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. It needs to be transferred to the next process for mechanical processing and made into various shapes of containers or other packages. In the machining process, the ink coating on the tinplate surface must be resistant to mechanical impact and extension. For special products such as canned containers, the ink coating must also have physical properties that withstand high pressure cooking. This requires excellent adhesion between the ink coating and the tinplate surface.
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 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 glossy and bright. Tin-plated steel sheet is divided into hot dip tinplate and tin-plated tin 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 the tinned tinplate can be easily opened 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, ink coating incompatibility, 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. The tinplate should be pre-baked and printed.
The second layer of the tinplate surface structure is an oxide film. After the tinplate tin plating process is completed, there is a thin oxide film on the surface of the tin. The composition of the tinplate 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 exquisitely-packed metal containers, the manufacturer must correctly select the tinplate materials 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 is small, and when the curing agent is not added, it is a thermal-polymerizable resin, it is not deteriorated by heat, and the water resistance is strong.
The use of excellent primer paint, but also a scientific and rigorous production process. The paint should be evenly coated on the surface of the tinplate. The thickness of the coating should be controlled at about 1.5~2g/m2. If the film is too thin, it will not work. If it is too thick, it will cause shrinkage stress and affect adhesion. The actual baking temperature should be about 10~15°C lower than the theoretical standard baking temperature, artificially leaving some unreacted polar groups in the paint, so that when overprinting the first color ink, the organic solvent in the ink The primer swells to make the interfacial polymer chains of the ink film entangled and enhance the adhesion of the coating.
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 formed. 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 acid. 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 tinplate printability 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 titanium dioxide pigment in white ink accounts for a large proportion 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 after 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 the manufacturers of optical oil materials to adjust the formulation, and also obtain good water resistance through the isolation of the cross bond structure.
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-based varnish, commonly used hydroxy acrylic acid such as acrylic acid ethyl ester and acrylic acid copolymer, the introduction of hydroxyl and carboxyl groups in the resin, the amino tree tire as a cross-linking agent. The varnish is not yellowed by high-temperature baking, has good heat resistance, acid and alkali resistance and water resistance, and is commonly 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 be sticky and has an anti-printing. The next step is due to the adhesion of the ink to make the tinplate minute. Zhang difficult, shadow