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What is the difference between the extinction effect of fumed silica , precipitation silica and gel silica?

What is the difference between the extinction effect of fumed silica , precipitation silica and gel silica?

The matting agent in the paint is a kind of additive that can change the optical properties of the paint film surface. It can make the surface of the paint film form a fine and rough surface. When the light is projected onto the surface, it will produce diffuse reflection, so as to achieve a matte or even matte effect. . Two conditions are required for the coating to form a fine and rough surface: (1) there are a large number of matting agent particles with a suitable particle size in the paint film; (2) proper volume shrinkage occurs during the drying process of the paint film. At the same time, the matting agent for coatings also needs to meet: (1) The refractive index is as close as possible to the refractive index of the film-forming resin (1.4~1.6); (2) Good wear resistance and scratch resistance; (3) Dispersion in the coating Good performance and redispersibility.

Commonly used matting agents are divided into two types: organic and inorganic. Organic matting agents are mostly thermosetting polymers, and inorganic matting agents are mainly silica-based matting agents. In addition, there are wax matting agents, wax modified silica matting agents, etc. The refractive index of amorphous silica is 1.46, which is close to the refractive index of most of the resins commonly used in the paint industry. It has good optical properties when used in paint films, so it has become the material of choice for high-grade paint matting agents. The silica synthesis methods used for matting agents are: (1) gas phase method; (2) precipitation method; (3) gel method.

1. Fumed silica matting agent

Fumed silica is formed by the high-temperature hydrolysis and combustion of silicon halides in a hydrogen-oxygen flame. The primary particles of silicon dioxide collide with each other to form secondary particles and form chains. The surface has hydroxyl groups and adsorbed water. Powder, with the characteristics of large specific surface area, high chemical purity and good dispersibility. There are two common methods for preparing fumed silica, and the reaction principles are shown in equations (1) and (2):

2. Precipitation silica matting agent

Precipitated silica is produced by the reaction of sodium silicate and acid, and the precipitated hydrate after the reaction is washed, filtered and dried to form a product. Precipitated silica has the advantages of high oil absorption value, simple process conditions, and low cost, but has the problems of small specific surface area, small pore volume, and wide pore size distribution. The reaction principle is shown in equation (3):

3. Gel method silica matting agent

The preparation principle of silica airgel is similar to that of precipitated silica, and its preparation process usually includes two processes of sol-gel and drying. In the sol-gel process, precursors (siliconate, water glass, etc.) A gel with a fine porous network structure is generated through hydrolysis and polycondensation, and then a large amount of internal liquid can be taken out under the premise of maintaining the original structure of the gel through a suitable drying process (supercritical, freezing, normal pressure), so as to obtain the second gel. Silica airgel. This product has the advantages of large specific surface area, large pore volume, narrow particle size distribution, etc., but its oil absorption value is low, the preparation process is relatively complicated, and the preparation cost is high.