Components of Bitumen
Emulsions
Bitumen emulsions are
heterogeneous mixtures that often consist of two immiscible liquid phases,
bitumen and water, and are stabilized with an emulsifier. The stabilized
emulsifier disperses the bitumen throughout the continuous aqueous phase as
discrete particles that are often 1.0 to 10-microns in diameter. Bitumen is
often dispersed in the suspension using the electrostatic charges that an
emulsifier transfers to bitumen particles.
Bitumen emulsions can be
categorized into four groups:
• Cationic Emulsions
• Anionic Emulsions
• Nonionic Emulsions
• Clay-stabilized Emulsions
The first two types have the most
applications. The terms anionic and cationic refer to the electrical charges on
bitumen globules. This naming system is derived from one of the fundamental
laws of electricity, which states that like electrical charges repel each other
while unlike electrical charges attract.
If the electrical potential
between two electrodes floating in the emulsion containing negatively-charged
bitumen particles, they will move toward the anode electrode. In this case, the
emulsion is described as “anionic”.
However, if the system contains
positively-charged bitumen particles, they will move toward the cathode and the
emulsion is described as “cationic”. The
bitumen particles in an nonionic emulsion are neutral and will not move toward
either pole. This emulsion type is scarcely used in road construction.
Instead of road applications,
clay-stabilized emulsions are used for industrial applications. In these
materials, emulsifiers are fine natural or clay-processed particles, and
bentonites are much smaller than the emulsion's bitumen particles. Although
bitumen particles may have a weak electrical charge, their main mechanism which
prevents their accumulation is mechanical protection of the bitumen surface
using the powder and the thixotropic emulsion structure that prevents the bitumen
particles’ movement.
Emulsions’
Applications
Emulsifiers perform several functions inside the bitumen
emulsion. They facilitate emulsion
production by reducing the interfacial tension between bitumen and water.
They determine whether the
emulsion is water-in-oil or oil-in-water.
They stabilize the emulsion to
prevent particle coagulation.
They determine the emulsion’s
functional properties, including breaking time and adhesion.
Emulsifier is the most important component
of bitumen emulsions. For added effectiveness, the emulsifier should be
water-soluble with a correct hydrophilic (attraction to water) and lipophilic
balance (attraction to lipids (organic compounds that are water-insoluble but
soluble in organic solvents and fats)).
Emulsifiers are used separately or in mixtures to achieve specific
properties.
The emulsifier's ionic part is
present on bitumen particles, while the hydrocarbon chain is guided toward the
bitumen surface and strongly attaches to it, as illustrated by the following
figure.
Bitumen emulsions could also be
emulsifier independent, which could influence their final properties,
especially their breaking and adhesion properties. The emulsifier's ionic part
transfers its charge to the particles, and the opposite ions such as sodium or
chloride are dispersed in the aqueous phase.
Emulsification agents are large
organic molecules that have two separate parts, namely “head” and “tail.” The head
section contains a group of atoms that have positively-charged and
negatively-charged regions in terms of chemistry. These two regions cause the
head to become polar (e.g. magnetic poles).
The head is water-soluble due to
polarity and the nature of certain atoms present on the polar head. The tail is
an organic group with a long chain that is water-insoluble but soluble in other
minerals such as oil (bitumen). It is therefore a molecular emulsifier with
water-soluble and oil-soluble parts. This unique property provides it with the
ability to emulsify chemicals.
Schematic View of Electrical
Charges on Bitumen Particles
An
anionic emulsion contains billions of bitumen particles with the emulsifier
bonded to the water and bitumen. The tail
section balances its emulsifying agent toward the bitumen, while the positive
head section rotates in the water and leaves the rest of the head with a
negative charge on the drop surface, and then transfers the negative charge to
all particles. The following serves as an example.
R – COONa +
The cationic emulsifying agent
has a similar function to the anionic emulsifying agent.
The negative head section floats
in the water and creates a head with a positive charge, which is then
transferred to all particles.
Positive charges repulse each
other, as do all particles which remain suspended as bitumen particles. The
following is an example:
R–NH+ Cl−
In nonionic emulsifiers, the hydrophilic covalent head group (a
chemical bond involving the division of electron pairs between atoms) is polarized
and dissolved without ionization (a process in which an atom or a molecule
gains a negative or positive charge or gains or loses electrons), and any
electrical charge on the emulsion droplets are obtained from the ionic types in
the bitumen itself. The following is an example:
R–COO(CH2CH2O)xH
In ordinary emulsifiers with the aforementioned chemical
structure:
R represents the emulsifier’s
hydrophobic part (water repulsion) that often consists of a hydrocarbon with a
long 8-22 atom carbon chain comprised of natural fats and oils, such as animal
fat or oils like alkylbenzenes.
The hydrophilic head group may
include various amines, sulfonates, carboxylates, ether and alcohol.
Emulsifiers with multipurpose head groups that contain more than one of these
types are used widely.
Complex wood-derived emulsifiers
include vinsol resin, tannins, and lignosulfonates, which include polycyclic
hydrophobic moieties and several hydrophilic centers. Proteins like blood and
casein have also been used in bitumen emulsifiers. In general, these molecules
have complex structures, as shown by the figure for the lignosulfonate
structure.
The majority of emulsions
produced around the world are cationic, and they are produced from the
following types or mixtures: As the figure shows, monoamines, diamines,
quaternary ammonium compounds, amines and amidoamines, alkoxylate amines and
others can be used structurally.
Most of these materials are
produced in a preliminary neutral form and require reaction with acids to be
converted into water-soluble and natural cations.
Therefore, cationic emulsions
often have an acidic pH = 7. Hydrochloric acid is often used for this purpose
and reacts with the nitrogen atom to form ammonium ions. This reaction can be
stated as follows:
R−NH2 + HCl −−−"
R−NH+Cl-
Lignosulfonate Structure
The Chemical Properties of Cationic Emulsions (R=C8-22)
Although quaternary ammoniums do
not require acids since they are water-soluble salts, the emulsion’s
performance can be changed if necessary by adjusting the aqueous phase’s pH
with the acid. Anionic emulsifiers are the second most produced emulsions in
terms of volume and are often stabilized with fatty acids or sulfonate
emulsifiers.
Fatty acids are water-insoluble
and become soluble after reaction with a base, typically sodium or potassium
hydroxide, such that alkaline anionic emulsions are pH 7 (Cationic emulsions are
often pH 2-3, while anionic emulsions are pH 10-11).
A pH=7 liquid is neutral and
neither an acid nor an alkaline.
Sulfonates are often produced as
water-soluble sodium salt. While further neutralization is not necessary, a
large amount of sodium hydroxide is used to maintain the emulsion’s pH above 7
and neutralize the bitumen’s natural acids.
Nonionic emulsifiers are not
produced in significant amounts and are only used for modifying anionic and
cationic emulsions. Common nonionic emulsifiers include nonylphenol ethoxylates
and fatty acid ethoxylates.
Bitumen
Emulsion Production
Most bitumen emulsions are
produced with colloid mills using a continuous process (a machine used for
reducing the particle size of a solid suspended in a liquid or reducing the droplet
sizes of one liquid suspended in another).
This equipment has a high-speed
rotor that operates in a stator at 1000 to 6000 rpm. The distance between the
rotor and the stator is often 0.25-0.50 mm and adjustable.
Hot bitumen and emulsifier
solutions are separately yet simultaneously fed into the colloid mill, and
their temperature is crucial for the process. The viscosity of the bitumen that
enters the colloid mill should not exceed 0.2 PA.S, which is achieved with
100-140 °C bitumen in emulsions.
In order to avoid boiling the
water, the aqueous phase is adjusted to result in an emulsion temperature that
is lower than 908 °C. After the bitumen and emulsifier solutions enter the
colloid mill, they are subjected to tremendous shear forces that break the
bitumen into small globules. Individual globules are coated with the
emulsifier, which electrically charges droplet surfaces. The resulting
electrostatic forces prevent globule accumulation.
Read more at:
Bitumen Emulsions