Selection of Dispersants for Nano-Inorganic Pigments (Targeting Water-Based Systems; Suitable for Coatings and Inkjet Inks)
Characteristics of Nano-Inorganic Pigments: Small particle size and high specific surface area make them highly prone to agglomeration. Compared to conventional inorganic pigments, they are more difficult to disperse stably and are susceptible to issues such as particle coarsening, sedimentation, sudden viscosity spikes, and poor transparency.
I. Core Principles for Selection
1. Prioritize the surface chemical properties of the pigment
- Metal oxides (TiO₂, Fe₂O₃, ZnO, ZrO₂): Surfaces are predominantly hydroxylated; anionic polymeric dispersants (polyacrylates, ammonium polycarboxylates) are preferred to maintain dispersion stability via electrostatic repulsion. Recommended grades: RK-4008DH.
- Silicates/Kaolin/Calcium carbonate: Surfaces are relatively inert; anionic polymers or comb-type copolymer dispersants are recommended to combine electrostatic repulsion with steric hindrance effects. Recommended grades: RK-4017E.
- Hydrophobically modified nano-powders: Electrostatic stabilization alone is insufficient; steric hindrance-type dispersants (comb-type block copolymers) must be used. Recommended grades: RK-4010, RK-4017G.
2. The dispersant's anchoring groups must match the powder surface.
Anchoring groups (such as carboxyl or phosphate groups) adsorb onto the surface of inorganic powders, while solvating chains extend into the water to create steric hindrance—a critical factor in preventing agglomeration in nanoparticle systems. Phosphate-based dispersants exhibit strong adsorption on metal oxide nanopowders and are suitable for high-solids-content systems. Recommended grades: RK-4012, RK-4013A, RK-4013C.
3. Application Constraints
- Water-based digital inkjet systems: Require low viscosity and low foaming without damaging printheads; polymeric polyurethane dispersants are the preferred choice. Their core advantage lies in the synergy between electrostatic repulsion and steric hindrance stabilization mechanisms. They withstand high concentrations of diols, offer robust anchoring (resisting desorption from the pigment interface), are suitable for both organic and inorganic pigments, exhibit excellent compatibility with acrylic and PU resins, and are compatible with certain cationic systems. They also demonstrate outstanding shear stability—maintaining particle size without coarsening after repeated grinding cycles—and show a reduced tendency to foam. Recommended grades: RK-4059, RK-4056EF.
- Industrial water-based coatings: Polymeric comb-copolymer dispersants are recommended; these effectively inhibit pigment settling and significantly enhance the storage stability of the system. Recommended grades: RK-4039AB, RK-4058.
II. Comparison of Dispersant Types
1. Non-ionic block copolymer dispersants
✅ Advantages: Excellent electrolyte resistance and a wide pH operating range.
❌ Disadvantages: Relatively weak interfacial anchoring strength on inorganic powders; generally unsuitable for use as a sole primary dispersant and typically combined with a primary dispersant for synergistic effects.
2. Ammonium polycarboxylate (anionic type)
✅ Advantages: Excellent broad-spectrum applicability and outstanding cost-effectiveness; highly effective for dispersing nano-TiO₂ and iron oxide; allows for controllable system viscosity.
❌ Disadvantages: Reduced stability in high-salt environments; moderate steric hindrance; limited steric hindrance effect; prone to particle re-aggregation with nanoparticles possessing ultra-high specific surface areas.
Recommended grade: RK-4008DH.
3. Phosphate-based comb-structured polymeric dispersants
✅ Advantages: Exceptional anchoring capability, particularly well-suited for nano-metal oxide systems; maintains low viscosity at high solid content; offers excellent anti-flocculation and anti-settling performance.
❌ Disadvantages: Relatively high cost; certain grades may compromise the water resistance of the paint film.
Recommended grades: RK-4012, RK-4013A
III. Key Formulation Considerations
1. pH Control: The optimal pH range for dispersing most inorganic nanopowders is 7–9. They must be used in conjunction with wetting agents to enhance powder wetting efficiency and reduce the time required for bead milling (grinding).
2. Dosage Design: Due to their significantly higher specific surface area compared to conventional pigments, nanopigments require a higher dosage of dispersant. It is recommended to conduct small-scale gradient trials using 5%–12% of the powder mass, rather than relying on a fixed value.
3. Selection Pitfalls: Low-molecular-weight dispersants (such as sodium hexametaphosphate) should be avoided. While these dispersants may offer acceptable initial dispersion, they struggle to maintain long-term storage stability, making the system prone to flocculation and particle size coarsening.
IV. Small-scale Test Metrics (Dispersant Screening)
Evaluation after milling and dispersion: Measure fineness and system viscosity; assess particle size coarsening and phase separation/sedimentation after heat aging at 50°C for 7 days; evaluate coating gloss and transparency via film casting or inkjet proofing.
V. Summary of Selection Guidelines
- Surface-hydrophobized nano-inorganic powders: Comb-structured block copolymer dispersants are preferred.
- Nano-inorganic pigments for water-based digital inkjet inks: Low-molecular-weight ammonium polycarboxylates or low-viscosity comb-structured dispersants are recommended; foaming properties must be strictly controlled to avoid compromising jetting stability and printhead lifespan.
- Nano-TiO₂, iron oxide, and zirconium oxide (water-based coating systems): Preferred order of dispersants is phosphate-functionalized comb-structured dispersants > ammonium polycarboxylates.
