Functions and roles of Crown Flag Ordinary Series silica in coatings

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1. Rheology modifiers: Rheology is an important property of coatings, as it directly affects aspects such as the appearance of the coating, its handling properties, and storage stability. Different coating systems have varying requirements for rheology modifiers. In oil-based systems, most rheology modifiers function by forming hydrogen bonds. Untreated silica aggregates contain multiple such bonds; some of these are isolated and free from interference, while others are connected to each other via hydrogen bonds. In oil-based systems, these hydrogen bonds easily form three-dimensional network structures. When subjected to mechanical forces, these structures are disrupted, resulting in a decrease in viscosity and allowing the coating to regain its fluidity. Once the shear force is removed, the three-dimensional structure re-forms, and the viscosity increases again. In completely non-polar liquids, the viscosity returns to normal within just a fraction of a second; in polar liquids, the recovery time is longer, depending on the concentration of silica and the degree of its dispersion. This property confers excellent storage and handling characteristics to oil-based coatings, especially those that are applied in thick layers. It ensures that the coating remains fluid under the shear forces encountered during application, while also maintaining a consistent thickness of the coating layer. During application, solvent evaporation at the edges of the coating can lead to uneven surface tension, causing the coating to move toward those edges. The silica network effectively prevents this movement, preventing the formation of thick edges and also avoiding sagging during curing, thus ensuring a uniform coating. Additionally, silica’s ability to form hydrogen bonds increases the medium- and low-shear viscosity of the system, thereby acting as a thickening agent. Therefore, silica is widely used in oil-based systems.

2. Antisettling agents: Vapor-phase silica is an ideal antisettling agent, effective in preventing the precipitation of pigments in coating systems. Especially in pigment dispersions, an appropriate amount of silica can improve the stability of the dispersion, reduce the need for wetting dispersants, enhance the usability of the pigment dispersion, and minimize its impact on the coating system. The antisettling effect of silica is very beneficial for storing coatings. Certain pigments, such as metal powders and flakes, tend to precipitate easily and cannot remain suspended indefinitely. The use of silica helps ensure that these pigments stay dispersed and do not settle. Generally, the amount of silica used ranges from 0.4% to 0.8% of the total formula weight, but in special cases, such as zinc-rich paints, this amount may need to be increased to 2%.

3. Dispersion aid: In powder coating systems, due to their small particle size and high surface energy, silica particles can adhere to the surface of the coating powder, forming a protective layer that improves the dispersion of the powder. Thus, it can be used as a dispersant. In the same coating system, vapor-phase silica can significantly reduce the dispersion time and improve production efficiency. It’s important to note that it’s more effective to fully disperse the silica first; the amount added should not be too high, generally not exceeding 1%. Excessive amounts can result in strong thixotropic properties in the system, leading to insufficient shear forces at the edges during dispersion, which hinders dispersion efficiency. In special cases, such as zinc-rich paints where 2% silica is required, it can be used together with other rheology modifiers, with alcohol solvents being used to adjust the rheological properties of the silica.

4. Matting agents: Silica has a refractive index of 1.46–1.52, which is close to that of the resin used to form the coating film. As a result, it does not affect the color of the coating film. During the curing process, silica migrates to the surface of the film, creating the desired level of roughness and significantly reducing surface gloss. It is therefore an excellent matting agent. When using vapor-phase silica, it’s important to consider its compatibility with the thickness of the coating film. In thick-coat paints, using silica with very fine particles prevents the formation of adequate roughness on the film surface. Conversely, in thin-coat paints, using silica with larger particles results in excellent matting effects, but the resulting roughness on the film surface may not be acceptable to most users. Generally, silica particles with a size of 3–7 are suitable for coating systems with a dry film thickness of 15–40. When using silica to formulate paint, in addition to mixing it into a slurry and then adding it to the paint for dispersion, it can also be directly incorporated into the paint. Typically, in a container, sufficient dispersion can be achieved within 10–15 minutes using an agitator impeller at a linear speed of 20 m/s (approximately 1000 rpm). 5. Anti-wear agent: Silica, after being surface-treated with methacrylsilane, is added to polyurethane coatings to provide wear resistance. Adding 5%-15% of fumed silica can improve friction resistance by 10%-35%, without negatively affecting the rheological properties of the coating or the optical properties of the dried film. 6. Other functions: Silica can also enhance the weather resistance and scratch resistance of coatings, as well as improve the bonding strength between the coating and the substrate. Additionally, silica possesses strong UV absorption and infrared reflection properties; when incorporated into coatings, it helps to improve their aging resistance.