The role of fumed silica in coatings & paint

1. Rheological additives
Rheology is an important performance of coatings. It directly affects the appearance, construction performance and storage stability of the coating. Different coating systems have different requirements for rheology additives. For oily systems, most rheology additives They all work by forming hydrogen bonds. The untreated fumed silica aggregates on the surface contain multiple, among them, one is isolated, undisturbed and free, and the other is continuous, forming a hydrogen bond with each other. Hydrogen bonding in an oily system easily forms a three-dimensional network structure. This structure will be destroyed when affected by mechanical force, which will reduce the viscosity and restore the good fluidity of the coating; when the shear force is removed, the three-dimensional structure will be It recovers by itself and the viscosity rises. 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, which depends on the concentration of fumed silica and its degree of dispersion. This feature gives oil-based coatings very good storage and construction performance, especially thick-build coatings, which can not only ensure that the coating has good fluidity under a certain construction shear force, but also ensure the thickness of the coating film at one time. During the construction process, because the solvent at the edge of the coating evaporates quickly, resulting in uneven surface tension, it is easy to move the coating to the edge, and the silica network can effectively prevent the movement of the coating to form a thick edge, while also preventing the coating The sagging phenomenon in the curing process makes the coating uniform. At the same time, fumed silica can form hydrogen bonds to increase the medium and low shear viscosity in the system, thereby playing a thickening role. Therefore, fumed silica is in The application in oily systems is very extensive.

2. Anti-settling agent

Fumed silica is an ideal anti-settling agent. It is very effective in preventing 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 reduce wetting. The amount of dispersant to improve the applicability of the color paste and reduce the impact of the color paste on the coating system. The anti-settling effect of fumed silica is very beneficial to the storage of coatings, especially certain pigments, such as metal powder and flakes. It is easy to precipitate and cannot be completely suspended. The use of fumed silica can ensure its dispersion without precipitation. Based on the total formula, the amount of silica is in the range of 0.4%-0.8%, but in special circumstances, such as zinc-rich paint, Need to increase to 2%.

3. Additive dispersion
In the powder coating system, due to the small particle size and high surface energy of fumed silica, they can be adsorbed 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, so it can be used as a dispersion In the same coating system, adding fumed silica can significantly shorten the dispersion time and improve production efficiency. It is worth noting that it is better to disperse fumed silica first, and the amount of addition should not be too much. Not more than 1%. Because the addition of too much will cause the system to have strong thixotropic performance, resulting in insufficient edge dispersion shear force during dispersion, and it is frozen, which affects the dispersion efficiency. Special circumstances such as zinc-rich paint need to add 2%. At the same time, it is used with other rheological host additives, and the rheological properties of fumed silica are adjusted by alcohol solvents.

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. During the film formation process, it migrates to the surface of the paint film, which can produce the expected roughness on the surface and significantly reduce the surface gloss. A good matting agent, the use of fumed silica is to pay attention to the match with the thickness of the paint film. In thick film paint, the use of very fine fumed silica particles will not produce proper roughness on the surface of the coating film; on the contrary, For example, if fumed silica with coarse particles is used in film paint, although its matting effect is very good, the roughness of the paint film surface will not be acceptable to most users. Generally speaking, the particle size of fumed silica is about 3 7. Suitable for coating systems with a dry film thickness of 15-40. Use fumed silica to mix paint, in addition to mixing it into a slurry and then adding it to the paint for dispersion, it can also be directly mixed into the paint. Usually in the container, a stirring impeller with a linear speed of 20m/s (about 1000r/min) can be dispersed for 10-15 minutes to achieve sufficient dispersion.

5. Anti-wear agent
After the fumed silica is surface treated with methacryl silane, it is added to the polyurethane coating to have a friction-resistant effect. Adding 5%-15% fumed silica can increase the friction resistance by 10%-35%, while the rheological properties of the coating and the optical properties of the dry film are not negatively affected.

6. Other functions
Fumed silica can also improve the weather resistance and scratch resistance of the coating, and improve the bonding strength between the coating and the substrate. At the same time, the fumed silica has strong ultraviolet absorption and infrared light reflection characteristics, and it is added in The paint can improve the anti-aging performance of the paint