Better Roads Staff
Either way, the emulsifier producer applies the charge to the emulsifier molecule by making changes in the chemistry of the head section. This is accomplished by adding chemicals to the water, asphalt and emulsifier as it goes through the colloid mill.
Addition of the strong base sodium hydroxide to the mill produces a fatty acid sodium salt in the head with a negative charge, resulting in an anionic emulsion. When the materials are reacted with the strong acid hydrochloric acid, a fatty amide hydrochloride salt in the head with a positive charge, and a cationic emulsion, is produced.
Why use a cationic emulsifier instead of an anionic? Use of the right emulsifier ensures adhesion to aggregates. With rock chips used in chip seals, for example, the aggregates used will have a slight negative or positive charge; for good adhesion, use a cationic emulsion for stone that bears a negative charge, and anionic for stone that bears a positive charge.
Igneous chips of silicate mineralogy like granite, trap rock or basalt generally are negatively charged and react best with a cationic (positive) emulsion; positively charged sedimentary rocks of carbonate mineralogy like limestone or dolomite react best with an anionic (negative) emulsion.
The surface texture and microporosity of the aggregate will help the asphalt bond mechanically, whereas the chemical bond imparted by the negative/positive attributes seems to be more important when slick, siliceous gravels are used, which are best held with a cationic emulsion. Cationics tend to work with both good and bad aggregates, experts say; the anionics tend to work best with good limestone aggregates or those with lots of surface texture.
After placement on a pavement, the emulsified asphalt sets or “breaks” as the asphalt droplets precipitate or fall out of the water suspension and coat the aggregates. At this time the color of the emulsion turns from brown to black. As described by the Texas DOT, as more and more water is lost through evaporation, the particles are forced closer and closer together until they can no longer be separated by a film of water. At this point droplets coalesce into larger and larger drops and ultimately a sheet of asphalt on the road.
For rapid-setting emulsions, this process begins when they hit the chips. For medium-setting emulsions, the break starts sometime after mixing with the aggregate, depending upon the emulsion and the amount of fines in the aggregate. Slow-setting emulsions are very stable and normally break with the evaporation of the water.
Coalescence and Flocculation
The electrostatic charges on the heads of the emulsifier molecule sometimes may not be enough to keep the asphalt droplets apart. “The asphalt droplets in the emulsion have a small charge,” says Alan James, performance applications specialist, AkzoNobel Surface Chemistry, in his paper Overview of Asphalt Emulsion, Transportation Research Circular E-C102, Asphalt Emulsion Technology, August 2006.
“The source of the charge is the emulsifier, as well as ionizable components in the asphalt itself,” James says. “These small charges on the droplets normally provide an electrostatic barrier to their close approach to each other (like charges repel). However, when two droplets do achieve enough energy to overcome this barrier and approach closely then they adhere to each other (flocculate). This flocculation may sometimes be reversed by agitation, dilution or addition of more emulsifier.”
That’s why it’s important to use asphalt emulsions soon after they are produced. “Over a period of time the water layer between droplets in a floccule will thin and the droplets will coalesce,” James says. “The coalescence cannot be reversed. Factors that force the droplets together such as settlement under gravity, evaporation of the water, shear or freezing will accelerate the flocculation and coalescence process, as does anything that reduces the charge on the droplets. Lower viscosity asphalts coalesce more rapidly than high viscosity asphalts.”
An “alphabet soup” of letters designate the types of asphalt emulsions that are available, but it need not be confusing, as the letters correspond to the attributes of the emulsion.
Cationic emulsions begin with a “C.” If there is no C, the emulsion is usually an anionic, reports the Asphalt Institute (AI).
Emulsified asphalts come in rapid-, medium-, and slow-setting grades for different applications and are developed through the use of different emulsifying agents and the addition of some solvents. Still, their asphalt droplets particles will be either anionic or cationic. Rapid-setting emulsions are used mostly for chip sealing, while the medium- and slow-setting grades are used for emulsions mixes for recycling, fog seals or tack coats placed in advance of asphalt lifts.
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