Aluminum Surface Treatment Technology
China's aluminum product industry started slowly, but its development is also fast. There are only a few small enterprises in the early days of liberation, and now it has more than 1000 large enterprises all over the country, with an agreement output exceeding five hundred thousand thousand tons. However, with the continuous improvement of people’s living standards, aluminum products are still far from meeting the needs of people’s lives. In particular, compared with foreign products of the same type, there is a large gap between quality and fancy varieties, which shows that China’s aluminum products processing and surface treatment The technology is relatively backward. In recent years, through frequent domestic and foreign scientific and technological exchanges and the efforts of scientific and technological workers, new technologies and new technologies have been tested and adopted. However, in general, the surface treatment technology of aluminum products in China is still in a period of development to be developed. .
The effect of purity of the material composition of aluminum products on the quality of the oxidation film of aluminum oxide has long been understood by scientists and technologists. However, in the industrial production, the quality of the surface processing of domestic aluminum product plates has seriously affected the quality of anodized products. This contradiction has become increasingly prominent. The aluminum sheet of foreign countries has undergone milling processing, smooth as a mirror, excellent quality after oxidation. In China, aluminum materials are often not very glossy, and bubbles, scratches, and rough skins are very serious. After anodizing, these defects still appear. Decorative strong processes such as printing a variety of patterns, such as thermal transfer printing sheet due to the poor quality of the surface state caused by ineffective phenomenon is particularly serious. Therefore, improving the appearance quality of the plate becomes an important part of ensuring the best quality of the oxidation of aluminum products.
In the degreasing treatment of aluminum products prior to oxidation, most factories still use weak alkali chemical degreasing or caustic soda degreasing and whitewashing. However, in recent years, more and more factories have used a method of adding surfactants for degreasing. Utilizing the unique physical and chemical properties of surfactants, reducing surface tension, emulsifying, solubilizing foaming, and its own excellent chemical stability to alkalis, hard waters, etc., to improve the degreasing effect, and to form a layer of alkaline surface The foam layer inhibits the evolution of alkali mist, reduces pollution, and improves operating conditions.
For example, the AS anionic surfactant produced by Shanghai Synthetic Detergent Factory, which is commonly used by our company, is used to wash white aluminum tubs and it has received good economic results.
The specific process is: caustic soda (NaOH) 10%; temperature 97 ~ 100 °C; time 5 ~ 7 seconds. Surfactant AS is added in an amount of about 0.2 g/l, depending on the situation.
In order to make the surface of aluminum products smooth and smooth, the molded aluminum products are generally polished by machine. However, oxidation is performed after mechanical polishing, and only a smooth oxide film can be obtained, and glossiness is often insufficient. For products requiring a high gloss decorative oxide film, chemical polishing or electrolytic polishing is also performed after mechanical polishing. Pure aluminum or high-purity aluminum-magnesium alloys can obtain a glossy surface with a high reflection coefficient after chemical polishing or electrolytic polishing, and particularly electrolytic polishing can achieve satisfactory results. Unfortunately, the general tri-acid chemical polishing solution contains HNO 3 and heats to decompose nitrogen dioxide (NO 2 ) gas to produce the “Huanglong” hazard. The quality and stability of the electrolytic polishing solution contain chromic anhydride, resulting in chromium-containing wastewater, polluting the environment, resulting in three waste treatment problems. Naturally, people want to study chemical polishing that does not produce “Huanglong” and electrolytic polishing process that does not contain chromium. The "Wipe Additive for Aluminum Chemical Additives without Yellow Smoke, WXP" produced by Zhejiang Huangyan Fluorescence Chemical Factory passed the appraisal with the assistance of Beijing Environmental Protection Department. The additive WXP is used as a brightener for phosphoric acid and sulfuric acid-based polishing solutions, and has the function of inhibiting the acid mist.
The process recipe used is: phosphoric acid (specific gravity d = 1.7) industrial pure 800 ml / l;
Sulfuric acid (specific gravity d=1.84) industrial grade 200 ml/l;
WXP is 2 ml/l;
Three grams of aluminum should be added per liter of new solution.
Because the formula does not contain nitric acid, it does not produce "Huanglong" pollution during polishing; the effect of inhibiting the acid mist is also good.
As for electropolishing without chromic anhydride, many articles have listed many formulations. Most of them are organic acids (such as citric acid, tartaric acid, oxalic acid) or alcohols (such as ethanol, butanol, glycerol), etc. added to the polishing solution mainly containing phosphoric acid and sulfuric acid. The use of these additives in place of chromic anhydride to effect corrosion inhibition of aluminum surfaces. However, after testing, such formulations have the disadvantages of high cost and poor electrolyte stability, which are difficult to promote in large industrial production. According to reports, many factories in Shanghai have been working on electroless polishing tests for chromium-free anhydride. Some plants have also passed the appraisal recently. We are looking forward to breakthrough test results in this area.
Second, oxidation process
The sulfuric acid method, the oxalic acid method, and the chromic acid oxidation method, particularly the sulfuric acid anodic oxidation method, are almost unknown to anyone engaged in the surface treatment of aluminum products. Each of these oxidation methods has its own characteristics and scope of application.
Such as oxalic acid, can obtain a thick oxide film, oxide film itself with some decorative colors, but the method is costly and consumes large amounts of electricity.
Sulfuric acid oxide film is transparent, colorless, and has good color-absorption properties. In addition, the electrolyte composition is simple and stable, the process is easy to operate, and the cost is low, so it is widely used.
However, the sulfuric acid tank temperature control range is small, rapid heating, often need to install refrigeration equipment, which has become a difficult one.
In Japan, a sulfuric acid-oxalic acid mixed-acid oxidation method was developed for a long time and both were eliminated, and it became Japan's main oxidation bath. China's Shenyang Aluminum Products Factory introduced the use of mixed acid oxidation technology, their recommended formula is: sulfuric acid 10 ~ 20%;
Oxalic acid 1 to 2%; DC voltage 10 to 20 volts; anodized.
For the oxidizing solution heating and cooling method, Harbin Aluminum Products No. 1 Factory introduced the experience of adopting BXO pole heat exchanger produced by Siping General Machinery Plant in Jilin Province. The heat exchanger is considered to be effective. It has the advantages of less investment, less land occupation, quick installation, high heat exchange efficiency, strong corrosion resistance, and good sealing performance. The heat exchanger is more suitable for oxalic acid oxidation solution. As for the choice of model size, it can be calculated based on the amount of bath fluid, referring to the description of “Polar Heat Exchanger Selection Guide” edited by Siping General Machinery.
Third, After Oxidation Finishing
After oxidation, many aluminum products have to be dyed or printed and sprayed to obtain a variety of bright colors or beautiful patterns, increasing the number of fancy varieties. In 1980, the aluminum product transfer printing method identified by the Ministry of Light Industry was able to obtain a variety of color patterns with clear patterns, bright colors and rich levels. Aluminum products transfer printing method is also called sublimation transfer color technology, The basic principle is to use a disperse dye specially made transfer ink, according to the pattern requirements first printed on paper, made of color printing paper, and then paste the printing paper on the oxide film of aluminum products, Through warming and hot pressing, the disperse dyes on the printing paper are transferred into the pores of the oxide film in the vapor phase to form a color pattern. The transfer printing method obtains a color image on the surface of the aluminum product by its technology that is detached from the conventional oxidative dyeing method, and thus attracts many aluminum product companies to pay attention to trial and production. Aluminum products factories in Shanghai, Wuhan and Qingdao have all been put into industrial production. At this stage, the cost of the process is still high. To produce the transfer printing paper, the printing industry must rely on the printing industry. Most of the high-temperature pressurized transfer color patterns are still in the manual operation stage. Theoretically, the mechanism of the transfer of printing seems to have not been thoroughly studied. More people think that mainly in the high temperature heating conditions, the disperse dyes in the transfer ink sublimation to the oxide film. However, some experimental results show that the thickness and the adsorption performance of the oxide film also play a very important role, so it is not simply a phenomenon of sublimation of a disperse dye. It is believed that with the continuous exploration of the transfer mechanism and the continuous improvement of the application process and the accumulation of experience, the transfer printing method will open up a broad new field for the beautification of aluminum products.
After the oxidation of aluminum products, it has the advantages of stable technology, low cost and easy operation. In addition, through the improvement of the process, we can obtain a variety of beautiful or unique patterns and increase the fancy varieties. Therefore, the dyeing process has always been the subject of aluminum oxidation workers. "Infiltration method with color technology" "Marbling dyeing process" and so on belong to this category. The osmotic method uses the chromic anhydride to fade the understained color, while the marble patterning uses the grease to seal the underline osmotic method. The specific method is to paint the aluminum products after oxidation and tinting with chromic anhydride (industrial pure 350-450 g/l) or oxalic acid (industrial-grade 200-300 g/l), using asbestos, glass fiber, etc. With chromic anhydride wettability auxiliary exhibition products, the auxiliary exhibition part is discolored, and the faded image reaction is stopped immediately after rinsing with water, and then the second color is repeated or rubbing, rubbing, scouring, dyeing and other procedures are repeated. As a result, beautiful lines such as bright flowers, clouds and other colored lines can appear. As for the marble pattern dyeing method, aluminum oxide products are first stained with the first ground color and then immersed in water with a surface floating with grease (such as peanut oil). When lifting or immersing, due to the natural sagging of grease and water, the oxide layer is partially contaminated by irregular striped grease. When the second color is re-stained, the oxide film is stained with oil and stain. The other part that is not contaminated with grease is stained with a second shade. This gives the surface of the aluminum product an irregular and beautiful pattern in the form of a marble pattern.
Fourth, other surface treatment process
In addition to oxidative dyeing, aluminum products also have other surface treatment processes such as spray painting or spray printing. Spray painting process is to remove the oil after washing aluminum products, spraying amino baking, airing, drying at 150 ~ 170 °C for 15 to 20 minutes Serve. Spray paint spray is used nitro-color paint, spray primer, and then sprayed on the pattern flower board, and finally spray varnish, and baked at 60 °C temperature for 2 minutes. Spray painting and spray painting processes are widely used in aluminum products such as aluminum buckets, bowls, washbasins, tea trays, etc. in Jiangsu Province and other provinces.
In recent years, many new aluminum surface treatment technologies and new technologies have also been developed abroad. Such as electrolytic coloring, coated PTFE and enamel crafts, etc. And has been applied to industrial production, making aluminum products more colorful and adapt to various uses. Some domestic research institutes and factories have also tested and researched these processes. It is believed that these products will soon be available in the domestic market. Now these new processes are briefly described as follows:
(1) Electrolytic coloring
It is to add some additives directly in the oxidizing electrolyte, generally some organic acids such as thiosalicylic acid, cresol sulfonic acid, etc., to oxidize the colored oxide film. Strictly speaking, oxalic acid oxidation of yellow oxide film should also belong to this kind. The oxidized aluminum workpiece is also put into an electrolytic solution containing nickel, selenium, cobalt, tin and other metal salts to electrolyze the oxide film, and various colors can be obtained depending on the composition of the aluminum material and the electrolytic process. Although the color tone obtained by the electrolytic coloring method is still relatively monotonous and far less vivid than the organic dye, the electrolytic coloring is an oxidation film coloration, durability, and excellent weather resistance. Currently, electrolytically colored aluminum products have been widely used in building materials such as aluminum door and window frames.
(2) Process for coating resin such as PTFE and PPS
PTFE is a high-temperature engineering plastic, It is highly chemically stable, electrically insulating and anti-adhesive, and maintains good serviceability over a wide temperature range of -60°C to 250°C. Therefore, it is widely used in light industrial civilian bedding and other aluminum products. It is also very popular. Coating process is generally: first by a certain ratio of PTFE resin ball mill filter, made of dispersion, sprayed on the sandblasted, degreasing, washing white aluminum products, and then by sintering, plasticizing, quenching to make. This process has been tested in Shanghai, Harbin, Beijing and other places.
(3) Aluminum enamel process
The ordinary enamel powder used in steel enamels can be fused to the workpiece at 700-1000°C, while the melting point of aluminum and aluminum alloys is generally around 600°C. It is clear that ordinary enamels are not suitable for aluminum. Aluminium products, enamel temperature generally can not exceed 550 °C, it is also called low temperature enamel, which requires the use of some special enamel and special enamel method. It is said that Wuhu City Second Light Research Institute and Shandong Xintai Aluminum Products Factory have relatively comprehensive research on aluminum enamels, and preliminary tests have been successful.
Fifth, some superficial views
(1) The variety of aluminum products is annoying and the surface decoration requirements of the products are different. The conditions of manufacturers around the country are not the same. Therefore, we must choose new technologies and new technologies based on the environmental conditions, product requirements, and the actual production of the equipment. For example, the sulfuric acid-oxalic acid mixed acid oxidation method combines the advantages of the sulfuric acid oxalic acid method, has the advantages of energy saving, higher temperature, and thicker oxide film. However, it should be used flexibly in specific applications. Our factory has been using sulfuric acid oxidation production of multiple dyeing method to produce a variety of patterns of small knife alumina handle.
In 1993, due to the damage of the refrigeration equipment at the time, we used a sulfuric acid-oxalic acid mixed acid in a 1300-liter oxidation tank for trial production.
The practical process recipe at that time was 15% sulfuric acid;
Oxalic acid 1%; bath temperature 26 ~ 33 °C;
Current density 0.8 to 1 amps/dm2;
Anodizing, the effect is still good.
However, during the test, we also observed that when the oxalic acid content exceeds 1.5%, although a yellowish film that does not affect the appearance of oxidized oxalic acid has yet to be produced, one-time dyeing performance is also acceptable, but when dyed multiple times by printing, it is difficult to produce. Dyeing phenomenon.
Therefore, it is required to dye the decorative oxide film several times, and the content of oxalic acid in the mixed acid oxidation should not be too high.
In the north, some factories producing aluminum cookers are oxidized by oxalic acid. The products have a thick oxide film, a natural yellowish color, and they are also popular with people without dyeing.
Their experimental application of mixed acid oxidation aims to use less oxalic acid, save energy, and reduce costs.
The composition of the mixed acid is oxalic acid 2 to 3%; sulfuric acid 5 to 10%.
They also observed that when the content of oxalic acid is less than 2%, the oxide film no longer has a yellowish tone that gives a dense and thick feeling to the original oxidized oxalic acid, and some customers do not like the appearance of such products, affecting the product. market.
So they use 3% oxalic acid, Zhuojia sulfuric acid oxidation method.