If you work in the field of bitumen emulsions, you have probably already heard of the concept of emulsion “particle size”. This article provides you with the basic knowledge to understand what particle size means in the field of asphalt emulsions, and why it is of major importance when formulating and producing bitumen emulsions.

## Table of contents

## Why learning about the particle size ?

A bitumen emulsion must be as stable as possible before use, and then break effectively in contact with the road-s surface and the aggregates. The thing is that the stability and breaking behavior of the emulsion are strongly influenced by the particle size.

Bad particle size = Risk of phase seperation in the tanks + Non-homogeneous beaking

Good particle size = Maximized stability + Controlled breaking

## Particle size

In the road sector, the particle size mostly refers to determining the particle size curve of an aggregate formula, which means sieving the aggregates using several sieves, and the mass retained on each sieve is weighed to draw a particle size curve. For formulators, this curve provides crucial information on the quality of the aggregates, particularly through the fines content. The formulator adjusts this curve so that the final mix can correspond to the intended application and site specifications.

If you know these concepts, then you will not be disoriented with bitumen emulsions.

## Particle size of bitumen emulsions

In the field of asphalt emulsions, the particle size encompasses two concepts: the mean diameter and dispersity (or particle size distribution). These two concepts, which are explained in the following paragraphs, go hand in hand: together, they give information on the quality of an emulsion, which is key to adjust formulas and improve their properties. Many of the properties you know arise from the particle size of the emulsion, such as the stability of the emulsion, its viscosity, and even its breaking behavior. At VIALAB, we have carried out particle size analyzes to show the effect of bitumen temperature on the quality of emulsions. The particle size of emulsions is therefore major for formulators and quality control operators, and even plant managers: evaluating the particle size of the emulsions produced makes it possible to optimize the production parameters (temperature of the bitumen and the aqueous phase, maturation time of the aqueous phase, mill rotation speed) and limits the risks of phase seperation. Every plant manager has already had the experience of an emulsion that separated in a storage tank. Rest assured: the particle size of emulsions can help you.

## Mean diameter

The mean diameter of an emulsion is the size that separates the particles into two groups: 50% of the particles have a diameter greater than the mean diameter, and 50% of the particles have a smaller diameter.

The mean diameter is key for evaluating the quality of an emulsion. Indeed, the stability of an emulsion partly depends on its mean diameter: the larger the particles, the more they will tend to group together at the surface (creaming) or at the bottom (sedimentation) of the storage tank. This phenomenon is related to Stokes’ law: the migration speed of a particle in a dispersing phase is linked to the size of the particle, to the difference in densities between the two phases, and to the viscosity of the dispersing phase.

v = Migration speed (m/s)

g = earth’s attraction (m/s^{2})

r : particule radius (m)

ρ : density of the phase (kg/m^{3}) [p : particle ; f : dispersing phase]

μ : dynamic viscosity of the dispersing phase (kg/m.s)

The smaller the particle size (r ↘), the lower its migration speed (v ↘), and therefore the slower the phase separation. With a few assumptions, this law can be used to understand the parameters influencing the phase seperation of asphalt emulsions: in general, the lower the median diameter of the emulsion, the more stable it is. Of course, other phenomena can influence the stability of an emulsion (quality of the bitumen and the emulsifier, activation of the emulsifier, emulsifier content) but a small mean diameter between 1 and 20 μm will always be a stable work basis.

## Dispersity

If all the particles in the emulsion have the same size, the emulsion is said to be “monodisperse”. On the other hand, if the particles all have a different size, the emulsion is said to be “polydisperse”. In reality, emulsions are never completely monodisperse or completely polydisperse, but somewhere in between. The closer the particle size is to the mean diameter, the closer the emulsion is to monodispersity, which is a sign of quality. Values such as the standard deviation are used to evaluate the dispersity of an emulsion.

ε : standard deviation

D84 : Size for which 84% of the particles are bigger

D16 : Size for which 16% of the particles are bigger

There is no limit value for considering an emulsion as monodisperse or polydisperse, however the particle size of several emulsions can be compared to evaluate their difference in quality.

In the field of bitumen emulsions, the more monodisperse an emulsion is:

- The more stable it is ;
- The higher its viscosity ;
- The more homogeneous the breaking of the emulsion (less risk of water being trapped in the binder).

It is often better for an emulsion to be as monodisperse as possible to maximize its stability and optimize its breaking behavior. For example, bitumen emulsions from polymer modified bitumen (PMB) tend to have high dispersity, making them less stable and more fluid than bitumen emulsions from pure bitumen.

## Conclusion

The properties of a bitumen emulsion are all linked to its particle size, directly or indirectly. Particle size is therefore a central element for the formulation and quality control of emulsions. Being able to detect when the particle size starts to degrade giving yourself the opportunity to act quickly and avoid setbacks in the plant. On the formulation side, new formulas can be developed without fear of disrupting the essential properties of emulsions: their stability and their breaking behavior.