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The surface of the oxide film on the surface of the aluminum profile is composed of a porous layer, which has a large specific surface area and high chemical activity. Taking advantage of this feature, various coloring treatments can be performed on the surface of the anodized film. In addition to improving the decorativeness and corrosion resistance of the product, it also provides various functionalities to the surface of aluminum products.
There are generally three types of anodized film coloring methods.
Category 1: Chemical impregnation method
Interior decoration and handicrafts generally use chemical impregnation
Category 2: Electrolytic coloring method
Architectural aluminum doors and windows generally use electrolytic coloring method
Category 3: Overall coloring method
The overall coloring method was used in the early days, but due to the limited color types and relatively high cost, this technology is basically not used at home and abroad.
Taking the anodization of aluminum as an example, we will introduce how to solve the color difference problem caused by anodization. First understand the process.
Craftsmanship
1. Pre-processing
Degreasing → alkali cleaning → pickling → polishing
Pre-treatment includes degreasing, alkali washing, polishing, etc., with the purpose of making the surface smooth and shiny, and removing surface impurities, making it easier to oxidize and color.
Degreasing is to use the infiltration, separation, emulsification, saponification and other functions of the degreasing agent to separate the grease from the surface of the metal shell. Through degreasing, oil stains on the surface of parts can be removed to facilitate subsequent alkali cleaning, oxidation, etc.
Alkali cleaning removes excess grease through the saponification of sodium hydroxide (NaOH), and removes the natural oxide film, burrs, and impurities on the metal shell of the mobile phone. Reacts with aluminum parts to remove surface dirt and natural oxide film. The purpose is to activate the surface. It also has the function of removing extrusion streaks and obtaining different reflective surfaces.
Pickling is to remove stains and dust on the surface after alkali etching to obtain a relatively clean surface. At the same time, it neutralizes the chemical reaction between the residual alkaline solution and the surface of the aluminum parts.
There are two types of polishing, one is electrolytic polishing and the other is chemical polishing. Electrolytic polishing uses the action of electric current to cause an electrochemical reaction in the aluminum alloy, and to produce varying degrees of deep decomposition on the uneven parts of the aluminum alloy surface, producing a smooth mirror effect on the surface of the aluminum parts. Electrolytically polished aluminum parts can still retain most of their luster after subsequent anodizing. The higher the purity of the aluminum sheet, the higher the reflectivity obtained; chemical polishing is a chemical ion reaction between aluminum and chemical polishing liquid, which selectively dissolves the uneven areas on the sample surface to eliminate wear marks and etch and level it.
2. Anodizing
Anodizing is a process in which oxidation occurs on the anode under the action of external current to form an oxide film.
3. Post-processing
Wash → color → seal → dry
Water washing means that after anodizing, sulfuric acid solution remains in the pores of the oxide film. Therefore, aluminum products must be thoroughly cleaned before dyeing. Avoid bringing impurity ions into the dyeing tank, especially phosphate ions, fluoride ions, etc. Set up pure water cleaning before the dyeing tank, and monitor the water quality.
Coloring is a chemical impregnation method, which is generally considered to involve both physical adsorption and complexation reactions between organic dye functional groups and alumina.
Sealing treatment is an indispensable part of anodizing. After oxidation dyeing, only sealing treatment can ensure the original color of the dyed film. There are many sealing processes. Steam sealing, hot water sealing, and medium temperature sealing are all good choices. Some dyes can also choose cold sealing process. The surface of the oxide film is porous, and dyes and crystal water can be adsorbed in these pores. Due to its strong adsorption, impurities may be adsorbed and contaminated if not treated in time, so it must be filled in time to improve the porosity. The strength and other properties of the membrane. There are many methods for sealing treatment, such as boiling water method, high-pressure steam method, impregnating metal salt method and filling organic matter (oil, synthetic resin), etc.
