Micro-powder network news: gas-phase white carbon black (scientific name fumed silica) is a fine, special amorphous powder material obtained by high-temperature hydrolysis of chlorosilane by oxyhydrogen flame. The average primary particle size is about 7-40. Nano, aggregates with a particle size of about 200-300 nm, specific surface area of 50-380 m2 / g, high product purity, SiO2 content of not less than 99.8%, is a multifunctional additive, widely used in the coatings industry, can increase Thick, thixotropic, extinction, anti-sinking and so on. The fumed silica generally has both hydrophilic and hydrophobic products, wherein the hydrophobic product is obtained by surface chemical treatment using a hydrophilic product.

Application of fumed silica in coatings

Rheology additive

Rheology is an important property of coatings. It directly affects the appearance, performance and storage stability of coatings. Different coating systems have different requirements for rheological additives. For oily systems, most rheological additives work by forming hydrogen bonds. The surface untreated fumed silica aggregates contain multiple hydroxyl groups, one is an isolated, undisturbed free hydroxyl group; The second is a bonded hydroxyl group which forms hydrogen bonds with each other.

The isolated hydroxyl group accounts for 12-15% of the surface group of the fumed silica, and the hydrogen bonded hydroxyl group accounts for 28-35%. Hydrogen-bonded hydroxyl groups are easy to form a uniform three-dimensional network structure in an oily system. This three-dimensional network structure (hydrogen bond) is destroyed by mechanical influence (shearing force), viscosity is lowered, and the coating returns to good flow. When the shear force is removed, the three-dimensional structure (hydrogen bond) recovers itself and the viscosity rises. In a completely non-polar liquid, the viscosity recovery time is only a fraction of a second, and in a polar liquid, the recovery time is longer, depending on the concentration of the fumed silica and its degree of dispersion. This property gives oily coatings very good storage properties and construction properties, especially for high-build coatings (such as marine paint), which not only ensures good fluidity of the coating under certain construction shearing forces, but also ensures coating. The thickness of a construction. During the construction process, due to the faster evaporation of the solvent at the edge of the coating, the surface tension is not uniform, and the coating is easily moved to the edge. The silica network can effectively prevent the movement of the coating to form a thick edge, and at the same time prevent the coating from being The sag during the curing process makes the coating uniform. At the same time, fumed silica is thickened by increasing the medium to low shear viscosity of the system by forming hydrogen bonds. Therefore, fumed silica is widely used in oily systems. However, in aqueous systems, since water molecules also form hydrogen bonds with fumed silica, which greatly affects their effects, their surfaces are usually terminated and modified by alumina, etc., using coordination effects. Rheology to avoid the effects of water. However, from the current application situation, it is not very ideal, it is necessary to repeatedly match the experiment, and the improper use will lead to the trend of liver system.

Anti-settling agent

The fumed silica is an ideal anti-settling agent, which forms a hydrogen bond structure which is very uniform and stable, and is a three-dimensional network structure. Therefore, it is very effective for preventing precipitation of pigments in the coating system. Especially for the color paste system, the proper addition amount will greatly improve the stability of the color paste, and can reduce the amount of the wetting and dispersing agent, thereby improving the applicability of the color paste and reducing the influence of the color paste on the coating system. The anti-settling effect of fumed silica is very beneficial for coating storage. Especially some pigments such as metal powder and flakes are easy to precipitate and cannot be completely suspended. The use of fumed silica can ensure that the dispersion does not precipitate and improve obviously. The can opening performance of the coating. The amount of fumed silica used in the total amount of the formula is in the range of 0.4% to 0.8%, but in special cases, such as zinc-rich paint, it needs to be increased to 2%.

Co-dispersant

In the powder system, when the dry (low moisture content) particles are less than 75 μm, the force between the particles causes the powder to be usually agglomerated (especially particles below 25 μm in size), and it is difficult to achieve fluidization. Method white carbon black solves this problem. The small particle size and high surface energy of the fumed silica make it adsorb 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. In order to improve the adhesion of the dispersant to the powder particles, it is preferred to use hydrophobic silica because they can maximize the adsorption of the powder particles in a small micron size range, and at the same time, because of the hydrophobic product, Covers the powder particles to reduce the adsorption of moisture, which prevents agglomeration during flow.

In the same coating system, the addition of fumed silica can significantly shorten the dispersion time and increase production efficiency. However, it is worth noting that the first effect of dispersing the white carbon black is better. The amount of white carbon black added as a dispersing agent should not be too much, generally not more than 1%, because too much addition will cause the system to have too strong thixotropic properties, resulting in insufficient edge shearing force during dispersion and freezing, affecting dispersion efficiency. . In special cases, if the zinc-rich paint needs to be added 2%, it can be used together with other rheological additives, and the rheological properties of the fumed silica can be adjusted by using an alcohol solvent.

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 film. It is a good matting agent to migrate to the surface of the paint film during the film formation process, which can produce the expected roughness on the surface of the paint film and significantly reduce the surface gloss. The matting effect depends primarily on the three properties of the product: porosity, particle size and surface treatment. When using fumed silica, pay attention to the matching of the thickness of the paint film. In the thick film paint, the fine-grained fumed silica is a fine surface which cannot produce appropriate roughness, and vice versa. The coarse-grained white carbon black, although its matting effect is very good, the roughness of the surface of the paint film will not be accepted by most users.

Anti-abrasion agent

The fumed silica is surface-treated with methacryl silane and can be added to the polyurethane coating to provide a friction-resistant effect. Adding 5%-15% fumed silica can increase the friction resistance by 10% to 35%, while the rheological properties of the coating and the optical properties of the dry film are not adversely affected. At this time, the addition of a large amount of fumed silica should be considered as an active filler rather than an auxiliary.

Anti-weathering agent

The fumed silica has certain characteristics of ultraviolet light absorption and infrared reflection. The surface of the film is tested by a spectrophotometer, and the UV shielding property of the vapor-based white carbon black film is improved, and the shielding rate to UVA (320-400 nm) is improved. It reaches 88%; the shielding rate to UVB (290-320nm) is 85%; the shielding to UVC (200-290nm) is still 70-80%. When the amount of fumed silica is 3%, the UV shielding of the coating film is the best, and the artificial accelerated weathering and artificial radiation exposure aging time is increased from the original 250 hours (powdering grade 1, discoloration grade 2) to 600 hours. (No powdering, no discoloration of the paint film, color difference 4.8).

Other effects

The fumed silica can coalesce into a network structure when the coating film is formed, which can greatly improve the strength and compactness of the coating, improve the scrub resistance and scratch resistance, and improve the coating between the substrate and the substrate. Bond strength. (Source: Network)