Selection principles and methods for organic pigment dispersants in water-based and oil-based digital inkjet systems
Characteristics of organic pigments: For pigments such as phthalocyanine, azo, quinacridone, permanent pigments and carbon black, which have high specific surface area, are easy to crystallize and flocculate, and have significant differences in surface polarity (usually lacking hydroxyl groups), traditional electrostatic small molecule dispersants cannot meet the stability requirements because they mainly rely on aromatic ring adsorption, hydrogen bonding and polar blocks for anchoring. Therefore, high molecular weight superdispersants must be used to achieve effective dispersion.
I. Water-based Organic Pigment Dispersants (Paints/Water-based Pigments/Water-based Inkjet Prints)
1. Comb-like Anionic Polycarboxylate Dispersants (General-purpose, Economical)
Representatives: RK-4047, RK-4008DH, RK-4017C
- Anchoring Principle: Utilizing a structure with polar carboxyl groups and aromatic side chains, it achieves efficient adsorption of polar organic pigments such as phthalocyanine blue/green, scarlet, and medium yellow through electrostatic interactions and π-π stacking effects.
- Applications: Covers color pastes for azo and phthalocyanine organic pigments, water-based architectural coatings, and industrial paint systems.
- Features: Economical, excellent defoaming, strong weather resistance, and maintains the water resistance of the paint film.
- Disadvantages: Due to the lack of active functional groups on the surface of low-polarity pigments such as benzidine yellow, transparent violet, and quinacridone, their physical adsorption with the medium is weak and prone to reversible desorption, leading to floating color. In inkjet systems with high alcohol content, the dynamically changing hydrogen bond network of alcohol molecules further weakens the stability of the pigment interface, exacerbating the risk of desorption.
- Addition Amount: Pigment 10%–20%, Carbon Black 15%–30%
2. Amphoteric Polyurethane Dispersants (High-end general-purpose, preferred for inkjet printing)
Representatives: RK-4039AB, RK-4044
- Anchoring: Utilizing a block structure rich in aromatic amines and amides, it achieves broad compatibility and efficient dispersion of various polar organic pigments.
- Advantages:
1. All-purpose dispersant, suitable for all color pigments, effectively preventing floating and layering, ensuring system stability and uniformity;
2. High-boiling-point alcohol co-solvents such as ethylene glycol and propylene glycol, suitable for water-based digital inkjet systems, with good compatibility and stability;
3. Excellent broad-spectrum compatibility, capable of stable blending with cationic emulsions, water-based polyurethanes, and various water-based resin systems such as acrylics;
4. Ultra-fine transparent organic pigments exhibit excellent anti-crystallization and anti-flocculation effects.
- Applications: Water-based inkjet inks, high-concentration general-purpose color pastes, plastic paints, leather coatings, multi-pigment compounding systems
- Addition amount: 10%–25%, high-pigment carbon black 25%–70%
3. Styrene-maleic anhydride copolymer (for high-concentration pigment pastes and carbon black)
Features: Its benzene-rich molecular structure allows for strong adsorption with carbon black and phthalocyanine blue, significantly reducing system viscosity.
Disadvantages: Limited hydrolysis resistance, making it difficult to meet the stability requirements of long-term high-temperature storage in inkjet systems. Primarily used in industrial water-based pigment pastes.
4. Small molecule wetting agents (auxiliary only, not as primary dispersant)
Fat alcohol ethers or phosphate ester wetting agents are compounded with polymeric dispersants. Adding 0.2%–1% effectively wets pigment aggregates and inhibits bubble formation.
Water-based dispersant selection breakdown:
1. For high-polarity pigments such as phthalocyanine blue, phthalocyanine green, and permanent red, comb-type polycarboxylate dispersants are selected to achieve cost-effectiveness.
2. For low-polarity transparent pigments (quinacridone, transparent yellow, violet), carbon black, and multi-color blends: amphoteric polyurethane dispersants are selected.
3. For water-based digital inkjet organic inks: only polyurethane dispersants are suitable, as polycarboxylate dispersants easily agglomerate and clog printheads when exposed to high-alcohol solvents.
4. For cationic water-based resin systems: only cationic compatible polyurethanes are suitable, as anionic polycarboxylate dispersants directly flocculate due to charge neutralization.
II. Oil-based/Solvent-based Organic Pigment Dispersants (Baking Paint, Plastic Oil, UV Paint, Ink)
1. Acrylic Block Superdispersant (Mainstream on the Market)
Models: RK-4016A, RK-4016C
Anchoring: Utilizes the synergistic effect of aromatic blocks and carboxyl groups to wet and stably disperse various medium- to high-polarity organic pigments and carbon black.
Applications: Solvent-based acrylic, alkyd, epoxy, PU, oil-based UV paints
Dosage: Pigment 8%–25%
2. Polyamide Superdispersant (For Low-Polarity Pigments)
Models: RK-4028, RK-4029, RK-4042, RK-4064
Utilizes strong aromatic amide groups as anchoring ends to achieve efficient adsorption and stable dispersion of weakly polar azo pigments, transparent organic pigments, and halogen-free color powders.
Advantages: Achieves uniform color without floating color or blooming, ensures constant grinding fineness and prevents coarsening during long-term storage.
Disadvantages: Light colors are prone to slight yellowing; use with caution in pure white systems.
3. Phosphate Ester Dispersant (Low viscosity, light colors, high gloss systems)
RK-4013 and RK-4011 are used as auxiliary components in combination to enhance wetting energy; their effect on organic pigment dispersion is generally average when used alone.
4. Water- and Oil-Based Nonionic Polymer (UV Transparent Varnish, No Yellowing Required)
Utilizing a nonionic molecular structure, it avoids the risk of charge-induced flocculation at the source, perfectly meeting the stringent requirements for extreme stability in high-end transparent organic pigments.
Oil-Based Detail Matching
1. Phthalocyanine, High-Saturation Red/Yellow: Acrylic Block Dispersant
2. Low-Polarity Transparent Pigments, Multi-Color Mixing: Polyamide Dispersant Blended with Acrylic Acid
3. Oil-Based UV Inks, Transparent Coatings: RK-4039AC, RK-4061CF, RK-4061ED Water-Oil Universal Dispersant
III. Mild Solvent Digital Inkjet Organic Pigments (Outdoor Advertising, Leather Mild Solvent Inks)
Special Structured Acrylic Dispersant: RK-4039ACB
1. Due to the encapsulation effect of its long hydrophobic chains on organic pigment crystals, it can be completely dissolved in mild solvent systems such as esters, ketones, and alcohol ethers;
2. Optimizes ink stability to prevent pigment sedimentation and ensures unobstructed printhead operation during continuous printing;
3. Addition amount is much higher than ordinary oil-based paints: 12%–25% for pigments;
4. Incompatible with: water-based polycarboxylic acid and polyurethane dispersants, which are insoluble in mild solvents and will directly separate and clump.
IV. Precise Selection Based on Organic Pigment Type
1. Carbon Black (Most Difficult to Disperse)
Water-based: RK-4057, Styrene Maleic Anhydride / 1. Amphoteric Polyurethane; Oil-based: RK-4039AC; Inkjet: Special Polyurethane Polymer
Features: Extremely high specific surface area, high dispersant addition rate across all product categories
2. Phthalocyanine Blue, Phthalocyanine Green (High Polarity) Cost-effective, suitable for both water-based polycarboxylic acid and oil-based acrylic dispersants.
3. Azo Pigments (Medium Yellow, Permanent Red) General-purpose dispersants applicable to all types of products; for applications with lower transparency requirements, polyurethane or polyamide dispersants are recommended.
4. Quinacridone, Perylene, Transparent Violet/Pink (Low Polarity Ultrafine Pigments)
When using polyurethane (water-based) or polyamide (oil-based) systems, combining them with short-chain dispersants can lead to insufficient steric hindrance, causing pigment particles to re-aggregate (flocculate), resulting in a significant decrease in surface roughness and gloss of the paint film.
5.Heterocyclic High Weather-Resistant Pigments (DPP Red, Pyrrolopyrrole Dione) Use good polyurethane or polyamide-based dispersants to ensure good weather resistance and no risk of precipitation.
V. Key General Usage Points
1. Higher Dosage than Inorganic Powders
Organic pigments have high porosity and high oil/water absorption values, requiring significantly more dispersant than titanium dioxide and calcium carbonate (approximately 2 to 4 times the latter). It is recommended to test the dosage in gradients of 9%, 14%, 20%, and 28%. Insufficient dispersant will lead to incomplete pigment dispersion, resulting in coarse particles, reduced gloss, and floating color defects.
Excessive additives reduce the water resistance and hardness of the paint film, and cause inkjet scattering and pinhole defects.
2. Compounding Scheme
By compounding a polymeric dispersant with a small amount of low-foaming wetting agent, pigment agglomeration and bubbles can be effectively broken, significantly improving grinding efficiency and shortening grinding time.
3.Storage Stability Test Standards
It exhibits excellent thermal stability and printing reliability, remaining uniform and stable even after 15 days of accelerated aging at 50℃, with no sedimentation or stratification. Furthermore, it performs smoothly in a 48-hour continuous inkjet test, effectively eliminating the risk of printhead clogging due to crystallization.
