The most scientific, precise, and complete definition of emulsion is given by P. Becher in Emulsion Theory and Practice (1961):
An emulsion is a thermodynamically unstable heterogeneous system including at least two immiscible liquid phases of which one is dispersed in the other in the form of droplets whose diameter is generally greater than 0.1 micrometers. The minimum stability inherent to this type of system may be increased by adding appropriate agents, such as surfactants or finely divided solids (1).
A more specific definition, with respect to asphalt emulsions, might be A heterogeneous system with two or more liquid phases, made up of a continuous liquid phase (water) and at least a second liquid phase* (asphalt) dispersed in the former in fine droplets. (1)
(* Although typical asphalts used to manufacture most emulsions may appear to be a solid at room temperature, they are actually viscous liquids, also referred to as viscoelastic.)
Asphalt emulsion production is a science in which much research, time, and money are invested. Asphalt emulsion production is as much art as it is science; the art of asphalt emulsions production is in understanding the science and putting it to practical use.
The components of an asphalt emulsion include the following.
Asphalt is defined as the residual product of nondestructive distillation of crude oil in petroleum refining. Asphalt is an engineering material and is produced to meet a variety of end-use specifications based upon physical properties. This basic product is sometimes referred to as “straight run” asphalt. The vast majority of asphalt produced in North America and Europe conform to the characteristics of straight run, however, another common product often referred to as “oxidized” asphalt is produced by blowing air through the asphalt at elevated temperatures to alter its physical properties for commercial applications. Asphalt used in the production of asphalt emulsions is generally of the straight run variety.
Diluents and Fluxes
Other refinery products that may be blended with basic asphalts without altering their properties include, but are not limited to raffinates, vacuum residuum, asphaltenes, and petroleum resins.
The addition of any refinery streams or products other than these, and all nonrefinery products introduced into asphalt in significant quantities, may affect asphalt properties and should be considered prior to emulsion manufacture.
Water used to manufacture emulsions may be from various sources: municipal systems, wells, etc. Whatever the source of water, it must contain a minimum amount of mineral and organic impurities. It is often necessary to add salts to the water to create an ion exchange in the water. Ion exchange generally consists of replacing the magnesium and calcium ions in the water by adding sodium ions. Magnesium and calcium ions tend to react with some types of emulsifiers to form compounds which no longer have emulsifying properties.
Surface active agents (surfactants), also known as emulsifiers or emulsifying agents, are needed to provide the stability required over time. The stability of the emulsion determines its appropriate use. Surfactants are chemical compounds with a surface activity which, when dissolved in a liquid, especially water, lowers its interfacial or surface tension by preferential adsorption at the vapor/liquid surface or other interfaces.
Surfactant molecules have two opposite affinities, one part is made up of a polar group yielding hydrophilic (water-loving) properties and one part is made up of a non polar radical giving it lipophilic (oil-loving) properties.
There are numerous natural compounds that act as surfacants, but more commonly chemical compounds are synthesized to produce the desired characteristics.
Surfactants can be grouped according to the type emulsions they yield. The typical types of emulsions are anionic, cationic, amphoteric, and nonionic. Commonly used surfactants are amine class chemicals that are of a liquid or paste consistency not soluble in water.
Zeta Potential of Cationic Emulsifiers
As defined, asphalt emulsions are colloidal systems in which asphalt is dispersed in a continuous aqueous phase. The asphaltic-dispersed phase has certain properties with respect to electrical charge. Knowledge of these electrical properties makes understanding the behavior of the emulsion colloidal system easier. In the case of cationic asphalt emulsions, each asphalt droplet has multiple positive charges due to the emulsifier cation. Around each positively charged asphalt droplet, there is a negative ion layer whose electric charge exactly compensates for the positive particle charge (1).
The zeta potential can be determined by placing the colloid particles in an electric field and measuring their movement energy. This is usually done with a microelectrophoresis device where the electrophoretic mobility is measured in millivolts (mV). The behavior of the emulsions with respect to measurement of zeta potential is similar to that which is observed when aggregates and emulsions are mixed.
Zeta potential is both a function of emulsifier concentration and emulsion pH. Cationic asphalt emulsions range in pH from 1 to 7, more specifically in the 2- to 4-pH range, and typically have zeta potentials in the range of 15 to 150 mV. As a general rule, the higher the cationic zeta potential is, the more cationic the emulsion is and the more rapid is the breaking speed. Lower zeta potential yields a more stable emulsion with a much slower breaking speed.
Therefore, emulsion breaking speed onto aggregates is directly proportional to the zeta potential of the emulsion.
As emulsifiers are insoluble in water, it is necessary to convert them into salts so as to dissolve them in the dispersing phase. This is generally done by reacting with acid, most often hydrochloric acid in a 20°C to 22°C solution. Acid dosing is the regulatory factor which determines final emulsion pH.
The parameters to consider in the manufacture of asphalt emulsions are outlined below. Dispersion Energy Emulsion dispersion is caused by mechanical energy and physicochemical energy. The mechanical energy (provided by the mill) divides the asphalt into fine particles and the emulsion fineness increases with fractioning capacity (mill capabilities). The physicochemical energy is provided by the emulsifier and must
- Reduce the interfacial tension between the hydrocarbon phase (asphalt) and the
aqueous phase (water) so as to facilitate emulsification and
- Create a protective film around the particles.
In simple terms, there must be sufficient mechanical energy (mill energy) to provide
asphalt particles of the correct size and concentration. And there must be sufficient surfactant to maintain stability.
Particle Size Distribution
Particle size and particle size distribution are important variables and are controllable with formulation, raw materials, and the equipment used to manufacture the emulsion. Many of the processes of breaking and curing are directly dependant on particle size and particle size distribution (4). The importance of particle size in emulsions has been discussed in many papers (2–4). It is a determinant of emulsion stability, coating, break rate, and cure rate. Methods to improve particle sizing of emulsions by formulation and adjustment of asphalt chemistry are described in the literature (2, 3). These methods usually involve improvement of the dispersing phase, doping of asphalt with surfactants, and tailoring asphalt composition and optimization of manufacturing conditions. The intention is to improve formation and dispersion of asphalt particles by the mill and stabilize the resulting emulsion. The mill and milling process are the main determinants of initial particles size for a given asphalt–emulsifier system as particle size is determined by the shear in the mill and mill residence time (2, 4, 5, 7).
The relationship of the milling process to particle size can be expressed by
Shear rate = (2πRV / 60 E) (1)
R = colloid mill radius (rotor and stator combination);
V = velocity of rotation or rotation speed (rpm); and
E = gap dimension.
From Equation 1, it can be concluded that particle size is a function of mill diameter, gap, and peripheral speed. A correlation between shearing and the d50 value, a correlation between particle size distribution and initial particle size in the mill, has been reported by Holleran in U.S. Patent 5,518,538.
Internal mill configurations are quite different, with varying tooling and effective gap dimension, therefore, the relationship presented in Equation 1 should be changed to:
Shear rate = (2πRV / 60 E) Mf (2)
where Mf = mill factor, and the mill factor is the increase or decrease in shear created by the mill configuration and tooling.
There are several methods available to determine the average diameter of emulsion asphalt particles which are all aimed at measuring the particle size and distribution in water.
These methods range from sophisticated laser measurement of particle sizes and distribution to simple opacity measurements.
Component Viscosity and Temperature
Often the mechanical energy required to provide asphalt particles of correct size and concentration is aided by increases in the temperature of the hydrocarbon phase (asphalt). In other words, in order to enable the asphalt binder to properly disperse in the aqueous phase, it is necessary that its viscosity be relatively low. From practical experience, the optimal viscosity is 200 centipoises, which is obtained by maintaining the asphalt at a temperature that yields this viscosity; it is called the equiviscous temperature (EVT). The EVT of some common asphalts are given in Table 1.
The EVT limit of 200 centipoises is obtained from practical experience. In actual practice we know this works because emulsions from the asphalts listed above are typically produced at these temperatures. If these temperatures are significantly exceeded, unwanted and undesirable effects may occur. In fact, if the emulsion at the mill outlet is at a temperature greater than 100ºC (212ºF) the emulsion aqueous phase will boil, as water typically has a boiling point of 100ºC (212ºF). At higher altitudes the boiling point of water will be lower. A good rule of thumb for emulsion manufacturing purposes is to not exceed 95ºC (203ºF) at the mill outlet.
Often manufacture of the aqueous phase requires that the water (soap solution) be at a temperature of around 40ºC (104ºF) or higher to provide proper activation of the emulsifying agent, therefore, it is necessary that the asphalt binder temperature does not exceed a certain temperature to prevent boiling of the emulsion. Keeping in mind that the asphalt has a characteristic EVT, which is important for proper particle size and concentration, it may not be possible to successfully manufacture quality emulsion at a temperature below the boiling point of the emulsion.
The relationship of asphalt binder temperature to temperature of the aqueous phase and resulting emulsion temperature is represented by:
As the boiling point of water varies directly with respect to decreases and increases in pressure of its environment, we can increase the boiling point of water by increasing the pressure of its environment. Put more simply, if we maintain a certain amount of pressure on the emulsion, until the temperature is below its boiling point, boiling of the emulsion can be prevented. Therefore, if temperature requirements dictate, e.g., minimum soap temperature and asphalt EVT, the emulsion can be manufactured under pressure without fear of boiling the emulsion.
Relatively high EVTs of some asphalt binders or minimum soap temperatures require that the emulsions be manufactured under a pressure of a few bars (30–60 psi) to satisfy the mandatory parameters corresponding to these components and simultaneously prevent boiling of the emulsion. The EVT of an asphalt and its importance in the manufacture of asphalt emulsions has been presented. It is obvious that another important emulsion manufacturing parameter is the emulsion exit temperature or what is defined as the minimum emulsion exit temperature (MEET).
MEET is considered to be equivalent to the temperature of the asphalt where the viscosity is approximately equal to 20,000 centipoises. As with EVT this limit is obtained from practical experience. It is a common belief that if the emulsion exit temperature is maintained at or above the MEET, emulsion stability is improved during manufacturing, cooling, and storage.
The EVT and MEET can be obtained from a viscosity profile of the asphalt. An example of rotational viscosity profiles of a neat AC-30 (PG 67-22) and a 4.0% styrene-butadiene-styrene (SBS) -modified asphalt are given in Figure 1.
From Figure 1, the MEET can be obtained as the point where the viscosity of the asphalt is equal to 20,000 centipoises; for the PG 67-22, the MEET equals 82ºC (180ºF), and for the 4.0% SBS-modified asphalt, the MEET equals 118ºC (245ºF). The EVT can be obtained as the point where the viscosity of the asphalt is equal to 200 centipoises; for the PG 67-22, the EVT equals 177ºC (350ºF), and for the 4.0% SBS modified asphalt, the EVT = 218ºC (425ºF). (Note: Often it may be necessary to actually heat asphalt to about 6ºC (25ºF) higher than the EVT to ensure that the asphalt is at EVT at the milling surfaces.)
An emulsion manufactured from the PG 67-22 represented in Figure 1 could be manufactured under normal atmospheric conditions, while an emulsion manufactured from the 4% SBS asphalt must be manufactured under pressure. As a general rule, if the MEET is greater than 95ºC (203ºF) the emulsion will have to be produced under pressure. The amount of pressure required can be obtained by consulting steam data temperature tables (8) using the absolute pressure value plus 20%. It should be noted that this pressure is absolute pressure and not gauge pressure; gauge pressure would be equivalent to absolute pressure less 1 bar (14.5 psi). For demonstration purposes, consider the SBS-modified asphalt presented in Figure 1. The manufacturing pressure of the SBS-modified asphalt emulsion based on the MEET of 118ºC (245ºF) can be obtained from Figure 2.
From Figure 2 we see that the emulsion manufacturing pressure for the MEET of 118ºC (245ºF) is approximately 1.3 bars (19 psi) gauge pressure. At this point, it is important to note that the actual emulsion exit temperature for this asphalt may be higher than 118ºC (245ºF) depending on the soap temperature requirements and the recommended asphalt temperature of 218ºC (425ºF) based on EVT. The mill outlet temperature may also be slightly higher than calculated from Figure 1 as the milling process, mill resonance time and pressure effects will contribute to the final emulsion outlet temperature.
The following equation yields the soap temperature for a given emulsion.
Using Equation 4, a 65% residue emulsion with an asphalt binder having a 218ºC (425ºF) EVT and a 118ºC (245ºF) MEET, the soap temperature would be 25ºC (77ºF). It is not likely that the soap solution would be properly activated at this temperature.
As component dosing in emulsions must be extremely precise, especially the emulsifier and the activator (e.g., HCl or NaOH), proper activation and temperature of the soap solution is important. Variation, even very slight, may have far reaching consequences.
Soap solutions for typical cationic emulsions are usually in the 40ºC (104ºF) to 70ºC (160ºF) range. Understanding this, consider a 65% residue emulsion with the 218ºC (425ºF) EVT asphalt from Figure 1 and a soap temperature of 60ºC (140ºF). This emulsion would have an emulsion exit temperature of 136ºC (277ºF), from Equation 3, which would require an emulsion manufacturing gauge pressure of 2.8 bars (40 psi) (Figure 2). Note that the actual outlet temperature could possibly be 10–15 degrees higher due to milling energy and pressure effects.
The following is a step by step summary of the emulsion manufacturing parameters using the information presented thus far.
- Determine the rotational viscosity profile of asphalt.
- Determine asphalt EVT (asphalt temperature).
- Determine MEET (minimum mill exit temperature).
- Determine pressure requirements (from steam tables).
- Establish soap temperature based on emulsifier chemistry and/or manufacturer’s recommendations.
There are also parameters to consider in the manufacture of asphalt emulsions, including the following.
Asphalt is practically solid at ambient temperature; therefore, it must be heated and maintained in a liquid state to facilitate transfer and emulsification. In addition, water used in the emulsion manufacturing process is generally added above ambient temperature; thus a heat source is required for both asphalt and water.
For many years, steam was used for heat in emulsion manufacturing. Today the range of heating methods is widely extended. Heating methods currently used are steam, heat-exchange oil, and electric heat.
Dispersing Phase Preparation
The asphalt emulsion dispersing phase, also know as the “soap” phase, consists of water and various emulsifying agents which make up most often a sodium fatty acid salt for anionic emulsions or an amine hydrochloride for the cationic emulsions.
Depending on the manufacturing mode, the dispersing phase is either in one operation, meaning the emulsifiers and acid are added to the water in their exact doses, or in two operations where a concentrated product of emulsifiers and acid are added at a high dose rate to water which is diluted with hot water during emulsion manufacture at a dosing corresponding to the final dosing required.
To prepare the dispersing phase, the manufacturing facility must provide a method of dosing the components (by weight or by volume), mixing and diluting, if necessary, and heating of water. Dosing of components in the preparation of the soap solution may be controlled by simple graduated vessels or highly accurate load cells and mass flow meters. Some plants are equipped with a pH meter to be able to continuously check compliance of the pH indicated to that provided by formulation during the dispersing phase production and subsequent emulsion production.
Dispersed Phase Preparation
The dispersed phase “asphalt” may be either pure asphalt or a blend of asphalt in predetermined proportions with a flux or diluents and possibly other agents, such as elastomer in the case of polymer-modified emulsions. In some cases the emulsifier is fully or partially added to the asphalt prior to it being emulsified.
The various dispersed phase compositions may require equipment whose sophistication may vary from a simple “in-line” mixer to a more elaborate high-performance mixer. Continuous viscosity measurements can be obtained through incorporation of in-line viscometers such as rotational or vibrating (sphere or rod) technology, the later of which is less susceptible to turbulent flow errors.
Industrial manufacture of asphalt emulsion uses custom made equipment to provide thorough mixing capability to provide the fineness and stability of dispersion required to meet the desired asphalt emulsion properties. While high-pressure static mixers and high shear mechanical mixer may be used, colloid mills are more common in the manufacture of asphalt emulsion.
While there are various types of colloid mills, the most common characteristics are an adjustable or fixed air gap between the rotor and stator and fixed or variable rotor rotation speed.
The air gap spacing has a direct effect on emulsion fineness, while rotor speed affects the size and distribution of the asphalt particles. At certain critical speeds, a decantation phenomena may occur causing a reduction in emulsion quality.
The emulsion manufacturing process is a continuous process whereby the dispersing medium is continuously fed with the dispersing and dispersed phases by adjustable flow rate pumps. Mass flow meters may be incorporated to regulate material feed. As stated previously, certain emulsions have special characteristics and must be manufactured under pressure in specially designed colloid mills with necessary cooling provide to prevent boiling of the finished emulsion.
The production rate of emulsion plants is generally greater than demand; therefore, storage facilities make it possible to have longer production runs, thus improving plant productivity.
Present day emulsions may be stored for up to several months without major changes in physical properties. It is advisable to use small diameter vertical storage tanks with a minimum horizontal cross section with a dip tube filling pipe which reaches to or near the bottom of the storage tank.
Emulsions of different ionic types should never be mixed and tanks should be thoroughly cleaned before refilling with a different ionic type. Provisions should also be made to ensure proper agitation of stored emulsion to prevent settling, decantation or creaming.
Emulsions are sensitive to frost which can cause irreversible breaking; therefore, provisions should also be made to prevent stored emulsion from freezing.
From the information given in the preceding discussion one can see why the art of emulsion manufacturing is in the practical use of available science. Understand that this is strictly a guideline based on discussions with industry professionals and their experience and information from the SFERB text, Bitumen Emulsions: General Information and Applications (1). Remember, “Nothing under the sun is absolute, except death, and we are not sure about that.” In science nothing is absolute, therefore, these guidelines should be used as a starting point in the venture to manufacture asphalt emulsion with adjustments made, within limits, to obtain desired outcome.
1. Bitumen Emulsions: General Information and Applications. Syndicat des Fabricants d’Emulsions Routieres de Bitume (SFERB), 1991.
2. Durand, G., and J. E. Piorier. AEMA International Symposium on Asphalt Emulsions, Washington, D.C., 1996.
3. Booth, E. H., G. Gaughan, and G. Holleran. Australian Road Research Board International Conference, Perth, 1994.
4. Holleran, G. AEMA International Symposium on Asphalt Emulsions, Washington, D.C., 1999.
5. Holleran, G. AEMA International Symposium on Asphalt Emulsions, Washington, D.C., 2004.
6. Holleran, G. International Slurry Surfacing Meeting, Puerto Vallarta, Mexico, 1999.
7. Province, R. Workshop on Bitumen Emulsions, Melbourne, Australia, 1986.
8. Thomas J. Glover Pocket Reference, Second Edition. Sequoia Publishing, 2002.
GAYLON L. BAUMGARDNER