Emulsion Chemistry is About Control
by Glynn Holleran
Use a deliberate system to manufacture the emulsion you need
Manufacturing the right emulsion for a project depends on what you expect the emulsion to do for you. The combination of method of manufacture, asphalt, emulsifier, and storage and handling determines how coarse or how fine the emulsion will be, so you have to choose your course of action early on to get the desired result.
Asphalt: Choose which asphalt you want to use based on its final application and performance properties. In durability tests, emulsifiers have been shown to increase durability of asphalt, but they may also soften the final binder. Select your asphalt and emulsifier based on the performance requirements, such as rheological, or flow-of-material, requirements. For example, choose higher penetration asphalt for cooler conditions, polymer modified (latex) for cool and hot conditions, and harder asphalt for high temperatures.
Besides performance requirements, researchers must take the method of manufacture into consideration, too. Oxidized asphalt contains depleted levels of aromatic oils and polar aromatic (resin) fractions. This means that the emulsification could be harder to achieve, thus emulsion produced with oxidized asphalt could be less stable than emulsion produced with straight-run asphalt (non-oxidized).
PPA-based asphalt has very high levels of aromatic resins, but relatively low amounts of napthenic and paraffinic oils. This means that you usually get an easy and rapid emulsification with the PPA-based asphalt, but the emulsifier tends to move out of the asphalt and destabilize the emulsion over several days.
In cases where instability could occur, consider using co-emulsifiers or adding aromatics or other chemicals to the mix. Straight-run materials are almost always preferred for emulsification.
Water: The water must be potable. High levels of dissolved salts are not generally a problem in mostcationic emulsions; however, metal salts such as iron chloride, iron oxide and similar materials may cause precipitation of some fatty, diamine-type emulsifiers. Citric acid added at 0.1 to 0.3 percent is one solution to this problem. Otherwise, the water should be clear of suspended clay or any organic materials.
Additives: There are certain additives to consider for specific problems. The source of the problem has to be clearly defined to choose the correct additive. Here is an abbreviated list of additives, and issues they address, from which to choose.
Forming the emulsion
Once you know what problem your emulsion should solve, the method of manufacture comes into play. The basis of emulsification is the creation of small (1 to 5 micron) particles of asphalt that are coated with a chemical that allows the particles to stay apart. The emulsion must, however, break back to asphalt films to be able to perform its functions of coating, water proofing, adhering, etc.
Most of the time, combinations of emulsifiers and additives will be more effective than single emulsifiers. In most asphalt cases, the blending is already done by the manufacturer.
Double layers
The emulsified particles have emulsifier ions that are partly dissolved in the asphalt and partly inthe water. This creates a surface charge, such as when two particles approach each other and they are repelled. However, the particles cannot exist in a charged state and this must be balanced by counter-ions. So each particle sets up a double layer (See Figures 1 and 2, below). The charge is neutralised over the thickness of this layer.
At the interface, the charge density, as measured by the zeta potential, determines the stability of the emulsion. The thickness of the double layers are determined by the molecular size of the emulsifier, its ability to dissolve in the asphalt and its ability to dissolve in the water phase. This is its Hydrophile/Lipophile Balance (HLB). The higher the number, the more the emulsifier will be in the water phase. The lower the number, the more it will be in the asphalt, thus you want a low HLB number. HLB shows the ability of the emulsifier to maximize its presence in the interface.
| Type | Concentration (%) | Zeta Potential (mv) | Aggregate change (mv) | HLB |
| CRS RC 400/401/405 |
0.25-0.5 | 100-140 | -14 to -80 | 16-18 |
| CMS/CSS 500/501/502/505 |
0.8-1.5 | 20-80 | -14 to -80 | 18-20 |
| CQS 300/301/305 |
1.0-1.8 | 22-50 | -30 to -80 | 17-19 |
| Micro 200/201/205 |
1.0-1.8 | 40-60 | -40 to -80 | 15-16 |
| Coemulsifier 705 |
0.3-0.5 | 22 | -14 to -60 | 20 |
| SS/MS RC 1000 |
1.0-1.5 | -30 | evaporation and absorption |
25 |
In cationic, rapid-set systems, only about 30 percent of the added surfactant is available at the interface within several hours of emulsification. In medium-set systems, it is about 45 percent and in slow-set systems, about 60 percent. Having only a small percent of the added surfactant available at the interface means the system could be unstable. If the surface charge density is insufficient, the emulsion might flocculate and coalesce, or, in layman's terms, particles in the emulsion might bump up against one another and combine. Using calcium chloride, a non-ionic surfactant, or blending emulsifier with the asphalt, could solve the flocculating and coalescing problem.
Forming films
The application of emulsions relies on their ability to form films on aggregates. Film formation basically means the particle's coalescence, without entrapping water.
Film formation is a function of kinetic factors such as temperature, viscosity and internal stability, and thermodynamic factors such as interfacial energy. Films will form more slowly at low temperatures, with larger particles and with higher viscosity. Adding solvents or coalescing agents will help speed film formation, especially if you're working at a low temperature. Adhesion may be enhanced by adhesion agents doped into the asphalt.
Getting stable and staying there
In asphalt emulsions, the oil droplets are relatively large. Because of the large surface area per drop, the excess energy per drop is high and cannot be compensated for by entropy contributions. The stability of such a disperse system is characterized by how well it's dispersed, what its surface charge is and how small the particles are. The stability mechanisms of most interest for asphalt emulsions are sedimentation, flocculation, coalescence, inversion and Ostwald Ripening.
Sedimentation: The sedimentation of an emulsion determines how long it may be stored. It is tested by a simple separation test. There are several mechanisms that affect sedimentation. Its rate is determined by density and particle size. Buoyancy depends on density. Increased density equals a lower buoyancy because the particles have to displace more liquid to float. This factor (mechanism) can be changed by changing the density of the water with calcium chloride.
Brownian motion is another mechanism that affects sedimentation. This is a sort of internal mixing, but can speed up the rate of flocculation, which makes the mix more coarse because it allows the particles to collide. For stable, highly dispersed systems, sedimentation can be slow at first; however, anything that makes the emulsion coarser, will alter the sedimentation rate.
Flocculation: When two particles approach each other, several types of interactions may occur. There are two main ways that the interactions can influence flocculation. The first is collision efficiency. This is the likelihood that a pair of colliding particles will stick together. The second is the strength of such aggregations once they form.
Almost all interactions are of short range. In double layers, the thickness of the layers is critical to these interactions. Gelled dispersing phases will slow down collisions between particles, making flocculation more difficult. The method of movement, or transport, varies depending on pumping, shearing and changing temperatures. The method of attachment varies depending on zeta potential, double layer thickness and emulsifier type.
Coalescence: The coalescence of flocculated particles is a function of the surface charge and factors such as shearing and temperature. It is the next step from flocculation and occurs because a spherical shape is energetically lower. It is a second-order reaction. At this stage, the effects of shear can be used to break up the flocculated particles or cause coalescence. This is described in the Bernoulli equation:
Rate of coalescence = da/dt = k1cn - k1c (di3/D + dj3/D)3 n2
In the Bernoulli equation, kd is the disruption rate constant, or the known rate at which shear is breaking up the aggregates. The value kc is the coalescence rate constant, or the known rate at which the particles are sticking together. The value kc is independent of droplet sizes di and dj. D is the fractal dimensionality and n is the number of aggregated particles per volume. This is the reasonthat low shear mixing often breaks up flocculated groups in electrically stable emulsions.
Inversion: In water entrapment, the particles of the emulsion come together so quickly that the water gets trapped. This occurs in rapidly coalescing systems, especially those with very high binder emulsions (See Figure 6) or emulsions that are unstable.
Ostwald Ripening: An emulsion might be stable against flocculation and coalescence with time, but may still become coarse over time. This will cause increased sedimentation, which is a phenomenon related to the dependence of solubility on droplet radius. The Ostwald Ripening rate depends on the asphalt chemistry and may be adjusted by adding parrafinic solvents to the asphalt. The rate is a constant that depends on the interfacial tension. One of the effects is to create a bimodal distribution.
This section of asphalt illustrates fatigue
due to water damage and freeze-thaw conditions.
Emulsion breaking and cure
Break: When the double layer of an emulsion gets destroyed, the emulsion breaks. Mechanisms that contribute to double layer destruction are flocculation and coalescence - which are accelerated by evaporation, shearing, etc. - and aggregate interaction.
Aggregate interaction is interesting for road emulsions because it "describes" the chemical interaction between the emulsion and the aggregate. The mechanism of aggregate interaction is not completely understood, but appears to involve the stable emulsion particles being attracted to the surface of the aggregate where the emulsifier from the bulk solution interacts with the negative charges (See Figure 7). This interaction with negative charges changes the equilibrium of the emulsifier particles and the emulsion gets destabilized. This, in turn, leads to deposition of asphalt onto the aggregate surface. This is where the wetting and film formation become critical. The rate of film formation will determine the adhesion and the emulsifier/formulation will determine the breaking rate.
The internal flocculation and coalescence will also have an effect. Within the forming film, the particles are in closer proximity and these mechanisms will accelerate. However, if the mechanisms accelerate too fast, the water will be trapped and the binder film forming could be diminished. The bulk part of theemulsion, which is remote from the aggregate surface, will break by flocculation and coalescence. In slurry and Micro-surfacing, the rate will be controlled by additives.
Cure: The curing of an emulsion film is often confused with breaking. Curing is simply the loss of water from the film and bulk emulsion. Cure rates aredependent on water content, rate of evaporation and the diffusion of water through the curing binder. In systems with strong energy differences between the aggregate surface and the emulsified binder, an extra driving force is present to push water away from the aggregate/binder interface. This is especially true in Micro-surfacing. In light of this curing rate, emulsifier choice relative to aggregate is a key issue.
Figure 3. Effect of shearing on particle size
Equipment effects: The effect of equipment that is used to handle and apply the emulsion is significant. In short, sources of shear will coarsen and break the emulsion, depending on the level of shear, which is temperature and equipment dependent (See Figure 3).
In spraying applications, the nozzles used for spraying will also exert a shear. Because this application is for a short time, a stable emulsion will usually not break. Its rheology will change, however, because asphalt emulsions are shear-thinning - they reduce in viscosity with shear. Depending on the asphalt, the emulsifier and additives, and their levels, the material may also be thixotropic - its viscosity will recover rapidly after spraying. As spraying is the last action in application, no long-term effects are relevant.
In cold mix, slurry or other pugmill mixing applications, the shear stability is of much greater importance. The pugmill creates a high level of shear. In such operations, the pugmill should coat the aggregate with emulsion, not break the emulsion. If the emulsion is not sufficiently stable, there will be a tendency for the emulsion to have an accelerated break. This leads to poor coating and decreases strength, durability and resistance to water in the final mix. Freeze-thaw resistance is also greatly reduced.
In order to create an emulsion that will stay stable, the factors discussed above must be controlled. Remember that asphalt is a chemically complex material, and emulsification increases this complexity. Therefore, it is important in all applications that the chemistry of the system be considered. This will optimize the performance of the emulsion product you choose to manufacture.
| PLAN CAREFULLY FOR THE EMULSIFIER YOU WANT | ||
| The factors | Check the desired qualities of the factors | Restrictions to be aware of |
| slurry emulsion |
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| emulsifiers |
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| equipment |
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| RAW MATERIALS | ||
| asphalt |
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| water |
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| additives |
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Information for this article was provided by Glynn Holleran, vice president of Valley Slurry Seal Co., Sacramento, Calif.
Last Updated (Wednesday, 21 October 2009 16:34)