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Water is a major component of bitumen emulsions. It is the vector of the chemical energy provided by the surfactant and its role in the emulsion is a major element to consider, in close connection with the quality of the materials used.
Water represents most of the dispersing. During the preparation of the aqueous phase, it serves as vessel for the chemical equilibrium activating the emulsifier under the action of an acid for cationic emulsions, or of a base for anionic emulsions. This balance (Figure 1) must be mastered throughout the life of the emulsion in order to guarantee the expected shelf life as well as the breaking behavior.
Temperature, impurities contained in the water and exchanges with the bitumen phase all affect this equilibrium.
Water allows for the dispersion of bitumen droplets within a continuous phase with the help of an emulsifier. An increase in binder content (and therefore a decrease in the percentage of aqueous phase) strongly influences the viscosity of emulsions (Figure 2). That is why chip seal emulsions often have a higher binder content.
Emulsions that are too thin could run off the surface due to the slope and cause the application to fail. On the other hand, impregnation emulsions must be as fluid as possible to flow in between the aggregates. Of course, the water content (and binder content) is not solely responsible: particle size and mono-polydispersity play a fundamental role.
Role in unwanted phenomenons
But sometimes, things still go wrong ! Once produced, an emulsion has a given binder content. However the viscosity can still increase, which can lead to a pasty emulsion. Such observations are due to osmotic exchanges. The water penetrates the bitumen droplets by osmosis, and thus causes them to swell. Without any increase of the binder content, the volume fraction occupied by the bitumen phase increases, thus increasing the viscosity of the emulsion (because the amount of water available for dispersion decreases). Such phenomena are linked to the salt content of bitumen binders, which can vary depending on the origin of the crudes and the refining conditions. Very often, they come with a strong alkalinity, which can also affect the chemical balance of the emulsifier activation reaction. In addition to being pasty, sometimes not even pumpable, the emulsion becomes chemically unstable, the risk of breaking is high and an industrial incident is near.
Although workarounds (more or less easy to implement) exist, the preferred solution is anticipation. The formulator must take the greatest care in choosing the materials, particularly the binder used for the production. In other words, an “a priori” control is better than an “a posteriori” solution. Production of pilot batches using Laboratory Pilots Atomix Inside can be carried out to validate binder supplies. The performances measured are then compared with the values declared in the quality control system. Such an approach implies that the entity in charge of production and control has the appropriate laboratory equipment, and teams fully trained in these eventualities.
And for the most demanding laboratories, coupling these control methods with the use of analytical photocentrifugation (LUM Gmbh technology) makes it possible to precisely refine the formulas to have full control on the behavior of the binder once emulsified.
Vincent HESRY, Ph.D and Antonin RICHARD, Chemist Engineer.