Application of Pre-coated Steel Sheet in Food Container Packaging

In the production of metal cans, tinplate is widely used as a raw material, and the tinplate is applied to the tinplate prior to its formation. In general, a colored pattern is printed on the front surface of the can, an anti-corrosive paint is printed on the back surface, and then the can manufacturing is performed. .

The pre-coated steel sheets mentioned here are different from those of tinplate. It is a cold-rolled steel sheet, and most of it is pre-painted before leaving the factory. Normally, it is coiled steel sheet, and the whole coil is supplied. Various kinds of coatings are pre-coated on both sides of the steel sheet. It is not only widely used, but also has a much lower material cost than tinplate. This material is often called coated iron in China.

Pre-coated steel sheets used in the production of food cans are usually coated with organic coatings to meet the requirements of hygiene, corrosion resistance, and chemical resistance.

There are many types of coatings and coatings used in metal can coatings. The usual coating is the addition of curing agents, plasticizers, toners, and other ingredients to organic polymers, dissolved in solvents and thinners.

The main coatings are the following:

First, epoxy coatings, including epoxy phenols, epoxy ureas, epoxy esters, epoxy propylene, etc.;

The second is vinyl coating, including pure ethylene, modified ethylene, ethylene organosol, etc.;

The third is acrylic polyester coating, including diacrylamide, propylene epoxy resin and so on.

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. From the standpoint of processability and potability, polyesters typified by polyethylene terephthalate (PET) and resins represented by polypropylene (PP) are best used.

Pre-coated steel sheets can be used for the manufacture of a variety of food cans, and the requirements for coatings for different types of food cans are also different. The main applications are the following:

One is the adhesive can. The can body is bonded using a film-like nylon-based hot metal adhesive, which combines the metal plate of the can with the organic material primer and adhesive. Adhesive cans are generally used only for canned beer, carbonated drinks and other cold cans. If high-temperature (90°C) canning or high-temperature sterilization (120°C) is required, TFS, organic coating primers, and adhesives need to be improved to ensure excellent performance under severe high-temperature environments. Bonding properties.

The second is the welding tank. The two ends of the plate are pressed together and heated by an external resistor or a 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 inventions the seam welding method, in which the seam welding method has become the mainstream.

The third is a deep-drawn tank. Deep-flush pots require that the steel plates have good speed, processability, and corrosion resistance.

After stamping, the wall thickness of the can body is only 40% of the thick plate. The next development trend of deep-flush tanks is: thin-walled, lightweight, improved processing performance and improved corrosion resistance.

Food cans are available in three-piece cans and two-piece cans, and their coating is also different. The inner surface of the three-piece can body can be canned by cans or cans. The outer surface of the can body is treated with roller coating in consideration of rust-proof performance, appearance, and scratches during loading and unloading of cans. The welded metal cans need to be partially repaired to improve the inner corrosion resistance and the outer rust resistance.

The deep-impregnated cans in the two-piece cans are sprayed inside and outside after the cans are formed, while the outer surfaces are coated with curved rollers. The number of inside spraying varies depending on the corrosion resistance of the raw materials. Generally, the aluminum cans are sprayed once and the steel cans are sprayed twice. The outer surface of the bottom of the rust-proof steel can also be painted. In recent years, the development of organic coatings for the precoating of raw materials for metal containers has been very rapid, and environmental issues have increasingly attracted people's attention. Therefore, organic coating production processes must adapt to this trend.

First of all, to reduce the volatile components in the paint, use new paints such as water-based paints, quick-drying paints, and powder paints; second, instead of using paints that have been discharged by solvents, turn to use pre-coated organic boards on metal plates. . At present, China has begun to use thin film coated plates as deep-drawing tanks and welded tank thick plates, the amount of which is gradually expanding.

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 tanks, and sprayers. It is coated with a polyester film coated steel sheet, which has excellent processing adhesion and processing corrosion resistance. It 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, aroma and taste in food are determined by the complex flavoring agent in the production process. The polyester film coating has aroma in the food. Absorption rate is small, can make the various ingredients in the food tank to maintain a delicate balance between the long-term in good condition.

In addition, some experts believe that the use of coated steel cans can be dry-molded without excessive lubricant. In this way, large amounts of waste water can be prevented from polluting the environment by cleaning the lubricant. In short, plastic coated steel plate can improve the performance of food containers, reduce production costs, market demand will further increase.