After printing on plastic film or transparent paper, the imprint appears to be dry, but the back side is sticky during storage after printing on the reel or bag. In severe cases, the prints are inseparable, and the graphic ink layer is transferred. , Causing product scrapping. This phenomenon is more likely to occur in high temperature seasons, but high temperature is not the only cause of back sticking. It is also closely related to a series of factors such as plastic, ink, solvent evaporation rate, operating environment air volume, and light.
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1. The binder resin in the ink
The binder resin used in the ink is different, and the anti-blocking properties of the ink are also different. At present, the inks used in plastic film gravure are all solvent-based inks, which are composed of binder resins, pigments, and solvents. As a binder resin, it is required to adhere firmly to the printing film, not to stick after printing and winding, have a certain flexibility, and tolerate a certain high temperature, and not be affected by changes in external factors within a certain range.
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The softening point of the binder resin directly affects the performance of the printing ink layer. If the softening point of the binder resin is too low, the printing ink layer will be in a slightly molten state under a high-temperature environment, so that the printed product will be blocked after being rolled up. Increasing the softening point of the binder resin is obviously helpful to prevent blocking, but if the softening point is too high, the flexibility of the printing ink layer will be poor and it will be brittle. Take the commonly used polyamide ink for PE and PP film surface printing as an example. The connecting material of the ink is polyamide resin, which is generally a low molecular weight linear polymer, which is composed of dimer acid and alkyl (or aryl). The softening point of polyamine obtained by polycondensation is usually 100 ~ 110 ℃.
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Generally, in order to meet the needs of printing or economic considerations, a mixed solvent is often added to the ink during printing. Each link material has its own true solvent, secondary solvent, and non-solvent. The better the solubility, the greater the affinity between the binder resin and the solvent molecules; and the better the solubility of the solvent to the resin, the worse the release of the resin to the solvent. Poor release will directly cause the residual solvent problem mentioned below. Therefore, when preparing the mixed solvent, the solvent should be controlled within a certain range. Both the printability requirements of the ink must be considered, as well as the comprehensive release of the solvent.
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For the same type of binder resin, the lower the softening point of the resin, the easier it is to dissolve the resin, and the worse the desolvation of the resin. From this point of view, the softening point of the binder resin should also be increased accordingly.
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In addition, when processing the ink, if the polishing time is too long, the temperature generated by the polishing will be too high, resulting in partial denaturation of the binder resin, and will also have a certain effect on adhesion.
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2. The solvent in the gravure ink failed to get completely evaporated after printing
After the printing ink is transferred to the plastic film, the solvent on the surface of the printing ink layer will evaporate first, and the solvent inside the printing ink layer must diffuse and penetrate the surface of the ink film before it can be evaporated and dried. When the drying of the printing ink layer enters the final stage, the surface layer of the printing ink film has been cured, which limits the continued diffusion and evaporation of the solvent inside the printing ink layer, thereby causing residual solvent problems.
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Plastic film printing is to dry the ink by solvent evaporation. Its volatilization speed is an important factor that determines the drying condition of the ink layer and affects the printing quality. The slower the solvent volatilization, the better the reproducibility of the plate and the more beautiful the color of the printed product, but it is easy to stick; on the contrary, it will produce whitening and other phenomena. Therefore, choosing a solvent with an appropriate volatilization rate is the key to plastic film gravure printing. In continuous register printing, the solvent must be completely evaporated during the running time of the film between the two printing stations, otherwise, the roller will stick when printing in the next station. After the film is printed, the organic solvent in the ink is not completely volatilized in the drying system, and there is still heat after the film is rolled up, which brings the conditions of continued volatilization to the solvent remaining in the ink layer and forms adhesion. In the hot and humid season, the film is too tight or compressed after printing.
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When the residual solvent in the printing ink layer reaches a certain concentration, the printing ink layer remains in a slightly melted (wet) state, which directly leads to adhesion problems. Therefore, the amount of residual solvent must be controlled.
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The solvent used for plastic film gravure printing is mainly alcohols, with benzene and alcohol multi-purpose ethanol and isopropanol, and benzene multi-purpose toluene and xylene. Due to the needs of nitrocellulose, esters such as ethyl acetate and butyl acetate should also be added. In short, the organic solvent in the ink should be completely volatilized before winding. Slow down the vehicle speed properly and increase the air volume. After the film is printed, it is loosely stored in a wire braided basket to allow the ink layer to continue to be ventilated and dried. After curing, it is cut and bagged. After the film is printed and bagged, it is wrapped and placed vertically in the carton to reduce the film surface Pressure.
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3. Adhesion fastness of printing ink layer
The adhesion fastness and adhesion of the printing ink layer are also closely related. When the printing ink layer has poor adhesion fastness, it is easy for the printing ink film to be transferred to another film in contact with it under pressure, thus causing adhesion. Therefore, it is necessary to ensure good ink adhesion fastness. The reasons for poor adhesion of ink are as follows.
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1. Whether the ink is misused or different types of ink are mixed.
2. Poor corona treatment of plastic film or excessive moisture absorption.
3. The additives in the plastic film are precipitated, or the air is attracted to the film by dust, which affects the adhesion of the ink.
4. The ink is whitened and deteriorated.
5. Poor drying.
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Source: Plastic Flexible Packaging Interactive
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