Talking about Some Problems Affecting the Adhesion of Metal Printing

The object of metal printing is non-absorbent material—tinplate tinplate. The printing method is similar to lithographic offset printing, but it also has its unique aspect in the process, such as:

1. After the tinplate is printed, the ink stays on the surface and must be heated to cure;

2. Because the tin layer on the surface of the tinplate is bright and shiny, it is customary to apply a layer of white paint to the iron sheet before the color printing. Of course, this must be determined according to the requirements of the printed matter, or it can be directly applied to the front side of the iron sheet, which is also known as the tin-plated surface;

3. After all the processes are printed, they need to be machined to produce packages of various shapes and sizes. After the coating is mechanically processed, it must meet the requirements of impact resistance and elongation resistance; The grades also need to be autoclaved; this requires good adhesion between the coating and the substrate. The adhesion of the coating to the substrate is related to the surface properties of the tinplate itself, the type of ink, the baking temperature, and the thickness of the ink coating.

Now on how to enhance the adhesion between the ink coating and the substrate (ie, tin plated tinplate), learn from experts in your industry:

One, the relationship between the surface properties of tinplate and the adhesion of ink coating

Tinplate is made of thin steel plate as the base material. Both sides are coated with pure tin to make it bright and bright. According to the different process, it can be divided into hot dip plate and plated plate; mainly by steel substrate, tin-iron alloy layer, oxide film, Oil film and tin layer.

First of all, the surface of tinplate is usually coated with a thin film of oil. Most of its components are dioctyldmonate or acetyl tributyl citrate. The purpose is mainly to prevent the tinplate from rusting, and it is also easy to separate the tines into one piece when printing. Usually the thickness of the oil film is not fixed and should be controlled around 3-5mg/m2. Should not be too thick so as not to affect the wettability of the coating on the surface of the substrate, to prevent printing defects such as shrinkage cavities after the baking and drying, due to changes in the seasonal climate such as the environment, tinplate also easily damp, directly affect the coating The incompatibility leads to poor adhesion between layers. For this reason, many can makers pretreat tinplates with a film over 5-7mg/m2 and make the oil film completely volatile before use. The pretreatment temperature is generally within 200°C/15min, and the speed is controlled at 4000r. Less than /h is appropriate.

The most common and straightforward way to check the quality of tinplate film is to take a small amount of paint and place it on a flat tinplate. Observe its diffusion. Generally, the radius around the center point is roughly equal. At this time, one corner of the tinplate is lifted. At an angle of 45°C, the paint can flow evenly in the other direction, which means that the amount of oil film meets the printing requirements. If it spreads or irregularly spreads to the surroundings, it means that it does not meet the printing requirements and should be pre-baked before it can be used.

The second layer of tinplate is an oxide film. After the tin plating process of tinplate is completed, a thin oxide film is left on the surface of the tin. The structure of the tinplate is very unstable, which easily causes the coating to fall off and affect its adhesion. Therefore, the tinplate must be passivated after each tin plating is completed, in order to increase its stability; too thick oxide film tends to make the tinplate color, especially after high-temperature baking; in general, plating The board is passivated, while the hot dip board is just the opposite. Therefore, to choose the appropriate tinplate must be decided according to the customer's different requirements for the printed matter.

Second, the relationship between printing ink ink and adhesion

Tinplate printing is commonly called printing iron. The production of such products is called printing iron ink. It belongs to lithographic offset printing method and uses the working principle of oil and water repulsion. The requirements for such products are also relatively high. They must have the following properties: Heat resistance , Solvent resistance, mechanical processing resistance and resistance to retort, etc. Therefore, the printing plant should be required to selectively use Indian iron products according to the different requirements of the printed products.

Base coat

Primer coating is the first process in tinplate printing. It helps the adhesion between the surface coating and the ink, so that the coating should not be damaged during various mechanical processing and molding processes. Therefore, the use of primer is recommended. The coating must first consider its wetting properties on the metal surface and have good mechanical processing properties after film formation.

It is usually formulated with alkyd, acrylic, and polyester resins, supplemented by an epoxy resin, because the structure of these resins contains aliphatic hydroxyl groups, ether groups, and extremely reactive epoxy groups, making the resin molecules Electromagnetic attraction occurs between adjacent surfaces, and the epoxy group reacts with the surface of the metal containing active hydrogen to form a chemical bond. Therefore, the adhesion and water resistance are particularly strong. No by-products are generated, no bubbles are generated, and the shrinkage is small. , will not deteriorate due to heat.

In the normal printing process, in addition to the selection of excellent primer, scientific and rational production process, the thickness of the film, the uniform coating and the baking temperature are all key, especially the thickness of the film. Thinning does not work, thick coating can easily cause shrinkage of the coating film, affect its adhesion; baking temperature in theory should also be a bit lower than the actual temperature, but the amplitude should not be too large, generally controlled at 10 ~ 15 °C Left and right, artificially left unreacted completely polar groups, so that in the overprint color, the organic solvents in the ink can swell the primer, so that the two interfaces between the coating polymer chain is more encrypted, enhanced between the coating Adhesion.

2. Printing ink

Since the tin plating layer of the tinplate is relatively smooth, although a layer of base oil has been used, if the adhesiveness of the synthetic resin to the surface is ignored during the ink production process, the produced ink cannot satisfy the printing adaptability. .

Usually, the metal printing ink is selected from the oil-modified alkyd resin, which is modified and polymerized with vegetable oil or fatty acid. According to its performance, it can be divided into two major categories: dry and non-dry; The amount of vegetable oil or fatty acid can be divided into short, medium, and long oil degrees. The dry resin is mainly composed of soybean oil, dehydrated castor oil and other ingredients. It can be used dry or it can be used together with amino resin. After curing, it has the characteristics of firm adhesion, high gloss, strong hydrophobicity, good gloss and color retention, etc. It can basically meet the printability of tinplate and the requirements of mechanical processing of products.

Due to the particularity of its process, the printing of tinplate often requires overprinting, drying and non-absorptive smooth surfaces. Therefore, the thickness of the ink layers should be controlled during the printing process. If the layer is too thick, the cohesion between the layers is large, virtually affecting the adhesion of the entire coating to the surface of the substrate, directly leading to the decrease of the adhesion of the entire coating, especially when printing white ink, due to the white ink. The proportion of titanium dioxide is relatively large, and its ink layer thickness should be controlled at 14±2g/m2. Only in this way can the color of the white ink be guaranteed, and the adhesion of the ink layer can be maintained.

3. Cover oil

After a variety of color overprinting, the surface of the ink needs to be coated with a protective film, which is a varnish. After applying varnish, it not only increases the aesthetics of the entire print, but also achieves the desired adhesion. The key is that its gloss, flexibility, wear resistance, impact strength and so on have been greatly improved.

The commonly used varnish is composed of the following three major types of resins:

A. Epoxy resin: Because of its relatively high polarity and the aliphatic hydroxyl and ether bonds that are not easily hydrolyzed, it not only has good adhesion and chemical resistance, but also has a rigid benzene core and its structure. The alternating arrangement of flexible hydrocarbon chains promotes enhanced physical and mechanical processing properties. However, due to the presence of polar groups, the coating film has a certain degree of hydrophilicity, and the water resistance can generally be solved by the isolation of the cross-linking structure.

B. Alkyd resin: Because it contains castor oil, coconut oil, and short-chain products of synthetic fatty acids, it has high saturation, good oxidation resistance, etc.; it is made by copolymerizing with amino resin, epoxy resin, etc. The coating has the characteristics of firm adhesion, toughness, heat resistance, weather resistance, and solvent resistance, and is widely used in the printing iron industry.

C. Acrylic resin: This kind of resin generally uses hydroxy acrylates, such as hydroxy ethyl acrylate and acrylic acid copolymerization, the introduction of hydroxyl and carboxyl groups in the resin, amino resin as a cross-linking agent, such varnish oil is commonly used in the printing of food cans. , Has the characteristics of acid and alkali resistance, good water resistance, no yellowing under high temperature, and meets the health standards.

In fact, no matter which type of varnish oil has its own advantages and disadvantages, it is necessary to select the appropriate varnish according to the different requirements of the printed matter, but no matter how you choose, you must first understand the various technical parameters of the product. So that printing can proceed normally.

Third, the baking temperature on the impact of adhesion

Under normal circumstances, the coating of the printed iron products is dried in the form of heat curing, which is one of the main differences between offset printing inks. Since the raw materials used in each product are different, the baking temperature is also followed. Variety. The baking temperature is the key to the quality of the product throughout the printing process:

a. If the temperature is too low, the chemical molecular bonds in the resin are not sufficiently cross-linked and cured to form a film, not only in terms of non-adhesion, but also cause backside smudging and reverse printing; and in the next printing process, due to the ink Difficulties arise in the adhesion; it is particularly critical that the back of the sheet is the inner wall of the container. If it is a food can, it will directly contaminate the content it holds.

b. If the temperature is too high, the molecular bonds will break, the coating will become brittle, and the coating will fail to withstand mechanical processing and high-temperature cooking to produce explosive paint. Therefore, the baking temperature error should be controlled between ± 3 °C, the circulation of the air volume should also be controlled at 21600m3 / h, in addition, the machine should also be regularly checked the speed of the machine to ensure that each coating Have a suitable baking time.

All in all, the quality of its adherence involves many factors, both objectively and subjectively. The answer to this question is that people are knowledgeable and they have their own ideas. The above text is only representative of my personal point of view. Due to the limitations of knowledge, there are places where people are unsatisfactory. I urge my colleagues in the industry to criticize and correct me and learn together.