Printing ink

Iron printing ink belongs to the lithographic ink category, because it is mainly printed on the principle of lithographic printing with a lithographic offset printing machine (printing speed is 40-80 rpm). Almost all tin products (such as food cans, thermos shells, toys, cosmetic boxes, etc.) can be printed with ink to beautify their packaging and decoration . In order to avoid unnecessary repetition, we will mainly discuss food can inks because it is widely representative.

Iron printing inks (some are called coatings) for food cans can be roughly divided into two contents , namely inner coatings and outer coatings. The outer coating can be divided into white ink (including printing type and roller coating type - the so-called Bai Ke Ding), color ink, varnish and so on.

(1) Internal coating

The content of the inner coating is more, this is because there are more types of food cans . For example, the inner coating of canned fish is not suitable for canned meat, the inner coating of beverage cans is not suitable for fruit and so on. This is determined by the different properties of food. For example, the inner coating used for canned meat must be able to withstand (resist) the action of fat and protein. The inner coating used for fish and crabs should be resistant to (anti) hydrogen sulfide corrosion. Beverages (alcohol, spit, etc.) must be resistant to solvents, carbon dioxide, etc. The fruit should be resistant to (anti) fruit acid and so on.

In fact, it depends on what kind of fruit you put on fruit, and what kind of fish you put on it. There are almost all kinds of fish. Therefore, the content of internal information is more natural. For example, the inner coating of fruit should mainly dry vegetable oil mixed with some rosin modified phenolic resin. For food containing inorganic acid and high-grade fatty acid, the inner coating based on phenolic resin should be used. The inner coating of beverage cans is mainly composed of epoxy resin and vinyl resin, etc.

In summary, the inner coating of food cans can be divided into four categories:

1. Fruit-type paints are packed with high-pigment, sulfur-free, jam-type plants , and low-acid products. This type of coating is mainly based on oleoresin-based binders.

2. Corn and grain type coatings: loaded with sulfur-containing foods such as corn and beans. Sulfur can form black iron sulfide with iron. This type of paint is also based on oleoresin-based binders, with some small particles of zinc oxide added. Zinc oxide can react with sulfur released from food to form white zinc sulfide .

3. General type: oleoresin type link material plus some phenolic resin.

4. Meat type coating; loaded meat or fish. Due to the need for corrosion protection, such coatings need to be highly crosslinkable and resistant to oils, greases and sulfides. This type of coating is mainly based on phenolic and epoxy-phenolic binders. The former is used for fish and livestock food, and the latter is mainly used for human meat food. Such paints are often mixed with some aluminum pigments to prevent sulfur pollution.

Some waxes can also be dispersed in this kind of Xu material to achieve the purpose of easy pouring out of the food in the can. This product is called release coating in my country's food industry. The release coating currently used in CHINA is composed of synthetic wax and cyclized rubber produced by the reaction of stearic acid and ethylenediamine . The interior coatings for aluminum cans containing wine and carbonated soft drinks are generally based on vinyl-based organic sols. The baking temperature is about 200 °C to 5 °C . The time is about 8 minutes. The viscosity index of the inner paint is 40-50 seconds (coating -4 cups/20°C). The thickness of the coating is about 0.31-0.93 mg/cm". Usually, Wuxi steel plates are coated thicker, while horse iron and aluminum plates are coated thinner .

It should be pointed out that the inner coating is in direct contact with food, so it needs to be non-toxic , tasteless, odorless, and cannot chemically react with the food it contains. Because the function of the inner coating is simply to protect the food.

(2) Exterior coating

In a nutshell, there are two functions of exterior coatings, that is, to increase the appearance and improve the durability . Therefore, the characteristics of the outer coating should be considered around the above two contents. The white ink is used as a primer to make the colored ink printed on the white more vivid. Of course, not all cans need to be primed. Some transparent colored inks are more distinctive when printed on metal.

Printing white ink can also be made , but the requirements for its color and the tendency to reduce yellowing are higher. However, due to the limitation of conditions, the white ink of printing type is still not as white as the white ink of roller coating type. It is no exaggeration to say that a large part of the appearance quality of printed iron products depends on the white oil printed on the bottom layer. Because this layer of white ink occupies a large area in most patterns , and the whiteness of the white ink will affect the vividness of the colored ink. We often see that people do not pay much attention to colored inks, but put a lot of energy into researching white inks. This is the reason.

Due to the white ink printed on the iron sheet by printing, it is not easy to get a very white result of trembling color (this is determined by the structure of the ink), and the printed ink layer is very thin, so it is often necessary to print two or three times. costs and so on. Therefore, the white ink of roller coating type was studied again . The roller method can apply the ink thicker, and in the formulation design the color of the oil can be made white as much as possible.

The white ink of this roller coating method is called White Coating (White Coating) in foreign countries. In fact, it can indeed be counted as a coating. The roller-coating type Baimeng is rolled on the iron sheet, so a large amount of solvent . Allowing the use of solvent provides convenience for the selection of light-colored resins, so that it is possible to design a product with a relatively white color.

The composition of white paint has three main parts, namely titanium dioxide, resin, and solvent.

There are two types of titanium dioxide: anatase type and all red stone type. The former is whiter in color but easy to pulverize . The latter has relatively large covering power and is relatively stable . In practical applications, the two are generally combined and used according to the situation.

Since solvents can be used, the choice of resins is expanded. It should be noted that, in addition to titanium dioxide, the color of the white paint mainly depends on the color of the resin in the joint. At present, the resins used in white paints generally include ethylene or ethylene (such as styrene ) modified alkyd resins, melamine resins, urea resins, epoxy resins, acrylic resins and polyester resins. Some ink factories in CHINA use coconut oil-modified alkyd resin, epoxy resin and melamine resin to mix, and satisfactory .

The viscosity of roller coating white paint is about 120-140 seconds (coating -4 cups/20°C). Generally speaking, the coating thickness is about 1.55-1.86 mg/cm*. The oolitic color of the white paint must not turn yellow after being baked at high temperature. The softness and adhesion are better, so as to be suitable for punching and pressing processing in the subsequent process. Gloss should be excellent. Due to the good quality and high gloss of the white paint, some two-piece cans do not need to be covered.

The use of solvents is relatively wide, and benzene, alcohols, ketones, esters, and petroleum solvents (aliphatic hydrocarbons) can be used. It depends on the structural requirements and drying requirements of the ink formulation.

→ Generally speaking, if the printed matter is required to have brightness, fastness, leveling and softness , a milder solvent can be used. Because the whiteness of the white pigment is still unsatisfactory. The coating film may . Therefore, some color pigments are generally added to this type of product to adjust the white hue, which is called a color correcting agent. If you want to get a warm color tone, you can add some red or purple pigments (such as quinazinone red, dioxazine purple, etc.). If you want to get a cool tone , you can add some phthalocyanine blue and other pigments. It is true that their dosage is very small, generally only a few thousandths or ten thousandths.