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
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).
& dispersing additives
have an influence on this stage, by modifying the surface tension at the interface they can help and accelerate the pigment wetting.
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.
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
- 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
- Liquid phase soluble polymer chains adsorbed to the pigment particles through the anchoring groups
- Strong stabilizing mechanism
- OK for waterborne and solventborne systems