When metal cans are used for foods and beverages, organ […]
When metal cans are used for foods and beverages, organic protective coatings are usually applied inside and outside the tank wall. The inner wall coating is used to prevent the contents from corroding the tank wall, to avoid the elution of metal ions, and to protect the quality of the contents during storage. The coating film is often required to have excellent corrosion resistance, adhesion, mechanical resistance, heat sterilization resistance, and toxicological hygiene regulations. The outer wall coating is used to prevent rust outside the can, protect the printed film, increase the appearance and enhance the value of the product. The coating film is required to have good gloss, hardness, adhesion, color retention, and retort resistance.
Main types of inner wall coatings
The main types of interior wall coatings are: 1 oleoresin coating. It is usually prepared by reacting a drying oil (such as tung oil, linseed oil, etc.) with a resin (such as an oil-soluble phenolic resin) at a high ratio in a certain ratio. The oleoresin coating was the earliest (1903) used in-can coating. At that time, in order to solve the problem of red fruit fading, oleoic coatings were prepared using natural and dry oils, commonly known as hygienic tank coatings (R-coatings). In 1921, the addition of zinc oxide to the hygienic tank coating solved the blackening problem of corn cans, which was called C-magnetic paint. The oleoresin coating has been used up to now because of its simple preparation process and low price, and the coating film has good corrosion resistance and adhesion. The disadvantage is that the solder heat resistance is poor, and it is easy to bring odor to the food. The varnish varnish (sanitary tank coating) can be used in fruit, vegetable and juice beverage cans; the C-magnetic lacquer with zinc oxide can be used in sulphur-containing food cans such as meat, poultry, seafood, corn and asparagus. 2 epoxy-phenolic coating. It is prepared by epoxy resin and phenolic resin (such as phenol formaldehyde resin, bisphenol formaldehyde resin, polymethylol phenol ether resin, etc.) in a certain ratio. These coatings have comprehensive performance, especially adhesion, flexibility and corrosion resistance, so they can be used as general-purpose coatings for food cans, such as fruits, vegetables, meat, baby food, tomato products, etc. If zinc oxide or aluminum powder is added to the epoxy-phenolic paint, it can also be used for sulfur-containing foods such as fish and meat. 3 vinyl coatings. It is a coating containing a vinyl chloride copolymer (such as a vinyl chloride-vinyl acetate copolymer) and/or a polyvinyl chloride resin as a main component. There are two types of solution type and organosol type. Vinyl paint was originally used to solve the problem of beer turbidity and flavor deterioration caused by the incorporation of trace iron and tin ions in beer cans. The coating film has good compactness and flexibility, is colorless and has no odor, and has the disadvantages of poor adhesion to metal and thermal stability, and is usually only used as a surface coating. Currently used mainly in beer and soft drink cans. In addition, the inner wall coating of the can is also an acrylic paint, an alkyd paint, a phenolic paint, a bisphenolic acid paint, an epoxy amino paint, a polybutadiene paint, and the like.