Surface performance and ink adhesion in tinplate printing

The object of metal printing is a non-absorbent material, a low-carbon tinplate, commonly known as tinplate. The printing principle of tinplate is similar to that of lithographic offset printing, but there is a special aspect in the process, such as:

1. After the tinplate is printed, the ink stays on the surface and must be dried and solidified by some form of heat energy;

2. The tin layer on the surface of the tinplate is bright and shiny, and its spectral characteristics are considered as the black phase. Therefore, before the color printing, the surface of tinplate must be coated with white paint. Of course, depending on the special artistic effect of the product pattern, it may be partially coated with white paint or directly using the tinplate surface itself;

3. Tinplate after printing the finished product. Need to go to the next process for machining, made into a variety of shapes of containers or other packaging. In the machining process, the ink coating on the tinplate surface must be resistant to mechanical impact and extension. Special products such as cans and containers, the ink coating also has the physical properties of autoclave resistance. This requires excellent adhesion between the ink coating and the tinplate surface.

4. The quality of the ink coating on the surface of tinplate has always been a long-term discussion and careful study by professionals in practice. It is related to the surface properties of tinplate itself, the types of ink used in the printing process, the type of coating, the baking temperature, and the thickness of the ink coating. Now on how to enhance the adhesion between the ink coating and tinplate, talk about personal superficial views.

First, the relationship between tinplate surface properties and ink coating adhesion

Tinplate is a low-carbon steel sheet with a steel plate as base material and pure tin plated on both sides to make it bright and shiny. Tin-plated steel sheet is divided into hot-dip tinplate and tin-plated plate by tin plating process.

The structure of tinplate is composed of a steel substrate, a tin-iron alloy layer, a tin layer, an oxide film, and an oil film.

From the structural point of view of the tinplate, its surface is coated with a thin layer of oil film in order to prevent the tinplate from being rusted during storage and transportation, so that it is easy to lift the tinplates of the tinplate one by one during printing. Most oil film components are dioctyl sebacate or acetyl tributyl citrate. At present, tinplate produced in various countries in the world has no certain standard for the thickness of the oil film. Generally, the amount of oil applied is controlled at 2 to 4 mg/m2, but some of them are above 6 mg/m2. If too much oil will affect the quality of tinplate erosion, so that the paint, ink layer can not fully wet the surface of tinplate, in the curing of the ink layer after the formation of circular or oval pore defects, commonly known as "eyehole" . The "eyehole" will cause the adhesion between the ink layer and the tinplate to decrease. Therefore, when the tinplate surface oil is over 6mg/m2, the tinplate must be pre-baked once before printing to make excessive oil film volatilize. The pre-baking temperature is generally between 200°C and 15min, and the speed is 4200r/h.

In addition, due to the poor production environment, such as the tinplate covered with fine water droplets before printing due to moisture, the seasonal climate is too cold, etc., will prompt the tinplate surface coating and ink coating insoluble, affecting Adhesion. Similar to this phenomenon, pre-baking before printing is basically ruled out.

The easiest way to detect the quality of the tinplate surface oil film is the liquid tension and casting test of the paint. If you take a small amount of paint, pour it slowly onto the flat surface of the tinplate and observe the paint. At this point, the coating liquid spreads rapidly to the surroundings, and the radius of the center point of the liquid is approximately equal to the circumference. Then, the tinplate is lifted up to make a 45-degree angle with the plane. At this time, the liquid flows evenly to the other end. Then the tinplate The amount of film meets the printing requirements. If the paint spreads on the surface of the tinplate or spreads irregularly to the surroundings, the amount of oil film does not meet the requirements before printing, and the tinplate should be pre-baked and printed.

The second layer of the tinplate surface structure is an oxide film. After tinplate tin plating process is completed, there is a very thin oxide film on the surface of tin. Its composition is mostly divalent tin or tetravalent tin oxide and its hydrate, and the structure is very unstable. Some people think that the thinner the oxide film, the better the adhesion, and vice versa. This argument is not without reason. After the tinplate is tin-plated, passivation treatment is performed. The purpose of the passivation treatment is to stabilize the surface of the tinplate and prevent the growth of the surface oxide film. Excessive oxide film will discolor tinplate during long-term storage, especially after printing and baking. Due to the structural instability, the ink layer on the oxide film is peeled off, the ink adhesion performance is lost, and the product quality problem occurs. Electroplated tinplate surfaces are usually chemically or electrochemically passivated. Hot dip tinplate is not passivated during production. Therefore, when producing various kinds of exquisite packaging metal containers, the manufacturer must correctly select the material of tinplate according to the characteristics of product modeling and machining.

Second , printing paint. Ink and the relationship between

Tinplate printing, also known as imprinting iron, is lithographic offset printing. The basic principle is to use the oil-water insoluble physical phenomenon to print. Therefore, in addition to the printability of offset printing inks, iron printing inks must also possess several properties such as heat resistance, solvent resistance, processability, and resistance to retort. The team printed tinplate products should use paints and inks selectively.

1. Primer coating

Primer coating is an indispensable first coating in the tinplate surface printing process. Its main function is to firmly adhere to the surface of the tinplate, and at the same time, it can be easily attached to and adhered to the various inks on the tinplate so that the tinplate printing product can be bent, impacted, stretched, crimped, etc., and the ink is formed. The coating will not be damaged by machining. Therefore, the choice of primer coating requires the paint to have good wettability on the surface of the gold crumb, and it has a very good mechanical strength after film formation. It is generally believed that alkyd, epoxy and other resins have these properties. This is due to the fact that the resin structure contains aliphatic hydroxyl groups, ether groups, and extremely reactive epoxy groups. The polarity of the hydroxyl group and the ether group causes the electromagnetic attraction between the resin molecule and the adjacent surface, and the epoxy group reacts with the surface of the metal containing the active hydrogen to form a chemical bond, so the adhesion is particularly strong. In addition, there is no generation of by-products during the curing of the resin, and no bubbles are generated. Therefore, the shrinkage rate is small, and when the curing agent is not added, the resin is a heat-polymerizable resin, does not deteriorate due to heat, and has strong water resistance.

2. Printing ink

It is necessary to reasonably select the ink type. The surface of the tinplate is smooth like a pound. Although the surface of the tinplate is coated with a primer before color overprinting, if the adhesiveness of the synthetic resin to the tinplate surface of the connecting material is not taken into consideration, the ink adhesion produced cannot be produced. Satisfy the printability of tinplate. It is generally advisable to use an oil-modified alkyd resin binder-based ink.

Alkyd resins are polyester resins modified with vegetable oils or fatty acids. They can be classified into dry and non-dried properties according to their properties; they can be classified as short, medium, and large according to the amount of vegetable oil or fatty acids. Three kinds of oil length. Dry intoxication resin in the ink mainly soybean oil, dehydrated castor oil and other ingredients, this resin can be dry, it can also join the amino tree and tire combination. The printing ink produced by the high temperature curing, has a strong adhesion, high gloss, strong hydrophobicity, color retention and other characteristics, to meet the printability of tinplate and product processing requirements.

Due to the particularity of its process, tinplate printing often requires multiple color registrations, drying, and non-absorption of the smooth surface. Therefore, the thickness of each ink layer should be controlled during the construction process. Thick ink layer, the cohesion between the ink layer is large. Imperceptibly offsets the adhesion between the primer coating and the tinplate surface, resulting in reduced adhesion of the entire coating layer. In particular, when coating or printing white ink, it is appropriate to maintain the white ink color and maintain the ink because the proportion of dioxy titanium (titanium oxide) pigment in white ink is large and the ink layer thickness should be controlled at (14±2) g/m 2 . Layer adhesion.

3. Varnish coating

Tinplate after multicolor overprint. Also apply a layer of varnish to the surface of the ink. The coated varnish not only adds luster to the appearance of the tinplate product, but also makes it bright and dazzling. Moreover, some of the hydroxyl and ether bonds in the mooring penetrate the ink layer under the effect of the surface tension of the ink, enhancing the flexibility of the ink layer surface. Impact strength, wear resistance, to achieve the ideal adhesion. In addition, the coating film formed by the high-temperature curing of the varnish isolates the ink coating from the outside, so that the surface of the tinplate product is resistant to physical and chemical substances.

The commonly used varnish is mostly epoxy type, alkyd type, acrylate type. Epoxy varnish contains high-polarity aliphatic hydroxyl and ether bonds that are not easily hydrolyzed, making the coating not only has good adhesion to the surface of the article to be coated, but also has high chemical resistance. Due to the alternating arrangement of the rigid benzene core and the flexible hydrocarbon chain, the physical and mechanical properties of the coating film are enhanced. Of course, due to the presence of polar groups, the coating film will have a certain degree of hydrophilicity, which requires manufacturers to adjust the gloss oil material to adjust the formula, through the isolation of the bond structure, but also get good water resistance.

Inulin resin type varnish containing medium and short chain products of castor oil, coconut oil, and synthetic fatty acids. It is a medium and short oil alkyd resin. High saturation speed, good resistance to oxidation, co-polymerization with nitrocellulose, amino resin, penta-oxide resin, etc., making baked dry coating film with firm adhesion, toughness, heat resistance, solvent resistance, weather resistance, etc. Features. It is widely used in tinplate varnish.

Acrylic type varnish, generally used hydroxy acrylic acid such as acrylic acid ethyl ester and acrylic acid copolymerization, the introduction of hydroxyl and carboxyl groups in the resin, amino tree tire as a crosslinking agent. The varnish is not yellowed by high-temperature baking, has good heat resistance, acid and alkali resistance, and water resistance, and is often used for coating on tinplate used for food cans.

All types of Tuoshi Varnish have their own advantages and disadvantages. In actual production, the type of varnish should be used correctly according to the characteristics and application of the printed product. However, no matter which type of varnish is selected, the solid content of the varnish, the purity of the varnish, the viscosity of the coating, the baking temperature and other technical data should be known before use so as to ensure the hardness (abrasion) and luster of the coating film. Degrees and adhesion.

Third, the baking temperature on the impact of tinplate adhesion

In tinplate printing, the surface ink is dried using a heat-cured form, which is one of the main-to-temporal signs that distinguishes metal printing from lithographic offset printing. Since the paint and ink used in each process contain different resins and pigments, the baking temperature is not the same. Strictly controlling the baking temperature of each process is the key to the quality of tinplate products. It will directly affect the adhesion of the entire ink film layer. If the baking temperature is low, the chemical molecular bonds in the resin are not sufficiently cross-linked and cured to form a film. Not only does it have no adhesive force, but it also causes the back of the tinplate to become sticky, and the reverse printing occurs. The next process is due to ink stickiness, which makes the tinplate minute. Difficulties in the sheeting affect the color registration (the back of the tinplate is the inner wall of the container and the presence of sticky stains can contaminate the contents of the container). However, the baking temperature is too high, the chemical breaks down at the bond, the ink film layer becomes brittle and cracks, and can not withstand the mechanical processing and high temperature sterilization. Therefore, it is necessary to strictly control the baking temperature of each process, and regularly check the temperature inside the drying room, and strive to achieve the error between the measured temperature value in the drying room and the temperature value of the drying room temperature display instrument at 13°C. At the same time, during the printing process, the circulating air volume should reach 21600m3/h. The electric heating tube, automatic thermostat, and temperature indicator of the drying room must be normal. The speed of the machine should also be controlled within a certain range to ensure that the ink coating has a suitable baking time.

The printing of tinplate, its adhesion is good or bad, involves many factors, there is an objective existence, but also subjective. This article is only a personal opinion, which has certain deficiencies, but also hope to study and discuss with industry colleagues.

Tinplate printing, good adhesion