dispersantWhat is the mechanism of action?
In latex coatings, dispersants are adsorbed on the surfaces of pigments and fillers, causing ion charges, physical shielding, hydrogen bonding, and dipole interactions between particles, thereby preventing aggregation and flocculation between pigment and filler particles.
1. Ionic charge
Dispersants are mostly ionized electrolytes, and their anionic parts preferentially adsorb on the surface of the pigment, forming a negative charge layer at the interface between the pigment and the liquid. In the liquid phase, this negative potential attracts positively charged ion clouds (i.e. positive ion regions), forming a double layer that effectively repels pigment particles and largely dominates the stability of the colloidal system.
2. Physical shielding (protective adhesive)
It works through the size of the dispersant molecules themselves. The molecules of synthetic resin dispersants are relatively large, and they are adsorbed on pigment particles to create physical barriers or buffer layers between pigment particles, forming steric hindrance and preventing particle contact, thus playing a protective role in the adhesive.
3. Hydrogen bond
Due to the positively and negatively charged terminals of dispersant molecules, they can cause a special directional arrangement of adjacent water, forming a weak but important bond hydrogen bond, which establishes an additional buffer layer near the pigment particles. This can increase the viscosity of the system and help stabilize the pigment dispersion.
4. Dipole effect
Under the action of an electric field, the positive negative centers inside the non dissociated dipole molecules shift, resulting in the formation of dipole molecules, with one end of the dipole molecule arranged along the surface of the pigment; The other end extends towards the liquid phase to repel other pigment particles, thus playing a dispersing and protective role.
How to choose dispersants reasonably?
No dispersant can have good dispersibility for all pigments and fillers. And it has high selectivity for pigments and base materials, so selecting dispersants is a meticulous task.
Each dispersant has its own HLB value (hydrophilic lipophilic balance). Generally speaking, most inorganic oxides, sulfates, and carbonates require dispersants with HLB values ranging from 13 to 20, while organic pigments such as phthalocyanine pigments and Han Sha Huang. The HLB value should be around 14, while carbon black, toluene amine red, etc. require an HLB value of 10-13. HLB systems can be used for pigment dispersion by selecting surfactants.
In latex coatings, two or more dispersants are generally used, mostly inorganic dispersants and organic polymer dispersants are used in combination, and the dispersion effect is relatively ideal. Although low molecular weight organic and inorganic dispersants have good dispersion or wetting effects, their low molecular weight reduces the water resistance and scrub resistance of the coating film. Phosphate dispersants are easily hydrolyzed in alkaline media, causing them to lose their dispersibility and potentially leading to salt precipitation during film formation. This can cause pigment particles to coagulate before film formation, resulting in decreased coloring power and coating discoloration. Using organic high molecular weight dispersants not only avoids the drawbacks of small molecule dispersants, but also helps improve gloss.
The commonly used amount of dispersant is 0.2-1.0% of the pigment amount. It should be noted that when calculating the dosage, it is important to convert the solid content of the dispersant before determining the actual dosage.
