Prediction of emulsion properties from binder/emulsifier ... - Nynas

Prediction of emulsion properties from binder/emulsifier characteristics
B.Eckmann
AB Nynas Petroleum, Group Competence Center (GCC), Brussels, Belgium
K. Van Nieuwenhuyze / T. Tanghe / P. Verlhac
NYNAS NV, R&D Dept., Antwerpen, Belgium
Technical Session : Development in Bituminous Products & Techniques
Keywords : Emulsions, Testing, Cold Asphalt
Abstract.
The study reported in this paper has been realised in the frame of the European “BRITE-EURAM” Research
Project OPTEL BE-1516 : “Slow-setting cationic bituminous emulsions for construction and maintenance of
roads”. One particular task within this project was aimed at a better understanding on how emulsion properties
such as particle size distribution, viscosity, breaking behaviour and storage stability are related to binder and
emulsifier properties.
A major effort has been devoted to the measurement of interfacial tension via drop shape analysis and to the
titration of residual emulsifier. The strengths and weaknesses of these tests have been identified and resulted in
practical guidelines for their use. A number of laboratory emulsification trials performed with various binders
and emulsifiers confirmed the importance of particle size distribution and residual emulsifier content for key
emulsion properties such as the breaking index and storage stability.
It could be shown that, under fixed manufacturing conditions , particle size distribution seems indeed to be
essentially governed by interfacial tension and binder viscosity. It seems however that the prediction of the
residual amount of emulsifier ( from interfacial data and the total developed surface of the binder droplets ) is not
possible at this stage. This is likely to be due to the fact that a certain fraction of the emulsifier may also get
trapped inside the binder during the emulsification process.
Although the generated knowledge needs to be confirmed on a wider set of products it can already be
converted into more efficient and meaningful protocols for the assessment of binders, surfactants and
emulsions. It does also provide immediately applicable practical guidelines for the control of emulsion
properties via particle size distribution and emulsifier content.
Résumé.
Dans le cadre d’un projet européen “BRITE-EURAM” ( projet OPTEL BE-1516) intitulé “Optimisation des
émulsions cationiques de bitume à rupture lente pour la construction et la maintenance des chaussées”, une tâche
particulière était consacrée à une meilleure compréhension des liens entre les propriétés du liant et de
l’émulsifiant et les principales caractéristiques de l’émulsion (granulométrie, viscosité, indice de rupture, …).
Un effort important a été consacré à la mesure de la tension interfaciale bitume/eau par la méthode du
tensiomètre à goutte et à la détermination de la teneur en émulsifiant résiduel. Les avantages et points faibles
de ces méthodes ont été identifiés et ont conduit à des recommandations pratiques pour leur utilisation. Un
programme de fabrication et d’analyse d’émulsions en laboratoire a permis de confirmer le rôle clé de la
granulométrie et de la teneur en émulsifiant résiduel vis-à-vis de propriétés telles que l’indice de rupture ou
la sédimentation.
Nous avons pu montrer que, pour des conditions de fabrication données, la distribution granulométrique
semble effectivement être principalement déterminée par la tension interfaciale et la viscosité du liant. Il semble
cependant que la prédiction de la teneur en émulsifiant résiduel (à partir des données interfaciales et de la surface
développée totale des gouttelettes de bitume) ne soit pas possible à ce stade. Cela est probablement dû au fait
qu’une certaine quantité d’émulsifiant se trouve “piégée” à l’intérieur des gouttelettes lors de fabrication.
Bien que les résultats obtenus doivent être confirmés à plus grande échelle, ils peuvent néanmoins d’ores et
déjà conduire au développement de procédures mieux appropriées pour l’évaluation des liants, émulsifiants
et émulsions. Ils donnent également des indications pratiques directement utilisables pour un meilleur
contrôle des propriétés des émulsions par le biais de la granulométrie et de la teneur en émulsifiant.

Prediction of emulsion properties from binder/emulsifier characteristics
Introduction
B.Eckmann
AB Nynas Petroleum, Group Competence Center (GCC), Brussels, Belgium
K. Van Nieuwenhuyze / T. Tanghe / P. Verlhac
NYNAS NV, R&D Dept., Antwerpen, Belgium
The purpose of the study reported here was to improve the understanding on how emulsion properties
such as particle size distribution, viscosity, breaking behaviour and storage stability are related to
binder and emulsifier properties. It is however important to underline that this ambition was restricted
to the prediction of emulsion properties under fixed manufacturing conditions.
Starting basis – Original assumptions
The work-programme and its adaptations have been aimed at the verification of a number of initial
assumptions on how key emulsion properties are related to binder and emulsifier characteristics.
These initial assumptions are summarised in Figure 1 and briefly explained hereafter :
Emulsion viscosity should be mainly
related to the volume concentration of
the binder and the particle size
distribution (how the total binder
volume is split between droplets of
different sizes).
The pH value (more precisely the
often observed increase in comparison
to the original pH of the water-phase)
is supposed to be related to
“exchanges” between the binder and
the water-phase and should therefore
be related to the amount of total
created surface (specific surface area
of the PSD) and specific binder and/or
surfactant properties.
The Breaking Index describes the
behaviour of the emulsion as increasing
Particle Size
Distribution
Residual Emulsifier
Content
Binder Content
Binder/Waterphase
Interactions
Binder Rheology
Binder Density
Figure 1 : Original assumptions
Emulsion Viscosity
Increase in pH
Breaking
Index
Storage
Stability
amounts of a silica filler are stirred into a fixed amount of emulsion. This process involves chemical
reactions (increase in pH, migration of “free” surfactant to the filler, de-sorption of the surfactant
present at the surface of the droplet ) as well as mechanical actions (deformation and coalescence of
the droplets under the applied shearing forces). Breaking index should therefore be affected by
several interacting factors such as residual emulsifier content, pH value, particle sizes (small
droplets being more difficult to “break” than large ones) and binder rheology.
Storage Stability should be essentially conditioned by the amount of large particles and the density
differential between the binder and the water-phase (Stoke’s law). The settlement of the largest
particles may however also be affected by the overall “viscosity” of the emulsion which is
conditioned by the particle size distribution as a whole. Due to the relatively high concentration of
the dispersed phase, particles do often collide and may coalesce, which leads to the formation of

larger droplets. These coalescence phenomena are conditioned by the amount of available
emulsifier and its repartition between the interface and the water-phase. Particle size distribution
and residual emulsifier content appear thus again as the major potential explanatory parameters.
This analysis led to the conclusion that Particle Size Distribution ( PSD ) and Residual Emulsifier
Content should be the key parameters to be considered. Furthermore, Interfacial Tension
Characteristics were seen as key factors governing both parameters.
Indeed, together with binder viscosity, interfacial tension (which depends on the nature and the
“affinity “ of the used binder and surfactant) should be the main material parameter conditioning
particle size distribution. The residual amount of emulsifier (difference between the original amount
and the amount adsorbed at the interface) may theoretically be calculated from the total developed
surface of the binder droplets (which is easily obtained from the PSD) and the “polar head surface
area“ of the emulsifier. Also this last parameter may be derived from the so-called “adsorption
isotherm” which describes the evolution of interfacial tension with increasing concentrations of
surfactant.
These considerations have naturally led the research activities to be focused on the implementation
and further development of specific test methods and on the verification of the originally made
assumptions through the investigation of a number of laboratory made emulsions.
Interfacial tension measurements with a drop tensiometer
The principle of the drop tensiometer
method consists in deriving the interfacial
tension (via the Laplace equation) from the
digital analysis of the shape of a binder
droplet which is formed, by means of a
syringe, in a cell containing the surfactant
solution. The interest of the method is that a
prior dilution of the binder is not required if
measurements are done at a high enough
temperature. A French company (IT
Concept) has developed a suitable
equipment for bituminous binders which
allows to work at binder temperatures up to
90°C. The fact that this temperature is
reasonably close to emulsification conditions
and that possible artefacts due to the heavy dilution required by the Du Noüy ring method could be
avoided have motivated our interest for this method in the frame of the OPTEL project. The
extensive use of the equipment did however rapidly reveal a number of problems related to either
the method in general or specific to bituminous binders. Those could however be overcome and
resulted in a number of practical recommendations which are briefly summarised hereafter :
External pollution, by either surface active matters (e.g. sweat, ..) or other impurities which may
fix themselves on the drop and alter the image analysis, must be avoided through very strict
handling and cleaning precautions. Working at high temperatures for longer times may lead to a
partial evaporation of the water-phase, hence to uncontrolled variations in the actual surfactant
concentration. This difficulty could be overcome by the use of a cooled cover lid and frequent
renewals of the surfactant solution.

Density of bituminous binders at elevated temperature. The density differential between the binder
and the surfactant solution is the major factor conditioning the accuracy of the measurement. A
special attention has therefore been given to the measurement of the density of bituminous binders
at elevated temperatures. An existing method ( Bingham pycnometer method, ASTM-D1480) has
been adapted for that purpose.
Stabilisation time. It has been constantly observed that, after the drop has been formed, a certain
amount of time is required before the interfacial tension (γ ) reaches a stable value. This time is
particularly important when the droplet is formed into pure water (no surfactant) and may easily
exceed the hour. Stabilisation time as well as the difference between “instantaneous” and
“stabilised” γ values are significantly reduced as a function of the nature and concentration of
surfactant. These phenomena are supposed to be caused by surface active bitumen compounds
migrating to the surface and entering into competition with the surfactant molecules.
Interpretation of adsorption isotherms
A typical adsorption isotherm is shown in
Figure 3. As the surfactant concentration
increases, the interfacial tension decreases
with an increasing slope. A sudden
plateau is reached when the amount of
emulsifier reaches the “Critical Micellar
Concentration” (CMC). This theoretical
behaviour can be modelled by the “Von
Szyszkowski” equation in which Γ ∞ is
the maximum surface concentration (in
moles/m²) of the emulsifier :
⎛ c ⎞
γ − γ = − R. T. Γ . ln⎜1+
0
∞ ⎟
⎝ a ⎠
Interfacial Tension (mN.m-1)
50.0
40.0
30.0
20.0
10.0
0.0
ADSORPTION ISOTHERMS
0.0001 0.001 0.01 0.1 1 10
Log (c) (g/l)
Figure 2
"Instantaneous" values
After stabilisation
Γ ∞ is directly related to the slope of the adsorption isotherm at the CMC and allows to calculate the
minimum surface occupied by the polar head at the interface.
In the case of bituminous binders and industrial emulsifiers, this theoretical interpretation is
however made difficult by the presence of more than one species at the interface. Moreover, the
used emulsifiers are complex and do often incorporate more than one type of molecule. This may
explain why interfacial tension does often slightly increase again after the CMC, due to a selective
de-sorption of some emulsifier or bitumen compounds (Fig. 2). The experimentally determined Γ ∞
values and the corresponding minimum “surface area per polar head” are therefore affected by a
certain uncertainty. At the high emulsifier concentrations (several times the CMC) used for slowsetting
emulsions, it is likely that the surface active bitumen compounds can hardly compete with
the surfactant molecules. Our guess is therefore that the adequate Γ ∞ value to be considered is
closest to the one determined from the “instantaneous” adsorption isotherm as this curve is less
affected (especially at lower surfactant concentrations) by bitumen compounds. Unfortunately, it is
much more difficult to get reliable “instantaneous” interfacial values than stabilised values !

Titration of residual emulsifier
The determination of the residual emulsifier has mostly been done with the “EPTON” colour
titration method ( ISO 2871-2 or NFT 73-258 ). This method proved to be rather delicate as the
colour change is sometimes difficult to detect and may be perceived differently by different
operators. To ensure a sufficient accuracy, a number of precautions need to be taken, such as the use
of an adequate ratio between the volume of the solution to be analysed and the volume of titrant. It
is also recommended to keep solutions of known emulsifier concentration as a reference for the
detection of the colour change. The major issues with regard to the determination of residual
emulsifier content are however related to the separation of the water-phase from the original
emulsion. This is done via centrifugation and requests the control of two parameters :
- Centrifugation speed and duration need to be adjusted in such a way that one gets a clear
water-phase without breaking the emulsion. Best results were obtained with a two steps
process ( first separation under moderate acceleration followed by a more intensive
centrifugation of the recuperated solution ). In all cases, the PSD of the remaining
centrifuged emulsion should be checked so as to detect a possible bias due to coalescence.
- Certain emulsifiers ( e.g. diamines ) tend to crystallise at ambient temperature and the
formed crystals may stay in the centrifuged binder rich phase. Centrifugation should thus
always be conducted at a temperature above the crystallisation point of the emulsifier.
Prediction of residual emulsifier content
From the total developed surface area (derived from the PSD ) and the measured amount of residual
emulsifier, one may calculate the theoretical surface area (As) occupied by a surfactant molecule at the
interface. This value can then be compared to the one derived from the adsorption isotherm. Although
the As values are affected by some uncertainty (due to the cumulating of errors on PSD and on the
titration) they were, except in a few cases, consistently found to be quite low in comparison to the
values predicted from the “stabilised” adsorption isotherms. Supporting the earlier made interpretation
of adsorption isotherms, the polar head areas predicted from “instantaneous” isotherms are much closer
to the measured values. Polymer modification leads to higher specific surface areas (more fine
particles) and lower residual emulsifier contents but to a lower relative surfactant consumption. The
polar head surface area tends to increase ( Figure 3 ) ! Another interesting finding is related to the
evolution of As with manufacturing conditions and original emulsifier content. As tends to decrease
(more consumed emulsifier) when larger particles are created ( at lower rotation speeds or higher
temperatures ). As is even more substantially decreased for higher initial amounts of surfactant ( 0.6%
vs 0.3% ) (Figure 4). This suggests that some surfactant molecules may get « trapped » inside binder
droplets (formation of an inverted emulsion) during the emulsification process.
Area of Polar Head [ A² ]
160
140
120
100
80
60
40
20
0
B017
B017+EVA
Area of Polar Head - 65% Binder - Polyamine
B017+SBS
B020
B020+EVA
B020+SBS
Measured (mN.m-1) From "instantaneous" tension values (mN.m-1) From "stabilised" tension values (mN.m-1)
Polar Head Area (A²)
35
30
25
20
15
10
5
0
Area of Polar Head - Impact of Manufacturing Conditions
Unmodified Bitumen / Polyamine
165 °C 150°C 138°C
138°C
0.6% Emulsifier - 5000 rpm 0.6% Emulsifier - 8500 rpm
0.3% Emulsifier - 5000 rpm 0.3% Emulsifier - 8500 rpm
Figure 3 Figure 4

Prediction of Particle Size Distribution
Figure 5 shows the total developed surface
area in relation to the interfacial tension at
saturation ( γsat = tension at surfactant
concentrations above the CMC ). It is to be
mentioned that the pure bitumen do all
have a similar viscosity at emulsification
temperature. The fact that they all fall more
or less on the same line shows that the
variations in binder/emulsifier
combinations are indeed reflected by the
corresponding γsat values. The right trend
( lower γsat leading to a higher specific
surface ) is also observed for the polymer
40000
35000
30000
25000
20000
15000
10000
5000
modified binders. In this case, however, differences in
viscosity explain (to a large extent) the fact that the
lines are not confounded.
Surface Area ( cm²/ml binder )
0
Specific Surface Area / Interfacial tension
0 5 10 15 20
Interfacial Tension ( g sat ) (mN.m-1)
Figure 5
Unmodified Bitumen - Polyamine
Unmodified Bitumen - Diamine
EVA modified Bitumen - Polyamine
EVA modified Bitumen - Diamine
SBS modified Bitumen - Polyamine
SBS modified Bitumen - Diamine
These findings comfort the assumption that (under given manufacturing conditions) interfacial tension
and binder viscosity, irrespective of the nature of the binder and emulsifier, should be the main
parameters governing particle size distribution. These two parameters could thus be sufficient to
establish (from a limited number of actual emulsification trials ), empirical models allowing the
prediction of specific PSD characteristics ( e.g. specific surface, mean particle size, ..) for different
binder/surfactant combinations. Such a model would however have to be established for each new set
of operating parameters (temperatures, flow rates, rotation speed, ..).
Relationship between emulsion properties, particle size distribution and residual emulsifier
Emulsion viscosity. As explained in the introduction, most emulsions manufactured within OPTEL
were formulated with a binder content of 65% or less. A bit surprisingly, almost no differences in
viscosity could be evidenced. This was valid for both unmodified and modified emulsions, even if the
later showed considerable differences in their PSD (larger amount of fine particles and higher surface
areas). These findings suggest that at 65% binder content and lower, the STV method may not be
sensitive enough to evidence the impact of particle size distribution. This is apparently in contradiction
with practical experience since, even at lower binder contents, significant differences are sometimes
observed. This may however be due to other factors than PSD, for instance osmotic pressure resulting
from high salt contents in the bitumen, which were not represented in the OPTEL study.
Emulsion pH. All the studied emulsions have been manufactured from a water-phase at a pH of 1.8 or
2. The increase in pH after emulsification ranged between 0.2 and 2. This increase was found to be
mainly dependent on the origin of the bitumen. One may interpret the increase in pH as the result of
exchanges between the binder and the water-phase. As for the residual emulsifier content, a deeper
interpretation has been made by calculating the amount of “consumed H+ ions” (which are responsible
for the pH increase) and relating it to the total created surface. This analysis showed that polymer
modification seems to reduce these exchanges. As for the amount of residual emulsifier, it seems that
more exchanges occur (larger increases in pH) when larger particles are created ( impact of
manufacturing conditions ).

Breaking Index. The relation between residual emulsifier content and breaking index is shown in
Figure 6. The expected trend (higher breaking index for higher amounts of available surfactant) is
indeed noticed but a large scatter is observed as soon as the amount of surfactant exceeds a certain
value. Considering the original assumption that small droplets should be more difficult to “break”
(Laplace pressure), one may look at the total volume of the droplets greater than, for instance, 3µm.
Figure 7 shows that, above a certain volume fraction of larger droplets, the breaking index clearly
decreases with the total amount of large particles.
Breaking Index ( g / 100g emulsion )
180
160
140
120
100
80
Breaking Index / Residual Emulsifier
Unmodified Bitumen - Polyamine
Unmodified Bitumen - Diamine
EVA modified Bitumen - Polyamine
EVA modified Bitumen - Diamine
SBS modified Bitumen - Polyamine
SBS modified Bitumen - Diamine
60
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50
Residual Emulsifier ( g/l emulsion )
Breaking Index ( g / 100g emulsion )
180
160
140
120
100
80
60
50
45
40
35
30
25
20
15
10
5
0
Breaking Index / Binder Volume for D > 3µm
Unmodified Bitumen - Polyamine
Unmodified Bitumen - Diamine
EVA modified Bitumen - Polyamine
EVA modified Bitumen - Diamine
SBS modified Bitumen - Polyamine
SBS modified Bitumen - Diamine
0 100 200 300 400 500 600 700 800
Binder Volume for D > 3 µm ( g/l emulsion )
Figure 6 Figure 7
These findings suggest that the breaking behaviour, as measured by this specific method, is
predominantly governed by residual emulsifier content and the amount of larger particles. Both are of
course correlated to a certain extent since the generation of a large amount of small particles will
inevitably lead to a reduction in the residual emulsifier content. For the emulsions which have been
analysed within OPTEL, it seems also that the nature of the emulsifier is mainly acting through its
impact on the particle size distribution.
Storage Stability. The main parameter explaining
the storage behaviour was supposed to be the
volume fraction of the largest droplets, since those
are likely to settle the quickest. As it can be seen on
Figure 8 the results were however rather
disappointing. The right trends seem to exist but the
results apparently belong to different families. A full
explanation could not be found but one may
however notice that the worst decanting behaviour is
observed for :
- Emulsions with a low binder concentration
( 60% ) which one could explain by the fact
that more « free water » is available for the
decanting of the large droplets. This is
comforted by the low decanting observed for
some of the 69% emulsions.
Decantation ( %-% )
Storage Stability / Volume of droplets > 12 µm
69% Unmodified Bitumen - Polyamine
65% Unmodified Bitumen - Polyamine
65% Unmodified Bitumen - Diamine
60% Unmodified Bitumen - Polyamine
65% Modified Bitumen - Polyamine
65% modified Bitumen - Diamine
0 50 100 150 200 250 300 350 400
Volume of droplets > 12 µm ( ml )
Figure 8
- The polymer modified emulsions for which the poor stability is suspected to be related to the
low amount of residual emulsifier or to specific features of the particle size distribution.

Conclusion
With regard to “binder emulsifiability”, the main findings of OPTEL can be summarised as follows :
- Evaluation and implementation of test methodologies for the titration of residual emulsifier
content and the determination of interfacial tension characteristics ( drop tensiometer and Du
Noüy ring method ). The strengths and weaknesses of these tests have been identified and
resulted in practical guidelines for their use.
- The prediction of residual emulsifier content from the knowledge of particle size distribution
and the ad-hoc adsorption isotherm does not seem possible at this stage. This is due on one
hand to the difficulty to derive the right Γ ∞ value from the adsorption isotherm. On the other
hand, our investigations suggest that some emulsifier might get trapped inside the binder during
the emulsification process.
- The obtained data do however confirm the importance of particle size distribution and residual
emulsifier content for key emulsion properties. They also suggest that interfacial tension and
binder viscosity are indeed the essential parameters controlling particle size distribution (for
given manufacturing conditions).
The results of this study need of course to be confirmed on a wider set of products and emulsion
formulations. But the generated knowledge can already be converted into more efficient and
meaningful protocols for the assessment of binders, surfactants and emulsions. It does also provide
immediately applicable practical guidelines for the control of emulsion properties via particle size
distribution and emulsifier content.
Acknowledgements
This study has been performed in the frame of the European Research Program BRITE-EURAM
project “OPTEL” BE-1516 : “Slow-setting cationic bituminous emulsions for construction and
maintenance of roads” (http://www.cordis.lu/brite-euram). The development of the OPTEL project has
been made possible thanks to the financing contribution of the European Community (DGXII), in the
frame of the IVth Framework Programme BRITE-EURAM III. The OPTEL Consortium expresses
herewith his thanks for this contribution. The Consortium included following organisations : Probisa,
Entreprise Jean Lefebvre, Nynas, CECA, Laboratoire Central des Ponts et Chaussées (LCPC), Centre
de Recherche Paul Pascal (CNRS) and the University of Las Palmas ( Canarian Islands).
The authors would like to thank herewith Mrs. Juan José Potti, Alain Beghin, Bernard Brûlé, Bruno
Simaillaud, Fernando Leal-Calderon, Jean-Claude Fabre, Maurice Bourrel, Francis Verzaro for their
advice as well as Mr. A. Cagna (IT Concept) for its support on the interfacial tension measurements.
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