These are pigments made from metals or alloys. Global annual production reaches 70,000 tons, and they are widely used in the coatings industry. The main types include aluminum powder, copper powder, zinc powder, and stainless steel powder. Aluminum powder is used in the largest quantity, while copper powder is used less and has slightly lower light reflection intensity. Aluminum powder, zinc powder, and stainless steel powder all have anti-corrosion properties, while aluminum powder has better decorative properties.
Aluminum Powder: A powder with a silvery-white metallic luster made from metallic aluminum. Under cooling conditions, aluminum powder is ground to approximately 40 μm in a petroleum solvent, then filtered, and a suitable amount of wetting agent and solvent is added to adjust to the standard content (65%) to obtain aluminum powder slurry. If the aluminum powder is ground, the solvent is filtered out, and then vacuum dried, dry aluminum powder can be produced. Dry aluminum powder exists in two types: floating and non-floating. It is mainly used in the manufacture of coatings for painting oil storage tanks and bridges, and also in paper, rubber, and inks. Aluminum powder reflects sunlight, ultraviolet rays, and infrared rays, reducing photodegradation of polymers and extending their lifespan. Due to its low water vapor transmittance, aluminum powder also offers good rust resistance.
Copper powder is a powder with a golden to bronze luster, made from metallic copper or alloys of zinc, tin, etc. The coarse metal powder is mixed with stearic acid or its esters, ball-milled to a bright fine powder, and then post-treated to achieve a glossy finish. Copper-zinc alloy powder can be made into a reddish or greenish-gold color; copper-tin alloy powder can be made into bronze to golden-bronze colors. These are used in stationery, inks, and coatings.
Zinc powder is a pigment made from metallic zinc, appearing as bluish-gray spherical particles. Zinc plating on iron surfaces provides cathodic protection. Zinc powder has been used in coatings since 1840, and zinc-rich primers with high corrosion resistance have gradually been developed. The pigment volume concentration of zinc powder in zinc-rich primers can reach 30%–45%. Using metallic zinc as raw material, the zinc is heated and melted in a sealed environment, evaporating into zinc vapor. This vapor is then rapidly condensed to form spherical zinc particles. The faster the condensation rate, the finer the particle size. The condensed zinc powder settles at the bottom of the equipment, is collected, and then sieved or air-classified to obtain a product with uniform particle size. The oxygen content in the condenser must be strictly controlled to ensure product purity, keeping the zinc oxide content below 6%.