Pigment Dispersion

Pigment dispersion

1. Pigment dispersion process

There are three processes in pigment dispersion: wetting, dispersion and stabilization.

⑴ Wetting

The replacement of the moisture and air on the surface of the pigment by the solvent (paint) is called wetting. Wetting of solvent-based paints is not a problem because the surface tension of the solvent (paint) is generally always lower than that of the pigment. But wetting requires a process, especially since the pigment is an aggregate, and the solvent needs to flow into the voids of the pigment. When the solvent viscosity is low, the speed of wetting can be very fast. Pay attention to the order of adding the pigment and solvent, add the solvent first and then add the pigment. In water-based paints, due to the high surface tension of water, it is difficult to wet organic pigments, and a wetting agent needs to be added to reduce the surface tension of water.

(2) Grinding and dispersing

In the preparation process of the pigment, the particle size of the pigment is controlled according to the specified requirements, but because of the van der Waals force between the particles, the fine particles of the pigment will aggregate with each other and become aggregates, so they need to be re-dispersed. Shear or impact force is required. Abrasives in coatings are primarily shear forces.

When learning viscosity, it has been known that when the shear speed (D) is constant, the shear force (τ) is proportional to the viscosity (η), namely: τ=Dη

It can be seen that the viscosity is high and the shear force is large, which is favorable for grinding, but the load capacity of the motor of the grinding equipment determines the maximum value of η of the system, so the viscosity cannot be too high.

Wetting requires a low viscosity medium (paint) when dispersing pigments by impact force, and requires high viscosity when grinding; in order to make full use of the dispersion equipment, it is desirable to have a large amount of dispersed pigment per batch. How to balance these three requirements? According to Mooney's formula, adding as much pigment as possible and less polymer in the system is a best-of-three approach.

⑶ stable

After the pigment is dispersed, there is still a tendency to aggregate with each other, that is, flocculation tendency. For this reason, the dispersed particles need to be stabilized, that is, protected. Otherwise, due to flocculation, the covering power, tinting power, etc. may decrease, or even coalesce. There are two main ways to stabilize pigment particles:

①The charge stabilization makes the surface of the pigment charged, that is, an electric double layer is formed on the surface, and the repulsive force of the opposite charge is used to keep the particles stable. Adding some surfactants or inorganic dispersants, such as polyphosphates and hydroxylamines, can achieve this purpose.

②Three-dimensional protection The three-dimensional protection is also called entropy protection. There is an adsorption layer on the surface of the pigment. When the adsorption layer reaches a certain thickness (>8-9nm), the repulsive force between them can protect the particles from agglomeration. If there is only solvent in the system, because the adsorption layer is too thin, the repulsive force is not enough to stabilize the particles. If some long-chain surfactant is added to the solvent-based paint, the polar group of the surfactant is adsorbed on the pigment, and the non-polar end faces the paint, which can form a thicker adsorption layer (8-9nm), but The surfactant has only one adsorption point on the pigment, and it is easily replaced by the solvent molecules. If some polymers are added, the polymer will be adsorbed on the surface of the pigment particles to form an adsorption layer with a thickness of 50 nm, and the polymer has multiple adsorption properties. The dots can be placed on top of each other, and they will not be separated from the pigment, so they can play a good protective role.