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Fuel 87 (2008) 1763–1775
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 2008Available online 20 February 2008
Abstract
Most adhesives and binders, including binders for asphalt mixture production, are presently produced from petrochemicals throughthe refining of crude oil. The fact that crude oil reserves are a finite resource means that in the future it may become necessary to producethese materials from alternative and probably renewable sources. Suitable resources of this kind may include polysaccharides, plant oilsand proteins. This paper deals with the synthesis of polymer binders from monomers that could in future be derived from renewableresources. These binders consist of polyethyl acrylate (PEA) of different molecular weight, polymethyl acrylate (PMA) and polybutylacrylate (PBA), which were synthesised from ethyl acrylate, methyl acrylate and butyl acrylate, respectively, by atom transfer radicalpolymerization (ATRP). The fundamental rheological properties of these binders were determined by means of a dynamic shear rheom-eter (DSR) using a combination of temperature and frequency sweeps. The results indicate that PEA has rheological properties similar tothat 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 werealso combined with conventional penetration grade bitumen to produce a range of bitumen–synthetic polymer binder blends. Theseblends 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 compatibleand therefore stable and produced rheological properties that combined the properties of the two components. The synthetic binders andparticularly the extended bitumen samples (blends) produced rheological properties that showed similar characteristics to elastomericSBS PMBs, although their precise viscoelastic properties were not identical.� 2008 Elsevier Ltd. All rights reserved.
Keywords: Bitumen; Synthetic polymers; Rheological properties; Viscosity
1. Introduction
Most adhesives and binders, including bituminous bind-ers that are used for road building, are derived mainly fromfossil fuels. However, with petroleum reserves becomingdepleted and the subsequent need to reduce fossil fuelusage, there is a drive to develop adhesives and bindersfrom alternative sources, especially from renewable
0016-2361/$ - see front matter � 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.fuel.2008.01.012
* Corresponding author. Tel.: +44 115 9513913; fax: +44 115 9513909.E-mail addresses: [email protected] (G.D. Airey),
[email protected] (M.H. Mohammed), [email protected] (C. Fichter).
1 Tel.: +44 115 98466077; fax: +44 115 9513909.2 Tel.: +44 115 9513905; fax: +44 115 9513909.
sources. Adhesives based on soy protein, starch, celluloseand other polysaccharides have been used over the yearsfor a wide range of adherents such as wood, paper, plastic,metal, leather and glass [1]. Renewable natural resourcesincluding sugars, triglyceride oils and proteins have beentested as alternative sources for producing adhesives andbinders [1–7]. Large quantities of renewable sources suchas triglyceride oils, proteins, starch and other carbohy-drates are available from various botanical sources, photo-synthetic micro-organisms and algae and there are goodtechnical and economic prospects in utilizing them fromthese sources. A range of different vegetable oils have beendeveloped in recent years with the knowledge of their phys-ical and chemical properties obtained through the applica-tion of scientific research and development [7,8].
1764 G.D. Airey et al. / Fuel 87 (2008) 1763–1775
This paper presents an initial study of the synthesis ofpolymer binders from monomers that could in future beproduced from triglyceride oils and carbohydrates. Theobjective of the project is to investigate if these polymersshow rheological properties similar to bitumen and if theycan replace bituminous materials. Synthetic binders can beused in three ways to ease the demand for fossil fuel basedbituminous binders. Firstly, they can be used as a directalternative binder to traditionally used bitumen (100%replacement). Secondly, these synthetic binders can be usedas bitumen modifiers (usually in the order of <10% bitu-men replacement) and, thirdly, they can be used as bitumenextenders (part bitumen replacement with percentagesbetween 25% and 75%).
The bitumen modifier market is already very well devel-oped with the use of petroleum derived polymers to mod-ify the performance of conventional bituminous bindersdating back to the early 1970s [9], with these modifiedbinders subsequently having decreased temperature sus-ceptibility, increased cohesion and modified rheologicalcharacteristics [10–15]. Globally, approximately 75 percentof modified binders can be classified as elastomeric, 15percent as plastomeric with the remaining 10 percent beingeither rubber or miscellaneously modified [16,17]. Elasto-mers modify bitumen by having a characteristically highelastic response and, therefore, resist permanent deforma-tion by stretching and recovering their initial shape. Plas-tomers modify bitumen by forming a tough, rigid, threedimensional network to resist deformation. Within theelastomeric group, styrenic block copolymers have shownthe greatest potential when blended with bitumen [18].Other examples of elastomers used in bitumen modifica-tion include natural rubber, polybutadiene, polyisoprene,isobutene isoprene copolymer, polychloroprene and sty-rene butadiene rubber. One of the principal plastomersused in pavement applications is the semi-crystallinecopolymer, ethylene vinyl acetate (EVA). EVA polymershave been used in road construction for over 25 years inorder to improve both the workability of the asphalt dur-ing 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 pro-duced in this study were only considered as bitumenreplacements and bitumen extenders. Three types of acry-late based polymer binders were produced using atomtransfer radical polymerization (ATRP) and their funda-mental rheological (viscoelastic) properties determined bymeans of dynamic (oscillatory) mechanical analysis usinga dynamic shear rheometer (DSR) and presented in theform of temperature and frequency dependent rheologicalparameters. The synthetic polymer binders were also usedwith conventional bitumen to produce a range of blends(extended bitumen) and their rheological properties werealso quantified using the DSR. Finally the synthetic bind-ers and blends were compared with conventional bitumensand standard polymer modified bitumens (PMBs).
2. Synthesis of binders
Samples of polybutyl acrylate (PBA), polyethyl acrylate(PEA) and polymethyl acrylate (PMA) were synthesisedfrom butyl acrylate, ethyl acrylate and methyl acrylaterespectively, by ATRP. The ATRP technique involves theabstraction of a halogen from an alkyl halide, methyl-2-bromopropionate (MBP), by a transition metal compoundsuch as copper bromide (CuBr) and a ligand N,N,N0,N0,N00-pentamethyl diethylenetriamine (PMDETA) in a redoxprocess. This produces an alkyl radical that undergoespropagation as in conventional free radical polymerization.However, the free radicals are also able to abstract the hal-ogen back from the metal, reproducing the dormant spe-cies. These processes are rapid, and the dynamicequilibrium that is established favours the dormant species.The concentration of the active radicals is therefore verylow, limiting radical–radical coupling/disproportionationreactions as the principal mode of termination.
Butyl acrylate (BA), ethyl acrylate (EA) and methylacrylate (MA) (all from Aldrich, 99.9%) were distilled atatmospheric pressure over calcium hydride (CaH2). MBP(Aldrich, 99.9%) and PMDETA (Aldrich, 99.9%) wereused as received. CuBr (Aldrich, 98%) was also directlyused as received without any further treatment in orderto avoid an oxidation of the Cu(I) compound in the openair. A required amount of CuBr was introduced to athree-necked round bottom flask containing a magneticstirrer and connected with a three-way stopcock and a con-denser. The flask was then sealed with a rubber septum andwas cycled five times between vacuum and nitrogen, using ahigh purity nitrogen gas. The mixture containing requiredamounts of monomer, initiator and ligand was degassedby nitrogen purging for 30 min before it was injected tothe reaction flask using a syringe. The reaction flask wasthen placed in a preheated oil bath at a desired tempera-ture. After a given time, the reaction was stopped byquenching and the reaction mixture was dissolved in tetra-hydrofuran (THF). The reaction conditions are given inTable 1. The dissolved polymer solution was passedthrough a neutral alumina column to remove copper bro-mide catalyst. The polymer solution was precipitated intoa large amount of methanol and water (1:3) mixture. Theprecipitated polymer was then dried and was characterizedby dynamic mechanical analysis (DMA).
Table 1 shows the condition of polymerisation, percent-age yield, molecular weight, reaction temperature and reac-tion time of the polymerized butyl acrylate, ethyl acrylateand methyl acrylate. At a reaction temperature of 50 �C,only a trace amount of product yield was obtained. There-fore, a higher reaction temperature was chosen as lowerreaction temperatures produced less radical species, owingto a poor dissociation of C–X (X refers to an halogen atomsuch as Br, Cl, etc.) bonds in the initiator and in the prop-agating chain ends. As a result, a few active species wereproduced and thus only a few monomers experienced apropagation step. This resulted in low percentage yield
Table 1Condition of polymerisation, percentage yield and molecular weight of thepolymerized ethyl acrylate, butyl acrylate and methyl acrylate
Polymersample
Condition[Ma]:[MBPb]:[CuBr]:[PMDEAT]d
Temp(�C)
Time(h)
Yield(%)
Mthc
(g/mol)
PEA 1 400:1:1:1 100 4 87 34,967PEA 2 1000:1:1:1 100 6 89 89,167PEA 3 600:1:1:1 100 4 82 49,367PEA 4 600:1:1:1 100 5 84 50,567PBA 400:1:1:1 100 4 76 39,079PMA 1000:1:1:1 100 6 90 77,567
a M (monomer) = ethyl acrylate or butyl acrylate or methyl acrylate.b 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 � conversionor % yield.
d []0 = initial concentration; [M]0/[I]0 = 400/1 for PEA 1.
G.D. Airey et al. / Fuel 87 (2008) 1763–1775 1765
and molecular weight of the product. On the other hand,bimolecular termination became more significant at higherreaction temperatures due to more propagating chains anda 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 gradebitumen with synthetic binder blends were produced. Theseblends were made from combinations of the three differentpolymer binder types (PMA having ‘high’ stiffness (com-plex modulus), PEA1 with ‘medium’ stiffness and PBAwith ‘low’ stiffness) and three ‘pure’ penetration grade bit-umens (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 combina-tions of the eight blends in terms of their weight propor-tions are listed in Table 2. Six of the eight blends wereproduced by mixing one of the conventional (penetrationgrade) bitumens with a synthetic binder, while two wereproduced by mixing two different synthetic binderstogether. The combinations were chosen to cover as wide
Table 2Proportions of blended binders
Blendedbinder
Proportions by mass
10/20 pen 70/100 pen 100/150 pen PBA PEA1 PMA
Blend 1 0.75 0.25Blend 2 0.75 0.25Blend 3 0.75 0.25Blend 4 0.50 0.50Blend 5 0.50 0.50Blend 6 0.25 0.75Blend 7 0.25 0.75Blend 8 0.75 0.25
a range of possible blends within the restrains of the testingmatrix.
The penetration grade bitumen with synthetic binderblends, as well as the combination synthetic binder blends,were all produced by mixing two of the components inratios of 1:1 or 3:1 to produce a blended sample of 16 gtotal mass (8 g by 8 g or 12 g by 4 g). The blending processconsisted of heating both components to 160 �C for 30 minand pouring the required masses into a small container.The two components were then manually stirred togetherfor approximately 60 s to produce a uniformly distributedbinder blend. The blends were then poured into samplecontainers and stored at 5 �C prior to rheological testing.
3.2. Rheological characterisation
The rheological properties of the synthetic polymerbinders and bitumen blends were determined by means ofdynamic mechanical methods consisting of temperatureand frequency sweeps in an oscillatory-type testing modeperformed within the region of linear viscoelastic (LVE)response [23–25]. The oscillatory tests were conducted ona Bohlin CVO 100 dynamic shear rheometer (DSR) usingtwo parallel plate testing geometries consisting of 8 mmdiameter plates with a 2 mm testing gap and 25 mm diam-eter plates with a 1 mm testing gap.
The procedure used to prepare the samples for rheolog-ical testing is shown in Fig. 1. For the synthetic binders,samples were prepared by heating the binder to 150 �Cand then pouring the hot binder into 8 mm or 25 mm diam-eter silicone moulds as required. For the blends, the sam-ples were heated to 160 �C in their sample containers forat least 15 min in order to ensure that the material wasliquid. The blend was then mixed with a spatula to homog-enise the material before pouring the blended binder into asilicone mould.
Two silicone moulds were used for the rheological test-ing, namely a 25 mm diameter and 8 mm diameter. Bothmoulds allow slightly more material to be placed in themould than required in the final testing geometry (1 mmfor the 25 mm geometry and 2 mm for the 8 mm geometry).The gap between the upper and lower spindles of the DSRwas set to a height of 25 lm plus the required testing gap atthe mid-point of the testing temperature range. Once thesamples had cooled and solidified, they were removed fromthe moulds and placed on the lower (bottom) plate of theDSR. The upper plate of the DSR was then gradually low-ered to the required testing gap plus 25 lm. The binder thatwas squeezed out between the plates was then trimmed offfrom the edge of the plates using a hot blade. Finally, thegap was set as required for the test and the slightlysqueezed binder was left around the circumference of thetesting geometry [26].
The rheological properties of the binders were mea-sured in terms of their complex shear modulus (stiffnessand overall resistance to deformation), G*, and phaseangle, d, (viscoelastic balance of rheological behaviour).
Fig. 1. Methodology for preparing and loading synthetic binders and blended binders into the DSR: (a) homogenising binder blends, (b) 25 mm and 8 mmsilicone moulds and binder samples, (c) pouring and placing binder samples in DSR and (d) setting final testing gap and trimming excess binder aroundtesting geometry.
0
50,000
100,000
150,000
200,000
250,000
300,000
350,000
400,000
450,000
0.0001 0.001 0.01 0.1 1 10 100
Strain
Co
mp
lex
Mo
du
lus
(Pa)
PEA1 10°C, 1 HzPEA2 10°C, 1 HzPEA3 10°C, 1 Hz
50 pen 40°C, 5 Hz50 pen 30°C, 1 Hz
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 50penetration grade bitumen.
0.1
1
10
100
80 100 120 140 160 180 200 220
Temperature (°C)
Vis
cosi
ty (
Pa.
s)
PEA1
PBA
70/100 pen
SBS PMB
Ideal compaction viscosity
Ideal mixing viscosity
Fig. 2. High temperature viscosity versus temperature for synthetic binders, conventional bitumen and polymer modified bitumen.
1766 G.D. Airey et al. / Fuel 87 (2008) 1763–1775
G.D. Airey et al. / Fuel 87 (2008) 1763–1775 1767
The selection of the testing geometry is based on the oper-ational conditions with the 8 mm plate geometry generallybeing used at low temperatures (�5 �C to 35 �C) and the
1.E+01
1.E+02
1.E+03
1.E+04
1.E+05
1.E+06
1.E+07
1.E+08
1.E+09
-30 -10 10 30
Temperat
Co
mp
lex
Mo
du
lus
(Pa)
Fig. 4. Temperature sweeps for PEA2 and 10/20 penetration gr
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E-04 1E-03 1E-02 1E-01
Reduced Fre
Co
mp
lex
Mo
du
lus
(Pa)
20
30
40
50
60
70
80
90
1E-04 1E-03 1E-02 1E-01
Reduced Fr
Ph
ase
An
gle
(d
egre
es)
PEA1 PEA2
PEA3 PEA4
a
b
Fig. 5. Master curves of (a) complex modulus and (b) phase angle for different25 �C.
25 mm geometry at intermediate to high temperatures(25–80 �C). Three types of DSR tests were performed inthe study:
50 70 90
ure (°C)
10
20
30
40
50
60
70
80
90
Ph
ase
An
gle
(d
egre
es)
G* for PEA2 @ 1HzG* for 10/20 pen @ 1Hzdelta for PEA2 @ 1Hzdelta for 10/20 pen @ 1Hz
ade bitumen in terms of complex modulus and phase angle.
1E+00 1E+01 1E+02 1E+03
quency (Hz)
PEA1 PEA2
PEA3 PEA4
1E+00 1E+01 1E+02 1E+03
equency (Hz)
molecular weight polyethyl acrylate binders at a reference temperature of
1768 G.D. Airey et al. / Fuel 87 (2008) 1763–1775
� Amplitude sweeps (stress sweeps),� temperature sweeps, and� frequency sweeps.
The amplitude sweeps were undertaken using stresssweeps at 10 �C and 1 Hz for the 8 mm testing geometryand at 40 �C and 1 Hz for the 25 mm testing geometry.These stress sweeps were then used to determine the limitof the LVE response based on the point where complexmodulus has decreased to 95% of its initial value as pre-scribed during the SHRP study [27].
The temperature sweeps were undertaken at a constantfrequency of 1 Hz over a temperature range from �25 �Cto 80 �C using the 8 mm parallel plate testing geometry.The frequency sweep tests were performed under con-trolled strain loading conditions using frequenciesbetween 0.1 to 10 Hz at 5 �C temperature intervalsbetween 5 and 75 �C. The tests between 5 and 35 �C wereundertaken with the 8 mm diameter and 2 mm testing gapgeometry and from 25 to 75 �C with the 25 mm diameterand 1 mm testing gap geometry. The strain amplitude for
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
1E-05 1E-04 1E-03 1E-02 1
Reduced Fr
Co
mp
lex
Mo
du
lus
(Pa)
10/20 pen 70/100 100/150 pen PEA1PMA PBA
10
20
30
40
50
60
70
80
90
1E-05 1E-04 1E-03 1E-02 1
Reduced Fr
Ph
ase
An
gle
(d
egre
es)
10/20 pen 70/100 100/150 pen PEA1PMA PBA
Fig. 6. Master curves of (a) complex modulus and (b) phase angle for differenpolybutyl acrylate binders at a reference temperature of 25 �C.
all the temperature and frequency tests was confinedwithin the linear viscoelastic (LVE) response of the binder[28].
4. Synthetic polymer binders
4.1. High temperature viscosity
The high temperature viscosities of the synthetic bindersat typical road material mixing and compaction tempera-tures were determined using a Brookfield rotational vis-cometer at temperatures between 100 and 200 �C. Thehigh temperature viscosity relationships for PEA1 andPBA are shown in Fig. 2 together with relationships for aconventional 70/100 penetration grade bitumen and aSBS PMB. The flatter slopes for PEA1 and PBA illustratea lower temperature susceptibility for the synthetic binderscompared to the steeper slopes shown for the 70/100 penbitumen and SBS PMB. The ideal asphalt mixture mixingand compaction viscosities have also been added toFig. 2 and show that the temperatures associated with these
E-01 1E+00 1E+01 1E+02 1E+03
equency (Hz)
pen
E-01 1E+00 1E+01 1E+02 1E+03
equency (Hz)
pen
t penetration grade bitumens, polyethyl acrylate, polymethyl acrylate and
G.D. Airey et al. / Fuel 87 (2008) 1763–1775 1769
practical viscosity requirements are similar for the syntheticand traditional binders, although relatively high tempera-tures (>200 �C) would be required for ideal mixing ofPEA1 with aggregate.
4.2. Stress/strain response
The amplitude sweeps undertaken on the synthetic bind-ers and binder blends were used to determine the LVE lim-its for the different binders as well as to quantify the strain(or stress) dependency of the materials. The amplitudesweeps for three of the synthetic binders at 10 �C are pre-sented in Fig. 3.
The three synthetic binders show similar strain depen-dency compared to conventional bitumen as well as similarLVE limits both in terms of strain and stress (not shownfor brevity). Similar results were found for the other syn-thetic binders and blends where the LVE strain and stresslimits were shown to be functions of complex modulus(stiffness) of the binders similar to that found for conven-tional bitumens [24,27,28].
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
1E-05 1E-04 1E-03 1E-02 1
Reduced Fr
Co
mp
lex
Mo
du
lus
(Pa)
10/20 pen10/20 pen + PEA1 (3:1)10/20 pen + PBA (3:1)PEA1PBA
20
30
40
50
60
70
80
90
1E-05 1E-04 1E-03 1E-02 1
Reduced Fr
Ph
ase
An
gle
(d
egre
es)
10/20 pen10/20 pen + PEA1 (3:1)10/20 pen + PBA (3:1)PEA1PBA
Fig. 7. Master curves of (a) complex modulus and (b) phase angle for 10/20binders at a reference temperature of 25 �C.
4.3. Temperature dependency
The temperature dependency of the synthetic bindershas been assessed by means of isochronal plots of complexmodulus (G*) and phase angle (d) versus temperature at1 Hz as shown in Fig. 4. Fig. 4 shows the data for PEA2,which can be considered to be a medium stiffness syntheticpolymer binder, compared to the ‘hard’ 10/20 penetrationgrade bitumen. Similar plots were produced for the othersynthetic binders but are not shown here for brevity.
The results in Fig. 4 show that the temperature depen-dency of the synthetic binder differs from that found forconventional, unmodified bitumen. The PEA2 sampleshows less temperature susceptibility compared to the 10/20 pen bitumen as shown by the flatter slope of the com-plex modulus versus temperature relationship. In addition,the viscoelastic behaviour of the synthetic binder (as repre-sented by the phase angle versus temperature relationship)is more complex than the uniform transition from elasticresponse (low phase angles) to viscous response (high phaseangles) with increasing temperature for the conventional
E-01 1E+00 1E+01 1E+02 1E+03
equency (Hz)
E-01 1E+001 E+01 1E+02 1E+03
equency (Hz)
penetration grade with polyethyl acrylate and polybutyl acrylate blended
1770 G.D. Airey et al. / Fuel 87 (2008) 1763–1775
bitumen. Thermal analysis of the synthetic binder usingdifferential scanning calorimetry (DSC) would provide amore detailed analysis of the temperature dependencyand the presence of the G* plateau between 0 and 30 �C.
4.4. Frequency dependency
The frequency dependency of the synthetic binders interms of complex modulus and phase angle has beenassessed by producing rheological master curves at a refer-ence temperature of 25 �C using the time–temperaturesuperposition principle (TTSP) [29] and shift factors deter-mined for both the G* and d master curves. The mastercurves for the four PEA synthetic binders are shown inFig. 5. Due to the ‘‘thermo-rheological simplicity” of thesepolymeric materials (rheological properties being tempera-ture and time equivalent), smooth master curves for bothcomplex modulus and phase angle have been produced.The complex modulus and phase angle master curves showan increase in complex modulus (stiffening) and a decreasein phase angle (increasing elastic response) with increasingmolecular weight for the PEAs. However, these changes
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
1E-05 1E-04 1E-03 1E-02 1
Reduced Fr
Co
mp
lex
Mo
du
lus
(Pa)
10
20
30
40
50
60
70
80
90
1E-05 1E-04 1E-03 1E-02 1
Reduced Fr
Ph
ase
An
gle
(d
egre
es)
Fig. 8. Master curves of (a) complex modulus and (b) phase angle for 70/100temperature of 25 �C.
are relatively minor and do not alter the overall rheologicalresponse of the polyethyl acrylate binders.
The three types of acrylate binder (PEA, PMA andPBA) are compared in Fig. 6 together with three controlbitumens (a ‘hard’ 10/20 pen bitumen, a ‘medium’ stiffness70/100 pen bitumen and a ‘soft’ 100/150 pen bitumen). Thecomplex modulus master curves show that the polyethylacrylate binder (PEA1) has comparable G* values to the‘soft’ 100/150 pen bitumen over the reduced frequencyrange from 0.001 to 1 Hz, although the material tends tobe softer than the 100/150 pen bitumen at high frequencies.The PMA binder is more comparable to the ‘hard’ 10/20pen bitumen, although due to its lower frequency suscepti-bility (and related temperature susceptibility) it tends tohave higher complex modulus values at low reduced fre-quencies. The rheological behaviour of the PBA binder dif-fers considerable from both the PEA and PMA, as well asthe conventional bitumens, with this very soft syntheticbinder having consistently lower G* values over the entirereduced frequency domain.
The viscoelastic nature of the synthetic binders, asshown through the phase angle master curves in Fig. 6b,
E-01 1E+00 1E+01 1E+02 1E+03
equency (Hz)
70/100 pen + PMA (3:1)70/100 pen + PMA (1:1)PMA70/100 pen
E-01 1E+001 E+01 1E+02 1E+03
equency (Hz)
70/100 pen + PMA (3:1)70/100 pen + PMA (1:1)PMA70/100 pen
penetration grade with polymethyl acrylate blended binders at a reference
G.D. Airey et al. / Fuel 87 (2008) 1763–1775 1771
do differ significantly from those of the conventional bitu-mens. Although PBA shows a continuous decrease ofphase angle with frequency similar to that found for bitu-men, its response is predominantly viscous in nature withphase angles between 80� and 90� over the majority ofthe frequency domain. The PEA and PMA binders tendto show patently different viscoelastic behaviour to bitu-men with associated transitions resulting in their phaseangles initially decreasing with increasing frequency, mov-ing from a viscous to an increasingly elastic rheologicalresponse, then increasing before finally decreasing again.This results in the PMA binder showing a minimum andmaximum phase angle transition within the reduced fre-quency range depicted in Fig. 6b. It is, therefore, probablythat PEA and PMA may be less compatible in bitumencompared to PBA.
The results show that synthetic polymer binders canpartly replicate the rheological properties of conventionalbitumens in terms of being able to demonstrate comparablecomplex modulus values, but they do show significantlydifferent viscoelastic response as represented by their phaseangles as a function of both temperature and frequency.
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
1E-04 1E-03 1E-02 1E-01 1
Reduced Fr
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mp
lex
Mo
du
lus
(Pa)
10
20
30
40
50
60
70
80
90
1E-05 1E-04 1E-03 1E-02 1
Reduced Fr
Ph
ase
An
gle
(d
egre
es)
Fig. 9. Master curves of (a) complex modulus and (b) phase angle for 100/150temperature of 25 �C.
5. Blended binders
5.1. Hard bitumen blends
To investigate what the effect would be of using the syn-thetic binders as bitumen extenders rather than simply asreplacement binders, PBA and PEA1 were blended withthe ‘hard’ 10/20 penetration grade bitumen. High percent-ages (25% to 75% synthetic binder) were used to investigatethe ability of the synthetic binders to act as a partial binderreplacement. The rheological properties of the blends arepresented in Fig. 7 in terms of rheological master curvesof complex modulus and phase angle.
The effect of blending 25% by mass of the softer syn-thetic binders with 75% by mass of the hard bitumen hasnot significantly altered the overall complex modulus val-ues of the blended binder compared to the 10/20 pen bitu-men. The only significant effect seen in Fig. 7a is that theblended binders have a lower frequency (and related tem-perature) susceptibility compared to the conventional bitu-men. The changes in terms of the phase angle master curvesin Fig. 7b are however fairly dramatic with the master
E+00 1E+01 1E+02 1E+03 1E+04
equency (Hz)
100/150 pen + PMA (1:3)
100/150 pen + PMA (1:1)
PMA
100/150 pen
E-01 1E+001 E+01 1E+02 1E+03
equency (Hz)
100/150 pen + PMA (1:3)
100/150 pen + PMA (1:1)
PMA
100/150 pen
penetration grade with polymethyl acrylate blended binders at a reference
1772 G.D. Airey et al. / Fuel 87 (2008) 1763–1775
curves for the blends not only showing a reduction in fre-quency susceptibility but also a considerable increase inelastic response compared to the 10/20 pen bitumen. Thisis unexpected as the viscoelastic response of the blendedbitumen would be expected to lie between the hard bitumenand the more viscous synthetic binders. One possible rea-son for the viscoelastic response of the blended binders isthe lack of compatibility (stability) between the hard bitu-men and soft synthetic binders which would tend to mani-fest itself more in terms of the phase angle than complexmodulus.
5.2. Intermediate stiffness bitumen blends
As an alternative to blending a hard bitumen with a softsynthetic binder, an intermediate stiffness bitumen (70/100pen) was blended with the hard PMA synthetic binder attwo percentages (75% bitumen to 25% PMA and a 50:50blend). The rheological master curves of the blends areshown in Fig. 8.
The rheological properties of the blends show the com-bined properties of the individual bitumen and synthetic
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
1E-05 1E-04 1E-03 1E-02 1
Reduced F
Co
mp
lex
Mo
du
lus
(Pa)
PBA + PMA (1:3)PEA1 + PMA (3:1)PEA1PMAPBA
10
20
30
40
50
60
70
80
90
1E-05 1E-04 1E-03 1E-02 1
Reduced F
Ph
ase
An
gle
(d
egre
es)
Fig. 10. Master curves of (a) complex modulus and (b) phase angle for polymetat a reference temperature of 25 �C.
binder components. The complex modulus master curvesare situated between the 70/100 pen bitumen and the ‘hard’PMA binder, while the same pattern can be seen for thephase angle master curves. The overall observation forthese two blends is that the material appeared uniformlymixed and compatible.
5.3. Soft bitumen blends
Similar to the intermediate stiffness bitumen and PMAblends, blends were also produced using the soft 100/150pen bitumen with the hard PMA synthetic binder at twopercentages (25% bitumen to 75% PMA and a 50:50 blend).The rheological master curves of the blends can be seen inFig. 9.
Once again, as with the previous set of blends, the twomaterials appear to be very compatible and producedblends with rheological properties between those of thepenetration grade bitumen and the PMA synthetic binder.The higher percentage of PMA (75%) in one of the blendsas well as the softer nature of the 100/150 pen bitumenmeant that the rheological properties, particularly the
E-01 1E+00 1E+01 1E+02 1E+03
requency (Hz)
E-01 1E+001 E+01 1E+02 1E+03
requency (Hz)
PBA + PMA (1:3)PEA1 + PMA (3:1)PEA1PMAPBA
hyl acrylate with polyethyl acrylate and polybutyl acrylate blended binders
G.D. Airey et al. / Fuel 87 (2008) 1763–1775 1773
phase angle master curves, tend to be more indicative of thePMA binder than that seen for the blends in Fig. 8.
5.4. Synthetic binder blends
In addition to the bitumen–synthetic binder blends inFigs. 7–9, two blends were produced using only combina-tions of synthetic binder. The two blends both used PMAcombined with either 25% of the soft PBA binder or 75%of the medium stiffness PEA1. The rheological propertiesof the blends in the form of master curves can be seen inFig. 10. The rheological properties of both blends are dom-inated by the larger (higher percentage) component withthe PMA–PEA1 blend being very similar to the rheologicalproperties of PEA1 and the PMA–PBA blend being verysimilar to the rheological properties of PMA. For thePMA–PEA1 blend, the effect of adding 25% of the hardPMA–PEA1 results in an increase in complex modulus(Fig. 10a) and a decrease in phase angle (Fig. 10b). Con-versely for the PMA–PBA blend, the addition of 25% of
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
1E+08
1E+09
0 10 20 30 40
Phase Ang
Co
mp
lex
Mo
du
lus
(Pa)
SBS PMB
EVA PMB
PEA1
PMA
Fig. 11. Black diagrams of rheological data for SBS and E
1E+00
1E+01
1E+02
1E+03
1E+04
1E+05
1E+06
1E+07
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1E+09
0 10 20 30 40
Phase Ang
Co
mp
lex
Mo
du
lus
(Pa)
SBS PMB
EVA PMB
70/100 pen + PMA (1:1)
100/150 pen + PMA (1:3)
PEA1 + PMA (3:1)
Fig. 12. Black diagrams of rheological data for SBS and EVA PMBs com
the soft PBA–PMA results in a decrease in complex mod-ulus and an increase in phase angle.
6. Comparison with traditional polymer modified bitumens
Although the synthetic binders have not been used asbitumen modifiers in this study, it is interesting to comparetheir rheological properties to those associated with tradi-tional PMBs. However, as polymer modification tends toreduce the thermo-rheological simplicity of bitumen andtherefore the ability to produce master curves, the rheolog-ical data was assessed using Black diagrams as shown inFig. 11 [30].
The rheological properties of PEA1 and PMA are com-pared to high polymer content SBS and EVA PMBs. TheSBS PMB shows a relatively smooth curve in the Blackspace with the effect of the elastomeric polymer being seenthrough the increase in elastic response (decrease in phaseangles) at low complex modulus values. The EVA PMBshows typical semi-crystalline rheological behaviour with
50 60 70 80 90
le (degrees)
VA PMBs compared to synthesised polymer binders.
50 60 70 80 90
le (degrees)
pared to blended penetration grade and synthesised polymer binders.
1774 G.D. Airey et al. / Fuel 87 (2008) 1763–1775
a series of discrete curves within the intermediate tempera-ture and frequency range (centre of the Black space). Blackdiagrams can be considered as rheological ‘fingerprints’and this can be clearly seen in Fig. 11 where the rheologicalcurves for the two acrylate binders (PEA1 and PMA) showthe same pattern. The two synthetic binders bear a closerresemblance to the SBS PMB than the EVA PMB,although rheologically they are still very different fromthe PMB.
Three of the blended binders were also compared to theSBS and EVA PMBs in Fig. 12. The three blends (PEA1–PMA, 100/150 pen – PMA and 70/100 pen – PMA) allshow a similar rheological pattern in the Black diagramwith a shift towards more elastic response (lower phaseangles) being associated with an increase in the percentageof synthetic binder. Similar to the observations in Fig. 11,the blended bitumens are rheologically closer to the SBSPMB than the plastomeric EVA PMB.
7. Conclusions
The DSR results indicate that polyethyl acrylate syn-thetic polymer binders have rheological properties, in termsof complex modulus values, similar to that of a ‘soft’ 100/150 penetration grade bitumen, while the polymethyl acry-late synthetic polymer binder showed complex modulusvalues comparable with a ‘hard’ 10/20 penetration gradebitumen. However, in terms of their viscoelastic response,both acrylate binder types showed considerably more poly-mer-like rheological behaviour than that found for conven-tional, unmodified bitumens.
The polybutyl acrylate synthetic polymer binder wasfound to be considerably softer and more viscous in natureat ambient temperatures compared to conventional bitu-mens and therefore cannot be used by itself as an asphaltbinder but could be used to modify (soften) stiffer gradebitumen. Overall the results showed that synthetic polymerbinders can partly replicate the rheological properties ofconventional bitumens, although they should not be con-sidered to be a direct rheological replacement for bitumen.
The synthetic polymer binders were successfully usedtogether with conventional, penetration grade bitumen toproduce bitumen–synthetic binder as well as combinationsynthetic 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 rhe-ological properties that combined the properties of the twocomponents. The synthetic binders, and particularly theextended bitumen samples (blends), produced rheologicalproperties that showed similar characteristics to elasto-meric SBS PMBs, although the precise viscoelastic proper-ties were not identical.
Although the rheological properties of the syntheticbinders and blends are important in terms of characterisingthese binders, other physical and mechanical propertiessuch as high temperature viscosity, thermal stability, UVresistance, adhesion and durability considerations in terms
of ageing and moisture damage will also need to beassessed before these binders can be considered suitablefor asphalt mixture application.
Acknowledgements
The authors would like to thank the UK Engineeringand Physical Sciences Research Council (EPSRC) for sup-porting this research under a Platform Grant awarded tothe Nottingham Transportation Engineering Centre. Theywould also like to acknowledge the contribution of Dr.Christopher Hayes of the School of Chemistry at the Uni-versity of Nottingham through his involvement with thesynthesis work described in the paper.
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