A dispersant or a dispersing agent is a substance, typically a surfactant, that is added to a suspension of solid or liquid particles in a liquid (such as a colloid or emulsion) to improve the separation of the particles and to prevent their settling or clumping.
What is a dispersing agent or dispersant:
A dispersing agent by definition is a paint ingredient which is used to avoid the flocculation of pigments and fillers. In other words, dispersants promote and stabilize the suspension of solid particles in the paint medium. All dispersing agents have two parts:
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Anchoring groups
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Soluble tails
The choice of your dispersant will greatly influence the quality of your dispersion. They improve, Milling time and viscosity, Tint strength, Hiding power, Gloss, Water resistance and Batch-to-batch consistency
How does a dispersant work?
Dispersants function by reducing the attraction between solid particles like pigments, fillers, or additives in any dispersion. This prevents the solid particles from agglomerating or settling out. Generally, when solid particles are added to any liquid medium, they tend to attract each other due to intramolecular forces. Dispersants function is to initially separate these particles from each other and continuously avoid further agglomeration. First, anchoring groups adsorb to the pigments and fillers surface. They form strong physical bonds with chemical groups that are present on the surface of the particles, the so-called anchor sites. Second, soluble tails create a layer around the solid particles preventing dispersed particles to glue together again and ensuring colloidal stability. These repulsive forces working against the natural attraction of particles are of two kinds:
- Electrostatic repulsion: The surface of particles is equally charged and they repel each other.
- Steric repulsion: The tails form a layer around the solid particles and prevent particles to collide with each other.
Types of Dispersing Agents:
There are many dispersing agents chemistries today in the coatings market, and choosing the right one for your system is tricky. The chemistry of your dispersing agent plays an important role in the performance and success of your final coating. The optimal dispersing agent chemistry can impact the stability, quality, and performance of your dispersion or coating. Various dispersing agent classes can be classified as below:
- Conventional dispersing agents (High Mw, Low Mw)
- Polymeric dispersing agents
- Ionic and non-ionic dispersing agents
Conventional dispersing agents:
Mainly low molecular weight, they are based on polyesters, polyamides, polyglycols and fatty acid chemistry (FAME). They have general characteristics as listed below:
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Surfactant effect, reduction of solid / liquid interface surface tension
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Anchoring groups adsorbed at the pigment surface
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Good compatibility with the media
High molecular mass dispersants:
With a molecular weight of approximately 5000-30,000 g/mol, these high molecular weight dispersants are most widely used in industrial paints. As compared to low molecular mass dispersants, they typically provide superior performances, Workability, Best coloristic properties, Gloss, Film transparency, Film integrity and Low risk of being extracted from the dried film, and Minimal side effects.
High versus low molecular mass-based dispersants:
High molecular mass-based additives tend to be more system specific as compared to low molecular mass dispersants and a careful selection and evaluation procedure is required. Oligomers of medium high molecular mass (Mw around 1000-2000 g/mol) typically show widest range of compatibility and superior (fast) pigment wetting properties in comparison to high molecular mass products, whereas still outperforming low molecular mass products in dispersion stability and film consistency properties. With few exceptions, monomolecular surfactant-based additives are less commonly used as dispersing agent. This is because, this group typically is inferior in contributing to dispersion stability and has high risk on effecting film properties, such as:
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Water sensibility
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Hardness, and
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Substrate adhesion
If you struggling at achieving the desired characteristics - hardness, resistance to water and solvents or adhesion even you have the right chemistry, then it may come from an overuse of dispersants. Take this quick tutorial to save on troubleshooting and understand if you are using optimum level of dispersants and hence, lower formulation costs.
Polymeric dispersants:
Main dispersing agents that are polymeric include polyacrylates, polyester, polyether or polyurethane-based systems. The classification of polymeric types dispersants is based on their Anchoring mechanism. Chemical structure (polyacrylic, polyurethane, copolymer…), and Molecular weight.
This type is also influenced by the polymer design (linear, branched, star designed) and the polymerization process (controlled polymerization process types offer high performances products but are also more expensive).
