Relationship between gravure ink and back tack and countermeasures
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In this paper, the most common "back-stick" phenomenon in plastic gravure printing, the reasons for its appearance, especially the ink manufacturing and use process, are described in more detail. Some suggestions and methods are also given from the perspective of ink production.
By blocking, it is meant that after printing, the ink layer on the print is adhered to the other contact surface (usually the back side of the film) or the ink is attached to the other contact surface. There are two kinds of sticking phenomenon: one is the adhesion of the plastic film on the back surface, which is usually called “back tack”, which is the illusion that the print is dry after the film is printed. After printing the roll or bag, the back side sticks again during the storage period. Dirty phenomenon. Specifically, it is not possible to smoothly unwind, or damage the printed pattern and contaminate the contact surface due to ink peeling. When the adhesion is light (the previous stage of the adhesion), it appears as a peeling sound when rewinding, or an abnormal gloss on the surface of the ink in the overlapping portion. In severe cases, the printed materials cannot be torn apart, and the ink layer of the graphic is dyed, which causes the products to be scrapped. Second, the surface of the plastic tubular film adheres to each other, and the mouth of the bag is difficult to be uncovered, and the user refuses to use it. Both of the above cases are more likely to occur in summer. The high temperature in the operation workshop and the relatively high wetness are one of the main causes of adhesion. However, it is sometimes related to the low content of the PE resin itself, 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 print has a dry illusion, but the back side is dirty during storage after printing the roll or bag. In severe cases, the print is not broken, and the ink layer is dyed. , resulting in product retirement. This phenomenon is more likely to occur during the hot season, but high temperature is not the only cause of back sticking. It is also closely related to a series of factors such as plastics, inks, solvent evaporation rate, air volume in the operating environment, and illumination.
First, the binder resin in the ink
The binder resin used in the ink is different, and the blocking resistance of the ink is also different. At present, the inks used for the gravure printing of plastic films are all solvent-based inks, and are composed of a binder resin, a pigment, a solvent, and the like. As the binder resin, it is required to be firmly bonded to the printing film, not adhered after printing, has certain flexibility, and can withstand a certain high temperature, and is not affected by 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 is slightly melted in a high temperature environment, so that the printed product is stuck after winding. Increasing the softening point of the binder resin is obviously advantageous for preventing blocking, but the softening point is too high, and the flexural property of the printing ink layer is deteriorated, and it is easy to be brittle. For example, a commonly used polyamide ink is printed on a PE film or a PP film. The binder of the ink is a polyamide resin, generally a low molecular weight linear polymer, which is composed of a dimer acid and an alkyl group (or an aryl group). The polyamine polycondensation results in a softening point of usually 100 to 110 ° C.
In general, in order to meet the needs of printing or to take economic considerations, it is often necessary to add a mixed solvent to the ink during printing. Each binder 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; 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 a mixed solvent, the solvent should be controlled within a certain range. It is necessary to take into account the printability requirements of the ink, 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 more easily the resin dissolves and the worse the solvent desolvability of the resin. In this regard, the softening point of the binder resin should also be increased accordingly.
In addition, when the ink is processed, if the polishing time is too long, the temperature generated by the polishing is too high, and the resin of the binder is partially denatured, which also has a certain influence on the adhesion.
Second, the solvent in the gravure ink failed to be 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 first evaporate, and the solvent inside the printing ink layer must first diffuse and penetrate the surface of the ink film before evaporative drying. When the drying of the printing ink layer enters the final stage, the surface layer of the printing ink film has solidified, which limits the continued diffusion and evaporation of the solvent inside the printing ink layer, thereby causing residual solvent problems.
Plastic film printing is the drying of the ink by solvent evaporation. Its volatilization speed is an important factor that determines the drying of the ink layer and affects the printing quality. The slower the solvent evaporates, the better the reproducibility of the plate, the more beautiful the color of the printed matter, but it is easy to stick; on the contrary, it will cause whitening of the print. Therefore, choosing the solvent with the proper volatilization speed is the key to gravure printing of plastic film. In continuous color printing, the film must be completely evaporated during the running time between the two printing stations. Otherwise, the roller will stick when printing at the next station. After the film printing, the organic solvent in the ink is not completely volatilized in the drying system, and there is still residual heat after the film is wound, which causes the solvent remaining in the ink layer to continue to volatilize and form blocking. During 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 maintains a micro-melt (wetting) state, which directly leads to the problem of blocking. Therefore, the amount of residual solvent must be controlled. The amount of the residual solvent is affected by the solvent release property of the above-mentioned binder resin, and is also affected by the following factors.
1, the solvent's ability to volatilize
The rate of volatilization of a single solvent is determined by its physical parameters. The correlation between volatilization capacity and solvent parameters is:
Where E is the solvent evaporation 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 volatilization rate of each solvent is different. In this case, the volatilization of the solvent will change, the volatile component will escape first, and the weakly volatile component will remain, causing the solvent to change composition, making it impossible to act like a single solvent. Thus, at a constant temperature, at a single rate of volatilization, the rate of evaporation of the solvent will gradually slow down. If the purity of the solvent is not satisfactory (such as containing too much high-boiling components) or if the slow-drying solvent is used excessively, severe residual solvent problems will occur under normal conditions. Therefore, the rational design of the mixed solvent formulation is a very important technical issue, and should be used with great care when using substitutes.
In addition, the surface characteristics, specific surface area and concentration of the pigment also have a certain influence on the volatilization of the solvent. For the same pigment, the solvent evaporation rate will decrease as the pigment concentration increases; for different pigments, in general, the pigment density is small, the solvent volatilization rate is low, the pigment particles are small, and the solvent volatilization rate Also low.
2, drying conditions
Drying conditions include dry air temperature, air volume (wind speed), drying device structure, and the like. Poor drying, the amount of residual solvent will increase. Therefore, the best possible drying conditions should be created. Increasing the air temperature and air volume (wind speed) will undoubtedly strengthen the drying conditions. However, it should be noted that in the case where the printing ink layer is thick, if the drying is too fast, the surface of the ink layer is quickly conjuncted, resulting in the inability of the internal solvent to escape, and the amount of residual solvent is increased. Further, if the temperature is too high, the ink film is easily softened.
3, printing speed
The speed of printing determines the length of drying time that the print can get. Therefore, the printing speed should be increased only when the printing ink layer is sufficiently dried first.
4, the humidity of the drying medium (air)
The humidity of the drying medium (air) has a large influence on the amount of residual solvent. First, if the moisture in the air enters the ink, the overall volatility of the solvent will be deteriorated. Second, the presence of a large amount of moisture in the drying medium (air) also inhibits the volatilization of the solvent. The ambient humidity is doubled, and the drying speed of the ink is generally nearly two times slower. It is also for this reason that adhesion defects are most likely to occur when printing plastic films during the rainy season. Therefore, in a high-humidity environment, the printing speed should be slowed down as much as possible, and the contact between the ink and the air should be reduced to ensure that the ink can be sufficiently dried. The relative humidity of the printing shop 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 tendencies for solvents. Aluminum foil, polyester, etc. are non-absorbent films, and the residual solvent is generally small; the polypropylene-based film tends to leave a hydrocarbon-based solvent, and the water-absorbent film (such as nylon or cellophane) tends to remain an alcohol-based solvent. In addition, additives added to some films may also affect the evaporation of the solvent. This increases the amount of residual solvent.
Countermeasure, the evaporation rate of the solvent in the ink depends not only on the boiling point of the solvent, vapor pressure, latent heat of vapor, but also on the temperature, humidity, wind base, solute and thickness of the ink layer in the operating environment. Therefore, the evaporation rate of the solvent in the ink should be adjusted at any time according to the change of the conditions. At normal temperatures, if the ink layer is too dry, a slow-releasing solvent should be added; otherwise, a solvent that evaporates quickly is added. It can also be diluted with a mixed solvent of xylene, ethanol and isopropanol (the three solvents are each mixed in a ratio of 1/3, and it is better to use a single solvent than the mixed solvent). If you feel too dry, you can use a little alcohol instead of some ethanol (butanol has the effect of increasing the gloss of the ink). But be sure to pay attention to the amount of addition, so as not to affect the drying of the print.
The solvent for gravure printing of plastic film is mainly composed of alcohol, benzene, alcohol, ethanol, isopropanol, benzene, toluene and xylene. Due to the need of nitrocellulose, esters such as ethyl acetate and butyl acetate are also added. In short, the organic solvent in the ink is required to be completely volatilized before winding. Appropriately slow down the speed and increase the air volume. After the film is printed, it is loosely placed in the wire woven basket, so that the ink layer can be continuously ventilated and dried, and then cut and bagged after curing. After film printing, the bag is wrapped and placed vertically in the carton to reduce the film surface. The pressure between.
Third, the adhesion fastness of the printed ink layer
The adhesion fastness of the printed ink layer is also closely related to the adhesion. When the adhesion of the printing ink layer is poor, it is easy to transfer the printing ink film under pressure to another film in contact with it, thereby causing blocking. Therefore, good ink adhesion fastness must be ensured. The reasons for the poor adhesion of the ink are as follows.
1. Is the ink used incorrectly or mixed with different types of ink?
2. The plastic film is poorly treated by corona or excessive moisture absorption.
3. The additive in the plastic film is precipitated, or the air is adsorbed to the film by the dust, thereby affecting the adhesion of the ink.
4. The ink is whitened and deteriorated.
5, poor drying.
Fourth, some other major issues
1. Poor ink in plastic gravure ink. The binder 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: 1 When the conditions permit, install the air conditioner in the operation workshop, control the room temperature between 18 °C and 20 °C, and control the relative humidity to less than 65%; 2 Replace the ink with good ink.
2, plastic film thin printing package is not suitable
Reasons: 1 Plastic film processed from non-packaging resin; 2 insufficient opening agent in resin.
Countermeasure: Replace the plastic film.
3, the impact of static electricity
Cause: The plastic film generates static electricity and forms a film to adhere to each other. Countermeasure: Add an antistatic agent to the plastic.
4, cooling conditions and storage environment
The printed matter has a high heat after passing through the drying box. The film that has passed through the drying system still has a certain amount of residual heat. If it is not cooled back, it will cause residual heat accumulation inside the finished semi-finished product, and then increase due to softening of the ink. The possibility of adhesion. Therefore, the printed film must be cooled before being wound up.
Cooling rolls are now commonly used for cooling. The smooth running speed of the chill roll, the printing line speed, etc. are the main factors affecting the cooling effect. If you forget to use tap water to cool during operation, the cooling roller loses its cooling effect. After the plastic film is completely printed, the heat will accumulate in the film roll and the temperature will rise to 50~60 °C. There are many examples of such adhesions.
In addition, during storage and handling, excessive temperature or poor ventilation may also cause adhesions.
5, the winding tension is too large. Excessive winding tension can directly increase the tendency of transfer between the printing ink layer and the contact surface. Therefore, under the premise of ensuring that the winding is neat, the winding tension should be minimized, and the winding diameter should not be too large.
6. The cohesive force of the ink layer is very weak (the cohesive force is extremely poor). When the cohesive force of the ink film is very weak, the separation of the ink film occurs under a small pressure, causing adhesion.
7. The pressure on the film roll is too high, and the pressure on the film roll tends to increase the tendency of the ink layer to transfer to the contact surface. The straight storage of the film roll can effectively reduce the pressure between the ink film and the contact layer.
8. If 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, a part (or all) of the ink layer is easily transferred to the contact surface under pressure to cause adhesion.
Fifth, the solution and precautions to solve the adhesion failure
Through the analysis of the above factors affecting adhesion, we can summarize the solutions and precautions for plastic gravure adhesion failure.
1. Adding additives to the resin can effectively prevent internal sticking, and the test effect is good. The specific method is as follows. The oleic acid amide is selected, the chemical structure is C17H33CONH2, the iodine value is ≤86g iodine/100g, the acid value is ≤0.8mgKOH/g, the melting point is 72~76°C, white or yellow waxy. It is a kind of slip agent, added to PE resin, heated by screw and kneaded, melted and uniformly dispersed in plastic at about 50 °C. After high temperature extrusion, some oleic acid amide oozes out the surface to form a The extremely thin wax film makes the film layer and the layer not in direct contact with each other, and does not have affinity, so it can play an anti-blocking effect.
The operation method is as follows: the oleic acid amide is finely ground or crushed in a pulverizer. Because the beginning of the incoming material is uneven, no smashing will cause local concentration of the additive. After pulverization, the sieve was sieved through a 50-mesh sieve to remove the coarse chips, and added to the LDPE pellets, and the ratio of 100 parts of the resin was added to 0.1 part of oleic acid amide, which was simply stirred, and added to a hopper for direct blow molding. Other operating processes are unchanged.
The effect: after adding the additive, not only effectively prevents the inner layer of the film from sticking, but also because the film forms a uniform layer of extremely thin wax, the smoothness and transparency of the film are greatly improved.
After gravure printing, the brightness of the ink is greatly improved, and it has no effect on the ink. However, after the amount exceeds 0.3%, the surface tension of the film is difficult to control, and the ink repellency is lowered.
Therefore, the dosage should be controlled below 0.3%, and the film should be corona treated after blow molding to control the surface tension of the printing film to be 38×10-5 N/cm 2 or more. When the content of the formula is 0.2%, the screw and the machine head are easy to be removed, and the lubricity of the screw protects the screw, the barrel and the machine head. Oleic acid amide also has a certain antistatic effect in plastics. Blown color film, adding oleic acid amide, can improve the coloring dispersibility, eliminate the phenomenon of color masterbatch accumulation, and have no adverse effect on color. In addition, there is no effect on the process of heat sealing the bag, cutting 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 weight. 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. At the time of printing or re-winding inspection, special attention is paid to films such as PET and NY. During the transportation and handling process, keep the film that has been printed and rolled up upright, and do not put it down. Otherwise, the partial pressure of the film roll will be too large and cause adhesion. Do not concentrate on printed parts on specific parts. For multi-color printing, it should be considered that the pattern design should not be too much color. If possible, it can be printed with spot color ink 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 of the film on which the printed film is printed is certainly relatively large, and it is very likely to cause adhesion. Therefore, it is especially important for such prints that the rolls are not too tight. Take corresponding measures to improve the adhesion fastness of the ink layer. The ink containing the metal powder in the gravure printing of the plastic film has a weak cohesive force of the ink layer and poor adhesion, and it is easy to cause adhesion under a small pressure, and sufficient attention must be paid.
4. Cellophane, nylon or corona-treated film on both sides. After winding, the back side has a strong affinity for contact with the ink layer, and the risk of blocking is high. Therefore, 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 raised, the printed film must be sufficiently cooled by a cooling roll before being wound up. For polyolefin-based films, which are easy to stretch, pay particular attention not to roll too tightly.
5. The printed matter should be kept in a cool place. Keep it ventilated and dry during storage, and the storage time should not be too long. It should avoid storing the wounded printed matter under direct sunlight or near the heat source. The warehouse should adopt ventilation and cooling measures when the climate is hot.
Summary
Plastic film gravure printing is a comprehensive technology, and there are many problems that are easy to occur in the printing process. Adhesion is one of the most common problems. In the high temperature, high humidity plum rain season, adhesion is particularly prone to occur. Because adhesions are not immediately noticeable (non-intuitive), they often cause significant losses.
Summarizing the experience of some seniors, whether it is for our ink manufacturers, or for the printing industry, there are many benefits, this is the original intention of this small thing.