Study on the process of coating PVF coating on steel drums for new foods (1)

Study on the process of coating PVF coating on steel drums for new foods (1)

Yang Wenliang

When steel drums are used to hold food, steel drums are generally required to be internally coated, and the requirements for internal coatings are quite strict. In the past, the best internal coatings used in the domestic barrel industry were imported from Japan, but due to the high price, Gradually replaced by domestic coatings. However, in the past ten years, domestic coatings have been ups and downs, many internal coating products have been eliminated due to complicated construction methods or poor application results, but PVF coatings have been more and more widely used for their excellent performance and good process performance. Applications.

PVF coating is the abbreviation of polyvinyl fluoride coating. It has excellent physical and chemical properties, anti-aging, chemical resistance, good adhesion, good impact resistance and non-toxicity. It is a promising new variety. Polyvinyl fluoride coating has good adhesion to metal. Due to the introduction of fluorine atoms in the structure, the thermal stability and chemical stability are significantly improved compared with ethylene coatings. Compared with other fluorine-containing coatings (such as polychlorotrifluoroethylene, polytetrafluoroethylene and perfluoroethylene propylene), the performance and advantages of polyvinyl fluoride coatings are: lowest raw material cost, low heat treatment temperature, simple construction and dense coating. Easy to eliminate pinholes. The main disadvantage is that the plasticizing temperature is high, and it needs a special drying room or the original drying room to be modified. However, the polyvinyl fluoride coating is used as a three-proof coating, as well as an anti-corrosion coating and a food coating.

The polyvinyl fluoride coating is composed of a polyvinyl fluoride resin, a latent solvent, a pigment, a stabilizer, and the like. The solid content is ≥24% and the specific gravity is 1.2~1.3.

Polyfluoroethylene coatings are usually sprayed with high pressure airless spray and air spray paint. Air spray is highly adaptable and efficient. The main disadvantage is that the paint loss is large and the environmental pollution is large. The shaped steel drum products are preferably sprayed by mechanical means, and the high pressure airless spraying is most ideal.

PVF coating construction process has certain special characteristics, which are mainly controlled from three aspects: 1. Surface treatment; 2. Spraying; 3. Baking and safety. The illustration is as follows:

Workpiece

↓

Surface treatment

Degreasing, descaling, phosphating, etc.

↓

Spraying

Generally use two spray two baking

↓

Baking plasticization

240~270°C, 10~20 minutes

↓

cool down

Air or water

↓

Finished product

The coated barrels are required to be pre-treated to improve the bonding strength between the surface of the substrate and the coating; different processing steps are taken depending on the material and the environment. After surface treatment and drying, PVF coating can be directly sprayed, and then heat-treated in the drying room. When the solvent is evaporated and the plasticization is completed, it can be immediately cooled. When baking, it is required to directly enter the specified temperature, without going through the process of heating and cooling, the time is generally not more than 20 minutes. In some applications, the coating is required to have a certain protective thickness or fullness, so it is necessary to repeatedly spray once or twice. It is sprayed twice after each plasticizing and cooling.

The final performance of the polyvinyl fluoride coating is not only determined by the performance of the coating itself, but also closely related to the construction process of the barrel manufacturer. For the corrosive and permeable substances, it is necessary to ensure uniform coating. The two-spray and two-coating process is adopted to ensure that the coating has no pinholes.

First, the surface treatment process before painting

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 adhesion not only can withstand mechanical impact, but also does not easily spread to the surroundings even if some of it is subject to corrosion or harmful liquid penetration. . No matter what kind of material, surface grease and scale need to be removed. The steel sheet should preferably be phosphatized to enhance the adhesion of the coating.

The surface treatment of steel drums is generally divided into chemical and mechanical methods:

1. Chemical treatment General treatment process: degreasing→water washing→rust removal→water washing→phosphating→water washing→drying. Commercially available surface treatment agents are commercially available, and suitable processes are employed according to the instructions for use. Generally, for cold-rolled plates, the focus should be on degreasing to ensure that the oil is completely removed. For hot-rolled plates, degreasing and derusting are essential. After the phosphating treatment, the phosphating film should not be too thin or too thick. The main disadvantages of the chemical method are: large site occupancy; difficult sewage treatment; some degreasers and phosphating agents need to be effective at a certain temperature. The surface treatment agent produced by Sino-Japanese joint venture Shenyang Paka Yujing Co., Ltd. has good quality and now has matching flow production equipment.

2, mechanical methods: mechanical treatment methods, it can replace oil removal, rust, phosphating and other steps, the surface of the object after painting is soft and beautiful. Main disadvantages: noise, dust, serious environmental pollution; high energy consumption; the processed workpiece must be sprayed immediately to avoid rusting again.

There are many surface treatment methods, and the conditions and equipment of each unit are various. Fortunately, polyvinyl fluoride coatings have no special requirements for surface treatment, and are generally applicable to effective methods in the paint industry.