UV plastic printing adhesion
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UV ink printing or uv glazing is becoming increasingly popular on the surface of plastic substrates, and this is a challenging task. Plastic printing materials have been used in the printing industry for decades. With the popularity of uv ink/varnish, uv ink printing on plastic substrates has brought new topics to printers and ink manufacturers. The biggest problem is how to The uv ink/varnish adheres firmly to the surface of the plastic substrate, ie adhesion problems.
1. Surface tension of plastic substrates
The surface tension of plastic substrates is the first factor to consider when printing plastics. Many plastic films have a low surface tension before they are treated. Generally, they need to be surface treated to increase the surface tension of the plastic substrate, and generally reach 40 dynes/cm or higher after treatment. Usually, plastic substrate suppliers have surface treated plastics, but the treatment effect will soon be lost.
For printers, wire handling is the best solution to ensure that the surface tension of the plastic substrate is maintained within acceptable limits during printing. Corona discharge (oxidation) treatment is the most common treatment, and it is applied to the surface treatment of various plastic films without damaging those temperature sensitive plastic substrates. Corona discharge uses high-frequency high-voltage or medium-frequency high-voltage discharge to treat the surface of the plastic to activate its surface and to be porous, so as to improve the adhesion of the surface of the plastic film to the ink and improve the printability of the film. For example, a commonly used pe (polyethylene) film is subjected to corona discharge treatment to form an unsaturated bond such as a carboxyl group or a carbonyl group on the pe molecular chain. After the surface is oxidized, the pe molecule is converted into a polar molecule, which increases the surface energy. Generally, the surface tension of pe is increased from 31 dynes/cm to 39-40 dynes/cm, which increases the wettability and adhesion of the pe film to uv ink/varnish, thereby enhancing the firmness of the print. degree.
The 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 surface layer to improve the ink repellency under the action of instantaneous high temperature; chemical treatment is also used, but this method is usually Used in conjunction with corona discharge treatment, the chemical treatment method uses an oxidizing agent to treat the surface of the polyolefin plastic to form a polar group on the surface, so that the surface of the plastic substrate can be well wetted by the uv ink/varnish. . In order to detect the treatment effect of the surface of plastic substrates, plastic film printers are best equipped with Dyne test pens.
After the treated plastic film, the surface tension of the ink is lower than the surface tension of the plastic film. Similarly, in order to make the varnish wet well on the surface of the ink and firmly adhere to the surface of the ink, the surface tension of the varnish is lower than the surface tension of the ink. Therefore, the choice of raw materials for uv ink formulations is very sophisticated in order to achieve acceptable surface tension. Studies on various ink formulations have found that solvent-based inks generally have lower surface tension than other types of inks, so they can be spread on a variety of plastic film surfaces. Therefore, the surface treatment of plastic substrates, especially uv ink printing, is particularly critical.
2. Plastic substrate expansion and penetration properties
Unlike many commonly used printing papers and paperboards, the surface of plastic substrates has no micropores that allow ink/varnish to penetrate into its interior. However, some plastic substrates can swell when exposed to some uv ink/varnish raw materials. Therefore, this feature allows plastic substrates to be used with certain uv inks to allow ink/varnish to penetrate the interior of the plastic substrate. At the time of curing, due to the internal penetration of the ink/varnish, a strong and strong adhesion effect can be formed between the ink/varnish and the surface of the plastic substrate. In addition, the shop floor temperature can be increased to enhance the penetration between the plastic substrate and the ink/varnish, which is one of the ways to improve the adhesion between the uv ink/varnish and the plastic film.
3. Ink glass transition temperature (tg)
In physics, glass is formed when certain materials are below or equal to the "glass transition temperature" of the material. Compared to conventional inks/varnishes, uv inks/varnishes generally contain smaller molecular weight materials that are thicker and more crosslinked by the reaction when cured. At the same time, after the uv ink/varnish is cured, the film has a higher glass transition temperature (tg) and a harder film layer, so it has good abrasion resistance and chemical resistance.
If the tg of the film is higher than the working temperature in the gilding or laminating process, the uv film in the finished product will not adhere firmly to the gold foil or composite plastic film. The uv ink/varnish prepared using a low tg material adheres well to the gold foil and the composite film in the post-press process. Sometimes in order to make the bronzing or film smooth, I have to lower the working temperature of the process.
4. Degree of cure
For any uv ink formulation, the appropriate photoinitiator must be used to optimize the performance of the ink layer after curing. Each uv ink/varnish formulation works in conjunction 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, it may be felt that the uv ink/varnish has solidified, but in reality, it is likely that the entire film layer is not completely cured. The degree of cure near the bottom layer of the film layer is critical to achieving good adhesion. The inner and bottom layers of the film are not completely cured, so the penetration of uv ink/varnish into the interior of the plastic substrate is meaningless. Once the curing energy changes, the surface tension of the film will also change, which will eventually affect the subsequent post-press processing.
The plastic packaging market is an exciting market, and uv ink/varnish is a common material for plastic packaging printing. Fast curing means higher production efficiency, and no need to heat dry the ink means less impact on the environment and the work shop.
Adhesion is one of the few problems in uv ink/varnish printing in plastic packaging. The main reason is that printing customers are constantly needing new plastic substrate materials and new applications, so that the adhesion of the film becomes uv plastic. Common problems in printing. As long as the four problems mentioned above are fully considered, the problem of adhesion in plastic printing can be solved.