Applications & Functions of Fumed Silica in Coatings

1. Rheological additives

Rheology is an important performance of the coating, it directly affects the appearance, construction performance and storage stability of the coating, and different coating systems have different requirements for rheology additives. For oily systems, most rheology additives work by forming hydrogen bonds. The untreated fumed silica aggregates on the surface contain multiple hydroxyl groups, one is an isolated, undisturbed free hydroxyl group; The second is the bonded hydroxyl groups that are continuously formed and form hydrogen bonds with each other.

Isolated hydroxyl groups account for 12-15% of the surface groups of fumed silica, and hydrogen-bonded hydroxyl groups account for 28-35%. Hydrogen-bonded hydroxyl groups can easily form a uniform three-dimensional network structure in an oily system. This three-dimensional network structure (hydrogen bond) will be destroyed by mechanical influence (shearing force), the viscosity will decrease, and the paint will return to a good flow. When the shearing force is eliminated, the three-dimensional structure (hydrogen bond) will recover by itself and the viscosity will increase. In a completely non-polar liquid, the viscosity recovery time is only a fraction of a second. In a polar liquid, the recovery time is longer, depending on the concentration of fumed silica and its degree of dispersion. This feature gives oil-based coatings very good storage performance and construction performance, especially thick-build coatings (such as marine paint), which can not only ensure that the coating has good fluidity under a certain construction shear force, but also ensure the coating film The thickness of one-time construction. During the construction process, the solvent at the edge of the coating evaporates quickly, resulting in uneven surface tension and easy movement of the coating to the edge. The silica network can effectively prevent the movement of the coating to form a thick edge, and at the same time prevent the coating from being The sagging phenomenon in the curing process makes the coating uniform. At the same time, fumed silica can increase the medium and low shear viscosity of the system due to the formation of hydrogen bonds, thereby playing a thickening role. Therefore, the application of fumed silica in oily systems is very extensive.

2. Anti-settling agent

Fumed silica is an ideal anti-settling agent, the hydrogen bond structure it forms is very uniform and stable, and it is a three-dimensional network structure. Therefore, it is very effective to prevent the precipitation of pigments in the coating system. Especially for the color paste system, the appropriate amount of addition will greatly improve the stability of the color paste, and can reduce the amount of wetting and dispersing agent, so as to improve the applicability of the color paste and reduce the influence of the color paste on the coating system.

3. Dispersant

In the powder system, when the dry (low moisture content) particles are smaller than 75μm, the force between the particles causes the powder to usually cohesive and agglomerate (especially the particles below 25μm), making it difficult to achieve fluidization. The small particle size and high surface energy of fumed silica make it adsorb on the surface of the coating powder and form a surface layer on the surface of the powder to improve the dispersibility of the powder.

In the same coating system, adding fumed silica can significantly shorten the dispersion time and improve production efficiency. But it is worth noting that it is better to disperse the fumed silica completely first. The amount of fumed silica used as a dispersant should not be too much, generally not more than 1%, because too much added amount will cause the system to have too strong thixotropic properties, resulting in insufficient edge shearing force during dispersion and freezing, which affects dispersion efficient. In special cases such as zinc-rich paint needs to add 2%, it can be used together with other rheological additives, and alcohol solvents can be used to adjust the rheological properties of fumed silica.

4. Matting agent

The refractive index of fumed silica is 1.46, which is close to the refractive index of the film-forming resin, and has no effect on the color of the paint film. It migrates to the surface of the paint film during the film forming process, which can produce the expected roughness on the surface of the paint film and significantly reduce its surface gloss. It is a good matting agent. The matting effect mainly depends on the three properties of the product: porosity, particle size and surface treatment.

5. Anti-wear agent

Fumed silica uses methacryl silane for surface treatment, and can be added to polyurethane coatings to provide friction resistance. Adding 5% to 15% fumed silica can increase the friction resistance by 10% to 35%, while the rheological properties of the coating and the optical properties of the dry film are not negatively affected.

6. Anti-weathering agent

Fumed silica has certain characteristics of ultraviolet light absorption and infrared reflection. The UV shielding performance of fumed silica coating film is improved after the surface is tested by spectrophotometer, and the shielding rate of UVA (320-400nm) reaches 88 %; the shielding rate of UVB (290-320nm) reaches 85%; the shielding rate of UVC (200-290nm) is still 70-80%. When the amount of fumed silica used is 3%, the UV shielding of the coating film is the best, and the artificial accelerated weathering and artificial radiation exposure aging time is increased from 250 hours (powdering level 1, discoloration level 2) to 600 hours (No chalking, no discoloration of paint film, color difference value 4.8).

7. Other functions

Fumed silica can coalesce into a network structure when the coating is formed, which can greatly improve the strength and compactness of the coating, improve its scrubbing resistance and scratch resistance, and improve the bond strength between the coating and the substrate.