Selection of Anti-Sedimentation Dispersants for Water-Based Digital Inkjet Inks
The core issue is that inkjet ink viscosity remains only in the 5-20 mPa·s range, with no significant thixotropic thickening effect. The system relies entirely on dispersants to create strong steric hindrance and high Zeta potential to achieve anti-settling effects for pigment particles. It is unacceptable to use thickening components such as bentonite or fumed silica for anti-settling, as the tiny solid particles of these materials can easily cause printhead clogging. The anti-settling logic of this system is completely different from the conventional dispersion approach of coatings. The core priority is ensuring the long-term stability of the pigment particles themselves, and the introduction of large-particle anti-settling fillers into the system is strictly prohibited.
I. Core and stringent technical screening requirements that inkjet ink anti-settling dispersants must meet
1. After grinding, the system must meet the requirement of D99 < 300nm with no large particle residue. Introducing any foreign solid particles into the system is strictly prohibited.
2. Diol resistance requirements: When the concentration of ethylene glycol/diethylene glycol/propylene glycol in the ink system is between 20% and 40%, the dispersant must not detach from the pigment surface. Desorption will cause pigment agglomeration and sedimentation.
3. The system's Zeta potential must be controlled within the range of -35 to -50mV. The higher the absolute value of the potential, the better the system's anti-settling stability.
4. The system has good low-foaming properties, with no satellite droplet generation during the printing process, perfectly adapting to the jetting requirements of piezoelectric inkjet printheads.
5. Thermal stability requirements: After being placed at 50℃ for 7-14 days, the system particle size increase should be less than 10%, with no obvious sedimentation or stratification abnormalities.
6. The system must not damage the ink's original near-Newtonian fluid rheological properties, nor cause a significant increase in ink viscosity.
⚠️Important Reminder: Inkjet inks must not use bentonite, hydrogenated castor oil, or polyamide wax-based anti-settling agents commonly used in coatings. These materials will directly cause printhead clogging. The anti-settling ability of inkjet systems comes almost entirely from the stabilizing effect of dispersants on pigment particles.
II. Performance Comparison and Key Selection Considerations of Various Anti-Settling Dispersants Applicable to Inkjet Systems
1. Polyurethane dispersants are the preferred type of dispersant for anti-settling applications in inkjet inks.
The mechanism of action of this dispersant is that the polymer anchoring chains are firmly adsorbed onto the surface of pigment particles, and the solvated side chains form a thick steric hindrance layer in the water-polyol mixed medium, preventing particle collision, agglomeration, and sedimentation. It has strong universal compatibility with both organic and inorganic pigments.
- ✅ Core advantages: Resistant to high-content diol media, firmly adsorbed on the pigment surface and not easily desorbed, high system zeta potential, excellent anti-settling performance during thermal storage, and able to withstand high-shear grinding conditions. Its dispersion effect is particularly outstanding for carbon black and low-polarity organic pigments.
- ❌ Core disadvantages: Higher overall cost, some grades may slightly increase system viscosity, and precise control of the addition range is required in actual applications.
- Recommended addition amount: 25%-60% of carbon black mass, 12%-30% of organic pigment mass, and 4%-8% of inorganic pigment mass, all calculated based on the total mass of the corresponding pigments.
- Reference models: RK-4039AC, RK-4057, RK-4059, RK-4061EF
- Compatible with carbon black inks, colored organic pigment inks, ceramic inkjet inks, textile digital inkjet inks, and color paste and ink systems with high long-term storage requirements and prone to sedimentation.
2. SMA-type polymeric dispersants obtained from styrene-maleic anhydride copolymerization
- Suitable only for early-stage pigment preparation. Poor alcohol resistance and heat storage performance. Insufficient anti-settling properties in finished inks; not recommended for use as an anti-settling dispersant in finished inks.
3. Phosphate ester-type small-molecule wetting and dispersing agents
- Can only be used as auxiliary compounding components, not as the primary anti-settling dispersant. The steric hindrance provided by small molecules is insufficient; the system is prone to sedimentation under static conditions, and may introduce application risks such as foaming and printhead corrosion.
4. Polymer comb-type anionic polycarboxylate ammonium dispersant
The mechanism of action is primarily based on electrostatic stabilization, forming a relatively thin steric hindrance layer.
- ✅ Core advantages include excellent viscosity reduction effect, low procurement cost, and acceptable anti-settling performance for inorganic pigments such as zirconium oxide and titanium dioxide.
- ❌The core drawback is that the steric hindrance layer is easily compressed by the solvent in a high diol environment, leading to pigment desorption and significant subsequent sedimentation. It has poor anti-settling effect on carbon black and low-polarity organic pigments, and its performance depends on a pH range of 7-9.
- Recommended dosage: 3%-5% of the total pigment mass.
- Reference models: RK-4017C, RK-4047
- ✔Compatible with inorganic white ink and matte ink systems. Not suitable for finished inkjet inks containing carbon black or organic pigments; only applicable to the preparation of low-alcohol pigment pastes.
III. Quick Selection of Anti-Sedimentation Solutions for Ink Based on Different Pigments
1. Phthalocyanine Blue and Phthalocyanine Green: These are typical pigments with moderate dispersion difficulty in inkjet systems.
- In cost-priority scenarios, use comb-type polycarboxylate ammonium dispersants, but a thermal storage sedimentation stability verification test at 50℃ is mandatory.
- For scenarios where long-term storage and anti-sedimentation performance are the core priorities, polyurethane dispersants can be used directly.
2. Carbon Black and Quinacridone: These are low-polarity organic pigments most prone to sedimentation in inkjet systems.
These pigments must use polyurethane dispersants. While polycarboxylate systems may appear to perform well in the short term, hard precipitates will form at the bottom after thermal storage or long-term static storage.
3. Inkjet ink systems prepared by co-componenting organic and inorganic pigments
Use polyurethane dispersants throughout the system to avoid selective sedimentation problems caused by poor dispersion compatibility between different pigments.
4. Titanium dioxide and nano-zirconia inorganic white inks belong to inkjet ink systems with high pigment density and a high risk of sedimentation.
- When the budget is limited, comb-type polycarboxylate ammonium dispersant is selected. For those with high requirements for long-term anti-settling performance, polyurethane dispersant can be directly upgraded. Inorganic powders have high density, and electrostatic stabilization alone is insufficient to resist gravitational sedimentation. A thick steric hindrance layer is necessary to achieve stability.
IV. Key Control Points for Improving the Anti-Settling Effect of Inkjet Ink: Choosing the Right Dispersant Alone is Far From Enough to Achieve Long-Term Stability
1. Grind pigment particles to the finest possible size. The lower the D50 value, the slower the gravitational settling velocity of the pigment particles. For the same type of pigment, the settling velocity increases exponentially with increasing particle size.
2. The amount of dispersant added needs to be verified through gradient experiments: Insufficient dispersant dosage leads to insufficient pigment adsorption and easily causes sedimentation; excessive dosage will produce a large amount of free dispersant, which will damage the system stability, generate precipitation, and cause foaming problems.
3. It is strictly forbidden to introduce any solid particulate anti-settling additive components into the inkjet ink system.
4. Reasonably control the addition ratio of cosolvent. Excessive diol content will compress the solvation layer structure of the dispersant, further exacerbating the sedimentation risk of the system.
5. Core Verification Tests Required During the Development of Inkjet Ink Anti-settling Performance:
① Conduct accelerated sedimentation performance testing using centrifugation at 3000 r/min for 10 minutes;
② Place the sample in a 50℃ oven for 7-14 days and observe whether hard sediment forms at the bottom of the bottle. Soft sediment can be easily redispersed by gentle shaking, indicating acceptable performance; the presence of hard sediment indicates complete failure of the dispersion system;
③ Conduct continuous printing tests, observing the system throughout the process for any abnormal conditions such as printhead clogging or ink supply interruption.
V. Typical Pitfalls and Mistakes Frequently Encountered During the Development of Anti-Settling Agents for Inkjet Ink
1. Only observing the initial state of the pigment immediately after grinding, without conducting accelerated sedimentation and thermal storage stability verification tests.
2. Directly applying anti-settling dispersants from water-based coatings. Coating systems have high viscosity and contain thixotropic agents to aid anti-settling, while inkjet inks have extremely low viscosity. Coating dispersants lack sufficient alcohol resistance; short-term tests may appear satisfactory, but long-term storage will lead to the formation of hard deposits that clog the printhead.
