Rheological characteristics of synthetic road binders

Available online at www.sciencedirect.com

Fuel 87 (2008) 1763–1775 www.fuelfirst.com

Rheological characteristics of synthetic road binders Gordon D. Airey *, Musarrat H. Mohammed 1, Caroline Fichter 2 Nottingham Transportation Engineering Centre, University of Nottingham, Nottingham, NG7 2RD, UK Received 21 November 2007; received in revised form 13 January 2008; accepted 15 January 2008 Available online 20 February 2008

Abstract Most adhesives and binders, including binders for asphalt mixture production, are presently produced from petrochemicals through the refining of crude oil. The fact that crude oil reserves are a finite resource means that in the future it may become necessary to produce these materials from alternative and probably renewable sources. Suitable resources of this kind may include polysaccharides, plant oils and proteins. This paper deals with the synthesis of polymer binders from monomers that could in future be derived from renewable resources. These binders consist of polyethyl acrylate (PEA) of different molecular weight, polymethyl acrylate (PMA) and polybutyl acrylate (PBA), which were synthesised from ethyl acrylate, methyl acrylate and butyl acrylate, respectively, by atom transfer radical polymerization (ATRP). The fundamental rheological properties of these binders were determined by means of a dynamic shear rheometer (DSR) using a combination of temperature and frequency sweeps. The results indicate that PEA has rheological properties similar to that of 100/150 penetration grade bitumen, PMA similar rheological properties to that of 10/20 penetration grade bitumen, while PBA, due to its highly viscous nature and low complex modulus, cannot be used on its own as an asphalt binder. The synthetic binders were also combined with conventional penetration grade bitumen to produce a range of bitumen–synthetic polymer binder blends. These blends were batched by mass in the ratio of 1:1 or 3:1 and subjected to the same DSR rheological testing as the synthetic binders. The blends consisting of a softer bitumen (70/100 pen or 100/150 pen) with a hard synthetic binder (PMA) tended to be more compatible and therefore stable and produced rheological properties that combined the properties of the two components. The synthetic binders and particularly the extended bitumen samples (blends) produced rheological properties that showed similar characteristics to elastomeric SBS PMBs, although their precise viscoelastic properties were not identical. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Bitumen; Synthetic polymers; Rheological properties; Viscosity

1. Introduction Most adhesives and binders, including bituminous binders that are used for road building, are derived mainly from fossil fuels. However, with petroleum reserves becoming depleted and the subsequent need to reduce fossil fuel usage, there is a drive to develop adhesives and binders from alternative sources, especially from renewable *

Corresponding author. Tel.: +44 115 9513913; fax: +44 115 9513909. E-mail addresses: [email protected] (G.D. Airey), [email protected] (M.H. Mohammed), caroline. fi[email protected] (C. Fichter). 1 Tel.: +44 115 98466077; fax: +44 115 9513909. 2 Tel.: +44 115 9513905; fax: +44 115 9513909. 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.01.012

sources. Adhesives based on soy protein, starch, cellulose and other polysaccharides have been used over the years for a wide range of adherents such as wood, paper, plastic, metal, leather and glass [1]. Renewable natural resources including sugars, triglyceride oils and proteins have been tested as alternative sources for producing adhesives and binders [1–7]. Large quantities of renewable sources such as triglyceride oils, proteins, starch and other carbohydrates are available from various botanical sources, photosynthetic micro-organisms and algae and there are good technical and economic prospects in utilizing them from these sources. A range of different vegetable oils have been developed in recent years with the knowledge of their physical and chemical properties obtained through the application of scientific research and development [7,8].

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This paper presents an initial study of the synthesis of polymer binders from monomers that could in future be produced from triglyceride oils and carbohydrates. The objective of the project is to investigate if these polymers show rheological properties similar to bitumen and if they can replace bituminous materials. Synthetic binders can be used in three ways to ease the demand for fossil fuel based bituminous binders. Firstly, they can be used as a direct alternative binder to traditionally used bitumen (100% replacement). Secondly, these synthetic binders can be used as bitumen modifiers (usually in the order of <10% bitumen replacement) and, thirdly, they can be used as bitumen extenders (part bitumen replacement with percentages between 25% and 75%). The bitumen modifier market is already very well developed with the use of petroleum derived polymers to modify the performance of conventional bituminous binders dating back to the early 1970s [9], with these modified binders subsequently having decreased temperature susceptibility, increased cohesion and modified rheological characteristics [10–15]. Globally, approximately 75 percent of modified binders can be classified as elastomeric, 15 percent as plastomeric with the remaining 10 percent being either rubber or miscellaneously modified [16,17]. Elastomers modify bitumen by having a characteristically high elastic response and, therefore, resist permanent deformation by stretching and recovering their initial shape. Plastomers modify bitumen by forming a tough, rigid, three dimensional network to resist deformation. Within the elastomeric group, styrenic block copolymers have shown the greatest potential when blended with bitumen [18]. Other examples of elastomers used in bitumen modification include natural rubber, polybutadiene, polyisoprene, isobutene isoprene copolymer, polychloroprene and styrene butadiene rubber. One of the principal plastomers used in pavement applications is the semi-crystalline copolymer, ethylene vinyl acetate (EVA). EVA polymers have been used in road construction for over 25 years in order to improve both the workability of the asphalt during construction and its deformation resistance in service [19–22]. Due to the dominance of traditional polymer modifiers, such as SBS and EVA, the synthetic polymer binders produced in this study were only considered as bitumen replacements and bitumen extenders. Three types of acrylate based polymer binders were produced using atom transfer radical polymerization (ATRP) and their fundamental rheological (viscoelastic) properties determined by means of dynamic (oscillatory) mechanical analysis using a dynamic shear rheometer (DSR) and presented in the form of temperature and frequency dependent rheological parameters. The synthetic polymer binders were also used with conventional bitumen to produce a range of blends (extended bitumen) and their rheological properties were also quantified using the DSR. Finally the synthetic binders and blends were compared with conventional bitumens and standard polymer modified bitumens (PMBs).

2. Synthesis of binders Samples of polybutyl acrylate (PBA), polyethyl acrylate (PEA) and polymethyl acrylate (PMA) were synthesised from butyl acrylate, ethyl acrylate and methyl acrylate respectively, by ATRP. The ATRP technique involves the abstraction of a halogen from an alkyl halide, methyl-2bromopropionate (MBP), by a transition metal compound such as copper bromide (CuBr) and a ligand N,N,N0 ,N0 ,N00 pentamethyl diethylenetriamine (PMDETA) in a redox process. This produces an alkyl radical that undergoes propagation as in conventional free radical polymerization. However, the free radicals are also able to abstract the halogen back from the metal, reproducing the dormant species. These processes are rapid, and the dynamic equilibrium that is established favours the dormant species. The concentration of the active radicals is therefore very low, limiting radical–radical coupling/disproportionation reactions as the principal mode of termination. Butyl acrylate (BA), ethyl acrylate (EA) and methyl acrylate (MA) (all from Aldrich, 99.9%) were distilled at atmospheric pressure over calcium hydride (CaH2). MBP (Aldrich, 99.9%) and PMDETA (Aldrich, 99.9%) were used as received. CuBr (Aldrich, 98%) was also directly used as received without any further treatment in order to avoid an oxidation of the Cu(I) compound in the open air. A required amount of CuBr was introduced to a three-necked round bottom flask containing a magnetic stirrer and connected with a three-way stopcock and a condenser. The flask was then sealed with a rubber septum and was cycled five times between vacuum and nitrogen, using a high purity nitrogen gas. The mixture containing required amounts of monomer, initiator and ligand was degassed by nitrogen purging for 30 min before it was injected to the reaction flask using a syringe. The reaction flask was then placed in a preheated oil bath at a desired temperature. After a given time, the reaction was stopped by quenching and the reaction mixture was dissolved in tetrahydrofuran (THF). The reaction conditions are given in Table 1. The dissolved polymer solution was passed through a neutral alumina column to remove copper bromide catalyst. The polymer solution was precipitated into a large amount of methanol and water (1:3) mixture. The precipitated polymer was then dried and was characterized by dynamic mechanical analysis (DMA). Table 1 shows the condition of polymerisation, percentage yield, molecular weight, reaction temperature and reaction time of the polymerized butyl acrylate, ethyl acrylate and methyl acrylate. At a reaction temperature of 50 °C, only a trace amount of product yield was obtained. Therefore, a higher reaction temperature was chosen as lower reaction temperatures produced less radical species, owing to a poor dissociation of C–X (X refers to an halogen atom such as Br, Cl, etc.) bonds in the initiator and in the propagating chain ends. As a result, a few active species were produced and thus only a few monomers experienced a propagation step. This resulted in low percentage yield

G.D. Airey et al. / Fuel 87 (2008) 1763–1775 Table 1 Condition of polymerisation, percentage yield and molecular weight of the polymerized ethyl acrylate, butyl acrylate and methyl acrylate Polymer sample

Condition [Ma]:[MBPb]:[CuBr]: [PMDEAT]d

Temp (°C)

Time (h)

Yield (%)

Mthc (g/mol)

PEA 1 PEA 2 PEA 3 PEA 4 PBA PMA

400:1:1:1 1000:1:1:1 600:1:1:1 600:1:1:1 400:1:1:1 1000:1:1:1

100 100 100 100 100 100

4 6 4 5 4 6

87 89 82 84 76 90

34,967 89,167 49,367 50,567 39,079 77,567

a

M (monomer) = ethyl acrylate or butyl acrylate or methyl acrylate. Initiator (I) = methyl-2-bromopropionate (MBP). c Theoretical molecular weight (Mth) = Formula weight of MBP + ([M]0/[MBP]0)  formula weight of monomer  conversion or % yield = 167 + ([M]/[MBP])  formula weight of monomer  conversion or % yield. d []0 = initial concentration; [M]0/[I]0 = 400/1 for PEA 1. b

and molecular weight of the product. On the other hand, bimolecular termination became more significant at higher reaction temperatures due to more propagating chains and a higher rate of termination. 3. Testing programme 3.1. Materials In addition to the six synthesised polymer binders (PEA1–PEA4, PMA and PBA), eight penetration grade bitumen with synthetic binder blends were produced. These blends were made from combinations of the three different polymer binder types (PMA having ‘high’ stiffness (complex modulus), PEA1 with ‘medium’ stiffness and PBA with ‘low’ stiffness) and three ‘pure’ penetration grade bitumens (a ‘high’ stiffness 10/20 penetration grade bitumen, a ‘medium’ stiffness 70/100 penetration grade bitumen and a ‘soft’ 100/150 penetration grade bitumen). The combinations of the eight blends in terms of their weight proportions are listed in Table 2. Six of the eight blends were produced by mixing one of the conventional (penetration grade) bitumens with a synthetic binder, while two were produced by mixing two different synthetic binders together. The combinations were chosen to cover as wide Table 2 Proportions of blended binders Blended binder

Proportions by mass

Blend Blend Blend Blend Blend Blend Blend Blend

0.75 0.75

1 2 3 4 5 6 7 8

10/20 pen

70/100 pen

100/150 pen

PBA

PEA1

PMA

0.25 0.25 0.75 0.50 0.50 0.25 0.25 0.75

0.25 0.50 0.50 0.75 0.75 0.25

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a range of possible blends within the restrains of the testing matrix. The penetration grade bitumen with synthetic binder blends, as well as the combination synthetic binder blends, were all produced by mixing two of the components in ratios of 1:1 or 3:1 to produce a blended sample of 16 g total mass (8 g by 8 g or 12 g by 4 g). The blending process consisted of heating both components to 160 °C for 30 min and pouring the required masses into a small container. The two components were then manually stirred together for approximately 60 s to produce a uniformly distributed binder blend. The blends were then poured into sample containers and stored at 5 °C prior to rheological testing. 3.2. Rheological characterisation The rheological properties of the synthetic polymer binders and bitumen blends were determined by means of dynamic mechanical methods consisting of temperature and frequency sweeps in an oscillatory-type testing mode performed within the region of linear viscoelastic (LVE) response [23–25]. The oscillatory tests were conducted on a Bohlin CVO 100 dynamic shear rheometer (DSR) using two parallel plate testing geometries consisting of 8 mm diameter plates with a 2 mm testing gap and 25 mm diameter plates with a 1 mm testing gap. The procedure used to prepare the samples for rheological testing is shown in Fig. 1. For the synthetic binders, samples were prepared by heating the binder to 150 °C and then pouring the hot binder into 8 mm or 25 mm diameter silicone moulds as required. For the blends, the samples were heated to 160 °C in their sample containers for at least 15 min in order to ensure that the material was liquid. The blend was then mixed with a spatula to homogenise the material before pouring the blended binder into a silicone mould. Two silicone moulds were used for the rheological testing, namely a 25 mm diameter and 8 mm diameter. Both moulds allow slightly more material to be placed in the mould than required in the final testing geometry (1 mm for the 25 mm geometry and 2 mm for the 8 mm geometry). The gap between the upper and lower spindles of the DSR was set to a height of 25 lm plus the required testing gap at the mid-point of the testing temperature range. Once the samples had cooled and solidified, they were removed from the moulds and placed on the lower (bottom) plate of the DSR. The upper plate of the DSR was then gradually lowered to the required testing gap plus 25 lm. The binder that was squeezed out between the plates was then trimmed off from the edge of the plates using a hot blade. Finally, the gap was set as required for the test and the slightly squeezed binder was left around the circumference of the testing geometry [26]. The rheological properties of the binders were measured in terms of their complex shear modulus (stiffness and overall resistance to deformation), G*, and phase angle, d, (viscoelastic balance of rheological behaviour).

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Fig. 1. Methodology for preparing and loading synthetic binders and blended binders into the DSR: (a) homogenising binder blends, (b) 25 mm and 8 mm silicone moulds and binder samples, (c) pouring and placing binder samples in DSR and (d) setting final testing gap and trimming excess binder around testing geometry.

Viscosity (Pa.s)

100

PEA1 PBA 70/100 pen SBS PMB 10 Ideal compaction viscosity

1 Ideal mixing viscosity

0.1 80

100

120

140

160

180

200

220

Temperature (°C) Fig. 2. High temperature viscosity versus temperature for synthetic binders, conventional bitumen and polymer modified bitumen.

450,000 400,000

Complex Modulus (Pa)

350,000 300,000 250,000 200,000 150,000 100,000 50,000 0 0.0001

PEA1 10°C, 1 Hz PEA2 10°C, 1 Hz PEA3 10°C, 1 Hz 50 pen 40°C, 5 Hz 50 pen 30°C, 1 Hz 0.001

0.01

0.1

1

10

100

Strain

Fig. 3. Amplitude sweeps as a function of strain at 10 °C and 1 Hz for PEA1, PEA2 and PEA3 compared to equivalent complex modulus results for a 50 penetration grade bitumen.

G.D. Airey et al. / Fuel 87 (2008) 1763–1775

25 mm geometry at intermediate to high temperatures (25–80 °C). Three types of DSR tests were performed in the study:

1.E+09

90

1.E+08

80

1.E+07

70

1.E+06

60

1.E+05

50

1.E+04

40

G* for PEA2 @ 1Hz G* for 10/20 pen @ 1Hz delta for PEA2 @ 1Hz delta for 10/20 pen @ 1Hz

1.E+03 1.E+02

30

Phase Angle (degrees)

Complex Modulus (Pa)

The selection of the testing geometry is based on the operational conditions with the 8 mm plate geometry generally being used at low temperatures (5 °C to 35 °C) and the

1767

20

1.E+01

10 -30

-10

10

30

50

70

90

Temperature (°C) Fig. 4. Temperature sweeps for PEA2 and 10/20 penetration grade bitumen in terms of complex modulus and phase angle.

a

1E+07

Complex Modulus (Pa)

1E+06

1E+05

1E+04

1E+03

PEA1

PEA2

PEA3

PEA4

1E+02

1E+01 1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1E+01

1E+02

1E+03

Reduced Frequency (Hz) 90

b Phase Angle (degrees)

80 70 60 50 40

PEA1

PEA2

PEA3

PEA4

30 20 1E-04

1E-03

1E-02

1E-01

1E+00

Reduced Frequency (Hz) Fig. 5. Master curves of (a) complex modulus and (b) phase angle for different molecular weight polyethyl acrylate binders at a reference temperature of 25 °C.

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 Amplitude sweeps (stress sweeps),  temperature sweeps, and  frequency sweeps.

all the temperature and frequency tests was confined within the linear viscoelastic (LVE) response of the binder [28].

The amplitude sweeps were undertaken using stress sweeps at 10 °C and 1 Hz for the 8 mm testing geometry and at 40 °C and 1 Hz for the 25 mm testing geometry. These stress sweeps were then used to determine the limit of the LVE response based on the point where complex modulus has decreased to 95% of its initial value as prescribed during the SHRP study [27]. The temperature sweeps were undertaken at a constant frequency of 1 Hz over a temperature range from 25 °C to 80 °C using the 8 mm parallel plate testing geometry. The frequency sweep tests were performed under controlled strain loading conditions using frequencies between 0.1 to 10 Hz at 5 °C temperature intervals between 5 and 75 °C. The tests between 5 and 35 °C were undertaken with the 8 mm diameter and 2 mm testing gap geometry and from 25 to 75 °C with the 25 mm diameter and 1 mm testing gap geometry. The strain amplitude for

4. Synthetic polymer binders 4.1. High temperature viscosity The high temperature viscosities of the synthetic binders at typical road material mixing and compaction temperatures were determined using a Brookfield rotational viscometer at temperatures between 100 and 200 °C. The high temperature viscosity relationships for PEA1 and PBA are shown in Fig. 2 together with relationships for a conventional 70/100 penetration grade bitumen and a SBS PMB. The flatter slopes for PEA1 and PBA illustrate a lower temperature susceptibility for the synthetic binders compared to the steeper slopes shown for the 70/100 pen bitumen and SBS PMB. The ideal asphalt mixture mixing and compaction viscosities have also been added to Fig. 2 and show that the temperatures associated with these

1E+09

Complex Modulus (Pa)

1E+08 1E+07

10/20 pen 100/150 pen PMA

70/100 pen PEA1 PBA

1E+06 1E+05 1E+04 1E+03 1E+02 1E+01 1E+00 1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1E+01

1E+02

1E+03

Reduced Frequency (Hz) 90

Phase Angle (degrees)

80 70 60 50 40 30 20 10 1E-05

10/20 pen 100/150 pen PMA 1E-04

1E-03

70/100 pen PEA1 PBA 1E-02

1E-01

1E+00

Reduced Frequency (Hz)

Fig. 6. Master curves of (a) complex modulus and (b) phase angle for different penetration grade bitumens, polyethyl acrylate, polymethyl acrylate and polybutyl acrylate binders at a reference temperature of 25 °C.

G.D. Airey et al. / Fuel 87 (2008) 1763–1775

practical viscosity requirements are similar for the synthetic and traditional binders, although relatively high temperatures (>200 °C) would be required for ideal mixing of PEA1 with aggregate. 4.2. Stress/strain response The amplitude sweeps undertaken on the synthetic binders and binder blends were used to determine the LVE limits for the different binders as well as to quantify the strain (or stress) dependency of the materials. The amplitude sweeps for three of the synthetic binders at 10 °C are presented in Fig. 3. The three synthetic binders show similar strain dependency compared to conventional bitumen as well as similar LVE limits both in terms of strain and stress (not shown for brevity). Similar results were found for the other synthetic binders and blends where the LVE strain and stress limits were shown to be functions of complex modulus (stiffness) of the binders similar to that found for conventional bitumens [24,27,28].

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4.3. Temperature dependency The temperature dependency of the synthetic binders has been assessed by means of isochronal plots of complex modulus (G*) and phase angle (d) versus temperature at 1 Hz as shown in Fig. 4. Fig. 4 shows the data for PEA2, which can be considered to be a medium stiffness synthetic polymer binder, compared to the ‘hard’ 10/20 penetration grade bitumen. Similar plots were produced for the other synthetic binders but are not shown here for brevity. The results in Fig. 4 show that the temperature dependency of the synthetic binder differs from that found for conventional, unmodified bitumen. The PEA2 sample shows less temperature susceptibility compared to the 10/ 20 pen bitumen as shown by the flatter slope of the complex modulus versus temperature relationship. In addition, the viscoelastic behaviour of the synthetic binder (as represented by the phase angle versus temperature relationship) is more complex than the uniform transition from elastic response (low phase angles) to viscous response (high phase angles) with increasing temperature for the conventional

1E+09

Complex Modulus (Pa)

1E+08 1E+07

10/20 pen 10/20 pen + PEA1 (3:1) 10/20 pen + PBA (3:1) PEA1 PBA

1E+06 1E+05 1E+04 1E+03 1E+02 1E+01 1E+00 1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

E+01

1E+02

1E+03

Reduced Frequency (Hz) 90

Phase Angle (degrees)

80

70

60

50

40

30

20 1E-05

10/20 pen 10/20 pen + PEA1 (3:1) 10/20 pen + PBA (3:1) PEA1 PBA 1E-04

1E-03

1E-02

1E-01

1E+001

Reduced Frequency (Hz) Fig. 7. Master curves of (a) complex modulus and (b) phase angle for 10/20 penetration grade with polyethyl acrylate and polybutyl acrylate blended binders at a reference temperature of 25 °C.

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bitumen. Thermal analysis of the synthetic binder using differential scanning calorimetry (DSC) would provide a more detailed analysis of the temperature dependency and the presence of the G* plateau between 0 and 30 °C. 4.4. Frequency dependency The frequency dependency of the synthetic binders in terms of complex modulus and phase angle has been assessed by producing rheological master curves at a reference temperature of 25 °C using the time–temperature superposition principle (TTSP) [29] and shift factors determined for both the G* and d master curves. The master curves for the four PEA synthetic binders are shown in Fig. 5. Due to the ‘‘thermo-rheological simplicity” of these polymeric materials (rheological properties being temperature and time equivalent), smooth master curves for both complex modulus and phase angle have been produced. The complex modulus and phase angle master curves show an increase in complex modulus (stiffening) and a decrease in phase angle (increasing elastic response) with increasing molecular weight for the PEAs. However, these changes

are relatively minor and do not alter the overall rheological response of the polyethyl acrylate binders. The three types of acrylate binder (PEA, PMA and PBA) are compared in Fig. 6 together with three control bitumens (a ‘hard’ 10/20 pen bitumen, a ‘medium’ stiffness 70/100 pen bitumen and a ‘soft’ 100/150 pen bitumen). The complex modulus master curves show that the polyethyl acrylate binder (PEA1) has comparable G* values to the ‘soft’ 100/150 pen bitumen over the reduced frequency range from 0.001 to 1 Hz, although the material tends to be softer than the 100/150 pen bitumen at high frequencies. The PMA binder is more comparable to the ‘hard’ 10/20 pen bitumen, although due to its lower frequency susceptibility (and related temperature susceptibility) it tends to have higher complex modulus values at low reduced frequencies. The rheological behaviour of the PBA binder differs considerable from both the PEA and PMA, as well as the conventional bitumens, with this very soft synthetic binder having consistently lower G* values over the entire reduced frequency domain. The viscoelastic nature of the synthetic binders, as shown through the phase angle master curves in Fig. 6b,

1E+09

Complex Modulus (Pa)

1E+08 1E+07 1E+06 1E+05 1E+04

70/100 pen + PMA (3:1) 70/100 pen + PMA (1:1) PMA 70/100 pen

1E+03 1E+02 1E+01 1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

Reduced Frequency (Hz) 90

Phase Angle (degrees)

80 70 60 50 40

70/100 pen + PMA (3:1) 70/100 pen + PMA (1:1) PMA 70/100 pen

30 20 10 1E-05

1E-04

1E-03

1E-02

1E-01

1E+001

E+01

1E+02

1E+03

Reduced Frequency (Hz) Fig. 8. Master curves of (a) complex modulus and (b) phase angle for 70/100 penetration grade with polymethyl acrylate blended binders at a reference temperature of 25 °C.

G.D. Airey et al. / Fuel 87 (2008) 1763–1775

do differ significantly from those of the conventional bitumens. Although PBA shows a continuous decrease of phase angle with frequency similar to that found for bitumen, its response is predominantly viscous in nature with phase angles between 80° and 90° over the majority of the frequency domain. The PEA and PMA binders tend to show patently different viscoelastic behaviour to bitumen with associated transitions resulting in their phase angles initially decreasing with increasing frequency, moving from a viscous to an increasingly elastic rheological response, then increasing before finally decreasing again. This results in the PMA binder showing a minimum and maximum phase angle transition within the reduced frequency range depicted in Fig. 6b. It is, therefore, probably that PEA and PMA may be less compatible in bitumen compared to PBA. The results show that synthetic polymer binders can partly replicate the rheological properties of conventional bitumens in terms of being able to demonstrate comparable complex modulus values, but they do show significantly different viscoelastic response as represented by their phase angles as a function of both temperature and frequency.

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5. Blended binders 5.1. Hard bitumen blends To investigate what the effect would be of using the synthetic binders as bitumen extenders rather than simply as replacement binders, PBA and PEA1 were blended with the ‘hard’ 10/20 penetration grade bitumen. High percentages (25% to 75% synthetic binder) were used to investigate the ability of the synthetic binders to act as a partial binder replacement. The rheological properties of the blends are presented in Fig. 7 in terms of rheological master curves of complex modulus and phase angle. The effect of blending 25% by mass of the softer synthetic binders with 75% by mass of the hard bitumen has not significantly altered the overall complex modulus values of the blended binder compared to the 10/20 pen bitumen. The only significant effect seen in Fig. 7a is that the blended binders have a lower frequency (and related temperature) susceptibility compared to the conventional bitumen. The changes in terms of the phase angle master curves in Fig. 7b are however fairly dramatic with the master

1E+09

Complex Modulus (Pa)

1E+08 1E+07 1E+06 1E+05 1E+04

100/150 pen + PMA (1:3)

1E+03

100/150 pen + PMA (1:1) PMA

1E+02

100/150 pen 1E+01 1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

1E+04

1E+02

1E+03

Reduced Frequency (Hz) 90

Phase Angle (degrees)

80 70 60 50 40

100/150 pen + PMA (1:3) 30

100/150 pen + PMA (1:1) PMA

20

100/150 pen 10 1E-05

1E-04

1E-03

1E-02

1E-01

1E+001

E+01

Reduced Frequency (Hz) Fig. 9. Master curves of (a) complex modulus and (b) phase angle for 100/150 penetration grade with polymethyl acrylate blended binders at a reference temperature of 25 °C.

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curves for the blends not only showing a reduction in frequency susceptibility but also a considerable increase in elastic response compared to the 10/20 pen bitumen. This is unexpected as the viscoelastic response of the blended bitumen would be expected to lie between the hard bitumen and the more viscous synthetic binders. One possible reason for the viscoelastic response of the blended binders is the lack of compatibility (stability) between the hard bitumen and soft synthetic binders which would tend to manifest itself more in terms of the phase angle than complex modulus. 5.2. Intermediate stiffness bitumen blends As an alternative to blending a hard bitumen with a soft synthetic binder, an intermediate stiffness bitumen (70/100 pen) was blended with the hard PMA synthetic binder at two percentages (75% bitumen to 25% PMA and a 50:50 blend). The rheological master curves of the blends are shown in Fig. 8. The rheological properties of the blends show the combined properties of the individual bitumen and synthetic

binder components. The complex modulus master curves are situated between the 70/100 pen bitumen and the ‘hard’ PMA binder, while the same pattern can be seen for the phase angle master curves. The overall observation for these two blends is that the material appeared uniformly mixed and compatible. 5.3. Soft bitumen blends Similar to the intermediate stiffness bitumen and PMA blends, blends were also produced using the soft 100/150 pen bitumen with the hard PMA synthetic binder at two percentages (25% bitumen to 75% PMA and a 50:50 blend). The rheological master curves of the blends can be seen in Fig. 9. Once again, as with the previous set of blends, the two materials appear to be very compatible and produced blends with rheological properties between those of the penetration grade bitumen and the PMA synthetic binder. The higher percentage of PMA (75%) in one of the blends as well as the softer nature of the 100/150 pen bitumen meant that the rheological properties, particularly the

1E+09

Complex Modulus (Pa)

1E+08 1E+07 1E+06

PBA + PMA (1:3) PEA1 + PMA (3:1) PEA1 PMA PBA

1E+05 1E+04 1E+03 1E+02 1E+01 1E+00 1E-05

1E-04

1E-03

1E-02

1E-01

1E+00

1E+01

1E+02

1E+03

Reduced Frequency (Hz) 90

Phase Angle (degrees)

80 70 60 50 40

PBA + PMA (1:3) PEA1 + PMA (3:1) PEA1 PMA PBA

30 20 10 1E-05

1E-04

1E-03

1E-02

1E-01

1E+001

E+01

1E+02

1E+03

Reduced Frequency (Hz) Fig. 10. Master curves of (a) complex modulus and (b) phase angle for polymethyl acrylate with polyethyl acrylate and polybutyl acrylate blended binders at a reference temperature of 25 °C.

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phase angle master curves, tend to be more indicative of the PMA binder than that seen for the blends in Fig. 8.

the soft PBA–PMA results in a decrease in complex modulus and an increase in phase angle.

5.4. Synthetic binder blends

6. Comparison with traditional polymer modified bitumens

In addition to the bitumen–synthetic binder blends in Figs. 7–9, two blends were produced using only combinations of synthetic binder. The two blends both used PMA combined with either 25% of the soft PBA binder or 75% of the medium stiffness PEA1. The rheological properties of the blends in the form of master curves can be seen in Fig. 10. The rheological properties of both blends are dominated by the larger (higher percentage) component with the PMA–PEA1 blend being very similar to the rheological properties of PEA1 and the PMA–PBA blend being very similar to the rheological properties of PMA. For the PMA–PEA1 blend, the effect of adding 25% of the hard PMA–PEA1 results in an increase in complex modulus (Fig. 10a) and a decrease in phase angle (Fig. 10b). Conversely for the PMA–PBA blend, the addition of 25% of

Although the synthetic binders have not been used as bitumen modifiers in this study, it is interesting to compare their rheological properties to those associated with traditional PMBs. However, as polymer modification tends to reduce the thermo-rheological simplicity of bitumen and therefore the ability to produce master curves, the rheological data was assessed using Black diagrams as shown in Fig. 11 [30]. The rheological properties of PEA1 and PMA are compared to high polymer content SBS and EVA PMBs. The SBS PMB shows a relatively smooth curve in the Black space with the effect of the elastomeric polymer being seen through the increase in elastic response (decrease in phase angles) at low complex modulus values. The EVA PMB shows typical semi-crystalline rheological behaviour with

1E+09

SBS PMB EVA PMB PEA1 PMA

Complex Modulus (Pa)

1E+08 1E+07 1E+06 1E+05 1E+04 1E+03 1E+02 1E+01 1E+00 0

10

20

30

40

50

60

70

80

90

Phase Angle (degrees) Fig. 11. Black diagrams of rheological data for SBS and EVA PMBs compared to synthesised polymer binders.

1E+09

Complex Modulus (Pa)

1E+08 1E+07 1E+06 1E+05 1E+04

SBS PMB

1E+03

EVA PMB 70/100 pen + PMA (1:1)

1E+02

100/150 pen + PMA (1:3) 1E+01

PEA1 + PMA (3:1)

1E+00 0

10

20

30

40

50

60

70

80

90

Phase Angle (degrees) Fig. 12. Black diagrams of rheological data for SBS and EVA PMBs compared to blended penetration grade and synthesised polymer binders.

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a series of discrete curves within the intermediate temperature and frequency range (centre of the Black space). Black diagrams can be considered as rheological ‘fingerprints’ and this can be clearly seen in Fig. 11 where the rheological curves for the two acrylate binders (PEA1 and PMA) show the same pattern. The two synthetic binders bear a closer resemblance to the SBS PMB than the EVA PMB, although rheologically they are still very different from the PMB. Three of the blended binders were also compared to the SBS and EVA PMBs in Fig. 12. The three blends (PEA1– PMA, 100/150 pen – PMA and 70/100 pen – PMA) all show a similar rheological pattern in the Black diagram with a shift towards more elastic response (lower phase angles) being associated with an increase in the percentage of synthetic binder. Similar to the observations in Fig. 11, the blended bitumens are rheologically closer to the SBS PMB than the plastomeric EVA PMB. 7. Conclusions The DSR results indicate that polyethyl acrylate synthetic polymer binders have rheological properties, in terms of complex modulus values, similar to that of a ‘soft’ 100/ 150 penetration grade bitumen, while the polymethyl acrylate synthetic polymer binder showed complex modulus values comparable with a ‘hard’ 10/20 penetration grade bitumen. However, in terms of their viscoelastic response, both acrylate binder types showed considerably more polymer-like rheological behaviour than that found for conventional, unmodified bitumens. The polybutyl acrylate synthetic polymer binder was found to be considerably softer and more viscous in nature at ambient temperatures compared to conventional bitumens and therefore cannot be used by itself as an asphalt binder but could be used to modify (soften) stiffer grade bitumen. Overall the results showed that synthetic polymer binders can partly replicate the rheological properties of conventional bitumens, although they should not be considered to be a direct rheological replacement for bitumen. The synthetic polymer binders were successfully used together with conventional, penetration grade bitumen to produce bitumen–synthetic binder as well as combination synthetic binder blends. The use of a softer bitumen (70/ 100 pen or 100/150 pen) with a hard synthetic binder (PMA) tended to produce more consistent blends with rheological properties that combined the properties of the two components. The synthetic binders, and particularly the extended bitumen samples (blends), produced rheological properties that showed similar characteristics to elastomeric SBS PMBs, although the precise viscoelastic properties were not identical. Although the rheological properties of the synthetic binders and blends are important in terms of characterising these binders, other physical and mechanical properties such as high temperature viscosity, thermal stability, UV resistance, adhesion and durability considerations in terms

of ageing and moisture damage will also need to be assessed before these binders can be considered suitable for asphalt mixture application. Acknowledgements The authors would like to thank the UK Engineering and Physical Sciences Research Council (EPSRC) for supporting this research under a Platform Grant awarded to the Nottingham Transportation Engineering Centre. They would also like to acknowledge the contribution of Dr. Christopher Hayes of the School of Chemistry at the University of Nottingham through his involvement with the synthesis work described in the paper. References [1] Shields J. Adhesives handbook. London: Butterworth; 1976. p. 1. [2] Emengo FN, Chukwu SER, Mozie J. Tack and bonding strength of carbohydrate-based adhesives from different botanical sources. Int J Adhes Adhes 2002;22(2):93–100. [3] Uyama H, Kuwabara M, Tsujimoto T, Kobayashi S. Enzymatic synthesis and curing of biodegradable epoxide-containing polyesters from renewable resources. Biomacromolecules 2003;4:211–5. [4] Tsujimoto T, Uyama H, Kobayashi S. Synthesis and curing behaviors of cross-linkable polynaphthols from renewable resources: preparation of artificial urushi. Macromolecules 2004;37:1777–82. [5] Ahmad S, Haque MM, Ashraf SM, Ahmad S. Urethane modified boron filled polyesteramide: a novel anti-microbial polymer from a sustainable resource. Eur Polym J 2004;40:2097–104. [6] Chakrapani H, Liu C, Widenhoefer RA. Enantioselective cyclization hydrosilylation of 1,6-enynes catalyzed by a cationic rhodium bis(phosphine) complex. Org Lett 2003;5(2):157–9. [7] Kaplan DL. Biopolymers from renewable resources. Berlin: Springer; 1998. p. 1–3. [8] Tan CP, Che Man YB. Comparative differential scanning calorimetric analysis of vegetable oils: effects of heating rate variation. Phytochem Anal 2002;13:129–41. [9] Ajour AM. Several projects, several types of surfaces. Bull LCPC 1981;113:9–21. [10] Brule B, Brion Y, Tanguy A. Paving asphalt polymer blends: relationship between composition, structure and properties. Assoc Asphalt Paving Technologists 1988;57:41–64. [11] Brown SF, Rowlett RD, Boucher JL. Asphalt modification. In: Proceedings of the conference on us shrp highway research program: sharing the benefits, ICE, 1990. p. 181–203. [12] Isacsson U, Lu X. Testing and appraisal of polymer modified road bitumens – state of the art. Mater Struct 1995;28:139–59. [13] Collins JH, Bouldin MG, Gelles R, Berker A. Improved performance of paving asphalts by polymer modification. Assoc Asphalt Paving Technologists 1991;60:43–79. [14] Goodrich JL. Asphaltic binder rheology, asphalt concrete rheology and asphalt concrete mix properties. Assoc Asphalt Paving Technologists 1991;60:80–120. [15] King GN, King HW, Chaverot P, Planche JP, Harders O. Using European wheel-tracking and restrained tensile tests to validate SHRP performance-graded binder specifications for polymer modified asphalts. In: Proceedings of the 5th Eurobitume Congress, Stockholm, vol. 1A, 1.06, 1993. p. 51–5. [16] Diehl CF. Ethylene-styrene interpolymers for bitumen modification. In: Proceedings of the 2nd Eurasphalt and Eurobitume Congress, Barcelona, vol. 2, 2000. p. 93–102. [17] Bardesi A. Use of modified bituminous binders, special bitumens and bitumens with additives in pavement applications. Technical Committee Flexible Roads (C8), World Road Association (PIARC), 1999.

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