What is the role of fumed silica in coatings?
Fumed silica is an ideal anti settling agent, to prevent paint paint system of precipitation is very effective, especially for the paste system, add the appropriate amount of paste will greatly improve the stability, but also can reduce the amount of wetting and dispersing agent, in order to improve the applicability of the paste, and reduce the influence of coating paste system, sink prevention effect of fumed silica on coating deposited very favorable, especially some pigments, such as metal powder and sheet, are easy to precipitate and not fully suspended, the use of fumed silica can ensure the dispersed precipitation. In formula of total amount of silica content in the range of 0.4%-0.8%, but under special circumstances for example, zinc rich paints need to increase to 2%.
Rheology is the important properties of the coating, it directly affects the appearance of coatings, the performance of the construction performance and storage stability, and the requirements of different coating systems on rheological additives are also different. For oily system, most are rheological agent plays the role of hydrogen bond formation. The unmodified fumed silica containing aggregates a, which is an isolated, free two undisturbed is united, each other to form hydrogen bond bonded hydrogen bonding in the oil system, very easy to form a three-dimensional network structure, the structure of mechanical force effect will be damaged, so that the viscosity decreased, coating restore good fluidity; when the shear the force is eliminated, the three-dimensional structure will recover, viscosity increased. In a completely non polar liquid, only a fraction of a second viscosity recovery time; in a polar liquid, recovery time is longer, depending on the gas phase silica And the dispersion degree of concentration, storage and application performance of this feature gives oily coating is very good, especially thick conformal coatings, which can ensure the coating has good fluidity in the construction of the shear, a thickness of the construction, but also can ensure the film is usually in the construction process, due to the edge of the coating the solvent leads to faster, the surface tension is not uniform, easy to paint to the edge of mobile, and mobile network can prevent coating silica effectively and the formation of thick, while also preventing coating during the curing process of the sagging phenomenon, the coated layer is uniform. At the same time, fumed silica due to the formation of hydrogen bonds and improve the low shear viscosity in the system, and thus play a thickening effect. Therefore, fumed silica is widely used in the oil system.
In the powder coating system, due to the small size of fumed silica and high surface energy, which can be adsorbed on the surface of coating powder, and a surface layer formed on the surface of powder to improve powder scattered, so it can be used as the dispersing agent. In the same coating system, fumed silica can be added shorten the dispersion time, improve the production efficiency. It is worth noting that the fumed silica dispersed completely better effect, the amount should not be too much, generally not more than 1%. because of excessive amount of lead system lead to strong thixotropy, loose edge shear dispersion is not enough, and a jelly, influence the dispersion efficiency, special situations such as the need to add 2% zinc rich paint can and other additives used together with rheological host collocation, rheological properties and the use of alcohol solvent to adjust the fumed silica.
The refractive index of fumed silica 1.46, close to the binder resin and the refractive index, has no effect on the film color. Their migration to the film surface in the film-forming process, can make the surface roughness is expected, significantly reduced the surface gloss, is a kind of good matting agent, fumed silica is used, should pay attention to the film the thickness of the thick film. In the paint, silica with very fine particles and gas, can not produce proper film surface roughness; on the other hand, such as paint film in coarse particles by gas phase silica, while the extinction effect is very good, but the surface roughness of the vast majority of users will not be able to accept, in general, fumed silica particle size is about 3-7, suitable for dry film thickness of coating system of 15-40. Using fumed silica with paint, in addition with slurry added to paint dispersion, can be directly deployed to Usually in a container, 10-15min can be sufficiently dispersed by stirring the impeller at a linear speed of 20m/s (about 1000r/min)
