Development of Surface Treatment Technology for Domestic Aluminum Products

(a) Surface treatment before oxidation.

(b) Oxidation process.

(c) Oxidized finishes.

(d) Other surface treatment processes.

(five) some superficial views.

Overview

With the development of aluminum smelting and processing technology, aluminum products are now widely used in aviation, building materials, vehicles, ships, light industry and other departments. In the advanced industrialized countries abroad, such as the United States, Britain, and Japan, since the development of industrial production of anodized aluminum films for aluminum products in the 1920s, the surface treatment technology for aluminum products has developed extremely rapidly over the past 60 years. In terms of light industrial products, the use of automated equipment, rigorous process oxidation products, after offset printing, thermal transfer printing coloring, electrophoretic painting and other decorative treatment, will give the product a very beautiful, refined appearance. China's aluminum product industry started slowly, but its development is also fast. From the beginning of the liberation, only a few small companies have now owned more than one hundred large enterprises across the country, and the annual output has exceeded 50,000 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. .

First, the surface treatment before oxidation

The influence of the purity of aluminum material composition and purity on the quality of the aluminum oxide film has long been understood by scientists and technologists. However, in the industrial production, the quality of the surface treatment of domestic aluminum products 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. The strong decorative process such as a variety of pattern printing, thermal transfer printing, etc. due to poor quality of the surface quality of the plate caused by poor results is particularly serious. Therefore, improving the appearance quality of the plate becomes an important part of ensuring better quality of oxidation of the aluminum product.

In the degreasing treatment of aluminum products before oxidation, most factories still use weak alkali chemical degreasing or caustic soda degreasing and whitening, but in recent years, more and more factories have used the method of adding surfactants to 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, Tianjin Aluminium Products Factory introduced the use of AS anionic surfactant produced by Tianjin or Shanghai Synthetic Detergent Factory to wash white aluminum pots, and it 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 bright and smooth, the molded aluminum products are generally mechanically polished. However, oxidation is performed after mechanical polishing, only a smooth oxide film can be obtained, and the gloss is often not enough. 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 especially 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 recipes used were: 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 2 ml/l; temperature 95 to 120°C. 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 when 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 which are organic acids (such as citric acid, tartaric acid, oxalic acid) or alcohols (such as ethanol, butadiene) in phosphate-based, sulfuric acid-based polishing solutions. Alcohols, glycerol, etc., in order to replace the chromic anhydride by these additives, acts as a corrosion-inhibiting polishing on the aluminum surface. However, after tests, 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 units in Shanghai have been working on electroless polishing experiments without chromic anhydride. Some plants have also passed the appraisal. 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 application scope. For example, the oxalic acid method can obtain a thick oxide film layer, and the oxide film itself has some decorative colors. However, this method has a high cost and consumes a large amount of electric energy. Sulfuric acid oxide film is transparent, colorless, and has good color-absorption properties. In addition, the electrolyte composition is simple and stable, the process operation is easy, 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 to avoid both and became the main oxidation bath in Japan. Shenyang Aluminum Products Factory in China introduced the use of mixed acid oxidation technology. Their recommended formula is: sulfuric acid 10 to 20%; oxalic acid 1 to 2%; DC voltage 10 to 20 volts; anodizing.

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 size of the selected model, it can be calculated based on the amount of bath fluid, referring to the description of the "Polar Heat Exchanger Selection Guide" edited by Siping General Machinery.

III. Finished after oxidation

After oxidation, many aluminum products have to be dyed or printed and sprayed, in order to obtain a variety of bright colors or beautiful patterns, and to increase 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. The aluminum product transfer printing method is also called sublimation transfer color technology. Its basic principle is to use a disperse dye specially made to transfer ink, and then print on the paper according to the pattern requirements to make a color printing paper, and then attach the printing paper to the aluminum products. On the oxide film, by heating 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 continuing to improve the application process and accumulate experience, the transfer printing method will open up a vast new field for the beautification of aluminum products (see the materials 2 and 3 for the aluminum oxide transfer printing process).

After the oxidation of aluminum products, it has the advantages of stable technology, low cost and easy operation. In addition, it is possible to obtain a variety of beautiful or distinctive patterns through the improvement of the process and to increase the number of 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 undertone color, while the marble patterning uses the grease to seal the background color. The specific method is to spray 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. Chromium anhydride wettability auxiliary exhibition products, so that the auxiliary exhibition part faded, immediately after washing with water to stop the fading of the image reaction, and then dye the second time or repeated rubbing, rubbing, scouring, dyeing and other procedures. As a result, beautiful lines such as bright flowers, clouds and other colored lines can appear. As for the marble pattern dyeing method, the oxidized aluminum products are first dyed by the front 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 The oil stained part is not stained, and the other part that is not contaminated with grease is dyed with a second color, which makes the surface of the aluminum product appear irregular and beautiful like a marble pattern. pattern.

Fourth, other surface treatment process

In addition to the oxidation dyeing of aluminum products, there are other surface treatment processes such as spray painting or spray painting. The painting process is to remove the aluminum products after oil washing and whitening, spray the amino baking varnish, air dry, and bake at 150-170°C for 15-20 minutes. Spray paint is used nitro-color spray paint, spray primer, and then sprayed flowers against the pattern flower board, later spray cover light paint, and baked at 60 °C temperature conditions 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, a number of new aluminum surface treatment technologies have been developed abroad, such as electrolytic coloring, coated PTFE and enamel technology, and have been applied in industrial production, making aluminum products more colorful and adaptable to various uses. Some domestic research institutes and factories have also tested and researched these processes. It is believed that these products can be seen in the domestic market in large quantities soon. 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 polytetrafluoroethylene and polyphenylene sulfide

PTFE is a high-temperature resistant engineering plastic. It has high chemical stability, electrical insulation, and anti-adhesion. It can maintain good performance over a wide temperature range of -60°C to 250°C. Therefore, it is widely used. It is also very popular with aluminum products such as light industrial and civilian bedding. Coating process is generally: first by a certain ratio of PTFE resin ball mill, made into a dispersion, sprayed on the sandblasted, degreasing, washing white aluminum products, and then sintered, plasticized, quenched to make. This process has been tested in Shanghai, Harbin, Beijing and other places.

(3) Aluminum enamel process

The ordinary enamel powder used for 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, the surface decoration requirements of the products are different, and 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 (State-owned Yangjiang Knife Factory) has been using sulfuric acid oxidation to produce multiple dyeing methods to produce aluminum knife shanks with multiple patterns. In 1973, due to the destruction 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. At that time, the practical process formulation was sulfuric acid 15%; oxalic acid 1%; bath temperature 26-33°C; current density 0.8-1 ampere/square decimeter; anodizing, the effect was good. However, during the test, we also observed that when the oxalic acid content exceeds 1.5%, although a yellowish film that sufficiently affects the appearance of oxidized oxalic acid has not yet been produced, one-time dyeing performance is still possible, 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 the oxalic acid method. The products have thicker oxide films and naturally yellowish color. 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 oxidation film no longer has a yellowish tone that gives a dense and thick feeling to the original oxidized oxalic acid. Some customers do not like the appearance of such products, affecting the product. market. So they used 3% oxalic acid, Zhuojia sulfuric acid oxidation method.

(2) Seriously learn from advanced organizations and help introduce new technologies and materials. In recent years, the aluminum oxide transfer printing method is one of the more advanced technologies. Many units compete for tests, but the test levels are uneven and the product quality is very different. In fact, the key to the transfer printing process is to transfer the printing paper. The transfer printing paper production needs the assistance of the printing industry. The selection of the ink and the printing technology are not the specialty of professional oxidation workers. Some advanced units have trial-produced high-quality transfer printing papers, and the transfer printing process is relatively high (such as Shanghai, Qingdao, etc.), and they are willing to make transfer printing papers. Therefore, some small enterprises with poor conditions do not have to experiment from scratch. If they directly request the transfer of printing paper, they can shorten the trial production process and increase economic efficiency.

(3) Strengthen cooperation with universities and research institutes to develop new technologies and new technologies for surface treatment of aluminum products. The surface treatment of aluminum products has increasingly involved multidisciplinary technologies. Such as coating PTFE resin process, which has polymer chemical engineering issues. The scientific research department of universities and colleges has a complete set of disciplines, a large number of professional scientists and technicians, and a relatively high level. It is relatively easy to obtain support for them and it is relatively easy to produce results.

(4) Do a good job of scientific and technological intelligence exchanges. The aluminum products industry is divided into various departments, aviation, light industry, and building materials, and the exchange of scientific and technological information is strengthened. The new technology and new technologies at home and abroad are sent to the factories for trial production as soon as possible to improve the technological level of the entire aluminum product industry. It is necessary to improve the quality of aluminum products.