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Components of Bitumen Emulsions

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:


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.
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Bitumen Emulsions

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