Detailed explanation of the working principle and selection method of dispersants
What is a dispersant? A surfactant that can stably disperse solid particles in a solid-liquid suspension in the medium is called a dispersant.
The working principle of dispersant – dispersion is the process of uniformly distributing solid particles in the dispersion liquid, and the dispersion liquid has a certain stability.
Mainly manifested in:
1. Adsorbed on the surface of solid particles, making the surface of the agglomerated solid particles easy to wet.
2. Polymer dispersants form an adsorption layer on the surface of solid particles, which increases the charge on the surface of solid particles and increases the reaction force between particles that forms a three-dimensional hindrance.
3. Form a bilayer structure on the surface of the solid particles. The polar end of the outer dispersant has a strong affinity with water, which increases the degree to which the solid particles are wetted by water. The solid particles are kept apart due to electrostatic repulsion 4. The system is made uniform, the suspension performance is increased, and there is no precipitation, so that the physical and chemical properties of the entire system are the same
As mentioned above, the use of dispersants can stably disperse solid particles in liquids.
Dispersant dispersion mechanism
1
Electric double layer principle – the dispersants used in water-based coatings must be water-soluble, and they are selectively adsorbed to the interface between the powder and water. Currently, the anionic type is commonly used. They ionize in water to form anions, and have certain surface activity and are adsorbed on the surface of the powder. After the dispersant is adsorbed on the surface of the powdery particles, a double electric layer is formed. The anions are tightly adsorbed on the surface of the particles and are called surface ions. Oppositely charged ions in a medium are called counterions. They are electrostatically adsorbed by surface ions, and some of the counterions are more closely combined with particles and surface ions. They are called bound counterions. They become a moving whole in the medium, carrying a negative charge, and other counter ions are surrounded by them. They are called free counter ions, forming a diffusion layer. This forms an electric double layer between surface ions and counterions.
2
Electrokinetic potential – the double electric layer formed by the negative charge of the particles and the positive charge of the diffusion layer is called electrokinetic potential. Thermodynamic potential: the double electric layer formed between all anions and cations, the corresponding potential.
It is the electrokinetic potential rather than the thermodynamic potential that plays a role in dispersion. The charges in the electrokinetic potential are unbalanced and there is charge repulsion, while the thermodynamic potential is a charge balance phenomenon. If the concentration of counter ions in the medium increases, 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 decrease. When all the free counter ions become bound, After the counter ions, the electrokinetic potential is zero, which is called the isoelectric point. Without charge repulsion, the system has no stability and flocculation occurs.
3
steric hindrance effect – the formation of a stable dispersion system, in addition to the use of electrostatic repulsion, that is, the negative charges adsorbed on the particle surface repel each other to prevent the adsorption/aggregation between particles and finally form large particles and stratify /In addition to sedimentation, the theory of steric hindrance effect must also be used, that is, when the particles with adsorbed negative charges are close to each other, they are caused to slide and stagger each other. This type of surfactant that plays a steric hindrance effect is generally a non-ionic surfactant. . Flexible use of the theory of electrostatic repulsion and steric hindrance can form a highly stable dispersion system.
The polymer adsorption layer has a certain thickness, which can effectively block the mutual adsorption of particles, mainly relying on the solvation layer of the polymer. When the adsorption layer on the surface of the powder reaches 8-9nm, the repulsive force between them can Protect particles from flocculation. Therefore, polymer dispersants are better than ordinary surfactants.
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According to its surface structure and treatment, it has been tested that dispersed titanium dioxide can be used:
-Traditional wetting and dispersing agents include organic carboxylic acids with acid value AV, ammonia value AMV, amine salts of phosphoric acid and controlled flocculation type wetting and dispersing agents for floating colors
-Organophosphate
-Special PU high dispersant
-Specialized vinyl polymer titanium dioxide dispersant
－Polymeric surfactant that is widely effective in water-based applications
Among them, wetting and dispersing agents are a common choice. It has broad adaptability to the system, but cannot adapt to special requirements.
Organophosphates are often recommended to produce high-level pure whites to remove haze, while high relative mass dispersants are considered for their ability to control floating colors and their whiteness. New technologies for dispersants in water-based systems allow factory-made reference whites to accept commercially available color masterbatches. Therefore, it is now common knowledge that dispersants are used in titanium dioxide.
5. Transparent iron oxide
The particle size of transparent iron oxide is at the nanometer level, and its surface is amphoteric. It seems to be easily dispersed when the pigment concentration is low, and the viscosity of the color paste is very low, but the transparency is indeed not easy to achieve the best; and once it is slightly Above a critical pigment concentration, the paste immediately thickens to the point where it cannot be stirred, resulting in sand mill inefficiency.
The transparency of iron oxide, somewhat like the blackness of carbon black, seems to always continue to improve its transparency. Our experimental results show that a sample that we already think has good transparency may still have heavy haze when viewed at 45°;
So, which one is best? This question is another elusive doubt.
Coupled with the selectivity caused by different resin systems, there is more than one recommended solution.
For example, there are methods using phosphate esters; there are methods using high dispersants combined with acidic terminal wetting agents; there are also methods of separately designing high dispersants, etc. They show adaptability to the same transparent iron oxide pigment in different systems.
6. Matting powder
Matting powder itself is actually not difficult to disperse. It is pre-micronized during production. Some have surface wax treatment, some do not, and have polar hydroxyl groups. However, the dispersion problem of matting powder comes from application requirements.
Some require matte coatings to be adaptable to multiple construction methods with one formula, such as spraying with a consistent gloss;
Some require that the extinction uniformity will not be affected under high temperature and high humidity;
Some require the matting powder to have minimal settlement under low viscosity conditions;
Some require the highest level of transparency;
Some require excellent friction resistance and introduce hard quartz powder, so it needs to be dispersed together, etc.
This resulted in a consequent change in the dispersant. From traditional wetting and dispersing agents, to special high PU dispersing agents, to phosphate esters, to amine salts of phosphate esters, to other special polymers, they are all used to disperse matting powder. So which one is best? As mentioned before, it depends on what you ask for. One dispersant cannot be expected to address all requirements simultaneously.
In principle, wetting agents can improve the flow ability of the final system; high relative mass dispersants can prevent settling and control the movement of matting powder in the wet film, making it easier to orient and obtain uniform matting.
7. Metallic glitter pigments such as aluminum powder and pearlescent powder
A common solution is a wetting agent.
They can also be dispersed with high dispersants that are compatible with the resin. control their movements at the same time. These all have examples of successful recipes.
8. Determine a main dispersant
Generally, in a certain resin and solvent system, it is recommended to use this method to screen a suitable primary dispersant:
First, disperse four pigments: high-pigment carbon black, titanium white, DPP red, and ordinary iron oxide red.
Evaluate whether the dispersant has any difficulties in the preparation of these four conventional color masterbatches, such as whether the viscosity reducing behavior is sufficient.
Assess color intensity
Assess storage stability (flow plate and thermal storage).
If a dispersant can show good dispersion ability for the above four pigments in this specific system, then it can basically meet the requirements of various other pigments. can be selected as the primary dispersant for this system. Of course, exceptions may still be made for special pigments such as ferrite.
This method can also be used to evaluate the combined performance of two different dispersants to find the type of pigment that is suitable for it to handle.
