Introduction: UV ink printing or UV glazing on the surface of plastic substrates is becoming more and more popular, and this is a challenging job. Plastic printing materials have been used in the printing industry for decades. UV ink printing on plastic substrates poses new challenges for printers. Regarding UV plastic printing, this article shares four points for attention. The contents are for friends’ reference :

UV printing

UV printing, curing the ink containing photosensitizer on the surface of the printed matter by ultraviolet light drying, thus forming a layer of extremely light and artistic bright film on the surface of the printed matter; the surface is convex and elastic, which can better highlight the graphic parts Subtle levels and graphic outlines.
The English full name of UV is the abbreviation of ultraviolet (ultraviolet), which belongs to a special ink in printing, also known as UV ink. To UV means to pass a layer of UV ink on the surface of the printed matter. The standard term in the printing industry is called UV glazing.

Surface tension of plastic substrate
When printing plastics, the surface tension of the plastic substrate is the first factor to be considered first. Before being processed, many plastic films have low surface tension, and generally need to be surface-treated to increase the surface tension of the plastic substrate. After treatment, they generally reach 40 dynes/cm or even higher. Usually, plastic substrate suppliers have treated the surface of the plastic, but the processing effect will soon be lost.

For printing plants, online processing is the best solution to ensure that the surface tension of the plastic substrate during printing remains within an acceptable range. Corona discharge (oxidation) treatment is the most common treatment method. It is used in the surface treatment of various plastic films and does not damage those plastic substrates that are sensitive to temperature. Corona discharge adopts high-frequency high-pressure or medium-frequency high-pressure discharge to treat the plastic surface to activate the surface 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 commonly used PE (polyethylene) film undergoes corona discharge treatment, unsaturated bonds such as carboxyl group and carbonyl group are formed on the PE molecular chain. After the surface is oxidized, the PE molecules are converted into polar molecules, increasing the surface energy. Generally, the surface tension of PE increases from 31 dynes/cm to 39-40 dynes/cm, which increases the wettability and adhesion of the PE film to UV inks/varnishes, thereby improving the firmness of the imprint degree.

The flame method is more used in the surface treatment of high-temperature-resistant plastic products. Under the action of instantaneous high temperature, the plastic removes the oil stain on the surface and melts the thin layer of the surface to improve the inking ability. Used in conjunction with the corona discharge treatment method, the chemical treatment method uses an oxidizing agent to treat the surface of the polyolefin plastic to generate polar groups on the surface, so that the surface of the plastic substrate can be well wetted by UV ink/varnish adhesion . In order to detect the surface treatment effect of plastic substrates, plastic film printing manufacturers are best equipped with Dyne test pens.

After treatment of the 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 surface of the ink and adhere firmly to the surface of the ink, the surface tension of the varnish is lower than that of the ink. Therefore, the UV ink formulation is very careful about the selection of raw materials, which is to obtain acceptable surface tension. Researches on various ink formulations have found that the surface tension of solvent-based inks is generally lower than other types of inks, so it can be spread on a variety of plastic film surfaces. Therefore, the surface treatment of plastic substrates, especially when printing with UV inks, is particularly critical.

Plastic substrate swelling and penetration properties
Unlike many commonly used printing papers and paperboards, the surface of plastic substrates does not have micropores that allow ink/varnish to penetrate into it. However, some plastic substrates will swell when they come in contact with some raw materials of UV ink/varnish. Therefore, this feature is used to make plastic substrates work with certain 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 and powerful adhesion effect can be formed between the ink/varnish and the surface of the plastic substrate. In addition, the temperature of the workshop 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.

Ink glass transition temperature (Tg)
In physics, certain substances form glass when they are below or equal to the substance’s “Glass Transition Temperature”. Compared with ordinary inks/varnishes, the components of UV inks/varnishes generally contain materials with smaller molecular weights. The ink layers formed by the reaction of these materials during curing are thicker and more cross-linked. At the same time, the glass transition temperature (Tg) of the film layer after UV ink/varnish curing is higher and the film layer is harder, so it has good abrasion resistance and chemical resistance.

If the Tg of the film layer is higher than the working temperature in the hot stamping or laminating process, then the UV film layer in the finished product will not be able to firmly adhere to the gold foil or composite plastic film. UV inks/varnishes made with low Tg materials can adhere well to gold foil and composite films in the post-press processing process. Sometimes, in order to enable the bronzing or lamination to proceed smoothly, it is necessary to lower the working temperature of the process.

Curing degree
For any UV ink formulation, the appropriate photoinitiator must be selected to optimize the performance of the ink layer after curing. Each UV ink/varnish formulation works with a specific, sufficient amount of UV energy. If the spectral band and power of UV energy change, it will affect the performance of the ink layer after curing.

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 the entire film layer has not been completely cured. The degree of curing close to the bottom layer of the film is critical to achieving good adhesion. The film layer and the bottom layer are not completely cured, so the penetration of UV ink/varnish inside the plastic substrate is meaningless. Once the curing energy changes, the surface tension of the film layer will also change, which ultimately affects the successful completion of the subsequent post-press processing process.

to sum up
UV ink/varnish is a common material for plastic packaging printing. Fast curing means higher production efficiency, and drying ink without heat means less impact on the environment and workshop.

Adhesion is one of the few problems that UV inks/varnishes have in plastic packaging printing. The main reason is that printing customers continue to need new plastic substrate materials and new applications, making the adhesion of the film layer UV plastic Common problems in printing. As long as the four issues mentioned above are fully taken into account, the problem of adhesion in plastic printing can be solved.