Better Roads Staff
“When you emulsify asphalt, you are creating a vehicle that will transport asphalt without having to heat it, or use high mechanical energy to spread it in a thin layer, or coat the surface of a rock,” says Codrin Daranga, Ph.D., technical manager, Blacklidge Emulsions. “The water part or side of the emulsion is nothing more than a vehicle. It is used to get the asphalt in place, and then it’s gone.” And once the water is gone, all that’s left is the residual asphalt, traces of the emulsifier, and traces of other additives.
“We emulsify asphalt to reduce its viscosity to make it liquid at ambient temperatures,” Bickford says. “Benefits include reduced energy use and costs, as not as much heat is required to get the asphalt usable. Worker exposure is less as they are not exposed to vapors and fumes from either the asphalt itself, or from cutback asphalt solvents. And there is less opportunity for burn hazards and jobsite odor, which is important in residential areas.”
In addition, experts say, if you properly formulate the emulsion based on the kind of materials involved, the complete system will have long-term performance benefits that a conventional hot mix asphalt mix will not have. Lastly, emulsions allow expensive liquid asphalt to be spread more thinly, saving valuable road maintenance funds.
Powerful blenders or colloid mills create asphalt, or polymer modified asphalt, emulsions. The mill consists of two parts: a stationary element called the stator, and a rotating part called the rotor. A small gap separates the two, on the order of a millimeter in diameter, up to 75/1000 of an inch across. Hot liquid asphalt, water and an emulsifying agent are brought together in the mill, where the spinning blades of the rotor break or shear the liquid asphalt against the stator into suspended micro-droplets. The asphalt globules first elongate, then break into two or three smaller particles, which themselves break up into even smaller particles. The dwell time within the mill is measured in thousandths of a second.
The emulsifier – commonly a surfactant or surface-active agent – maintains the microscopic asphalt droplets in a stable suspension within the water, keeping them from recombining. Emulsifiers also control “break” time following placement on a road, in which the water evaporates, leaving the asphalt behind
“Anybody who works with liquid asphalt knows it sticks to everything,” Bickford says. “It also wants to stick to itself. The moment you produce the asphalt particles, and they are close to each other in the emulsion, they will try to stick back together, and the emulsion as separate particles won’t last very long.”
To keep the asphalt droplets apart, a chemical additive, or emulsifier, is used to coat the droplets, and protect them from each other. The average droplet size is from 3 to 7 microns in diameter, and asphalt usually is 57 to 70 percent of the emulsion, less than half being water, Bickford explains.
One gram of asphalt – about the size of a sugar cube, Bickford says – will form about 10 billion particles with a total surface area of 1 to 2 square meters, yet in an asphalt emulsion a third or less of the volume is water. The emulsifier must protect or stabilize all this material. “One drop of emulsifier will stabilize 100 billion particles, for a surface area of 20 square meters, or a 14-by-14-foot square,” Bickford says.
Chemistry of Emulsions
There are hundreds of emulsifiers available, each chemically a little different from the other, but they all have elements in common. Part of an emulsifier molecule is oil-soluble, and another is water-soluble. And the molecules will have a characteristic – most often an electrical charge – that keeps the asphalt surfaces from rejoining.
Typically the emulsifier molecule will be in the form of a head-and-tail, in which the long and skinny tail group is oil-soluble, with the properties of a hydrocarbon. The head will be water-soluble, with carbon and hydrogen atoms, but also contain nitrogen, oxygen or other elements. “It’s one big molecule, but has two different characteristics,” Bickford says.
“There is something about the nitrogen and oxygen that causes the head to want to hold an electric charge,” he adds. “Nitrogen kind of likes to hold a positive charge, and oxygen likes to hold a negative charge. And the charge makes the ‘head’ part of the molecule water-soluble.”
The oil-soluble tail portion of the molecule is derived from a fat source, for example, tallow or beef fat, or vegetable fat, like coconut oil. “These fats have chemically reactive sites on them that allow us to test the head groups and change the way they perform,” Bickford says. And that includes giving the head an electrical charge.
There are two main types of emulsions, cationic (positively charged) and anionic (negatively charged). Because like charges repel, droplets bearing the positive (cationic) emulsifier will repel each other, keeping them from recombining, thus providing a storage-stable asphalt emulsion product. The same is true for droplets bearing the negative (anioic) emulsifier.
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