China Aluminum Industry Net reports that as production, construction, and technological and cultural progress continue to develop, people's demand for the aesthetic appeal of products has also significantly increased. Color aluminum powder is widely used in various industrial sectors such as gas, coatings, printing, and dyeing due to its bright colors, strong corrosion resistance, high insulation properties, non-fading nature, strong metallic luster, and low cost. Its application has opened up new possibilities for expanding the color range of metallic pigments. Colored aluminum powder has been used for decorative purposes, and research on coloring aluminum powders began overseas in the 1930s [1], with some successful experiences gained. In recent years, there have been domestic reports [2] indicating that anodizing can be used to prepare colored aluminum powder with a particle size of 320 mesh, although it is still in the experimental stage. With increasing environmental awareness in the 21st century, water-based paints have shown great development potential. The successful application of aluminum powder pigments in water-based systems has significantly improved the weather resistance of coatings. Using sol-gel methods, an inert silica film is coated on the surface of sheet aluminum, and an iron oxide film is deposited on the SiOâ‚‚/Al composite particles through liquid deposition. This process successfully transforms aluminum powder into colored aluminum powder, promoting the development of environmentally friendly water-based coatings. Below, I will review three common methods for coloring aluminum powder. 1. Anodizing to Prepare Colored Aluminum Powder
Anodizing is an electrochemical process where an aluminum oxide film is formed on the surface of aluminum powder through the anode reaction in an electrolyte. This oxide film is then colored using organic dyes or inorganic pigments. The aluminum powder is placed in a sulfuric acid electrolyte and continuously stirred so that the powder remains in motion, ensuring constant contact with the anode. This allows for the formation of a colored aluminum oxide film on the surface of the powder. During the anode reaction, oxygen generated at the anode reacts with aluminum atoms to form alumina, some of which is immediately hydrated to create hydrated alumina. This forms the aluminum oxide layer. At the same time, the oxide film may dissolve in the sulfuric acid electrolyte, meaning both film formation and dissolution occur simultaneously during anodizing. Therefore, proper conditions must be controlled to achieve a specific thickness of the oxide film. Meanwhile, hydrogen is produced at the cathode, resulting in a porous structure in the oxide film, which enhances its adsorption capacity. The coloring of aluminum powder is a physical and chemical process. After anodizing, the aluminum powder is soaked in an organic dye solution, allowing the aluminum oxide film to absorb organic dye molecules. These molecules can form complexes with the aluminum oxide through covalent, coordination, or hydrogen bonds, resulting in a colored oxide film. During the anodizing process, many factors influence the formation of the oxide film, and different coloring solutions yield varying results. Key factors include electrolyte concentration, reaction time, temperature, and the type of coloring solution. Research shows: (1) Sulfuric acid concentration significantly affects oxide film formation. Too low a concentration leads to poor conductivity and slow film formation, while too high a concentration causes excessive dissolution of the oxide film. The optimal concentration is 5-10%. (2) Anode current density is directly proportional to the rate of film formation. Higher current density promotes better oxidation and creates a looser, more porous oxide film, which is beneficial for coloring. Experiments suggest that anodic oxidation should be carried out in 7% sulfuric acid with a current density of at least 5 A/dm² and a voltage of no less than 40 V. (3) Oxidation time should be around 60–90 minutes, with a temperature of 25–35°C. (4) The coloring effect depends on the oxide film thickness and the concentration of the coloring solution. Thicker films are easier to color, and higher concentrations result in deeper colors. Typically, a thicker coloring solution is used, with the concentration adjusted based on the desired color depth. The coloring temperature should be between 50–60°C, and the process should last 20–40 minutes with a pH of 4.5–6.0. 2. Chemical Oxidation Method to Prepare Colored Aluminum Powder
The chemical oxidation method involves placing aluminum powder in a weakly alkaline oxygen solution to form a certain thickness of oxide film on its surface. The main reactions during this process include: 2Al + (3+x)H₂O → Al₂O₃·xH₂O + 3H₂ (1) Al₂O₃·xH₂O → Al₂O₃·xH₂O + (x−1)H₂O (2) Al₂O₃·H₂O → Al₂O₃ + H₂O (3) Al₂O₃·xH₂O + 2OH⻠→ 2[Al(OH)₄]⻠+ (x−3)H₂O (4) 2Al₂O₃ + 4OH⻠+ 6H₂O → 4[Al(OH)₄]⻠(5)
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