Dispersant is a surfactant that has two opposite properties: lipophilicity and hydrophilicity within the molecule. It can uniformly disperse solid and liquid particles of inorganic and organic pigments that are difficult to dissolve in liquid. It can also prevent the sedimentation and agglomeration of particles and form an amphiphilic reagent required for a stable suspension.
The role of the dispersant
The role of the dispersant is to use a wetting and dispersing agent to reduce the amount of The time and energy required for the process stabilize the dispersed pigment dispersion, modify the surface properties of the pigment particles, and adjust the mobility of the pigment particles.
The specific manifestations are as follows: ① Improve gloss and increase leveling effect ② Prevent floating color and bloom ③ Improve tinting strength ④ Reduce viscosity and increase pigment loading ⑤ Reduce flocculation ⑥ Increase storage stability ⑦ Increase color development properties, increase color saturation, increase transparency (organic pigments) or hiding power (inorganic pigments)
The mechanism of dispersants
1. Adsorbed on the surface of solid particles, making the surface of the condensed solid particles easy to wet. 2. Polymer dispersant forms an adsorption layer on the surface of solid particles, which increases the charge on the surface of solid particles. 3. Increases the reaction force between particles that forms a three-dimensional hindrance. 4. 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. Solid particles are kept apart by electrostatic repulsion. 6. Make the system uniform, increase the suspension performance, prevent precipitation, and make the physical and chemical properties of the entire system the same.
How dispersants work
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.
The anionic type is commonly used at present. They ionize in water to form anions, and have a 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. Some of the counterions are more tightly combined with the 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 negative charge of the particles and the positive charge of the diffusion layer form a double electrical layer, which is called electrokinetic potential. Thermodynamic potential: the double electrical layer formed between all anions and cations, corresponding to Potential.
It is the electrokinetic potential rather than the thermodynamic potential that plays the role of 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, so that the electric double layer is compressed and the electrokinetic potential decreases. After all free counterions become bound counterions, 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 requires the use of electrostatic repulsion, that is, the negative charges adsorbed on the surface of particles repel each other to prevent adsorption/aggregation between particles and the final formation of large particles. In addition to the delamination/sedimentation of particles, 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 non- Ionic surfactants.
Flexibly using 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 gap between them The repulsive force protects particles from flocculation. Therefore, polymer dispersants are better than ordinary surfactants.
The action process of dispersant
1. Solid particle dispersion process The dispersion process of solid particles in the medium is generally It is divided into three stages: Moistening of solid particles Moisturizing is the most basic condition for solid particle dispersion. If solid particles are to be evenly dispersed in the medium, each solid particle or particle group must first be fully moistened.
Dispersion or fragmentation of ion clusters. This process involves the dispersion or fragmentation of particle clusters, which involves the separation of particle clusters and internal solid-solid interfaces. Different types of surfactants play different roles in the dispersion or fragmentation of particle clusters.
Prevent the re-aggregation of solid particles. Once solid particles are dispersed in a liquid, a uniform dispersion is obtained, but whether it is stable or not depends on whether the dispersed solid particles can re-aggregate to form agglomerates. .
2. The dispersion and stabilization effect of surfactants in aqueous medium a. The dispersion effect on non-polar solid particles Adding surfactantAfter the floating body, since the surfactant can reduce the surface tension of water, and the hydrophobic bonds of the surfactant can be adsorbed on the surface of non-polar solid particles through van der Waals force, the hydrophilic group extends into the water to increase the hydrophilicity of the surface, making it The wettability of non-polar solid particles is improved. b. Dispersion and stabilization of charged particles
3. Dispersion and stabilization of surfactants in organic media. The dispersion of particles in organic media is mainly achieved by entropic repulsion generated by steric hindrance. For non-polar particles, they can be stably dispersed in organic media by overcoming the van der Waals force between particles. Surface treatment of organic pigments can be achieved in the following ways.
a. Surface treatment of organic pigments using organic amines. b. Surface treatment of organic pigments using pigment derivatives.
Classification of dispersants (by structure)
Dispersants are also called wetting dispersants. In addition to its wetting effect, one end of its active group can be adsorbed on the surface of the pigment crushed into fine particles, and the other end is solvated into the paint base to form an adsorption layer (the more adsorption groups, the longer the chain links, and the thicker the adsorption layer). Generate charge repulsion (water-based paint) or entropy repulsion (solvent-based paint), so that the pigment particles can be dispersed and suspended in the paint base for a long time to avoid re-flocculation, thus ensuring the storage stability of the finished paint system.
There are many kinds of dispersants. According to preliminary estimates, there are more than 1,000 substances with dispersing effects in the world. Now divided according to their structure, they can be divided into:
1. Anionic wetting and dispersing agents are mostly composed of non-polar negatively charged lipophilic hydrocarbon chain parts and polar hydrophilic ones. group composition. The two groups are located at both ends of the molecule, forming an asymmetric hydrophilic and lipophilic molecular structure.
Its varieties include: sodium oleate C17H33COONa, carboxylate, sulfate ester salt (R—O—SO3Na), sulfonate R—SO3Na), etc. Anionic dispersants have good compatibility and are widely used in water-based coatings and inks.
Polycarboxylic acid polymers can also be used in solvent-based coatings and are widely used as controlled flocculation dispersants.
2. Cationic wetting and dispersing agents are non-polar and positively charged compounds, mainly including amine salts, quaternary ammonium salts, pyridinium salts, etc. Cationic surfactants have strong adsorption power and have good dispersion effects on carbon black, various iron oxides, and organic pigments. However, attention should be paid to its chemical reaction with the carboxyl groups in the base material, and care should be taken not to use it at the same time with anionic dispersants.
3. Non-ionic wetting and dispersing agents are non-ionized and uncharged in water, and have weak adsorption on the pigment surface. They are mainly used in water-based paints. Mainly divided into glycol type and polyol type, which reduce surface tension and improve wettability. Used in conjunction with anionic dispersants as wetting agents or emulsifiers, it is widely used in water-based color pastes, water-based coatings and inks.
4. Amphoteric wetting and dispersing agents are compounds composed of anions and cations. Typical applications are phosphate ester salt type polymers. This type of polymer has a higher acid value, which may affect interlayer adhesion.
5. The sizes of the anionic and cationic organic groups in the electroneutral wetting and dispersing agent molecules are basically equal, and the entire molecule is neutral but polar, such as oil amino oil Acid ester C18H35NH3OOCC17H33.
6. Polymer hyperdispersant Polymer dispersant is the most commonly used and has the best stability. It is divided into polycaprolactone polyester polyol-polyethyleneimine block copolymer type, acrylate polymer type, polyurethane or polyester type, etc.
7. The controlled free radical hyperdispersant uses the latest controlled free radical polymerization technology (CFPP) to make the structure of the dispersant more regular. Commonly used methods include: GTP, ATRP, RAFT (reversible addition fragmentation chain transfer controlled radical polymerization, including C-RAFT, S-RAFT, NMP, SFRP, TEMPO), etc. Controlled radical polymerization technology is used to make the molecular weight distribution of the dispersant more concentrated and more efficient. </p
