How to choose a dispersant for nano pigments?

How to choose a dispersant for nano pigments?

Complete Selection Guide for Nano Organic/Inorganic Pigment Dispersants (Water-Based/Oil-Based Full Coverage + Pitfall Avoidance + Process)

Foreword

Core characteristics of nano-pigments (both inorganic and organic) include an extremely high specific surface area and surface energy, as well as a strong tendency toward secondary agglomeration.

They differ fundamentally from conventional micron-scale pigments: in nano-systems, the use of small-molecule wetting and dispersing agents as the primary dispersant is strictly prohibited. While initial fineness may meet specifications, storage inevitably leads to coarsening, flocculation, and hard sedimentation.

The core logic for selection distinguishes between two major pigment systems:

1. Nano-inorganic pigments (titanium dioxide, iron oxide, zirconium oxide, silica powder, ceramic powder, etc.): Surfaces are rich in hydroxyl groups; stability is achieved through electrostatic adsorption combined with strong anchoring via phosphate groups.

2. Nano-organic pigments (phthalocyanine, DPP, quinacridone, azo, carbon black, etc.): Surfaces have low polarity and lack hydroxyl groups; stability is achieved through anchoring via affinity blocks combined with thick-layer steric hindrance.

I. Oil-based/Solvent-based/Solvent-free Systems (solvent-based paints, inks, UV-cured systems, epoxy systems, high-solids systems)

1. Oil-based Systems — Nano-organic Pigments

Suitable Dispersants

Preferred: Polyurethane polymeric dispersants; pigment-affinic acrylic block dispersants

RK-4035AB, RK-4036

Advantages

Contains aromatic-affinic groups that match and adsorb onto the conjugated structures of organic pigments;

Provides strong steric hindrance; prevents flocculation, floating, and flooding; delivers high tinting strength and low color paste viscosity.

Considerations

Some long-chain polyurethane dispersants may slightly affect recoat adhesion and surface gloss; small-scale trials are recommended for verification.

2. Solvent-based — Nano-inorganic pigments

Suitable dispersants

Preferred: Phosphate-anchored polyester dispersants; phosphate-modified acrylic polymeric dispersants

Recommended grades: RK-4013A, RK-4013C

Mechanism

In solvent-based systems, stabilization relies entirely on anchoring/adsorption and steric hindrance, rather than electrostatic stabilization.

Phosphate groups serve as the strongest anchoring groups for metal oxides and ceramic nanopowders—far superior to standard carboxyl groups.

Applications

Aromatics, esters, weak solvents, and solvent-free high-solids systems.

II. Water-based Systems (Water-based coatings, inks, colorants, digital inkjet inks, dip coatings)

1. Water-based – Nano-organic pigments

Suitable Dispersants

Preferred: Anionic/non-ionic block acrylic polymeric dispersants (featuring pigment-affinic block structures)

Recommended Grades: RK-4039AC, RK-4059

Selection Rationale (Key Points)

Standard polycarboxylate water-based dispersants offer virtually no anchoring capability for organic pigments, relying solely on electrostatic stabilization.

Observed Issues: Initial grind fineness meets specifications, but after one day, the system exhibits re-coarsening, pigment floating, colorant thickening, and reduced tinting strength.

Block-type Dispersants: These feature a structure where one end firmly adsorbs onto the conjugated structure of the organic pigment while the other end is solvated in the aqueous phase, creating a thick steric hindrance layer—the key to stabilizing nano-organic pigments.

Applications

High-end water-based colorants, decorative paints, inkjet inks, and weather-resistant water-based topcoats.

Formulation Advice

Can be combined with low-foaming acetylenic diol or silicone wetting agents to enhance powder wetting and penetration; these additives do not replace the dispersant itself.

2. Water-based — Nano-inorganic Pigments

Suitable Dispersants

Preferred: Ammonium polycarboxylate polymeric dispersants; phosphate-modified anionic acrylic dispersants.

Recommended Grades: RK-4008DH, RK-4017F0.

Stabilization Mechanism: Strong electrostatic repulsion combined with the superior anchoring capability of phosphate groups onto metal oxide powders; de-agglomerates nano-particles and prevents flocculation caused by particle proximity.

Applications

Water-based industrial coatings, water-based color pastes, water-based ceramic slurries, dispersion of water-based photovoltaic powders, and general water-based systems.

Advantages

Controllable particle size, high tinting strength, excellent cost-performance ratio, and high grinding efficiency.

Limitations

Stability decreases in systems with high electrolyte or salt content; standard polycarboxylates lack resistance to strong alkalis.

Additional Selection Guidelines

High pH/alkali-resistant systems: Avoid standard polycarboxylates; use specialized alkali-resistant phosphate-based dispersants.

Strict Prohibitions

Do not use cationic dispersants (causes immediate flocculation and agglomeration due to charge neutralization).

III. Specialized Selection for Specific Systems (High-Frequency, Challenging Scenarios)

3.1 UV / Solvent-free / High-solids Low-viscosity Systems

- Nano-organic pigments: Select block dispersants modified with reactive double-bond groups.

Recommended grades: RK-4061 CF, RK-4054EF

Prevents issues associated with standard polyurethanes, such as migration, bleeding, impact on curing rates, and residual cratering.

- Nano-inorganic pigments: Specialized solvent-free phosphate-based dispersants.

Controls viscosity in high-solids systems; prevents abnormal thixotropy, settling, and phase separation.

3.2 Water-based Digital Inkjet Systems

Specific requirements:

1. Low molecular weight and low ionic content (to prevent nozzle clogging and crystallization);

2. Hydrolysis resistance, bleed resistance, and migration resistance;

3. Avoidance of high-viscosity, long-chain dispersants.

Recommended grades: RK-4061EC, RK-4059, RK-4054EF

3.3 Weak-solvent systems (alcohol-ether based)

Not suitable for pure oil-based polyurethanes or pure water-based polycarboxylates.

Selection: Alcohol-ether modified block acrylic dispersants; offering moderate compatibility and optimal stability.

Recommended grades: RK-4035AB, RK-4036B, RK-4061ED

IV. Key Process Points (90% of dispersion issues stem from the order of addition)

Correct Order of Addition

Solvent/Water → Dispersant → Wetting Agent → Mix thoroughly → Slowly add nano-powder → Low-speed pre-wetting → High-speed dispersion → Bead milling

Incorrect Order

Grinding nano-powder with solvent first, then adding dispersant later

Consequence: Powder agglomerates become compacted and the dispersant cannot penetrate them; the particle size cannot be reduced effectively, and the material will subsequently re-coarsen.

V. Additive Synergy and Formulation Strategy (Comprehensive Formulation Logic)

1 .Wetting Agents (Essential)

Due to their extremely high specific surface area, nanopowders are far more difficult to wet than conventional pigments.

- Water-based: Acetylenic diols, low-foam silicone wetting agents

- Solvent-based: Non-ionic solvent-borne wetting agents

Wetting agents facilitate rapid wetting and air release, while dispersants handle de-agglomeration and long-term stability; the two functions are not interchangeable.

2 .Rheology Modifiers / Anti-settling Agents

Use in conjunction with dispersed and stabilized systems:

- Water-based: Associative thickeners, bentonite

- Solvent-based: Fumed silica, polyamide wax

Note: Anionic dispersant systems must not be combined with cationic rheology modifiers, as charge incompatibility will cause instability.

3 .Defoamers

Nano-milling processes involve high shear and high air entrapment; therefore, system-specific defoamers compatible with the dispersant must be used to prevent the defoamer from disrupting the adsorbed dispersant layer, which could lead to flocculation.

VI. List of Absolute Prohibitions and Non-Recommended Dispersants (Key Pitfalls to Avoid)

1. Prohibitions for Solvent-Based Systems

(1) Do not use low-molecular-weight fatty acid dispersants (insufficient steric hindrance leads to rapid, hard settling);

(2) Do not use highly polar dispersants containing free carboxyl groups in non-polar solvents (causes loss of gloss, cratering, and poor compatibility).

2. Prohibitions for Water-Based Systems

(1) Nano-organic pigments: Do not use standard non-block polycarboxylates or low-molecular-weight naphthalene sulfonates (provides short-term stability, but coarsening/flocculation is inevitable over the long term);

(2) Nano-inorganic pigments: Do not use cationic dispersants or low-end dispersants with poor salt tolerance;

(3) All nano-systems: Do not use low-molecular-weight dispersants alone as the primary dispersing agent.

VII. Selection and Performance Trade-offs for Dispersants (Formulation Considerations)

1.  Low-molecular-weight dispersants: Result in low color paste viscosity, high tinting strength, and fine particle size, but offer relatively weak heat-storage stability;

2.  Phosphate-based dispersants (for inorganic pigments): Provide the strongest anchoring, but carry a slight risk of hydrolysis and desorption in long-term high-temperature, high-humidity environments;

3.  High-molecular-weight dispersants: Offer excellent stability and effectively prevent color reversion and flocculation, but result in higher viscosity, making them unsuitable for inkjet or ultra-high-solids systems;

4.  Aromatic-anchoring dispersants (for organic pigments): Deliver extremely high color strength, but exhibit average weatherability and yellowing resistance; suitability for outdoor systems requires evaluation.

VIII. Troubleshooting Common Failure Modes

1.  Grinding fineness remains unsatisfactory: Insufficient wetting / Mismatch between the dispersant's anchoring groups and the pigment.

2.  Fine initially but coarsens upon storage: Insufficient steric hindrance from the dispersant / Adsorption layer is too thin.

3.  Color floating and flooding: Uneven dispersion/adsorption of organic pigments / Insufficient steric hindrance.

4.  Separation and soft settling: Dispersion stability is adequate, but the rheological anti-settling system is insufficient.

5.  Hard settling, caking, and flocculation: Incorrect dispersant selection / Charge incompatibility / Complete incompatibility.

IX. Ultimate Quick-Reference Comparison Table