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Adhesion problem in tinplate printing
The object of metal printing is a non-absorbent material, a low-carbon tin-plated steel sheet, commonly known as tinplate. The principle of tinplate printing is similar to that of lithographic offset printing, but there are special aspects 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;
2. The tin layer on the surface of the tinplate is bright and bright, and the black phase is analyzed by spectral characteristics. Therefore, a layer of white paint must be applied to the surface of the tinplate before color printing. Of course, sometimes according to the special artistic effect of the product pattern, it can also be partially coated with white paint or directly using the appearance of the tinplate tinplate itself;
3. After the tinplate is printed, the finished product. It needs to be transferred to the next process for machining, and it can be made into containers or other packages of various shapes. In the process of machining, the ink coating on the surface of the tinplate must be resistant to mechanical shock and stretch. Special products such as canned containers, etc., the ink coating must also have the physical properties of high pressure cooking. This requires excellent adhesion between the ink coating and the surface of the tinplate.
The adhesion of ink coating on the surface of tinplate has always been a problem that professionals have been discussing and studying in practice for a long time. It is related to the surface properties of the tinplate itself, the ink used in the printing process, the type of coating, the baking temperature and the thickness of the ink coating. Now I will talk about how to enhance the adhesion between ink coating and tinplate.
First, the relationship between the surface properties of tinplate and the adhesion of ink coating
Tinplate is a low-carbon steel sheet based on a steel plate and coated with pure tin on both sides to make it bright and shiny. The tin-plated steel sheets are divided into hot-dip tinplates and electroplated tinplates by different tin plating processes.
The structure of the tinplate is composed of a steel substrate, a tin-iron alloy layer, a tin layer, an oxide film, and an oil film. as the picture shows:
From the structure of the tinplate, the surface is coated with a very thin oil film, in order to prevent the tinplate from rusting during storage and transportation, so that it is easy to open the tinplate of the enamel during printing. Most of the 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 oil film thickness. Generally, the oil amount is controlled at 2 to 4 mg/m2, but some are above 6 mg/m2. If the oil is too much, it will affect the quality of the tinplate scouring, so that the coating and ink layer can not completely wet the surface of the tinplate, and a circular or elliptical pore defect is formed on the ink layer after baking and curing, commonly known as "eye hole". . The "eye hole" causes a decrease in the adhesion between the ink layer and the tinplate. Therefore, if the surface oil content of the tinplate is above 6 mg/m2, the tinplate is pre-baked before printing to evaporate the excess oil film. The pre-baking temperature is generally between 200 ° C and 15 min, and the vehicle speed is 4200 r / h.
In addition, due to poor production environment, such as tinplate due to moisture before the printing, the surface is covered with tiny water droplets, the seasonal climate is too cold, etc., will promote the incompatibility of the surface of the tinplate to the coating and ink coating, affecting its Adhesion. Similar to this phenomenon, pre-baking once before printing will basically be ruled out.
To test whether the oil film on the surface of the tinplate is qualified, the most easy way to master is the liquid tension and casting test of the coating. If you take a small amount of paint, slowly pour it onto the flat surface of the tinplate and observe the paint. At this point, the coating liquid quickly spreads to the surroundings, and the radius of the liquid center point is almost equal to the surrounding, and then the tinplate is lifted up to a 45 degree angle with the plane, and the liquid flows evenly to the other end, then the tinplate The amount of oil film meets the printing requirements. If the coating does not condense or irregularly spread around the surface of the tinplate, 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 surface structure of the tinplate is an oxide film. After the tinplate tinning process is completed, there is a thin oxide film on the surface of the tin, which is mostly composed of 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 is not unreasonable. After the tinplate is tinned, the passivation treatment is carried out. 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 can cause tinplate to change color during long-term storage, especially after printing and baking. Moreover, due to the structural instability, the ink layer on the oxide film is peeled off, so that the ink adhesion performance is lost, and product quality problems occur. Electroplated tinplate surfaces are typically chemically or electrochemically passivated. The hot dip tinplate is not passivated during the production process. Therefore, when manufacturing various kinds of exquisite packaging metal containers, the manufacturer should correctly select the tinplate materials according to the characteristics of product modeling and machining.
Second, printing paint. The relationship between ink and just focus
Tinplate printing, also known as printing iron, is a lithographic offset printing. The basic principle is to use the physical phenomenon of oil and water incompatibility to print. Therefore, in addition to the printing suitability of offset ink, the printing ink must also have several properties, such as heat resistance, solvent resistance, processability, and retort resistance. The team's printed tinplate products should be selectively used with paints and inks.
1. Primer coating
Primer coating is the first coating that is essential in the tinplate surface printing process. Its main function is to adhere to the surface of the tinplate firmly, and at the same time, it can easily adhere and bond with the inks of various colors on it, so that the tinplate printing products can be bent, impacted, stretched, crimped, etc. The coating is not damaged by machining. Therefore, the choice of primer coating requires a good wetting property on the surface of the gold chip, and it has an ideal mechanical strength after film formation. It is generally believed that resins such as alkyd and epoxy have these properties. This is because the resin structure contains an aliphatic hydroxyl group, an ether group, and an extremely active epoxy group. The polarity of the hydroxyl group and the ether group causes electromagnetic attraction between the resin molecule and the adjacent surface, and the epoxy group reacts with the metal surface containing the active hydrogen to form a chemical bond, and thus the adhesion is particularly strong. Further, when the resin is cured, no by-products are formed and bubbles are not generated, so that the shrinkage ratio is small, and when the curing agent is not added, it is a thermopolymerizable resin, which is not deteriorated by heat, and has high water resistance.
Choose a good primer coating, but also have a scientific and rigorous production process. The paint should be uniformly coated on the surface of the tinplate, and the thickness of the coating should be controlled at about 1.5 to 2 g/m2. If the coating film is too thin, it will not work. If it is too thick, it will cause shrinkage stress and affect the adhesion. The actual baking temperature should be lower than the theoretical standard baking temperature by about 10-15 °C, artificially leaving some unreacted polar groups in the paint, so that when the first color ink is overprinted, the organic solvent in the ink The primer is swollen to entangle the polymer chains at both interfaces of the film and ink to enhance the adhesion of the coating.
2. Printing ink
To choose the ink type reasonably. The surface of the tinplate is smooth and pound-like. Although the surface of the tinplate is coated with a primer coating before the color overprinting, if the adhesion of the synthetic resin to the surface of the tinplate is insufficient, the adhesion of the ink can not be produced. Meet the tinplate printing suitability. Generally, an oil-based alkyd resin binder is generally used.
Alkyd resin is a polyester resin modified with vegetable oil or fatty acid. It can be divided into two categories according to its properties: dryness and non-dryness. According to the amount of vegetable oil or fatty acid, it can be divided into short, medium and Three oil levels. Dry drunk acid resin mainly contains ingredients such as soybean oil and dehydrated castor oil in the ink. The resin can be self-dried or combined with amino tree tread. The printed iron ink produced by the high temperature curing has the characteristics of firm adhesion, high gloss, strong hydrophobicity, color retention and light retention, etc., which can meet the requirements of tinplate printing suitability and product processing.
Due to the special nature of the process, tinplate printing often prints a certain product, often multiple times of coloring, drying, and is a non-absorbent smooth surface. Therefore, the thickness of each color layer should be controlled during the construction process. The ink layer is thicker and the cohesion between the ink layers is large. Invisibly offsets the adhesion between the primer coating and the surface of the tinplate, resulting in a decrease in adhesion of the entire coating layer. Especially when coating or printing white ink, it is suitable because the proportion of titanium dioxide (titanium dioxide) pigment in white ink is large, and the thickness of ink layer should be controlled at (14±2) g/m2, which maintains the color of white ink and maintains ink. The adhesion of the layer.
3. Coated varnish
After the multi-color overprint of the tinplate is over. Also apply a layer of varnish to the surface of the ink. The coating varnish not only increases the gloss of the appearance of the tinplate product, but also makes it bright and dazzling, and some of the hydroxyl and ether bonds in the mooring penetrate into the ink layer under the surface tension of the ink, which enhances the flexibility of the surface of the ink layer. Impact strength, wear resistance, and ideal adhesion. In addition, the coating film formed by the varnish after high temperature curing isolates the ink coating from the outside, so that the surface of the tinplate product can resist physical and chemical substances.
Commonly used varnishes are mostly epoxy type, alkyd type, and acrylate type. The epoxy resin type varnish contains a fatty hydroxy group and an ether bond which are not easily hydrolyzed by the polar varnish, so that the coating film not only has good adhesion to the surface of the object to be coated, but also has high chemical resistance. Moreover, due to the rigid arrangement of the benzene nucleus 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 hydrophilicity, which requires the manufacturer of the varnish material to adjust the formulation, and also obtain good water resistance by the isolation of the cross-linking structure.
It is a medium- and short-oil alkyd resin containing linseed oil, coconut oil and medium and short-chain products of synthetic fatty acids. It has high saturation speed and good oxidation resistance. It is copolymerized with nitrocellulose, amino resin and pentaoxy resin to form a baked and dried coating film with strong adhesion, toughness, heat resistance, solvent resistance and weather resistance. Features. Used a lot in tinplate varnish...
Acrylic varnish, generally using hydroxy acrylic acid such as acrylic acid copolymerized with ethyl ester and acrylic acid, introducing a hydroxyl group and a carboxyl group into the resin, and using 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 commonly used for coating on tinplate for food cans.
Each type of Tushi varnish has its own advantages and disadvantages. In actual production, the type of varnish should be correctly used according to the characteristics and use of the printed product. However, no matter which type of varnish is used, the solid content of varnish, the purity of oil and the viscosity of coating, baking temperature and other technical data should be known before use to ensure the hardness (wear resistance) and gloss of the coating film. Degree and adhesion.
Third, the effect of baking temperature on the adhesion of tinplate
Tinplate printing, the surface ink is dried by means of heat curing, which is one of the main signs of metal printing that is different from lithographic offset printing. Since the paint and ink used in each process are different in resin and pigment composition, the baking temperature is not the same. Strict control of the baking temperature of each process is the key to the quality of tinplate products, which 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 to form a film. Not only does it have no adhesion, but also causes the backside of the tinplate to become dirty, and the reverse printing occurs. The next process is caused by ink sticking, so that the tinplate is divided. Difficulty, affecting the color (the back of the tinplate is the inner wall of the container, and the appearance of sticky dirt will contaminate the contents of the container). However, the baking temperature is too high, the chemical is broken at the bond, the ink film layer is brittle and dry, and it cannot withstand mechanical processing and high temperature sterilization and fall off. Therefore, it is necessary to strictly control the baking temperature of each process, and regularly check the temperature in 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 at 13 °C. At the same time, during the printing process, the circulating air volume should reach 21600m3/h. The electric heating tube, automatic temperature controller and temperature display of the drying room must be normal. The running speed of the machine should also be controlled within a certain range to ensure proper baking time of the ink coating.
Tinplate printing, its adhesion is good or bad, involving many aspects of the cause, there are objective existence, but also subjective. This article is only a personal opinion, and there are inadequacies, and it is expected to study and discuss with industry colleagues.
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