Controlling Emulsion Viscosity and Residue

By: S. Brewster, A. Walker

Emulsion viscosity is primarily controlled by the amount of asphalt (or “dispersed phase”) within an emulsion – the more asphalt, the higher the emulsion viscosity. Plain and simple! This is why chip sealing grades of emulsion, such as CRS-2, carry higher emulsion “residue” requirements - a surrogate measure of the amount of asphalt in a given emulsion. Greater asphalt content yields greater emulsion viscosity. Greater emulsion viscosity ensures chip embedment and reduces loss due to emulsion runoff. Still, wild fluctuations can occur due to processing. While base asphalts have caused emulsion viscosities to grow out of specification, equally as frequent are low viscosities due to either processing or formulation parameters. Included below are some of the inputs that can be tweaked to ensure a good performing emulsion.

First and foremost, make sure experienced eyes are looking at the emulsion and that a good QC program is in place during manufacturing. When inexperienced operators are faced with a low viscosity emulsion, they may not realize that the emulsion may be “falling apart”, manifesting itself as low viscosity. As an example, not long ago a producer of an RS-2 emulsion had a viscosity of only 38 SFS which was reported as “passing all other specifications”. Viscosities were never run off of the mill and so there was no way of knowing if the emulsion had lost viscosity or if it never had it in the first place. After another 24 hours in the tank, the emulsion had a lot of sieve and a low pH. It was later determined that not enough caustic soda was used during manufacturing, thus causing the viscosity to plummet.

A change in asphalt source can also lead to low emulsion viscosity. Just because source A was capable of hitting spec viscosity at 67% residue, does not mean that your next delivery will be the same. Source B may produce very thin emulsion at 67% residue, but a slight increase to 67.5% may do the trick. 

Order of addition of modifiers can also lead to swings in viscosity. Latex is the big one that comes to mind. When added to the soap phase, viscosities tend to be lower. When injected into the asphalt line, viscosities tend to be higher. This could be a good way to hold residues constant if you have the capability to add on both sides of the emulsion mill.

Mixing emulsions can also cause emulsion viscosity to fall. Occasionally circulation of the emulsion through pumps, in and out of tanks, is used to keep an emulsion mixed. With each pump pass, the emulsion can lose viscosity. Without emulsion tank mixer blades, the best course of action here would be to only produce as needed to avoid holding the emulsion for any length of time. 

In terms of high (or growing) emulsion viscosity, salts can be very effective. Here, small amounts of calcium chloride in cationic emulsions or sodium chloride in anionic emulsions can do the trick when incorporated at 0.05-0.20% by weight of emulsion (that is, about 25-200 lbs per 6,000 gal finished emulsion).

Lastly, modifiers can help control viscosity. For some low residue emulsions, a “thickener” or “stabilizer” is necessary to not just get the viscosity for the specification, but to also keep the emulsion together. Tracking the total use of all chemicals and asphalt used in your formulations is important to ensure repeatable quality in your manufacturing process. Some of your chemical packages will add to the overall residue. When investigating a bad emulsion run, it is important to separate the solids provided by chemical vs. the solids from your asphalt base. At times, you may find that you thought your total residue was normal, when in actuality you had overloaded on either the chemical side or the asphalt side. “Asphalt content” is not the same as “residue %”!