Often times the
distinction
given to
particle sizes
of materials
also reflects
its ability to
perform better
in the context
of bituminous
cold
technologies
for roads and
specifically in
applications of
neat and
polymer
modified
bitumen
emulsions. It
is very common
in most
scientific
conversations
to mention the
importance of
particle size.
To answer the
question posed,
this discussion
supports the
notion and
narrative that
that size
matters. The
contestations
for why size
matters seek to
evaluate
microemulsions
as the length
scale prefix to
the word
emulsion
focuses
specifically on
the
micro-scale.
Key is the
understanding
of
microemulsion
constituents
which are
macroscopically
homogenous
mixtures of
water, bitumen
and
surface-active
agent.
Microscopically,
the mixture is
made up of
water-abundant
and
bitumen-abundant
phases
separated by an
amphiphilic
surface-active
film.
This
article will
focus on
understanding
the components
of
microemulsions,
and their
formulation and
behaviour as is
related to
applications in
cold mix
asphalt through
cold bitumen
emulsion slurry
dispersions.
Background
of Emulsion
Science
Historically,
the first
documented
extraordinarily
stable emulsion
was formulated
by Ramsden and
Pickering in
1901 and was
known as the
Pickering
emulsion, which
used solid
particles at
the interface
of two
immiscible
liquids.
Schulman and
coworkers
originally
proposed the
word
microemulsion
although the
first
publication had
appeared in the
early 1940s.
Development of
surfactant
science in the
1960s which
focused on bulk
phases paved
way for
microemulsion
research in the
1970s and
1980s.
Microemulsions
bear their name
from the
diameter of
emulsion
droplets which
are determined
by the amount
of surfactant.
It consists of
thermodynamically
stable oil
droplets of
diameters
ranging from
100 to 1000A.
Figuratively,
the interfacial
tension at the
amphiphilic
surface-active
film is less
than 0.0001
mJ/m relative
to just an
oil/water
interface
without the
surfactant
which is 50
mJ/m. Clearly
the role of the
surfactant is
critical in
decreasing
interfacial
tension which
leads to a
significant
reduction in
capillary
retention force
and allows for
oil droplets to
deform and move
easily through
pores allowing
for better
penetration and
binding. Given
this background
the options of
microemulsions
can either be
oil/water or
water/oil both
determined by
volume fraction
of the
dispersed and
continuous
phase. Bitumen
or
polymer-modified
bitumen is
represented as
the oil for
applications
unique to the
context of the
discussion.
Micro
vs MacroThe
knowledge of
microemulsion
phase behaviour
can lead to a
direct
correlation
with
macroemulsion
properties. A
good example of
the differences
between
microemulsions
and
macroemulsions
is that the
former has
droplets that
are too small
to scatter
light whereas
the latter has
large enough
droplets to
scatter light.
Therefore,
microemulsions
are transparent
whereas
macroemulsions
are opaque. In
order to
understand the
molecular
behavior of
microemulsions
the phenomenon
of
self-diffusion
is key in the
transition from
an oil in water
to a water in
oil droplet
structure and
support for
bicontinuous
structures at a
molecular
level. At high
temperatures
the water in
oil structure
is dominant.
Emulsion
breaking which
is at a
macroscopic
level can
happen through
coalescence
driven by a
decrease in
surface area
with a
concurrent gain
in surface free
energy,
nucleation as a
result of
droplet
proximity and
Ostwald
ripening in
which few
emulsion
droplets grow
larger at the
expense of the
majority
shrinking and
disappearing.
They are three
types of
bitumen
emulsions with
the
classification
based on
setting on
contact with
unbound
granular
material
surfaces.
Rapid-setting
emulsions are
very reactive
and set quickly
when in contact
with clean
aggregates of
low surface
area such as
chip seal
chippings for
surface
dressings.
Medium-setting
emulsions set
slightly slower
than
rapid-setting
and are
commonly used
in open-graded
mixes. Lastly,
slow setting
emulsions used
with reactive
aggregates of
high surface
area.
Emulsion
Formulation and
Application
Formulation
of conventional
bitumen is
performed in a
colloidal mill,
with hot liquid
bitumen at
1400C and
aqueous
surfactant are
mixed for a
very short
period of time
and pass
through a
rotor-stator
system with
very miniscule
openings of
less than a
millimeter and
high shear of
up to 3000
revolution per
minute. On the
other hand,
formulation of
high internal
phase ratio
bitumen
emulsions which
lead to bitumen
microemulsions
is performed in
special
reactor. The
bitumen is
placed in the
reactor until
reaching a
uniform
temperature of
900C whilst the
aqueous
surfactant
phase is
prepared at the
selected
concentration
and pH set at
2.5 using
hydrochloric
acid. Once the
bitumen has
reached the
required
temperature,
the aqueous
phase is
introduced at
low agitation
then slowly
increased to
680 rpm and is
maintained
until the
desired
particle size.
Water with an
acid pH is used
to dilute to
60% bitumen
content.
Typically for
cationic
bitumen
emulsions the
average
diameter is
about 3 to 6
micrometers.
Slurry
or
microemulsion
is the cold mix
asphalt
specific
application of
bitumen
emulsion either
in their neat
or modified
form in
microsurfacing
placed over the
cheap seal to
eliminate risks
associated with
loose chips as
well as
achieving the
desired surface
texture.
Improved
coating quality
in cold
recycling can
be achieved
with recycled
asphalt
pavement
millings when
bitumen
emulsion is
used since it
provides a
greater
specific
surface area.
Preference for
cold recycling
is because
majority of the
projects are
performed
in-place
rendering it
economical as
opposed to
plant recycling
which gives
better results
with regards to
homogeneity.
Grave emulsions
originally
developed in
France use
bitumen
emulsion as a
binder at low
binder content
than other cold
mix asphalt. It
finds its
applications in
base courses
and wearing
courses for
light traffic
loads.
So
it does
matter!
As
well as their
functionality,
the benefits of
using
microemulsions
in cold mix
asphalt
technologies
include reduced
fuel usage and
greenhouse gas
emissions which
conforms with
sustainable
development,
better working
conditions and
improved field
compaction.
Practically
speaking, the
fact that
aggregates do
not require
heating as
mixing and
laying can
occur at
ambient
temperature
renders the
technology to
be more
environmentally
friendly.
However, it is
important to
mention that
cold asphalt
mixtures
contain
substantial
air-voids that
could possibly
trap enough
water to impair
short and
long-term
bonding between
the aggregates
and bitumen
leading to a
weakened
asphalt layer.
All things
being equal,
cold mix
asphalt and
cold bitumen
emulsion slurry
dispersions
would not
practically
work if size
was not of the
essence – so
we can state
that ‘Yes,
size does
matter’.
REFERENCES
- Khan, A. 2016. Fundamental Investigation to Improve the Quality of Cold Mix Asphalt. Thesis from KTH Royal Institute of Technology.
- Kumar, P., and Mittal, K.L. 1999. Handbook of Microemulsion Science and Technology. Marcel Dekker, Inc.
- Lesueur, D., Herrero, L., Uguet, N., Hurtado, J., Pena, J.L., Potti, J.J., Walter, J., and Lancaster. 2006. Bitumen nanoemulsions and their application in the cold recycling of asphalt mixes. Revue Gėnėral Des Routes Des Aėrodromes. 850. 79-81.
- Prince, L.M. 1977. Microemulsions: Theory and Practice. Academic Press, Inc. New York.
- Salomon, D.R. 2006. Asphalt Emulsion Technology. Transportation Research Circular E-C102.
- Schick, M.J., and Hubbard, A.T. 2009. Surfactant Science Series: Microemulsions Properties and Applications. CRC Press, Taylor & Francis Group.
- Shinoda, K., and Kunieda, H. 1973. Conditions to Produce So-called Microemulsions: Factors to Increase the Mutual Solubility of Oil and Water by Solubilizer. Journal of Colloid and Interface Science. 42:2. 381-388.
- Sjöblom, J. 2001. Encyclopedic Handbook of Emulsion Technology. Marcel Dekker, Inc. New York.
- Strey, R. 1994. Microemulsion microstructure and interfacial curvature. Colloid Polym Sci. 272. 1005-1019.
- Stubenrauch, C. 2009. Microemulsions: Background, New Concepts, Applications, Perspectives. First Edition. John Wiley and Sons, Ltd. Oxford.
- Xing, F. 2016. A fundamental research on cold mix asphalt modified with cementitious materials. Doctoral thesis from EIT Zurich.