The Universal Selection Source: Coatings Ingredients

Pigment dispersion : Wetting & Dispersing Agents Selection

Bright, vivid and durable colors are one of the most important part of the coatings’ aesthetic definition. Formulators are using pigments in order to bring this color in the paint. But keeping stable these solid particles in a liquid is a complex mechanism which consists in wetting, dispersing and stabilizing the solid particles in the system. This guide will help you to understand this mechanism in order to select the best product for your formulation. Selecting the right wetting and dispersing agent can enhance your paint’s quality, but also lower your production cost !

Dispersion Process and Dispersing Agents


The dispersion process is crucial. Indeed, it will influence many parameters such as the appearance (color strength, transparency, gloss...) but also the paint physical properties (rheology, stability flocculation…). Keeping problems at this stage can dramatically reduce the final paint quality. Understanding this process is a great help to prevent many coatings defects. Mastered Wetting, Dispersing and Stabilizing are the keys for high quality coatings.

The 3 stages of the pigment dispersion


Wetting:



Wetting


 The formulator introduces the solid pigments into the liquid phase. In this phase, air and moisture entrapped at the pigment surface are displaced to the grinding medium liquid phase. The pigment / air interfaces become pigment / liquid interface. To proceed, the liquid needs to wet the pigment surface (please visit the “Wetting Agent Guide” for more information on the wetting theory). 


Wetting & dispersing additives have an influence on this stage, by modifying the surface tension at the interface they can help and accelerate the pigment wetting.

Dispersing:



Dispersing


 The grinding equipment (dissolvers, grinding mills, sand mills…) introduce 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. 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.


Wetting & dispersing agents lower the forces between the pigment particles and help in crushing the agglomerates in smaller particles.


Stabilizing:



Stabilizing


 Naturally the newly obtained small particles have a tendency to re-agglomerate. It is the “flocculation”. 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. 


Wetting & dispersing agents will prevent this unwanted effect and provide a long-term stabilization.



There are mainly two mechanisms of 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




Pigment Dispersion

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