Powder surface modification changes the surface state of particles through physical or chemical means, with the core being to break the agglomeration force between particles. When the particle size of the powder decreases to the micrometer or nanometer level, the surface energy sharply increases, and van der Waals forces, hydrogen bonds, and other gravitational forces cause the particles to spontaneously aggregate, forming secondary particles and losing the surface area effect and volume effect of ultrafine powder. Surface modification improves dispersibility from three key dimensions by introducing modifiers: firstly, using coupling agents to construct "molecular bridges" and reduce particle surface energy; The second is to generate spatial hindrance through the coating layer to prevent particle contact; The third is to regulate surface charge, increase electrostatic repulsion, and ultimately achieve uniform dispersion of particles in the medium.

First、 Core mechanism: From molecular action to macroscopic dispersion

1. Reduce surface energy and gravity
The surface of unmodified powders is usually rich in polar groups such as hydroxyl groups, which can easily form hydrogen bonds or electrostatic adsorption between particles. For example, due to the dense hydroxyl groups on the surface, the agglomerate strength of nano zirconia powder can reach several hundred megapascals. By reacting the alkoxy group of silane coupling agent (general formula RnSiX (4-n)) with the hydroxyl group on the surface of the powder, the polar surface can be transformed into a non-polar surface covered with organic groups. The contact angle is increased from hydrophilic 0 ° to hydrophobic 114 ° or more, and the surface energy is reduced by more than 60%. Titanium ester coupling agents anchor the surface of the powder through titanium oxygen bonds, and long carbon chains provide steric hindrance, reducing the dispersed particle size of calcium carbonate in polypropylene from 50 μ m to 2 μ m.

2. Build a spatial obstruction barrier

Physical coating modification forms an elastic shell on the surface of particles through polymer chains, which generates entropy repulsion when particles approach. For example, the MoO3 layer coated with polyacrylic acid can increase the minimum distance between zirconia particles from 5nm to 20nm, effectively preventing agglomeration. Precipitation reaction modification involves the growth of inorganic coatings (such as Al2O3, SiO2) on the surface of particles, and the dispersion is adjusted by controlling the coating thickness (usually 5-50nm). This method increases the coverage power of titanium dioxide in coatings by 30%.

3. Regulating surface charge and Zeta potential

Surfactants form a double layer on the surface of particles by dissociating functional groups, increasing the absolute value of Zeta potential. For example, sodium stearate modified calcium carbonate increased the Zeta potential from 14.1mV to 30.2mV, and electrostatic repulsion improved the stability of the suspension for more than 24 hours. Composite modification (such as aluminum ester+SDS) can synergistically enhance the charge effect and steric hindrance effect, increasing the absolute Zeta potential of silicon carbide powder from 30.5mV to 60mV, and maintaining low viscosity even when the solid content of the slurry reaches 57% (volume fraction).

Second、 Key Technology Paths and Typical Applications

1. Chemical modification: the "molecular bridging" effect of coupling agents
Silane coupling agent: suitable for silicon/hydroxyl containing powders (quartz, kaolin), reacting with resins through functional groups such as amino and epoxy groups. For example, KH-550 treated silicon micro powder can maintain the insulation performance of epoxy sealant at 85 ℃/85% RH environment with a retention rate of>95%.
Titanium ester coupling agent: For calcium carbonate, talc powder, etc., the monoalkoxy type can improve the impact strength of PP composite materials to 45 kJ/m ², while the pyrophosphate type is suitable for powders with a moisture content greater than 0.5%.
Aluminum ester coupling agent: The cost is only 50% of titanium ester, the thermal stability is improved by 20 ℃, and the modified PVC/calcium carbonate system has a notch impact strength of 8kJ/m ².

2. Physical modification: encapsulation and mechanochemistry
Polymer coating: In situ polymerization modification of calcium carbonate with polyvinyl acetate can reduce the melt viscosity of PVC composite materials by 40% and improve processing flowability.
Mechanochemical modification: By high-energy ball milling, the surface lattice of particles is distorted, the active sites are increased, and the reaction efficiency with stearic acid is doubled. The modification time is shortened from 2 hours to 30 minutes.

3. Composite modification: synergistic enhancement strategy
The composite modification of stearic acid titanate (ratio 1:3) was used to treat heavy calcium carbonate, with an activation degree of 99.4% and an oil absorption value reduced to 0.267g/g. The filling amount in PP can reach 30% without reducing the mechanical properties. Salicylic acid and acrylamide are combined to modify silicon carbide, and the isoelectric point is adjusted to pH=12.5 to achieve stable dispersion in alkaline media.

Third、 Effect verification and process optimization

1. Core evaluation indicators

Contact angle and activation index: After modification, the contact angle of the powder is greater than 90 ° (hydrophobic), and an activation index close to 1 indicates complete coating. For example, aluminum ester modified calcium carbonate has a contact angle of 136.3 ° and an activation index of 100%.

Particle size and Zeta potential: The laser particle size analyzer shows a 30% -70% decrease in D50, and a Zeta potential absolute value greater than 30mV is a sign of good dispersion.
Application performance correlation: manifested as a decrease in settling volume in coatings (such as from 4.1mL/g to 1.0mL/g), and an improvement in impact strength in composite materials (such as a 41.97% increase in PP/calcium carbonate system).

2. Key points of the process

Dosage of modifier: 2% -3% for ultrafine powder (particle size<5 μ m) and 0.3% -1.5% for ordinary powder. For example, when using silane coupling agent for nano zinc oxide, a dosage of 2.5% is required to fully cover the surface hydroxyl groups.

Equipment matching: High speed mixers (with a speed of 1000-3000r/min) are suitable for dry modification, while ball mills or sand mills are used for wet modification. The integrated equipment of airflow crushing and modification can increase the discharge rate by 170%.

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