Coating Science: general knowledge of aluminum painting

In the era of 2.0, low-grade bathrooms are generally difficult to operate Huida bathroom set off the tide of nanotechnology Farnsha fb1676 one-piece toilet Jiumu 1129-1 siphon one-piece toilet toto pearl bathtub ppy1523w, whether it's for aircraft dashboard or common external use, pretreatment is crucial to the success of coating adhesion Because of its excellent corrosion resistance, aluminum is the most popular industrial and architectural substrate It is also the most difficult substrate for successful painting The naturally formed thin layer of alumina, invisible to the naked eye and visible under the microscope, is covered on the surface, providing the inert to atmospheric corrosion However, because of this inertia, paint and coating often can not adhere firmly The key to good adhesion is to remove the oxide and apply passivation layer immediately to prevent the oxide from forming again It's easier said than done Aluminum railings, windows, sidings, wheels and other products exposed to the outside must be pretreated by a complex process consisting of a series of chemicals In the first step of the five to seven step process, the alkali solution degreases the surface to remove the grease The next step is to rinse with at least one tap to remove the contaminants The third step contains strong acid solution (reducing agent) to remove the stubborn oxide Then there is the washing step of washing with water (step 4) In the fifth step, apply the conversion coating on the aluminum The most common trade names are alodine (Henkel Henkel Henkel) and iridite (McDermid) Chemical membranes are also used This process is often referred to as chemical coating After the conversion coating is applied, the aluminum surface is flushed and sealed in one or more aqueous steps Where high quality is required, the final rinse step consists of deionized or reverse osmosis water Finally, clean pretreated aluminum enters an oven where water evaporates to dry the surface Aluminium is now ready for painting As the coating is easy to decompose, the furnace temperature should not rise above 60 ° C Fortunately, the process chemicals used before entering the oven are often heated, so that when the product leaves the final flushing step, the metal hot sink promotes rapid evaporation Dry and oil-free compressed air shall be used to blow away stagnant water at cracks, channels, etc In the electroplating industry where the final coat is often decorative, the conversion coating process often consists of 10 steps Conversion coatings for military and high-grade commercial applications with very important corrosion resistance must meet mil-c-55411a standard or grade 3 standard of "chemical conversion coatings for aluminum materials and aluminum alloys" Grade 1A standard requires conversion coating to contain hexavalent chromium; however, because this chemical is a carcinogen, the industry has vigorously promoted chromium free coating to pretreat aluminum in the past few years There are several new products on the market, some based on siloxane technology They may be used in fewer cleaning steps, many of which are dried in the right place In other words, they don't need to be cleaned before entering the oven Many different kinds of organic paint can be applied to the pretreated aluminum In the construction industry, which needs to be exposed to the external environment for a long time, it is common to coat aluminum plates and profiles with polyvinylidene fluoride (PVDF) layer AAMA 2605-05 stipulates "independent specifications, performance requirements and test procedures for high performance organic coatings on aluminum profiles and plates", and AAMA 620-02 "independent specifications for high performance organic coatings on coated aluminum coils" are two examples For less demanding applications, such as indoors, epoxy or polyester coating is common The army paints airplanes, tanks and other materials made of aluminum For example, in the U.S air force, a layer of strontium chromate epoxy primer and a layer of polyurethane finish are often applied The army has different requirements On the outer surface, it is applied with epoxy primer and then with chemical resistant coating In the commercial field, epoxy primer is often used with two-component polyurethane finish Most paint manufacturers supply products for these markets It is also possible to coat the pretreated aluminum with porcelain paint Some coatings are applied directly to the passivated surfaces, while others are applied with a primer followed by a color coat Porcelain enamels are cured at 120 ℃, but they have excellent durability and fastness In the past 25 years, powder coating has been applied to pretreated aluminum In most cases, powder coatings, often epoxy, polyester, TGIC or polyurethane, are applied directly to the prepared surfaces without primer at the beginning When aluminum plates or extruded aluminum profiles are to be used outdoors, especially when exposed to the sea, it is recommended to apply anti-corrosion primer first Most aluminum plates and profiles used for armrests, benches, billboards, etc are powder coated in this way Unfortunately, in my experience, professional painters don't always tell powder coating contractors that their products will be exposed outdoors, especially near the sea Shortly after the aluminum plates and profiles were installed, corrosion began, and large pieces of powder paint fell off (see Figure 2) The exposed metal was then corroded There is a dispute between the painting of aluminum ships and self-service painting The most frequently asked questions of the house owners and the on-site painting contractors are related to the painting of aluminum surfaces Because there is no simple way to get rid of the natural oxides on aluminum, there is a real problem DIY fanatics and field painters cannot apply irritating and often harmful chemicals, which can be controlled and applied in industrial situations After the faucet is cleaned, the aluminum can be degreased with a strong detergent containing soap solution For small amount of work, chemical degreaser can be applied through oil-free cloth and sponge Water cleaning is also relatively easy Although the surface is now free of grease, the stubborn and invisible oxides still exist, and the chemical conversion coating applied in industry cannot adhere It is also possible to rub the surface with non-metallic abrasive paper For thick aluminum plates and profiles, the surface can be sandblasted with non-metallic abrasives such as coarse sand and garnet, but this is time-consuming and expensive Moreover, the primer must be applied as soon as possible to prevent the re formation of natural oxides Grind the surface by hand or sand blasting with sandpaper to bring the shape shown in Fig 3 and Fig 4 to the surface Now it is mainly through mechanical adhesion rather than chemical adhesion to attach the primer layer Among the two adhesion principles, chemical adhesion is generally considered to be better, but the best adhesion comes from the combination of the two methods If you need to sand blast aluminum, do not use steel shot; some Abrasives will remain inside aluminum and form rust after rust and discoloration This may not cause any obvious paint problems, but it will at least damage the surface quality The vinyl wash resistant primer conforming to dod-p-15328 "primer (wash resistant) pretreatment (formula No 117 for metals)" was first determined during or after World War II and is still in use today The two-component primer contains less water and so much isopropyl alcohol that it is no longer allowed to be used by air pollution control agencies in most states of the United States For example, this highly volatile organic compound product is banned in California; however, historically, it has been very useful for pretreatment of aluminum substrates for painting purposes In the early 1980s, when light metals for computers flourished in Silicon Valley, California, thousands of gallons of wash resistant primers were used in substrates that were not easy to handle with conversion coatings By the mid-1980s, the product had been banned, and paint suppliers were trying to mix similar water-based products Although I have never tried them, I believe some products meet the performance requirements of dod-p-15328 specification It should be realized that no solvent based or water-based wash resistant primer can be applied on the existing conversion coating Wash resistant primers must be in direct contact with the aluminum substrate, otherwise they will fall off Moreover, the paint data sheet reminds you that the dry film thickness must be in the range of 8 – 13 microns The wash primer will not provide corrosion protection at thinner thickness, but at 13 microns and above, the film tends to crack, allowing all coatings above it to layer What's more, it's a regrettable experience for me to apply the wash resistant primer to the pre-treated surface after the thick epoxy primer; when the coating system is tested in the salt spray tank, there is a large area of blistering between the wash resistant primer and the epoxy resin There is no blistering when the same test is performed on paint systems that do not have a wash resistant primer applied Obviously, some new transparent siloxanes can be used in the field, and they are reported to produce excellent corrosion resistance One example is the distribution of prekotex it laboratory test results in April 2007, while carpenter chemical company published a paper on plaforization coating in 2006 Unfortunately, at this time I don't know if these products have been sold to the consumer market I recently used prekotex it on the handrails of buildings that have been severely corroded, but it's too early to publish the results In the initial laboratory tests, they passed the 1000 hour salt spray experiment (astmb-117 "operation standard of salt spray (FOG) instrument") Typical paint problems based on my experience, the most common problems on handrails and windows are due to the insufficient thickness of the paint layer at the cut end and sharp edge (90 degree bend) of the profile It is undeniable that because the paint is easy to separate, it is difficult to fully paint along the sharp edge For example, if a specification requires a film thickness of 366 microns, it is likely that the measured film thickness will be thinner than 305 microns along the sharp edge Corrosion often begins when moisture penetrates the paint along the sharp edges Aluminum oxide (white corrosion) begins to form under the paint layer and lifts the paint layer from the base layer After a period of time, often months rather than years, corrosion spreads under the paint film, constantly destroying more paint layers In the end, the problem is so widespread that all or most of the paint layers Extruded profiles usually have complex shapes, especially in products such as aluminum windows The areas susceptible to attack in the early stage of corrosion are internal angles, cracks, 90 degree bends and even more sharp angles When liquid paint and powder paint are electrostatic coated, these cracks, or the electric shielding area called Faraday framework, and the paint layer usually do not exist Obviously, if these areas will be exposed to corrosive environments, you can expect corrosion to start fairly quickly Once started, corrosion can gradually work under the adjacent well brushed paint film, causing corrosion and peeling Another common cause of paint problems is improper or inadequate