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Dispersion Stability in Paints & Coatings: The Formulator's Necessity

Dispersion Stability in Paints and Coatings
  1. Why does proper dispersion in paints & coatings matter?
  2. 3-stages for properly dispersing solid particles
  3. Important principles for effective particle stabilization
  4. Reasons why dispersing & wetting agents are important

Particles Dispersion and Efficiency

Paint quality and final appearance rely on good pigment dispersion. Any paint or coating is a blend of solid and liquid components, certain type of processes and equipment are used to make sure that the pigments and fillers are dispersed and stabilized well.

The dispersion and stabilization of pigments and fillers are one of the most important aspects of the coatings’ optimal performance. The proper dispersion and stabilization of solid particles in coatings is essential for optimization properties, such as:

Proper dispersion assures consistent color, quality and durability
throughout manufacture, storage, application, and film formation.

Pigment dispersion can be a challenging process during the production of paints, coatings, and inks. Waterborne systems are particularly challenging due to environmental regulations, demand for premium color coatings, and cost considerations. Moreover, the quality of the dry paint film or ink is very much related to the uniformity of the distribution of the pigment. Inferior mechanical film properties and low film transparency are often related to insufficient pigment dispersion.

Pigment dispersion & stabilization root causes

Many performance-related problems can be prevented as well as time and money can be saved when fundamental knowledge, key information about the raw materials & their selection and equipment used is understood well.

If you are already well versed with basics, then get detailed tips and guidelines for selecting to find the right dispersant for your system in our free guide. Else, let's begin with three-stage process of dispersion.

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3-Stages of Dispersion Process

Generally, dispersion is described as a three-stage process:

  • Wetting: particle surfaces must be wetted, and adsorbed air replaced with liquid medium
  • Separation/Dispersing: breaking up clumps of flocculated, agglomerated, and aggregated particles
  • Stabilization: avoiding re-flocculation

The whole process of dispersion and stabilization starts from dry powdery material, consisting of agglomerates of solid particles. The powdery solid is mixed with liquid and wetting of the particles takes place. The solid particles are separated from each other by applying mechanical force. Immediately after separation, the particles have to be stabilized to prevent flocculation, the gluing together of solid particles in a liquid.

These three steps lead the formulator from dry pigment agglomerate to a stable pigment or filler dispersion

Understand each stage in detail.

Stage One: Wetting

In the wetting step, the solid pigments are added into the liquid phase. In this phase, the air and moisture entrapped in the dry pigment agglomerates is substituted by the liquid used in the dispersion process. The pigment | air interface becomes pigment | liquid interface.

It is important to note that wetting can only proceed when the surface tension of the liquid is lower than the surface energy of the solid particles.

Related Read: Go deeper into the underlying science of Surface Tension to see how it determines overall effectiveness of any coating.

Unfortunately, the information about surface energy of pigments and fillers is not always easy to get. In many cases this key property is governed by the type and amount of post-treatment used in the pigment production process. Insight into surface energy of materials can be obtained in a fast and easy way by doing simple experiments.

Proceeding further, the liquid needs to wet the pigment surface i.e., replace the adsorbed air and water onto pigment particle by the liquid medium in the mill-base, by lowering the surface tension of a liquid.

Wetting Stage of Pigment Dispersion

Unwanted Challenges You Might Face at Wetting Stage

Wetting agents are additives that are used to facilitate complete wetting of the particle. However, wetting agents can cause problems like foam, insufficient intercoat adhesion, increased water sensitivity and reduced film hardness. Fortunately, in many cases, wetting can happen naturally and wetting agent is not needed in the process. For cases where you cannot achieve wetting, then wetting agents come to the rescue. It is recommended to check first if they are needed in your system...Take a this free tutorial to check if your system is already achieving good wetting of pigments or fillers before adding any additive.

The right types of pigments and fillers must be selected in order to be able to develop a system without wetting agents. Explore here, the detailed knowledge on various pigment families used in inks, paints, and coatings and select the ideal product satisfying the requirements of your coating formulation.

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
Optimal Pigment Wetting is Essential for Proper Dispersion Conditions

Stage Two: Separation

In the separation step, the wetted pigment agglomerates (solid particles glued together) are split into individual pigment particles. Because of the strong cohesive forces between the pigment particles, this requires a high amount of mechanical energy that is provided by suitable equipment. The smaller the pigment particles the more energy is needed to separate them. The dispersing energy (related to the dispersing time) is an extremely important parameter to reach an optimal separation of all pigment agglomerates to the ideal primary pigment particles.

Separation of the wetted pigment agglomerates

Principles Used to Split Agglomerates

Shear and impact are the two main principles used to split agglomerates.

  1. Shear - The grinding equipment, disk disperser, often called 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 in the system. This shear forces energy breaks and separates the pigment agglomerates in smaller particles. This new created surface is wetted by the grinding medium liquid.

    Separating Equipments

    One key aspect to note here is temperature, as it turns out to be a key parameter when using a disk disperser in the dispersion process. The temperature rises when high shear stress is applied on a high viscous liquid and, because of that, viscosity goes down. And, when a low amount of shear is applied, the agglomerates are not further split into smaller particles. The obvious solution to this problem is to cool the vessel in which separation is done.

  2. Impact - In a bead mill, often called pearl mill, the impact principle (combined with crunching) is used to split agglomerates. Beads (pearls) collide on each other with 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
    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.

Solving Surface Tension Issues

Stage Three: Stabilization

Naturally the newly obtained small particles tend to re-agglomerate. It is the “flocculation”. Particles that have been separated from each other have to be stabilized in order to prevent flocculation.

In the absence of repulsive forces, the disrupted and suspended pigment particles will flocculate in response to attractive forces (London-van der Waals forces) between approaching particles. Explained by Brownian motion and contributing to a lesser extend gravitational forces, particles may collide and form flocculates: the smaller the particle size, the stronger the Brownian movement of the particle and the higher the probability to encounter a second particle. Flocculation due to Brownian movement is most pronounced for small particles sized organic pigments. Flocculation is prevented by introducing repulsive forces onto the pigment particles.

Uncontrolled aggregates and flocculates have poor influence on the paint quality and can reduce the paint stability, reduce the color strength and gloss, or change the paint’s rheology.

Stabilizing aggregates and flocculates in pigment dispersion process

A main contribution of dispersing agents is avoiding re-flocculation of the freshly dispersed particles.

Key Principles of Particle Stabilization

Two principles of stabilization are used to prevent the pigment particles to flocculate - Electrostatic Stabilization and Steric Stabilization.

Electrostatic Stabilization

All particles can have the same electrostatic charge making them repel each other. This stabilization mechanism involves the presence of a repulsive potential created by interaction of diffuse electrical charge double layers at the particle and liquid interface. Coulombic repulsion of equally charged particles keeps the particles apart.

This stabilization mechanism depends upon key system properties like pH and the amount of electrolyte that is present. It is most effective in systems of high polarity (notably aqueous systems), enabling full dissociation of ionic compounds, brought onto the pigment surface. In water-based paints and inks mostly anionic stabilization is used, implying that all particles have a negative charge.

Steric Stabilization

This stabilization mechanism is explained by adsorbtion of monomeric or polymeric species onto the pigment surface. The adsorbed layer provides a steric barrier preventing approaching particles from attraction and collision. Obviously, the effectiveness of steric stabilization is related to the structure and thickness of the adsorbed layer.

Overall, steric stabilization is the most secure principle to use in both solvent-based and water-based systems. Although steric stabilization being the dominant mechanism for stabilization in apolar systems, it applies also to aqueous systems, where it is typically being applied in conjunction with electrostatic stabilization.

Electrostatic Stabilization Steric Stabilization
Electrostatic Stabilization
  • Pigment particles have the same surface charge
  • Charges are arranged in a double layer causing the repulsion
  • Van-der-Waals forces cause the attraction
  • Mainly for inorganic pigments and dispersions in water
  • Stability can be affected by high salt concentrations
Steric Stabilization
  • Liquid phase soluble polymer chains adsorbed to the pigment particles through the anchoring groups
  • Strong stabilizing mechanism
  • OK for waterborne and solventborne systems

Practical Tips for Pigment Dispersion & Stabilization

How can you get right dispersion?

It is clear that dispersion process results in three important stages - Wetting, Separation, Stabilizing – and a correct wetting & dispersing agent should fulfill the requirement for all these three stages.

Wetting agents are additives that are generally used to facilitate complete wetting of the particle

During the mechanical disruption process, fresh pigment/ liquid interfaces are being formed; spontaneous wetting of the newly formed surfaces is essential for maintaining optimal rheological conditions. A correct and effective wetting agent enables to increase the speed at which the liquid phase wets the pigment surface. This is valid to aqueous as well as non-aqueous mill-bases.

Dispersing agents or Dispersants are the additives used for stabilizing pigment particles

An effective dispersant adsorbs strongly onto the surface of the pigment particles and provides electrostatic- and/or steric stabilization. Dispersant molecules must adsorb strongly at the surface of the particles. This adsorption process is often referred to as anchoring, and the groups, sticking to the surface, are called anchoring groups.

Anchoring Process

In order to be effective, the stabilizing species (pigment dispersant) must provide strong anchoring groups for interaction to the pigment surface. Furthermore, the non-adsorbed moiety of the molecule is preferably solvated and highly compatible with the liquid phase, as otherwise the stabilizing layer collapses leading to low layer thickness and poor stabilization. This is an important criterion for the selection of dispersing agents that should provide steric stabilization! The solvent or resin molecules need to be able to migrate easily into the penetrating layers.

The effectiveness of the dispersant is much related to the compatibility of the
dispersant with the solvent and binder molecules.

Problems resulting from poor dispersion stability are flocculation, color change, sedimentation, pigment separation during film formation and poor compatibility with other additives. There are many possible causes of these problems, such as poor adhesion of the dispersant, insufficient molecular weight of the dispersant and too much electrolyte being present.

Selecting the right dispersing additive according to the pigment you are using
is a strong dominator with respect to stability of the resulting pigment dispersion

Our Coatings Selector features a full range of dispersing agents available today for all types of coating and ink formulations (aqueous, solvent-based, high-solid or 100% solids systems and for pigment concentrates). You can check tech profile for each product, ask for samples or discuss your case with producer’s tech staff.

What's next?

In the practical guide, you can find hands-on tips to select the right dispersant for your system as well as methods to check the performance of a dispersing agent.

Pigment Dispersion: Wetting & Dispersing Agents Selection

Dispersing Agents – Commercial Grades for Paints, Coatings and Inks

View the full range of dispersing agents available today, analyze technical data of each product, get technical assistance or request samples.

We would like to acknowledge our experts Jochum Beetsma, Johan Bieleman & Vincent Makala for providing technical information needed to develop this page.

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