Background of bitumen emulsion development
On 9 May, 1922, an English chemist, Hugh Alan Mackay, filed a patent for the production of bitumen emulsion. He claimed in his patent application that a stable dispersion of bitumen in water can be obtained if the bitumen is dispersed with high shear in water containing a surfactant or soap. The advantage of the bitumen emulsion was that it was liquid at ambient temperature in contrast to bitumen that is semi-solid at ambient temperatures. This new technology rapidly spread throughout the world and by 1926 it was estimated that a cumulative total of 150,000 tons of bitumen emulsion was produced by England, Germany, Denmark, Australia and India.
The soaps or emulsifiers used for the production of the first emulsions were obtained by neutralising fatty acids such as Oleic Acid or Tall Oil with inorganic alkalis such as caustic soda or caustic potash. These neutralised fatty acid salts consisted of long organic alkyl chains with a carboxyl end group which imparted a negative charge to the bitumen droplets. The organic end of the chain is hydrophobic and has and affinity for the bitumen surface to which it adsorbs. The carboxyl end of the chain is hydrophilic and extends into the water phase, thereby imparting a negative charge to the bitumen droplets. The size of the bitumen droplets produced varied from 4 to 7 microns, depending on the type and quantity of emulsifier used.
RCOOH + NaOH → RCOO– + H2O + (Na+
Fatty acid + Caustic soda Anionic soap
Anionic soap molecules adsorbed on a bitumen droplet
Up to the early 1950’s, the anionic emulsions were the only bitumen emulsions available to the road building industry and the scope of applications were limited due the limited range of emulsifiers available. The only emulsions that could be produced with the fatty acids were the rapid and medium set variations. The rapid set emulsions were used for construction of single and double chip seals and for tack coat applications under hot-mix asphalt. The medium set emulsions were used for preparation of patching mixes using clean, dust-free aggregate fractions. Due to the lack of a suitable emulsifier it was not possible to produce emulsions capable of mixing with aggregates containing large proportions of mineral fines and cement. In the late 1950’s the bitumen emulsion industry was revolutionised when a new rosin based emulsifier, derived from pine tree stumps, became available from America. This emulsifier, known as Vinsol Resin, enabled the industry to produce emulsions capable of mixing with aggregates containing large quantities of mineral fines. Emulsions produced with Vinsol Resin were also capable of mixing with pure cement. New technologies such as Slurry Sealing and Emulsion Treated Basecourse (ETB) thus became available to the road building/surfacing industry. The new type of emulsion was classified a Stable Grade due to its great chemical stability.
Two common problems were experienced with the anionic emulsions produced from fatty acids. These were:
- poor dilutability – the emulsions would break prematurely when diluted with water containing traces of heavy metal ions such as calcium, magnesium or aluminium. It was thus difficult to perform activities such as fog sprays or diluted tack coats
- Chip seals constructed with acidic aggregates such as quartzites and granites were prone to stripping or aggregate loss during prolonged wet conditions.
Further investigation by emulsion scientists revealed that most acidic aggregates are predominantly negatively charged under wet conditions, due to slight solubility of the silica dioxide component in water. These negative charges would repel the negatively charged residue of the binder deposited by the anionic emulsions, leading to adhesion failure on the aggregate/binder interface.
The scientists realised that if the charge on the bitumen droplet could be reversed, strong electrostatic attraction between the aggregate and the bitumen droplets would be possible. They found that chemical compounds known as amines could solve this problem.
Long-chain amines, when neutralised with an inorganic acid such as Hydrochloric or Sulphuric Acid, would impart a positive charge to hydrophilic part of the molecule extending into the water phase. The positively charged emulsions are called cationic emulsions. Since the late 1950’s, cationic spray and medium grade emulsions have virtually replaced the anionic equivalents. Unlike anionic rapid and medium set emulsions, cationic emulsions are universal in terms of adhesion and will adhere to all types of aggregate, irrespective of the geological origin.
RNH2 + HCl → RNH3 + + (Cl –
Amine + Hydrochloric Acid Cationic soap
Cationic soap molecules adsorbed on a bitumen droplet
In the late 1970’s special cationic emulsifiers became available that enabled the emulsion industry to produce stable type of cationic emulsions capable of the same applications as the emulsions produced with Vinsol Resin. The cationic stable grade emulsions were popular for a number of years, but due to the higher cost of these emulsions, the industry reverted back to the anionic equivalents which had a proven track record. South Africa is one of the few countries in the world that still produce anionic stable grade emulsions. Anionic stable grade emulsion is the most popular emulsion currently being produced in South Africa.
Anionic emulsions generally have a pH value ranging from 10 to 12 due to the inclusion of alkali in the formulation, whilst cationic emulsions have a pH value ranging from 2 to 4 due to the inclusion of the inorganic acid in the formulation.
Types of emulsions
As mentioned above, anionic and cationic emulsions can be divided into three distinct groups
The characteristics of anionic emulsions are specified in SABS 309 and the following classifications are used:
- Rapid-set (RS)
- Medium-set (MS)
- Slow-set (SS)
The characteristics of cationic emulsions are specified in SABS 548 and the following classifications are used:
- Rapid-set (CRS)
- Medium-set (CMS)
- Slow-set (CSS)
Cationic spray grade emulsions may contain up to 5% by mass hydrocarbon solvent (flux) to assist with the breaking or coalescence of the emulsion. During the summer months the fluxant level is reduced. Cationic premix grade emulsions may contain up to 10% fluxant to enhance the stockpile life of cold mixes prepared with this emulsion. Anionic emulsions contain no fluxants.
Bitumen emulsions are generally prepared with 70/100 penetration grade bitumen in South Africa. In special cases, if required, the emulsions can also be prepared with 50/70 penetration grade bitumen.
Bitumen emulsions can only be manufactured with harder grades of bitumen if the emulsion plant is equipped with a heat exchanger or a cooler. To emulsify the harder grades of bitumen, the hard bitumen must be heated to excessively high temperatures before introduction into the mill. The exit temperature of the mill will then exceed 100 °C, which is the boiling point of water at sea level. It is thus necessary to cool the emulsion to below the boiling point of water, before storage.
The bitumen content of bitumen emulsions vary from 60 to 70%. The viscosity increases as the bitumen content increases. Emulsions that are used in applications such as mixing, where a low viscosity is required, will thus have a bitumen content of 60%. Cationic emulsions used for hand spray applications will also have a bitumen content of 60% to facilitate pumping and spraying of the product. Cationic spray grade emulsions used for application by binder distributors, will have either 65 or 70% bitumen contents. The 70% emulsions are generally very viscous and will not flow excessively when applied on inclines. Emulsions with binder contents exceeding 73% are too viscous for handling with normal application equipment.
Anionic emulsions (SABS 309)
|Spray (RS)||Medium(MS)||Stable (SS)|
|Binder Content % m/m||60 -62||60 -62||60 -62||ASTM D244|
|Viscosity @ 50 °C (SFs)||20- 50||20 – 50||–||ASTM D244|
|Residue on sieving (g/100 ml)||0,25 max.||0,25 max.||0,25 max.||SABS 309|
|Coagulation value when mixed with dolerite chips % m/m||25 min||25 max.||–||SABS 309|
|Coagulation when mixed with Portland Cement % m/m||–||–||2,0 max||SABS 309|
|Sedimentation after 60 rotations||nil||nil||Nil||SABS 309|
Cationic emulsions (SABS 548)
|Spray (CRS)||Medium(CMS)||Stable (CSS)|
|Binder Content, %m/m||60-63||65-68||70-73||60-63||65-68||60-63||ASTM D244|
|Viscosity @ 50 °C (SFs)||20-50||51-200||51-400||20-50||51-200||50 max.||ASTM D244|
|Residue on sieving, g /100 ml||0,25 max||0,25 max||0,25 max||0,25 max||0,25 max||0,25 max||SABS 548|
|Cathode Deposit after 30 minutes, g||1||1,0 min.||1,0 min.||–||–||–||SABS 548|
|Particle charge Standard procedure Modified procedure|
|Fluxant level, % m/m of binder||5 max.||5 max.||5 max.||5 – 10||5 – 10||5 – 10||ASTM D244|
|Coagulation when mixed with Silica flour, % m/m max||—||—||—||—||—||2,0 max.||SABS 548|
|Sedimentation after 60 rotations||nil||nil||nil||Nil||nil||nil||SABS 548|
Manufacture of emulsions
Bitumen emulsions are manufactured by dispersing molten bitumen in hot water containing a surfactant. The dispersion occurs in an apparatus known as a homogeniser or a colloid mill. Most emulsion plants using a batch system, have a heated bitumen batch tank and heated water tank (emulsifier tank) into which the surfactants are introduced. The temperature of the bitumen is controlled between 130 and 140 °C, whilst the temperature in the emulsifier tank is controlled between 45 and 50 °C. These components are then accurately fed into the mill by proportioning with positive displacement pumps. It is important that the flow rates are accurately controlled to ensure that the correct bitumen content is obtained on the final emulsion. The exit temperature of the final emulsion produced is between 85 and 95 °C. The emulsion produced is pumped to storage tanks from where it is transferred to delivery tankers or placed in drums, when required.
Schematic diagram of a bitumen emulsion batch plant
Modern continuous plants are fully computer controlled. Molten bitumen and heated water is fed to the mill. En-route to the mill, the necessary surfactants are accurately injected into the bitumen and water streams. The modern plants produce emulsions of extremely high quality and the final bitumen content is accurately controlled by electronic mass flow meters.
Breaking or coalescence of bitumen emulsions
Bitumen emulsions break through the following mechanisms:
- Evaporation of water – when the water phase evaporates from a bitumen emulsion, the bitumen droplets come into closer proximity and eventually come into contact with each other. When further evaporation occurs, the droplets eventually flow together and breaking or coalescence occurs. The bitumen droplets thus return to a solid mass similar to the state before emulsification occurred. Anionic emulsions predominantly break through the evaporation of the water phase. The different stages of coalescence are depicted in the diagram below.
- Mechanical action – during the rolling of a chip seal constructed with bitumen emulsion, the particles are forced together due to the mechanical action of the rolling process. If the bitumen in the emulsion has been softened with fluxants, the droplets will coalesce easier. The breaking induced by mechanical action is very effective if the emulsion has already lost part of its water and the particles are in close proximity to each other
- The breaking of cationic emulsions is induced by both of the mechanisms mentioned above, but the destabilisation of cationic emulsions by electrostatic attraction between the bitumen droplets and the negatively charged aggregate particles, also play an important role in the breaking of the emulsion. See diagram below.
Uses of emulsions
Bitumen emulsions are very versatile materials and its use is thus diverse. The general application and uses of bitumen emulsions are discussed below:
Cationic spray grade emulsions and anionic stable grade emulsions are commonly used in diluted form as a tack coat when placing new hot mix surfaces and overlays. It facilitates the bonding of the various layers and prevents slippage. Care should be taken when using cationic spray grade emulsions for this purpose as the emulsion residue contains hydrocarbon fluxants which softens the residue deposited from the emulsion. In winter months, when the fluxant level is highest, the residue could be excessively soft. The fluxants cannot evaporate if entrapped below the asphalt layer.
Surface dressings (chip and spray)
Cationic spray grade emulsions are used as the binder for construction of single and double seals. Because of its rapid breaking rate, seals constructed with these emulsions can be opened to traffick in a short period after application of the stone. In South Africa the use of hot, polymer modified binders is generally preferred for construction of seals during the summer months. A minimum road surface temperature of 25 °C is required when using hot binders. In the cooler months, hot binders cannot be used for surface dressings as the road surface temperature seldom reaches 25 °C. Bitumen emulsions can be used at road surface temperatures as low as 10°C, and the use of spray grade emulsions thus becomes more popular in winter.
Bitumen emulsions are used to prepare cold mixes that can be used in overlays and patching of potholes. In Europe, semi-stable cationic emulsions are used to prepare dense graded mixes that can be used for overlays on roads carrying light to medium traffic levels. Stable grade emulsions (SS) are used to prepare dense-graded mixes, whilst premix grade (CMS) emulsions are used to prepare open graded mixes that can be stockpiled.
Anionic stable mix is the most popular emulsion being produced in South Africa. The vast majority of this emulsion is used in the preparation of slurries. Slurry consists of crusher dust, Portland Cement, stable emulsion and water. It is, therefore, important that the emulsion has sufficient chemical stability to mix with the mineral fines and the reactive cement component.
Slurries are used for texture treatments, overlays and in the Cape Seal application.
Another important application for stable grade emulsions is in soil stabilisation. By incorporating bitumen emulsion in base course materials, the following benefits are derived:
- Improvement in bearing capacity of the stabilised material
- Improvement in the water resistance of the material
- Reduction in compactive effort required to densify the base material
- Improved shear properties
A small quantity of cement is normally incorporated when base course materials are stabilised with bitumen emulsion. The cement increases the early strength of the stabilised material and also enhances the breaking behaviour of the stable grade emulsion.
Special stable grade emulsions are used in dust suppression applications. The emulsions are used in highly diluted form to generally stabilise the upper 50 – 70 mm of mine haul roads and other roads requiring dust suppression. The water used for dilution of the emulsion on the mines are often chemically very aggressive and the highly diluted emulsion thus needs to be exceptionally stable in order to withstand the effect of the water on the emulsion.
Bitumen emulsions can easily be modified by incorporation of fillers, rubber latex, pigments etc. The rheological properties of the compounded blends can be modified by the addition of thickeners. These industrial emulsions are commonly used for:
- Waterproofing of flat roofs
- Waterproofing of water retaining structures
- Floor tile/wood block adhesives
- Underbody sealants for vehicles
- Pruning and grafting compounds for fruit trees in the agricultural sector
Handling of emulsions
When handling emulsions, it must always be remembered that the product consists of bitumen droplets suspended in water. The density of bitumen at ambient temperature is approximately 1,02 kg/l. The density of water at ambient temperature is 1,0 kg/l. There will thus be a natural tendency for the bitumen droplets to settle at the bottom of the container or vessel in which it is stored. Spray grade emulsions generally have a storage life of approximately 3 months, whilst medium and slow set emulsions can generally be stored for up to six months.
If it is envisaged that the emulsion will be stored for an extended period, it is advisable to circulate emulsion stored in tanks once a week by pump circulation to re-disperse settled particles. Air can also be blown into the bottom of storage tanks to re-disperse settled particles. If the emulsion is stored in drums, the drums should be rolled for approximately one minute every week. During prolonged storage, the emulsion particles will also coagulate or grow, thus causing settlement of the emulsion. Emulsion that has been in storage for a very long period, will settle irreversibly. It will then not be possible to use the product. This happens when all the bitumen globules have flowed together to form a solid mass of bitumen. If necessary, the bitumen can be recovered by boiling off the water phase.
When it is necessary to heat bitumen emulsion, care should be taken not exceed 90 °C, as the emulsion could boil at certain altitudes at this temperature. Once the emulsion has boiled it will have broken irreversibly as evidenced by lumps in the product.
Care should be taken when working with emulsions in regions where the temperatures could fall below the freezing point of water. Emulsions that have frozen will not revert back to the original emulsion once thawed and have to be discarded.
If anionic and cationic emulsions are mixed accidentally, the opposite charges on the bitumen particles neutralize each other and the emulsifier systems of the emulsions collapse completely. The droplets coalesce and solid bitumen lumps are formed. Care should thus be taken to prevent cationic and anionic emulsion coming into contact with each other.
Synthetic and natural rubber latex is commonly used in bitumen emulsions to enhance the elastic and ductile properties of the residual binder. The most popular latex used is synthetic Styrene Butadiene Rubber (SBR) Latex. Latex modified emulsions are mainly used in:
- Surface dressing applications on cracked roads carrying medium to heavy traffick.
- Tack coats for adhering Ultra Thin Frictional Course Surfacings (UTFC) to the substrate.
- Microsurfacing and rutfilling applications
- Crack sealing emulsions
Emulsions of cutback bitumens are also available. These emulsions are used for priming applications.
Advantages of emulsions
- Emulsions can be used cold or slightly heated
- Emulsions are non-flammable
- Greater worker safety is ensured when handling emulsions as no harmful fumes are emitted and the there is no risk of burning
- Emulsions require less energy to produce and apply, due to the lower temperatures required
- Emulsions can be diluted with water
- Chemical adhesion between the binder residue and the aggregate
- Cationic rapid- and medium-set emulsions can be used with all types of aggregates.
Compiled by: Kobus Louw
Research and Development Manager
Colas South Africa (Pty) Ltd