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Corona Treatment

 

Corona treatment is a way to alter surface adhesion properties of the materials to be printed, making them more suitable for the adhesion and coating of printing inks.

In other words, corona treatment is a kind of electrical discharge, which enhances surface adhesion properties of the materials to be printed. Corona is produced by discharge through high voltage and high frequency, via grounded and inductive air jets of the corona treatment equipment. The electric current does not pass through the two air jets until voltage reaches 9,000~15,000V per square millimeter. Electric discharge molecules burst out from the air jets carrying high-capacity ions, and rush at the anode. Corona treatment is carried out by ions intensively and powerfully burst out.

Ions penetrate into surfaces of the materials to be printed and destroy their molecular structures via electric discharge, and further, oxidize and polarize the surfaces. By corroding the surfaces, ion discharge enhances surface adhesion level. However, to learn about the physical features of corona treatment is not so important. What we really need to learn is the treatment itself and how corona treatment can help printing-related industries with their work.

What does corona treatment do?
What corona treatment really does is to change the adhesion energy of film surfaces, enabling them to better accept printing inks or meet coating needs. Adhesion property measuring unit is "dyne." To achieve effective ink adhesion, coating or varnishing, surfaces of materials to be printed need to feature higher than 10 dynes (of inks).

Generally, PE film surface features 31 dynes under normal circumstances. However, if PE film needs to be printed with flexo ink featuring 36 dynes, it should feature 46 dynes to be in a better printing condition (36 dynes + 10 dynes). The gap between 31 dynes and 46 dynes exactly requires corona treatment to fill up. We all know that it is more difficult for flexo inks to be printed on film than solvent-based ones. But why is it? It is because the former features higher surface energy than the latter. The surface energy of solvent-based inks features 28 dynes, UV inks 32~34 dynes, while that of flexo inks 36 dynes. That is to say, if solvent-based inks are applied to film printing, the film surface needs to be corona treated to achieve 38 dynes; or 46 dynes, while flexo inks are utilized. Partially because of the development of flexo inks, corona treatment is widely known today.

Flexo inks--water-based ink is the only solution
In the 1970s, only blown-film making plants have corona treatment equipment. Firms in the packaging and printing industries used to utilize solvent-based inks to print, because film surface treatment is easy--the only thing that needs to be done is to slightly raise film-surface energy level during blown-film making--enabling the adhesion of solvent-based inks, which feature low surface energy level. However, in the 1980s, new environmental protection treaties were established, and have since strongly impacted upon the packaging and printing industries. Environment protectionists worry about how solvent volatilization may affect the atmospheric boundary layer, not to mention affecting human lives and the living environments. That was the time when flexo inks were developed and produced to meet the urgent needs.

The wide-web printing industry is the one who suffers the impact the most in the field due to its utilization of high-volatilization solvent-based inks; and the first one as well to use flexo inks in plastic-film printing. This helps advance technologies in terms of inks, pattern rollers, ink transfer methods, pressure rollers, and rapid drying systems, and has greatly improved the environment for flexo-ink printing. The improvement has also played an essential part in the exploitation of corona treatment, as well as the change of flexo-ink properties and materials for printing.

Though blown film is corona treated, the surface energy level drops along with time in conveyance and storage, and thus will not be able to meet requirements for flexo-ink printing as much as before, and what's worse, making high-speed printing and tiny-printing-dot transferring very difficult. This is why firms in the printing industry need to have corona treating machines.

Now the narrow-web printing industry also has a tendency to use flexo inks for printing, because it has been confronted with concerns from the environmental protection organizations, occupational safety organizations, and blue-collar health organizations. Paper tags and labels need no corona treatment before printing. Unlike plastic film, paper has no adhesion problems in flexo-ink printing. However, the use of plastic film has obviously been increasing among companies in such industries as aerospace, in-mold tag and label forming, as well as packaging. The narrow-web printing industry needs to face the change as well-to print on plastic film, with flexo inks. They have two options; to either use high-price plastic lamination film, or to add a corona treating machine to their printing lines for corona treatment. Corona treatment is absolutely necessary when it comes to printing with UV or EB inks. Featuring high-energy surfaces, UV and EB inks require high-speed printing, and corona treatment is the key to achieving instant ink transferring and drying, without causing pasty ink, ink flowing, and half tone problems. It's not hard for us to find out now why corona treatment has become so popular in the packaging and printing industries.

Who Needs Corona Treatment?
Using flexo or UV inks instead of solvent-based ones to avoid environmental pollution is a must today for the printing and packaging industries. This has made corona treatment even more indispensable than before. Corona treatment is now widely applied, to plastic sheets, metallic foils (including tin foils), etc. in film lamination and plastic sheet mounting. A great number of film-material manufacturers have corona treatment equipment, because they need to do surface treatment on materials to be printed for downstream collaborators. Some of them also need to take care of their own surface treatment business, such as destroying surface-layer fibers of paper to allow it to receive thin and smooth lamination of film (or i.e. film lamination for aluminum foil), and to lower production costs. Potentials and opportunities for corona treatment have been welling out like springs and are never going to end.

Innovation and R&D
Corona treatment equipment has been improved in terms of dimensions, efficiency, and capacity, as well as treatment speeds and standards, in order to meet the needs for in-line operation; and has also been innovated in various ways to correspond to the demand of various types of materials for printing. As a result, corona treatment has become so sophisticated. Since the design of corona treatment equipment can be affected by a lot of factors, manufacturers are seeking to build a standard model to meet all demands. However, customers love only what meets their individual needs. To achieve this, we at Chaang Horng are willing to invest on corona treatment equipment R&D, and to innovate the equipment both technologically and in terms of assembly. Basically, corona treatment equipment is available in three types, including conventional models, models with exterior-mounted rolls, and three-dimensional operating models. These are all designed to better meet myriad printing requirements.

What will the industry do in the future?
We believe what the industry will do in the future is to emphasize on enhancing treatment speed without damaging film surfaces. In fact, up to the present, corona treatment has been fully exploited. We believe there is no need for manufacturers to worry about whatever new or different machine models competitors will develop. Everyone in the field is challenged with new technologies. We are proud that what Chaang-Horng has accomplished in recent years is more than just changing the machine appearance, but has developed innovative and practical models.

In fact, the use of flexo inks instead of solvent-based inks has resulted in the intensive utilization of corona treatment equipment in recent years. Ozone produced by such a large number of corona treatment machines is everywhere, which might have become another environmental pollution problem. Maybe, in the early '90s, corona treatment machinery was one of the tools serving for competition, but now, it is only a basic tool-a necessity that leads to the close cooperation relationships between buyers and suppliers.