Silver paste is a flake like metallic pigment made from atomized spherical aluminum powder that is ground, graded, and coated on the surface to form a paste like paste. It is widely used in automotive coatings, coil coatings, inks, plastic colorants, hot stamping paper, and imitation electroplating decorations. Its industrial production is mainly based on wet ball milling method, and the entire production line revolves around the core goal of “grinding spherical aluminum powder into thin sheets and then protecting it”.
1、 Raw material preparation
Add atomized aluminum powder (particle size of tens of microns), 200 # solvent oil (or high boiling mineral oil), and fatty acid additives (stearic acid, etc.) into the ball milling kettle according to the ratio, and stir to form a uniform slurry. On the one hand, additives control the direction of flaking during the grinding process, and on the other hand, they form an initial protective film on the surface of the aluminum sheet to prevent cold welding and oxidation.
2、 Wet ball milling
The slurry enters the horizontal ball mill, and under the impact and rolling of the steel grinding medium, the spherical aluminum powder is gradually flattened and stretched into scales. This step is the core of the entire line: the grading of the grinding medium, the amount of balls loaded, the rotation speed, the material to ball ratio, and the grinding time jointly determine the diameter to thickness ratio of the aluminum sheet – the larger the diameter to thickness ratio, the stronger the reflection of the aluminum sheet in the paint film, and the brighter the metallic feeling. To prevent spontaneous combustion of aluminum powder, the mill is filled with nitrogen throughout the entire process, and the oxygen content is monitored online.
3、 Particle size classification
After grinding, the slurry is subjected to particle size classification using a settling tank or centrifugal classifier, removing coarse particles and fine powders to ensure that the final particle size distribution (D50, D90) falls within the target window. There are significant differences in particle size requirements for different applications: fine particle size (several microns) is used for automotive topcoats, medium particle size is used for imitation electroplating, and coarse particle size is used for steel structure anti-corrosion.
4、 Surface coating
The graded qualified slurry enters the coating kettle, and silane coupling agent, oleic acid or other organic/inorganic coating agents are added to construct a dense protective film on the surface of the aluminum sheet. The coating formula determines whether the product is floating (floating on the surface of the paint film, with a strong metallic feel) or non floating (uniformly dispersed in the paint film), and also determines its acid and alkali resistance, weather resistance, and adhesion level.
5、 Pressure filtration dehydration
After the coating is completed, the thin slurry is dehydrated by a plate and frame filter press or a horizontal scraper centrifuge to obtain a filter cake with an aluminum content of about 65% (± 2%). The extracted solvent is recycled after condensation to reduce material consumption and VOCs emissions.
6、 Mixing and testing packaging
According to the customer’s viscosity requirements, solvent is added to the filter cake and stirred into a slurry. After passing the inspection of color difference, particle size, bottom cover force, floating rate and other indicators, it is sealed and stored in barrels. Some high-end powder products will be transformed into aluminum powder form through vacuum drying under nitrogen protection.