High Molecular Weight Dispersants
When dispersing pigments, in particular organic pigments, one frequently encounters problems such as flocculation, insufficient color or transparency, poor rheological qualities or stability. These issues arise along every step of the coatings life cycle from production and storage through to the end-use performance of the film. The dispersing of solid pigments or fillers into the liquid phase of binder solutions is an important step in paint and coatings production influencing optical properties like gloss and color strength. Dispersion control additives are used to improve and accelerate the dispersion process and to stabilize the dispersion during storage. For the design and the use of these additives, it is essential to have knowledge about the dispersion process.
High molecular weight wetting and dispersing agents properties:
- 5,000 g/mol < Molecular weight < 30,000 g/mol
- They are built of branched or long linear molecules, which in general have a polyurethane or polyacrylate structure.
- Designed to adsorb via special groups with high affinity towards specific sites on the pigment surface. These are called anchoring groups, and are built in at strategic points on the polymer backbone.
- Suited for inorganic and organic pigments, in addition to carbon black pigments.
Mode of action:
- Anchoring groups enable strong interaction between the dispersion control additive and the pigment surface.
- This interaction is much stronger than in the case of the low molecular weight types as the dispersion control additive is bound to numerous sites on the surface via the anchoring groups monoadsorption assuring an efficient steric hindrance between the solid particles by keeping them apart.
- The remaining parts of the dispersion control additive act as a steric barrier to the surroundings by stretching into the liquid phase preventing flocculation.
- These parts have a good affinity with the solvent and the resin system, so the compatibility with the liquid phase is increased and the viscosity of the paint system is reduced as a desired side-effect.
- Organic pigments molecules consist mainly of the elements C, H, O and N. These atoms are not charged and are connected to each other through covalent bonds. Despite the fact that the main interaction between the anchoring groups and pigment surface is hydrogen bonding, this mechanism applies for both organic and inorganic pigments. Inorganic pigments are usually treated with different types of polar organic compounds which enable this interaction to take place as it does by the organic pigments.