Application Characteristics of Silica in Coatings
In the field of coatings, silica has the functions of thickening, thixotropy, anti-precipitation and extinction, and can improve the process applicability of coatings in production, storage, application and after curing. It is an important functional filler.
1. Use as rheological aid
Silicon dioxide has a large number of hydroxyl groups on its surface. These hydroxyl groups form hydrogen bonds between its aggregates. When they are fully dispersed in coatings, they form a network structure of silicon dioxide.
On the one hand, in the construction process, the solvent volatilization at the edge of the coating is faster, resulting in uneven surface tension and easy to make the coating move towards the edge. The silica network can effectively prevent the coating from moving and forming edges, and at the same time, it can prevent the coating from hanging in the curing process and make the coating uniform.
On the other hand, silicon dioxide can form hydrogen bonds, which can increase the low and medium shear viscosity of the system, play a thickening role, and produce thixotropic phenomena. Therefore, silicon dioxide is widely used in oily systems.
In addition, silica has large specific surface area, high surface energy and is very easy to reunite. In order to achieve the best results, it must be properly dispersed in the application process. If the dispersion is insufficient, the three-dimensional network structure can not be fully formed, but excessive dispersion will cause the network structure between silica particles to be completely destroyed, even if the shear force is stopped for a long time, its network structure will be difficult to recover.
In the production process of coatings, the surface treatment of silica, the way of adding silica, the choice of dispersing equipment, etc. will affect the dispersion state of gaseous silica in coatings. Generally speaking, it is better to disperse silica into pre-slurry first, and then put into powder to disperse completely.
2. Used as anti-sinking agent
Silica has small particle size, large specific surface area and silane alcohol groups on the surface. These silane alcohols interact with adjacent silica particles to form hydrogen bonds.
Hydrogen bonding makes it form thixotropic structure. The suspension of silica is due to the formation of network structure when it is dispersed in solution. This network structure makes the particles in the system difficult to coalesce and phase separation. At the same time, the increase of the viscosity of the system is also beneficial to prevent the movement of the components in the system.
As an ideal anti-sedimentation agent, on the one hand, fumed silica is very effective in preventing the precipitation of pigments in paint system. Especially for the color paste system, proper addition 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 influence of the color paste on the paint system. Hydrogen bonding makes it form thixotropic structure.
On the other hand, the anti-sedimentation effect of silicon dioxide is very beneficial to the storage of coatings, especially some pigments, such as metal powder and thin sheets, are easy to precipitate and can not be completely suspended. The use of gas-phase silicon dioxide can ensure its dispersion without precipitation. According to the total 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. Use as dispersant
In powder coatings system, because of the small particle size and high surface energy of silica, it can be adsorbed on the surface of powder coatings and form a surface layer on the surface of powder coatings to improve the dispersion and spraying performance of coatings, so it can be used as a dispersant.
It is noteworthy to control the addition of silica, generally not more than 1%. Because the thixotropic property of the system is too strong, the edge shear force is not enough when the system is dispersed, and it is frozen, which affects the dispersion efficiency.
Hydrophobic silica is the best way to improve the adhesion of dispersant to powder particles, because it can maximize the adsorption of powder particles in the small micron size range. At the same time, because it is a hydrophobic product, it can reduce the water absorption when covering the powder particles, which can prevent the coating from caking.
4. Use as a matting agent
Silica has obvious light scattering ability because of its small size and network structure. Its refractive index is similar to that of most resins used in paint industry, which is 1.46. Therefore, silica has good optical properties in varnish and becomes the first choice of high-grade paint extinction agent.
The main characteristic parameters affecting silica extinction agent are pore volume (porosity), average particle size and particle size distribution, surface treatment, etc. For a given silica extinction agent, its extinction efficiency increases with the increase of pore volume. The average particle size and particle size distribution of silica play an important role in reducing the gloss of coatings. For a given coating and film thickness, the extinction effect is the best when the particle size of extinction agent corresponds to the thickness of dry film. The narrower the frequency band of particle size distribution, the higher the extinction efficiency.
5. Other application features
(1) Silica surface treated with methylpropylene silane and added to polyurethane coatings can play a role of friction resistance. Adding 5%-15% of fumed silica, on the one hand, it is firmly embedded in the film. The particles of aggregates are partly exposed to the surface of the coatings after film formation, resulting in micro-roughness. This micro-structure makes the surface of the coatings non-slippery and non-adherent, and the friction coefficient is greatly increased by 10%-35%. At the same time, the rheological properties of the coating and the optical properties of the dry film are not affected negatively.
(2) Silica can also improve the weatherability and scratch resistance of coatings and the bonding strength between coatings and substrates.
(3) Silicon dioxide has strong ultraviolet absorption and infrared light reflection characteristics, which can improve the anti-aging performance of coatings.
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