The relationship between gravure ink and stick back and countermeasures

Abstract: In this paper, the most common "back tack" phenomenon in plastic gravure printing is explained in detail in all aspects of its appearance, especially the process of ink manufacturing and use. From the perspective of ink production, some suggestions and methods are given.

Keywords: gravure, plastic, ink, adhesion, additives

The so-called adhesion refers to the ink layer on the printed product and another contact surface (usually the back side of the film) adhere to each other after printing and winding, or the ink adheres to the other contact surface. There are two kinds of adhesion phenomena: one is the adhesion of the back surface of the plastic film printing, usually called "stick back", that is, the film has a dry impression after printing, and after printing the reel or bag, the back adhesion occurs during storage Dirty phenomenon. The specific performance is that it can not be unrolled smoothly, or the printed pattern is damaged due to ink peeling, and the contact surface is contaminated. When the adhesion is light (the previous stage of adhesion), it appears as a peeling sound when rewinding, or the ink surface of the overlapping part shows abnormal gloss. In severe cases, the printed parts cannot be torn apart, and the graphic ink layer is stained, causing the product to be scrapped. Second, the surfaces of the plastic tubular films are stuck to each other, and the bag mouth is difficult to uncover, and the user refuses to use it. The above two situations are more likely to happen in summer. The high temperature of the operation workshop and the relatively high humidity are one of the main causes of adhesion. But sometimes it is related to the content of the PE resin slip agent is too small, and it is also closely related to a series of factors such as air volume and speed.

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.

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.

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 ℃.

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.

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.

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.

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.

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. The amount of residual solvent is affected by the following factors in addition to the solvent release properties of the binder resin described above.

1. The volatility of the solvent

The evaporation rate of a single solvent is determined by its physical parameters. The correlation between volatility and solvent parameters is:

In the formula, E is the solvent volatilization rate; P25 is the saturated vapor pressure of the solvent at 25 ° C; d25 is the density of the solvent at 25 ° C; M is the relative molecular weight; K is a constant.

The solvent contained in the printing ink film is a mixed solvent, and the evaporation rate of each solvent is different. In this case, the volatility of the solvent will change, the more volatile components will escape first, and the less volatile components will stay behind, resulting in a change in the solvent v. Composition, it is impossible to be like a single solvent In this way, the single volatilization rate will escape at a constant temperature, and the volatilization rate of the solvent will gradually slow down. If the purity of the solvent is unsatisfactory (such as containing too many high-boiling components) or excessive use of slow-drying solvents, serious residual solvent problems will occur under normal conditions. Therefore, the rational design of mixed solvent formulations is a very important technical issue, and more care should be taken when using substitutes.

In addition, the surface characteristics, specific surface area and concentration of the pigment also have a certain effect on the volatilization of the solvent. For the same pigment, the solvent volatilization rate will decrease with the increase of the pigment concentration; for different pigments, in general, the pigment density is small, the solvent volatilization rate is low, the pigment particles are small, the solvent volatilization rate Also low.

2. Drying conditions

Drying conditions include drying air temperature, air volume (wind speed), drying device structure, etc. Poor drying will increase the amount of residual solvent. Therefore, the best drying conditions should be created. Increasing the wind temperature and volume (wind speed) can undoubtedly strengthen the drying conditions. However, it should be noted that in the case of a thick printing ink layer, if it dries too fast, the surface of the ink layer will quickly form a film, resulting in the internal solvent cannot escape, but will increase the amount of residual solvent. Furthermore, if the temperature is too high, the ink film may be easily softened.

3. Printing speed

The speed of the printing speed determines the length of drying time the printed product can get. Therefore, the printing speed can be increased only after ensuring that the printing ink layer is sufficiently dry.

4. Humidity of drying medium (air)

The humidity of the drying medium (air) has a great influence on the amount of residual solvent. First, if the moisture in the air enters the ink, it will cause the overall volatility of the solvent to deteriorate. Second, the large amount of moisture in the drying medium (air) also suppresses the evaporation of the solvent. The environmental humidity doubles, and the drying speed of the ink is generally slowed by nearly twice. It is also for this reason that adhesion faults are most likely to occur when printing plastic films during the rainy season. Therefore, in a high-humidity environment, the printing speed should be reduced as much as possible, and the contact between the ink and the air should be reduced to ensure that the ink can be fully dried. The relative humidity of the printing workshop should generally not exceed 70%. Of course, it should not be too dry, otherwise, it will easily cause static problems.

5. Printing film

Films of different materials have different selective absorption tendency towards solvents. Aluminum foil, polyester, etc. are non-absorbent films and generally have few residual solvents; polypropylene-based films are prone to residual hydrocarbon-based solvents, while water-absorbing films (such as nylon and cellophane) are prone to residual alcohol-based solvents. In addition, some additives added to the film will also affect the evaporation of the solvent. This increases the amount of residual solvent.

Countermeasures, the volatilization rate of the solvent in the ink depends not only on the boiling point of the solvent, vapor pressure, vapor latent heat, etc., but also on the temperature, humidity, wind-based, solute and ink layer thickness of the operating environment. Therefore, the volatilization rate of the solvent in the ink should be adjusted at any time according to the change of conditions. At normal temperature, if the ink layer dries too fast, a solvent that evaporates slowly should be added; otherwise, a solvent that evaporates quickly should be added. It can also be diluted with a mixed solvent of xylene, ethanol and isopropanol (the three solvents are mixed in a ratio of 1/3, using a single solvent is not as good as a mixed solvent). If you feel the drying is too fast, you can use a little alcohol to replace part of the ethanol (butanol has the effect of increasing the gloss of the ink). But be sure to add the appropriate amount to avoid affecting the drying of the printed parts.

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.

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.

4. Some other major issues

1. The ink of plastic gravure ink is bad. The bonding material in some plastic gravure inks has a low melting point and is easy to stick when the operating environment temperature is high and the relative humidity is large.

Countermeasures: ①When conditions permit, install air conditioners in the operation workshop, control the room temperature between 18 ℃ -20 ℃ and the relative humidity below 65%; ②Replace the ink with good ink quality.

2. Poor adaptability of plastic film thin printing and packaging

Reasons: â‘  Plastic film made of non-packaging resin; â‘¡ Insufficient opening agent in resin.

Countermeasure: Replace the plastic film.

3. Effect of static electricity

Cause: The plastic film generates static electricity, which forms adhesion between the films. Countermeasure: Add antistatic agent to the plastic.

4. Cooling conditions and storage environment

The printed matter has a high heat after passing through the drying box, and the film coming out of the drying system still has a certain amount of residual heat. If it is not cooled, it will cause the accumulation of residual heat inside the rolled semi-finished product, which will increase due to the softening of the ink. The possibility of adhesion. Therefore, the printed film must be cooled before being wound.

Now cooling rolls are commonly used for cooling. The temperature running smoothness of the cooling roller and the printing line speed are the main factors that affect the cooling effect. If you forget to use tap water for cooling during operation, the cooling roller will lose the cooling effect. After all the plastic film is printed and received, the heat will accumulate in the film roll and the temperature will rise to 50 ~ 60 ℃. There are many examples of such adhesions.

In addition, during storage and handling, excessive temperature or poor ventilation may also cause adhesion.

5. The winding tension is too large. Excessive winding tension can directly aggravate the transfer tendency between the printing ink layer and the contact surface. Therefore, under the premise of ensuring a neat winding, the winding tension should be reduced as much as possible, and the winding diameter should not be too large.

6. The cohesion of the ink layer is very weak (the cohesion is extremely poor) When the cohesion of the ink film is very weak, the separation of the ink film will occur under a small pressure, causing adhesion.

7. Excessive pressure on the film roll Too much pressure on the film roll will aggravate the tendency of the ink layer to transfer to the contact surface. Straight storage of the film roll can effectively reduce the pressure between the ink film and the contact layer.

8. The affinity between the printing surface and the contact surface is too strong. When the affinity between the printing surface and the contact surface is too strong, part (or all) of the ink layer is easily transferred to the contact surface under pressure, causing adhesion.

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V. Methods and precautions for solving adhesion failure

Through the analysis of the various factors that affect the above adhesion, we can summarize the solutions and precautions for the failure of plastic gravure adhesion.

1. Adding additives to the resin can effectively prevent internal sticking, and the effect after trial is good. The specific methods are as follows. Choose oleic acid amide, chemical structure is C17H33CONH2, iodine value≤86g iodine / 100g, acid value≤0.8mgKOH / g, melting point 72 ~ 76 ℃, white or yellow wax. It is a slip agent. It is added to PE resin and heated and kneaded by a screw. It melts at about 50 ° C and is evenly dispersed in the plastic. After high temperature extrusion, some oleic acid amide oozes out of the surface, forming a The extremely thin wax film prevents direct contact and incompatibility between the film layers, so it can play an anti-blocking role.

The operation method is: firstly grind the oleic acid amide finely, or add it to the crusher to crush it. Because the beginning of the incoming material is uneven, if it is not crushed, it will cause local concentration of additives. After crushing, use a 50-mesh sieve to remove the coarse debris, add it to the LDPE pellets, mix with 100 parts of resin, add 0.1 parts of oleic acid amide, stir briefly, add to the hopper, and blow directly. Other operating techniques remain unchanged.

Effect: After adding additives, it not only effectively prevents the inner layer of the film from sticking, but because the film forms a uniform and extremely thin wax layer, the smoothness and transparency of the film are greatly improved.

After gravure printing, the brightness of the ink has been greatly improved, and it has no effect on the inking performance. Only after the amount exceeds 0.3%, the surface tension of the film is difficult to control, and the ink-injection property decreases. Therefore, the dosage should be controlled below 0.3%, and corona treatment should be carried out after the film is blown, so that the surface tension of the printed film is controlled above 38 × 10-5N / cm2. When the content of the formula is 0.2%, it is much easier to disassemble and wash the screw and the machine head, and its lubricity protects the screw, the barrel and the head. Oleic acid amide also has a certain antistatic effect in plastics. Blown color film, adding oleic acid amide, can improve coloring and dispersibility, eliminate the phenomenon of color masterbatch, without adversely affecting color. In addition, it has no effect on the processes of heat-sealing bags, slitting and cutting.

2. Control the use of slow-drying solvents and residual solvents. Try to improve the performance of the drying oven so that it can supply sufficient heat and quantity. Make the cooling roller rotate smoothly. When rewinding, pay attention to the abnormal temperature rise caused by the sliding of the paper tube and the roller. Pay special attention to thin films such as PET and NY when printing or rewinding inspection. During the transportation and handling, the printed film should be kept upright and do not fall down, otherwise the local pressure of the film roll will be too large and cause adhesion. Do not concentrate printed patterns on specific parts. For multi-color printing, when designing a pattern, it should be considered that there should not be too many colors. If possible, spot color ink printing can be used to prevent the local ink layer from being too thick.

3. In the case where the printed pattern on the plastic film is biased to one side, the pressure on the side with the printed pattern on the rolled film must be relatively high, and it is very easy to cause blocking. Therefore, pay special attention to this kind of printed matter not to be too tight on the roll. Take corresponding measures to improve the adhesion fastness of the ink layer. Ink containing metal powder in plastic film gravure has weak cohesion and poor adhesion. It is easy to cause adhesion under a small pressure, and it must be paid attention to.

4. Cellophane, nylon or a film with corona treatment on both sides has a strong affinity for contact with the ink layer on the back after winding, and there is a high risk of adhesion, so appropriate measures should be taken before printing . When only one-sided printing is performed, the corona treatment device should be adjusted to perform only one-sided processing. Slow down the printing speed. When the temperature of the drying oven is increased, the printed film must be fully cooled by a cooling roller before being wound up. For films that are easily stretched, such as polyolefin films, be especially careful not to wind them too tightly.

5. The printed matter should be kept in a cool place. During the storage process, it should be kept ventilated and dry, and the storage time should not be too long. It should avoid storing the printed materials under direct sunlight or near the heat source. When the warehouse is hot, ventilation and cooling measures should be taken.

6. Summary

Plastic film gravure printing is a comprehensive technology. There are many problems that can easily occur in the printing process. Blocking is one of the most common problems. In the rainy season of high temperature and high humidity, adhesion is particularly prone to occur. Because adhesion is not immediately noticeable (non-intuitive), it often causes great losses.

Summing up the experience of some predecessors, whether it is for our ink manufacturers or for printing professionals, there are many benefits, which is the original intention of this little thing.

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