Common causes of bonding between flexo and gravure prints
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"Bonding" is an unnecessary bond between the ink layers of the printing ink, and such problems may occur in the inside of the printed web, between the sheets, or in the composite structure. Bonding can occur for several reasons. The most common ones are web tension, large solvent residue, high plasticizer content in the ink, and excessive corona discharge treatment, depending on the process conditions of the company.
In packaging printing, the amount of solvent residue is the most common cause of bonding, and the absence of volatile solvents in the printing causes the ink layer to soften under pressure and adhere to the back side of the web. Most plants sample the product and measure the number of milligrams of residual solvent per unit area to monitor solvent residue.
The ink does not dry out or "crust on the surface of the ink layer" can cause adhesion. The phenomenon of crust on the surface of the ink layer is related to the heat generation of the ink layer. The surface of the ink is "dry", and the ink layer contains a solvent. When using modern water-soluble inks, the problem is more serious because the drying of such inks is an irreversible process.
Solvents with higher boiling points in the ink layer or bonding agent are difficult to volatilize and often result in a relatively large amount of residual solvent. Some solvents such as toluene, xylene, ethylene glycol, etc., can cause not only adhesion but also an unpleasant odor. The first step in analyzing the cause of adhesion should be to determine the amount of solvent residue by gas chromatography (G.C.). If the amount of residue is high, the blocking resistance of the ink is lowered. If the print is further dried to reduce the bond, solvent residue is likely to be the culprit.
An ink having a high plasticizer content or a low molecular weight of the binder may become soft and sticky after drying, and bonding may occur once the surface of the ink layer becomes too soft. Nitro-based inks often contain a large amount of plasticizer, and ink formulators must strictly control the ratio of plasticizer to nitrocellulose. In addition, the choice of modified resin must take into account the plasticizer content of the ink formulation. If a soft resin is used, the proportion of plasticizer should be reduced accordingly.
In the case where the plasticizer causes bonding, the proportion of the plasticizer is high, and the degree of solvent retention is also high; the printing ink layer becomes soft and becomes softer with time. The adhesion test can explain the phenomenon that the ink layer becomes soft and deteriorates with time, and the adhesion is almost or not improved after the sample is heated or further dried.
Web bonding due to excessive corona discharge treatment is not too common, but it also causes trouble. The corona discharge treatment oxidizes the surface of the ink layer and increases the polarity, eventually increasing the surface tension. Such surfaces naturally adhere to each other.
After this bonding occurs, it indicates that the corona discharge treatment is excessive, for example, the PE film corona treatment is close to 50 or even 60. Adhesion estimation is partially related to the reduction in resin properties in the ink and appears to be an oxidative decomposition process, often associated with certain resins. Under high corona discharge treatment, the oxidation resistance of the film is impaired. In any case, there are many reasons for bonding, and specific phenomena must be carefully analyzed.