Bitumen Emulsion

A schematic diagram of a typical emulsion system is show in process flow diagram.

The main features are, Molten Asphalt is sheared to fine droplets by a high shear system called colloidal mill.

Within the colloidal 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 is 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.

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 (K OH or Na OH) to form a salt. It is this salt is the active emulsifier.

The non polar tall 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 phases. 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.

b) Cationic Emulsifiers are based on acid salts of amines prepared form fatty acids. These may be fatty demines, fatty quaternary ammonium compounds or ethoxylated derivatives. The type of emulsifier determines the number of charges that are on the surface of the Asphalt. Thus they determine the zeta potential.

Commercial emulsifiers are mixtures and give a balance of properties.

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 counter ions 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.

All the properties of emulsions and their behaviour 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.

4.1. BREAKING

a) ANIONIC

As anionic emulsions have a negative charge as does almost every mineral there can be no electrostatic attraction. So for an ionic emulsion to break requires that the particles get so close to each other that the repulsion force 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 setting.

b) CATIONIC

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, ie. 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. After Break is completed the water must still be completely evaporated for the residual Asphalt to achieve full strength.

4.2. STABILITY

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 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.

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:

• Asphalt type and source.
• Chemical selection and formulation to meet specification and application requirements.
• Equipment for manufacture.

The main areas of advantage of emulsions are related to this can be summarized in terms of:

• Energy Conservation/ Pollution Control/ Safety
• Versatility
• Ease Of Use.
• Performance

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.

The advantages of Asphalt emulsions are related to their chemical and physical makeup, these advantages lead directly to the applications to the applications in which emulsions are used.

• Sealing (C- Rs-₁, RS-₂,)
• Slurry Surfacing (C - MS, SS₁, SS₂)
• Patching/ Crack filling (C- SS₁, SS₂, MS,)
• Fog Seal (C - SS₁, SS₂)
• Cold Mix (C - MS, specialty emulsions)
• Tack Coating (C - RS₁, specialty emulsions).
• Dust laying (C -SS-₁, specialty emulsions)
• Stabilization/Recycling. (C- SS, MS, Specialty emulsions).
• Waterproofing/Roofing/Home. (specialty emulsions).
• Agriculture. (C- SS₁, MS, specialty emulsions).

EMULSIONS ARE VERSATILE AND COMPLEX PRODUCTS BUT WITH THE RIGHT TECHNOLOGY, CHEMICALS AND EQUIPMENT EMULSIONS CAN SOLVE MANY ROAD PROBLEMS IN AN ECONOMIC,SAFE AND ENVIRONMENTALLY FRIENDLY WAY.