UNDERSTANDING EMULSION PRODUCTS
It is the purpose of this page to give a better understanding of the basics of emulsion physical chemistry so that problems in testing, handling or specification become less difficult.
2. WHAT IS EMULSION?
If you know exactly what an emulsion is it is far easier to predict what it might do under certain circumstances.
An emulsion is one phenomena that occurs when you mix two components together.
At one extreme a solution results, in this the dispersed component takes up an elemental form, such as a molecule or an ion.
At the other end of the scale a slurry or suspension forms in which the dispersed phase will fall out.
Emulsions are somewhere in the middle. Common examples of emulsions are milk, margarine, butter, beer and paint.
Emulsions are thus by definition made up of two components with one dispersed through the other. The dispersed component is not soluble in the continuous component.
Because of this to maintain the dispersion requires some way of overcoming this lack of compatibility. The methods that have been found to work over many years are high shear and chemical treatment.
An Asphalt emulsion is thus Asphalt dispersed through water and chemically stabilized as shown.
Emulsions are Liquids
Asphalt Dispersed in Water
3. MANUFACTURING EMULSIONS
The main features are:
Molten Asphalt is sheared to fine droplets by a high shear system. In VSS we use a
colloid mill designed for the purpose. Go to www.slurry.com for more detail.
Within the colloid mill the Asphalt is brought into intimate contact with a chemical solution. This is the chemical stabilization.
After discharge the emulsion consists of water with fine particles of Asphalt dispersed in it, all that is between the particles and each other is water and chemicals.
Asphalt is not soluble in water and so to keep it dispersed in fine particles is a significant feat.
It may be understood from this that the viscosity of the Asphalt when it enters the mill is important, hence the temperature of the Asphalt is important.
Also the temperature of the soap is important. If either are too low the emulsion may become more coarse. If , on the other hand the temperatures are too high the emulsion may boil, lose water and cause coarsening.
4. Chemical Systems
VSS manufactures three basic types of chemically stabilized materials:
The last, clay, is the filled type and it's structure is rather different from the other two, they are mostly used for industrial applications such as roofing and underbody sealing and use an activated clay as the emulsifier system. They are essentially anionic emulsions.
The other two categories get their name from the chemical type used to stabilize them. It is important to note that other chemicals may be added for specific effects but these do not change the basic chemistry, only modify it.
4.1 How Emulsifiers Work
Emulsifiers of both the cationic and anionic type are based on salts of fatty long chain molecules, these may be synthetic or derivatives of fatty acids as found in oils and fats.
a) Anionic emulsifiers are based on fatty acids, these are reacted with a base such as caustic potash or caustic soda (KOH or NaOH ) to form a salt. It is this salt that is the active emulsifier. Figure 4 shows a schematic of such a molecule. The non polar tail is hydrophobic and hence aligns itself inward to the Asphalt. The polar end is hydrophilic and hence provides the solubility in water. The emulsifier thus attaches itself to the Asphalt particle. The number and density of emulsifier molecules that do this will impart a charge to the surface of the Asphalt particle. This charge will be exactly balanced by the free charges in the water phase. This will be the sodium or potassium ion of the salt.
The products that are used for this process are usually mixtures of fatty acids, these will impart different levels of charge or Zeta Potential to the particles and give a balance of properties.
Another emulsifier for slow setting emulsions worth mentioning is Vinsol. This is a wood resin. The emulsifying species is the salt of a long chain derivative of abietic acid.
b) Cationic Emulsifiers are based on acid salts of amines prepared from fatty acids. These may be fatty diamines, fatty quaternary ammonium compounds or ethoxylated derivatives. The main emulsifier types in slurry and microsurfacing emulsions are amidoamines and immadazolines. These are reaction products of polyamines and carboxylic acids. The type of emulsifier determines the number of charges that are on the surface of the Asphalt. Thus they determine the zeta potential.. Again, commercial emulsifiers are mixtures and give a balance of properties.
c) Zeta Potential
Zeta potential is an important concept to grasp as most emulsion properties flow from it or as a result of it.
Zeta potential is the electrical potential between the surface of the Asphalt particle and the bulk solution. The zeta potential is determined by the emulsifier adsorbed onto the surface of the Asphalt. A double layer of ions and counterions exists in solution surrounding each particle of Asphalt. The form of the double layer depends on the concentration and ionic density of the emulsifier and the pH.
A large zeta potential indicates a greater double layer, faster movement and greater repulsion between particles. Larger repulsion produces more stable emulsions.
The pH affects the way in which the Asphalt adsorbs emulsifier and so is critical to the double layer and thus Zeta potential.
Increasing the concentration of the emulsifier compresses the double layer this in fact decreases zeta potential but the increased amount of surfactant increases stability by increasing colloidal protection. For this reason it is always better to choose the emulsifier for the application rather than attempting to slow break or make it more rapid by adjusting emulsifier concentration. In cationic emulsions it is often not possible to make a slow set using a rapid set emulsifier due to the very high zeta potential of such emulsifiers.
5.PROPERTIES OF EMULSIONS
All the properties of emulsions and their behavior under various conditions may be directly related to the structure as discussed above. This goes for any application. If the structure is understood then everything else follows.
As anionic emulsions have a negative charge as does almost every mineral there can be no electrostatic attraction. So for an anionic emulsion to break requires that the particles get so close to each other that the repulsion forces are overcome by the attractive forces that exist between all things. This is a stepwise mechanism that involves coarsening into larger and larger particles until the particles are macroscopic.(see fig 8 and 9) This is normally broken up into two steps of flocculation and coalescence.
This can occur by forcing the particles together in any way. This may by an outside force such as pumping at high shear, taking away water by heating and boiling or pushing particles together by freezing. Breaking can also occur if the emulsion is packed into a smaller area by sedimenting or settling.
Cationic emulsions have a positive charge and hence a direct and very rapid reaction between the emulsion and an aggregate or pavement is possible(figure 10). The size of the charge , or the Zeta potential affects stability, i.e. the larger the charge the greater the repulsion, but as the aggregate is negatively charged the higher the zeta potential the more rapid the reaction.
So it is possible to stabilize a cationic emulsion in the same way that makes it a more rapid break.
The other mechanism of evaporation is available too but as the emulsion is stabilized this form of break becomes slower. Thus a balance must be struck.
After the electrostatic part of the reaction is complete the emulsion will rely on flocculation and coalescence to complete break.(figure 8, 9).
After break is completed the water must still be completely evaporated for the residual Asphalt to achieve full strength.
Stability may be understood in terms of the structure and the mechanism of break.
Firstly, the density of all Asphalts is around one but always higher. Thus the emulsions will always settle unless this density can be adjusted below that of water or if the water is modified to a higher density. (figure 12) In general it is not practical to do this aside from relatively small adjustments. Cutter is added for such an adjustment as is calcium chloride. The former changes the density of the Asphalt, the latter the density of the water.
As the break down in storage of an emulsion is by flocculation and coalescence any measure to retard this or to begin the process at a finer particle size will increase stability.
Also finer particles settle more slowly, if they are less than 3 microns they even have as sort of internal mixing called Brownian motion.
6. CONCLUSIONS Asphalt Emulsions are fine dispersions of Asphalt in water. Manufacturing is done by high shear in specially designed plants.
The key features and considerations when choosing a system therefore are:
a) Asphalt type and source.
b) Chemical selection and formulation to meet specification and application requirements.
c) Equipment for manufacture.
The next part deals with the advantages of such systems.
ADVANTAGES OF ASPHALT EMULSIONS
In part 1 the basics of emulsions were considered. Restated, Asphalt emulsions are dispersions of Asphalt in water and stabilized by a chemical system. All of the advantages of emulsions flow directly from this fact.
As a way of comparison they will be compared to cutback Asphalts and to hot Asphalt.
Keep in mind that the reason we use Asphalt in road applications is because it is waterproof and adheres to stone. To get it into a form that we can apply it requires the viscosity to be reduced. This may be done by:
- Making a solution with a solvent such as kerosene or naptha
- Making an emulsion
The main areas of advantage of emulsions are related to this can be summarized in terms of:
- Energy Conservation
- Pollution Control
- Ease Of Use
2. ENERGY CONSERVATION
2.1 Avoids waste of fuel
Cutback Asphalts can contain anything up to 50% kerosene or gasoil. Asphalt emulsions contain 0-2%. Thus there is a significant saving in the use of valuable white fuel products.
In a cutback the solvent is added merely to reduce viscosity of the Asphalt to a level where it can be poured and sprayed. It is expected that this will evaporate into the atmosphere; in fact if it does not then the Asphalt is too soft.
In an emulsion the viscosity characteristics are determined by the water phase and hence the viscosity is low, only water will evaporate away.
2.2 Reduces Overall Energy Requirement
(Source: US EPA REPORT 450/2-78-004 JANUARY 1978).
Comparing an emulsion with a cutback of the same solids content:
It takes about 700KJ to process a liter of Asphalt for paving according to the Asphalt Institute.
Added to this is the energy of the cutter added. This is 40,000 KJ per liter.
For a 60% cutback the total energy requirement is:
16,700 KJ = ( 700 + 0.4x 40,000) per liter.
567 kj is required to produce a liter of emulsion.
Emulsifier energy requirement to produce 1 liter of emulsion is 584 KJ.
This gives a total of 1151 KJ per liter of emulsion.
Compare to 16,700 KJ for cutback.
Most of the energy is in the fuel lost!
There is another saving however and that is in heating on the road.
To spray a liter of emulsion at ambient requires 41 KJ per liter.
To spray the cutback requires an extra 127 KJ per liter for heating.
Before cutbacks were largely banned in the USA it was estimated that the energy loss was enough to fuel 588,000 cars for one year! (at 12 miles per gallon!)
2.3. Pollution Control
Kerosene and gas oil fumes are green house gases. In a cutback they evaporate into the air and become pollutants. The cutbacks are designed this way. In an emulsion there is no such evaporation.
Emulsions are water based. They have no flash point, they are not flammable or explosive. Drums of emulsion kept in the sun will not expand or burst.
Being water based emulsions do not pose any health risk to workers and, being used cold they cannot cause burns.
Spillages are of no environmental importance. If a spill occurs in water way the emulsion will break and settle to the bottom, a harmless organic mass. Grass and other plant life can grow through it. A spill on the side of the road will actually encourage plant growth by keeping the moisture and warmth in the ground plants can simply break through.
Emulsions have many different applications. They can, with proper selection, be used for a wide range of applications. This will be discussed in a later session. But the same emulsion that is sprayed in large amounts for a sealing application can be used for small patching jobs.
Because they can be stored in drums for long periods they can be used in remote areas.
4. EASE OF USE
Application of emulsions for specialized applications will require specialized equipment such as a sprayer , however for small jobs the emulsion, being handled directly from the drum can be poured or spread by hand. For small patching jobs, crack sealing, small amounts of cold mix only the most basic of equipment is required. For example a watering can with a baffle and a shovel can be used to seal pathways and small areas such as school grounds etc.
Asphalt emulsions are sophisticated chemical systems. This creates the opportunity of creating systems that suit the conditions of application and the materials available.
The advantages compared to cutbacks or hot Asphalt is as a result of this.
The performance advantages can be summarized as:
- Improvement in adhesion to aggregates
- Extension of range of conditions of application.
- Economy of materials.
Aggregates can be classified by their mineralogical type, however when moisture is present all aggregates have a net negative charge. If a cationic emulsion is used the breaking mechanism is by a physio- chemical reaction with the stone in which the emulsified particles electrostatically plate out on the surface of the stone. The greater the charge on the aggregate the faster the reaction will be. This can also be controlled by the emulsifier system.
Thus the emulsion makes a strong chemical bond. The rest of the break is by flocculation and coalescence of the emulsion that is the water evaporates and the particles come closer together and form groups of particles then ultimately become Asphalt.
The fine nature of the Asphalt particles makes this process fast and even and the low viscosity makes the process able to happen at ordinary temperatures.
AT THE END OF THE PROCESS THE RESIDUAL MATERIAL IS THE ASPHALT THAT YOU STARTED WITH.
With a cutback the coating is purely physical and an adhesion agent is needed for aggregates of high surface charge. This is also true of hot Asphalt.
With cutbacks the loss of kerosene is slow and can take years. This leaves a Asphalt that is soft and subject to bleeding in the summer.
5.2 RANGE Of CONDITIONS
Because an emulsion is water based the aggregates need not be dry for application. In fact it is an advantage if they are damp. Cutbacks will coat well wet aggregates because kerosene and water have a large surface energy difference. They do not mix.
Hot Asphalt behaves similarly.
At lower temperature conditions the viscosity to wet aggregate and form a bond depends on the viscosity and the ability to form a continuous film.
An emulsion’s viscosity is low hence wetting can occur to temperatures well below 10C. For a cutback higher levels of kerosene are required (20-30%) and for Asphalt alone, higher temperature can be used.
The drawbacks of higher cutter content are that the residual Asphalt is even softer virtually ensuring bleeding later on. For Asphalt the rate of temperature loss is such that the viscosity increases at a very rapid rate, and it has been estimated that the Asphalt will reach the road temperature in about 2.5 minutes. This gives little time for penetration or wetting to occur, virtually ensuring stone stripping or patch disintegration in a short time.
The rate of curing of cutbacks compared to emulsions is shown in figure 13. The more rapid evaporation of water makes the total cure faster.
5.3 ECONOMY Of MATERIALS
A lower viscosity emulsion that has good adhesion will not only improve adhesion but will coat stone better than hot Asphalt or even a cutback.
This means that less emulsion is required in many instances than hot Asphalt or cutback. This can be 5?10% depending on the situation.
For example in penetration patching the emulsion will fill the voids more easily and not get hung up in the top layers. This can save Asphalt and create a patch that is not as soft or likely to bleed.
In sealing the emulsion goes further up the sides of the stone creating a better bond. In residential streets this can save 10% on binder requirement. For highway work or remote areas other considerations will need to be taken into account.
Emulsifiers in Asphalt are present in very small concentrations. However work has been carried out that shows that emulsifiers can have a positive benefit on the durability that is resistance to oxidation of Asphalt.
Also, it is relatively easy in an emulsion to incorporate additives to improve durability without affecting other properties.
The major advantages of asphalt emulsion relate to their chemistry and physical properties. They are a handy way of storing, transporting and applying asphalt. They save energy and materials and are simple to use.