, showing a cross-over point around 30 8C from which G 00 stands above G 0 , and thus, the viscous properties prevail over the elastic properties at this fixed frequency. However, binders having 9 wt% polymer content behave in a different way. Thus, sample B9 presents a cross-over point at a higher temperature than sample B5, about 35 8C, due to the presence of a larger amount of polymer which provides it a more significant elastic capacity. On the 10

107

tan δ

106

G',G'' [Pa]

1

ω = 10 rads–1 10

0

A5 B5 10

–1

20

105

104

30

40

50

60

70

80

T [°C]

G'

G'' A5 B5

103 10

1

G',G'' [Pa]

tan δ

107 10

0

A9 B9

106 10

–1

20

30

40

50

60

70

80

T [°C]

105 G'

G'' A9 B9

104 20

30

40

50

60

70

80

T [°C] Fig. 5. Temperature dependence of the linear viscoelasticity functions, at 10 rad sK1, for blends A5, A9, B5 and B9.

contrary, binder A9 shows quite similar values of both linear viscoelasticity functions in a range of temperature comprised between 40 and 80 8C. Once again, polymer A appears more suitable as bitumen modifier. A reduction in the thermal susceptibility of the binder is observed as polymer concentration increases. Thermal susceptibility may be easily deduced by the change in G 0 and G 00 values with temperature, as well as from plots of tan d, for the modified bitumens A5, B5, A9 and B9 (inset Fig. 5). For systems having a polymer content of 5 wt%, tan d increases as temperature does, because the differences between the loss and storage moduli become larger. The same behaviour is shown for the sample B9, although the variation in the loss tangent with temperature is less pronounced than in the former cases. Finally, sample A9 exhibits a behaviour that does not match any of the above mentioned ones, with a plateau region from about 45 8C where the loss tangent reaches the value of 1. The oscillatory shear tests performed at high in-service temperatures (Fig. 6) confirm that bitumen A9 must have a different microstructure. A particular behaviour is shown by sample A9 at 50 8C, in which both G 0 and G 00 curves are parallel and almost coincident in the whole range of frequencies. Furthermore, a value of 0.5 for the slope of both curves is obtained when G 0 and G 00 are represented versus u in a log–log plot, which is the typical response of a critical gel [43]. Fig. 6 also demonstrates that a ‘plateau’ region, that is, a zone in which G 0 keeps constant, tends to appear at low frequency as temperature increases in sample A9, unlike sample B9 that shows a viscous flow region under the above mentioned conditions. The appearance of the ‘plateau’ region causes an apparent shift of the terminal zone of the mechanical spectrum to lower frequencies. It is worth pointing out that the appearance of a ‘plateau’ region has been commonly related to the development of entanglements [1] among the macromolecular components of the modified bitumen, which provide the enhanced elastic response to the material. For that reason, it may be assumed that a polymer network that extends all over the polymer–bitumen composite is formed in the blend A9. Such a polymer network seems to control the linear viscoelastic rheology of the binder at temperatures at which the bitumen softening point is overcome by far, but the polymer has not been melted yet. Such microstructural considerations are confirmed in Fig. 7. The photomicrographs were taken at 75 8C, temperature at which the polymer has not reached its melting point yet. Fig. 7(a) reveals that a polymer network, which extends throughout the polymer–bitumen composite, has been formed at this polymer concentration (A9). The appearance of the polymer network is the responsible for the results obtained in the oscillatory shear tests, in which the binder exhibited highly improved elastic properties. On the contrary, sample A5 (see Fig. 7(c)) presents a dispersion of small droplets of polymer into a continuous phase of bitumen. This lack of entanglements among different points of the binder structure makes its mechanical properties worsen as temperature increases above the bitumen softening point. A dispersed polymer phase is observed for modified bitumens having 5 and 9 wt% EVA

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Fig. 6. Frequency dependence of the linear viscoelasticity functions, at 50, 70 and 80 8C, for binders A9 and B9.

(Fig. 7(b) and (d)), although a polymer network tends to appear at the highest concentration. The above described effects may be explained on the basis of polymer–bitumen compatibility. Modulated differential scanning calorimetry (MDSC) experiments were developed to analyse thermal transitions related to the polymer–bitumen blends (Fig. 8). Fig. 8(a) presents the total heat flow curves for polymers A, B and ABS, neat and processed bitumen, as well as bitumens modified by polymers A and B. As can be seen by the appearance of two melting peaks, polymer A is a blend composed of EVA with a low content in vinyl acetate (larger peak at 109 8C) and LDPE (smaller peak at 122 8C), whilst one only peak at 112.5 8C arises for polymer B, corresponding to EVA with a low content in vinyl acetate [19–21]. ABS, instead, does not present any melting process, although a thermal event

appears at about 95 8C corresponding to the glass transition of the rigid domains of the ABS terpolymer, that is, the styrene– acrylonitrile. The remaining glass transition, corresponding to the elastic domains of the terpolymer, that is, butadiene, which happens at about K80 8C, could not be detected due to technical limitations. Modified bitumens with polymers A and B behave in a very different way. A clearly defined peak, at 102 8C, corresponding to the melting of the polymer appears after blending the polymer B with bitumen, regardless the polymer concentration used. Hence, a shift in the polymer melting point of about 10 8C, approximately the same for both concentrations, happens due to the migration of some bitumen light components to the polymer domains. Unlike that, a wide endotherm is observed as polymer A is mixed with bitumen,

Fig. 7. The microstructure, by optical microscopy, of modified bitumens: (A) A9; (B) B9; (C) A5; (D) B5.

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Total Heat Flow [W·g–1] Y=0.05

A

ABS

B

Tg= 95˚C

[1] [2]

A B 109 0

50

122 112.5 100 150 200 T [˚C]

[3] [4]

[1] [2]

[5]

[3]

[1] Neat bitumen [2] Processed bitumen [3] A5 [4] A9 [5] B5 [6] B9

–80

–40

0

[4]

[6]

[5]

[7] [8]

[6] Endo

40

80

120

T [°C]

Exotherm

160 –60 –40 –20 0

[1] Neat bitumen [2] Proc.bitumen [3] A5 [4] A9

[5] B5 [6] B9 [7] S9 [8] R9

Y=0.02 Non-rev Heat Flow [W·g–1]

–1

Heat Flow [W·g ]

942

20 40 60 80 100 120

T [°C]

Fig. 8. Modulated calorimetry experiments for neat and processed bitumen, and a set of different blends of bitumen and waste polymers: (A) Total heat flow; (B) Non-reversing heat flow. For the sake of comparison, standard calorimetry for the polymers A, B and ABS are also included (inset Fig. 8(A)).

though the peak related to the melting process of the crystalline fractions disappears. This fact might be explained on the basis of polymer–bitumen compatibility. In other words, a more favourable mixing process must be taking place when bitumen is blended with polymer A. Hence, the components of the maltenic fraction of the bitumen are able to penetrate inside the polymer microstructure [19–20] and, as a consequence, its crystalline phase is to a great extent altered. Fig. 8(b) displays the non-reversing heat flow curves for unmodified and modified bitumens. Non-reversing curves also allow the assessment of the degree in which bitumen components penetrate inside the polymer domains. It is noteworthy that thermal characterisation of bituminous blends have been carried out after the samples were subjected to heating above the melting temperature and further cooling, and thus, all the samples presented the same thermal history. A very different thermal response would have been obtained from the as-received bitumen, in which enough time has elapsed for the bitumen phases to attain equilibrium [4]. On one hand, upon heating, an endothermic background partially arises from the crystallization of saturates and, above all, from the ordering of simple aromatic structures such as those known to occur in liquid crystalline mesophases [4]. This broad endotherm ranges from K40 to 80 8C, approximately, and corresponds to an immediate ordering process that occurs when bitumen is quenched from the melt. On the other hand, a second thermal event related to the diffusion and assembly of structures such as naphthalene-like aromatics takes place, which may arise from the twisting and alignment of aromatic centres along an order axis to produce a mesophase [4]. The extent of this ordering in the bitumen is inversely proportional to the height of the exotherm at about 0 8C. Thus, it is maximum at the conditions used in our experiments, in which no annealing time (that is, minimum ordering) is considered. As can be observed in Fig. 8(b), unmodified bitumens show both the large endothermic background and the high exotherm, because the compounds that originate the above mentioned thermal events belong to the maltenic fraction of the bitumen.

On the contrary, both events undergo a reduction in bitumens modified by polymers A and B, more significant as the amount of polymer increases, due probably to the migration of some maltenic components to the polymer phase, which reflects the polymer affinity. According to Masson and Polomark [4], 85% of the total endotherm arises from the isotropisation of the aromatics, whereas only 15% arises from the melting of partially crystalline saturates. As a result, the swelling process of polymer would be mainly attributed to the aromatic fraction of the maltenes. An intermediate situation takes place in sample R9 (Fig. 8(b)), in which the scrap rubber is able to be swollen by the lighter compounds of the bitumen and, as has been already proved, a minor part of it (about 15 wt%) is dissolved [23] by the bituminous oily matrix at 180 8C. Finally, it could be concluded that binder S9 does not present any modification in relation to the unmodified bitumen, what makes evident its extremely poor affinity (see Fig. 8(b)). As a result, the elastic filler, crumb rubber swollen by the maltenic fraction, provides improved viscoelastic characteristics to the modified bitumen, which results in an enhancement of the asphalt mechanical properties. The results obtained are in good agreement with the solubility parameters shown by saturates (14.3 MPa0.5), aromatics (17.8 MPa 0.5), recycled polyolefins 0.5 (z16.2 MPa ), crumb rubber (z16.6–17.6 MPa0.5) and ABS (z21.1 MPa0.5) [44,45]. 4. Conclusions The addition of four different waste polymers to petroleum bitumen, in order to improve its performance for pavement applications has been studied. Scrap tire rubber became the most interesting additive for the modification of bitumen at low in-service temperature. A higher dissipation of energy prevents the pavement from thermal cracking as stressed by the heavy traffic loadings. A polymer blend composed of EVA and LDPE resulted quite suitable mainly at high in-service temperatures, showing favourable mechanical properties at temperatures for

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which neat bitumen undergoes permanent deformation processes. Nevertheless, the amount of polymer in the blend should be painstakingly adjusted, in order to get a proper value of viscosity at the temperatures involved in bitumen application and compactation. Enhanced rheological properties at both low and high inservice temperature may be obtained by mixing polyolefins and crumb rubber. Thus, the blend containing 3.5 wt% EVA/LDPE and 3.5 wt% crumb rubber (referred to as AR7) appears to be an interesting formulation in order to obtain a better behaviour of bitumen in a wide range of temperature. The studied blends of bitumen and different types of waste polymers showed a shear-thinning behaviour at 50 8C, more significant for the highest polymer concentrations. The shearthinning behaviour is strongly linked to the size and shape of the dispersed phase. In addition, the viscous properties of the modified bitumens, at 135 8C, depend on the polymer behaviour at high temperature. Finally, calorimetry experiments proved that different degrees of compatibility exist between the bitumen and the polymers used. Polymers B and, especially, A showed a large compatibility with the bitumen, since it seems to exist a clear interaction between both materials. On the contrary, scrap computer shell does not show any kind of affinity by the maltenic compounds of the bitumen. Acknowledgements This work is part of a research project sponsored by a MECFEDER programme (Research Project MAT2004-06299-C0202). The authors gratefully acknowledge its financial support. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

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