Bitumen Emulsion Destabilization Kinetics: Importance of the

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Bitumen emulsion destabilization kinetics: importance of the crystallized wax content Laure Boucard, Vincent Gaudefroy, Emmanuel Chailleux, Fabienne FARCAS, and Véronique Schmitt Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01578 • Publication Date (Web): 26 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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Bitumen emulsion destabilization kinetics: importance of the crystallized wax content Laure Boucard, Vincent Gaudefroy*, Emmanuel Chailleux, Fabienne Farcas†, Véronique Schmitt § ‡

IFSTTAR, MAST, Route de Bouaye, CS4, 44344 Bouguenais, France

†IFSTTAR, Université Paris Est, MAST, Boulevard Newton, 77447 Marne-la-Vallée, France §

CRPP CNRS, Université de Bordeaux, UPR CNRS 8641, 115 Av. Albert Schweitzer, 33600

Pessac, France

E-mail: [email protected], [email protected] [email protected] [email protected] [email protected]

Corresponding author * To whom correspondence should be addressed. E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT We study the kinetics of bitumen emulsion destabilization after addition of sodium hydroxide (NaOH) using macroscopic observations and rheology. Destabilization occurs in a two step process: first emulsion flocculates forming a percolated network of contacting drops and then coalescence provokes the irreversible connection of bitumen drops leading to a bitumen continuous network that further relaxes the shape. We show that the destabilization kinetics exhibits a rheological easily identifiable signature allowing reproducible and accurate measurement of the connection/coalescence time trc (which corresponds to the time, determined by rheology, required to form the network made of drops connected by non relaxed coalescence). Using this powerful tool, we show that, even if viscosity is thought to govern the shape relaxation of the connected network it does not determine the connection kinetics. Indeed emulsions with similar rheological behaviors exhibit very different destabilization times. Instead, we evidence a good correlation between the bitumen crystallized wax content and trc. From these experimental results we discuss the stabilizing effect against coalescence of crystals in bitumen emulsions.

INTRODUCTION In the road pavement domain, the use of bitumen emulsion cold mixtures presents several advantages in comparison with hot mixtures. Indeed, it reduces energy consumption and also the environmental impact is lower1,2. However the mechanical properties are often lower than those of hot mixtures3 they are therefore mainly used for lower traffic roads. Bitumen emulsions used in these applications correspond to direct bitumen-in-water emulsions. The bitumen is dispersed


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into droplets in an aqueous phase containing a cationic emulsifier. The breaking process of the bitumen emulsion is initiated when put into contact with mineral aggregates. It is due to interactions with the mineral aggregates and water loss (evaporation and gravity flow). Some mechanisms, supposed to play a role in the breaking process, like emulsifier adsorption at minerals surface, rise in pH or ionic species release, have been studied4–11. Indeed the contact between bitumen emulsion and aggregates leads to mineral dissolution phenomena which occur through hydrolysis reactions at mineral aggregates surface and ionic species are released in solution (Na2+, Ca2+, Mg2+, K+… )9,12–14. With specific mineral aggregates, a rise in pH is observed which corresponds to an increase of OH- concentration in solution15. This phenomenon can directly impact the surfactant interfacial properties and thus the emulsion stability16. Moreover, the bitumen composition can also influence the destabilization mechanisms17. However emulsion breaking process is not fully understood, it is difficult to know which parameters have a preponderant role on emulsion breaking and how the kinetics of the different involved phenomena affect the global destabilization. Emulsions can be destabilized through different mechanisms: creaming or sedimentation, flocculation, coalescence and Oswald ripening. Creaming, sedimentation and flocculation are reversible mechanisms. Flocculation is a reversible process whereby droplets stick together, without losing their individual integrity, and form aggregates of droplets. The irreversible destabilization of emulsions can only be due to coalescence or Ostwald ripening. Ostwald ripening is not observed in bitumen emulsions since the solubility of those oils in water is almost zero. Coalescence is a breaking process in which droplets merge together to form bigger ones. Coalescence requires contact between initial droplets and therefore may be favored by


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flocculation17. The presence of ions at sufficient concentration in the aqueous phase can lead to emulsion destabilization by flocculation and coalescence18–20. Destabilization of bitumen emulsions by addition of electrolytes has been the topic of various articles10,21–24. Philip et al.21,23 and Bonakdar et al.22 have demonstrated the "gelation" phenomenon of bitumen emulsions when coalescence is initiated by addition of NaOH. When coalescence occurs, the thin liquid film between two adjacent droplets breaks through the nucleation of a small channel. Nucleation is followed by shape relaxation of the droplets driven by surface tension which leads the two droplets to fuse into a unique one. The shape relaxation characteristic time ‫ݐ‬௥ depends on viscosity through: ఎோ

‫ݐ‬௥ ∝ ఊ

(Equation 1)

೔೙೟

where ߟ is the oil viscosity, R the droplet radius and ߛ௜௡௧ the surface tension. When the viscosity increases, ‫ݐ‬௥ increases as well. So, when coalescence happens, if ‫ݐ‬௥ is sufficiently high compared to the coalescence kinetics, a network of coalesced droplets can form before shape relaxation is completed which leads to the formation of a network of percolated and interconnected droplets. The term "interconnected drops" should be understood as: there exists a continuous path for the bitumen from one drop to its neighbors. The percolation time or "gelation time" is defined as the delay between the introduction of the rupturing agent, NaOH, and the moment when the sample doesn’t flow anymore. The term "gelation" has often been chosen by the authors because macroscopically the sample did not flow anymore on a short timescale and because of a sharp increase in the viscosity. However no visco-elastic characterization has been carried out to confirm the gel nature of the sample. The shape relaxation went with a homothetic contraction of the sample that kept the shape of the initial vial, also called viscous sintering.


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The aim of this work was to get more insight about the destabilization mechanisms and kinetics and to determine the role of various parameters such as the bitumen viscosity or its crystal content. We first propose a rheological protocol to follow the bitumen emulsion destabilization kinetics. This experiment can only be performed when there is at least a delay of 3 min between the initiation of the destabilization and the increase in viscosity of the sample, meaning the destabilization is not instantaneous after a rupture agent addition. We have chosen the conditions for which the "gelation" phenomenon can be macroscopically observed and the "gelation" time can be measured and controlled17. These conditions have been previously determined using various electrolytes and bitumen17. Therefore, in the present study, we chose a paraffinic bitumen (with a penetration grade of 160/220 and an acid value of 0.3 mg KOH / g of bitumen (according to NF T66-066) as the dispersed oily phase and a quaternary ammonium salt surfactant tetradecyltrimethylammonium bromide (TTAB) as the cationic emulsifier. To destabilize the emulsions, NaOH solutions were used. Once the rheological procedure established, the impact of different parameters was studied in order to have a better understanding of bitumen emulsions breaking mechanisms. The NaOH concentration has been varied to modify the destabilization kinetics. Despite its influence on viscosity, temperature is chosen as a way to tune the amount of crystallized wax content and a fluxing agent has been added to vary the bitumen viscosity at constant wax content. The impact of these parameters on the emulsion kinetic evolution was measured using the proposed rheological protocol. Finally, based on these new results, an interpretation of the destabilization process and kinetics is proposed.


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EXPERIMENTAL SECTION Emulsion composition and preparation Bitumen used for road materials with penetration grade 160/220 has been chosen as the dispersed phase of the emulsions referred to as EB. The same bitumen has been fluxed with Greenflux SD® (petroleum distillates from TOTAL) at 5 wt% in order to decrease its viscosity. The emulsions prepared thereof are noted EFB. Both types of emulsions were stabilized with the same cationic emulsifiers, a quaternary ammonium salt; tetradecyltrimethylammonium bromide (TTAB) from Fluka, its critical micellar concentration is equal to 3.8x10-3 M. Bitumen-in-water emulsions were prepared with a colloidal mill Rink Elektro. The apparatus allows the manufacture of 1L of emulsion. It is composed of a rotor-stator system with a shear rate of 14 160 s-1 and a 0.3 mm gap. The aqueous phase at 45°C containing 0.4wt% (equal to 8.7 CMC) of TTAB, a concentration corresponding to typical bitumen emulsion formula used in pavement industry, is first introduced in the colloidal mill and then, the bitumen, also called binder, preheated at 110°C, is added stepwise. The mixture thus progressively concentrated in dispersed phase until it reached 65wt% with respect to the total emulsion weight. The dispersed phase mass fraction was kept constant for all the prepared emulsions. Emulsions were diluted after their fabrication, with an aqueous phase containing or not NaOH at various concentrations, to obtain bitumen content of 58 wt% (corresponding to a volume fraction equal to 57 vol%) keeping the same surfactant concentration. Therefore the emulsion composition was constant (58wt% of bitumen with respect to the total emulsion and TTAB 0.4wt% with respect to the aqueous phase) throughout the study except for NaOH concentration. In the absence of NaOH, the emulsions remained stable during the experimental period as evidenced by regular droplet size distribution measurements.


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Droplet size distribution determination Droplets size measurements were used to assess the emulsions stability during the experimental period. The size distribution was measured by static light-scattering using a Malvern Mastersizer Hydro 2000 SM apparatus using Mie theory. The samples were strongly diluted (to avoid multiple scattering) in the measuring cell either with distilled water or, if required, with the aqueous surfactant solution to avoid emulsion coalescence during the size measurement. The samples were in-situ homogenized thanks to the flow generated by the apparatus pump. Oil droplets size distribution was characterized by the volume-averaged droplet diameter noted d4,3. Refractive index for bitumen and distilled water were taken equal to 1.625 and 1.330 respectively and the absorption coefficient of bitumen was set to 0.0035 (usual value). The volume-averaged droplet diameter of EB and EFB were equal to 3.1 µm and 3.8 µm respectively (see Figure S1). Emulsion destabilization protocol An aqueous NaOH solution at 5 M was prepared by dissolving NaOH salt in deionized water purified by a Millipore system. This solution was used as destabilizing agent for the emulsions. We chose such a high concentration to avoid emulsion dilution when adding this rupturing agent. To evaluate the NaOH influence on emulsion stability in the rheometer, the following protocol has been used: first the emulsion of known initial mass fraction was introduced in a 20 ml glass jar, and secondly a known amount of NaOH solution was added fitting the targeted concentration. Then the mixture was manually stirred for 2-3 s. Finally, the sample was loaded in the rheometer and the measurement was started 180 s ±30 s after NaOH addition in the sample. Destabilization behavior of the emulsion was then rheologically assessed versus time. All the samples were prepared at ambient temperature before being loaded in the rheometer, previously


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thermalized at the measuring temperature. In parallel, macroscopic observations were carried out. Initially the emulsions flowed under their own weight when the vial was tilted. The time tm, (to indicate a macroscopic determination and previously named percolation time or "gelation" time) at which a sample of 15 ml did not flow anymore, has been determined by regularly and gently taping the surface of the sample with a small spatula, tm was reached when the surface of the sample became "rigid", then tilting the vial did not lead to sample flowing anymore. Differential scanning calorimetry (DSC) Differential scanning calorimetry (DSC) measurements were performed with a TA Instrument Q100 to evaluate the fraction of crystallized wax in the pure and fluxed bitumen. The used methodology was the same as the one described by Lesueur et al.

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. For each measurement,

approximately 14 mg of bitumen was weighed and heated to 90°C in a DSC sample pan in order to obtain a flat surface and a uniform bitumen distribution in the pan. The wax content was determined from the data recorded during heating from - 80°C to 120°C, at a rate of 5°C/min. The wax content Xc, expressed in wt% with respect to the oil phase, was calculated from the endothermic peak during the heating scan. In the calculation, a constant melting enthalpy of 200 J/g for pure paraffin was used as reference26. By DSC the glass transition temperature Tg, wax melting starting temperature Ts and melting out temperature Tm were also determined. Table 1 gathers the results obtained for both studied bitumen. The results are the mean values of two tests. Table 1. Glass transition temperature (Tg), Total wax content (total Xc), wax melting starting temperature (Ts) and melting out temperature (Tm) for pure bitumen and fluxed bitumen obtained by DSC measurements analysis.


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Tg (°C)

Total Xc (%)

Ts (°C)

Tm (°C)

Pure bitumen (B)

-24.4±1.2

2.6±0.1

5.0±0.2

80.5±0.7

Fluxed bitumen (FB)

-37.9±0.9

2.7±0.1

-6.2±1.5

79.1±0.1

The wax content was also determined as a function of the temperature by measuring the area under the endothermic peak. Fig. 1 shows a decrease in the wax fraction Xc with increasing temperature. For the fluxed bitumen, melting of the crystallized fractions occurred between 6.2°C and 79.1 °C whereas for pure bitumen melting was observed between 5.0 °C and 80.5 °C, meaning that the endothermic peak was slightly shifted towards lower temperature by incorporating the fluxing agent. Owing the vicinity of the curves, one can consider that the wax content is almost the same for the two bitumen. In both cases, there is no more wax above 80°C

Figure 1. Percentage of crystallized wax, Xc, remaining in the bitumen as a function of temperature. Rheological measurements for bituminous materials characterization Rheological measurements were assessed by dynamic oscillatory rheometry using a controlled stress Malvern Kinexus Pro rheometer. The elastic modulus G', the loss modulus G" and the


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phase angle δ were recorded. Depending on the sample type, different geometries were used. A 40 mm diameter cone-plate geometry for the less viscous samples, meaning that the initial emulsion is not destabilized. A 20 mm diameter plate-plate geometry for measurements on bitumen sample between 30°C and 60°C and a 8 mm diameter plate-plate for lower temperature. We checked that the rheological data did not depend on the chosen geometry. Despite the fact that the stress and strain are not constant in a plate-plate unlike in a cone-plate, we used a plateplate geometry in order to tune the gap geometry. For monitoring the emulsion destabilization over time, a 20 mm diameter sand-blasted plate-plate geometry was preferred in order to minimize wall slip effects. Rheological tests were performed on the emulsion using various gaps and no significant gap influence (0.5, 0.75, 1 and 1.5 mm) on the obtained flow curves was observed (see Figure S2). As a consequence, the gap has been fixed at 1 mm for the plate-plate geometries and at the required imposed gap for the cone-plate geometry (0.147mm). There was no strong frequency dependence for the elastic modulus for both the bitumen and the emulsions so that we chose to fix the frequency at 1 Hz. To regulate the sample temperature, the apparatus was equipped with a Peltier module incorporated in the lower plate (precision of 0.01°C). A solvent trap was also used for measurement on emulsions in order to prevent drying of the sample. For the emulsion destabilization experiments, the emulsion at 58 wt% of bitumen was placed within the geometry gap after NaOH addition in the sample. Then the measurement was started 180 s ± 30 s after addition of NaOH to the sample. In the case where the destabilization was not instantaneous, meaning there was at least three minutes delay between NaOH addition and emulsion phase separation, the destabilization could be monitored rheologically. The delay of three minutes corresponded to the time needed to prepare and load the sample in the rheometer. It is worth noticing that in the present work, viscous sintering

22, 23

also occured but


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on a timescale much larger than the rheological measurements so that the sample volume was preserved during the experiment and no water expulsion has been observed.

RESULTS AND DISCUSSION To follow the time evolution of the material using rheology, it is necessary to carry out the experiments in the linear visco-elastic regime (LVER) at each step of the material evolution. To determine such conditions, we first determined the limit of the LVER of both the two extremes that is to say the initial emulsion without addition of NaOH that is kinetically stable and the final material after emulsion destabilization and contraction. This allowed defining the conditions to stay in the LVER throughout emulsion destabilization. The LVER corresponds to the range of stress and strain for which no structural modification of the sample is generated by application of an oscillatory perturbation. ߛ௅௏ாோ is defined as the LVER limit, it corresponds to the maximal strain that can be applied to the sample without changing its structure i.e. without variation of G' and G". Bitumen and bitumen emulsion before and after destabilization characterization Bitumen is a material whose visco-elastic behavior evolves with temperature. Fig. 2a represents the evolution with temperature, from 5 to 60°C, of the rheological characteristics of bitumen (G’, G’’ and δ). From the evolution of the phase angle, one can conclude that the liquid behavior dominates in the whole explored temperature range (δ larger than 45°) and that the liquid nature increases with temperature to the detriment of the elastic contribution. At 60°C, bitumen tends to behave as an almost pure liquid with a phase angle approaching 90°.


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Figure 2. Evolution as a function of temperature of elastic modulus G' (full squares), loss modulus G'' (empty squares) and phase angle δ (triangle symbol) at 1Hz of: a) the bitumen and b) the fluxed bitumen. Bitumen viscosity is very high at room temperature as demonstrated by the loss modulus equal to 105 Pa at 1Hz. This makes bitumen very difficult to handle, requiring heating at high temperature. Dispersing bitumen in an emulsion results in a much easier workability of the binder owing to a large viscosity drop, this makes emulsions very useful in aggregate coatings. In the present case, once emulsified in water at 58wt%, the loss modulus has dropped by a factor of about 104. Fig. 3 shows the evolution of the rheological quantities as a function of strain or stress amplitude for bitumen, bitumen-in-water kinetically stable emulsion at 25°C and destabilized emulsion at 25°C, 5 hours and 30 minutes after NaOH addition at 0.07M.


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Figure 3. Evolution of elastic modulus G' (full squares), loss modulus G'' (empty squares) and phase angle δ (triangles) at 1Hz and 25°C as a function of the oscillatory amplitude of either stress (a, c, e) or strain (b, d, f) for the bitumen (top), the initial emulsion EB containing 58 wt%


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of bitumen (middle) an for the destabilized emulsion EB 5 hours and 30 minutes after NaOH addition at 0.07 M (bottom). These results have been obtained by applying an oscillatory shear with a stress amplitude varying from 10-4 Pa to 105 Pa at 1 Hz to the samples. As the rheometer is stress-controlled, we chose to apply the stress rather than the strain in order to avoid using the feedback loop. For a better reading, the results have been plotted as a function of both stress and strain. Fig. 3 also evidences the narrower linear visco-elastic regime for the non destabilized bitumen emulsions compared to the dispersed phase alone i.e. bitumen. Indeed, the linear visco-elastic regime stops at a shear strain of 0.15% for the emulsion (compared to 20% for the bitumen). This limited range of LVER is usual for emulsions27–30. For larger strains or stresses, neighboring drops may switch or adopt different trajectories31,32 leading to the emulsion flow. It is worth noticing that the rheological behavior of the destabilized emulsion is very close to the bitumen, the elastic and loss moduli are relatively high with a larger contribution of the loss modulus compared to the elastic modulus. However the values of the moduli do not completely reach the pure bitumen ones, there are about a decade lower. The ߛ௅௏ாோ is of the order of 2%, higher than the value for the initial emulsion but lower than that of pure bitumen. Emulsion destabilization monitored in situ As the initial emulsion and the final destabilized samples exhibit very different rheological behaviors and in order to reliably measure the rheological quantities during destabilization, we adopted a protocol allowing applying high enough stresses to get a sufficient sensitivity on the measured strains but also low enough stresses to remain in the linear visco-elastic regime. The shear stress values were chosen in the range from 3.10-3 Pa to 1 Pa belonging to the LVER and we also added the condition that the measured strain should belong to the range extending from


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0.001% to 0.15 % to ensure a good measurement sensitivity implying that the shear stress should not be too low. As the rheological behavior of the sample evolved, the shear strain decreased for a fixed shear stress. Thus the adopted procedure has been chosen to progressively increase the applied stress keeping a minimal strain level. This protocol is as following: the applied stress is equal to 3.10-3 Pa as long as ߛ < 0.001 %. If this condition on the measured strain is no more fulfilled, then the applied stress becomes equal to 0.05 Pa. This is valid as long as ߛ < 0.001 %. If this condition is no more fulfilled then the applied stress becomes equal to 0.1 Pa as long as ߛ < 0.001 %. If this condition is no more fulfilled then the applied stress is equal to 0.5 Pa as long as ߛ < 0.005 %. If this condition is no more fulfilled then the applied stress is equal to 1 Pa. We checked that a modification of the stress did not impact the rheological quantities thus confirming that the measurements were performed in the LVER whatever the stage of the evolving material. Figure 4 shows the rheological evolution versus time of the EB emulsion destabilization after NaOH addition to the sample (concentration of 0.07 M), which corresponds to a macroscopic tm time around 35 min. The evolution of the loss (G’’) and elastic (G’) moduli as well as the phase angle (δ) were registered over time. Fig. 4a gives a general overview of the phenomenon whereas Fig. 4b provides more details focusing on the early stage of the emulsion evolution and Fig. 4c shows size distribution profile evolution of bitumen emulsion after NaOH addition (0.07M) and ongoing destabilization (adapted from Boucard et al.17). The same experiments were carried out several times on the same emulsions to check the repeatability of the measurements and also to take pictures at the different stages. Indeed, for each picture the experiment was stopped and the upper plate removed before taking the picture.


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Figure 4. Characteristic evolution of G’ (full squares), G'' (empty squares) and phase angle (triangles) of EB during destabilization induced by addition of NaOH at 0.07M. Time 0 corresponds to the beginning of the rheological measurement i.e 180 s after addition of NaOH. a) Semi-logarithmic plot of the time over 5 hours and 30 minutes. b) Time is plotted linearly to focus over the first 1000 s. The pictures A, B and C show the samples after removal of the rheometer upper plate at 30°, 68° and 78° phase angle respectively. c) Size distribution profile of bitumen emulsion after NaOH addition (0.07M) and ongoing destabilization. Pictures A, B and C reported on Fig. 4a were taken at different destabilization steps. In the first stages of the measurement, the sample exhibits the aspect of an emulsion (Fig. 4a.A). It is worth noticing that G' is larger than G" (the initial phase angle of the emulsions is about 30°) in the early stages showing the dominant elastic behavior of the emulsion. This was not the case in Fig.


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3a where G" was larger than G' (δ was around 51°) for the initial emulsion, without the addition of NaOH. The reason for this difference comes from the addition of NaOH. Indeed, we previously showed that adding NaOH induces attractive interactions between drops leading to flocculation of the emulsion, that gives rise to a percolated 3D network of aggregated droplets17 (See Figure S3). This network of contacting drops confers an elastic behavior to the sample that behaves then as a classical colloidal gel33. On Fig. 4b, we observe that elastic and loss moduli increase progressively versus time before stabilizing at plateau values. At a time around 240 s after rheological measurement started, the phase angle begins to rise. This change can likely be attributed to the beginning of connections between drops already at contact. At 360 s there is a crossover of the moduli and G’’ becomes larger than G'. The phase angle stabilizes at around 78°, which is the value of the pure bitumen at 25°C. The time corresponding to the phase angle stabilization, which 180 s have been added to, is noted “trc”, rheological connection time. It corresponds to the delay for fusing and connecting drops together so that the percolated network becomes made of a continuous bitumen pathway. For the loss and elastic moduli, a gradual increase is observed until stabilization of values. The loss modulus increases more significantly than the elastic modulus and the system gradually gets closer to bitumen properties (Fig. 4a.C). As time passes the connections between drops become larger and the system evolves towards the rheological behavior of the bitumen. An evolution of the distribution towards the larger droplets is also observed (Fig 4c.). Of course, the drops size distribution by itself is not very meaningful as it likely does not catch the largest bitumen domains. However it clearly demonstrates that the emulsion has evolved evidencing its destabilization. The behavior at long time after destabilization (Fig. 3b) shows that the loss modulus is again dominant. However, the moduli values are still lower than those for original pure bitumen as some water could remain trapped in


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the bitumen matrix. For the sample tested here trc was about 25 min whereas tm measured manually on larger samples was about 32 minutes17. Discussion on the rheological behavior evolution with time The phase angle appears as a reliable indicator to monitor and study the kinetics of bitumen emulsion destabilization since it may be used to determine the time where the sample has turned to a percolated network of connected drops. Moreover it exhibits a good accuracy and repeatability: on three replicate tests the trc obtained was 25 ± 2min. The first effect of NaOH addition was to transform the emulsion made of individual drops and exhibiting a visco-elastic behavior with a main liquid contribution (see Fig. 3 middle) into a flocculated emulsion where the drops slightly adhere to each other forming a gel of drops in contact (see Supporting Information S3) and exhibiting a visco-elastic behavior with a dominant elastic contribution (see Fig. 4b). As a result the phase angle decreased from 52° to a value around 30° with the addition of NaOH. Then drops in contact begun to coalesce1–4. On a macroscopic scale, after the delay tm the emulsion did not flow anymore showing a change in the macroscopic mechanical behavior. Examining the rheological behavior, one can observe an increase of the phase angle that becomes larger than 45° and increases until reaching a plateau value at trc close to the bitumen's phase angle. As the emulsion evolution is mainly characterized by a phase angle larger than 45°, to be rigorous, the term "gelation" often used in literature21,23,22,17 is inappropriate. It is worth noticing the good correlation between tm and trc despite the difference in the sample volume (15 ml in the glass container and 0.3 ml in the rheometer). Even if a priori counterintuitive, this observation of a non-flowing sample and an increase of the phase angle can be understood as following: as long as drop adhere keeping their individuality, the elastic modulus dominates the


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rheological behavior. Emulsion elasticity is known to be given by the total amount of interface in contact27,30. At the macroscopic scale, when the vial is tilted the emulsion is able to flow under its own weight by rupture of these weak links between drops. This behavior is completely reversible after cessation of the emulsion perturbation. When coalescence occurs, the nature of the links between drops evolves towards bitumen channels. It is our understanding that, once drops have percolated and are interconnected, the rheometer is sensitive to the bitumen composing the formed network. Therefore the measured phase angle becomes similar to the bitumen one. Remaining water has almost no effect. The final material can be seen as a bicontinuous phase (an illustrative picture is given in Supporting Information S4): indeed due to percolation there is a continuous path in the bitumen from lower to upper plate as well as in the continuous water. Therefore both components (water and bitumen) are subjected to the same strain corresponding to water and bitumen associated in parallel on a rheological point of view. As a consequence, the moduli are additive and as water is characterized by very low moduli, the composite material is dominated by the bitumen high moduli. The new links seem to be much stronger than the former contacts between drops: they do not break when the vial is tilted as it was the case previously and the linear visco-elastic regime becomes more extended (γLVER=2%). At the macroscopic scale, when the vial is tilted or if the emulsion is gently taped with a spatula the very high viscosity of the interconnected bitumen gives the appearance of a "rigidification" of the sample that does not flow at this macroscopic observation timescale. In a third step (after connection of the percolated contacting drops i.e for time larger than trc), the network made of interconnected drops contract owing to shape relaxation of the coalesced droplets and the water is expelled from the network. If we assume that all the water is expelled, the final obtained material should be the initial bitumen. As a consequence rheology, and more


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precisely the phase angle measurement, is a very useful tool to objectively determine the connection time of the percolated contacting drops with good reproducibility and good accuracy. It also allows a better control over the experimental conditions (i.e solvent evaporation, temperature and sample dimensions). Influence of various parameters on the emulsion destabilization kinetics Different parameters may influence the destabilization kinetics like NaOH concentration and oil viscosity. It has been observed in previous works17,21 that NaOH concentration in the emulsion has an impact on the macroscopically measured tm. On the other hand, the destabilization mechanism strongly depends on the oil phase viscosity, as the shape relaxation characteristic time ‫ݐ‬௥ depends on viscosity (see Eq. 1)21. Since the temperature has a direct impact on the bitumen viscosity, it is another important parameter which can influence the destabilization evolution. Therefore, NaOH concentration, bitumen viscosity by addition of a fluxing agent and temperature have been varied and their impact on the destabilization kinetics determined. The evolution of the phase angle during the destabilization has been monitored and the connection times of the percolated contacting drops have been measured varying these parameters. (a) NaOH concentration. First, the influence of NaOH concentration on emulsion destabilization has been studied. Different amounts of concentrated solution of NaOH have been added to bitumen emulsions EB at 25°C to reach final NaOH concentrations ranging from 0.055 to 0.081M. Destabilization phenomenon and kinetics have been assessed using the rheological protocol previously described. Phase angle evolution versus time is reported in Fig. 5. For all studied NaOH concentrations, the phase angle stabilizes at 78° in agreement with pure bitumen at 25°C. Regarding the kinetic evolution, the phase angle increased more rapidly for higher


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NaOH concentrations which resulted in shorter trc. Fig. 5b reports the rheological connection time trc and the manually measured time tm evolutions as a function of NaOH concentration in the samples. The two characteristic times correlates very well even if trc are generally slightly lower than tm.

Figure 5. Influence of NaOH concentration on the emulsion destabilization. Evolution of: a) the phase angle with time and b) Times tm and trc versus NaOH concentration in the sample. The higher the NaOH concentration in the emulsion, the faster the destabilization. Moreover, the coalescence of the percolated network has been observed only in presence of hydroxide anions in the emulsion17. Addition of other electrolytes like KBr or NaCl only induced flocculation17. Thus, the hydroxide anions must be interacting with the interfacial layer of the bitumen droplet and favors coalescence. (b) Influence of the temperature and the addition of a fluxing agent. Temperature has two different effects: heating induces a decrease of both the bitumen viscosity and the bitumen crystallized fraction content. In order to discriminate these two effects, the destabilization as a function of temperature has been studied for two different emulsions having similar drop size and dispersed phase volume fraction, one being composed of pure bitumen (EB) and the other composed of bitumen fluxed by addition of 5% of Greenflux SD® (EFB). Our idea in doing that is


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the following: temperature has two effects: it decreases both the viscosity and the wax content of the bitumen. Addition of a fluxing agent only decreases the viscosity without changing the amount of crystallized wax as can be deduced from Fig. 1. We can therefore compare the kinetics of emulsions composed of: i) two bitumen with the same amount of crystals and different viscosities (by comparing EB and EFB at the same temperature) and ii) two bitumen with the same viscosities and different amount of crystals (EB and EFB at different temperatures). This second comparison is made possible because as we will show, there exists a temperature shift that leads to analogous rheological behaviors. By comparing the destabilization kinetics of these systems we will be able to determine whether rheology or crystallize fraction content is the most determining characteristics. The phase angle evolution for a fixed NaOH concentration equal to 0.07 M in EB samples has been measured at different temperatures (from 10°C to 35°C). Results are plotted in Fig. 6a. As temperature is increased, the phase angle reaches its plateau value earlier meaning that the destabilization kinetics is faster. Also, the final phase angle value increases, in qualitative agreement with the bitumen visco-elastic behavior evolution with temperature (Fig. 2a). However, for the lower temperatures (10°C and 13°C) there is a significant difference between the value of the phase angle stabilization (around 45°) — it could also be noted that as the angles remain lower than or become equal to 45° the term "gelation" may be used for these specific cases — and the value of the bitumen phase angle at the same temperature (of the order of 70°). Therefore it seems that the bitumen properties at low temperatures are not recovered by the NaOH addition. To better highlight this feature, we plotted in Fig. 6b the recovered phase angle as a function of temperature. This observation could indicate that coalescence did not occur completely so that channel-like links did not replace all contacts between drops.


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Figure 6. a) Time evolution of the phase angle of Ep (with a NaOH concentration of 0.07 M) at various temperatures ranging from 10°C to 35°C and b) Plateau value of the emulsion phase angle after destabilization compared to the bitume phase angle at the same temperature. As described previously trc could be deduced from Fig. 6a as a function of temperature. The extracted values are reported in Fig. 7. The acceleration of the destabilization by the temperature increase is clearly evidenced.

Figure 7. Influence of temperature on trc for the bitumen-in-water emulsion EB (full diamonds) and fluxed bitumen-in-water emulsion EFB (empty squares). The concentration of NaOH in the sample is fixed at 0.07 M. For EFB, the real temperature is written next to each point while the


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abscissa corresponds to the pure bitumen equivalent temperature (see text and Table 3 for further explanations). To determine if this acceleration can be attributed to the only bitumen viscosity decrease through the shape relaxation time decrease as suggested by Eq. 1, an emulsion made of fluxed bitumen, EFB, was also studied. Indeed it is known that the viscosity of bitumen may be decreased by addition of Greenflux SD®. In the present case, we added 5% of petroleum distillate, the effect of this addition on the temperature-dependent rheological behavior can be seen by comparing Fig.2 a and b. Rheology of fluxed bitumen exhibits the same trend as pure bitumen. However for the fluxed bitumen the moduli curves are shifted by one decade downwards in comparison to pure bitumen, while the phase angles are globally higher showing a more liquid dominating behavior. The difference in phase angle vanishes as the temperature is increased (the phase angle difference is equal to 20°, 5° and 1° for a temperature equal to 5°C, 30°C and 60°C respectively). The very interesting point is that it is possible to find temperature equivalences for which the moduli and phase angle match for the two bitumen. Therefore equivalent behaviors may be encountered for pure and fluxed bitumen at two different temperatures. This is evidenced in Table that gathers some of the results.

Table 2. Pure bitumen and fluxed bitumen corresponding temperatures for equivalent moduli and phase angle.

Bitumen type

Temperature (°C) G’’ (Pa)

Pure Fluxed Pure Fluxed bitumen bitumen bitumen bitumen 20.0 3.5.105

7.5

22.0

10.0 1.7.105

Pure Fluxed bitumen bitumen 25.0

12.0 1.1.105


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G’ (Pa) δ (°)

7.5.104

3.4.104

1.9.104

77

79

80

As a consequence, we can consider that, just by adjusting the temperature, we have two bitumen exhibiting similar rheological behaviors at our disposal. The EFB destabilization (0.07 M of NaOH) was rheologically determined at 7.5, 10.0 and 12.0°C; time evolution of the phase angle is reported on Fig. 8. It can be compared to EB destabilization at 20.0, 22.0 and 25.0 °C respectively (Fig. 6a).

Figure 8. Phase angle evolution of EFB at 0.07 M NaOH at different temperature as a function of time. As for EB, the phase angle increases with time and then reaches a stationary value. The phase angle plateau value increases with temperature. Again the time trc can be extracted for 7.5, 10.0 and 12.0°C (Fig. 7). Note that the X-axis values are the corresponding temperature for the pure bitumen and not the testing temperatures which are noted in the vicinity of the corresponding points. As demonstrated on the Fig. 7, the times trc for EB and EFB do not match even if rheological behaviors of the two dispersed phases are the same. The connecting times are higher


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than expected for EFB and the deviation increases when temperature decreases. This demonstrates that the rheological behavior of the bitumen is not the main parameter governing the emulsion destabilization kinetics. Temperature has a strong influence on the destabilization kinetics: the emulsion destabilizes faster when temperature is increased. As just seen, this cannot just be explained by the temperature induced decrease of the bitumen viscosity. As temperature not only influences the bitumen viscosity but also controls the amount of crystallized wax fractions, the connection time of the percolated contacting drops trc can be plotted as a function of the crystallized fractions (Fig. 9), XC, remaining in the bitumen at the corresponding temperature and measured by DSC (Fig. 1). The data from the two systems superimpose quite well showing the link between trc and XC independently of the dispersed phase viscosity. The lower Xc, the faster the destabilization kinetics. The crystallized fraction content seems therefore to be the most important parameter (rather than the bitumen viscosity) even if a sufficiently high viscosity is still necessary for coalescence occurring much faster than shape relaxation 17.

Figure 9. trc versus Xc percentage remaining in the bitumen.


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These experiments highlight the important role of crystallized wax content in bitumen-in-water emulsion on the destabilization kinetics of these systems. In literature, the presence of fat crystals has been reported to affect emulsions stability in various manners. For example Thivilliers et al.34,35 showed that the presence of crystals is detrimental for the anhydrous milk fat-in-water emulsion stability with a non-monotonic behavior as a function of the amount of crystals. To promote connections through coalescence the dispersed phase had to be composed of about 15% of crystals and 85% of liquid oil. In opposition, Rousseau et al.36 have put into evidence the stabilization effect of fat crystals adsorbed at the oil-water interface in O/W emulsions when the contact angle at solid/water/oil interface across the water phase is smaller than 90°. In the present case, the crystallized fractions improve emulsion stability likely by preventing the connections between drops already at contact. This conclusion can also explain the very fast destabilization observed for naphtenic bitumen-in-water emulsions. Indeed naphtenic bitumen, exhibiting similar rheological behavior as paraffinic bitumen, is deprived of crystallized wax and naphtenic bitumen-in-water emulsions destabilized so fast (faster than the required 3 min) after addition of NaOH 17 that sample loading in a rheometer was not conceivable.

CONCLUSIONS To conclude, we developed a rheological tool to follow the bitumen emulsion destabilization evolution and kinetics. We showed that the phase angle is a good indicator of the emulsion destabilization kinetics. Indeed once emulsions drops are connected through no shape relaxed coalescence events, the phase angle stabilizes at a plateau value. This allows a sharp definition of the time trc assessing the emulsion destabilization kinetics. However this methodology requires a delay between the destabilization initiation and the creation of connection between contacting


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bitumen drops in the percolated network in order to allow the sample loading. This methodology was then used to study the influence of various parameters on the destabilization kinetics. The experimental results show a strong dependence with temperature. The lower the temperature, the slower the destabilization. We demonstrated that this temperature effect is not governed by the viscosity variation. Instead, the destabilization kinetics is mostly related to the proportion of crystals in the bitumen everything else being fixed. The stabilizing effect of crystallized wax likely results from a crystal adsorption at the bitumen water interface.

Supporting Informations Figure S1: Size distribution of EB and EFB determined by static light scattering Figure S2: Flow curves of EB obtained for different gaps (0.5, 0.75, 1 and 1.5 mm) with a 20 mm diameter sand-blasted plate-plate geometry to assess possible wall slip effects Figure S3: Evolution of an emulsion previously diluted with a 0.5M NaOH solution to reach a drop volume fraction equal to 10wt%. The optical microscopy observations show the formation of a network of droplets. Adapted from [1]. Figure S4: Optical microscopy observation showing a 3D network of bitumen resulting from the connection of the percolated bitumen droplets during emulsion destabilization

AUTHOR INFORMATION Corresponding Author * To whom correspondence should be addressed. E-mail: [email protected] Present Addresses †IFSTTAR, Université Paris Est, Boulevard Newton, F-77447 Marne-la-Vallée, France


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§ CRPP

CNRS, Université de Bordeaux, UPR CNRS 8641, 115 Av. Albert Schweitzer, F-33600

Pessac, France Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources This research was partially funded by the French Région Pays de la Loire.

ACKNOWLEDGMENT The authors acknowledge Anke Lindner and Lionel Odie for valuable discussions and Flore Vendé and Lucie Marius for their experimental contributions to this study.

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