In recent years, the varieties and output of coated steel plates in China have been increasing, and a complete industrial structure has been formed. In the world, with the continuous expansion of the application of coated steel, it has begun to be applied to the
Packaging industry as a new type of packaging material. In China, as the coated steel plate has just been developed, its scope of application is still expanding. Due to the limitations of metal container packaging and processing technologies and costs, it is mainly applied in the fields of metal container cans and packing boxes, and the metal container packaging industry is widely used. The application has not yet begun, but with the continuous development of the metal container packaging industry, the widespread application of coated steel sheets in the metal container packaging industry will be just around the corner.
Because the coated steel plate is generally produced by the metallurgical enterprises, the painting process in the production of the product is omitted, and the cost of the packaging manufacturing enterprise is greatly reduced. According to calculations, the cost of packaged steel sheet packaging products has been reduced by 5% to 10%, saving energy 1/6 to 1/5. In particular, the pretreatment of thin steel sheet packaging products and the large investment in painting equipment have been saved, and the company's environment and workers' working conditions have been improved.
The coated steel plate combines the advantages of both organic coating and steel plate. It has both good colorability, formability, corrosion resistance, and decorative properties of the organic coating, as well as high strength and easy processing of the steel plate. Punching, bending, deep-drawing, welding and other processing, which makes the coated steel plate packaging products have excellent practicality, decorative, processability, durability.
First, the corrosion resistance of coated steel plate
1, the coating of the film under the corrosion process
Although the organic coating has the ability to isolate the steel and the external environment, generally speaking, the organic coating often has tiny pinholes. External moisture, steam, oxygen and various ions can penetrate the steel plate and the coating through the pinholes. The interface between, and the formation of local cells, sub-film corrosion occurs.
When the corrosion occurs under the coating film, the organic coating blisters. The bubbling position is consistent with the location of the anode and cathode of the local cell, the anode region is acidic and the coating tends to become brittle. The alkaline region is alkaline, and the coating softens and expands, deteriorating the adhesion. At the same time, due to the intrusion of water in the cathode region, the blisters are likely to grow up and blisters rupture, resulting in a large area of corrosion.
In addition to the physicochemical properties of organic coatings, corrosion rates of coated steel plates also depend on the resistance of organic coatings to gas, liquid, and ion permeation.
2, the coating's anti-corrosion mechanism
The corrosive reaction of steel in contact with water and air is the result of the action of local cells, the so-called electrochemical corrosion. The organic coating has the function of isolating the corrosive environment of steel and the outside world. When the ions are coated, infiltrated into the steel and coating interface and formed into a partial battery, the organic coating with high resistance value is equivalent to inserting a resistor between the two electrodes of the partial battery, making the local current difficult to flow and thus impeding the steel material. Corrosion.
It can be seen that the corrosion resistance of the coating is closely related to the composition, thickness, uniformity of the coating, and the binding force of the coating and the steel. Increasing the coating thickness and number of layers will reduce the actual presence of tiny pinholes in the coating. This will also increase the corrosion resistance of the coating. For example, a two-coat, two-coat, or three-coat, three-bake process can yield excellent corrosion resistance. Coated steel plate.
3, corrosion resistance of coated steel plate
Corrosion resistance of coated steel plates is usually expressed in the degree of corrosion damage such as loss of light, discoloration, powdering, cracking, cracking blisters, and rust.
There are many factors that affect the corrosion resistance of coated steel plates, including the types of substrates, types of coatings, the number and thickness of coating layers, the use environment of coated steel plates, and the machining, transportation, storage, and other aspects of coated steel plate applications. the elements of. From the analysis of the coating characteristics of the coated steel plate, the fundamental factor that determines the corrosion resistance of the coated steel plate is the structural characteristics of the coating and the corrosion resistance of the substrate. The protective properties of the coating under atmospheric corrosion conditions are mainly determined by the corrosion resistance or aging resistance of the coating.
Second, the application of coated steel plate in the metal container packaging industry
1, the performance of the metal container packaging on the coated steel plate
Since the metal container packaging product is the outerwear of the interior product, it must be beautiful and practical. The general requirements are mainly the higher requirements for the performance and quality of the coating film and the adhesion of the coating film.
Coating thickness requirements: In order to ensure adequate protection and service life of the coating, the coating thickness is 15~25μm.
Film color difference requirements: For different parts of the same roll of steel or different coils of the same batch of products, the requirements are the same or the difference is as small as possible, which can be monitored by the color difference meter.
Film gloss requirements: The gloss of the coating on the surface of the coated steel plate is related to the type of coating. Different users have different requirements for glossiness.
Film adhesion requirements: When the coated steel plate is used in the production of metal container packaging products, bending, deep drawing and other mechanical processing are required. Therefore, the adhesion of the coating film to the steel plate is required to be strong, and it is required to have good impact resistance and depth. Punching performance, coating process does not produce cracks or shedding.
In addition, the coated steel coating film must have a certain hardness, weather resistance, moisture resistance, water resistance, boiling water resistance, pollution resistance and heat resistance, cold resistance, self-extinguishing, fire resistance.
2. Processing method of coated steel plate in metal container packaging production
In order to prevent damage to the coating, producers of metal container packaging are most concerned with the method of connection between the various product components. At present, in the production of coated steel plates in metal boxes, metal barrels, metal cans, etc., the main methods of joining are crimping, riveting, welding, etc. Bonding methods are also available.
The method of sealing and sealing the bottom cover of steel drums is similar to the production of steel drums using ordinary steel plates. Riveting is used in the production of packaging boxes and other products, the same as the production of steel boxes using ordinary steel.
In order to make the coating non-destructive welding, there are currently two types of welding equipment.
The first is the capacitor discharge bolt welding. The electricity stored in the capacitor is added to the end with spiky bolts and coated steel plates. The capacitor generates an arc through the spur discharge and the bolt and the coated steel plate surface within a few milliseconds. Melted and welded together.
The second is resistance welding. First, several bumps are pressed with a punch at the place where it is welded, and then pressed with a copper electrode. The pulse current is applied. The cycle is only a few milliseconds. When the pulse current passes, heat is generated and the metal is melted and welded. Such as the commonly used projection welding machine and seam welding machine.
The common feature of these two kinds of welding machines is that welding is completed within 10 milliseconds. If heat is transmitted to the other side of the steel plate, the welding is completed, thus ensuring that the coating on the other side will not be damaged by high temperatures.
3, some common problems of coated steel plate in the production of metal container packaging
The raw material coated steel plate produced by metal container packaging should be used as soon as possible after entering the factory. Generally, it should be used within two months. Even if it is to be stored, the storage environment should be dry and kept relatively constant. The coils require vertical storage and transportation, and do not allow multiple stacking during storage. Do not store in the open after leaving the factory.
During storage and transportation, when subjected to pressure, the surface of the coating film loses its luster due to pressure. After the pressure is relieved, after a period of time, this phenomenon can be reduced or eliminated. If the temperature is above 30°C, the recovery time can be shortened.
The cuts in the coated steel plate or the small holes punched in the production of the metal container packaging reveal a metal cut. The incision is easily corroded and often causes the paint film to peel off. Can be painted at the incision, or use a rubber gasket or sealant at the punching area.
In the transport and processing of coated steel plates, individual scratches or contamination are unavoidable. If it is lightly polluted, wipe it with a rag and a cleaner. If it is heavier and scratches, wash the surface first. , and then painted to repair the paint. The repair coating must have good adhesion to the original coating and be consistent with the original color after application.
Third, the application and development trend of coated steel plate for metal container packaging
Metal cans are often used in food packaging and are usually coated to meet the requirements for hygiene, corrosion resistance and chemical resistance.
(1) Coating type:
There are many types of coatings and coatings used in metal can coatings. The usual coating is the addition of curing agents, plasticizers, and other ingredients to organic polymers, dissolved in solvents and thinners.
The main coatings are: 1 epoxy coatings, including epoxy phenols, epoxy ureas, epoxy esters, epoxy propylene, etc.; 2 vinyl coatings, including pure ethylene, modified ethylene, ethylene organosol, etc.; 3 Ester-based paints, including propylene ammonia, propylene epoxy resin.
In general, the inner surface of the can is mainly made of epoxy, vinyl, and polyester paints, and the outer surface of the can is mainly made of acrylic and polyester paints. In addition, a thermoplastic resin film having a thickness of several tens of micrometers can also be used on the coating sheet. Polyester represented by polyethylene titanate (PET) and resins typified by polypropylene (PP) are most preferably used from the viewpoints of processability and suitability for cans.
(2) Production and Coating of Cans
Adhesive cans are made of film-like nylon-based hot metal adhesive for can body bonding, combining the metal plate of the can with the organic material primer and adhesive. Adhesive cans are generally used for canned beer, carbonated beverages and other cold drink cans. In the case of high temperature (90°C) canning or high temperature sterilization (120°C), TFS, coating primers and adhesives need to be improved to ensure excellent performance in harsh environments at high temperatures. Bonding properties.
The solder cans are pressed together at both ends with a flat plate and heated by an external resistor or laser (small diameter can) to fuse the metal materials together. In order to avoid the adverse effects of lead on human health, the United States invents the fillet welding method and the Swiss invention seam welding method, in which the seam welding method has long been the mainstream.
Deep-flush pots require steel plates with good strength, processing properties, and corrosion resistance. After stamping, the wall thickness of the can body is only 40% of the plate thickness. The further development of deep-flush pots is: thin-walled lightweight, improved processability and improved corrosion performance.
Environmental issues are increasingly being taken seriously by people, and the production process of coated steel plates is also required to adapt to this trend. First of all, to reduce the volatile components in the paint, new paints such as water-based paints, quick-drying paints, and powder paints are used; secondly, paints with solvent discharge are not used, and instead, coating boards pre-coated on metal plates are used instead. . In 1990, Japan began to use thin film coated plates as deep-flush pots and plates for welding tanks, and its use gradually increased. In recent years, China has begun to promote its use.
The plastic coating layer of the alternative coating has good functions of rust prevention, decoration, heat insulation, shock absorption, and the like. The tin-free and aluminum alloy plates coated with polyethylene terephthalate (PET) and polypropylene (PP) can be used for caps for caps, DRD cans and spray cans.
It is coated with a polyester film coated steel sheet, which has excellent processing adhesion and processing corrosion resistance, and is suitable for the production of deep-impregnated tanks and welding tanks with large deformation and high corrosive contents. Another feature of the polyester film coated steel plate is its adaptability. The color, fragrance and taste in food are determined by the complex flavoring agent in the production process. The polyester film coating has the flavor in the food. Absorption rate is the smallest, which can maintain the delicate balance between the food ingredients in the tank and keep it in good condition for a long time.
In addition, the use of coated steel plate can be used for dry molding without excessive lubricant. This avoids the large amount of waste water polluting the environment caused by the cleaning of the lubricant. In short, plastic coated steel plate can improve the performance of food containers, reduce production costs, market demand will inevitably increase.
2005-11-14
