Selecting a water-based paint dispersant requires comprehensive consideration of the dispersion system characteristics
(pigment type, resin, pH value, etc.), performance requirements (dispersion efficiency, stability, compatibility), and application
scenarios (latex paint, industrial paint, etc.). The following is a systematic selection method and key points:
I. Clarify the core function and type of dispersant.
The core function of a dispersant is to reduce the surface tension between the
pigment/filler and water (or dispersion medium), preventing pigment aggregation through steric hindrance or electrostatic repulsion,
and improving dispersion uniformity, stability, and coating performance (such as color strength, gloss, and storage stability).
1. Selection based on ion type (most basic classification): Water-based dispersants are mainly divided into anionic, non-ionic, cationic,
and amphoteric types. Anionic and non-ionic types are the most commonly used and need to be matched according to the pigment
charge and system pH value:
Anionic Dispersants (most mainstream)
Structural characteristics: Contains hydrophilic groups such as carboxyl groups (-COOH), sulfonic acid groups (-SO₃H), and phosphoric
acid groups (-PO₄H₂), which strongly adsorb to the pigment surface (especially inorganic pigments) through ionic bonds.
Advantages: Excellent dispersion effect on inorganic pigments (such as titanium dioxide, calcium carbonate, carbon black), strong
salt and electrolyte resistance, and wide applicability.
Limitations: Easily partially protonated in acidic systems (pH<6), resulting in reduced adsorption capacity; may conflict with
cationic additives (such as bactericides and emulsifiers).
Representative products: Polycarboxylic acids (such as RK-4008Y), lignin sulfonates, phosphate esters (such as RK-4012).
Non-ionic Dispersants
Structural characteristics: Contains hydrophilic groups such as polyoxyethylene ether (-O(CH₂CH₂O)ₙH), which stabilize pigments
through steric hindrance (ether chain hydration), without relying on ionic bonds.
Advantages: Excellent compatibility (miscible with various resins and additives), no charge interference, suitable for pH-sensitive
systems or systems requiring high gloss (such as water-based wood coatings and varnishes). Limitations: Weak adsorption to
inorganic pigments (especially high surface area pigments such as carbon black), requiring combination with other dispersants
or adjustment of dosage.
Representative products: Polyethers (e.g., RK-4018), fatty alcohol polyoxyethylene ethers (e.g., non-ionic surfactants), esters
(e.g., RK-4017C).
Cationic/Amphoteric Dispersants (for specific applications)
Cationic type: Contains cationic groups such as amine groups (-NH₂), suitable for negatively charged organic pigments
(such as some azo pigments) or acidic resin systems. Care must be taken to avoid charge conflicts with anionic additives
(such as defoamers and thickeners).
Amphoteric type: Possesses both anionic and cationic groups, adaptable to pigments with various charges or complex pH
environments (such as leather coatings, inks), but at a higher cost.
II. Precise Matching of Dispersants Based on Dispersion System Characteristics: The selection of dispersants needs to be
closely linked to the core parameters of the coating system, such as pigment type, resin characteristics, pH value, and salt
content, avoiding the blind use of "general-purpose" types.
1. Selection based on pigment type (most crucial): The surface charge and specific surface area of the pigment determine the
"anchoring ability" of the dispersant, requiring targeted matching:
Inorganic pigments (e.g., titanium dioxide, carbon black, iron oxide, calcium carbonate)
Titanium dioxide (anatase/rutile): High specific surface area (50-100 m²/g), containing a large number of hydroxyl groups
(-OH) on the surface, weakly negatively charged, requiring strong adsorption anionic dispersants (such as polycarboxylic
acids, containing multiple -COOH groups, anchored through ionic bonds).
Carbon black: High pigment content, strong hydrophobicity, weakly negatively charged surface (small amount of carboxyl
and phenolic hydroxyl groups), requiring high anchoring force dispersants (such as RK-4010 containing phosphate ester
groups, or polycarboxylic acid-phosphate ester composite types) to prevent carbon black flocculation leading to "black phase"
and "reduced color strength". Calcium Carbonate/Talc: Low specific surface area (<10 m²/g), weakly negatively charged surface.
Use a common anionic dispersant (such as lignin sulfonate NNO, low cost, sufficient dispersion efficiency).
Organic Pigments (e.g., phthalocyanine blue, azo red, quinacridone violet)
High polarity, complex charge: Some organic pigments (such as azo pigments) may have a weakly positive or neutral surface charge.
A non-ionic dispersant (such as polyether, stabilized by steric hindrance) or a combination of anionic and non-ionic dispersants
(to improve compatibility) should be selected. If the pigment is negatively charged (such as some disazo pigments), anionic
dispersants are preferred.
Functional Pigments (e.g., nanopigments, fluorescent pigments)
Nanopigments (e.g., nano-TiO₂, nano-ZnO) have a very high specific surface area and are prone to agglomeration. A
specialized dispersant is required (such as RK-4058, containing multiple anchoring groups + long branched chains, providing
strong steric hindrance); for fluorescent pigments, avoid dispersants that compete with pigment molecules for adsorption,
and choose a non-ionic type with good compatibility.
2. Selection based on resin type (to avoid compatibility issues): The chemical structure (acid value, polarity, Tg) of water-based
resins (emulsions, epoxies, polyurethanes, etc.) directly affects the adsorption and compatibility of the dispersant.
"Resin-dispersant structure matching" is necessary:
Water-based emulsions (styrene-acrylic, pure acrylic, vinyl acetate-acrylic):
Emulsions contain carboxyl groups (acid value 10-30 mg KOH/g), negatively charged. An anionic dispersant
(such as polycarboxylic acid) should be selected. The -COO⁻ forms ionic bonds with -NH₃⁺ (after amine neutralization)
in the emulsion, enhancing adsorption stability and preventing the dispersant from being "displaced".
Water-based epoxy/polyurethane resins:
Water-based epoxy: Contains epoxy groups and a small amount of hydroxyl groups, with medium polarity. Phosphate
ester dispersants (such as RK-4013) are suitable; the phosphate group can react with the epoxy hydroxyl group,
enhancing anchoring. Waterborne Polyurethane: Contains a large number of amino (-NH-) and urea (-NH-CO-NH-) groups,
which are highly polar. A dispersant with a structure similar to polyurethane should be selected (such as non-ionic
dispersants containing ester/ether groups, like RK-4020) to avoid turbidity and stratification due to poor compatibility.
Waterborne Amino Resin/Amino Baking Paint:
Amino resins (such as melamine formaldehyde resin) are highly polar and pH-sensitive. Non-ionic dispersants
(such as RK-4017E) should be selected to avoid insufficient dissociation of anionic dispersants under weakly acidic
conditions, which would affect dispersion stability.
3. Selection based on system pH and salt content
pH value:
Alkaline systems (pH>8, such as latex paint, waterborne industrial primers): Anionic dispersants are completely dissociated
and have strong adsorption capacity; therefore, anionic types should be selected.
Neutral/weakly acidic systems (pH=6-8, such as waterborne wood coatings, varnishes): Non-ionic dispersants should be
selected (to avoid reduced adsorption due to partial protonation of anionic dispersants), or a "anionic + non-ionic blend"
(such as polycarboxylic acid + polyether blend, to balance adsorption and compatibility).
Strongly acidic systems (pH<6, such as some waterborne inks): Cationic dispersants should be selected (such as dispersants
containing amine groups) to avoid inactivation of anionic dispersants due to protonation.
Salt content:
Waterborne coatings (especially latex paints) contain a large amount of pigments, fillers, and electrolytes (such as ammonia
water, salts in thickeners), which can easily lead to "salt precipitation" of the dispersant (electrolytes stripping the hydration
layer of the dispersant). Therefore, salt-resistant dispersants should be selected:
Among anionic dispersants, polycarboxylic acids (such as RK-4008) have better salt resistance than lignin sulfonates (due to
higher density of carboxylic acid groups and stronger adsorption).
Non-ionic dispersants (such as polyethers) have better salt resistance than anionic dispersants, but their "cloud point"
(precipitation at elevated temperatures) should be considered to avoid affecting storage stability. III. Core Performance
Indicators and Verification Methods: Selecting a dispersant requires testing key performance indicators through
small-scale trials to avoid risks in mass production:
1. Dispersion Efficiency (Particle Size)
Test Method: Mix the pigment, dispersant, and base material, grind with a sand mill, and measure the D50 (median particle size)
using a particle size analyzer. A smaller D50 indicates better dispersion (generally, latex paint requires D50 < 10 μm, and colored
paint < 5 μm).
Note: The dispersant dosage should be at the "dispersion efficiency saturation point" (excessive amounts may cause the
dispersant to "foam" or "shrink," while insufficient amounts will result in incomplete dispersion).
2. Storage Stability (Anti-flocculation/Sedimentation)
Test Method: Seal the dispersed coating and store it (at room temperature/high temperature aging), observe whether
stratification or clumping occurs, or check the sedimentation volume through a centrifugation test (3000 rpm for 30 minutes).
Key Point: Non-ionic dispersants need to improve stability through "steric hindrance" (long branched chains), avoiding
reliance solely on electrostatic repulsion (which is easily affected by salts).
3. Compatibility (No "Turbidity" or "Cratering")
Test Method: Add the dispersant to the coating base material (without pigment), stir evenly, and observe whether it is turbid
or stratified, or whether "cratering" or "pinholes" appear after coating (poor dispersant compatibility can lead to abnormal
surface tension of the base material).
Judgment Standard: Transparent, no precipitation, and no cratering (especially for high-gloss clear coats, poor compatibility
can lead to reduced gloss).
4. Impact on Other Coating Properties
Gloss: For colored paints, pay attention to the dispersant's effect on pigment orientation (selecting non-ionic dispersants
can reduce the impact on gloss);
Adhesion: Avoid dispersants containing strongly polar groups (such as phosphate esters) that cause the coating to
"absorb moisture," affecting adhesion;
VOC/Environmental Protection: Food contact coatings require FDA-certified dispersants (such as food-grade), avoiding
dispersants containing APEO (restricted by EU REACH regulations). IV. Typical Application Cases and Recommendations
for Dispersant Types and Parameters
| Coating Type/System | Key Considerations | Recommended Dispersant Type |
| Latex Paint (Architectural Paint) | High filler content (titanium dioxide, calcium carbonate), high salt content, pH=8-9 |
Polycarboxylic acid type (e.g., RK-4008), lignosulfonate | Salt resistance, low cost, balancing dispersion efficiency and
storage stability. |
| Water-based Color Paint (Automotive Paint) | High gloss, mainly organic pigments, pH=7-8 | Non-ionic polyether type
(e.g., RK-4018), polyether-modified silicone | Good compatibility, does not affect gloss, avoids color strength reduction. |
| Water-based Industrial Paint (Epoxy Primer) | Epoxy resin, contains amine neutralizing agent, pH=8-9 | Phosphate ester
type (e.g., RK-4013C), polycarboxylic acid type | Reacts with epoxy groups, strong anchoring force, baking resistance
(120℃/30min). |
| Water-based Wood Coating (PU Varnish) | Polyurethane resin, strong polarity, requires high compatibility | Polyether
type (e.g., RK-4020), polyether-polyester blend | No cratering, improved leveling, avoids affecting gloss. |
Summary: Selection Steps
1. **Analyze the system:** Clarify the pigment type (inorganic/organic), resin type (emulsion/epoxy/polyurethane),
pH value, and salt content;
2. **Match the type:** Choose anionic dispersants for inorganic pigments, and non-ionic dispersants (polyether type)
for organic pigments/high compatibility requirements;
3. **Small-scale testing:** Measure particle size (dispersion efficiency), stability (storage/centrifugation), and
compatibility (no turbidity/cratering);
4. **Optimization and adjustment:** If performance is insufficient, a blend can be used (e.g., anionic + non-ionic)
or a specialized model can be selected (e.g., high steric hindrance dispersant for nano-pigments).
Following this logic, the dispersant needs of water-based coatings can be accurately matched, avoiding
"ineffective addition" or "performance defects".
