How should a dispersant be selected for an ink system?
Selection Guide for Dispersants in Coating Systems
Core Selection Logic: The fundamental function of a dispersant is to anchor and adsorb onto the surface of pigment particles, stabilizing the system through electrostatic repulsion or steric hindrance. Selection should primarily be based on two key dimensions: the type of pigment (inorganic vs. organic, including carbon black) and the nature of the system (water-borne vs. solvent-borne). Furthermore, the choice must align with the specific resin system and the required performance criteria for the final paint film—such as water resistance, gloss, prevention of pigment floating or flooding, and resistance to settling during storage.
I. Quick Selection Guide Based on Pigment Type (Key Points)
1. Inorganic Pigments (Titanium dioxide, calcium carbonate, talc, iron oxides, etc.): For water-based systems, use polyacrylate or low-molecular-weight polymeric dispersants (Recommended: Dongguan Hongrui RK-4008DH, RK-4010). For solvent-based systems, use phosphate ester or polyester-type dispersants (Recommended: Dongguan Hongrui RK-4012, RK-4013A, RK-4013C).
2. Organic Pigments (Organic colorants such as red, yellow, blue, violet, etc.): For both water-based and solvent-based systems, polymeric block-type hyperdispersants are the preferred choice (Recommended: Dongguan Hongrui RK-4035AC, RK-4029, RK-4054EF).
3. Aluminum Pigments and Pearlescent Pigments: Use phosphate ester-type dispersants (Recommended: Dongguan Hongrui RK-4012, RK-4013A, RK-4013C) to avoid damaging the structural integrity of the metallic flakes, thereby ensuring the desired metallic effect upon proper orientation.
4. Carbon Black (High-pigment carbon black): Polymeric hyperdispersants must be used (Recommended: Dongguan Hongrui RK-4035AC, RK-4029, RK-4059). Conventional polycarboxylate dispersants are generally insufficient for achieving full dispersion, which can lead to pigment re-agglomeration (coarsening) and failure to meet blackness/jetness specifications.
II. Mainstream Dispersant Categories and Application Scenarios
Water-based Systems
1. Non-ionic Dispersants
✅ Suitable for: pH-sensitive pigments and certain transparent iron oxides; often used in combination with anionic dispersants.
⚠ Limitations: Limited dispersion efficiency when used alone; may negatively affect the water resistance of the paint film.
2. Polyacrylates (Anionic Dispersants) (Recommended grades: Dongguan Hongrui RK-4008Y)
✅ Suitable for: Inorganic pigments such as titanium dioxide, calcium carbonate, talc, and iron oxide; also used in water-based interior primers and latex paints.
✅ Advantages: Excellent cost-performance ratio, superior viscosity-reduction efficiency, and effective at preventing the settling of inorganic powders during storage.
⚠ Limitations: Inadequate dispersion capability for organic pigments and high-pigment-content carbon black; moderate water resistance—use with caution in exterior weather-resistant topcoat systems.
Dosage: 0.2%–1.0% based on the weight of the pigment powder.
3. Polymeric Block Copolymers (Water-based Hyperdispersants; recommended grades: Dongguan Hongrui RK-4039AC, RK-4057, RK-4059)
✅ Suitable for: Organic pigments, carbon black, and nano-scale inorganic pigments; used in water-based polyurethane/acrylic topcoats and water-based industrial coatings.
✅ Advantages: Provides strong anchoring to pigment surfaces and ensures system stability through steric hindrance. Effectively prevents pigment flooding and floating, flocculation, and coarsening during storage; significantly enhances the gloss of the paint film.
⚠ Limitations: Higher cost compared to polyacrylates.
Dosage: 2%–8% based on the weight of the pigment (higher dosages required for carbon black and nano-powders).
Oil-based/Solvent-borne Systems
1. Fatty Acid-based Dispersants
✅ Suitable for: Conventional inorganic fillers and low-end alkyd primer systems.
⚠ Limitations: Poor weather resistance; prone to causing yellowing in the paint film. Not recommended for use in topcoats or exterior-grade coatings.
2. Polyester-based Hyperdispersants (General-purpose)
✅ Suitable for: Organic pigments, carbon black, and titanium dioxide; compatible with a wide range of solvent-borne coating systems, including acrylic, alkyd, polyurethane, and epoxy.
✅ Advantages: Features a diverse range of anchoring groups, offers excellent colorant efficiency, effectively inhibits pigment flocculation and color flooding/floating, and provides broad system compatibility.
Recommended dosage: 3%–10% based on the total weight of the pigment; for high-pigment carbon black systems, the dosage can be up to 12%.
3. Phosphate Ester Salt-based Dispersants (Recommended grades: Dongguan Hongrui RK-4012, RK-4013A, RK-4017F0)
✅ Suitable for: Inorganic pigments such as titanium dioxide and aluminum flakes; used in solvent-borne metallic paints and aluminum-pigmented coatings.
✅ Advantages: Provides strong anchoring for inorganic oxide pigments, promotes the proper orientation of aluminum flakes, and effectively prevents the darkening of aluminum flakes.
⚠ Limitations: Shows only moderate performance/compatibility when dispersing organic pigments.
III. Key Evaluation Criteria for Selection
1. Viscosity Reduction: The viscosity of the pigment dispersion (mill base) is the primary metric. At a constant pigment solids content, a lower system viscosity indicates superior grinding and dispersing efficiency for the dispersant.
2. System Compatibility: The dispersant must be fully compatible with the primary resin and various additives—such as wetting agents, defoamers, and leveling agents—to prevent surface defects in the paint film, including haze and pinholes.
3. Storage Stability: Following accelerated heat-aging tests, the system should be monitored for issues such as an increase in viscosity, phase separation/settling, or pigment flocculation and particle coarsening.
4. Prevention of Floating and Flooding: For multi-pigment paint systems—particularly gray or light-colored shades—polymeric, sterically stabilizing dispersants are preferred.
5. Tinting Strength, Color Development, and Gloss: These are critical performance indicators for topcoat systems; inadequate dispersion can lead to a loss of gloss and insufficient color intensity in the paint film.
6. Overall Paint Film Performance: Key parameters include water resistance, salt-spray resistance, weather resistance, and adhesion to the substrate. Note that an excessive dosage of polycarboxylate-based dispersants can compromise the water resistance of the system.
IV. Addition Guidelines and Common Misconceptions
1. Basis for Dosage: The dosage of a dispersant should be calculated based on the total mass of the pigments or powders, rather than the total mass of the entire formulation.
2. Timing of Addition: It is recommended to add the dispersant during the initial charging stage of the sand-milling process, alongside the pigments and fillers. Adding the dispersant after the milling process is complete is not advised.
3. Common Misconceptions
(1) Using the same dispersant for both inorganic and organic pigments: Polycarboxylate-based dispersants, when used in systems containing organic pigments, can easily trigger pigment flocculation and instability.
(2) Assuming that a higher dosage of dispersant always yields better results: Excess dispersant will remain in a free state within the paint film, directly compromising the system's water and chemical resistance.
(3) Relying solely on viscosity reduction as the evaluation criterion, while overlooking the risk of pigment flooding and floating: Many low-molecular-weight dispersants are highly effective at reducing viscosity, but they frequently cause flooding and floating issues in multi-pigment (mixed-color) paint systems.
