We would like to acknowledge Jochum Beetsma for sharing his technical expertise and validating the page.
- Pigment Dispersion: A quick definition for formulators
- 14 signs your pigment dispersion is poor
- The dispersion process in more details
- How to assess the quality of your dispersion?
| Navigating Pigment Dispersion in Paints and Coatings |
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Pigment Dispersion: A Quick Definition for Busy Paints & Inks Formulators
Pigments and fillers are powdery materials consisting of insoluble solid particles. In the powdery dry state, the solid particles are surrounded by air. Separated solid particles in liquids tend to flocculate, creating lumps.
In the paints and inks world, dispersing pigments and fillers is the action of:
- replacing air around the particles with the paint/ink medium (wetting)
- breaking the agglomerates of solid particles to create a suspension of smaller particles (separation)
- ensuring the suspension remains stable over time (stabilization)
Proper dispersion assures stability of the dispersion throughout manufacture, storage, application and film formation (in chemistry).
14 Signs Your Pigment Dispersion is Poor
Achieving good dispersion of all solid particles is crucial for the paint's final aesthetics and performance. Here is what you may experience when your dispersion is poor:
- Color inconsistency: From batch to batch you do not get the exact same color.
- Discoloration after drying: Once solvent and/or water has evaporated, the tone of your dried coating is whiter than it used to be in the can.
- Lack of gloss: The painted surface is not as shiny as it should be. Achieve glossy finish in your coatings.
- Floating - Benard Cells: In case you use two or more pigments, you may observe the formation of hexagonal patterns with color differences at the surface of your coating.
- Specks: Noticeable color defects may be visible at the surface of your dried coating.
- Lack of hiding power: Your coating may fail to properly hide the substrate after film application. Get the desired hiding power in your coatings.
- Flooding: Your coating film color is uniform, but lighter or darker than intended. It is linked to a vertical separation of different particles.
- Bad rub-out: The color and/or gloss difference between the test and the control section is too high.
- Flocculation: Particles glue together during storage and/or application and/or film formation.
- Sedimentation / Phase separation: After storage or transport, two phases are visible instead of one.
- Settling: After storage or transport, there is a formation of a hard layer at the bottom of the can. Check how to improve paints storage stability.
- Pigment shock: Sudden flocculation and/or increase in viscosity when adding pigment concentrates to base paint.
- Too long dispersion time: It may take ages to reach the desired fineness of grind and color strength.
- Foaming: Air bubbles may form when the wetting phase is not fully mastered and/or when too much wetting agent is used and/or when the wrong type/amount of dispersant is used.
Else, if you prefer to upskill and troubleshoot on your own, this online course will be beneficial:
![Pigment dispersion & stabilization root causes]()
The Dispersion Process in More Details
From a dry powdery material, the goal is to create a stable suspension of fine solid particles. Three phenomena take place at the same time:
- Wetting
- Separation
- Stabilization
For the sake of a clear explanation, we will cover each phenomenon one by one.
STEP 1 - What is pigment wetting?
The wetting step consists of replacing the air on the surface of the particles and entrapping the pigment agglomerates with liquid. The {pigment/air} interface becomes {pigment/liquid} interface.
Why does surface tension matter for good wetting results?
Wetting can only occur when the
surface tension of the liquid is low enough, compared to the surface energy of the solid particles.
How do I know about the surface energy of my pigments and fillers?
You may find it in the product datasheet. However, often, there is limited information about the surface energy of pigments and fillers. No worries though; you can easily assess wettability with water or the chosen solvent with a quick test in the lab:
Many commercial pigments and fillers are surface treated to improve dispersibility. When you formulate paints, make sure to select additives suited for your paint type!
What should I do in case of holdout?
In case you experience holdout (liquid does not wet the particles), you have two options:
Why is it better to achieve wetting without adding extra additives?
When wetting is insufficient or not possible, you may use wetting agents as facilitators.
They are useful additives; however, they can cause problems like:
This is why it is often a better choice to select particles with a better-suited surface treatment.
How to further optimize the wetting of your solid particles?
Wetting proceeds faster when the viscosity of the liquid is lower. The importance of wetting is often misjudged giving problems in the steps to follow in the dispersion and stabilization process.
Optimal Pigment Wetting is Essential for Proper Dispersion Conditions
STEP 2 - What is happening during the separation phase?
After wetting particles, it is time to split the agglomerates. The goal of the separation phase is to get smaller particles. Breaking agglomerates calls for a high amount of mechanical energy. This is where dispersing equipment comes into play.
Principles Used to Split Agglomerates
Shear and impact are the two main principles used to split agglomerates.
- Shear – A disk disperser, often called a dissolver, works by means of shear forces. A disk, with teeth on the edge, rotates with high speed in a liquid mill base of high viscosity. The dissolver introduces mechanical energy into the system. This shear forces energy splits agglomerates into smaller particles.
- Impact – In a bead mill, often called a pearl mill, the impact principle (combined with crunching) is used to split agglomerates. Beads (pearls) collide with each other at high speed and agglomerates, finding themselves between two beads, are separated into smaller pieces. A moderate to low viscosity is needed in order to make high-velocity collisions possible.
Mill Chamber of a Bead Mill and Beads
In the center of the mill, an axis with rotor blades rotates at high speed in the mill chamber. The input of a bead mill is a pre-dispersion that is made by using, for example, a disk disperser. The output of a bead mill is a dispersion of solid particles in a liquid, containing particles that have the required particle size.
STEP 3 - What is pigment stabilization?
After achieving smaller particles, the stabilization step consists of avoiding flocculation.
Why does flocculation occur?
Because of attractive van der Waals forces, that are always present, separated particles tend to glue together. This process is called flocculation.
So, how to get a stable dispersion?
The goal is to introduce repulsive forces. For good dispersion stability, repulsive forces must be stronger than attractive forces. In practice, we use dispersants (also called
dispersing agents).
There are two main approaches to create such repulsive forces:
- Steric stabilization
- Electrostatic stabilization
| Electrostatic Stabilization |
Steric Stabilization |
- Possible when pigment particles have the same surface charge or when charged molecules are adsorbed at the surface of the particles
- Mainly for dispersions in water
- Stability depends on pH
- Stability can be affected by high salt concentrations
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- Liquid phase soluble polymeric tails chains are at the surface of the particles
- Strong stabilizing mechanism
- Suited for waterborne and solventborne systems
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Choosing one or the other is an important starting point for the selection of your dispersant. Let's see these two stabilization methods in more detail.
Steric Stabilization of Solid Particles
Stabilization happens with adsorption of monomeric or polymeric species at the pigment surface. The adsorbed layer provides a steric barrier. It prevents approaching particles from collision.
Dispersant must have good affinity with the pigment, so adsorption takes place. The steric layer depends on the structure of the dispersants.
Find more details on how to choose your dispersant!
Steric stabilization is the most secure principle to use in both solvent-based and water-based systems. In apolar system, this is the only stabilization method available.
Electrostatic Stabilization
Particles that have an electrostatic charge of the same sign repel each other. Flocculation is prevented when the repulsive forces are strong enough. This stabilization mechanism depends upon key system properties like pH and the amount of electrolyte.
Electrostatic stabilization is effective in systems of high polarity (like waterborne coatings). It enables full dissociation of ionic compounds, brought onto the pigment surface. In water-based paints and inks, anionic stabilization is the most common. It means that all particles have a negative charge.
Else if you prefer to grow your understanding first, this course combo is a good next step:
![Practical Tips for Pigment Dispersion & Stabilization]()
How to Assess the Quality of Your Dispersion?
Fineness of Grind
A
Hegman gauge is an automatic machine using the draw-down method. It contains a stainless-steel block with either one or two channels. It also has a precision ground scraper with an increasing depth from zero to a specified depth. The procedure includes:
- Place the gauge on a flat, horizontal and non-slip surface with the zero mark on the scale closest to you.
- Then place a suitable amount of paint sample on the deep ends of the channels.
- Hold the precision scraper behind the paint sample with both hands. Keep the scraper angled towards you or in a perpendicular direction.
- Pull the scraper along the length of the gauge at a constant speed using downward pressure. Ensure that the sample scraps over the entire length of the channel.
- Stop the pull at a point beyond zero depth and view the angle at 20 to 30° to view the result.
The reading is further identified on the scale of the gauge. This instrument measures particle dispersion indicated in microns or Hegman. This method also assures the absence of agglomerates.
Some standards used to calculate the fineness are ISO 1524, ASTM D 3333, ASTM D 1210, ASTM D 1316, and DIN EN 21524.
Discover more about the fineness of grind in coatings »
Rub-out Test
The rub-out test checks flooding and
tinting strength. Apply the sample paint on the plate and scrap it over the entire length using a scrapper. As soon as the paint starts to dry the rub-out test begins. Rub the thumb or index finger in a circular or linear direction over the applied paint. This ensures the even distribution of the pigment. The color difference between the rubbed and unrubbed parts is then evaluated.
(Source: BYK)
Developing Knowledge About Pigment Dispersion
Here is a selection of resources to learn about the dispersion process:
