How to choose a digital inkjet metal oxide dispersant?
Metal oxides (TiO ₂, Fe ₂ O3, Fe ∝ O3, Al ₂ O3, SiO ₂ ZnO、 The surface of cobalt blue/chromium green ceramic oxides, ITO, etc. is rich in hydroxyl groups and exhibits polarity and acidity. The core logic requirements for dispersants are:
1) The "anchor head" of dispersant molecules (such as carboxyl, phosphate, amino, etc.) needs to form strong interactions (ionic bonds, hydrogen bonds, or coordination bonds) with the surface charge properties (positive/negative charges) and chemical environment of nanoparticles (such as indium tin oxide, titanium dioxide, etc.) to achieve firm adsorption and prevent detachment between dispersants and particles.
2) The "tail" (solvation chain) of the dispersant must be highly compatible with the continuous phase (water, organic solvents, or UV monomers). Choose hydrophilic chains (polyether, polyacrylic acid) for water-based systems, non-polar chains (polyolefin) for solvent based systems, and polymerizable double bonds for UV systems; Only when the chain segments are fully extended can effective spatial hindrance be provided.
3) The molecular weight of the dispersant needs to be geometrically matched with the size of the nanoparticles. The molecular weight is too small, and the steric hindrance layer is too thin to prevent aggregation; Excessive molecular weight can easily lead to a surge in system viscosity or the formation of bridging flocculation. Usually, the optimal adsorption capacity and chain length need to be calculated based on the specific surface area of the particles.
4) In combination with the above three steps, the ink has low viscosity (conducive to high-speed nozzle spraying), anti flocculation (long-term non settling), non clogging (narrow particle size distribution, no large particles), and stable storage (thermal storage without layering).
I、 First, master the key points for selecting the two major structures of dispersants
1) Anchor group (firmly grasp oxide particles)
Metal oxides will form M-OH hydroxyl groups on the surface according to the environment. Dispersants with acidic anchoring groups are preferred, and they are firmly adsorbed by hydrogen bonds and ionic bonds, making it difficult to desorb and agglomerate
1. Phosphoric acid/phosphonic acid based dispersants: have the strongest adsorption force, usually 3-5 times that of carboxyl groups, and are more firmly bound. Suitable for TiO ₂, iron oxide, aluminum oxide, zirconium oxide, ceramic pigments; Good temperature resistance, excellent grinding and viscosity reduction, preferred solvent based;
2. Carboxylic acid/carboxylate dispersant: high cost-effectiveness, mainly used in water-based systems, suitable for the vast majority of oxides; PH sensitive, the optimal use is pH 7.0-9.0;
3. Sulfonic acid based dispersant: Strong electrolyte and salt resistance, suitable for inkjet systems containing alcohols, polyols, and additives, as well as doped metal oxides;
4. Hydroxyl and amide based dispersants are usually difficult to independently serve as the main dispersants for high solid content or difficult to disperse pigment systems due to their low anchoring energy and weak adsorption force. They play more roles in assisting wetting, stabilizing steric hindrance, or enhancing compounding efficiency, and cannot be used as the main dispersants alone;
❌ Amino and alkaline anchoring groups are prohibited: the oxide surface carries weak positive charges, and alkaline groups repel and cannot adsorb.
2) Solvation chain (forming steric hindrance to prevent agglomeration)
The solvation chain must be highly compatible with the ink medium (polarity matching) in order to fully stretch and effectively form steric hindrance:
-In water-based ink systems, polyethylene oxide (PEO) and polyether hydrophilic chains are mainly used as dispersants, solvent chains, rheological modifiers, or resin hydrophilic modification groups to maintain pigment stability and regulate ink properties through steric hindrance and hydrogen bonding effects;
-In the alcohol ether/solvent based oil-based ceramic inkjet system, the polyester chain is not a typical "hydrophobic chain" choice, but exists as an anchoring group for polar linking materials or dispersants; The true hydrophobic solvation chains of this system are usually long-chain alkyl, hydrocarbon, or low polarity polyester/polyacrylate segments;
-In UV curable inkjet ink, "polyester and modified polyether acrylic chains" refer to two types of polymer skeleton structures that act as photoactive prepolymers (oligomers). They participate in UV curing reactions through end grafted acrylic double bonds, jointly determining the physical properties of the ink film;
Molecular weight range: Mw 2000-8000 is the optimal polymer dispersant for inkjet printing; The chain is too short to settle due to insufficient steric hindrance, the viscosity of the ink slurry increases due to the entanglement of molecules that are too long, and the piezoelectric nozzle cannot spray.
II、 Classification and selection by ink system (three major inkjet systems)
（1） Water based inkjet ink (pigment inkjet, functional oxide inkjet)
Applicable: paper inkjet, textile digital printing, water-based ceramic ink, transparent oxide coating inkjet
1. Preferred type: Anionic comb shaped polycarboxylate dispersant
Anchoring: a large number of carboxyl groups; Side chain polyether; Double stability of static electricity and spatial resistance; Suitable for TiO ₂, transparent iron oxide, Al ₂ O3, SiO ₂ ZnO。
Representative: Polyacrylate ammonium salt, styrene maleic anhydride SMA ammonium salt, modified polycarboxylate dispersant (RK-4039AC, RK-4057, RK-4059).
2. Difficult to disperse nano oxides and systems with high alcohol content: Phosphate modified polycarboxylates and dispersants containing sulfonic acid groups enhance alcohol stability and inhibit alcohol induced flocculation.
3. Taboo: Small molecule inorganic dispersant (sodium hexametaphosphate) disperses well in the short term, but is prone to water separation and nozzle blockage during long-term storage. It is strictly prohibited to use it alone for inkjet printing.
4. Dosage: 2% to 6% by mass of oxide powder; The upper limit of the specific surface area for nano oxides is 3% -6%, while for micro oxides it is 2% -4%.
（2） Solvent based oil-based inkjet (ceramic tile inkjet, industrial functional inkjet)
Media: alcohol ether, diethylene glycol methyl ether DBE、 Organic solvents such as cyclohexanone; The most commonly used ceramic inkjet in the industry
1. Main selection: Acid phosphate super dispersant
Phosphate groups strongly anchor various ceramic metal oxides (titanium dioxide, iron red and iron yellow, cobalt aluminum oxide, chromium oxide, zirconium oxide); Outstanding viscosity reducing performance, capable of achieving high solid content and low viscosity ink slurry; High temperature firing results in minimal residual ash and does not affect color development.
Representative: (RK-4013A, RK-4013C, RK-4011) phosphate dispersant.
2. Light colored/transparent oxides (transparent TiO ₂, transparent iron): modified polyester carboxylic acid dispersant to avoid trace residual yellowing of phosphate esters.
3. Compound scheme: Phosphate ester as the main dispersant+a small amount of non-ionic polyester dispersant, balancing dispersion stability and ink rheological spray properties.
4. Dosage: Powder mass 3% to 9%; Ceramic colored oxide 6% -9%, titanium dioxide 4% -7%.
（3） UV curing inkjet (UV white ink, UV color oxide ink, coating inkjet)
Characteristics: solvent-free, pure active monomer system, unable to introduce migratory small molecules; White ink TiO ₂ is highly prone to precipitation and flocculation
1. White UV ink (mainly TiO ₂): phosphoric acid anchored polyester block dispersant, no free amine (amine UV system later turns yellow); Alkaline dispersants cannot be used.
2. Color metal oxide UV ink (iron oxide, composite oxide): carboxylic acid anchored polyester type super dispersant; The system consists of epoxy monomers and polyether modified polyester dispersants.
3. Mandatory requirement: Dispersants with reactive double bonds are preferred, and after curing, they should be embedded in the coating without migration or precipitation; Narrow distribution of molecular weight between 3000-6000.
4. Dosage: TiO ₂ white ink 7% -14%; Colored oxide 5% -13%.
III、 One on one precise selection of common metal oxides
1. Titanium dioxide TiO ₂ (inkjet white ink)
Water based polycarboxylate anion dispersant; Oil based → phosphate ester dispersant; UV → amine free phosphopolyester dispersant; The surface hydroxyl density of TiO ₂ is high, requiring a large amount of anchoring, and the dosage of dispersant is relatively high.
2. Iron oxide (Fe ₂ O ∝ red/yellow/black, transparent iron oxide)
Surface acidity is relatively weak; Water based: polycarboxylate; Oiliness: Intermediate acidic polyester dispersant; Transparent nano iron oxide should be used with caution for strong phosphate esters, which are prone to fogging. Carboxylic acid modified dispersants should be preferred.
3. Aluminum oxide Al ₂ O3, zirconium oxide ZrO ₂
The surface hydroxyl activity is extremely high; Phosphate ester dispersant for oil-based ceramic inkjet printing; Water based comb dispersants with high carboxyl content.
4. Zinc oxide ZnO
Amphoteric oxides, pH sensitive; Water based control pH 7.5-8.5, using sulfonic acid modified polycarboxylic acid; Avoid strong acid dispersants from corroding ZnO.
5. Ceramic composite oxides (cobalt blue, cobalt green, chromium brown, iron manganese black)
Complex composition and multiple hydroxyl groups on the surface; Oil phosphate ester dispersant must be selected; Water based dispersant using a combination of carboxylic acid and sulfonic acid; A single dispersant is difficult to stabilize, and most binary compounds are used.
6. Silicon dioxide SiO ₂
Extremely hydrophilic; Water based polyether polycarboxylate; Oil based polyester carboxylic acid dispersants should not be used with high phosphate esters (excessive adsorption leads to a sudden increase in viscosity).
IV、 Inkjet exclusive additional screening criteria (distinguishing paint dispersants)
1. Rheological properties: After dispersion, the ink has low viscosity and becomes thinner when sheared. It does not thicken when left to stand and is suitable for piezoelectric inkjet spraying; Refuse to disperse and use sticky wire drawing;
2. Storage stability: No delamination, settling, or coarsening at room temperature for 2-3 months; Cold and hot cycles without layering;
3. Nozzle compatibility: does not contain excessive silicone, does not corrode piezoelectric crystals, and does not precipitate crystal clogging nozzles;
4. Burning/curing performance: Ceramic inkjet dispersant has extremely low residual carbon after high-temperature burning, without producing color spots or pinholes; UV ink does not affect the curing speed;
5. Environmentally friendly, free of APEO, heavy metals, polycyclic aromatic hydrocarbons, in compliance with REACH and RoHS;
6. pH adaptation: The oxide system strictly controls the pH, and the anionic dispersant avoids the system being too acidic or too alkaline.
V、 Simple optimization steps
1. Determine the type of oxide → measure the specific surface area of the powder;
2. Determine the ink system (water/solvent/UV);
3. Preliminary selection of 2-3 different anchoring dispersants for grinding tests;
4. Testing: Particle size D50/D90, Zeta potential, ink viscosity, static settling;
5. Conduct gradient dosage tests to determine the optimal addition ratio;
6. Conduct inkjet testing on the machine to determine ink breakage, oblique spraying, and nozzle durability, and finally finalize the design.
