Plastic packaging and decoration screen printing technology (1)

Plastics have long been known as a class of polymer materials, but plastics that have been processed as packaging materials appeared in the 1960s. In addition to the basic properties of general packaging materials, plastics also have a variety of excellent properties that other packaging materials do not have, and no other packaging material is heavy, easy to rust, crushing, rot, leakage of the disadvantages, the scope of application continues to expand.
The plastic packaging forms are mainly divided into plastic films (thickness of 250 μm or less) and plastic sheets, hollow plastic containers, plastic turnover boxes, and plastic woven bags.
First, the plastic film screen printing decorative plastic film packaging is light and transparent, moisture, oxygen, airtight, can effectively protect the goods, show its style and color, and other advantages, more and more by the food, textile, clothing, etc. Many industries welcome.
Plastic film printing methods can be divided into two types, for large print batches, there is no special requirements of the printing of large-scale gravure printing linkage production line or flexographic printing production line; and for those who require special, fast-paced and small number of Prints are more suitable for manual screen printing.
1. Prepress processing technology of plastic film 1) Principle of prepress processing of plastic film Theoretically, the surface tension of the plastic film should be higher than the surface tension of the ink in order to obtain good printability of the surface of the plastic film. The PP and PE film-based non-polar high-molecular materials have stable chemical properties and small surface tension. In addition, the addition of an opening agent, an antistatic agent, and an anti-aging agent during the synthesis of the resin are difficult to adhere to the ink. The surface tension and printability of several commonly used films are as follows:
(Note: The surface tension of water is 0.0725N/m, and the surface tension of oil is 0.025 to 0.050N/m.) Therefore, the surface of the plastic should be treated appropriately according to the nature of the surface of the plastic in order to obtain excellent printability.
2) Surface Treatment Methods Prepress methods for plastics are mainly the following:
1 chemical treatment. The surface treatment with a chemical method produces a polar group on the surface of the solid and also gives the surface a certain degree of roughening. For example, PP, PE plastic film, because the surface is non-polar, and the chemical stability is high, it is difficult to penetrate or absorb ink solvent, and the bonding ink with the printing ink is relatively low, and even the ink may not be printed. . Therefore, the surface of the PP and PE plastic films should be surface-treated with an oxidizing agent prior to printing to form hydroxyl groups, carbonyl groups, and other polar groups on the surface, so as to improve the surface fastness of the ink and the plastic film.
Printing plants often use potassium dichromate-sulfuric acid solutions to treat polyolefin surfaces. The mechanism of this method can be explained by the fact that washing polyethylene with potassium dichromate-sulfuric acid can form complex root-like cavities on the cleaned surface, and the specific shape is related to the crystal shape of the surface being cleaned. This results in a certain degree of roughening of the plastic surface, which greatly improves the wetting and bonding properties of the surface. At the same time, oxidation occurs on the surface, which oxidizes the surface of the plastic film and generates oxygen-containing polar groups.
The use of this method for surface treatment, the formulation of the treatment liquid, the time and temperature of the treatment process all have a certain influence on the treatment effect. In general, the processing time is longer, the processing temperature is higher, and the effect is better. However, if the time or temperature is too high, although the surface polarity is increased, the surface of the film will also be oxidized and the bond strength will be reduced.
2 Corona treatment. The corona treatment is to apply high-frequency high-voltage electricity of 2 to 20 kV and 3000 to 20000 Hz to the discharge electrode to generate a large amount of plasma gas and ozone. These plasma gases and ozone interact with the surface of the plastic film to achieve the purpose of surface treatment. As the degraded small-molecule products continue to be discharged, the weight of the polymer also decreases. As processing time increases directly, the treated polymer surface becomes rough, forming small round pits. Actually equal to increase the surface contact area, so that wetting and adhesion improve and increase the bond strength.
3 photochemical treatment. Photochemical treatment is generally the use of ultraviolet radiation on the polymer surface caused by chemical changes, to improve the surface tension, improve the wettability and adhesion purposes. In general, the changes in chemical wettability and cohesiveness obtained by photochemical treatment of polyethylene and corona treated polyethylene are essentially the same. If you want a good photochemical treatment, you must choose the appropriate UV wavelength. For example, irradiation of a surface of polyethylene with ultraviolet rays having a wavelength of 180 nm can crosslink the surface of the polyethylene; however, if ultraviolet rays having a wavelength of 253.7 nm are used, it is difficult to have the same effect. Like corona treatment, UV irradiation can also cause cracking, cross-linking, and oxidation of the polymer surface, improving their wettability and bondability. Photosensitizers are used to make the photochemical treatment more effective. Benzophenone is a better photosensitizer. When treated with UV, it is sublimated and removed.
4 flame treatment. When the flame-retardant polyolefin film is treated, its surface is oxidized and some polar oxygen-containing groups are introduced. The oxidation process is carried out by a free-radical mechanism with the occurrence of a chain scission reaction. The reason for the improved wettability is the formation of polar groups; the improvement in adhesion is due to the polar groups improving their wettability and the corresponding diffusion of the chain-broken products.
The flame contains radicals such as O, NO, OH, and NH in the excited state. They can extract hydrogen from the surface of the polymer, and then undergo surface oxidation by the free radical mechanism. The reaction process has commonality with the corona treatment. The antioxidant in the polymer does not have much effect on the flame, but it can prevent thermal oxidation because the flame oxidation is caused by the large amount of free radicals present in the flame.
Flame Treated Polyolefin Table 1 Surface Tension Tester
2. Plastic film screen printing process 1) Screen printing ink should be based on different types of film selection of the corresponding type of ink, commonly used plastic screen printing ink type, suitable for printing substrates see Table 2. Table 2 Common Plastic Screen Printing Ink Models, Substrates, Manufacturers