How to Choose Dispersants for Water-Based Coatings and Inks?
Choosing dispersants for water-based coatings and inks requires comprehensive consideration of various factors,
including pigment characteristics, system environment, and application performance, to ensure uniform pigment
dispersion, system stability, and satisfactory final product performance. The following are specific selection points
and methods:
I. Clarify Pigment Type and Surface Properties: The core function of a dispersant is to reduce surface energy by
adsorbing onto the pigment surface, preventing pigment flocculation. Its structure must match the pigment's
surface characteristics.
1. Pigment Type
Inorganic Pigments (e.g., carbon black, iron oxide, titanium dioxide, calcium carbonate):
The surface usually has polar groups (such as hydroxyl and carboxyl groups), which easily combine with water or
polar dispersants. Anionic dispersants (such as polycarboxylic acid ammonium salts, phosphates) or non-ionic
dispersants (such as fatty alcohol polyoxyethylene ethers) are preferred, providing stable dispersion through
electrostatic repulsion (anionic) or steric hindrance (non-ionic).
Example: Polycarboxylic acid ammonium salt dispersants (such as the RK-4008Y series) are commonly used for
carbon black dispersion, while phosphate ester dispersants (such as RK-4113) can be used for titanium dioxide
dispersion.
Organic Pigments (e.g., azo dyes, xanthene dyes, quinacridone dyes, and other high-performance organic pigments):
The surface has weaker polarity and often contains non-polar groups (such as aromatic rings, alkyl chains), requiring
stronger dispersion capabilities. High-molecular-weight dispersants (such as polyurethane dispersants containing
anchoring groups, acrylate copolymer dispersants) are preferred, enhancing dispersion stability through a "
anchoring-solvation" structure (anchoring groups adsorb onto the pigment surface, and the solvating chains
provide steric hindrance).
Example: High-performance organic pigment dispersion commonly uses RK-4117EA series high-molecular-weight
dispersants or polycarboxylic acid dispersants from Dongguan Ruikun Materials Technology Co., Ltd. Functional
pigments (such as nanopigments, fluorescent pigments, and pearlescent pigments):
Nanopigments have a large specific surface area and are prone to agglomeration, requiring the selection of
high-adsorption capacity polymer dispersants (such as polyurethane dispersants containing multiple anchoring
groups); fluorescent and pearlescent pigments are sensitive to gloss, and it is necessary to avoid color shift caused
by dispersant residue. Low-VOC, low-migration dispersants should be selected.
2. Pigment particle size and specific surface area: The smaller the pigment particle size (e.g., nanoscale) and the larger
the specific surface area, the higher the surface energy, requiring dispersants to provide stronger adsorption capacity
and steric hindrance. In this case, polymer dispersants (large molecular weight, high adsorption capacity, and significant
steric hindrance effect) are preferred over small molecule dispersants (easy to desorb, poor stability).
3. Pigment surface treatment: Some pigments (such as titanium dioxide and organic pigments) have reduced surface
polarity after surface treatment (such as silane coupling agents, titanate treatment), requiring the selection of dispersants
that match the treated surface. For example, for silane-treated pigments, dispersants containing polar groups (such as
amino groups, epoxy groups) should be selected to enhance adsorption.
II. Analysis of the pH value and medium composition of the system: The pH value and medium composition (water,
additives, organic solvents, etc.) of the aqueous system directly affect the stability and effectiveness of the dispersant.
1. pH value
Alkaline systems (pH 8-10, such as latex paints, water-based coatings):
Anionic dispersants (such as polycarboxylic acid ammonium salts, carboxylates) are preferred. The carboxylate groups
(-COO⁻) on their molecular chains ionize under alkaline conditions, generating electrostatic repulsion and stabilizing
pigment dispersion. Acidic systems (pH 4-6, such as some inks and acid-catalyzed curing systems):
The carboxylic acid groups of anionic dispersants are easily protonated (-COOH), weakening electrostatic repulsion.
Non-ionic dispersants (such as fatty alcohol polyoxyethylene ether phosphates) or cationic dispersants (such as those
containing amino groups or quaternary ammonium salts) can be used to stabilize the dispersion through hydrogen
bonding or electrostatic attraction.
Neutral/wide pH range systems:
Non-ionic dispersants (such as polyethers) are preferred, as their molecular chains (e.g., EO/PO blocks) are less
affected by pH, offering wider applicability.
2. Dispersion Medium and Other Additives
Water-based systems, without organic solvents:
Choose water-soluble dispersants (such as polycarboxylic acids, polyurethanes) to ensure the dispersant is miscible
with water and avoid precipitation or flocculation due to solubility issues.
Systems containing small amounts of organic solvents (such as alcohols, ethers, esters):
The solubility of the dispersant in the mixed solvent needs to be balanced. The appropriate dispersant should be
selected by matching the HLB value (hydrophilic-lipophilic balance). For example, in systems containing ethanol, a
non-ionic dispersant with an HLB value of 8-12 can be used.
Systems containing thickeners/biocides/defoamers:
The dispersant needs to have good compatibility with these additives to avoid interactions that could lead to dispersant
failure or system turbidity. For example, in systems containing alkali-swellable thickeners, the dispersant needs to be
alkali-resistant (e.g., using ammonium polycarboxylate salts instead of carboxylic acids).
III. Clarify Application Performance Requirements Based on the final use of the coating/ink (e.g., color paste, topcoat,
packaging ink), determine the key performance indicators and select the dispersant accordingly.
1. Dispersion Stability
Long-term storage stability (e.g., color pastes, industrial coatings):
The dispersant needs to provide strong steric hindrance or high charge density. Polymeric dispersants (such as
polyurethane dispersants) have solvated chains that can form a thick adsorbed layer, providing strong anti-agglomeration
capabilities; small molecule dispersants (such as phosphates) are suitable for short-term storage or low-viscosity
systems. Water Resistance/Electrolyte Resistance (e.g., architectural coatings, water-based inks):
Choose high-molecular-weight dispersants whose anchoring groups (such as carboxylic acids, sulfonic acids) strongly
bind to the pigment, and whose solvating chains are not easily displaced in water, thus resisting flocculation caused
by electrolytes or water.
2. Color Strength and Gloss
High Color Strength/High Gloss (e.g., automotive paints, high-grade inks):
Dispersants are needed to ensure small and uniformly distributed primary pigment particles. High-molecular-weight
dispersants (such as those containing epoxy groups) are preferred, achieving pigment deflocculation through precise
adsorption, improving gloss and color saturation.
3. Viscosity and Leveling
Low Viscosity/High Leveling (e.g., screen printing inks, varnishes):
Choose low-molecular-weight dispersants (such as RK-4139AC) to reduce the impact of the dispersant on system
viscosity; or choose polyether-modified dispersants (such as RK-4117C), which combine dispersion and leveling
functions, reducing system viscosity.
4. Cost and Environmental Requirements
General-purpose systems (e.g., interior wall paints):
Small-molecule dispersants (such as sodium lignosulfonate) are preferred due to their low cost and high
cost-effectiveness.
Environmentally friendly systems (e.g., food packaging inks, children's toy coatings):
Choose low-VOC, formaldehyde-free, and biodegradable dispersants (such as polyaspartate esters), avoiding
dispersants containing benzene rings or heavy metals.
IV. Dispersant Type Selection Based on System Characteristics: Dispersants can be divided into small-molecule
dispersants and high-molecular-weight dispersants, each with its own applicable scenarios:
Small-molecule dispersants (e.g., phosphates, polycarboxylic acids):
Advantages: Low cost, easy to compound; Disadvantages: Low dispersion efficiency, easily affected by system pH
or electrolytes, poor stability. Suitable for low-requirement, low-cost systems (such as primers, color pastes).
High-molecular-weight dispersants (e.g., polyurethanes, acrylate copolymers):
Advantages: High dispersion efficiency, strong steric hindrance effect, good stability; Disadvantages: Higher cost,
compatibility with the resin needs to be considered. Suitable for demanding systems (such as topcoats and
high-end inks).
V. Experimental Verification and Optimization
Small-scale testing:
Take a small amount of pigment, resin, and dispersant, and grind them according to the actual formula (sand
milling or three-roll milling). Test the dispersibility (particle size distribution, detected by a particle size analyzer),
stability (centrifuge test, viscosity change after storage), and color strength (detected by a colorimeter).
Adjusting dispersant type/amount:
If the pigment flocculates after dispersion, try changing the dispersant (e.g., from anionic to polymeric); if the
system viscosity is abnormal, reduce the amount of dispersant or replace it with a lower viscosity dispersant.
Summary of Selection Process Flowchart
graph TD
A --> [Pigment Type] -->|Inorganic/Organic? Nano/Conventional?|
B --> [Dispersant Type Selection] -->|Surface Polarity? Treatment?|
C --> [System pH/Medium] -->|pH Range? Solvent-containing? Additives?|
D --> [Application Performance] -->|Stability/Gloss/Viscosity?| B -->
E --> [Small-scale Testing]
F --> [Optimizing Dispersant Type/Amount] --> Final Determination Key Considerations
Avoid conflicts between dispersant and resin: The dispersant needs to have good compatibility with the base
material (such as acrylic resin, polyurethane resin), otherwise it may lead to the dispersant detaching from the
pigment surface, causing flocculation.
Control the dosage: Too little dispersant (not covering the pigment surface) or too much (affecting leveling,
causing bubbles) will both lead to problems. The usual dosage is 0.5%-5% of the pigment weight (the specific
amount needs to be determined experimentally).
Pay attention to synergistic effects: Dispersants can be compounded with wetting agents (such as RK-8126) to
improve initial dispersion efficiency; when combined with defoamers (such as RK-8415), ensure that the
dispersant does not affect the stability of the defoamer.
Through the above steps, a dispersant suitable for water-based coatings/ink systems can be systematically
selected, achieving uniform pigment dispersion, system stability, and performance optimization. In actual
applications, it is recommended to combine with supplier technical support and verify through small-scale
tests before mass production.
