What causes sedimentation in water-based inks, and how should anti-settling dispersants be selected?
I. Formation mechanisms of sedimentation in water-based inks and actual operating conditions
Root causes of redispersible soft sediment versus irreversible hard sediment:
1. Mismatched dispersant selection or dosage:Insufficient adsorption strength of the dispersant's anchoring groups leads to desorption from the pigment surface after milling, triggering pigment particle flocculation and agglomeration; this results in a significant increase in particle size and rapid sedimentation. Poor compatibility between the dispersant and the ink resin, co-solvents, or system pH can directly disrupt the stable structure of the electrical double layer.
2. Inefficient sand milling:The system fails to meet final fineness standards, resulting in a high proportion of large residual particles and a broad particle size distribution; coarse particles settle preferentially under the influence of gravity.
3. Sedimentation driven by pigment density:Inorganic pigments such as titanium dioxide and iron oxide have densities significantly higher than the aqueous phase, creating a strong inherent tendency to settle under gravity. Organic pigments and carbon black also exhibit sedimentation behavior when dispersion stability fails and particle agglomeration increases particle size.
4. Instability in system rheology and pH:Failure to establish an effective thixotropic network under static conditions results in low viscosity during storage. pH drift (e.g., a drop in pH due to the volatilization of amine additives) can cause acrylic resin precipitation; pigment particles lose their protective coating and are highly prone to forming irreversible hard sediment.
5. Improper storage and application conditions:Long-term static storage, storage in high-temperature environments, repeated thermal cycling, or prolonged periods without agitation in the printing press ink trough can all significantly accelerate the sedimentation process. Two types of sedimentation can be clearly distinguished: soft settling is a physical process driven solely by gravity, where pigment particles do not undergo flocculation or aggregation; hard settling results from pigment flocculation combined with the failure of the dispersion system—a problem that cannot be fully resolved by thickeners alone, necessitating the optimization of the dispersant system as a priority.
II. Selection Guide for Anti-Settling Dispersants for Water-Based Flexographic and Gravure Inks
Core Selection Principle: Prioritize the use of polymeric dispersants featuring anchoring groups; these achieve long-term stability through strong anchoring adsorption and steric hindrance effects. Standard electrostatic small-molecule dispersants are not recommended (as they exhibit poor storage stability, tend to desorb from pigment surfaces, and can lead to flocculation and settling over time).
1. Systems based on organic pigments (phthalocyanines, Pigment Red), and high-pigment carbon black
Prioritize block copolymer-type polymeric hyperdispersants containing pigment-affinic groups (including anionic and zwitterionic types).
Key Characteristics: Aromatic anchoring groups adsorb tightly onto the surfaces of organic pigments and carbon black; they resist displacement by glycols and various co-solvents, preventing molecular desorption. They effectively inhibit flocculation and settling while simultaneously enhancing color development. Suitable for water-based flexographic and gravure ink systems, as well as resin-free color paste preparation. Representative products: RK-4017F0, RK-4039AB, RK-4058.
2. Inorganic systems based on titanium dioxide, iron oxide pigments, and functional fillers
Prioritize comb-structured ammonium polycarboxylate and polyacrylic acid-based anionic polymeric dispersants.
Performance Characteristics: Strong adsorption and anchoring capabilities form a thick, hydrated stabilization layer. Dual stabilization is achieved through electrostatic repulsion and steric hindrance, effectively inhibiting the agglomeration and settling of inorganic pigments. Representative products: RK-4008DH and RK-4010 (ammonium polycarboxylate polymeric dispersants).
Note: These dispersants are incompatible with cationic resins. 3. Composite pigment systems comprising blends of organic and inorganic pigments
Preferentially select amphoteric polymeric block polyurethane dispersants; these offer excellent versatility and compatibility, preventing flocculation caused by charge neutralization between different pigments while simultaneously reducing the risk of both pigment floating and settling. Representative products: RK-4044, RK-4039AC, and RK-4059.
❌ The use of ordinary small-molecule surfactants or low-molecular-weight polyacrylate salts is not recommended; these agents achieve only a temporary state of dispersion and tend to desorb from the pigment surface during long-term storage, leading to the formation of hard, compacted sediment.
III. Key Points for System Formulation: A Single Dispersant Cannot Fully Eliminate All Risks of Sedimentation
1.Dispersants primarily function to inhibit the flocculation and agglomeration of pigment particles. When combined with low dosages of thixotropic rheology modifiers (such as fumed silica, bentonite, or associative thickeners)—which provide static suspension capabilities—these two types of components work synergistically to prevent sedimentation.
2. It is recommended to add the dispersant during the grinding stage rather than during the subsequent paint-letdown or tinting phase. Dosage is calculated based on the mass of the pigment powder: 5–12% is recommended for inorganic pigment systems, while 15–30% is recommended for organic pigment and carbon black systems; gradient-based laboratory trials should be conducted to determine the optimal dosage.
3.The system's pH should be controlled to remain stable within the 8.0–9.5 range. High-temperature storage stability tests (storage at 50°C for 7 or 14 days) should be performed to determine whether any sediment formed is a soft, redispersible deposit or a hard, compacted cake.
