Analysis of Common Causes of Bonding of Flexographic and Intaglio Printed Products

"Bonding" is the unnecessary adhesion between the inks of the ink. Such problems may occur in the interior of the printed web, between the sheets of paper, or in the composite structure substrate. Bonding may occur for several reasons. The most common are web tensions when rewinding, many solvent residues, high levels of plasticizer in the ink, and strong corona discharge handling, depending on the process conditions of the company.
In packaging printing, the solvent residue is the most common cause of adhesion. The solvent that is not volatilized during printing causes the ink layer to soften under pressure and stick to the back of the web. Most plants sample the product and measure the residual solvent in milligrams per unit area to monitor the residual solvent.
Ink does not dry out or "ink surface crust" can cause adhesion. The phenomenon of crusting on the surface of the ink layer is related to troubles caused by the ink layer. The ink surface is "dried" and the ink layer contains solvent inside. With modern water-soluble inks, the problem is even more serious because the drying of such inks is an irreversible process.
Solvents with higher boiling points in the ink layer or the linking agent are difficult to evaporate and often lead to relatively more residual solvent. Some solvents, such as toluene, xylene, ethylene glycol, etc., can not only cause adhesions, but can also produce an unpleasant odor. The first step in the analysis of the cause of adhesion should be the use of gas chromatography (GC) to determine the amount of solvent remaining. If the amount of residue is high, the ink's blocking resistance is reduced. If the print is further dried to reduce adhesion, solvent residue is likely to be the culprit of adhesion.
An ink with a high plasticizer content or a lower molecular weight of the linker may become soft and sticky after drying. Adhesion may occur once the surface of the ink layer becomes too soft. Nitro inks often contain a large amount of plasticizers. The ink formulator must have strict control over the ratio of plasticizer to nitrocellulose. In addition, the plasticizer content in the ink formulation must be taken into account when selecting the modified resin. If soft resin is used, the proportion of plasticizer should be reduced accordingly.
In the case where the plasticizer causes adhesion, the proportion of the plasticizer is high, and the degree of retention of the solvent is also high; the printing ink layer becomes soft and becomes softer over time. The adhesion test can indicate that the ink layer is soft and deteriorates over time. After the sample is heated or further dried, the blocking property is not improved at all or at all.
The web sticking caused by excessive corona discharge treatment is not too common, but it also brings trouble. The corona discharge treatment oxidizes the surface of the ink layer and increases polarity, eventually increasing the surface tension. Such surfaces naturally easily adhere to one another.
After this adhesion occurred, it was demonstrated that the corona discharge treatment was excessive, for example, the corona treatment of the PE film was 50 or even 60. The sticky edge estimate is partly related to the reduction of the resin properties in the ink and appears to be a oxidative decomposition process, often associated with certain resins. Under high corona discharge treatment, the film's oxidation resistance is also weakened. In any case, there are many reasons for bonding, and it is necessary to carefully analyze the specific phenomenon.

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