Application of fumed silica in coatings

October 12, 2024

What is fumed silica
Fumed silica is silicon dioxide produced by the vapor phase method, commonly known as fumed silica. It is amorphous nano-sized particles with surface hydroxyl and adsorbed water generated by the high-temperature hydrolysis of silicon halide in the hydrogen and oxygen flame. Under normal conditions, it is a white flocculent powder. It is a non-toxic, tasteless, odorless and pollution-free non-metallic oxide.
Principle of action
Fumed silica is one of the most important high-tech ultra-fine inorganic materials. The particle size of fumed silica is very small, so it has large specific surface area, strong surface adsorption, high surface energy and is very easy to agglomerate. Fumed silica has high chemical purity, good dispersion performance, and special performance in thermal resistance and resistance. It has unique characteristics in many disciplines and fields with its superior stability, reinforcement, thickening and thixotropy, and plays an irreplaceable role.
performance
Fumed silica can 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, fumed silica has strong ultraviolet absorption and infrared light reflection characteristics, which can improve the anti-aging performance of the coating.
After surface treatment with methacrylosilane, fumed silica can be added to polyurethane coatings, which can play a role in friction resistance. Adding 5% ～ 15% fumed silica, on the one hand, it is firmly embedded in the coating film, and some particles of the aggregate are exposed to the coating surface after film forming, forming a micro roughness. This microstructure makes the coating surface non slippery and non adhesive, and the friction coefficient is greatly improved, which can be increased by 10% ～ 35%. At the same time, the rheological properties of the coating and the optical properties of the dry film are not negatively affected.
(2) 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.
(3) Fumed silica has the characteristics of strong ultraviolet absorption and infrared light reflection. Adding fumed silica in the coating can improve the anti-aging performance of the coating.
The role of fumed silica in coatings
1. Rheological additives
Rheological property is an important property of coatings, which directly affects the appearance, construction performance, storage stability and other properties of coatings. The requirements of rheological additives for different coating systems are also different. For oil-based systems, most rheological additives work by forming hydrogen bonds. The surface untreated gaseous silica aggregates contain several, one is isolated and free from interference, the other is continuous, and the bonding hydrogen bonds that form hydrogen bonds with each other are bonded in the oil-based system, which is very easy to form a three-dimensional network structure. This structure will be destroyed when affected by mechanical force, so that the viscosity will drop, and the coating will restore good fluidity; When the shear force is eliminated, the three-dimensional structure will recover and the viscosity will rise. In completely nonpolar liquids, the viscosity recovery time is only a fraction of a second; In polar liquids, the recovery time is long, which depends on the concentration and dispersion of fumed silica. This property gives oil-based coatings very good storage and application performance, especially thick paste coatings, which can not only ensure the good fluidity of the coatings under a certain construction shear force, but also ensure the primary construction thickness of the film. Generally, during the construction process, due to the rapid volatilization of the solvent at the edge of the coating, the surface tension is uneven, which is easy to make the coating move to the edge, while the silica network can effectively prevent the movement of the coating and form a thick edge. At the same time, it can also prevent the sagging phenomenon of the coating during the curing process and make the coating uniform. At the same time, fumed silica can improve the medium and low shear viscosity of the system due to the formation of hydrogen bonds.Therefore, fumed silica is widely used in oily systems.
2. Anti sedimentation agent
Fumed silica is an ideal anti sedimentation agent, which is very effective in preventing the precipitation of pigments in the coating system, especially for the color paste system. The appropriate addition amount will greatly improve the stability of the color paste, and can reduce the amount of wetting dispersant, so as to improve the applicability of the color paste and reduce the impact of the color paste on the coating system. The anti sedimentation effect of fumed silica is very beneficial to the storage of coatings, especially for some pigments, such as metal powder and flakes, which are very easy to precipitate and cannot be completely suspended. The use of fumed silica can ensure that they are dispersed without precipitation. Based on the total amount of formula, the amount of silica is in the range of 0.4% -0.8%, but in special cases, such as zinc rich paint, it needs to be increased 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 they can be used as a dispersant. In the same coating system, adding fumed silica can significantly shorten the dispersion time and improve the production efficiency. It is worth noting that it is better to completely disperse the fumed silica first, and the amount of fumed silica should not be too much, generally not more than 1%. Because too much addition will lead to strong thixotropy of the system, resulting in insufficient shear force at the edge of the dispersion, which will affect the dispersion efficiency. In special cases, if 2% zinc rich paint needs to be added, it can be added at the same time.The rheological properties of fumed silica were adjusted by alcohol solvent
4. Matting agent
The refractive index of fumed silica is 1.46, which is close to the refractive index of 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 of the surface and significantly reduce the surface gloss. It is a good matting agent. The use of fumed silica should be matched with the thickness of the paint film; On the contrary, if the gas-phase silica with coarse particles is used in the film paint, although its extinction effect is very good, the roughness of the film surface will not be accepted by the vast majority of users. Generally speaking, the particle size of the gas-phase silica is about 3-7, which is suitable for the paint system with a dry film thickness of 15-40. When using fumed silica to prepare paint, it can be mixed directly into the paint in addition to being mixed into a slurry and added to the paint for dispersion. Generally, in the container, sufficient dispersion can be achieved by dispersing the impeller with a linear speed of 20m/s (about 1000r/min) for 10-15min.
5. Anti abrasion agent
After surface treatment with methacrylosilane, fumed silica can be added to polyurethane coatings, which can play a role in friction resistance. Adding 5% -15% fumed silica, the friction resistance can be improved by 10% -35%, and the rheological properties and optical properties of the dry film are not negatively affected.
6. Other roles
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, fumed silica has strong ultraviolet absorption and infrared light reflection characteristics, which can improve the anti-aging performance of the coating.

Author:

Ms. Yolanda

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info@jinhetec.com

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