The Action Principle of Dispersants

The role of dispersants

The role of dispersants is to use wetting dispersants to reduce the time and energy required to complete the dispersion process, stabilize the dispersed pigment dispersion, modify the surface properties of pigment particles, and adjust the mobility of pigment particles.

Specific manifestations are as follows: ① Improve gloss and increase leveling effect ② Prevent floating color ③ Improve tinting power ④ Reduce viscosity and increase pigment loading ⑤ Reduce flocculation ⑥ Increase storage stability ⑦ Increase color development, increase color saturation, increase transparency (organic pigments) or hiding power (inorganic pigments)

Mechanism of dispersant

● Adsorbed on the surface of solid particles, making the surface of condensed solid particles easy to wet.

● High molecular weight dispersant forms an adsorption layer on the surface of solid particles, which increases the charge on the surface of solid particles and improves the reaction force between particles that form steric hindrance.

● Forms a double-layer structure on the surface of solid particles. The polar end of the outer dispersant has a strong affinity with water, which increases the degree of wetting of solid particles by water. Solid particles are kept away from each other due to electrostatic repulsion.

● Makes the system uniform, increases suspension performance, does not precipitate, and makes the physical and chemical properties of the entire system the same.

The working principle of dispersant

1.Double layer principle

The dispersants used in water-based paints must be water-soluble, and they are selectively adsorbed to the interface between powder and water.

Currently, anionic types are commonly used. They ionize in water to form anions, and have a certain surface activity, which is adsorbed by the surface of the powder. After the powder particles adsorb the dispersant on the surface, a double layer is formed. The anions are tightly adsorbed on the surface of the particles and are called surface ions. Ions with opposite charges in the medium are called counterions.

They are electrostatically adsorbed by the surface ions. Some of the counterions are tightly bound to the particles and surface ions, and they are called bound counterions. They become a moving whole in the medium, with negative charges, and the other part of the counterions are surrounded by them. They are called free counterions and form a diffusion layer. In this way, a double layer is formed between the surface ions and the counterions.

2. Kinetic potential

 The negative charge of the particles and the positive charge of the diffusion layer form a double layer, which is called kinetic potential. Thermodynamic potential: the double layer formed between all anions and cations, and the corresponding potential.

It is the kinetic potential rather than the thermodynamic potential that plays a dispersing role. The kinetic potential has an unbalanced charge and a charge repulsion phenomenon, while the thermodynamic potential belongs to a charge balance phenomenon.

If the concentration of counterions in the medium is increased, the free counterions in the diffusion layer will be forced to enter the bound counterion layer due to electrostatic repulsion, so that the double electric layer is compressed and the kinetic potential decreases. When all free counterions become bound counterions, the kinetic potential is zero, which is called the isoelectric point. Without charge repulsion, the system has no stability and flocculation occurs.

3. Steric effect 

The formation of a stable dispersed system, in addition to utilizing electrostatic repulsion, that is, the negative charges adsorbed on the surface of the particles repel each other to prevent the adsorption/aggregation between particles and finally form large particles and stratification/sedimentation, also requires the use of the theory of steric hindrance effect, that is, when the particles that have been adsorbed with negative charges approach each other, they are made to slide and stagger each other. This type of surfactant that plays a steric hindrance role is generally a non-ionic surfactant.

Flexible use of the theory of electrostatic repulsion combined with steric hindrance can form a highly stable dispersed system.

The polymer adsorption layer has a certain thickness, which can effectively prevent the mutual adsorption of particles, mainly relying on the polymer solvation layer. When the adsorption layer on the powder surface reaches 8-9nm, the repulsive force between them can protect the particles from flocculation. Therefore, polymer dispersants are better than ordinary surfactants.

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