Selection of dispersant
Dispersant, as its name suggests, is to reasonably disperse various powders in the solvent, so that various solids can be stably suspended in the solvent (or dispersion) through certain charge repulsion principle or polymer steric effect.
In the coating production process, pigment dispersion is a very important production link, which is directly related to the coating storage, construction, appearance and film performance, so the reasonable selection of dispersant is a very important production link. But the dispersion of coating paste is not only related to dispersant, but also to formulation of coating formula and selection of raw materials.
Electric double layer principle
Dispersants used in waterborne coatings must be water-soluble and selectively adsorbed on the interface between powder and water. At present, anions are commonly used. They ionize in water to form anions and have certain surface activity. They are adsorbed on the surface of powders. After the dispersant is adsorbed on the surface of the powder particles, an electric double layer is formed, and the anions are tightly adsorbed on the surface of the particles, which is called surface ions. Ions with opposite charges in the medium are called counter ions. They are electrostatically adsorbed by surface ions, and some of the counter ions are closely bound to the particles and surface ions, which are called bound counter ions. They become a moving whole in the medium with negative charge, and the other part of the counter ions are surrounded by them. They are called free counter ions, forming a diffusion layer. In this way, an electric double layer is formed between the surface ions and counter ions.
Potentiodynamic: the negative charge of the particle and the positive charge of the diffusion layer form a double electric layer, which is called potentiodynamic. Thermoelectric potential: the electric double layer formed between all anions and cations, corresponding potential
It is the electrokinetic potential rather than the thermoelectric potential that plays the role of dispersion. The charge of the electrokinetic potential is unbalanced and there is a phenomenon of charge repulsion, while the thermoelectric potential belongs to the phenomenon of charge balance. If the concentration of counter ions is increased in the medium, the free counter ions in the diffusion layer will be forced into the bound counter ion layer due to electrostatic repulsion. In this way, the electric double layer will be compressed and the electrokinetic potential will drop. When all free counter ions become bound counter ions, the electrokinetic potential will be zero, which is called isoelectric point. There is no charge repulsion and the system has no stability.
The formation of a stable dispersion system not only uses electrostatic repulsion, that is, the negative charges adsorbed on the surface of particles repel each other, so as to prevent the adsorption / aggregation between particles and finally form large particles for stratification / sedimentation, but also uses the theory of steric hindrance effect, that is, when the adsorbed negative charges are close to each other, they slide and stagger each other Sterically hindered surfactants are generally nonionic surfactants. By using the theory of electrostatic repulsion and steric hindrance flexibly, a highly stable dispersion system can be formed.
The polymer adsorption layer has a certain thickness, which can effectively block the mutual adsorption of particles. It mainly depends on the solvation layer of polymer. When the adsorption layer reaches 8-9nm, the repulsive force between them can protect the particles from flocculation. So polymer dispersant is better than common surfactant.
Edit the determination method of this paragraph
Dosage of dispersant
(1) Daniel flow point determination method: gradually drop the water solvent of dispersant into a certain amount of pigment / pigment mixture with a dropper, and grind it with a small scraper carefully until the ground pigment slurry can flow down from the scraper as the end point, and calculate the ratio of pigment fraction agent to pigment. 
(2) Dosage curve method: to a certain amount of pigment / pigment mixture made of a very thick slurry, under stirring, gradually drop in a relatively thick pigment concentration solution, and measure the viscosity every time. The lowest point is the best dosage of dispersant.
(3) Concentration / flocculation method: a certain amount of pigment / pigment mixture is used to make a thick water slurry, the pigment dispersant solution is added step by step, and the mixture is uniform until it can flow completely from the scraper. 2 R 9 U 1 N 0 N 2 ^ 6 o-z 1 t 9 t, 1 ml of ionic thickener and a drop of dispersed pigment paste were dropped on the glass plate with black background. Then mix gently. If flocculation occurs, the pigment dispersant is added to the pigment slurry until no flocculation occurs. The amount of pigment dispersant at this point is calculated according to the pigment, which is called C-A value (concentration flocculation value).
Note: Daniel flow point is suitable for solvent based paint, but not suitable for latex paint. The addition curve method is only suitable for the slurry itself, and there is often a lack of dispersant in latex paint, so it is necessary to add more dispersant (up to double) in practical application. C-A value is more comprehensive.
The optimum dispersant concentration (ODC) is usually expressed in terms of the amount of dispersant required per unit mass of pigment. The larger the surface area of pigment per unit volume, the higher the ODC.
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