13 May An Introduction to Bitumen Emulsions
The first image that pops up into most people’s minds when they hear the word “asphalt” is probably an image like the one that is featured in this blog post: a brand spanking new road. However, “bitumen emulsions” or its interchangeable countertype, “asphalt emulsions,” has many other applications that just road paving. In this introduction to bitumen emulsions, the following questions will be answered:
- What is Bitumen?
- What are Bitumen Emulsions?
- What is an Emulsifying Agent and What Purpose does an Emulsifier have in an Asphalt Emulsion?
- What is an Example of a Product that is an Anionic Bitumen Emulsifier?
- What are the Main Components of a Bitumen Emulsion?
- What is the Manufacturing Process of Asphalt Emulsions?
- How are Bitumen Emulsions Classified?
- Who is Responsible for the Classification of Bitumen Emulsions?
- How are Anionic Bitumen Emulsions Classified?
- Who is Responsible for the Classification of Bitumen Emulsions?
- How are Anionic Bitumen Emulsions Classified?
- What are the Charges of Asphalt Emulsions?
- What is the Difference between Cationic and Anionic Emulsions?
- What are the Setting Times of Bitumen Emulsions?
What is Bitumen?
First, some terminology. The term bitumen is commonly known as “asphalt cement” or “asphalt” in North America. While in other parts of the world, the road paving mixture of sand, stones of various sizes, filler and bitumen is known as “asphalt.” For the purposes of this post, the terms will be used interchangeably.
At ambient temperatures, bitumen is a stable, semi-solid substance and in order to make a beautiful highway like the one displayed above, it has to be applied in an aqueous state. To make asphalt malleable and versatile, bitumen can be heated, dissolved in fuel oils or emulsified in water. However, for different reasons not all of these options are viable. Heating asphalt is expensive and dissolving it in kerosene, gasoline or the like results in pollution from volatile organic compounds (VOCs) and possible occupational hazards. So, the most cost-effective, environmental-friendly and worker-safe option is emulsifying the asphalt in water. By dissolving asphalt in water, a bitumen emulsion is created.
What are Bitumen Emulsions?
An asphalt emulsion is mainly a stable mixture of asphalt cement, water and an emulsifying agent were the bitumen droplets are suspended in water with the aid of an emulsifier. This aqueous solution can be pumped into tanks for storage, mixed with aggregates to be applied to pavement, and can be used in several different ways.
The main characteristic of all asphalt emulsion is to eventually “break,” independently if its upon contact with aggregates (sand, filler, stones, etc…) when mixing or after being sprayed onto a surface. The terminology “breaking” is used to describe the process in which water separates from bitumen after the asphalt emulsion is applied or mixed.
Also, bitumen emulsions can break in 3 different ways: 1) by chemical destabilization of the emulsifying agent, 2) simple evaporation of water or 3) a combination of both above-mentioned options. It all depends on how the emulsion will be used and what is the desired breaking speed. Consequently, after the asphalt emulsion breaks and cures, the bitumen residue has the same durability, adhesion, and water-resistance properties of the original base asphalt.
If imagining a bitumen emulsion is hard, think of paint and shampoo. These are common examples of emulsions we use every day.
What is an Emulsifying Agent and What Purpose Does an Emulsifier have in an Asphalt Emulsion?
The emulsifier, or the interchangeable term surfactant, is a chemical surface-active agent and is formed of large molecules. These soap molecules are like tadpoles, they have an electrically charged head soluble in water and a hydrocarbon tail soluble in the liquid bitumen. In short, the asphalt droplets are surrounded by the ionic charge of the emulsifying agent, resulting in the bitumen particles repelling each other and remaining suspended in the water solution. A typical emulsifier in action is shown below.
Emulsifiers are also called chemical surface-active agents and are classified by their electrochemical charge. There are 3 types of charges: anionic, cationic and non-ionic. In the case of anionic emulsifying agents, the electrochemical charge is negative, while for cationic emulsifiers, the charge is positive. In the case of non-ionic, there is no charge. All three types are surfactants and in their own way, give stability to the bitumen emulsion by not allowing the asphalt to separate from the water in the solution.
For instance, by mixing an anionic bitumen emulsifying agent with water and asphalt, the anionic surfactant will give a negative charge to all the bitumen particles. Since opposite charges attract and same charges repel, all of the negatively charged droplets repel each other and remain suspended as distinct asphalt droplets in the solution. The same is true for cationic emulsifiers, only with positive charges imparted to all asphalt particles. As an example, an anionic emulsifier is highlighted below.
What is an Example of a Product that is an Anionic Bitumen Emulsifier?
Within Polytrade’s Carbofen product line there are anionic bitumen emulsifying agents, such as Carbofen Gold, Carbofen 6060 and Carbofen 5055. As anionic surfactants, the Carbofen resins keep the asphalt droplets of microscopic size suspended in water after stirring. As a result, the bitumen becomes liquid and can be applied to a wall or road in several ways and will turn solid once again after the rupture of the emulsion as water leaves the solution by evaporation.
All products in the Carbofen product line that are emulsifying agents impart negative electric charges at the external surface of asphalt drops. Consequently, the Carbofen resins produce anionic emulsions which are widely used for waterproofing in several commercial and residential sites, dam sealing and dust control in mines.
What are the Main Components of a Bitumen Emulsion?
Since a bitumen emulsion is essentially asphalt particles suspended in water, there are 3 main components to this kind of emulsion: asphalt cement, water and an emulsifying agent.
The first and main element of bitumen emulsions is asphalt cement. Usually, 50 to 75% of the total emulsion consists of asphalt cement. Thus, it is not a surprise that some of its particular characteristics significantly affect the finished bitumen emulsion, which also have an impact on the efficiency of the base asphalt after application. For instance, the type of asphalt cement has an effect on the initial waterproofing ability of the pavement when applied to a road. However, there is no exact correlation between the potential to emulsify the asphalt cement and its properties, so trial and laboratory testing is essential to find the ideal mix for the desired application.
The second component of bitumen emulsions is water. However, it is not normal drinking water. Conventional water used for consumption may not be suitable for asphalt emulsions, because minerals or other materials in the water can react with the properties of the asphalt cement and the emulsifier, resulting in an adverse reaction to the stability of the bitumen emulsion. Consequently, pH neutral, distilled water and the like are recommended when producing asphalt emulsions.
The third component of a bitumen emulsion is the emulsifying agent. The product (or products) used as emulsifiers significantly impacts the properties of an asphalt emulsion. Not only does an emulsifying agent keep bitumen droplets suspended in water and determines its breaking time, the chemical compatibility of the asphalt cement with the emulsifier is also vital for creating a stable emulsion. Consequently, the chemical type and quantity of the surfactant used in the manufacturing process of the bitumen emulsion determine whether it is cationic, anionic or non-ionic and what applications the emulsion can be used for.
Finally, there are other components, like polymers, latex, acids and other materials, that can be added to an asphalt emulsion to enhance or modify its properties for a variety of applications. But in a nutshell, asphalt cement, water and at least one surfactant are the base ingredients that compose a bitumen emulsion.
What is the Manufacturing Process of Asphalt Emulsions?
Asphalt emulsions can be composed of as little as three ingredients, such as asphalt cement, water and an emulsifying agent, to several components to enhance the properties of the final application. But how are bitumen emulsions made? Here is a straightforward illustration that summarizes how asphalt emulsions are made, independently of their type and classification.
In order to suspend the asphalt cement in water, it has to be sheared into microscopic droplets ranging from 1 to 10 microns. This can be accomplished with the aid of a colloid mill or a rotor & stator. The process of shearing (or “cutting” bitumen) into microscopic particles happens in specialized plants that usually have a colloid mill to create these droplets that are smaller than the average 70 to 100 microns of thickness of a human hair.
Consequently, a bitumen emulsion is most commonly prepared by separately metering the heated asphalt cement, water, surfactant and any other components into a colloid mill at preset temperatures and rates. The temperature of the bitumen, rates of component addition and timing of the blend is closely monitored because the size of the asphalt particles and the quantity of surfactant affects the physical and chemical properties of the finished emulsion.
How are Bitumen Emulsions Classified?
In a nutshell, asphalt emulsions are mainly identified by their charge, them being anionic, cationic or nonionic, and their set, which are rapid, medium, slow or quick setting times.
Other than charge and breaking time, bitumen emulsions are classified by asphalt penetration ranges, modification with special additives, their viscosity and storage temperatures. Below is a table displaying some of the most common prefixes, suffixes and their abbreviations.
Who is Responsible for the Classification of Bitumen Emulsions?
In the US, there are two main institutions that have standardized and defined how asphalt emulsions are classified, the American Association of State Highway and Transportation Officials (AASHTO) and the American Society for Testing and Materials (ASTM). The vast majority of highway agencies have adopted the harmonized system ASTM and AASHTO have established. However, like every rule has an exception, there are some proprietary and/or local designations.
Around the world, there are domestic institutions like AASHTO and ASTM that regulate and standardize how asphalt emulsions, pavements and its derivatives are classified, identified and tested. For example, in Brazil, the Brazilian Association of Technical Standards (free translation), or ABNT for short, is the organization that is responsible for standardizing testing and classifications for all types of materials. From paints to concrete additives to emulsions and everything in between, they mostly follow the lead of ASTM and AASHTO, but ABNT adapts standards set by the American institutions to the reality of Brazil.
How are Anionic Bitumen Emulsions Classified?
As discussed before, asphalt emulsions are classified by their charge, setting time, modification, viscosity and storage. Below is a table summarizing the most common types of anionic bitumen emulsions.
In the following sections, we will discuss in more detail the properties and behavior of asphalt emulsions, how they are identified, the main applications of anionic bitumen emulsions, their ideal storage conditions and standardized testing according to ASTM and AASHTO.
What are the Charges of Asphalt Emulsions?
There are three possible charges for a bitumen emulsion, positive, negative or neutral. Positively charged asphalt emulsions are identified as Cationic Emulsions, while negatively charged emulsions are classified as Anionic Emulsions. The third and less common group of asphalt emulsions is Nonionic, which have a neutral charge.
The importance of knowing the charge of a bitumen emulsion lies on its affinity to particular aggregates. Also, having knowledge of the charge is imperative in order to not mix positive and negative charges together. Since opposites attract, the emulsion would be useless if it sticks to itself instead of adhering to the aggregates during application.
What is the Difference between Cationic and Anionic Emulsions?
There are three main differences between cationic and anionic emulsions. The first and most obvious is their charge – which is also determined by the choice of emulsifying agent – the second difference is their solubility in distinct systems and lastly, their resistance to certain materials.
Not all emulsions are made the same way. Cationic bitumen emulsions are positively charged while anionic bitumen emulsions are negatively charged. Not only does the surfactant determine the charge of the emulsion, its choice also impacts the reliability and efficiency of the final product.
Moreover, the types of materials that these emulsions dissolve in are also different. Due to their chemical makeup, anionic bitumen emulsions readily dissipate in high pH or alkaline detergents. On the other hand, they have a good resistance to acids. The opposite is true for cationic emulsions, which have a high tolerance for alkaline detergents and are susceptible to dissolving in acid solutions.
What are the Setting Times of Bitumen Emulsions?
Most asphalt emulsions fall under one of the following terms for setting time: rapid set (RS), medium set (MS), slow set (SS) and quick set (QS). Setting time is a term used to describe the coalescing rate in which the emulsion will bond to its application – the aggregates in a pavement to form a highway, for instance.
Due to rapid setting emulsion’s characteristic of breaking fast, this category has little to no stability and RS bitumen emulsions are rarely used to mix aggregates. Consequently, spray applications like sand and chip seals are the main utility for this type of asphalt emulsions.
Next, there are the medium set emulsions, which are most commonly referred to as mixing grade emulsions due to their ability to mix with aggregates. By design, MS bitumen emulsions are formulated not to break immediately upon contact with graded aggregates. Also, depending on the formulation, this type of asphalt emulsion will continue to retain its workability for a few minutes to several months. As a result, MS emulsions are primarily used in pugmills for cold recycling, patch mixes, both cold and warm aggregate mixes, and other mixture applications.
Following the above-mentioned types of breaking time, next are slow set bitumen emulsions. Yep, you guessed it, they are called slow set because this type of asphalt emulsion produces the most stable of bitumen emulsions and take the longest to break. Due to this characteristic, SS emulsions are utilized to mix with fine aggregates, while still allowing for extended workability and maximum mixing time. Since this category is designed for dense aggregate mixing, SS emulsions have several applications in slurry seals, base stabilization, some recycling and certain asphalt surface courses (like driveway sealers). Also, when diluted with water in order to reduce its viscosity, SS bitumen emulsions can be used in tack coats, dust control and fog seal applications.
Last but not least are quick set emulsions. Like the name suggests, this type of asphalt emulsion sets quickly, like RS emulsions. However, this category of asphalt emulsions is a special one, since very few emulsions fit its 3 criteria. Emulsions can only be classified as QS if they have a reactivity between MS and SS, do not need to pass the cement mix test and are primarily used in quick-set slurry surfacing applications. In addition to slurry seals, QS emulsions are designed for instances where a quick breaking time is needed to fix pavements while still minimizing closed traffic time, like micro-surfacing.
Below is an illustration summarizing the types of setting times for bitumen emulsions. Of course, one has to take into account the nature of the surfactant used and what it was designed to specifically do, but the higher the dose of the emulsifying agent added to the emulsion, the slower is the setting time.
More Information and Resources about Asphalt Emulsions
Still foaming at the mouth for more information about bitumen emulsions? Here are some institutions to check out:
- The Asphalt Institute and the Asphalt Emulsion Manufacturers Association (AEMA),
- International Slurry Surfacing Association (ISSA),
- Asphalt Recycling and Reclaiming Association (ARRA),
- Foundation for Pavement Preservation (FP2).
Let us know by leaving a comment below or talk to our team.