How should a dispersant be selected for mixing colors in printing inks?
Selection Guide for Dispersing Agents Used in Ink Color-Blending Pastes (Carbon Black + Organic Pigments + Inorganic Pigments)
Scenario Characteristics: In a single ink system, inorganic pigments, carbon black, and organic pigments are combined. These three types of components exhibit significantly different surface chemical properties: inorganic pigments possess high surface polarity and are hydrophilic; carbon black has a large specific surface area and a complex structure of surface functional groups; and organic pigments generally have low polarity, typically presenting hydrophobic surfaces. Since a single dispersant is rarely capable of effectively accommodating all three types of pigments, the core selection strategy involves prioritizing broad-spectrum polymeric dispersants or employing a blend of dispersants. This approach balances the system’s wetting, dispersing, anti-flocculation, and anti-floating/flooding performance, while also addressing the need to prevent pigment settling during storage.
I. Differences in Surface Characteristics Among Three Categories of Pigments (Foundational Basis for Selection)
1. Organic Pigments (Red, Yellow, Blue, Violet, etc.): Generally exhibit low polarity and hydrophobic crystal surfaces. During use, they are prone to issues such as color floating, flocculation, and hue drift, necessitating a high degree of steric stabilization within the dispersion system.
2. Inorganic Pigments (Titanium Dioxide, Iron Oxides, Extender Pigments, etc.): Characterized by high surface polarity and an abundance of active hydroxyl (-OH) sites. They tend to settle during storage. The primary requirement here is for the anchoring groups of the dispersant to adsorb specifically onto these hydroxyl-rich surfaces, providing electrostatic repulsion to prevent settling.
3. Carbon Black: Possesses an extremely large specific surface area, with active groups—such as carboxyl and phenolic hydroxyl groups—distributed across its surface. It is highly prone to agglomeration and difficult to wet. Residual micro-agglomerates can lead to insufficient blackness and an increase in system viscosity over time. The key requirements are strong anchoring and thorough wetting, which help to effectively reduce the viscosity of the system.
If a dispersant designed for only one type of pigment is used, it often leads to an imbalance: some pigments may be well-dispersed, while others undergo flocculation. This can result in various issues, including phase separation during storage, color floating or mottling, an increase in pigment paste viscosity over time, and poor consistency in printed hues.
II. Types of Dispersants and Their Compatibility (Water-based vs. Solvent-based Inks)
✅ Solvent-based Inks (Surface/Reverse-print Solvent-based Gravure Inks, Screen Printing Inks)
1. Polymeric Polyester-type Dispersants (Reference Grade: RK-4029)
Featuring broad-spectrum anchoring capabilities, these dispersants effectively adsorb onto organic pigments, carbon black, and inorganic fillers. In solvent-based systems, they form a stable steric hindrance layer that inhibits heteroflocculation between different pigments and reduces the risk of color floating. They are compatible with ink systems based on esters, ketones, and aromatic solvents.
2. Modified Polyurethane Polymeric Dispersants (Reference Grades: RK-4031, RK-4036B)
Suitable for low-polarity solvent systems (such as those containing alcohols), these dispersants offer excellent dispersion for high-pigment-content organic pigments and high-surface-area carbon black, while imparting superior flow properties to pigment pastes.
❌ Small-molecule Fatty Acid-type Dispersants: These are only compatible with inorganic pigments. When used in combination with organic pigments or carbon black, they result in poor storage stability, often leading to pigment re-coarsening (flocculation/agglomeration).
✅ Water-based Ink Systems (Most common: Water-based Flexographic and Gravure Inks)
1. Polyurethane Polymeric Dispersants (PU-based, high-end solution) (Reference Grade: RK-4061EC)
Characterized by multiple anchoring groups and long hydrophilic side chains, these dispersants utilize steric hindrance as their primary stabilization mechanism. They provide exceptional dispersion stability for organic pigments and carbon black, while also effectively anchoring inorganic pigments.
- Advantages: Enables low-viscosity pigment dispersions with excellent storage stability; inhibits heteroflocculation between mixed pigments, ensuring consistent hue; provides outstanding resistance to pigment floating and flooding; suitable for high-concentration pigment dispersions, digital inkjet inks, and high-end water-based inks.
- Limitations: Relatively higher procurement cost; some grades are sensitive to the system's pH, requiring it to be maintained within the 7.5–9.0 range.
2. Polymeric polyacrylate-based dispersants (a preferred broad-spectrum choice; reference grade: RK-4059)
The molecular chains contain various anchoring groups, such as carboxylates, which can simultaneously adsorb onto hydroxyl groups on the surface of inorganic pigments, oxygen-containing active groups on carbon black, and polar sites on organic pigments. The system is stabilized through a combination of electrostatic repulsion and steric hindrance.
- Advantages: A single dispersant can accommodate ternary mixed-pigment systems and reduce the likelihood of floating and flooding; offers excellent cost-performance; suitable for water-based flexographic and rotogravure inks.
- Limitations: When used alone, it may struggle to achieve peak blackness (jetness) in systems containing carbon black with high pigment content or high specific surface area.
3. Combined Dispersant System (recommended for hard-to-disperse systems); suggested grades: RK-4017EA, RK-4054EF.
This approach utilizes a polyurethane (PU) polymeric dispersant as the primary agent to ensure the dispersion and stabilization of carbon black and organic pigments, supplemented by a small amount of a polyacrylic dispersant to effectively anchor inorganic pigments.
❌ It is not recommended to use low-molecular-weight wetting and dispersing agents as the sole primary dispersant. Such small-molecule components can only reduce the surface tension of the system and lack sufficient steric hindrance; consequently, in mixed-pigment systems, this often leads to issues such as flocculation, sedimentation, and color floating (or flooding) during long-term storage.
III. Key Selection Criteria (Focus: Mixed-Pigment Inks)
1. pH Control in Aqueous Systems: The efficacy of carboxylic acid-based dispersants is highly dependent on the system's pH. If the pH is too low, the anchoring groups fail to dissociate sufficiently, leading to a complete loss of dispersing performance.
2. Viscosity Reduction in Pigment Concentrates: Mixed-pigment systems are prone to high viscosity; therefore, dispersants must provide excellent viscosity-reducing capabilities to allow for a higher maximum pigment loading.
3. Prevention of Floating and Flooding: The system must remain stable throughout the entire process—from post-milling and long-term storage to actual printing—without phase separation or the preferential migration (floating/enrichment) of individual pigments. This is critical for avoiding common issues such as the floating of carbon black or the settling of inorganic pigments.
4. Inhibition of Hetero-flocculation (Top Priority): It is essential to prevent the mutual adsorption and agglomeration of different pigment particles. Key evaluation metrics include the degree of particle coarsening, hue drift, and any increase in particle size (loss of fineness) when the pigment concentrate is left to stand.
5. Storage Stability: Systems should be evaluated under accelerated aging conditions (e.g., 50°C) to monitor changes in fineness and viscosity, as well as any phase separation or settling. For inorganic pigments—which are inherently prone to settling—the dispersion system must also provide effective anti-settling properties.
6. Compatibility with Finished Inks: The dispersant must not negatively impact the ink’s adhesion, gloss, water resistance, rub resistance, or transfer performance.
IV. Addition Guidelines and Processing Points
1. Dosage: Calculated based on the total mass of the pigments, not the total mass of the ink.
- For broad-spectrum polymeric dispersants in water-based systems: Use 3%–10% of the total pigment mass. The dosage should lean toward the upper limit when the proportion of carbon black is high, while the lower limit may be used when the proportion of inorganic pigments is high.
2. Order of Addition: Pre-mix the dispersant with the carrier resin and water/solvent before adding the pigments. This ensures that the dispersant wets the pigment surfaces first, preventing the dry pigment powder from clumping together.
3. Milling Process: Ensure thorough bead milling. When dealing with mixed pigment systems, it is often difficult to achieve the target fineness in a single pass. Once the required fineness is reached, re-measure it after 24 hours to check for any latent flocculation.
V. Anticipation of Typical Faults
1. Fineness meets specifications after pigment paste grinding, but deteriorates (increases) after standing overnight: This is caused by heteroflocculation between different pigments. The recommended solution is to switch to a polymeric polyurethane (PU) dispersant or employ a combination of dispersants.
2. Surface floating or streaking (mottling) in the system: This results from poor compatibility between the surface potentials of different pigment particles. It can be resolved by switching to a polymeric dispersant that provides steric stabilization.
3. Formation of hard, compacted sediment at the bottom of the pigment paste: This indicates insufficient dispersion stability of inorganic pigments. It can be addressed by incorporating a small amount of an anti-settling agent, or by adjusting and optimizing the anchoring groups of the dispersant.
VI. Screening Procedure (Small-Scale Trials)
1. Maintain a fixed resin-to-pigment ratio in the system, and conduct parallel trials using varying dosage levels of the candidate dispersants.
2. Perform bead milling (sand milling) until the system reaches the target fineness.
3. Once milling is complete, measure the initial viscosity, fineness, and hue of the samples.
4. Store the samples at 50°C for 7 days; subsequently, re-evaluate the fineness and viscosity, while simultaneously checking for phase separation (layering) and pigment floating.
5. Prepare ink films and conduct printing tests to evaluate the hue, gloss, and any signs of pigment floating.
6. Simulate actual printing shear conditions to verify the stability of the hue during dynamic operation.
