How to choose the ideal dispersant for a polyurethane resin system?
When selecting a dispersant for a polyurethane resin system, a comprehensive assessment is required, considering
the system type (solvent-based/water-based), pigment characteristics (surface charge, polarity), polyurethane resin
structure (functional groups, polarity), and application performance (dispersibility, stability, film formation). The
following are selection strategies and key indicators for different scenarios: I. Polyurethane System Classification
and Dispersant Matching Principles Polyurethane resin systems are mainly divided into two categories: solvent-based
(solvent-based coatings/inks) and water-based (water-based coatings/adhesives). The selection of dispersants needs
to be specifically matched to the system characteristics:
1. Solvent-based Polyurethane Systems Solvent-based polyurethanes commonly use organic solvents (such as ethyl
acetate, methyl ethyl ketone, toluene, xylene, etc.). The dispersant needs to meet the following requirements:
Compatibility with the solvent and polyurethane resin: The solvating chain of the dispersant needs to dissolve well in
the target solvent and have good miscibility with the polyurethane resin (such as aromatic MDI type, aliphatic IPDI type)
(to avoid dispersant precipitation or affecting resin curing due to incompatibility).
Selection of charge type based on pigment surface charge:
Inorganic pigments (such as carbon black, iron oxide red/black, barium sulfate): The surface usually carries a weakly
acidic or neutral charge. Anionic dispersants (carboxylic acid, sulfonic acid groups, anchored by electrostatic attraction)
or high molecular weight non-ionic dispersants (stabilized by steric hindrance) are preferred.
Organic pigments (such as azo dyes, xanthene dyes): The surface may contain polar groups (such as -COOH, -SO₃H).
Anionic dispersants (electrostatic + steric hindrance) or non-ionic dispersants (polyether chains, avoiding charge repulsion)
can be used. 2. Waterborne Polyurethane Systems: Waterborne polyurethanes form aqueous dispersions through
neutralization (e.g., triethylamine neutralizing carboxylic acid groups). The dispersion medium is water + a small
amount of co-solvent. The dispersant must meet the following requirements:
Aqueous stability: Easily dispersible in water and does not affect the stability of the polyurethane emulsion (avoiding
salting out and demulsification).
Compatibility with water and polyurethane: The hydrophilic/lipophilic balance (HLB value) of the dispersant must
match the aqueous phase environment (e.g., non-ionic dispersants containing polyether chains), while avoiding
reactions with functional groups such as amino and hydroxyl groups in the polyurethane (e.g., avoiding dispersants
containing isocyanate groups reacting with polyurethane hydroxyl groups).
Regarding pigment charge:
Inorganic pigments (e.g., carbon black, calcium carbonate): The surface is negatively charged in the aqueous phase;
select anionic dispersants (carboxylic acid, sulfonic acid groups) or zwitterionic dispersants (carrying both positive
and negative charges, adapting to complex charge environments).
Organic pigments (e.g., phthalocyanine blue, quinacridone): The surface may be neutral or weakly polar; select
non-ionic dispersants (polyether chains, steric hindrance effect) or high molecular weight superdispersants
(multiple anchoring groups, high adsorption force).
II. Key Dispersant Types and Typical Applications: Based on structural characteristics and performance, the following
types of dispersants are widely used in polyurethane systems: 1. High Molecular Weight Superdispersants (most
recommended, especially for high-viscosity systems)
Structure: Contains "anchoring groups" (e.g., carboxylic acid, sulfonic acid, phosphate ester) and "solvation chains"
(e.g., polyester, polyether, polymethacrylate), which can stabilize pigments through strong adsorption and steric hindrance.
Advantages:
High adsorption capacity, low dosage (typically 5%-20% of pigment amount), suitable for high-viscosity polyurethane
systems (reduces the risk of system thickening).
Wide adaptability to pigment surfaces; effectively disperses both inorganic and organic pigments.
Typical Applications:
Solvent-based systems: Ruikun RK-4112 (carboxylic acid ester anchoring, aliphatic solvent-based, suitable for carbon black
and iron oxide dispersion); Ruikun RK-4133 (super-dispersant, suitable for aromatic MDI-type polyurethanes, high shear
stability).
Water-based systems: RK-4117C (water-based super-dispersant, carboxylic acid anchoring + polyether chain, suitable for
carbon black and organic pigment dispersion in water-based polyurethanes); RK-4008Y (polyether modified, suitable for
water-based polyurethane emulsions, avoids affecting system pH).
2. Anionic Dispersants (General purpose, lower cost)
Structure: Contains carboxylic acid (-COOH), sulfonic acid (-SO₃H), or phosphate ester (-OPO(OH)₂) groups, adsorbed onto
the pigment surface through electrostatic interaction.
Advantages:
Excellent dispersion effect on inorganic pigments (such as carbon black and iron oxide), lower cost than super-dispersants.
Limitations:
May have insufficient dispersion power for highly polar pigments (such as organic pigments containing amine groups), and
is susceptible to electrolytes (such as salting out).
Typical Applications:
Solvent-based systems: RK-4116A (carboxylic acid ester type, suitable for aromatic polyurethane + carbon black dispersion,
low viscosity); RK-4008DF (polycarboxylic acid ammonium salt, suitable for aliphatic polyurethanes, high gloss).
Water-based systems: RK-4117E (water-based anionic dispersant, carboxylic acid group, suitable for water-based polyurethane
+ calcium carbonate, barium sulfate dispersion); RK-4117 (water-based system, sulfonic acid group, electrolyte resistant,
suitable for carbon black dispersion). 3. Nonionic Dispersants (low charge interference, suitable for sensitive systems)
Structure: Contains ether chains such as polyoxyethylene (EO) and polyoxypropylene (PO), stabilizing pigments through
steric hindrance without electrostatic interactions.
Advantages:
Excellent dispersion effect on polar pigments (such as organic pigments containing hydroxyl and amino groups), no
electrostatic interference, avoiding interference with polyurethane curing reactions (such as the neutralization reaction
between amino groups and anionic dispersants).
Typical Applications:
Solvent-based systems: RK-4133 (polyether type, suitable for aromatic polyurethanes + xanthene organic pigments,
low foaming); RK-8126 (polyether-modified silicone oil, suitable for high-viscosity polyurethanes, improving fluidity).
Water-based systems: RK-4117C (water-based non-ionic dispersant, polyether chain, suitable for water-based polyurethanes
+ phthalocyanine blue, quinacridone and other organic pigments, no charge repulsion); RK-4008EF (non-ionic, suitable
for water-based systems, avoiding affecting emulsion stability).
4. Zwitterionic Dispersants (Suitable for Complex Systems)
Structure: Contains both positively charged (e.g., amine groups) and negatively charged (e.g., carboxylic acid) groups,
adaptable to situations where pigment surface charge changes with pH.
Advantages: Provides stable dispersion even when pH fluctuates or pigment charge is complex (e.g., charge changes
of carbon black in water at different pH values).
Typical Applications: RK-4010 (zwitterionic dispersant, suitable for high-salt water-based polyurethane systems, such
as thick-film coatings containing a large amount of pigments and fillers).
III. Key Verification Indicators and Experimental Methods When selecting dispersants, the following properties need
to be verified through experiments to avoid affecting the final quality of the polyurethane system: 1. Dispersion Verification
Dispersion effect: Tested by pigment particle size distribution after grinding (laser particle size analyzer). For inorganic
pigments (such as carbon black), D50 usually needs to be <5 μm; for organic pigments (such as red organic pigments),
D50 <3 μm.
Storage stability: The dispersed ink/coating is left to stand for 7 days and 14 days to observe whether stratification,
precipitation, or thickening occurs (low-viscosity systems at 25°C, high-viscosity systems at 50°C aging test). 2. Impact on
Polyurethane Performance
Viscosity Change: After adding the dispersant, the system viscosity fluctuation should be <10% (the dispersant itself should
have low viscosity to avoid thickening the system).
Curing Performance: In water-based polyurethanes, the dispersant should not react with amino or hydroxyl groups (e.g., by
detecting changes in -NCO or -OH content using infrared spectroscopy to ensure that the curing agent's reaction with amino
groups is not interfered with).
Adhesion/Gloss: For coatings/inks, test the adhesion using the cross-cut test (≥4B) and 60° gloss (fluctuation <5% compared
to the control), avoiding surface defects (such as craters and fisheyes) caused by dispersant migration.
IV. Practical Application Suggestions
Priority Testing: For specific systems (solvent-based/water-based, pigment type), prioritize the selection of hyperdispersants
(such as RK-4117E/4112) or anionic dispersants (such as RK-4116A), and determine the optimal dosage through small-scale
testing.
Pre-dispersion Treatment: If the dispersant is a solid (e.g., granular), it needs to be dispersed first with a small amount of
polyurethane resin or solvent (milled to a particle size <10 μm) to avoid agglomeration.
Control of Dosage: Generally, the dosage is 5%-15% of the pigment amount (e.g., 5-15 parts of dispersant for 100 parts of pigment).
Excess may lead to thickening of the system or affect gloss.
Supplier Technical Support: Provide the complete formula (resin type, pigment, solvent), and request the supplier to recommend a
suitable model (e.g., Dongguan Hongrui Chemical Co., Ltd. and Dongguan Ruikun Materials Technology Co., Ltd. both offer dispersant
solutions specifically for polyurethane systems).
In summary, the selection of dispersants for polyurethane systems should be based on "compatibility + charge matching +
performance suitability": for solvent-based systems, prioritize high-molecular-weight hyperdispersants or anionic dispersants;
for water-based systems, prioritize hyperdispersants or non-ionic dispersants. Verification should be conducted through
experiments such as particle size distribution, storage stability, and curing performance to ensure a balance between dispersion
effect and the film-forming properties and stability of the polyurethane.
