What is the difference between matting effect of silica, gas phase, precipitation method and gel method?
From HIFULL
The matting agent in the paint is a kind of assistant that can change the optical properties of the paint film surface. It can make the paint film surface form a slightly rough surface. When light is projected onto the surface, it will produce diffuse reflection, so as to achieve the effect of matte or even matting. There are two conditions for the coating to form a fine rough surface: (1) there are a large number of matting agent particles with appropriate particle size in the paint film; (2) Proper volume shrinkage occurs during film drying. At the same time, the matting agent for coating also needs to meet: (1) the refractive index should be close to the refractive index of film-forming resin (1.4 ~ 1.6); (2) Good wear resistance and scratch resistance; (3) It has good dispersibility and redispersibility in the coating.
Matting effect
The commonly used matting agents are divided into organic and inorganic. Most of the organic matting agents are thermosetting polymers, and the inorganic matting agents are mainly silica matting agents. In addition, there are wax matting agent, wax modified silica matting agent and so on. The refractive index of amorphous silica is 1.46, which is close to that of most resins commonly used in the field of coating industry. It has good optical properties in coating film, so it has become the preferred material for high-grade coating matting agent. The synthesis methods of silica used as matting agent include: (1) gas phase method; (2) Precipitation method; (3) gel method.
1、 Fumed silica matting agent
Gaseous silica is an ultra-fine powder with hydroxyl groups and adsorbed water on the surface, which is formed by the collision of primary silica particles formed during the high-temperature hydrolysis and combustion of silicon halide in hydrogen and oxygen flame to form secondary particles and form chains. It has the characteristics of large specific surface area, high chemical purity and good dispersion. There are two common preparation methods of fumed silica, and the reaction principle is shown in equations (1) and (2):
2、 Precipitated silica matting agent
Precipitated silica is formed by the reaction of sodium silicate and acid. The precipitated hydrate after reaction is formed after washing, pressure filtration and drying. Precipitated silica has the advantages of high oil absorption value, simple process conditions and low cost, but it has the problems of small specific surface area, small pore volume and wide pore size distribution. Its reaction principle is shown in equation (3):
Three. Gel silica matting agent
The preparation principle of silica aerogel is similar to that of precipitated silica. The preparation process usually includes two processes: sol-gel and drying. In the sol gel process, the precursor (silanol salt and sodium silicate) are formed by the hydrolysis and condensation reaction of the gel with a fine porous network structure. Then, through suitable drying process (supercritical, freezing, atmospheric pressure), the silica aerogel can be obtained by removing a large amount of liquid from the interior of the gel under the premise of maintaining the original structure of the gel. The product has the advantages of large specific surface area, large pore volume and narrow particle size distribution, but its oil absorption value is low, the preparation process is relatively complex and the preparation cost is high.
At present, the domestic market demand for silica matting agent is increasing year by year, but the domestic products are mainly concentrated in the middle and low-end fields. Although the domestic high-end products have reached the international high-end level in terms of pore volume, oil absorption value and average particle size, there is still an obvious gap in the control of microporous structure, such as wide particle size distribution, large gap in surface modification technology and low product stability. The high-end products of silica matting agent are still in the dilemma of relying on foreign imports