Application and construction of polyvinyl fluoride (PVF) coating on steel drum

Application and construction of polyvinyl fluoride (PVF) coating on steel drum

Chen Yanqing, Experimental Plant of Zhejiang Research Institute of Chemical Industry

First, the introduction of polyvinyl fluoride coating

Polyvinyl Fluoride is abbreviated as PVF. Molecular formula CHzCHFn. PVF is a thermoplastic fluoropolymer developed by our institute. Foreign developed countries such as the United States, Japan, Germany, and the Commonwealth of Independent States have small-scale production. Since a fluorine atom is introduced into the polymer molecular chain, the manufacturing process is simpler than other fluoroplastics, the raw materials are easy to obtain, the price is low, and the processing is relatively easy.

The raw material for preparing polyvinyl fluoride can be acetylene produced by adding calcium carbide with water, removing impurities such as sulfur, phosphorus and arsenic, and the other raw material is anhydrous hydrogen fluoride prepared by reacting fluorite with sulfuric acid, and using aluminum salt as catalyst in the tube reactor. Addition reaction is carried out. The prepared crude monomer is purified by pressure low temperature fractionation to a content of 99.9% or more. The polymerization pressure of fluoroethylene is low, using pure water as a medium, and a water-soluble azo salt as an initiator, which is formed by radical polymerization.

The polyvinyl fluoride resin is a white powder, and a latent solvent, a stabilizer, an antioxidant, a pigment and a small amount of other substances are thoroughly mixed and ground to form a thin paste-like dispersion.

Polyvinyl fluoride is a film former. Since the bond energy of the CF bond is large, the energy to be destroyed is also large, so its chemical stability is good. Fluorine has the smallest atomic radius in the halogen element, and is closer to the hydrogen atom, and it is easy to form a crystal lattice, so that the crystallinity of the polymer is high. Due to the presence of hydrogen bonds, the melting temperature of PVF is higher than that of ordinary plastics, and it is close to its decomposition temperature, so processing of fluorine-containing plastics is difficult. Due to the tendency of PVF to decompose to produce halides at higher temperatures, it will cause oxygen intrusion points and produce conjugated double bonds, which will lead to chain degradation reaction, which will cause PVF to rapidly crack discoloration, brittleness and coking, so it is necessary to add stability. Sex and antioxidants to absorb traces of halides that may be produced and to prevent oxygen from invading the polymer bonds. Sometimes a small amount of buffer is added to passivate or form a complex on the surface of the object to be coated, preventing the infiltration of water and corrosive media, and preventing diffusion even with a small amount of damage.

Second, the construction of polyvinyl fluoride coating

Polyvinyl fluoride coatings can be used as three anti-corrosion coatings, as well as anti-corrosion and food coatings. In the steel drum packaging of food, pesticides, chemicals, etc., it has been widely applied to the inner wall. The final properties of the PVF coating depend not only on the properties of the coating itself, but also on whether the construction process is correct and reasonable. Pay close attention to the quality of construction. When preserving corrosive and permeable substances, try to ensure the adhesion of the steel drum coating and no pinholes. In this way, you can get satisfactory performance and results.

1. Surface treatment technology

The bond between the metal surface and the coating film is an important factor in measuring the quality of the construction. The film with good adhesion not only can withstand the impact of the machine, but also does not spread to the surroundings even if it is rusted or liquid infiltrated somewhere. To remove the grease and scale on the metal surface, it is best to phosphating the metal surface to increase the adhesion of the film. There are many methods for surface treatment, and each unit is mixed. Generally, the effective method for the paint industry is applicable to PVF. It will not be stated below.

2. Spraying technology

PVF coatings can be applied by compressed air spraying, electrostatic spraying and high pressure airless spraying.

Compressed air spraying is available in both mechanical and manual spray applications. Manual spraying equipment is simple, but the thickness of the spraying is not uniform. Mechanical spraying can achieve semi-automatic operation, and the spraying efficiency is high. Electrostatic spraying method is a suitable construction method. The obtained coating film is uniform, and GDD-100 type fixed cup-type electrostatic spraying equipment can be used; Air spray is more suitable for the viscosity of the paint. A thicker coating film can be obtained at a time, and the coating film is uniform.

The spray booth is best attached. A device for recycling paint. Due to the difference between PVF coating and ordinary paint, the solvent volatilizes very slowly at normal temperature, and the surface will not harden and crust when exposed to air, so the scattered paint can be collected and recycled.

PVF coatings are not sticky and do not solidify at room temperature, so utensils, tools, clothing, and skin that have been exposed to paint and workpieces that require repainting are easily wiped off. Scrub with soapy water or alcohol. Since the PVF-coated workpiece and the baked workpiece are easily exposed to heat due to collision, the fixture, the shelf, and the like cannot be directly contacted. The spray site is required to be clean and prevent dust from adhering to the workpiece.

The working pressure of spraying construction is generally 0.15~0.25Mpa and the distance is 15-25cm. Do not spray too thick or too thin at one time. When it is too thick, the surface is prone to pitting and water wave under the impact of air current, and it is easy to cause sagging. It is applied to packaging steel drums, generally adopting two-jet two-bake process. Each coating thickness is controlled at 10^15 microns, and the thickness can be 20 to 30 microns twice. If the anti-corrosion performance of the steel drum is not high, it can be controlled at 15-20 microns. The surface is required to be sprayed evenly to improve the effect of the coating film. The polyvinyl fluoride coating is thicker than ordinary paint when it is left standing. Generally, it is possible to spray without adjusting the viscosity. If it is really viscous, add a small amount of dimethyl phthalate and mix well before use. The upper layer of the long-storing PVF coating has a clear liquid precipitated, and only needs to be stirred to restore the original without affecting the quality.

3. Baking and plasticizing technology and safety issues

The sprayed workpiece is allowed to be placed indoors, but it is easy to get dust, and it is best to bake it in time. Baking and plasticizing is to heat the sprayed workpiece in a drying tunnel or drying room to remove volatile components such as dimethyl ester. This is a key process for plasticizing film formation of PVF resin. The main factors affecting plasticized film formation are temperature and time.

1) Requirements for baking and plasticizing equipment. A continuous drying tunnel or a stationary drying oven can be selected depending on the yield and geometry of the workpiece to be coated. Continuous drying tunnels should be used for all units in condition. In order to ensure a good coating film and achieve safe production, baking equipment should meet the following requirements:

1 can reach a sufficiently high temperature (average greater than 270 ° C);

2 should set a good exhaust vent;

There should be no open flame in the 8 furnaces;

4 temperature control is sensitive and adjustable.

2) The temperature and period of baking. When the furnace temperature rises to the specified temperature, it can be fed into the workpiece. The baking temperature available is about 230-280 ° C, and the actual use is 240-260 ° C, depending on the specific conditions. The appropriate temperature depends on the thickness of the workpiece wall, the amount and material. If the wall thickness or the number is large, the heat capacity is large. After the workpiece enters and is easy to cool down, it is necessary to use a higher furnace temperature, sometimes 300 °C. It is not appropriate to lower the baking time by lowering the temperature. ―The baking temperature has a minimum limit. Bake at too low a temperature, and cracks appear at the corners, and the entire layer is cracked. Its adhesion is significantly reduced.

The baking time is generally 5 to 20 minutes. After the workpiece is placed in the drying tunnel for about 1-5 minutes, a large amount of smoke (dimethyl ester vapor) is started. After the latent solvent is volatilized, the baking is completed, and the workpiece can be taken out. The baking cycle is affected by factors such as furnace temperature, thickness and number of workpiece walls, and coating thickness. When the furnace temperature is the same, the wall thickness and the amount are large, and the coating thickness is prolonged. The same kind of workpiece has the same quantity and thickness. For example, if the furnace temperature is high, the baking time is short, and if the furnace temperature is low, the baking time is long. However, the baking time should not be too long, generally not more than 20 minutes. If it is more than 20 minutes, increase the furnace temperature. The temperature should not be too high, the furnace temperature is too high, the baking time is difficult to grasp, and the coating film is easily coked. According to different equipment, choose the best furnace temperature and baking time within a certain temperature range. This best condition is not difficult to master. If the baking time is too long, the film will be yellowed or coked, and the adhesion will be too short. Slight yellowing does not affect the quality of use.

The baked workpiece can be cooled naturally, blown or submerged. The advantage of quenching is to shorten the production cycle, reduce the hot and soft coating ash, reduce the coating of the coating by collision, and reduce the crystal mushroom of the polymer, thereby increasing the flexibility of the film.

Pay special attention to the safe work during the baking process. First of all, to prevent open flames. During the baking process, the dimethyl ester gas evaporated from the coating film contacts the open flame within a certain concentration range to cause combustion. Therefore, the following points must be noted during construction:

1 Use a heating wire below the rated voltage.

2 Prevent the heat element from being red.

8 Prevent the power connector from causing sparks due to poor contact. It is best to install the power connector outside the furnace wall.

4 Maintain sufficient exhaust air volume and discharge dimethyl ester gas in time.

5 control the number of coatings baked in the drying tunnel (drying room), can not be placed too much and too dense. Prevent the local concentration of solvent gas from being too high and catch fire.

4. Quality inspection method and defect treatment

1) Quality inspection method: The surface of qualified coating film should be flat, free of cracks, dirt and flaws, uniform color and not crisp. When you use your fingernails, leave no traces. Use your fingernails to scrape the corners and do not bulge or fall off. After cutting with a knife, you can't peel it from the crack.

Pinhole inspection:

Copper sulphate immersion method: immerse in 5% copper sulphate solution at room temperature for more than 24 hours. If copper particles are precipitated on the surface, it means pinholes.

Megohmmeter measurement method: use 1000 volt hand-operated megohmmeter, one pole is in contact with the metal of the workpiece, and one brush is connected to the salt water brush, and moves on the surface of the coating film. The pointer jumps to indicate that there is a pinhole in the location. A 1.5 volt dry battery can also be used with a sensitive galvanometer or microampere meter.

2) Common quality problems and treatment methods are as follows:

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