Fuel 115 (2014) 416–425

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Fuel

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Morphology, thermal analysis and rheology of Sasobit modified warmmix asphalt binders

0016-2361/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.fuel.2013.07.033

⇑ Corresponding author.E-mail address: qqin@uwyo.edu (Q. Qin).

Qian Qin ⇑, Michael J. Farrar, Adam T. Pauli, Jeramie J. AdamsWestern Research Institute, 365 North 9th Street, Laramie, WY 82072, United States

h i g h l i g h t s

�We explored microstructure–property relationships of Sasobit modified asphalts.� Network and dendrite structures are demonstrated for 3% and 1% Sasobit blends.� Gel formation is responsible for the breakdown of time–temperature superposition.� Network structure stiffens blends and expands high limiting temperature by 5–16 �C.� Sasobit does not exhibit undue adverse effect on low temperature performance.

a r t i c l e i n f o

Article history:Received 8 May 2013Received in revised form 4 July 2013Accepted 10 July 2013Available online 30 July 2013

Keywords:SasobitWaxAsphalt binderRheologyMorphology

a b s t r a c t

The microstructure–property relationship of Sasobit modified Warm Mix Asphalts (WMA) is investigatedin terms of thermal, rheological and morphological studies. Four asphalt binders with different types andgrades and two Sasobit concentrations (1% and 3% by weight) are included in this study. A 3-D networkstructure or pseudo-solid like behavior of 3% Sasobit modified WMA is demonstrated. The network struc-ture contributes to blend stiffening at high temperatures resulting in the high limiting temperature beingexpanded by 5–16 �C, and the breakdown of time–temperature superposition at temperatures above30 �C. The network microstructure is developed not only on the asphalt surface as depicted by AtomicForce Microscopy (AFM) images, but also in the bulk as implied by rheology. Considering the unchangedglass transition temperatures of asphalts after blending with Sasobit, the network formation is presum-ably due to the interactions among Sasobit crystals, which act as the physical crosslinks in the viscousasphalt liquid. For 1% Sasobit blends, dendrites rather than typical ‘‘bee structure’’ are observed, whichis at least partially due to the high molecular weight of Sasobit and its relatively large concentration com-pared with naturally occurring wax inside asphalts. The network or dendritic microstructure appearsonly dependent on the Sasobit concentration, regardless of asphalt types and grades investigated. In addi-tion, Sasobit is not expected to exhibit an undue negative effect on low temperature performance as sug-gested by 2 �C upshift of the limiting low temperatures.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Concerns with the health consequences of exposure to asphaltfumes and the environmental impact of hot mix asphalt (HMA)production have resulted in a major effort in Europe, and more re-cently the United States, to develop an alternative technology forHMA. The alternative technology is known as warm mix asphalt(WMA). WMA represents a number of processes and additives,some of which are proprietary, that allow production of asphaltmix at a significantly lower temperature than HMA. The lower pro-

duction temperature substantially reduces energy consumptionand asphalt fumes emissions [1,2].

The focus of this paper is on the WMA organic additive Sasobit�.Sasobit is a fine crystalline, long-chain aliphatic hydrocarbon com-pound produced from the Fischer–Tropsch (FT) process [3,4]. Itmelts between 70 �C and 120 �C [5]. The melting temperature ishigh enough to make Sasobit maintain its crystalline structure atpavement service temperatures, imparting a stiffening effect. Attemperatures above its melting point, Sasobit acts as a flow impro-ver by reducing the viscosity of the asphalt enabling mixing andcompaction temperatures to be reduced by 18–54 �C [4,6].

There have been numerous studies on Sasobit modified binders,which include its impact on the performance grade [7], agingbehavior [8–10] and rheological properties at high or low

Q. Qin et al. / Fuel 115 (2014) 416–425 417

temperatures of asphalt binders [11,12–14]. It has been found thatSasobit modified binders have higher complex modulus, lowercreep compliance and phase angles. It is reported that the perfor-mance grade of asphalts can be expanded by addition of Sasobitwith the upper temperature increased by up to 13.5 �C and lowertemperature increased by up to 6.9 �C for 3% Sasobit modifiedWMA [7]. In addition, resistance to oxidative aging propertieswas observed for Sasobit modified WMA [10].

In spite of a large body of research on Sasobit modified binders,only limited systematic studies on the correlation between struc-ture and viscoelasticity have been performed. Recently, Polaccoand coworkers conducted a comprehensive study regarding the ef-fect of different types of waxes on asphalt morphology, residualcrystallinity and mechanical properties [15]. A gel structure of as-phalt blends with Fischer–Tropsch (FT) type waxes was suggestedby observed high penetration index and low values of phase angle(55–65�) [15].

In this paper, we present direct rheological and morphologicalevidence for a gel-like or pseudo-solid structure of Sasobit modi-fied WMA. Further, we suggest the sol–gel transition not only onthe surface of those blends as depicted by atomic force microscopy(AFM), but also extends to the bulk. The effects from binder typesand grades are also investigated in terms of how they impactmicrostructures and viscoelasticity of Sasobit blends. The purposeof this work is to explore the microstructure–property relationshipof Sasobit modified binders so as to have a better understanding oftheir linear viscoelasticity and therefore provide an important in-sight for warm mix asphalt technologies and binder modifications.

2. Materials and methods

2.1. Materials

Four asphalt binders with different types and grades were usedas control binders in this study and they are listed in Table 1.Among the four binders, MN1-3, MN1-4 and MB are PG 58-28unmodified and from different crude sources. YNP is a styrene–butadiene–styrene (SBS) PG 58-34 modified binder.

One and three percent Sasobit loading concentrations (byweight) were selected for this study because 3% is typically themaximum loading concentration above which the low tempera-ture performance of Sasobit modified WMA would be negativelyaffected [4,16]. The manufacture of Sasobit, Sasol, also recom-mends a Sasobit concentration larger than 0.8%, but no more than3%.

Sasobit modified asphalt binders were prepared by heating as-phalt to 150 �C, followed by addition of pre-weighted Sasobit andsubsequent gentle stirring. The modified asphalt blend was main-tained at 150 �C for approximately 1 h with occasional stirring toachieve a homogenous blend. With this type of treatment asphaltshould only undergo mild oxidation.

Table 1Sasobit modified WMA control binders.

Asphaltbinder

PGgrade

Description

MB PG58–28

Unmodified binder from Manitoba, Canada, 150/200pen, Canadian blend

MN1-3 PG58–28

Unmodified binder from Rochester Minnesota,Canadian blend

MN1-4 PG58–28

Unmodified binder from Rochester Minnesota, Blend:Arab heavy, Arab medium, and Kirkuk

YNP PG58–34

SBS modified binder from yellow stone national park

2.2. Experimental methods

2.2.1. Characterization of thermal propertiesThe thermal properties were measured using a TA instruments

Q2000 Differential Scanning Calorimeter (DSC). The melting tem-perature (Tm) and heat of fusion (DHf) of Sasobit component weredetermined from the heating scan made at 10 �C/min, after coolingfrom 165 �C to �90 �C at 5 �C/min and subsequently isothermalholding at �90 �C for 5 min.

The glass transition temperatures of the asphalt componentwere measured using temperature modulated DSC (MDSC) tech-nique. MDSC can clearly separate glass transition from other over-lapped effects, such as cold-crystallization and enthalpy recoveryby dividing the total heat flow into reversing and non-reversingparts [17]. The sample was first heated and equilibrated at165 �C before cooling down to �90 �C. After holding at �90 �Cfor 5 min, a second heating scan was performed up to 165 �C.The cooling scan and the subsequent heating scan were run at2 �C/min with modulation amplitude of 0.5 �C every 80 s. The lim-iting fictive temperature T 0f was determined from the reversingheat flow curve during the second heating scan. The limiting fic-tive temperature T 0f is known to be approximately equal to theglass transition temperature Tg obtained from the cooling scan[18,19].

2.2.2. Rheological measurements and machine compliance correctionsThe rheological behavior of the asphalt binders and Sasobit

modified binders was studied using a Malvern Instruments Kinex-us rheometer. The 4 mm diameter parallel plate geometry and a1.75 mm gap were used for tests over a temperature range from�30 �C to 30 �C. At 50 �C and 70 �C, the 25 mm diameter parallelplate and a 1 mm gap were used. At each temperature of interest,a dynamic strain sweep was first performed to determine the lin-ear viscoelastic range prior to the isothermal dynamic frequencysweep. All measurements were performed under nitrogen protec-tion to avoid thermal oxidation and the temperature was stabilizedwithin ±0.1 �C. The dynamic data presented below 0 �C arecorrected for machine compliance. Neglecting or improperly cor-recting the machine compliance will lead to substantial errors forlow temperature rheological measurements or for very stiff sam-ples. It is essential to perform machine compliance correction fordynamic shear rheometer measurements [20–22].

With the assumption that the instrument components arepurely elastic, the machine compliance Jmachine was determinedby the ratio of angular displacement h and the generated torqueM with zero gap (Eq. (1)). The zero gap can be achieved by eitherusing a thin layer of superglue between plates or machining a solidrod made of the same material as the parallel plates.

Jmachine ¼hM

ð1Þ

Based on Schr}oter and others [20–23], the actual measuredstiffness Kmeasured is related to the sample and machine stiffnessas follows.

1Kmeasured

¼ 1Ksample

þ 1Kmachine

ð2Þ

For parallel plate geometry with gap of h and radius of R, theconversion between stiffness K and shear modulus G can be ex-pressed as:

K ¼ pR4G2h

ð3Þ

Therefore, the true complex, storage and loss moduli of thesample between the parallel plates can be calculated as follows:

418 Q. Qin et al. / Fuel 115 (2014) 416–425

G�sample ¼G�measured

1� JmachineG�measuredpR4

2h

ð4Þ

G0sample ¼G0measured 1� pR4 Jmachine

2h G0measured

� �� pR4 Jmachine

2h G00measured

� �2

1� pR4 Jmachine2h G0measured

� �2þ pR4 Jmachine

2h G00measured

� �2

ð5Þ

G00sample ¼G00measured

1� pR4 Jmachine2h G0measured

� �2þ pR4 Jmachine

2h G00measured

� �2 ð6Þ

2.2.3. Morphological characterization by AFM imagingA Quesant Q-Scope™ 250 Atomic force microscopy (AFM) was

used to image thin films of asphalt binders and Sasobit modifiedbinders in dynamic wave mode. The thin films were prepared byinitially dissolving 87 mg binder samples in 1 ml toluene, allowingfor complete dissolution by allowing the solutions to stand over-night. Thin films were then prepared from the sample solutionsby spin casting onto a borosilicate glass microscope slide at400 rpm. Resultant films were annealed at 64 �C for 2 h, then rap-idly cooled to room temperature prior to imaging. Samples wereimaged in the intermittent contact mode (BB Wavemode� perAmbios terminology). An ‘ultrasharp’ NSC-16 cantilever (nominalspring constant of 40 N/m) supplied from Micromasch was usedfor the imaging. Set point was typically either �0.5 V or �0.7 V giv-ing damping factors of �50% and �30% respectively. Imaging wasconducted under dry nitrogen at room temperature (�24 �C).Topography, phase contrast, and error (derivative) images werecollected simultaneously for each sample. All of the images shownare 80 � 80-lm, and all were collected using a scan rate of 1 Hz.

2.2.4. Nuclear magnetic resonance spectroscopy (NMR)The molecular structure of Sasobit was analyzed using a Bruker

Advance III 400 MHz NMR on saturated CDCl3/Sasobit solution. 1HNMR spectra were acquired with a delay of 1 s and 13C NMR spec-tra were acquired using a 2 s delay. The pulse programs were ag30to acquire proton spectra, and zgpg30 to obtain carbon spectra. 1H

Fig. 1. 1H NMR spectra of Sasobit in CDCl3. The NMR solvent was contamina

NMR spectra were calibrated to 7.24 ppm for CDCl3 while 13C NMRspectra was calibrated to 77.23 ppm.

3. Results and discussion

3.1. Structural characterization and thermal analysis

1H NMR and 13C NMR were used to identify the molecular struc-ture of Sasobit. 1H NMR spectra gave a major broadened singlet at1.24 ppm, a triplet centered at 0.86 ppm, and an apparent doubletat 0.81 ppm as shown in Fig. 1. The triplet corresponds to methylprotons next to methylene protons while the large singlet is anaverage of several very similar CH2 groups which was confirmedby 13C NMR (Fig. 2). The apparent doublet at 0.81 ppm is attributedto the CH2 groups attached directly to the terminal methyl groups.Based on the integration of the singlet peak against the triplet anddoublet peaks (12.2:1), an average chain length of 65 carbons forSasobit can be estimated.

DEPT (Distortionless Enhancement by Polarization Transfer) 13CNMR spectra were used to determine if the carbon resonances arefrom primary, secondary or tertiary carbon atoms. DEPT 45 13CNMR spectrum gives all carbons with attached protons, whose res-onances occur at 32.1, 29.9, 22.9 and 14.3 ppm. From the DEPT 13513C NMR spectrum, it can be deduced that carbons at 32.1, 29.9,and 22.9 ppm are methylene carbons, and the resonance at14.3 ppm corresponds to methyl groups.

The absence of other complex proton resonances with couplingCH and adjacent methylene resonances by 1H NMR, and the lack ofother carbons and CH carbons in the 13C NMR spectra, suggeststhat this Sasobit sample primarily contains linear hydrocarbonchains.

Temperature modulated DSC (MDSC) thermograms of the baseasphalt binders and their Sasobit blends are shown in Fig. 3. Theobtained limiting fictive temperatures T 0f are listed in Table 2.Due to the fact that the amount of different component has to beused for heat flow normalization, the glass transition and meltingcurves are plotted separately.

Both the heat flow curves and calculated limiting fictive tem-peratures suggest no significant shifts in glass transition tempera-tures of the Sasobit blend. Further, no broadening of glass

ted with a small amount of water as evidenced by the peak at 1.5 ppm.

Fig. 2. DEPT 45 13C NMR and DEPT 135 13C NMR spectra showing Sasobit methylene carbons out of phase at 32.1, 29.9 and 22.9 ppm, and methyl carbons in phase at14.3 ppm.

-50 0 50 -50 0 50 -50 0 500.00

0.01

0.02

0.03

0.04

-50 0 500.00

0.01

0.02

0.03

0.04

0.05

Endo

MB

dCP /dT (J/g/°C

2)

Temperature (°C)

Rev

ersi

ng h

eat f

low

(W/g

)

3%

1%

0% Sasobit MN 1-3 MN 1-4 YNP

Fig. 3. MDSC reversing heat flow (top graphs) and temperature derivative ofreversing heat capacity (bottom graphs) of asphalt binders below 60 �C. The heatflow was normalized by the binder weight.

Table 2Limiting fictive temperature (T 0f ) of Sasobit modified warm mix asphalts.

Samples T 0f (�C)

0% Sasobit 1% Sasobit 3% Sasobit

MB �18.6 �18.9 �19.0MN1–3 �18.3 �18.7 �19.0MN1–4 �23.0 �22.5 �23.3YNP �21.4 �21.2 �20.1

Note: T 0f obtained from heating scan is approximately equal to the glass transitiontemperature Tg from cooling scan [18,19].

-100 -50 0 50 100 150 200

0

50

100

150

200

250

300

100.0°C

74.2°C

100.6°C

75.2°C

100.0°C74.7°C

99.0°C73.8 °C

109.5°C

95.1 °C

Nor

mal

ized

hea

t flo

w (W

/g)

Endo

Hea

t of f

usio

n ΔH f (

J/g)

YNP/ 3% Sasobit

MN1-4/ 3% Sasobit

MN1-3/ 3% Sasobit

MB/ 3% Sasobit

Sasobit

Temperature (°C)

YNP

MN1-4

MN1-3MB

3% SasobitSasobit

Fig. 4. DSC endotherms and heat of fusion (inset plot) of Sasobit and 3% Sasobitmodified binders. The heat flow was normalized by the Sasobit weight.

Q. Qin et al. / Fuel 115 (2014) 416–425 419

transition is found upon the addition of Sasobit into asphalt bind-ers as evidenced by the unchanged breadth of temperature deriv-ative of heat capacity (dCP/dT). Those facts imply an absence ofstrong interactions between Sasobit and asphalt moieties.

The DSC endotherm of Sasobit wax exhibits two characteristicpeaks with the onset melting temperature of 77 �C and peak melt-ing temperatures of about 95 �C and 110 �C. Since Sasobit primarilyconsists of linear or n-paraffin structures without apparent

presence of branching chains or iso-paraffins as suggested byaforementioned NMR results, two DSC endo-peaks of Sasobit areascribed to the mixture of linear long chain aliphatic hydrocarbonswith melting temperatures of 95 �C and 110 �C.

Upon blending the binders with 3% Sasobit, both the meltingpoint and heat of fusion of Sasobit are depressed as seen inFig. 4. Based on the Gibbs–Thomson effect [24,25], the depressedmelting point suggests a smaller crystal size is favorable for Sasobitmodified asphalt binders. The observed decrease in the heat of fu-sion is attributed to the finite size effect or reduced Sasobit crystalsize, which is consistent with literature results for small moleculesconfined in controlled porous media with nanometer pore sizes[25–27] and melting of metallic nanocrystals [28].

3.2. Rheological properties

Fig. 5 is the rheological results for the base asphalt binders andSasobit blends. The van Gurp–Palmen plots [29] were used to testthe applicability of time–temperature superposition (tTS)

100 101 102 103 104 105 106 107 108 109 1010

0

10

20

30

40

50

60

70

80

90

100 102 104 106 108

10

20

30

40

50

60

7080

90

(a)δ(

0 )

G* (Pa)

MB/ 3% wax

MBTemperature MB MB /3% wax -30 0 C -10 0 C 10 0 C 30 0 C 50 0 C 70 0 C

Phas

e an

gle

δ (0 )

Complex modulus G* (Pa) Complex modulus G* (Pa)

MB/ 1% wax

101 102 103 104 105 106 107 108 109 1010

10

20

30

40

50

60

70

80

90

100 102 104 106 108

1020

30

4050

60

7080

90

(b)

MN1-3/ 3% wax

MN1-3

δ(0 )

G* (Pa)

Temperature MN1-3 MN1-3/3% wax -30 0 C -10 0 C 10 0 C 30 0 C 50 0 C 70 0 C

Phas

e an

gle

δ (0 )

MN1-3/ 1% wax

100 101 102 103 104 105 106 107 108 109 1010

10

20

30

40

50

60

70

80

90

100 102 104 106 108 10100

102030405060708090

(c)

MN1-4 / 3% wax

MN1-4

δ (0 )

G*(Pa)

Temperature MN1-4 MN1-4 /3% wax -30 0 C -10 0 C 10 0 C 30 0 C 50 0 C 70 0 C

Phas

e an

gle

δ (0 )

Complex modulus G* (Pa)

MN1-4 / 1% wax

100 101 102 103 104 105 106 107 108 109 1010

0

10

20

30

40

50

60

70

80

90

100 102 104 106 108 1010102030405060708090

(d)

YNP

YNP/ 3% Sasobit

δ(0 )

G*(Pa)

Temperature YNP YNP /3% wax -30 0 C -10 0 C 10 0 C 30 0 C 50 0 C 70 0 C

Phas

e an

gle

δ (0 )

Complex modulus G* (Pa)

YNP /1% Sasobit

Fig. 5. van Gurp–Palmen plots (phase angle vs. complex modulus) for binders and their blends with 3% Sasobits. The insets are the same plots for 1% Sasobit/asphalt binders.(a) MB binder and Sasobit blends; (b) MN1-3 binder and Sasobit blends; (c) MN1-4 binder and Sasobit blends; and (d) YNP binder and Sasobit blends.

0.1 1 10108

109

0.1 1 10108

109

MB MB/ 1% Sasobit MB/ 3% Sasobit

G* (P

a)

ω (rad/s)

-30 0C

G* (P

a)

ω (rad/s)

YNPYNP/ 1% SasobitYNP/ 3% Sasobit

-30 0C

Fig. 6. Complex shear modulus at �30 �C for MB and Sasobit modified MB binders.The inset is the same plot for YNP and Sasobit modified YNP binders.

420 Q. Qin et al. / Fuel 115 (2014) 416–425

principle. The van Gurp–Palmen plot is one of a family of similarplots such as the Cole–Cole, Han, and Wicket plots which are inde-pendent of reduced frequency and hence temperature. Failure of

tTS can be conveniently read from the van Gurp–Palmen plot.The Black space plot is similar to the van Gurp–Palmen plot exceptthe axes are reversed.

In Fig. 5, time–temperature superposition (tTS) holds for theMB, MN1-3 and MN1-4 base binders, but not for the YNP base bin-der. The YNP base binder is SBS modified and its deviation from tTSabove 30 �C can be attributed to the presence of SBS in the asphaltbinder matrix, which not only changes the absolute value of therelaxation times, but also alters the temperature dependence ofstress relaxation of the binder matrix.

Upon the addition of 3% Sasobit, the van Gurp–Palmen plotslose their single curve characteristics above 30 �C, suggesting thebreakdown of tTS. The DSC results on these samples indicatedthe onset melting temperature of Sasobit component is higher than30 �C, therefore, the melting of Sasobit crystals cannot totally ac-count for the deviation from tTS for 3% Sasobit modified binders.That deviation is thought to be due to microstructural changes inthe blends, leading to different relaxation functions within the bin-der. This concept will be discussed in more detail later in the paper.

3.2.1. Low temperature rheologyAt �30 �C, no significant increase in the absolute complex shear

modulus is observed for Sasobit (63% by weight) modified MBbinders as seen in Fig. 6. Although not shown, a similar lack of

Table 3The ‘‘limiting low temperatures’’ at which S(t) 6 300 MPa and creep rate m-value 6 �0.3 for the asphalt binders and Sasobit blends.

Samples Calculated ‘‘limiting low temperature’’ (�C) based on S(t) and m-value

0% Sasobit 1% Sasobit 3% Sasobit

MB �42.9 �42.1 �41.2MN1-3 �40.5 �41.7 �38.2MN1-4 �45.0 �45.0 �41.5YNP �45.2 �45.3 �43

0

10

20

30

40

50

60

70

80

90

YNPMN1-4MN1-3MB

Lim

iting

hig

h te

mpe

ratu

re (0 C

)

Control asphalt binders 1% Sasobit 3% Sasobit

Fig. 7. The limiting high temperatures corresponding to G�/sind = 1.0 kPa(x = 10 rad/s).

101

102

103

104

105

Slope<0.5

50 0C

MB/ 3% Sasobit

MB / 1% SasobitG' (

Pa)

Q. Qin et al. / Fuel 115 (2014) 416–425 421

significant increase in the complex modulus at �30 �C for theMN1-3 and MN1-4 binders was also observed. Rather surprisingly,Sasobit modified YNP binders became softer at �30 �C as shown inthe inset of Fig. 6.

Besides the dynamic shear moduli, creep stiffness S(t) and creeprate (m-value) have been widely accepted as two importantparameters to characterize the low temperature properties of as-phalt binders. Creep stiffness and m-value are generally obtainedby means of bending beam rheometer (BBR).

Implementation of machine compliance correction to low tem-perature or high modulus DSR measurements allows a DSR alter-native to BBR [21,30]. By using 4-mm DSR at low temperaturesafter machine compliance correction, an estimate of creep stiffnessand m-value can be obtained by converting the dynamic modulusto the relaxation data [30]. The limiting low temperature, corre-sponding to S(t) = 300 MPa and m-value = �0.30, for all four as-phalts and their Sasobit blends is tabulated in Table 3.

It should be mentioned that the ‘‘limiting low temperature’’ ofbase binders appear lower than the corresponding PG grade de-scribed in Table 1. That is because these binders did not undergothe standard rolling thin film oven (RTFO) and pressure aging ves-sel (PAV) oxidative aging processes. Table 3 ‘‘limiting low temper-atures’’ are used to consider the effect of the Sasobit on lowtemperature performance. The results suggest an increase in the‘‘limiting low temperature’’ by about 2 �C for 3% Sasobit modifiedMB, MN1-3 and YNP binders. The increase in the ‘‘limiting lowtemperature’’ of the YNP Sasobit blends is found to be associatedwith its slower relaxation of accumulated stress even though itscreep stiffness and modulus are lower than the base binder. Aslightly larger upshift in ‘‘limiting low temperature’’ (3.5 �C) isfound for the 3% MN1-4 Sasobit blend, which is attributed to theunusually large presence of naturally occurring wax1 having a syn-ergistice effect on the low limiting temperature.

Overall, it does not appear that Sasobit at concentration levelsof 3% or less exerts undue adverse effect on binders’ low tempera-ture properties.

0.1 1 10 10010-1

100

1

102

103

104

Slope~2MB

G" (

Pa)

Slope~1

MB / 1% Sasobit

MB/ 3% Sasobit

MB

3.2.2. High temperature rheologyThe limiting high temperatures are expanded by 5 �C, 9 �C, 13 �C

and 16 �C for Sasobit modified MN1-4, MB, MN1-3 and YNP bindersrespectively as shown in Fig. 7, which is highly desirable for hightemperature rutting resistance improvement.

At 50 �C, which is far above the glass transition temperature ofasphalt binders, the base asphalt binder MB displays typical liquidbehavior, i.e., the storage modulus (G0) is proportional to the fre-quency squared (x2) and the loss modulus (G00) is proportional tothe frequency (x) at low frequencies shown in Fig. 8. Upon theaddition of Sasobit, especially 3% Sasobit by weight, a dramaticchange in rheological properties occurs. Instead of keeping the

1 Significant appearance of ‘‘bee’’ structuring depicted in AFM images of MN1-4binder (left image below) suggests occurrence of at least 3% natural wax as comparedto the right image below for MN1-5 which contains no wax.

classic liquid response, 3% Sasobit modified MB exhibits the pseu-do-solid or semi-solid behavior with the G0�x slope of less than0.5. Although loss modulus is generally believed to be insensitiveto structure change, its sluggish variation as a function of fre-quency can be clearly observed for 3% Sasobit modified MB atlow frequencies. Those results strongly suggest the formation ofpseudo-solid like structures in 3% Sasobit modified binders at50 �C. Considering the previously observed unchanged glass transi-tion temperatures upon incorporation of Sasobit, the semi-solidstructure for 3% systems are more likely to be due to the interac-tions among Sasobit crystals, where Sasobit might act as physicalcrosslink joints inside asphalt matrix.

The typical pseudo-solid like rheological behavior is also foundfor YNP/Sasobit blends depicted in Fig. 9. The presence of a veryweak frequency dependence of storage modulus G0 at low

0.1 1 10 10010

Frequency ω (rad/s)

Fig. 8. Storage and loss moduli of MB and Sasobit modified MB binders at 50 �C.

422 Q. Qin et al. / Fuel 115 (2014) 416–425

frequencies with G0 exceeding G00 indicates gel formation in 3% Sas-obit modified YNP, which resembles the rheological responses ofcross-linked networks, and some percolated nanocomposites andhybrids [31–33]. The inset plots of Fig. 9 illustrate the evolutionof a gel structure as Sasobit loading concentration increases inthe YNP binder matrix. Although the gel point of most networksoccurs not exactly at the crossover of G0–G00, but in its vicinity[34], the inset plots clearly reveal a sol–gel transition from YNPbinder to 3% Sasobit modified YNP.

Such pseudo-solid like or gel-like rheological behavior can beseen in Fig. 10 where complex viscosity is plotted against complexmodulus. Without Sasobit modifications, all binders except YNPdemonstrate nearly Newtonian viscosity, which represents typicalliquid response. With 3% Sasobit modifications, the viscosity tendsto diverge at a finite value of complex modulus, which has beenconsidered as a measure of the yield stress [31,35]. The presenceof nearly diverging viscosity or yield stress is consistent withsemi-solid characteristics. Based on the above linear viscoelasticityat 50 �C, the previously observed deviation from time–temperaturesuperposition at temperatures above 30 �C for 3% Sasobit modified

0.1 1 10 100100

101

102

103

104

105

0 1 2 3101

102

103

104

Sasobit weight%

G';G

" (Pa

)

50 0C

G"G'

G"G'

YNP/ 3% Sasobit

YNP

G';G

" (Pa

)

ω (rad/s)

ω = 0.1 rad/s

G,,

G'

Fig. 9. Storage and loss moduli of YNP and 3% Sasobit/YNP at 50 �C. The insetrepresents the variation of shear moduli (G0 and G00) with Sasobit loadingconcentration, showing the transition from sol to gel as Sasobit fraction increases.

101 102 103 104 105102

103

104

105

50 0C

MN1-3MN1-4

YNP

MN1-4/ 3% Sasobit

MB/ 3% Sasobit

MB

MN1-3/ 3% Sasobit

YNP/ 3% Sasobit

η* (Pa

s)

G* (Pa)

Fig. 10. Plots of complex viscosity as a function of complex modulus for MB, MN1-3, MN1-4, YNP binders and their blends with 3% Sasobit at 50 �C.

binders is attributed to microstructure change, where a gel-like orsemi-solid structure is formed as opposed to isolated Sasobit crys-tals dispersed among binder liquids. The network or semi-solidstructure formation alters the temperature dependence of stressrelaxation and consequently results in the inapplicability oftime–temperature superposition principle. In addition to the rheo-logical analysis, a morphological study was conducted to providedirect visualization for the microstructure change within Sasobitmodified WMA.

3.3. Morphology of asphalt binders and Sasobit modified WMA

In the past decades the concept of a surface microstructurepresent in asphalt binders has attracted much attention regardingthe so-called ‘‘bee structures’’. Bee structures, characterized assmall (�10 lm) oblong shaped structures with rippled interiors,have been observed in a host of neat and aged asphalts [36–42].By AFM examination on asphalt fractionations extracted by meansof SARA (saturates, aromatics, resins and asphaltenes) chromatog-raphy and ion-exchange chromatography, followed by wax separa-tion, Pauli and coworkers were able to investigate the morphologyof asphalt fractions before and after wax removal as well as frac-tions with and without asphaltenes. Their work strongly suggeststhat the ‘‘bee structure’’ is attributed to wax rather than asphaltenemicelles [43]. The ‘‘bee structures’’ were observed in both n-paraf-fin and microcrystalline wax doped asphalts in their work.

AFM topography images of MB binder exhibit relative homoge-nous morphology without apparent structural features as seen inFig. 11a. Upon incorporation of 1% and 3% Sasobit, micro-phasesdevelop as seen in Fig. 11b and c. Therefore the development of adendritic microstructure is attributed to the introduction of Sasobitin the MB binder based on the work reported in Pauli et al. [43]. Butinstead of forming ‘‘bee structures’’, the presence of Sasobit givesrise to crystal dendrites in 1% systems [44]. By comparison, theimages in Fig. 11c illustrate the development of a network gelstructure resulting from a 3% Sasobit/MB binder. At 50 �C, mobilityof Sasobit and asphalt binders (left hand images) is greatly en-hanced leading to a more diffuse boundary between micro-phaseboundary interfaces compared to room temperature images (righthand images).

Dendritic and network surface microstructures are also ob-served for 1% and 3% Sasobit modified YNP binders, respectively.Compared with the unmodified MB binder, the YNP binder initiallyexhibits a micro-phase structure as shown in Fig. 12a. One possibleinterpretation for this structure is SBS reported to be present in theYNP binder. The other possibly may be the presence of natural waxin the neat binder. By comparison to 1% Sasobit/MB binder, 1% Sas-obit/YNP binder also exhibits dendrite structuring, and by compar-ison to 3% Sasobit/MB binder, 3% Sasobit/YNP also exhibits anetwork gel structure. These findings strongly illustrate Sasobitconcentration dependence with the observed surface structuringindependent of the binder source. Consistent with Pauli andcoworkers’ findings regarding the preference of wax microstruc-ture occurring on the film surface [43], the images depicted inFigs. 11b and 12b may also suggest that Sasobit dendrites preferen-tially develop at the surface due to interfacial thermodynamic ef-fects [44,45].

It is presently unknown why dendrite crystals rather than beestructures form in these particular materials with this type of highmolecular weight wax (Sasobit), but Pauli et al. [43] observed thathigher molecular weight waxes, specifically hexacontane dopedinto SHRP asphalts AAA-1 and AAG-1, exhibited both bee structur-ing as well as dendrite branching lending to speculation that waxmolecular weight may partially contribute to the structuring re-ported here. Dendritic crystallization is normally characterized asa rapid unstable solidification process driven by steep thermal

Fig. 11. AFM topography image of Sasobit modified MB binders at 50 �C (left images) and at room temperature (right images) with scan area of 80 � 80 lm. (a) MB binder; (b)1% Sasobit/MB; and (c) 3% Sasobit/MB.

Q. Qin et al. / Fuel 115 (2014) 416–425 423

and concentration gradients. The samples reported in the presentstudies were, for all practice purposes, prepared at moderate tohigh wax concentrations (compared to naturally occurring wax

concentrations) and quench cooled. Lower Sasobit concentrationsand slower cooling processes may produce the familiar bee struc-tures from this type of wax. Pauli et al. [43] also reported observing

Fig. 12. AFM topography image of YNP binder and its Sasobit blends. (a) YNP binder; (b) 1% Sasobit/YNP; (c) and (d) 3% Sasobit/YNP. (a)–(c) are scanned at room temperature.(d) is 50 �C image.

424 Q. Qin et al. / Fuel 115 (2014) 416–425

gel-like network structuring rather than bee structuring for paraf-fin wax doped asphalts at concentrations at and above 3% similarto the results reported here.

Regarding morphology and rheological results of 3% Sasobitmodified WMA, we remark that the network structure developsnot only on the sample surface, but also in the bulk region of theblends. On the one hand, our AFM images in Figs. 11 and 12 suggestthe surface microstructure belongs to much larger amount of Sas-obit component than 3%, implying the Sasobit structuring prefer-entially forms at the asphalt surface by molecules diffusing andtransporting to the film surface. On the other hand, our rheologicalresults indicate that the network structure does happen in thebulk, providing additional insight on Sasobit microstructures.

4. Conclusions

We examined the thermal, morphological and linear viscoelas-tic properties of Sasobit modified Warm Mix Asphalts (WMA) withSasobit weight concentration of no greater than 3%. Four asphaltbinders with different types and grades were investigated. ThoseSasobit/binder blends exhibit both depressed melting

temperatures and reduced heat of fusion, suggesting a smallercrystal size of Sasobit is favorable in the blends. The structurecharacterization of Sasobit reveals its linear aliphatic long chain(n-paraffin) characteristics without apparent chain branching (oriso-paraffin) structures.

The time–temperature superposition (tTS) fails for 3% Sasobitblends at temperatures above 30 �C, which is below their onsetmelting points. Therefore, the melting of Sasobit crystals doesnot simply account for the deviation from tTS. The results suggestit is the formation of network or semi-solid like structure that isresponsible for the breakdown of time–temperature superposition.

According to the creep stiffness and creep rate calculated fromlow temperature 4-mm DSR data, the ‘‘limiting low temperature’’of 3% Sasobit modified MB, MN1-3 and YNP is increased by about2 �C. A slightly larger increase in the ‘‘limiting low temperature’’of 3.5 �C is observed for the MN1-4 Sasobit blend. Therefore, Sas-obit at concentration levels of 3% or less is not expected to exertundue adverse effect on binder low temperature properties.

At high temperatures, Sasobit reinforces asphalt binders withthe limiting high temperature expanded by 5 �C, 9 �C, 13 �C and16 �C for 3% Sasobit modified MN1-4, MB, MN1-3 and YNP binders

Q. Qin et al. / Fuel 115 (2014) 416–425 425

respectively. More importantly, the stiffening or reinforcing effectis closely correlated with the microstructure of Sasobit blends. Lin-ear viscoelasticity analysis at 50 �C demonstrates the gel-like orpseudo-solid behavior of 3% Sasobit modified WMA, as evidencedby a very weak frequency dependence of G0 at low frequencies withG0 exceeding G00 as well as by the presence of nearly diverging vis-cosity at finite modulus.

Consistent with the rheological results, the AFM images depictan inter-connected network structure for 3% Sasobit modifiedWMA, which proves to be dependent on the Sasobit concentrationrather than asphalt sources. For 1% Sasobit blends, instead of form-ing the familiar isolated ‘‘bee structure’’ of wax, a dendritic micro-structure is observed, which is thought to be partially due to thehigh molecular weight of Sasobit as well as its relatively larger con-centration compared with natural wax in asphalts. Morphologyand rheological results suggest that a network structure existsnot only on the surface of the sample, but also inside the bulk.

Disclaimer

This paper is disseminated under the sponsorship of the Depart-ment of Transportation in the interest of information exchange.The United States Government assumes no liability for its contentsor use thereof. The contents do not necessarily reflect the officialviews of the policy of the United States Department of Transporta-tion. Mention of specific brand names of equipment does not implyendorsement by the United States Department of Transportation orWestern Research Institute.

Acknowledgements

The authors would like to thank Alec O. Cookman, Pamela J.Coles, Bruce E. Thomas, Gerald E. Forney for performing the testsand generating raw data. The authors also gratefully acknowledgethe Federal Highway Administration, US Department of Transpor-tation, for financial support of this project under Contract No.DTFH61-07-D-00005.

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