Surfactants are generally low molecular weight dispersants. Surfactant molecules have a modifying effect, especially reducing the surface tension between the pigment and the resin solution. The surfactant structure contains two groups with opposite solubility or polarity, which increases surface activity. In aqueous systems, polar groups are hydrophilic groups, while non-polar groups are hydrophobic or lipophilic groups. In non-aqueous systems, polar groups are lipophobic groups and non-polar groups are lipophilic groups. Surfactants are classified according to their chemical structure, especially polar groups including: anions, cations, electrically neutral particles and non-ions. The effectiveness of polymeric dispersants is determined by the following factors:
Adsorption of polar groups on the pigment surface. Anchor groups can be amino, carboxylic acid, sulfonic acid, phosphoric acid and their salts.
The behavior of non-polar chain segments surrounding particles in a medium. Parts of the molecule (aliphatic or aliphatic-aromatic fragments) must be highly compatible with the adhesive system.
The stabilization mechanism of surfactant-like dispersants is electrostatic stabilization: the polar groups surrounding the pigment particles form a double-layer charged structure. Due to Brownian motion, the pigment particles in the liquid medium often collide together, so they have a strong tendency to reflocculate during their deceleration process.
Based on their chemical structure (e.g. low molecular weight) and electrostatic stabilization theory, surfactants have the following disadvantages:
- Water sensitivity: Surfactants often make the final coating water-sensitive, making it unsuitable for outdoor applications.
- Easy to generate foam: Many surface modifiers can generate foam and cause defects in the coating (such as fish eyes, pits). If foam occurs during the grinding process, it will lead to a decrease in production capacity.
- Interfering with the adhesion between coatings.
After years of development, special surfactants have been improved to minimize coating defects, and some can also give the coating some other advantages, such as defoaming/corrosion resistance or making the substrate difficult to wet.
Commonly used surfactants used for pigment dispersion include the following varieties: fatty acid derivatives, phosphate esters, sodium polyacrylate/polyacrylic acid, acetylene glycol and soy lecithin.
