When steel drums are used to hold foods, steel drums are generally required to be internally coated, and the requirements for internal paints are also very strict. In the past, the best internal paint used in the domestic barrel industry was imported from Japan, but due to the high price, Gradually replaced by domestic coatings. However, in the past decade, domestic coatings have been ups and downs, and many internal coating products have been eliminated due to complex construction methods or poor application results. However, PVF coatings have become increasingly popular due to their excellent properties and good process performance. Applications.
PVF coating is the abbreviation of polyvinyl fluoride coating. It has excellent physical and chemical properties, aging resistance, chemical resistance, good adhesion, good impact resistance, and no toxicity. It is a new breed with promising development. Polyfluoroethylene coating has better bonding with metals. Due to the introduction of fluorine atoms in the structure, thermal stability and chemical stability have been significantly improved compared to vinyl coatings. Compared with other fluorine-containing coatings (such as polychlorotrifluoroethylene, polytetrafluoroethylene and perfluoroethylene propylene), polyvinyl fluoride coatings have the advantages of the lowest raw material cost, lower heat treatment temperature, simple construction, and dense coating. Easily eliminate pinholes. The main disadvantage is that the plasticizing temperature is relatively high, requiring a special drying room or the original drying room to be modified. However, the polyvinyl fluoride coating is used as a three-prevention coating as well as an anti-corrosion coating and a food coating.
Polyvinyl fluoride coating consists of polyvinyl fluoride resin, latent solvents, pigments and stabilizers. Solids ≥ 24%, specific gravity 1.2 ~ 1.3.
Polyvinyl fluoride coatings are usually applied by high pressure airless spray and air spray paint. Air spray has strong adaptability and high efficiency. The main disadvantages are large paint losses and large environmental pollution. Shaped steel drum products are best sprayed with mechanical devices. High pressure airless spraying is the best choice.
PVF coating construction process has a certain degree of particularity, mainly controlled from three links: 1. Surface treatment; 2. Spraying; 3. Baking and safety. The gesture is as follows:
Workpiece
↓
Surface treatment
Oil removal, rust removal, phosphatization, etc.
↓
Spraying
Two-spray two-bake
↓
Baking plasticization
240~270°C, 10~20 minutes
↓
cool down
Air or water
↓
Finished product
Barreled parts are required to be pre-treated in order to improve the adhesion of the substrate surface and the coating; different processing steps are taken depending on the material and the environment. After surface treatment and drying, PVF coating can be sprayed directly, and then heat treated in the drying room. When the solvent evaporates and the plasticization is completed, it can be cooled immediately. When baking, it is required to directly enter the specified temperature, without going through the heating and cooling process, and the time generally does not exceed 20 minutes. Some applications require the coating to have a certain protective thickness or give a full sense, so it is necessary to repeat the spraying once or twice Each time after spraying, it is heat-plasticized and cooled before it is sprayed twice.
The final use performance of polyvinyl fluoride coatings depends not only on the performance of the coatings themselves, but also on the construction process of the barrel manufacturers. For substances with strong corrosive and permeable properties, the coating must be uniform and a two-spray two-coat process is used, mainly to ensure that the coating does not have pinholes.
First, the surface treatment process before coating
How the bonding force between the surface of the steel drum and the coating is an important factor in measuring the quality of the construction. The coating with good bonding strength can not only withstand the mechanical impact, but even if some of the coating is subject to rust or harmful liquid penetration, it is not easy to spread to the surrounding. . No matter what kind of material, surface oils and scales need to be removed. The steel plate should preferably be phosphatized to enhance the adhesion of the coating.
Steel drum surface treatment is generally divided into chemical method and mechanical method:
1, the general method of chemical treatment process: degreasing → washing → to rust → washing → phosphating → washing → drying. Commercially available surface treatment agents are available on the market and suitable processes are used according to the instructions. Generally, for cold-rolled steel sheet, it should be focused on degreasing to ensure that the oil is completely removed; for hot-rolled sheet, degreasing and rust removal are indispensable. After phosphating, the phosphating film should not be too thin nor too thick. The main disadvantages of chemical methods are: large site occupancy; difficulty in wastewater treatment; some oil removers and phosphating agents require significant effects at a certain temperature. The Sino-Japanese joint venture Shenyang Paka Co., Ltd. produces a surface treatment agent with good quality, and now has matching flow production equipment.
2. Mechanical methods: There are many mechanical processing methods. It can replace several steps such as oil removal, rust removal, and phosphating. After coating, the surface of the object is soft and beautiful in appearance. The main disadvantages are: noise, dust, environmental pollution, high energy consumption, and the processed parts must be sprayed immediately to avoid rusting again.
There are many surface treatment methods, and the conditions and equipment of various units vary. Fortunately, polyvinyl fluoride coatings have no special requirements for surface treatment and are generally applicable to effective methods in the paint industry.
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