How to Choose Dispersants for Water-Based Coatings and Inks?
Choosing dispersants for water-based coatings and inks requires considering various factors, including
pigment characteristics, system environment, application performance, and cost. The core objective is
to achieve uniform dispersion of pigments in the water-based medium through the action of the dispersant
(reducing pigment surface energy, stabilizing the dispersion system, and improving storage and application
performance), thus avoiding problems such as flocculation, sedimentation, and color separation. The following
are specific selection points:
I. Clarify Pigment Type and Surface Properties: The chemical composition, surface polarity, and charge properties
of the pigment are fundamental to selecting a dispersant. Different pigments require dispersants with different
mechanisms of action.
1. Selection based on pigment type
Inorganic pigments (e.g., titanium dioxide, calcium carbonate, iron oxide red):
The surface is usually neutral or weakly acidic (e.g., titanium dioxide surface contains a small amount of hydroxyl groups),
requiring stabilization through electrostatic repulsion or steric hindrance.
Electrostatic dispersants (mainly anionic, such as carboxylic acid and sulfonic acid groups): After ionization, they carry a
negative charge and form electrostatic adsorption with the positive charge (or hydroxyl groups) on the pigment
surface, suitable for neutral to alkaline systems (pH 7-10).
Steric hindrance dispersants (e.g., polyether chains, polyester chains): If the inorganic pigment has low surface polarity,
steric hindrance dispersants can be used. The long-chain solvating groups extend in water, providing steric repulsion.
Organic pigments (e.g., azo dyes, phthalocyanines, quinacridones):
Surface polarity varies greatly, and some contain polar groups (e.g., amino groups, carboxyl groups), requiring
targeted matching of dispersant charge or polarity.
Polar organic pigments (containing carboxyl groups, sulfonic acid groups, etc.): Anionic dispersants are preferred
(forming hydrogen bonds or ionic bonds with the polar groups on the pigment surface).
Non-polar organic pigments (e.g., azo condensation type): The surface is almost non-polar, requiring non-ionic
dispersants (such as polyoxyethylene ethers) or high steric hindrance dispersants (such as dispersants containing
fluorinated segments), which provide steric hindrance through long-chain solvating groups. Functional Pigments
(such as nanopigments, pearlescent pigments, conductive pigments, etc.):
Nanopigments have a large specific surface area and are prone to agglomeration, requiring dispersants with high
adsorption capacity and high steric hindrance (such as high molecular weight polyetheramines). Pearlescent pigments
/flake pigments require avoiding strong shear forces that cause directional alignment; the dispersant needs to control
the adsorption strength to prevent over-dispersion from damaging their structure.
2. Pigment Oil Absorption and Specific Surface Area:
The higher the pigment oil absorption (OA) and the larger the specific surface area, the greater the amount of dispersant
required (to cover more pigment surface). For example:
Carbon black (high oil absorption) requires a higher amount of dispersant (usually 0.5%~5%, based on pigment weight);
Calcium carbonate (low oil absorption) requires a lower amount (0.1%~1%).
II. Analysis of System Environment and pH Value: The medium for water-based coatings and inks is water. The system pH,
temperature, and other additives (such as defoamers and thickeners) will affect the stability and dispersion effect of the
dispersant.
1. Influence of pH
Alkaline systems (pH 8~12, such as latex paint): Anionic dispersants are preferred (carboxylic acid and sulfonic acid groups
are completely ionized under alkaline conditions, resulting in stable adsorption).
Acidic systems (pH 4~6, such as some inks): Anionic dispersants may experience reduced adsorption due to protonation
(-COO⁻→-COOH). Non-ionic dispersants (such as polyoxyethylene ethers) or cationic dispersants (such as amine salts,
requiring attention to matching the pigment surface charge) can be used.
Neutral systems (pH 6~8): Non-ionic dispersants (such as polyethers) or zwitterionic dispersants (such as amino acid
derivatives) are more widely applicable, avoiding dispersant failure due to pH fluctuations.
2. Temperature and Shear Force
High-temperature systems (such as baking coatings, >60℃): Dispersants need to be stable at high temperatures, avoiding
dehydration or degradation of the solvated chain at high temperatures. Non-ionic dispersants containing ester and ether
groups (such as polyoxyethylene polyoxypropylene block copolymers) can be selected. High-shear applications (such as
spraying and roller coating): Dispersants need to withstand the strong shear forces during the application process,
preventing molecular chain breakage or adsorption detachment. High molecular weight dispersants (such as those with
a molecular weight of 10,000-50,000) or dispersants containing anchoring groups (such as phosphonic acid groups) can
be used to enhance adsorption strength.
3. Compatibility with other additives
Defoamers: If the system contains silicone oil-based defoamers, it is necessary to avoid "defoamer contamination" by
the dispersant (silicone oil will compete for adsorption on the pigment surface, reducing the effect of the dispersant).
Polyether-modified polysiloxane dispersants or silicone oil-resistant dispersants (such as those containing long-chain
fluorine) can be used.
Thickeners: Water-based systems commonly use alkali-swellable thickeners (ASE) or polyurethane thickeners (HEUR).
It is necessary to ensure that the dispersant and thickener do not conflict (e.g., whether the charge of the dispersant
repels the ionic groups of the thickener), avoiding abnormal system viscosity or decreased dispersion stability.
Biocides/Preservatives: Some biocides containing formaldehyde-releasing agents may damage the dispersant molecular
chain. Dispersants resistant to chemical additive contamination should be selected (such as aliphatic polyether types).
III. Focusing on Application Performance Requirements: Dispersants must meet the core performance requirements
of coatings and inks (storage stability, application performance, and final film performance), avoiding quality defects
caused by dispersion problems. 1. Storage Stability
For long-term storage (e.g., more than 6 months), high steric hindrance dispersants should be selected (such as high
molecular weight dispersants containing polyoxyethylene/polyoxypropylene chains) to resist pigment particle collision
and aggregation through the "steric hindrance effect";
If rapid dispersion is required (sand milling/ball milling), low molecular weight, highly active dispersants should be
prioritized (such as anionic carboxylic acid types with a molecular weight of 500-2000) to quickly adsorb and reduce
surface energy.
2. Application and Appearance Performance
Anti-floating and anti-blooming: In color paste or multi-color paint systems, the dispersant needs to have consistent
adsorption capacity for different pigments (i.e., "pigment matching") to avoid uneven color development caused by
the dispersant having a stronger adsorption to a particular pigment. General-purpose dispersants (such as polyether
amines) or dispersants optimized for mixed pigments can be selected.
Leveling and Gloss: Excessive addition of dispersants may lead to abnormal system viscosity or reduced leveling. The
addition amount needs to be controlled (usually 0.1% to 5% of the pigment weight is used as a reference, and the
optimal amount is determined through experiments).
3. Environmental Protection and Cost
Environmental Requirements: Prioritize low-VOC, low-irritant dispersants (such as solvent-free, biodegradable polyethers),
avoiding harmful components such as benzene rings and formaldehyde;
Cost-effectiveness: General-purpose dispersants (such as anionic carboxylic acids) are less expensive and suitable for
basic coatings; high-end applications (such as high-gloss coatings, weather-resistant coatings) require specialized
dispersants (such as high-molecular-weight polyether amines), but a balance between performance and cost is necessary.
IV. Experimental Verification of Dispersion Effect: The selection of dispersants requires small-scale testing in the
actual system. Key indicators include:
Particle Size Distribution: Use a laser particle size analyzer to detect the D50 and D90 of the dispersed pigment,
ensuring that the particle size is <5μm (to prevent sedimentation);
Viscosity and Stability: Measure the viscosity of the dispersed system using a rotational viscometer, and observe whether
stratification or sedimentation occurs after storage (standing/centrifugation);
Application Performance (e.g., brushing/spraying): Test for sagging, cratering, or mottling, and whether the viscosity
meets application requirements;
Film Performance: Test the gloss, adhesion, and water resistance of the final film, avoiding the impact of dispersant residue
on film performance (such as plasticizer migration).
Summary of Selection Logic
Step 1: Clarify the pigment type and surface charge (inorganic/organic, polar/non-polar);
Step 2: Analyze the system environment (pH, temperature, additive compatibility);
Step 3: Determine performance requirements (storage stability, applicability, environmental friendliness);
Step 4: Small-scale testing (particle size, viscosity, storage stability, film performance);
Step 5: Optimize cost and environmental protection (select the most cost-effective dispersant type).
Through the above steps, a suitable dispersant can be selected to ensure uniform dispersion and stable performance of
pigments in water-based coatings and ink systems. In practical applications, you can refer to the technical manual provided
by the dispersant supplier (Dongguan Ruikun Materials Technology Co., Ltd.) and select the appropriate product based on
the specific pigment and system.
