Complete selection guide for water-based nano metal oxide dispersants
Applicable powders:
nano metal oxides such as nano TiO ₂, transparent iron oxide, nano ZnO, Al ₂ O3, SiO ₂, zirconia, bismuth vanadate, etc;
Core issue:The material surface is rich in - OH groups, which makes it easy for particles to form hydrogen bonds. The nanoscale particle size results in a huge surface area per unit mass, with extremely high surface energy. The particles have a strong tendency to spontaneously aggregate to reduce surface energy. Once the particles approach, hydrogen bonds and van der Waals forces will cause them to form hard aggregates (hard aggregates), which are difficult to reopen by simple stirring. Due to their high density and tendency to aggregate into large particles, they will quickly settle in the liquid under the influence of gravity, resulting in unstable suspension. Traditional inorganic phosphates or small molecule dispersants mainly rely on a single electrostatic repulsion mechanism (double layer theory) to stabilize particles, so they are only suitable for short-term temporary use. In order to achieve industrial mass production and long-term stability, polymer dispersants must be used. The core mechanism is that the charged groups on the polymer chains still provide charge repulsion, while the polymer long chains adsorb on the surface of the particles, forming a thick physical barrier. When two particles approach, these polymer chains will compress or overlap, generating huge osmotic pressure and elastic recovery force (entropy repulsion), forcibly pushing the particles away.
Divided into four major sections: comparison of dispersant categories, precise selection by powder, scenario based (coating/color paste/water-based digital inkjet) solutions, and supporting process control.
I、 Four major categories of water-based dispersants: performance, adaptability, advantages and disadvantages
1. Inorganic phosphates (sodium hexametaphosphate, sodium tripolyphosphate) [only temporary slurry, prohibited for mass production]
Stability mechanism: relying on a single electrostatic repulsion mechanism (double layer theory) to stabilize particles, with no spatial hindrance and extremely short molecular chains
-Advantages: Extremely low cost, fast viscosity reduction
-Fatal flaw: High temperature/long-term storage will quickly thicken, leading to hard precipitation; Encountering calcium and magnesium ions will cause failure; Poor water resistance; Inadequate nano powder coating
-Only applicable: The laboratory prepares dilute slurry (solid content<20%) on the same day, and the use of coatings and inkjet products is strictly prohibited
-Addition amount: Powder 1%~2%
2. Low molecular weight polyacrylic acid salt (RK-4046, molecular weight<3000) [Transition version for low-end interior wall coatings]
Anionic short chain polycarboxylate molecules undergo chemical adsorption with hydroxyl groups on the surface of oxides through carboxyl groups, but due to the short molecular chain, the colloidal dispersion mechanism is weak in terms of steric hindrance stabilization effect.
-Advantages: High cost-effectiveness, suitable for aluminum coated nano titanium dioxide and ordinary water-based building coatings
-Shortcomings: high solid color paste, poor thermal cycling stability; Insufficient long-term anti settling effect of nano SiO ₂/ZnO; Alcohol solvent systems are prone to detachment and aggregation.
-Applicable: Interior latex paint, low solid content filling paste, not suitable for inkjet, exterior wall, industrial paint
-Amount added: 2.5%~4% powder (the amount of nano powder is twice that of micron filler)
3. Comb shaped polymer polycarboxylate (the preferred choice for water-based nano oxides)
Multi carboxylic multi-point anchoring+long hydrophilic polyether side chains, thick solvation layer, electrostatic+strong steric hindrance bistability
Recommended models:RK-4010, RK-4017E,
Core Advantages
1. Can prepare 30%~70% high solid nano color paste, with no hard block precipitation after 30 days of thermal storage at 50 ℃.
2. The pH 8-9 system adsorbs firmly, is resistant to cold and hot cycles, and does not affect the water resistance, weather resistance, and UV shielding function of the paint film;
3. Compatible with the vast majority of nano oxides (TiO ₂, iron oxide, Al ₂ O ∝, ZrO ₂);
4. Low foaming, APEO free, suitable for basic water-based inkjet ink.
shortcoming
Cationic resin systems can cause flocculation due to charge conflicts; The stability of high ethylene glycol/propylene glycol composite ink is weaker than that of polyurethane dispersant
Addition amount: Powder 3.0%~4.5%; High specific surface area nano SiO ₂ with an upper limit of 4%~5%
Core advantages
1. It can stably accommodate more than 30% of binary alcohol co solvents, with no detachment of particles during storage and no clogging of the inkjet nozzle during long-term circulation;
2. Nano oxides+organic pigments/carbon black are ground together to achieve uniform color without floating, and the particles of different pigments do not aggregate with each other due to van der Waals forces, surface charge imbalance, or competitive adsorption of dispersants, forming flocculation and soft hard aggregation, maintaining their independent dispersion states;
3. It is the only compatible cationic lotion and cationic primer system, and will not cause demulsification, flocculation, layering or failure;
4. Nano zinc oxide, gas-phase silica, and transparent iron oxide ultrafine powder can effectively suppress the irreversible hard agglomeration effect between nanoparticles due to chemical/hydrogen bonding in the system.
Disadvantages: Very high cost, high addition amount
Addition amount: 3.4%~6.1% of pigment powder
5. Pure non-ionic additives (only applicable for auxiliary compounding, not for main dispersion)
Polyvinylpyrrolidone and fatty alcohol polyether are only used for wetting and defoaming purposes, and if used alone, they will cause severe settling; Only 0.3%~1% of the main dispersant can improve the air entrapment of the powder and reduce the foam.
II、 Accurately match dispersants with different nano metal oxides
1. Nano titanium dioxide (aluminum/silicon coating, white ink, UV shielding coating) - building/industrial water-based coatings:
Comb shaped polymer polycarboxylates (RK-4008DF, RK-4017F0, RK-4039AB)
-Polymer polyurethane dispersant with dual cationic and anionic functional groups, particularly suitable for water-based digital inkjet white ink and high alcohol systems.
2. Transparent nano iron oxide (red, yellow, black, wood paint, color inkjet ink)
Polyurethane based polymer dispersants must be selected to utilize their strong anchoring and steric hindrance effects to prevent nanoparticle aggregation and oxidation due to heat, thereby maintaining high transparency and original color without dullness.
3. Nano gas-phase silica (matte, wear-resistant, inkjet coating)
SiO ₂ has a high specific surface area, which makes its oil absorption value very high, leading to thickening of the system, and even forming hard agglomeration. The preferred polymer polycarboxylic acid can avoid agglomeration by wrapping the surface of nano powder through the steric hindrance effect and electrostatic repulsion; PU matrix or modifier can modify the surface of SiO ₂, reducing its surface energy and significantly reducing local excessive thickening and agglomeration caused by high oil absorption values, while retaining the matte (extinction) and wear-resistant (reinforcement) properties of SiO ₂.
4. Nano ZnO, Al ₂ O3 (antibacterial, anti-corrosion, wear-resistant functional coating)
Polycarboxylic acid is the main component; Salt containing and anti-corrosion primer systems can be combined with trace amounts of phosphate dispersants, which can not only help nanoparticles disperse better, but also chemically interact with metal substrates to form microscopic phosphate anchoring points. This mechanism significantly increases the adhesion strength (anchoring force) between the coating and the metal surface, preventing the coating from peeling off in harsh environments.
5. When preparing ceramic oxide electronic pastes such as nano zirconia or ceramic inkjet inks, ammonium polyacrylate (sodium free polycarboxylate) is used as a dispersant to completely avoid residual sodium ions (Na ⁺) in traditional sodium salt dispersants, thereby preventing these impurity ions from damaging the electrical insulation performance of the material.
III、 Complete selection plan for three major application scenarios
Scenario 1: Water based building/industrial coatings, high solid nano concentrated color paste (stored for 1-3 months)
1. Low cost interior wall putty and low solid slurry: polyacrylate (short-term storage)
2. Ordinary exterior walls, plastic water-based paint, single nano titanium dioxide system: comb shaped polymer polycarboxylate
3. Anti corrosion and antibacterial ZnO/Al ₂ O3 system: a combination of polycarboxylic acid and a small amount of phosphate wetting and dispersing agent
4. Inorganic+organic pigment compound universal color paste: polymer polyurethane dispersant, anti floating color, hair floc coagulation
Scenario 2: Water based digital inkjet ink (textile, ceramic, decorative white ink, coating inkjet)
Inkjet hardness requirements: low foam, resistant to diols D50＜100nm、 Long term circulation without clogging or crystallization
1. Pure nano titanium dioxide white ink, matte silicon coating ink: comb shaped inkjet specific polycarboxylate
2. Transparent iron oxide color ink, multi pigment compound ink: amphoteric polyurethane dispersant (RK-4057, RK-4059)
3. Cationic water-based inkjet and ceramic functional ink: Only cation compatible PU dispersants (RK-4039AB, RK-4017F0) can be used, and ordinary anionic polycarboxylates can be directly layered and flocculated
Scenario 3: Water based electronic ceramic slurry (alumina, zirconia varistor substrate)
Select ammonium polyacrylate (RK-4008Y), free of sodium and potassium ions, high-temperature sintering with no residual conductive impurities, and no damage to insulation performance
IV、 Key supporting process control (determining long-term stability of dispersion)
1. Precise pH regulation (of utmost importance)
The isoelectric point of nano metal oxides is concentrated at pH 5-7; The stable range of the system is adjusted to pH 8.0~9.5, and strong electrostatic repulsion is generated between particles with the same negative charge, overcoming van der Waals attraction; Dispersant molecules can fully adsorb on the surface of particles to reach saturation, providing additional steric hindrance effect, dual function to prevent flocculation and aggregation, and improve stability.
-The pH of nano SiO ₂ system should not be greater than 10. Strong alkali will cause hydrolysis of silica fume, and at the same time, it will cause dramatic change of particle surface charge and condensation crosslinking, resulting in thickening of slurry viscosity and even gel;
The ZnO and Al ₂ O3 anti-corrosion systems only stably exhibit passivation and shielding effects in weakly alkaline environments (pH 7.5-8.5); Once exposed to a strong acid environment, the powder undergoes chemical dissolution and structural damage, resulting in the loss of its anti-corrosion function.
2. Dispersant addition gradient test (required for nano powder)
Gradient setting (powder mass ratio): 2.5%/3.5%/4.5%/6%
-Insufficient addition: In the powder surface modification (coating) process, due to insufficient or uneven dispersion of dispersants, a complete isolation layer is not formed on the particle surface, which leads to chain failure phenomena such as agglomeration, abnormal test data (bimodal), and deterioration of the rheological properties of the slurry (high viscosity);
-Excessive addition: When the dispersant is excessive or free without adsorption, it destroys the stability of the colloidal double layer, causing pigment aggregation, surface tension imbalance, resulting in a series of chain failures such as shrinkage of the paint film, deterioration of water and weather resistance, and inkjet printing breakage and foaming.
3. Compound efficiency enhancing formula
When the main dispersant is a polycarboxylate or polyurethane type dispersant, it has a long-chain structure and can adsorb on the surface of nanoparticles. Through steric hindrance effect and electrostatic repulsion of polycarboxylate, particles are prevented from approaching and aggregating with each other; Nanoparticles have a large specific surface area and high surface energy, making them difficult to be wetted by liquids. Adding a small amount of low foaming non-ionic wetting agent (such as RK-8005) can significantly reduce the interfacial tension between liquid and solid, helping the liquid quickly penetrate into the pores of the particles, replacing air, ensuring that the particles are fully wetted and initially dispersed; Long term storage system combined with associative polyurethane thickener to construct a thixotropic suspension system that physically blocks the gravitational settling of nanoparticles.
4. Dispersion stability verification standards
1. Particle size detection: 90% of the particles have a particle size less than 120nm, no double peaks, and no secondary hard agglomeration;
2. Thermal storage test: After being sealed and left to stand at 50 ℃ for 15 days, the sediment at the bottom can be completely dispersed by shaking without any hard lumps;
3. Inkjet special: 48 hour continuous cycle printing without ink breakage, nozzle blockage, crystal precipitation or other malfunctions.
