Selection of Dispersants for Water-Based Digital Inkjet Inks
The core technical pain point of the water-based digital inkjet system is: the system incorporates a high proportion of polyol co-solvents such as ethylene glycol, diethylene glycol, and propylene glycol. The micro-nozzle aperture of the inkjet printhead, operating in the micrometer range, is highly susceptible to physical clogging due to interfacial detachment of dispersants and Ostwald ripening of pigment particles under dynamic working conditions. The system's core technical requirements for additives include low dynamic foam characteristics, narrow particle size distribution, high shear resistance, and excellent thermal storage stability. Additionally, it must achieve complete compatibility with the ink matrix resin and other functional additives. Some compliant application scenarios further require meeting environmental standards for zero APEO residue and low volatile organic compound (VOC) emissions.
I. Core Hard Mandatory Screening Requirements to be Confirmed Prior to Inkjet Ink Selection
1. After grinding, the system must meet the particle size requirements of D50 < 150 nm, D99 < 300 nm, and PDI < 0.2 to fundamentally prevent nozzle clogging caused by large particles.
2. At 25°C, the system viscosity is maintained between 5-20 mPa·s, exhibiting near-Newtonian fluid rheological behavior, fully meeting the operational requirements of piezoelectric inkjet printheads.
3. The system viscosity must be controlled within the range of 5-20 mPa·s at 25°C, exhibiting near-Newtonian fluid characteristics to meet the operational requirements of piezoelectric inkjet printheads.
4. The zeta potential must be controlled within the range of -30 to -50 mV to ensure the pigment particles achieve dual stabilization effects of electrostatic and steric hindrance.
5. From an environmental compliance perspective, most inkjet ink products are required to meet standards for zero APEO residue, low free amine content, and low volatile organic compound (VOC) emissions.
6. The accelerated storage test adopts standard conditions of 50°C thermal storage for 7 days, requiring a particle size increase of less than 10%, viscosity fluctuations of less than 25%, and no abnormal phenomena such as phase separation or sedimentation in the system.
II、 Overview of Core Types and Applicable Scenarios of Dispersants
1. Product category of phosphate ester small molecule wetting and dispersing agents
-It can only be used as a compound auxiliary component and cannot be added as a main dispersant. Excessive use can cause problems such as system foaming, nozzle corrosion, and deterioration of storage stability.
2. Comb shaped ammonium polycarboxylate dispersant (anionic polymer)
-The core advantages are excellent viscosity reduction effect, moderate cost, and strong anchoring and adsorption ability for inorganic oxide powders;
-The core weakness is that desorption and flocculation are prone to occur in the high alcohol system, which is not suitable for low polarity organic pigments and carbon black. It is pH sensitive and has the best performance in the 7-9 range. It cannot be used in combination with cationic additives.
- ✅ Suitable for single nano titanium dioxide white ink and silica matte ink, ink system containing only a single component inorganic pigment.
- ❌ Not suitable for low polarity organic pigments such as carbon black and quinacridone, as well as ink systems with multiple pigment complexes.
-The recommended addition amount is within the range of 3% -5% of the total pigment mass.
-Reference: RK-4017C, RK-4047.
3. Polymer copolymer SMA product prepared by copolymerization reaction of styrene and maleic anhydride
-The core advantage is strong adsorption force for carbon black and phthalocyanine blue pigments, and significant system viscosity reduction effect;
-The core weakness is poor alcohol resistance and moisture and heat storage performance, and hydrolysis reactions are prone to occur in high-temperature storage environments. Therefore, it is not recommended to directly use it for finished inkjet ink, but more commonly used in the front-end grinding and color paste preparation process.
4. Amphoteric block polyurethane dispersant (mainstream preferred in water-based inkjet field)
-The core advantage is the dual stabilization effect of static electricity and steric hindrance, tolerance to high concentration diol environment, resistance to pigment surface detachment, compatibility with most acrylic and PU resins, adaptability to some cationic systems, excellent shear resistance, multiple grinding cycles, and better low foaming performance.
-The core weakness is that the market price is higher than that of ordinary polycarboxylate dispersant products.
- ✅ Suitable for carbon black, transparent organic pigments (quinacridone, transparent purple, permanent series), multi-color compound inks, textile inkjet, ceramic inkjet, as well as high-end ink systems that require long-term machine circulation without clogging.
-The recommended addition amount is 12% -30% of the organic pigment mass and 25% -60% of the high pigment carbon black mass, both based on the corresponding total pigment mass as the calculation benchmark..
-Reference: RK-4039AB, RK-4044.
III、 Practical reference guide for quick selection of different pigment categories
1. Nano titanium dioxide/silica white ink (single inorganic pigment system) is compatible with comb type polycarboxylate ammonium dispersant, and can be replaced with amphoteric polyurethane dispersant in scenarios with sufficient budget and strict storage requirements.
2. Low polarity pigment systems such as carbon black, quinacridone, and transparent yellow violet must use amphoteric polyurethane dispersants. The use of polycarboxylate dispersants can easily lead to problems such as storage return and clogging.
3. For highly polar organic pigment systems such as phthalocyanine blue and phthalocyanine green, ammonium polycarboxylate dispersants are preferred when cost is a priority. For scenarios that require long-term inkjet printing, it is recommended to use amphoteric polyurethane dispersants instead.
4. For ink-jet system containing cationic lotion components, only cationic compatible amphoteric polyurethane dispersant can be selected, and ordinary anionic polycarboxylic acid will directly generate charge neutralization and trigger flocculation and stratification.
5. For ink systems mixed with organic and inorganic pigments, amphoteric polyurethane dispersants can be directly selected as the main dispersing component.
IV、 Summary of common pitfalls
1. Insufficient or excessive addition of dispersant will lead to abnormal performance: the system with insufficient addition is prone to flocculation and agglomeration, and the system with excessive addition will lead to viscosity rise, foam generation, and water resistance deterioration. Gradient addition test must be carried out to verify the optimal amount.
2. Directly using polycarboxylate dispersants from ordinary coatings to prepare inkjet ink. The alcohol content in ordinary coating systems is low, and a large amount of diols in the inkjet system will strip the dispersant from the surface of the pigment, causing a significant increase in particle size and nozzle blockage after placement.
3. Only verifying the initial fineness index and not conducting thermal storage acceleration testing. The particle size test is qualified when the grinding is just completed, but after one week of storage, the system shows a phenomenon of coarsening, which is the most common cause of failure in the inkjet project promotion process.
V、 Four items are required for small-scale verification
1. Control index system for the median diameter, proportion of large particles, and polydispersity index of pigment particles after grinding;
After 7-14 days of thermal storage at 50 ℃, retest the two core performance indicators of particle size and viscosity of the system;
3. Centrifuge at a speed of 3000r/min for 10 minutes and observe the changes in the layering and precipitation state of the system;
4. Conduct on-board printing tests, continuously observe the spraying status, ink interruption phenomenon, satellite droplet generation, and nozzle blockage fault performance.
