Iron oxide Due to their inherent non-toxicity, resistance to color bleeding, low cost, and ability to produce a wide range of hues, pigments are widely used in coatings, Paint , in the ink. Coatings are composed of film-forming substances, pigments, fillers, solvents, and additives. They have evolved from oil-based coatings to Synthetic resin In the development of coatings, the application of pigments is indispensable across all types of coatings; in particular, iron oxide pigments have become an essential pigment category for the coatings industry.

Iron oxide pigments used in coatings include: Iron Yellow Iron red, iron brown, iron black, micaceous iron oxide, transparent iron yellow, transparent iron red, as well as semi-transparent products; among these, iron red is the most important in terms of both quantity and application.

Molecular formulas, particle shapes, and colors of various iron oxide pigments

1. Iron Red:

It exhibits excellent thermal stability, remaining unchanged at 500°C and retaining its chemical structure even at 1200°C, making it exceptionally stable. It can absorb the ultraviolet spectrum of sunlight, thereby providing protective benefits to coatings. Additionally, it is resistant to dilute acids, alkalis, water, and solvents, endowing it with outstanding weather resistance.

When the particle size of iron red is 0.2 μm, it exhibits a yellowish tint and has a relatively large specific surface area and oil absorption. As the particle size increases, the color shifts from a reddish hue to a purplish one, while the specific surface area and oil absorption decrease accordingly. Iron red is extensively used in anti-corrosion coatings due to its physical corrosion-inhibiting properties: it prevents atmospheric moisture and other corrosive agents from penetrating the metal substrate, and it enhances the coating’s density and mechanical strength.

Iron oxide red used in anti-corrosion paints should have low water solubility to enhance its anti-corrosion performance. In particular, when chloride ion levels increase, moisture readily penetrates the coating and accelerates metal corrosion. Moreover, metals are highly sensitive to acidity; therefore, if the pH of the resin, pigments, or solvents in the coating is below 7—indicating strong acidity—the likelihood of metal corrosion is further increased. After prolonged exposure to sunlight, coatings formulated with iron oxide red are prone to chalking, a phenomenon that occurs more rapidly with finer particle sizes. To improve weather resistance, it is advisable to use iron oxide red with larger particle sizes; however, this can also lead to a reduction in coating gloss.

    Topcoat Color changes are typically caused by flocculation of one or more components in the pigment formulation, as well as poor pigment wettability and Wetting agent Excessive amounts often lead to flocculation; pigments that have undergone calcination exhibit a strong tendency to flocculate. Therefore, to ensure uniform and consistent color in the topcoat, it is advisable to use wet-process synthetic iron oxide red. Coatings formulated with needle-shaped crystalline iron oxide red are prone to a silky sheen effect, and brush marks can appear as streaks whose color intensity varies depending on the viewing angle—this phenomenon is related to the differing refractive indices of the crystal forms.

Iron oxide red exhibits good heat resistance and is also suitable for applications in Powder Coating coloring. In paint-spraying applications, there is iron-red polyvinyl chloride primer, which is first formulated with iron oxide red, polyvinyl chloride resin, and a plasticizer ( Benzene Dibutyl formate, low-carbon barium carbonate, and fillers (barium sulfate and talc) are thoroughly mixed and then rolled into paint flakes on a mixing mill. After dissolution in a solvent, phenolic resin and alkyd resin are added, followed by paint adjustment and filtration to produce the final coating. Iron oxide red also holds great promise for use in latex paints, where it can replace solvent-based and oil-based coatings for application on various architectural structural components.

To ensure good storage stability of latex paints, the pH of the pigments should be close to neutral, and the content of water-soluble salts should be minimized; these pigments are also extensively used in floor coatings. Natural iron oxide red, owing to its low cost, is suitable for use in primers as well. After air-classification milling, more than 99.5% of the natural product passes through a 325-mesh screen, and it can be incorporated into paints via high-speed dispersion or sand milling, exhibiting excellent hiding power and UV resistance. Natural iron oxide red is primarily used in primer formulations; in addition to providing physical rust-inhibiting properties, it also acts as a weight-adding agent.

Compared with the natural product, synthetic iron oxide red has a higher density, finer particle size, higher purity, superior hiding power, greater oil absorption, and stronger tinting strength. In certain coating formulations, natural and synthetic iron oxide red are used in combination—for example, in alkyd primers containing iron oxide red that are applied as a primer on the surfaces of ferrous metals such as vehicles, machinery, and instruments.

Application of iron oxide pigments in functional nanocoatings:

Nanocoatings hold vast market potential; they not only significantly enhance the performance of conventional nanocoatings but also introduce new functionalities, such as improved storage stability, enhanced weather resistance, and superior mechanical properties—including increased coating strength, hardness, wear resistance, and scratch resistance—resulting in a wide array of outstanding characteristics. However, we should avoid excessive hype and overextension and instead focus on practical, down-to-earth development. At present, nanocoating research and application in China are still in their infancy and largely confined to the laboratory stage. The following provides a brief overview of nanocoatings related to iron oxide pigments.

⑴ Military stealth coatings: Certain nano-oxides (Fe₂O₃, ZnO, NiO₂, MoO₂) exhibit strong electromagnetic wave absorption properties; when compounded with organic coatings, they can be used to formulate military stealth coatings. Applied to aircraft, missiles, submarines, warships, and other weapon systems, these coatings can significantly enhance their combat survivability. Compared with other stealth materials, they offer advantages such as a broad absorption bandwidth, light weight, and thin profile, making them highly promising for extensive use in future military stealth applications.

(2) Electrostatic shielding coatings: By leveraging the electrostatic shielding properties of nanoparticles, electrostatic shielding coatings can be formulated for electrostatic protection in household appliances. Moreover, by selecting different nanoparticles (such as Fe₂O₃, ZnO, NiO₂, and MoO₂), the color of the antistatic coating can be tailored.

(3) Thermal-insulating coatings: These coatings are formulated by compounding nanomaterials such as Fe₂O₃—known for their strong absorption and reflection of infrared radiation—with organic binders. They find extensive applications in thermal insulation for glass curtain walls, automotive glass, offshore drilling platforms, oil tanks, petroleum pipelines, vehicle, train, and aircraft surfaces, ship hulls and decks, tanks, warships, and spacecraft surfaces, among others.

(4) Nano-self-healing coatings: This type of coating is particularly well suited for formulating high-performance automotive paints. During vehicle operation, the surface coating is inevitably subject to damage from foreign-object scratches or human-induced vandalism. Conventional repair methods for such minor blemishes often result in unsightly mottling, while large-scale repairs are time-consuming, labor-intensive, and economically inefficient. By leveraging nanotechnology to endow automotive coatings with self-healing capabilities, the performance and quality of existing automotive paints can be significantly enhanced.

⑸ Air-purifying coatings: Nanoparticles exhibit strong catalytic properties, which can be leveraged to develop air-purifying coatings. For example, coatings formulated with anatase TiO2 as the organic binder and pigments/fillers such as Fe2O3 demonstrate excellent photocatalytic activity toward atmospheric NOx. These coatings can be applied to the surfaces of highways, bridges, buildings, and billboards, and their air-purifying performance is regenerated through rainwater washing.

⑹ Other functional nanocoatings: magnetic coatings (utilizing nano- and micro-particles such as Fe3O4), infrared stealth coatings, and luxury car topcoats with angle-dependent color-shifting effects, among others.