Application conditions usually require bitumen to behave as a mobile liquid. Therefore, it becomes imperative that bitumen, which is a highly viscous substance, is converted into a low-viscosity liquid (able to flow freely).
There are three ways to make bitumen less viscous:
Heat it at high temperatures, >120 deg.C.,
Dissolve it in Solvents like Kerosene or Diesel, however, this process is not environmentally friendly,
We are interested in the last one. In simple terms Bitumen Emulsion is a liquid mixture, consisting of water and bitumen. In the mixture, the bitumen remains dispersed and suspended throughout the water phase in the form of globules (droplets). However, what would be confusing is the fact that bitumen (which is an oil product) cannot be mixed (is immiscible) with water. For the purpose of mixing these two components, we apply one or more additives to assist in the formation, stabilization and modification of properties of the resultant bitumen emulsion, which is known as ‘emulsifier’.
Bitumen Emulsion = Bitumen + Water + Emulsifier + Other Chemical Additives
The purpose behind adding an emulsifier to the mix is to reduce the interfacial tension between bitumen and water, which it does by positioning itself at the bitumen-water interface, thus reducing the imbalance of intermolecular forces at the boundary of two immiscible liquids.
Emulsifier particles consist of large Hydrophilic heads which are wetted in the water phase and flexible Hydrophobic tails which are embedded in the bitumen. They are responsible for providing ionic properties or the charge to the emulsion. Emulsifiers provide stability to the emulsion by not allowing the bitumen to flocculate.
The stability of the emulsion occurs due to the simple laws of physics – “like forces repel each other”. The bitumen droplets attain a positive or negative charge based on the emulsifier used. The identical charges on the bitumen droplets create electrostatic forces of repulsion between them, stopping them from coalescing with each other and allowing them to remain in suspension.
At ART, we employ an array of Emulsifiers to develop the suitable emulsion properties tailor-made to a particular application and site. This enables us to improve the performance of our products, and maintain cost effectiveness at the same time.
Based on Surface Charge
Based on Setting Time
On the basis of overall particle charge on the bitumen droplet that is imparted to the emulsion by the emulsifier, bitumen emulsions are classified into – “Anionic Emulsions” possessing a negative charge and “Cationic Emulsions” having a positive charge.
The charge on emulsion that should be used for application would depend on the mineral composition of the aggregates being used for construction.
Mostly, aggregates in India possess a net negative charge. Therefore, cationic emulsions are the type mostly used. When the positively-charged emulsion comes in contact with the negatively-charged aggregate, the bitumen particles get attracted to the aggregate surface due to the nature of charge present and begin to travel towards it. As the positively-charged bitumen (due to the emulsifier) reaches the surface, the charges get neutralized and the bitumen begins to coalesce on the aggregate surface, forming a continuous layer. This is known as the breaking of an Emulsion. Once this destabilization is complete, i.e. the bitumen is separated from the water; the water begins to evaporate causing the bitumen and the aggregate to develop an adhesion to each other.
The time taken for this process of breaking to occur is determined by the type of emulsifier used, and it indicates the “setting time” for the emulsion.
On the basis of setting time, bitumen emulsions are said to be of three types:
Rapid Setting Emulsions
Medium Setting Emulsions
Slow Setting Emulsions
Bitumen Emulsions are used for a wide variety of purposes, finding their application from surface dressing of pavements to Stabilization and Microsurfacing. Increasing concerns about energy conservation and protection of the environment have led to the generation of interest in Bitumen Emulsions.
Stay tuned for more knowledge headed your way.