Road Materials and Pavements Design. Volume X – No X/2007, pages 1 to n

Glass Transition and Phase Stability in Asphalt Binders Pavel Kriz* — Jiri Stastna* — Ludo Zanzotto* *Bituminous Materials Chair Schulich School of Engineering University of Calgary 2500 University Dr. NW Calgary, AB, T2N 1N4 Canada pkriz@ucalgary.ca stastna@ucalgary.ca zanzotto@ucalgary.ca

ABSTRACT. Major aspects of the glass transition of asphalt binders are described and an extensive literature review of the phenomenon and its relation to chemical composition is presented.

The glass transition of asphalt binders was studied by modulated differential scanning calorimetry and also via dynamic mechanical analysis. A certain analogy between the glass transition of amorphous polymers and asphalts is suggested. The overall transition was found to be very broad on the temperature scale.

The effects of evaporation of light-end components and oxidation on asphalt phase stability and glass transition were studied. It was suggested that phase incompatibility may exist in asphalts; however, the phase separation is observable after long-term isothermal conditioning at a temperature within the glass transition range.

Based on the presented results, it is suggested that phase incompatibility develops if there is a discontinuity in the molecular distribution. Such discontinuity may be present in some neat binders as well as in severely oxidized or aged asphalt binders.

KEYWORDS: glass transition, modulated differential scanning calorimetry, asphalt binder, molecular mobility, physical aging, oxidative aging.

2 Road Materials and Pavements Design. Volume X – No X/2007

1. Introduction

The glass transition is, perhaps, the most important factor that determines the viscoelastic properties of amorphous material (Williams et al., 1955, Ngai, 2004). It is a reversible change in an amorphous domain from a viscous or rubbery state to a hard and relatively brittle glassy state, and vice versa. The glass transition occurs when the characteristic time of molecular motions responsible for structural rearrangements becomes longer than the timescale of the experiment. The timescale for structural relaxation increases rapidly with decreasing temperature (Moynihan, 1994). Therefore, the glass transition is usually considered as a predominantly rate and non-equilibrium phenomenon.

The transition to a glassy state is accompanied by a sudden change in the mechanical, optical and thermodynamic properties of the material. The increase in viscosity is enormous (up to 1013 Pa.s). The material becomes glossy in appearance and extremely brittle and rigid. The thermodynamic change during the glass transition upon cooling is recognized as the stepwise drop in heat capacity and loss in free volume (Ngai, 2004). Note, that the free volume is not a real volume measurable in cm3/g, but an operational quantity dealing with the molecular mobility (Struik, 1991).

The glass transition temperature (Tg) is the temperature arbitrarily chosen to represent the temperature range over which the glass transition takes place: it is usually assigned to the temperature where half of the sample is already vitrified or devitrified (Wunderlich, 1994).

In the current study, the glass transition of asphalt is thoroughly described and investigated by two techniques – modulated differential scanning calorimetry (MDSC) and dynamic mechanical analysis (DMA). The similarities between the glass transition of amorphous polymer and asphalt are suggested.

2. Background

The glass transition is a function of molecular mobility. Upon cooling, molecular mobility decreases. Molecules are considered “glassy”, if their molecular motion has appeared to cease with respect to observation time. Different molecules have different degrees of motion at particular temperatures. The molecular motion is a function of molecular weight and structure, as well as of inter- and intra-molecular interactions. Before the glass transition is studied, the asphalt composition and inter-molecular relations should be understood.

Glass Transition in Asphalt Binders 3

2.1. Molecular Continuity

Asphalt consists of tens of thousands different chemical species (Speight, 1999). The whole scale of molecules ranges from non-polar fully saturated linear alkanes to highly polar polycyclic porphyrins or hetero-hydrocarbons. Each molecular functional group is essential to the phase stability of the asphalt system. It is, therefore, necessary to view the material as a continuum of molecules with a gradual transition in polarity, molecular weight and functionality.

It is a generally accepted fact that the molecular functional groups overlap and, thus, are impossible to separate (Andersen et al., 2001). It has been reported (Carlson et al., 1958) that there is no sharp transition between asphaltenes and aromates. It has also been shown that part of the asphaltene fraction (based on the solubility parameter) may cosolubilize the rest of the fraction and, thus, behave as a resin (Heithaus, 1962, Andersen, 1995). There is no sudden transition in the molecular structural properties between resins and asphaltenes (Groenzin et al., 1999). Due to the complexity of the material and the interactions between high molecular weight molecules, the commonly used methods for fraction separation of asphalt cannot provide well-defined chemical fractions (Selucky et al., 1981). If part of the material is removed from the system during the separation, the molecular distribution and interaction are affected. The solubility of a substance is not a function of its general hydrocarbon skeleton and its chemical functionality alone, but also depends on interactions with other substances that act as co-solvents. Whether a given compound appears, for instance, as a resin or an asphaltene depends upon the presence or absence of other substances (Farcasiu, 1977).

Separation into fractions has not proved to be a reliable predictor of asphalt performance on test roads (Goodrich et al., 1986). Indeed, asphalt refining is based on another type of fraction separation – distillation (Corbett, 1984). For the sake of simplicity, we may still use the terms “asphaltenes”, “resins”, and so on; but instead of relating these terms to solubility, we relate them to their functionality in the system. The asphaltenes represent the polyaromatic fraction, which tends to be insoluble in the continuous phase at certain conditions and needs to be stabilized (cosolubilized). This fraction consists of different molecules at different conditions (temperature, pressure, change in the continuous phase composition). Resins are unique aromates that are not qualitatively different from asphaltenes, yet restrict the asphaltenes from further association and, due to complex interaction, keep them in a single phase providing the molecular transition between the polar and non-polar ends. And finally, the continuous (or oil) phase, with composition gradually ranging from semi-polar resins to non-polar paraffins, is present.

4 Road Materials and Pavements Design. Volume X – No X/2007

2.2. Asphalt Structure

The concept of asphalt being a colloidal system was proposed by Nellensteyn (Nellensteyn, 1938). A colloidal system is an intermediate between a homogenous solution and a heterogenous system, with the particle size ranging from 1 nanometer to 1 micrometer (Hiemenz et al., 1997). There is strong experimental evidence that the biggest single molecules in asphalt do not exceed the molecular weight of 1000 g/mol (McKay et al., 1978, Groenzin et al., 2001), which corresponds to molecular diameters in the range of 1-2 nm (Katz et al., 1945, Groenzin et al., 1999). The most polar molecules usually associate into bigger particles. The size of these particles rarely exceeds 6 nm (van der Hart et al., 1990); however, if the system is destabilized, they may grow bigger and eventually separate from the system. The presence of molecules and aggregates bigger than 1 nm disqualify asphalt from being a homogenous solution. The upper limit of a colloidal system (1 μm) is generally met in asphalts, even though recent atomic microscopy experiments revealed different domains of a size exceeding 1μm (Masson et al., 2006a).

The molecular interactions (especially between asphaltenes and resins) and the whole structure of the system are still topics for discussion. The lyophobic models, where the insoluble asphaltenes are peptized by the resins that provide a protective sheet, were actually motivated by early observations of the solubilizing power of resins (Mack, 1932, Nellensteyn, 1938, Eilers, 1949, Koots et al., 1975). This is inconsistent with the fact that the asphaltene precipitates can be re-dissolved in a good solvent, even in the absence of resins (Andersen et al., 1990, Andersen et al., 1991). Also, the asphaltene molecules are not many times larger than the size of the resin molecules, as they would logically have to be in order for many resin molecules to be able to fit around them and form the protective layer, similar to the water-surfactant-oil emulsion. The asphaltene and resin molecules are, in fact, not much different in size (Storm et al., 1995, Porte et al., 2003). Whether a particular molecule behaves as an asphaltene or resin always depends on actual conditions (temperature, pressure, surrounding molecules, etc.).

2.2.1. The Molecular Forces

The interaction between the asphaltene molecules and/or resin molecules is crucial in order to keep the colloidal character of the material and prevent further aggregation towards bigger agglomerates, which would eventually lead to instability and potential phase separation. There is no reason to assume that the inter-molecular forces acting between asphaltenes, resins and the oil phase should be in any way different from those existing between other well-known organic molecules containing the same atomic groups. Inter- and intra-molecular forces affecting the phase stability and molecular structure are charge transfer forces, electrostatic interactions, van der Waals interaction, exchange-repulsion interaction, forces arising from induced dipole, and hydrogen bonding (Murgich, 2002).

Glass Transition in Asphalt Binders 5

2.2.2. Thermodynamics of the Micelle Formation and Further Association

The formation and stability of molecular aggregates, such as micelles or disperse solids, present in asphalt are determined by the changes in the total free energy of the system. The binding of molecules is always an unfavorable entropic process (reduction in degrees of freedom) (Vinter, 1996). This implies that the change in enthalpy must be negative (Murgich, 2002), since only processes with negative change in free energy are spontaneous. This was proven experimentally, and the number of interaction sites on an asphaltene molecule was estimated to be between 1 and 2 and the heat of association in the range of 2-7 kJ/mol (Merino-Garcia et al., 2004a, Merino-Garcia et al., 2004b). The association of heavy aromatics into bigger entities, or even more pronounced structures, is an exothermic process. The association becomes more pronounced with decreasing temperature. At low temperatures, however, the increasing viscosity of the continuous phase impedes the molecular diffusion and, therefore, the association.

The aggregates or micelles may further associate into more complex structures. In the case of a sufficiently high dissolving power of the medium, phase separation does not set in; and, with the accumulation of asphaltenes, internal structures (networks) may form (Rogacheva et al., 1980). The very complex distribution of the molecular aggregates (Sheu et al., 1992) suggests that asphalt cannot be described simply as a pure sol or gel system (Saal et al., 1940, Eilers, 1949) formed by solid asphaltene particles dispersed by resins, or as a simple and uniform micellar system similar to those found in aqueous solutions of pure surfactants. Diversity of asphalt molecules (in size, shape, polarity, etc.) disallows the organized structure (crystal) to be built, and most of the molecules conform in an amorphous organization (Murgich et al., 2001).

2.3. Parameters Affecting the Glass Transition

2.3.1. Molecular Weight

The molecular weight of the glass forming molecules is an important factor in glass transition. The effect is not straightforward, and it is also dependent on the molecular structure. For linear molecules, the chain ends exhibit greater mobility over the repeat inner units, because they are bonded only on the one side. The shorter molecules have more mobile chain ends per volume unit than longer molecules and, thus, exhibit greater molecular mobility. Molecules with higher mobility (shorter chains) become motionless at a lower temperature than those with lower mobility (longer chains). This means that Tg increases with the increasing molecular weight (Fox et al., 1955). Contrarily, the limited data for small polymer rings surprisingly predicted the increase in Tg with decreasing molecular weight (Clarson et al., 1985). The free volume theory (Fox et al., 1955) is inconsistent with these results; however, one cannot omit the simple explanation that the free volume

6 Road Materials and Pavements Design. Volume X – No X/2007

decreases with decreasing ring size (molecular weight), as the rings become tighter and tighter.

2.3.2. Functional and Structural Groups

The effect of various functional and structural groups on Tg may be very complex and difficult to estimate. In polymers, for instance, introduction of para-phenyl rings into the linear chain significantly increases Tg, due to fact that para-phenyl restricts the chain backbone rotation. On the other hand, introduction of a methylene group or oxygen (ether bond, C–O–C) into the backbone lowers Tg, due to an increase in flexibility of the chain. Introduction of side alkyl chains also significantly decreases Tg. It has been shown (Weyland et al., 1970, Krevelen et al., 1976) that Tg can be successfully predicted by summing the partial Tg (Tgi) for particular functional groups. Tgi is dependent on molecular weight and structure. Generally, saturated and linear molecules possess higher molecular mobility due to the rotational degree of freedom of the single bond. Therefore, saturated molecules vitrify at much lower temperatures than unsaturated molecules. Rings and multiple bonds, as well as heteroatoms (especially carbonyl groups), introduce a certain rigidity into the molecule and reduce the molecular mobility. For instance, Tgi of the carbonyl group is 964 K, while Tgi of the ether group is 232 K. The presence of sulfur in the chain (C–S–C, Tgi=234 K) may lower the overall Tg, while Tgi of the SO2 group (905 K) may do the opposite. Carbonyl and sulfonyl groups are often considered as products of oxidative aging of asphalts, and their contribution to Tg may be important in oxidized asphalts. The prediction of Tg using the method developed by Krevelen may be difficult for asphalt, due to the vast number of functional groups. It has been shown, however, that reasonable estimates of Tg can be obtained (Huynh et al., 1978).

2.3.3. Cross-linking and Molecular Interactions

It is known in polymer science that introduction of cross-links into a polymer strongly affects the local segmental relaxation and, hence, the glass transition. The cross-links reduce the configurational degree of freedom, thereby increasing Tg. The increase in Tg can be understood by a possible decrease in the molecular motion, because the van der Waals interactions are replaced by shorter and firmer covalent bonds (Chang, 1992). In asphalt, no formation of new inter-molecular covalent bonds is expected at ambient conditions. Major interactions and associations between asphalt molecules are due to weak van der Waals forces, hydrogen bonds and other forces. The weak inter-molecular interactions in asphalt may reduce the configurational degree of freedom, at least to some extent, as the covalent cross-links do, possibly contributing to an increase in Tg. One should not omit the fact that the association energies between the most polar molecules are in the order of 5 kJ/mol (Merino-Garcia et al., 2004a), while single covalent bonds (cross-links in polymers) are much stronger (approximately in the order of 180-450 kJ/mol (Israelachvili, 1985)).

Glass Transition in Asphalt Binders 7

2.3.4. Crystallinity

Some asphalts may contain a certain amount of crystalline phase (Claudy et al., 1992); therefore, the effect of crystallinity on Tg should be also considered. Tg of amorphous phase may increase as well as decrease with the degree of crystallinity, depending on the relative density of the crystalline and amorphous phases. In most cases, the ordered crystalline phase possesses higher density; and, the molecular chains of the amorphous phase are entrapped in the crystalline lattice. Their mobility is reduced, and Tg is increased (Flocke, 1962, Bair, 1994). The effect of crystalline phase on the glass transition of asphalts was found to be significant (Kriz et al., 2007a).

2.3.5. Dilution

The solvent power and Tg of the oily phase in asphalt are important factors in the overall Tg. In polymer science, the effect of diluents to lower or increase Tg is well known. Dilution with a solvent that has a lower Tg than the original material decreases Tg of the system. Essentially, solvent with a higher Tg than the original material increases the resulting Tg (Plazek et al., 1991). Furthermore, there are solutions in which the mobility of the solvent is increased by the presence of a polymer whose undiluted Tg is higher than that of the solvent. Free volume theory cannot explain these unpredictable effects, and the additional concept of inter-molecular coupling was introduced (Santangelo et al., 1994).

2.3.6. Other Effects

The effect of pressure on Tg is typically in the order of an increase by 20°C per 100 MPa of pressure (Ngai, 2004). Since the pressure applied on pavement by traffic is in the order of several hundreds of kPa or a few Mpa, the increase of Tg due to pressure would be in the order of several tenths °C.

It has also been shown that, for thin films of polystyrene and other polymers on aggregate (silicone wafers), Tg of a particular polymer is strongly dependent on the film thickness (Fryer et al., 2000). However, this effect was observable for film thicknesses of 50 nm and thinner. Since the average binder film thickness in asphalt mix is in the order of μm (Frolov et al., 1983), the effect of the film thickness on Tg is probably negligible.

3. Experiment and Methods

3.1. Instruments

A TA Instruments Q100 differential scanning calorimeter (DSC), equipped with the modulated temperature setup and liquid nitrogen cooling system, was used in

8 Road Materials and Pavements Design. Volume X – No X/2007

this study. Ultra pure helium (99.999 percent) was used to purge the experimental cell. The rate was 50 mL/min. Standard hermetic aluminum pans were used for all experiments. The sample mass was in the range of 7-10 mg. Sample pans were sealed under nitrogen. The thermal history of the sample, if not stated otherwise, was deleted prior the experiment. The procedure was as follows. The sample was heated up to 150°C and underwent isotherm for 15 minutes. The sample was then quenched to −100°C at a rate of 10°C/minute. The MDSC setup was developed and evaluated in our other papers (Kriz et al., 2007b, Kriz et al., 2007c). The modulation amplitude was 2°C; the modulation period, 60 seconds; and, the linear heating rate, 2°C/minute. The typical range was from −100 to 100 or 150°C. The glass transition temperature was assigned to a temperature where the reversible heat capacity reached half of the overall change.

Table 1. Basic properties of asphalt binders used in this study

Asphalt A Asphalt B AAC-11 AAV1

Crude oil source Cold Lake Ural Russia Red-water Alaska

North Slope

Performance Grade (PG) PG 58-282 PG 52-282 PG 58-16 PG 52-22

Penetration 25°C [dmm] 1623 1863 133 121

gT (MDSC) [°C] −25.82 −18.90 −23.59 −20.78

Wax (% wt) 0.704 4.624 5.06 3.13

1data obtained from the SHRP Materials Reference Library (Jones, 1993), except for the Tg determination. 2AASHTO (American Association of State Highway and Transportation Officials) MP320 (Asphalt Institute, 1994). 2,3data acquired from Bituminous Materials Chair at the University of Calgary. 4UOP (Universal Oil Products) 46-85 method (UOP Inc., 1985).

A Rheometric Scientific strain-controlled rheometer (ARES) was used for the dynamic mechanical analysis. The linear viscoelastic (LVE) region was determined as a linear region in the G′ , G ′′ versus testing strain (Figure not presented). The dynamic frequency sweeps were run (0.1 to 100 rad/s) at a constant temperature and a constant strain within the LVE region.

A TA Instruments Q500 thermogravimetric analyzer (TGA) was used. About 40 mg of binder was placed on standard platinum pan prior to the experiment. Each sample was first melted, and small droplets were prepared. These droplets were

Glass Transition in Asphalt Binders 9

stored in a freezer to minimize the evaporation and to ensure the same number of re-heatings for each sample. The purging gas was either nitrogen (non-oxidative evaporation) or air (oxidation) at a flow rate of 60 mL/min. The heating rate was 20°C/min (from ambient to test temperature), and test temperatures varied between 60 and 200°C, depending on the experiment. The sample was held at the test temperature for 50-1440 minutes.

3.2. Asphalt Samples

Four different asphalts were used in this study. Asphalt A was an asphalt from heavy Alberta crude oil (Cold Lake) of penetration grade 150/200, supplied by Husky Energy Inc. It represented a high-quality binder for low-temperature application. Asphalt B was an asphalt from Ural crude oil of penetration grade 150/200, supplied by SLOVNAFT Plc., Slovakia. It represented an asphalt binder produced from the crude oil typically processed in Central Europe. The other two binders were Asphalts AAC-1 (Redwater) and AAV (Alaska North Slope) from the original Strategic Highway Research Program (SHRP) Material Reference Library. These three binders had been stored in bulk (15kg) at room temperature for 15 years.

4.Results and Discussion

4.1. Dependence of the Glass Transition on Observation Time

According to the definition of a glassy state, the domain is considered glassy if the molecular motion appears as nonexistent within the observation time. This means that the glass transition is a kinetic process, and it is dependent on the particular experiment’s observation time.

Figure 1 presents a master curve constructed from several isothermal dynamic data measured for Asphalt A. The time-temperature superposition principle (tTs) was applied (Ferry, 1961), and horizontal shift factors ( Ta ) were determined. Shift factors were fitted with the Williams-Landel-Ferry equation (WLF):

( ) ( )rrT TTcTTca −+−−= 21 /log [1]

where, c1 and c2 are the empirical fitting parameters, T is the temperature, and Tr is an arbitrarily selected reference temperature. The frequency where the loss modulus ( G ′′ ) attains its maximum value is usually assigned as the glass transition frequency (Wada et al., 1959). With the use of tTs, the frequency can be converted to temperature:

10 Road Materials and Pavements Design. Volume X – No X/2007

ωω Ta= [2]

where, ω is the reduced frequency, and ω is the testing frequency. The combination of Equations 1 and 2 yields:

( )( )g

grg c

cTT

ωω

ωω

/log

/log

2

1

++= [3]

where, gω is the frequency at the maximum of G ′′ , and Tg is the glass transition temperature. Using Equation 3, the glass transition temperature of the material can be calculated as a function of testing frequency (Kriz et al., 2008). Results for Asphalt A are presented in Figure 2.

Figure 1. Asphalt A – master curve, time-temperature superposition, test temperatures from −30 to 80°C, G ′ (○), G ′′ (●), δtan (−). Constant strains within the linear viscoelastic region (LVE) were used. Geometry varied according to the viscosity of the specimen: 50mm cone and plate (T>50°C), 25mm cone and plate (T~20-50°C), 10mm plate-plate (T~0-20°C) and rectangular torsion bar (T<0°C).

It is apparent from Figure 2 that the glass transition temperature is strongly dependent on the testing frequency. A single oscillation (period) refers to the observation time. The frequency of 108 rad/s refers to the observation time in the order of nanoseconds. The time allowed for the response to deformation is extremely short at high frequencies; therefore, the response of relatively soft asphalt (at temperatures above 0°C) is glassy. On the other hand, a low testing frequency (e.g. 10-7

rad/s) corresponds to extremely long observation times (2 years). Therefore, a specimen has sufficient time to respond to deformation, and the asphalt appears as non-glassy, even at very low temperatures – below −50°C.

Glass Transition in Asphalt Binders 11

Figure 2. Dependence of glass transition temperature (Tg) on testing frequency (ω) in Asphalt A.

Figure 3. Effect of the heating rate on Tg (DSC experiment, Asphalt A).

A similar effect can be observed in a calorimetric experiment. In this case, we

can vary the heating or cooling rate. Examples of the effect of the heating rate on the glass transition of Asphalt A is presented in Figures 3 and 4.

The data indicates that the glass transition temperature is strongly dependent on the experimental conditions. Therefore, the glass transition temperature is valid only for a particular set of conditions. Relating viscoelastic or even performance parameters of the asphalt to a single glass transition temperature without further

-60

-50

-40

-30

-20

-10

0

10

-10 -5 0 5 10log ω [rad/s]

Tg [°

C]

12 Road Materials and Pavements Design. Volume X – No X/2007

specification of observation time, transition range, thermal history, etc. is impractical.

Figure 4. Tg as a function of the heating rate in DSC experiment – Asphalt A.

4.2. Phase Compatibility

The major difference between the modulated differential scanning calorimetry (MDSC) and the standard differential scanning calorimetry (DSC) is the thermal program. In MDSC, the conventional linear heating program is modulated by superimposing a sine wave (or other periodic waveform) of small amplitude on the linear temperature ramp. Portions of each cycle then involve heating, while other portions involve cooling. The overall trend, however, remains a linear change in average temperature with time. The resultant heat flow signal is analyzed to separate the response to the perturbation from the response to the underlying heating program. Thus, the equilibrium (e.g. change of state) and kinetic (e.g. glass transition) thermal events can be separated into two independent signals (Baur et al., 1998, Judovits et al., 1998). Those two signals are the irreversible heat flow (change of state) and the reversible heat flow (glass transition). The use of MDSC in asphalt research was pioneered by Memom and Chollar (Memon et al., 1997) and by J.-F. Masson (Masson et al., 2001, Masson et al., 2002, Masson et al., 2005a, Masson et al., 2005b, Masson et al., 2006b).

The reversible signal can be used to describe the kinetic processes associated with the amorphous phase, i.e. the glass transition. For polymers, it has already been shown that the derivative of reversible heat capacity, with respect to temperature signal, is very useful in the determination of the compatibility of the amorphous phase (Song et al., 1995, Song et al., 1998a, Song et al., 1998b). The signal itself represents the rate at which the molecules pass through the glass transition. If the amorphous phase is fully compatible and uniform, the shape of the signal should

y = 2.24Ln(x) - 33.48R2 = 0.98

-36

-32

-28

-24

-201 10 100

Heating rate [°C/min]

Tg [°

C]

Glass Transition in Asphalt Binders 13

correspond to a Gaussian type distribution with a single maximum (Song et al., 1998b). For incompatible blends of polymers, however, multiple peaks were observed suggesting that there were actually two or more separate amorphous domains, each of which passes through its own glass transition at different temperatures (Song et al., 1998b). Therefore, the signal can be used to study the compatibility of the amorphous domain and also evaluate the effect of various processes on the phase stability.

Figure 5. Compatible amorphous phase in Asphalt A (c) and incompatible amorphous phase in Asphalt AAV (a) and Asphalt AAC-1 (b). No separate glass transition outside the main glass transition was observed in these asphalts upon heating up to 150°C. Curves are vertically shifted.

Examples of compatible and incompatible amorphous phases are presented in

Figure 5. It has been shown that the multiphase system in asphalt also evolves with time in isothermal conditions (Masson et al., 2001, Masson et al., 2002). The compatibility and evolution of amorphous domain were found to be strongly dependent on the presence of crystalline phase (Kriz et al., 2007a). In the case of semi-crystalline asphalt, the multiphase system has to be considered. At least three separate components exist in asphalt based on its physical state: crystalline, glassy amorphous and non-glassy amorphous phases. Part of the glassy amorphous domain may be considered as rigid amorphous phase (RAPh), which has been found in semi-crystalline polymers and is also indicated in asphalt (Kriz et al., 2008). The physical state of a particular molecule, i.e. crystalline, glassy/non-glassy amorphous, depends on molecular weight and structure, the rest of the molecules in the sample (interaction, solubility, spatial interference), thermodynamic parameters (temperature, pressure) and time/thermal history. Each domain consists of molecules in the same state. The crystallization and vitrification of asphalt molecules define the physical state of the domains present in asphalt at low temperatures, i.e. at temperatures within the glass transition and/or melting range. These processes are fully reversible upon heating (above the temperature range of glass transition and/or

14 Road Materials and Pavements Design. Volume X – No X/2007

melting). The following section discusses the effect of irreversible change in composition – oxidation – on the phase compatibility and glass transition.

4.3. Effect of Oxidative Aging on Phase Compatibility and Glass Transition

The effect of oxidative aging on asphalt phase compatibility and the glass transition temperature was studied. Asphalt A, which has a negligible crystalline domain, was chosen for this experiment. The processes associated with the crystalline phase may not only overlap with the glass transition on the calorimetric signal, but also contribute to the incompatibility of the amorphous phase (RAPh) (Kriz et al., 2007a). The absence of the crystalline phase in Asphalt A permitted the evaluation of the effect of oxidation on phase stability and glass transition. The reversible calorimetric signal of neat Asphalt A (Figure 5) shows no phase separation and a uniform amorphous phase. The peak shape suggests a Gaussian molecular distribution. Each molecule contributes to the glass transition at a particular temperature based on the molecular weight, structure and inter-molecular interaction. An irreversible calorimetric signal (not presented) showed a very small crystalline domain (heat of melting in the order of 0.5 J/g), which was the lowest value we have ever observed in an asphalt binder. As more than 20 different binders were subjected to the MDSC experiment in our laboratory, a purely amorphous binder may be considered as a rarity.

4.3.1. Effect of Evaporation

In this section, the effect of evaporation on the phase stability and Tg is discussed. Evaporation plays a minor role in the overall aging process of pavement (Kemp et al., 1981). Our objective was to estimate the effect of evaporation and separate it from simultaneous oxidation. The data presented here may also be related to the refining process setup – how the low-temperature properties of the product are dependent on the degree of distillation.

Asphalt A was subjected to a thermogravimetric analyzer (TGA) experiment under nitrogen atmosphere. The two sets of samples were prepared in the TGA. First, the same soaking time was maintained (24 hours), and the temperatures, between 60 and 200°C, were held constant. Second, the same soaking temperature was maintained (200°C), and the soaking times were changed. Tg was determined for both sets of samples: the results indicate that there was no qualitative difference between the samples. It can be concluded, therefore, that the time and temperature can be interchanged, within reasonable limits, during such an experiment (Figure 6).

Typical straight run asphalt is a residuum after crude oil distillation, which is an equilibrium separation process; and, complete separation of the individual components, based on their normal boiling point, is impossible (Smith et al., 2005). Therefore, asphalt contains molecules with lower boiling points than the equilibrium temperature (Speight, 1999). The boiling point depends not only on the carbon

Glass Transition in Asphalt Binders 15

number, but also on the molecular structure (Wessel et al., 1995). The heavy oil distilled from asphalt consists predominantly of saturates (~50%) and naphthene-aromatics (~40%) (Noel et al., 1970). Single bonds in saturated molecules possess a rotational degree of freedom; and, a whole chain is, therefore, more flexible than in molecules with multiple bonds or rings. Higher molecular mobility of saturated molecules results in a lower Tg (Ngai, 2004), and removal of such a component essentially leads to an increase in overall Tg.

Figure 6. Effect of Evaporation and Oxidation on Tg in Asphalt A: evaporation at different temperatures (60 to 200°C) under nitrogen for 24 hours (●); evaporation at 200°C, time varied between 50 min and 24 h (▲); oxidation in air at 200°C (◊); oxidation in air at 275°C (□).

Figure 7. Effect of Evaporation on Glass Transition in Asphalt A. Original asphalt (---) and after 5 %wt. (·−·) and 18 %wt. (−) was evaporated.

y (●) = 0.66x + 247.02R2 = 0.99

y (▲) = 0.66x + 246.75R2 = 0.98

y (◊) = 0.50x + 246.93R2 = 0.80

y (□) = 0.65x + 247.92R2 = 0.90

246

248

250

252

254

256

258

260

262

0 5 10 15 20Weight loss [%]

T g [K

]

16 Road Materials and Pavements Design. Volume X – No X/2007

Figure 8. Effect of Oxidation on Glass Transition in Asphalt A .Original asphalt (---) and after 200 min at 200°C (·−·) and after 50 min at 275°C (−). The results obtained for Asphalt A (Figures 6 and 7) were consistent with the

literature, where Tg was already found to increase linearly with the decrease in penetration (du Bois, 1966) or the logarithm of penetration (Schmidt et al., 1965). It has also been found by several researchers that the overall Tg linearly increases with the amount of mass fraction of asphaltenes, the heaviest asphalt fraction (Wada et al., 1960, Giavarini, 1984). The simple linear behavior of Tg upon evaporation (Figure 6) suggests that the raise in overall Tg was directed predominantly by a loss of the “low Tgi” component. No separate glass transition outside the main transition was observed in the neat Asphalt A (Figure 5). The solubility factor plays an important role during the glass transition. The heaviest and most polar molecules vitrify at much lower temperature than they would normally do as a separate entity, due to being mobilized and prohibited to pack into a glass by molecules of a much lower Tg.

Figure 9 presents the reversible calorimetric signal of Asphalt A and its asphaltenes and maltenes. Asphaltenes were prepared using the American Society for Testing and Materials’ ASTM D-3279-97 method. The glass transition of asphaltenes is extremely broad, due to higher molecular weight and variety in molecular types (see section 4.3.4.). The signal change above 200°C was caused by the deformation of the pan bottom as the internal pressure increased upon heating. This was confirmed visually. The asphaltenes and maltenes had glass transition temperatures of 68 and −30.8°C, respectively. The weight fraction of the asphaltenes was 12.9 %wt. The glass transition temperature of Asphalt A was calculated from the glass transition temperatures of the asphaltenes and maltenes, using a simple mixing rule (Krevelen et al., 1976). The value of −18.1°C was obtained (original Asphalt A has Tg of −25.7°C). The difference was expected, as this approach omits molecular interactions and solubility effects that were present in the original asphalt.

Glass Transition in Asphalt Binders 17

Figure 9. Glass transition as observed in (a) Asphalt A, in separated (b) asphaltenes (12.9 %wt.) and (c) maltenes (87.1 %wt.).

4.3.2.Effect of Oxidation

Asphalt A was oxidized in air at 200 and 275°C. The glass transition temperature was determined and plotted versus the weight loss. The results are presented in Figures 6 and 8. An interesting fact was observed: there was only a negligible difference between the glass transition temperatures of the evaporated (under nitrogen) and oxidized samples. The glass transition still increased linearly with the weight loss and the slope, and a y-intercept of the linear fit was not affected by oxidation. The curves were very similar with a single Gaussian peak in both the oxidized and evaporated binders, as shown in Figures 7 and 8.

It was expected that oxidation would affect the phase stability of the system and that a phase separation would set in. This was, however, not observed in samples with no thermal history. Thermal history turned out to be critical in this phase stability issue (see section 4.4.).

4.3.3. No Phase Separation in Evaporated/Oxidized Asphalt?

Is the glass transition of evaporated/oxidized asphalt affected only by the loss of low Tg components or is there any other parameter involved? Further association/flocculation of polar molecules, for instance, may be one of the additional effects that may occur upon loss of the light components. Essentially, such association would lead to significantly bigger aggregates of polar molecules, as proposed in earlier studies (Saal et al., 1940, Eilers, 1949, Rogacheva et al., 1980). Molecular motion inside such an aggregate would be restricted, and the aggregated molecules cannot be mobilized/solvated by other components in the asphalt. Association of polar molecules would result in incompatibility with the rest of the amorphous phase. This newly developed amorphous domain should be identified by one of the two following features on the calorimetric signal. First, the separate

18 Road Materials and Pavements Design. Volume X – No X/2007

amorphous domain would be observable only if the associated molecules that form a new phase still possess some degree of the molecular mobility and, therefore, the ability to exhibit glass transition. The new phase should be observed as a new peak on the calorimetric signal. This was not observed (Figures 7 and 8). Second, the molecules forming the new phase have lost molecular motion (associated molecules may have significantly restricted mobility since they are closely packed and possess a very low degree of freedom) and now behave like an “immobilized” solid phase. This phase would have extremely low molecular mobility; and, its glass transition, if it exists, would be extremely weak and perhaps undetectable in the calorimetric experiment. In such a case, the removal of the high molecular weight molecules from the original amorphous phase would lower the glass transition of the remaining amorphous phase. This is in conflict with the peak broadening and increase in Tg upon evaporation/oxidation (Figures 7 and 8).

4.3.4. Transition Broadening

The glass transition was broadened with a higher degree of evaporation/oxidation (Figures 7 and 8). The broadening of the glass transition is described in polymer literature. Wunderlich (Wunderlich, 1994) reported that the solutions involving macromolecules always have a symmetrically broadened glass transition region, which is caused by the inability of the molecules to randomize the structure of their molecular backbone on a nanometer scale.

The concentration of polar molecules (with a high partial Tg) per volume increases as the continuous phase is being removed. With the absence of smaller mobile molecules, the packing of large polar molecules into the glass upon cooling is complicated and slow. On heating, devitrification of the glass is directed by molecules with higher Tg, which may postpone the devitrification of the low Tg molecules through spatial interference. The spatial interference becomes more pronounced and the glass transition broader the higher the average molecular weight of the system.

Other effects may also contribute. Peak broadening accompanied with the relatively small discontinuity in heat capacity has been observed in highly cross-linked polymeric systems (Chang, 1992). It has also been shown that the breadth of the glass transition increases as polymers are mixed into miscible poly-blends or covalently linked together into homogeneous copolymers (Bates et al., 1984). It is improbable that new covalent bonds (bonding energy in the range of 180-450 kJ/mol) in between molecules (cross-links) are being created during evaporation or oxidation in asphalt. Rather, very weak non-covalent “cross-links”, most probably based on the van der Waals forces (~1 kJ/mol) or H-bonds (5-10 kJ/mol) (Israelachvili, 1985), exist between the molecules. These forces are not strong enough to bond the aggregates/molecules together into bigger agglomerates permanently, yet strong enough to spatially coordinate them. The number of such interactions may increase as the amount of the continuous phase decreases (evaporation) or polarity increases (oxidation). Weak interactions may contribute to

Glass Transition in Asphalt Binders 19

peak broadening because higher energy/temperature would be needed to overcome these forces and set molecules or molecular chains in motion.

4.4. Effect of Time

Figures 6-8 indicate that there is no significant difference between oxidized and evaporated samples in terms of glass transition and Tg. Even the most severely oxidized sample (275°C in air, 300 min) showed neither phase separation nor the glass transition temperature deviated from the trend set by the evaporated samples.

It has been shown (Kriz et al., 2007a, Kriz et al., 2007b, Kriz et al., 2008) that isothermal conditioning at a temperature within the glass transition range affects the phase distribution and glass transition temperature. The glass transition temperature was found to increase linearly with the logarithm of conditioning time. The mechanical properties have also been shown to develop with time. A certain analogy to the physical aging of amorphous polymers was suggested. The studies indicated that changes within the amorphous phase at low temperatures are extremely slow, as the diffusion and molecular mobility are reduced.

Figure 10. Asphalt A conditioned at −20°C for 140 days. Percentage represents a weight loss of a particular sample during the “evaporation” experiment (200°C in nitrogen). 0% wt. represents the control sample (conditioned but not evaporated). Curves vertically shifted.

All samples (original, evaporated and oxidized) were subjected to isothermal conditioning at −20°C for 3,360 hours (140 days). Results are presented in Figures 10-12. The differences in glass transition among the original, oxidized and evaporated samples became more apparent. For example, with the conditioning, Tg

20 Road Materials and Pavements Design. Volume X – No X/2007

increased by almost 2°C in the original binder; while in the most oxidized sample (275°C, 300 min, 10.7 %wt. loss), Tg increased by 16.5°C. The change in Tg upon conditioning was greater in oxidized samples than in evaporated samples, perhaps due to the increased content of high Tgi components (carbonyl and sulfonyl functional groups and higher aromatic content The peak split at −20°C (conditioning temperature) was observed in severely evaporated and oxidized samples (Figures 10 and 11). Upon isothermal conditioning, phase separation occurred with two glassy domains, stable above and below the conditioning temperature. This indicates that the phase stability of the oxidized asphalt was affected, and phase separation occurred. The origin of the small peak observable at around 4°C in Figures 10 and 11 is not clear. It may represent melting enthalpy of water as frost may develop in the sample upon long term storage at sub-zero temperature.

Figure 11. Asphalt A conditioned at −20°C for 3,360 hours (140 days). Percentage represents weight loss of a particular sample during the “oxidation” experiment (200°C in air). 0% wt. represents the control sample (conditioned but not oxidized). Curves vertically shifted.

4.5.Thermo-reversibility

It has been shown that the changes in phase distribution and glass transition upon isothermal conditioning within the glass transition range are fully reversible, if the thermal history is deleted. This can be achieved by heating the sample to a sufficiently high temperature (150°C) where the solid structure is devitrified/melted/disassociate, and asphalt becomes a simple fluid (Kriz et al., 2007a, Kriz et al., 2008).

Glass Transition in Asphalt Binders 21

Figure 12. Asphalt A with no thermal history (unfilled symbols) and conditioned at −20°C for 140 days (filled symbols). Original asphalt (○), evaporated in nitrogen at 200°C (◊), and oxidized in air at 200°C (□).

Figure 13. The effect of low-temperature isothermal conditioning on glass transition was fully reversible in Asphalt A, and an identical result was obtained for the original samples (no isothermal conditioning) and for samples isothermally conditioned with the thermal history deleted (samples marked DTH). Curves are vertically shifted.

The thermal history was deleted in the evaporated and oxidized samples, and

MDSC data were recorded. The comparison of the original MDSC run (prior to isothermal conditioning) and the MDSC run of the conditioned samples with deleted thermal history is presented in Figure 13. The results indicate that the effect of isothermal conditioning was fully reversible, as the specimen was heated up to 150°C. Reversibility indicates that the processes observed in asphalt upon isothermal conditioning are controlled by changes in molecular mobility, which can be affected

-30

-25

-20

-15

-10

-5

00.0 2.0 4.0 6.0 8.0 10.0 12.0

Weight loss [% wt.]

T g [°

C]

22 Road Materials and Pavements Design. Volume X – No X/2007

by spatial interferences and molecular interactions (possibly stronger in oxidized samples). As the sample is heated, temporary spatial interferences and molecular interactions disappear, and the sample returns to the initial state. Changes in phase stability can be reversed by heating the asphalt to a sufficiently high temperature. The lowest temperature at which the asphalt retains its original properties was not determined. This should be the subject of future study.

4.6. Long-term Conditioning of Asphalt B

Asphalt B was subjected to long-term conditioning at two temperatures −20°C and +20°C. Results are presented in Figure 14. Interestingly, peak separation was observed in both cases, i.e. in samples conditioned at both −20°C and +20°C.

Figure 14. Effect of long-term isothermal conditioning on glass transition in Asphalt B. (a) conditioned at −20°C for 5,400 hours, (b) control sample (no thermal history), (c) conditioned at +20°C for 8,445 hours. Curves vertically shifted.

The sample conditioned at −20°C started to devitrify upon heating very rapidly. This suggests that molecules that normally devitrify between −50 and −20°C, were built into the glassy structure. The structure of the already glassy molecules perhaps obstructed (spatially or by molecular interaction) the molecular motion of these molecules, and they eventually “froze” in the structure and became glassy – note the analogy to the rigid amorphous phase (Kriz et al., 2008). Therefore, upon heating, the restricted molecules could not devitrify until the “constraining molecules” devitrified too. The restricted molecules are released instantaneously. This explains the rapid start of the transition at −20°C (Figure 14, curve (a)).

The sample conditioned at 20°C also showed separation into two phases – one above and one below the conditioning temperature. In this case, the conditioning

Glass Transition in Asphalt Binders 23

temperature is close to transition end (upon heating) yet within the glass transition range. Therefore, the glassy domain above the conditioning temperature is very small.

The phase separation was observed after long-term conditioning at the temperature within the glass transition range in neat Asphalt B and in the severely evaporated and oxidized samples of Asphalt A. These asphalts are considered as phase instable asphalts. Phase separation upon long-term conditioning may be an indictor of the phase stability of the whole asphalt system. If the binder is phase stable, separation does not occur at the temperature of conditioning. The stability may be affected by several processes, as shown in Figure 15.

If any disturbance occurs within the sensitive region, as indicated in Figure 15, a phase separation may occur. With evaporation, the sensitive region is affected only if significant fraction of light-end components is evaporated. Polar molecules are sensitive to oxidation, and their polarity is increased during oxidation. Less polar molecules are not easily oxidized, and their polarity is not significantly affected. Gradual transition is discontinued, and phase separation may occur. In thermal cracking, severe treatment at high temperatures leads to a cleavage of covalent bonds and results in a low boiling product (polarity decreased) and coke (carbonized residuum). With solvent recovery, the action of non-polar solvent affects the continuum within the sensitive region and allows the most polar molecules to associate: phase separation occurs.

Figure 15. Asphalt as a molecular continuum and the effect of 4 processes on phase stability. Shades of grey represent molecular continuity in polarity or molecular weight (white - non-polar or lightest, black - most polar or heaviest). The region between white dashed lines represents the sensitive region, where molecular continuity is essential for system stability.

As previously discussed, the phase compatible asphalt shows a single Gaussian peak on calorimetric signal as the glass transition smoothly proceeds from the most

24 Road Materials and Pavements Design. Volume X – No X/2007

mobile to least mobile molecules or molecular chains. In such asphalt, the molecular continuity from the lightest to heaviest molecules must exist. If it is broken, sooner or later the phase incompatibility will occur in the asphalt. This material will behave as a composite material with properties directed by a “weaker” constituent.

5. Conclusions

The glass transition in asphalt binders is always dependent on experimental conditions; therefore, relating asphalt properties or performance to a particular parameter may be impractical without further specification of the experiment.

The effects of evaporation of light-end components in an inert atmosphere and of oxidation in air on the glass transition of asphalt were studied. It was shown that the glass transition temperature increases linearly with the weight loss in both the evaporated and oxidized asphalt samples and there was virtually no difference between these two sets of samples.

The difference in glass transition temperature and phase stability between evaporated and oxidized asphalt specimens became more apparent after long-term isothermal conditioning at a temperature within the glass transition region. The increase of Tg was observed upon isothermal conditioning. The increase of Tg was higher in oxidized samples. The oxidized samples of Asphalt A showed wider glass transition on the temperature scale that the evaporated. Also phase separation at the conditioning temperature is somewhat more apparent in oxidized specimens. Two separate glassy domains, one above and one below the conditioning temperature, were observed, if a sufficient length of time was allowed for the conditioning.

Phase incompatibility may affect the low-temperature properties of asphalt binder, as the asphalt failure may be directed by a failure within a domain with higher rigidity/glass transition temperature.

Acknowledgements

The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada and Husky Energy Inc. for financial support, and the Bituminous Materials Chair at the University of Calgary for providing the performance grades for Asphalts A and B.

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