The notion of dispersion system provides for two methods of production: by dispergation and by aggregation, where molecules unite under the influence of traction forces and create the nucleus of the new phase, which grows to the required size. This principle can be used for production of foam bitumen.
The primary method of production of bitumen emulsion is dispergation of bitumen in water phase with mechanical energy in the presence of special surfactant emulsifiers.
The role of the emulsifier when producing emulsion is as follows:
- decrease of interfacial tension on the border of phase separation, which promotes emulsification process;
- formation of protective film around bitumen particles to ensure emulsion stability.
A lot of research has been conducted in the field of emulsification. It is the key to production of materials with required properties.
Bitumen emulsion must be produced in the simplest way possible, be easy to apply and break down in contact with mineral mix material or surface coating only within specific time, required for product quality. At the same time, bitumen binder must maintain or improve its original qualities after emulsion breakdown.
Inadequate consideration of the accepted emulsification principle characteristics and incorrect choice of emulsifier may lead to low quality product (mix or coating layer). Therefore we should look in more detail into the conditions of formation of bitumen emulsion by dispergation.
The first condition of emulsion formation is indissolubility of DP in DM. Therefore, the substances forming the two phases must be of different polarity. In this case, one of the phases is water, which is a highly polar liquid, the other phase is bitumen, which has not charge, and is usually called “oil”. Hence the two primary types of emulsion: dispersion of water in oil (W/O) and dispersion of oil in water (O/W).
The second condition for production of stable emulsions is a consequence of the first: it is necessary to equalize the charge of both liquids. It may be done by introduction of another component, called emulsifier. Emulsifier lowers interfacial tension on the border of phase separation, being adsorbed in the border layer.
Adsorption is the phenomenon of accumulation of one substance on the surface of another. It depends on the chemical and physical nature of the adsorbent (the substance, on the surface of which the accumulation takes place) and the adsorptive (the accumulated substance).
Accumulation of a substance within a volume of another substance is called absorption.
It is known that the higher the decrease of interfacial tension by a substance, the more of the substance will be accumulated in the surface layer.
Adsorption, accompanied by accumulation of substance in the surface layer, is called positive, whereas the positively adsorbed substance (accumulated in the layer) is called surfactant.
If the solved substance increases interfacial tension, it will be pushed out of the adsorbent volume to almost complete expulsion from the surface layer. Such adhesion is called negative, and the substance is called surface-inactive. Inorganic salts are often an example of such substance.
Surfactants, the molecules of which consist of charged “head” and neutral “tail” orient themselves on the border of water and bitumen so that the “tail” is turned to bitumen, and the “head” is turned to water. As a result of such orientation of the surfactant, it creates a layer which equalizes the charge of the phases and decreases interfacial tension of the border of separation.
Sometimes such diphilic molecules (having a hydrophilic charged and hydrophobic neutral parts) can have an excessively short hydrophobic part, and will therefore exhibit mostly hydrophilic qualities. In this case, the substance will solve well in water and will have negative adsorption, i.e. it will no longer act as a surfactant and will cease to be an emulsifier.
Lengthening of the hydrocarbon chain of the “tail” of the molecule, it’s hydrophobic qualities will increase, solubility in water will decrease, and emulsifying and stabilizing properties will improve.
Therefore, surfactants are diphilic substances with interfacial tension lower than that of the solvent, and positive adsorption, which lower interfacial tension.
Due to dissociation of charged group, a surfactant particle of the emulsion phase becomes charged, negatively or positively. Particles of the same charge repel each other, preventing adhesion and increasing emulsion stability.
Contemporary classification divides surfactants into four classes:
- anionic surfactants with positively charged “head”;
- cationic surfactants with negatively charged “head”;
- twin action surfactants (ampholytes and amphoters), in which the “head” contains two or more functional groups, which may change the charge depending of the ambient pH;
- non-ionic surfactants, which do not dispergate to ions in a water solution. Solubility of such surfactants is enabled by the presence of water affinity of functional group in the molecules. Emulsion is stabilized due to formation of hydrate layer.
Beside surfactants, emulsifier can be finely ground mineral powders.
Solid power emulsifiers play the same role as the surfactants, i.e. they adsorb on the surface of bitumen, create protective film and therefore prevent particle adhesion. In this case the power must be mixed with water in a very precise proportion prior to introduction of bitumen with intensive mechanic agitation to break bitumen into droplets and promote its contact with the emulsifier. Addition of electrolytes or surfactants allows to adjust the properties of such emulsions.
The mechanics of the solid emulsifier required yet more research. Some authors suggest that metal hydroxides form of the surface of the particles upon contact with water, which results in formation of double charged layer near the surface of the solid particle. Other authors consider the most important the adsorption effect of lowering solid particle strength when the surface defects of the solid body creates new surfaces under ambient factors. These surfaces are covered with adsorptive layers of surfactants, present water, at the moment of formation.
In both cases the same things are required from solid surfactant – fine grind and compulsory addition of water to the powder in the amount close to the upper limit of its fluidity. The above means that application of emulsifiers of different origins will lead to production of emulsions with different properties.