What is bitumen emulsions?
Whatever the end use, application conditions usually require bitumen to behave as a mobile liquid. In principle, there are three ways to make a highly viscous bitumen into a low-viscosity liquid:
- Heat it
- Dissolve it in solvents
- Emulsify it
Bitumen emulsions are two-phase systems consisting of bitumen, water and one or more additives to assist in formation and stabilization and to modify the properties of the emulsion. The bitumen is dispersed throughout the water phase in the form of discrete globules, typically 0.1 to 50 micrometer in diameter, which are held in suspension by electrostatic charges stabilized by an emulsifier.
Bitumen emulsions can be divided into four classes. The first two are, by far, the most widely used:
- Cationic emulsions
- Anionic emulsions
- Non-ionic emulsions
- Clay-stabilized emulsions
The terms anionic and cationic stem from the electrical charges on the bitumen globules. This identification system originates from one of the fundamental laws of electricity-like charges repel, unlike charges attract. If an electrical potential is applied between two electrodes immersed in an emulsion containing negatively charged particles of bitumen, they will migrate to the anode. In that case, the emulsion is described as “anionic”. Conversely, in a system containing positively charged particles of bitumen they will move to the cathode and the emulsion is described as “cationic”.
The bitumen particles in a non-ionic emulsion are neutral and therefore, will not migrate to either pole. These types of emulsion are rarely used on highways.
Clay-stabilized emulsions are used for industrial rather than for road applications. In these materials, the emulsifiers are fine powders, often natural or processed clays and bentonites, with a particle size much less than that of the bitumen particles in the emulsion. Although the bitumen particles may carry a weak electrical charge, the prime mechanism that inhibits their agglomeration is the mechanical protection of surface of the bitumen by the powder together with the thixotropic structure of the emulsion which hinders movement of the bitumen particles.
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