New subject of plastic film in UV ink printing

2022-06-22
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New topic of plastic film in UV ink printing

UV ink printing or UV glazing on the surface of plastic substrate is becoming more and more popular, and it is a challenging work. Plastic substrate materials have been used in the printing industry for decades. With the popularity of UV ink/varnish, UV ink printing on plastic substrate has put forward a new topic for printing plants and ink manufacturers. The biggest problem is how to make UV ink/varnish firmly adhere to the surface of plastic substrate, that is, adhesion

1. Surface tension of plastic substrate

in plastic printing, the surface tension of plastic substrate is the first factor to be considered. The surface tension of many plastic films is very low before treatment. Generally, surface treatment is required to improve the surface tension of the plastic substrate. After treatment, it is generally up to 40 dyne/cm or even higher. Usually, the plastic substrate supplier has carried out surface treatment on the plastic, but the treatment effect will be lost soon

for the printing plant, wiring processing is the best solution to ensure that the surface tension of the plastic substrate is still maintained within an acceptable range during printing. Corona discharge (oxidation) treatment is the most common treatment method. It is applied to the surface treatment of various plastic films without damaging those temperature sensitive plastic substrates. Corona discharge adopts high frequency and high voltage or medium frequency high voltage discharge to treat the plastic surface to activate it and make it porous, so as to improve the adhesion of the plastic film surface to the ink and improve the printability of the film. For example; After corona discharge treatment, the commonly used PE (polyethylene) films formed unsaturated bonds such as carboxyl group, carbonyl group and so on. After the surface is oxidized, PE molecules are converted into polar molecules, which increases the surface energy. Generally, the surface tension of PE is increased from 31 dyne/cm to dyne/cm after treatment. In this way, the dynamometer is attached to the movable 10 character head driven by electric motor or hydraulic device, and the wettability and adhesion of PE film to UV ink/varnish are added, so as to improve the firmness of imprinting

flame method is more used in the surface treatment of high-temperature resistant plastic products, so that the plastic can remove the oil on the surface and melt the thin layer on the surface under the action of instantaneous high temperature, so as to improve the inking ability; Chemical treatment is also used, but this method is usually used in conjunction with corona discharge treatment. In the chemical treatment method, the surface of polyolefin plastic is treated with oxidant to generate polar groups on its surface, so that the surface of plastic substrate can be well wetted and attached to UV ink/varnish. In order to test the treatment effect of the surface of the plastic substrate, the plastic film printing manufacturer had better be equipped with a dyne test pen

for the treated plastic film, the surface tension of the ink is lower than that of the plastic film. Similarly, in order to make the varnish wet well on the ink surface and firmly adhere to the ink surface, the surface tension of the varnish should be lower than that of the ink. Therefore, the UV ink formula is very exquisite in the selection of raw materials, in order to obtain acceptable surface tension. It is found that the surface tension of solvent based inks is generally lower than that of other types of inks, so they can be wetted and spread on the surface of a variety of plastic films. Therefore, surface treatment of the plastic substrate, especially UV ink printing, is particularly critical

2. Unlike many commonly used printing papers and paperboards, the surface of the plastic substrate does not allow ink/varnish to penetrate into its internal pores. However, some plastic substrates will swell when they come into contact with some raw materials of UV ink/varnish. Therefore, use this feature to make the plastic substrate work with some UV inks, so that the ink/varnish can penetrate into the interior of the plastic substrate. During curing, due to the internal penetration of the ink/varnish, a strong adhesion effect can be formed between the ink/varnish and the surface of the plastic substrate. In addition, the workshop temperature can be increased to enhance the penetration effect between the plastic substrate and the ink/varnish, which is also one of the ways to improve the adhesion between the UV ink/varnish and the plastic film

3. Glass transition temperature of ink (TG)

in physics, when some substances are lower than or equal to the "glass transition temperature" of the substance, glass will be formed. Compared with ordinary inks/varnishes, UV inks/varnishes generally contain some materials with smaller molecular weight, which form a thicker ink layer and a higher degree of crosslinking through reaction during curing. At the same time, after UV ink/varnish curing, the glass transition temperature (TG) of the film is higher and the film is harder, so it has good friction resistance and chemical corrosion resistance

if the Tg of the film layer is higher than the working temperature in the bronzing or coating process, the UV film layer in the finished product will not be able to firmly adhere to the gold foil or composite plastic film. UV ink/varnish prepared with low Tg materials can well adhere to gold foil and composite film in post press processing. Sometimes, in order to make bronzing or laminating work smoothly, the working temperature of the process has to be reduced

4. Curing degree

for any UV ink formulation, appropriate photoinitiators must be selected to optimize the performance of the cured ink layer. Each UV ink/varnish formulation works with a specific and sufficient amount of UV energy. If the spectral band and power of UV energy change, it will affect the performance of the cured ink layer

on the surface of the plastic substrate, you may feel that the UV ink/varnish has been cured, but in fact, it is likely that it has not been completely cured inside the entire film. The curing degree close to the bottom layer of the film is very important to obtain good adhesion. If the inside of the film and the bottom layer are not completely cured, the penetration of UV ink/varnish into the plastic substrate is meaningless. Once the curing energy changes, the surface tension of the film will also change, which will ultimately affect the successful completion of the subsequent post press processing

the plastic packaging market is an exciting market, and UV ink/varnish is a common material for plastic packaging and printing. Rapid curing means higher production efficiency. Drying ink without heat means less impact on the environment and workshop

adhesion is one of the few problems of UV ink/varnish in plastic packaging printing. The main reason is that printing customers constantly need new plastic substrate materials and new applications, which makes the adhesion of film become a common problem in UV plastic printing. As long as we fully consider the four problems mentioned above because this kind of equipment can meet a variety of standards, the problem of adhesion in plastic printing can be solved

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