Common causes of flexographic and gravure printed matter bonding

"Bonding" is the unnecessary adhesion between the ink layers of the printed products. Such problems may occur inside the rolled web, between the flat sheets or in the composite structure substrate. There are several reasons for sticking. The most common ones are the high tension when the web is being wound, the large amount of solvent residue, the high content of plasticizer in the ink, and the excessive corona discharge treatment, which depends on the company's process conditions.

In packaging printing, the large amount of residual solvent is the most common cause of adhesion. The non-volatile solvent in printing causes the ink layer to soften under pressure and stick to the back of the web. Most factories sample products and measure milligrams of residual solvent per unit area to monitor the amount of residual solvent.

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 will be more serious, because the drying of such inks is an irreversible process.

Solvents with higher boiling points in the ink layer or connecting agent are difficult to volatilize, and often result in more residual solvent. Some solvents such as toluene, xylene, ethylene glycol, etc., can not only cause adhesion, but also produce an unpleasant odor. The first step in analyzing the cause of adhesion should be to determine the residual amount of solvent by gas chromatography (GC). If the residual amount is high, the blocking resistance of the ink decreases. If the printed matter is further dried to reduce adhesion, the solvent residue is likely to be the cause of adhesion.

Ink with too high plasticizer content or low molecular weight of the linker may become soft and sticky after drying. Once the surface of the ink layer becomes too soft, adhesion will occur. Nitro inks often contain large amounts of plasticizers, and ink formulators must strictly control the ratio of plasticizers to nitrocellulose. In addition, when selecting the modified resin, the plasticizer content in the ink formulation must be considered. If soft resin is used, the proportion of plasticizer should be reduced accordingly.

In the case of adhesion caused by plasticizers, the proportion of plasticizers 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. After the sample is heated or further dried, the adhesion is hardly or not improved at all.

Web adhesion caused by excessive corona discharge treatment is not very common, but it also causes trouble. The corona discharge treatment oxidizes the surface of the ink layer and increases the polarity, which ultimately increases the surface tension. Such surfaces naturally tend to adhere to each other.

After this adhesion occurs, it indicates that the corona discharge treatment is excessive, for example, the PE film corona treatment is close to 50 or even 60. Blocking is estimated to be partially related to the reduction in resin properties in the ink, and it appears to be an oxidative decomposition process, often associated with certain resins. Under the high corona discharge treatment, the oxidation resistance of the film is weakened. In any case, there are many reasons for bonding, and specific phenomena must be carefully analyzed.

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