As mentioned above, there is an anti-corrosion coating in the anti-corrosion system of tinplate food cans, and its film-forming substance is mainly epoxy phenolic resin. Let's start with the anti-corrosion layer of epoxy phenolic resin.
In the national standard "Epoxy phenolic type coated tinplate steel sheet" (GB 8230-87), the coating amount of epoxy phenolic resin is divided into 3 levels.
The corrosion resistance of food cans is proportional to the thickness of the anti-corrosion coating. In order to ensure the final film-forming performance of the coating, the two-coat and two-bake or three-coat and two-bake coating process requires that the drying temperature during the first drying be 15-30℃ lower than the normal construction temperature. In addition, since the increase in coating thickness will reduce the adhesion of the coating, when the coating thickness exceeds the Class A standard, a matching inner coating primer is generally applied as the first coating to enhance the adhesion of the coating.
For tinplate food cans containing high-sulfur contents (such as asparagus, shrimps and crabs, and steamed eels), simply coating with epoxy phenolic resin is not enough to prevent tinplate and sulfur from reacting to form sulfides. Therefore, many manufacturers use the method of adding zinc oxide to the inner coating to form a high-sulfur resistant coating. The ratio of inner coating to zinc oxide is about 10:1 (the amount of zinc oxide can be appropriately increased or decreased according to the sulfur content of the content). It is required to be prepared and used according to the usage. Otherwise, it will thicken and deteriorate if it is placed for too long.
The anti-sulfur mechanism of adding zinc oxide to the inner coating is: zinc oxide absorbs H2S released from food and reacts to form white zinc sulfide, thereby preventing H2S and Fe from generating black iron sulfide, which plays an anti-sulfur role. The needle-shaped zinc oxide particles are also helpful in improving the sealing property of the coating.
For food cans containing luncheon meat, beef and mutton, in addition to paying attention to the above factors, some anti-sticking agents (such as synthetic wax) should be added to the coating to prevent the contents from sticking to the anti-corrosion coating.
The epoxy phenolic resin coating process mentioned above is mainly used for the coating of tinplate food cans, and is not recommended for the coating of aluminum food cans, because the density of epoxy phenolic resin and aluminum is not completely reliable. Therefore, the internal coating process of aluminum cans currently uses ethylene coatings, which have strong density.
