Photo-curing (uv) technology in flexographic printing

In recent years, the uv technology in flexo printing has played an important role in the printing of paper and film materials. The uv technology has moved from narrow web flexography to the flexographic printing market. In offset printing, we also saw the advantages of the uv ink quick drying system, which can shorten the printing cycle and reduce the workload. The most important feature of the uv curing system is that it improves printing stability (drying) and shortens the waiting time for oxidative drying during the printing process. In the past 10 years, uv printing has had a great impact on the traditional, mainly on the plastic substrate flexographic printing.

Nowadays, there are new ink-cationic inks. It increases the environmental protection and quality of flexographic printing and increases its guarantee. Compared with the free radical system, the cationic ink has the advantages of low volatility and small odor, and can be said to be a very good environmental protection ink. The odor of free radical inks is a big headache and it is harmful to the health of operators and the environment. Cationic inks (low-volatile inks) have great advantages in this regard, and are now even used in some food packaging industries. Cationic inks also have the benefit of being attached to a plastic substrate. Cationic inks and coating glosses exhibit particularly good adhesion to electro-smoothed films, which is the most critical of all advantages.

Cationic inks have a medium cure speed (which is slightly inferior to free radical inks) compared to free radical inks and can be re-cured (approx. extended to 24 hours) without blocking oxidation, slight moisture effects, no shrinkage, plastic attachment Excellent sexual characteristics.

Although in terms of curing speed, radical inks can be dried at high speeds, the curing speed of cationic inks is slightly behind. However, recent research and development shows that the improved cationic ink can also be adapted for printing from 150 m/minute to 200 m/minute.

The benefit of the secondary cure is that the ink film appears to have cured after the print has been output from the printer, but there is still some reaction (not dried). Cationic curing techniques can continue to react until most of these materials react. In contrast, radical inks can only be exposed to uv light sources and can be thoroughly dried. This process, in turn, leads to higher volatilization, especially if unreacted low molecular weight components are more volatile. The cured polymer is usually of high molecular weight and generally does not volatilize. Volatilization of volatiles is also reflected in the extractable components. The content of extractables is an increasingly important feature of testing food packaging systems. In particular, as an analytical technique, the content of the extractable component is one part per billion, and even an amount of one trillion parts is now being discussed as an indicator of the content of the packaged product. Cationic inks have a small amount of shrinkage and therefore have good adhesion characteristics.

In the current situation of increasingly loud environmental protection calls, the packaging and printing industry is also demanding environmentally friendly products. Food packaging should meet the cepe standard in industry standards. From the perspective of inks, it is necessary to constantly solve the problem of harmful substances such as lead, benzene, aromatic compounds, and ethyl glycol in the ink, and ensure the safety of foods, supplies, and the health of printing personnel. From the results achieved by the ink industry, the results are still very encouraging. In 1974, lead in ink pigments was removed; in the late 1970s, cellosolve (ethyl glycol) was removed; in the early 1980s, toluene in food packaging instructions was removed; in the late 1980s, the associated benzidine was yellow Pigments; In the early 1990s, the use of phthalic acid plasticizers was reduced. In the initial stage of development of cationic inks (in the mid-1980s), some of the above-mentioned hazardous substances did not meet the regulatory standards. For example, benzene is a by-product produced by the production of cationic inks, and the subsequent generation of by-products under initiator light is not. Popular aromatic compounds and so on, these are all prohibited ingredients listed in the printing and food packaging ink regulations. Therefore, cationic inks have not been widely used at the initial stage. Up till now, some units still adopt a wait-and-see attitude, but after continuous improvement, the content of these harmful substances has finally reached the industry standard. Such as benzene, in the current production of cationic ink, its content is extremely low, using ordinary analytical techniques, simply can not be detected. Many test results on printed products and printing environments also showed that no benzene was found. The content of these substances is generally in the range of a few parts per billion (usually a single digit), which indicates that the package using photoinitiators no longer produces benzene. Looking at the development prospects of cationic inks, I believe most printing units will still use cationic inks with excellent properties. Although some customers still have plans to abandon the use of cationic inks, they show great advantages in the printing process and will certainly be recognized by customers. We believe that with the development of science and technology and the improvement of the ink industry, some of the disadvantages of cationic inks will surely find a good solution, like the excellent records mentioned above. Therefore, we must have a positive attitude and technical direction for these and better adapt to the new standards.

(1) Cationic coated adhesive

Gravure printing is fast, and when it is coated with adhesive it can appear dry (the presence of solvent) or wait a day for it to cure to a certain strength. In flexo printing, an adhesive is generally used on a printing press unit, the film is covered and cured at a normal printing speed, the coating of the printed matter can be cured within a few minutes, and it is also strongly cut on the printing press to the finished product. And hand it directly to the customer. In this case, the use of the uv adhesive does not present the problem of the volatilization of the solvent and the generation of harmful odors. The printer used a newly developed white pigment binder to remove the traditional solvent problems caused by back-white, followed by a white ink film. Solve the problem of the original solvent retention, and now, using a cationic adhesive coating, it is easy to reduce the residual solvent content per square meter to a very low level. This coating is now available on polyethylene, polyethers, metal sheets and other materials.

(2) Cationic Ink - Shrink Film

The vast majority of the shrink film market is occupied by solvent-based gravure printing, and very few are derived from solvent-based flexo printing. In fact, water-free inks are used. Flexographic printers have discovered that the ink system that can achieve the desired quality in flexo printing is an uv-curable ink that has no solvent volatilization and a good print consistency. However, when the shrinkage amount reaches 70%, the free volatile type ink and the mixed dry type ink exhibit poor adhesion. Cationic inks have passed this required test and become a unique ink system for ops, pvc and opp materials. The advantages of cationic inks in management and storage are obvious.

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