What is bitumen emulsion?
The basic definition of a bitumen emulsion is that it is a mixture of bitumen (or asphalt) and water. Since bitumen and water don’t generally mix, an emulsifying agent is used to bind them together. This is done under a carefully controlled conditions using highly specialized equipment and chemical additives. Depending on its applications, it may contain some other additives, such as improvers, anti-strips, and also break control and stabilizing agents.
To add to the reasoning behind the need of bitumen emulsion, we have to go to the history of bitumen. Bitumen is the oldest known thermoplastic material in use today. This is because of its thermoplastic property of being a thin liquid at high temperature and a virtual solid at ambient temperature. For many applications, these conditions are both essential and desirable allowing rapid throughput from hot bulk bitumen delivery through to final use without any reheating. However, there are limitations to the coating film thickness when using hot-applied techniques and for some applications, it is even necessary to apply at a lower temperature and/or viscosity. The reason why hot-applied techniques is unfavorable is because of the combination of the process requirements, the application techniques, safety and environmental requirements or the final product properties. Thus, there is a need to modify the properties of the bitumen so to achieve cold-application. The two methods mostly used are emulsification with water and dilution with hydrocarbon solvent enabling the bitumen to be applied cold, up to about 80°C for emulsions or up to about 150°C for cutbacks, depending on the dilution and solvent used.
Bitumen emulsion is heterogeneous, two-phase systems consisting of two immiscible liquids, bitumen and water, stabilized by a third component, the emulsifier. The bitumen is dispersed throughout the continuous aqueous phase in the form of discrete droplets, typically 0.5 to 5 microns in diameter, which are held in suspension by electrostatic charges.
There are three different classes of bitumen emulsions of which the first two, in volume terms, is by far the most important:
- Cationic emulsions;
- Anionic emulsions;
- Nonionic emulsions;
The terms cationic bitumen emulsion, anionic bitumen emulsion and nonionic bitumen emulsion refer to the overall particle charge on the bitumen droplet imparted by the emulsifier. If an electric charge is passed through an emulsion containing negatively charged particles of bitumen, the droplets will migrate to the anode; hence the emulsion is termed anionic. Conversely, positively charged particles will migrate to the cathode and these emulsions are known as cationic emulsions. Bitumen droplets in nonionic emulsions are neutral and consequently will not migrate to either pole. Such nonionic emulsions include clay emulsions. In 1906 SC hade van Westrum took out a patent covering the application of bituminous dispersions in water for road building. Initially, efforts were made to form emulsions by purely mechanical means, but it soon became apparent that whilst a dispersion could be formed by mechanical shear, the dispersion was not stable unless a third component, the emulsifier, was present. Initially naturally occurring organic acids in naphthenic bitumen were utilized. The addition of sodium or potassium hydroxide to the aqueous phase saponified (turned into soap) the acids and stabilized the dispersion.
Manufacturing of bitumen emulsion
Generally, to manufacture bitumen emulsion, there are two primary steps, which is the mixing of water and additives as the first step (this mixture is usually called mix-water), and the mixing of bitumen and the mix-water as the second step.
In the first step, water is mixed with carefully controlled and calculated amount of additives such as the emulsifier and the stabilizers. The type of emulsifier is depending on the ionic nature of the mix.
Then, this mixture is mixed together with bitumen. This is done in a mill called the colloidal mill. Depending on the application of the end product, the amount of bitumen to be added with the mix-water may range from 40-70%. The colloidal mill then breaks the bitumen into tiny droplets with the average diameter of around 2 microns. The emulsifier in the mix-water mixture crates a coating of surface charge around the bitumen droplets and helps them to be apart of each other. It also helps to keep these particles in a dispersed form.
The end product (bitumen emulsion) is then pumped to storage.
Typical bitumen emulsion designations
|Emulsion grade||Cationic||Anionic||set||Minimum asphalt content||Notes|
|QS-1h||*||Slow||57||Meet ASTM D3910|
|CSS-1hP||*||Slow||57||Whith 3% polymer|
|CQS-1h||*||Slow||57||Meet ASTM D3910|
|HFMS-2s||*||Medium||65||Whith 1-7% solvent|
|CRS-2P||*||Rapid||65||Whith 3% polymer|