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Many formulations for water-based industrial paints, inorganic coatings, and primers use water-based silica sol, which is essentially a colloidal solution of nano silica particles dispersed in water. The particle size is generally 5-100 nm, with hydroxyl groups on the surface and stable dispersion in the aqueous phase. Many people confuse it with gas silica: silica sol is a ready-made stable water-based colloid; Gas silicon is a powder that needs to be dispersed into the system at high speed.

Core mechanism of action

The surface of silica sol particles is rich in silicon hydroxyl groups Si-OH. During the film formation stage, water evaporates and the nanoparticles approach each other, causing dehydration and condensation between silicon hydroxyl groups, forming an Si-O-Si inorganic network structure. At the same time, it can also form hydrogen bonds or even chemical bonds with hydroxyl groups on resins and substrates.

Main functions in coatings

1. Improve hardness, wear resistance, and scratch resistance. Nano silica itself has extremely high hardness. After film formation, particles are tightly packed inside the coating, which is equivalent to embedding inorganic hard particles in the paint film, significantly improving pencil hardness, wear resistance, and scratch resistance.

Attention: Adding too much can make the paint film brittle and reduce its flexibility.

1. Improve water resistance, moisture resistance, and salt spray resistance by dehydrating and condensing silica sol to form a dense inorganic network, reduce paint film pores, and decrease water and corrosive ion penetration; At the same time, silica sol has excellent adhesion to steel substrates, and the inorganic phase is firmly bonded to the metal interface, enhancing the long-term protective ability of anti-corrosion primers.

2. Enhance the adhesion of the substrate with a large amount of silicon hydroxyl groups on the surface, which can form hydrogen bonds with hydroxyl groups on inorganic substrates such as metals, cement, and glass. After dehydration, chemical bonds are formed, greatly improving the adhesion of the paint film to inorganic substrates, especially suitable for metal primers and inorganic architectural coatings.

3. Regulating rheology and preventing sedimentation. Small particle size silica sol has a certain thickening and thixotropic effect, assisting pigments and fillers in preventing sedimentation and improving storage stability; But its thickening ability is much weaker than that of gas silicon, and it cannot be used alone as a mainstream transformer.

4. Reducing VOC and improving high temperature resistance and flame retardancy belong to completely inorganic water-based components with zero VOC. Inorganic networks are heat-resistant and non flammable, suitable for inorganic fire-resistant coatings and high-temperature resistant coatings.

Shortcomings and pitfalls in the formula

1. Stability sensitivity: Silica sol is highly sensitive to pH, electrolytes, and multivalent metal ions. Once the pH exceeds the stable range (alkaline silica sol is generally pH 8.5~10.5) or a large number of electrolytes are mixed, the colloid is easy to agglomerate and the gel is scrapped.

2. Limited compatibility with organic lotion: direct mixing will break the emulsion, so organic modified silica sol or compatibilizer shall be selected to control the addition sequence.

3. Affects flexibility: Inorganic networks have high rigidity, and adding too much coating increases brittleness and is prone to cracking.

4. Surface hydroxyl hydrophilic: Residual silicon hydroxyl groups that have not fully condensed will absorb water in high humidity environments, so it needs to be combined with resin to form a synergistic film.

Selection Tips

Alkaline silica sol: the most commonly used coating with good stability;

· Acid silica sol: poor stability, rarely directly mixed with lotion, and mostly used for special substrate treatment;

· Organic modified silica sol: improve the compatibility with acrylic acid and epoxy lotion, and broaden the formula window.

Summary

Water based silica sol is a nano silica hydrogel. After dehydration, the silicon hydroxyl group condenses to form an inorganic Si-O-Si network, which is used to enhance the hardness, wear resistance, inorganic substrate adhesion, and salt spray resistance of the paint film; But it is sensitive to electrolytes and pH, and adding too much can make the coating brittle. It needs to be used in combination with organic resins to balance the rigidity and flexibility of the coating.