Aluminium paste origin
Aluminum silver paste is a widely used metallic pigment with a silver-gray metallic luster. In the coatings industry, it is commonly known as silver paste. Essentially, it is a metallic aluminum paste and does not contain silver components. It only has a silver metallic texture.
In 1910, American J. Hall invented aluminum silver paste: replacing the inert gas used in the dry process of aluminum powder with petroleum solvents. The aluminum powder was mixed with the solvent and ground into a paste-like state in a ball mill, becoming a paste-like aluminum powder pigment. This wet process significantly enhanced safety, avoiding the explosion risk of the dry process, and this paste-like product was convenient to use and quickly became popular worldwide.
The wet ball milling method adopted by the above-mentioned wet aluminum silver paste specifically involves using spherical or nearly spherical spray aluminum powder, adding lubricants such as ricinoleic acid, oleic acid, and other auxiliary agents, as well as organic solvents such as mineral oil, solvent oil, heavy aromatics, and isopropanol. The ball milling medium is 1-5mm steel balls, and the ball milling time is approximately 8 to 30 hours. Through continuous ball milling for a long time, the spherical aluminum powder is extended into sheet-like aluminum powder, presenting a silver-gray metallic luster. The sheet-like aluminum powder after ball milling is subjected to a screening process to make the particle size more uniform, and then undergoes solid-liquid separation (filtration) operations to obtain a filter cake. Finally, the filter cake is mixed with an appropriate surface treatment agent and organic solvents to obtain an aluminum silver paste product with a mass solid content of approximately 60-70%.
The purpose of using organic solvents in the above-mentioned wet ball milling is to improve the uniformity of aluminum powder particles and prevent aluminum powder from coming into contact with air to avoid the risk of explosion. The main role of the lubricant is to prevent the spherical aluminum powder from experiencing cold welding phenomena during ball milling due to the mechanical action of the steel ball medium. Other auxiliaries include stabilizers, dispersants, surface protective agents, and anti-settling agents, whose purpose is to improve the surface metallic luster of aluminum powder and meet the requirements of subsequent coating and other industries.
From the above description, aluminum silver paste is composed of flake-like aluminum sheets, solvents, and auxiliaries. The aluminum sheets generally have a thickness of 0.1-2μm and a diameter of 0.5-200μm. Their surfaces are smooth and flat, with various types such as snowflake-like, fish scale-like, and silver coin-like, with regular shapes, concentrated particle size distribution, and excellent light reflection ability and metallic luster. It has a high reflectivity of visible light, infrared light, and ultraviolet light, reaching 75-80%, and has a certain thermal insulation effect. When used alone in coatings, it can present a silver metallic flash effect. If mixed with transparent colored pigments, the light passing through, refracting, and reflecting on the paint film will present a certain depth effect, and the paint film with this effect will exhibit anisotropic color changes with the angle, presenting a magnificent and unique decorative effect.