Qual è il rapporto tra l'inchiostro rotocalco e l'appiccicosità?

In the world of gravure printing on plastic films, there is a tricky problem – blocking, which is like a hidden spectre that often inadvertently causes serious damage to the printed product.
Adhesion refers to the phenomenon of the ink layer on the printed product sticking to another contact surface (usually the back of the film) or the ink transferring to another contact surface after printing is complete and the roll is wound up. This phenomenon has caused major losses in the printing industry throughout history. For example, in the 1980s, due to a lack of understanding of the adhesion problem, large printing factories had to scrap a large number of printed products during the hot and humid summer months, resulting in huge economic losses.
There are two main forms of adhesion. One is the adhesion of the back surface of plastic film printing, which is commonly referred to as “back adhesion”. This situation is like the printed film after printing is cheating us, it seems to be dry, but in the storage stage after printing the roll or making bags, the problem of adhesion on the back surface comes up. Specifically, this can lead to problems with unrolling the film smoothly. In severe cases, the ink peeling off can damage the printed pattern and contaminate the contact surface. In the early stages of light adhesion, you can hear a peeling sound when rewinding, or an unusual sheen on the surface of the ink on the overlapping part. If the situation is serious, the printed items will stick together tightly, the graphic ink layers will contaminate each other, and the product will have to be scrapped. Another phenomenon is the surface of the plastic tubular film sticking to each other, making it difficult to open the bag. This will cause great dissatisfaction among users, who will refuse to use it. It is no coincidence that these two situations occur frequently in the summer. From a professional point of view, high temperatures and high relative humidity in the workshop are one of the main causes of adhesion. However, there are also complex factors behind this, such as the PE resin itself having too little slip agent, and a series of factors such as air volume and speed are also closely related.
After printing on plastic film or transparent paper, the print may appear to dry, but then the backside becomes sticky. In severe cases, the print cannot be separated and the ink layer of the graphics is transferred, resulting in product scrapping. This is more likely to occur during the hot season. However, high temperatures are by no means the only factor leading to stickiness. As revealed by many historical cases of printing technology innovation, it is inextricably linked to many factors such as the speed of volatilization of plastics, inks and solvents, the air volume of the operating environment, and light.
1. Binder resin in the ink
The ink used in gravure printing on plastic film is solvent-based and consists of binder resin, pigments, solvents, etc. For the binder resin, it is like the skeleton of the ink, which needs to adhere closely to the film substrate, not stick after printing and winding, and also have a certain degree of flexibility and high temperature resistance, and be unaffected by external factors within a certain range.
The softening point of the binder resin is the key to the performance of the printed ink layer. Take a large-scale printing project in history as an example. Due to the use of a binder resin with too low a softening point, the printed ink layer was in a slightly melted state in a high-temperature environment, causing the printed product to stick together after winding. Raising the softening point seems like a good solution, but if the softening point is too high, the flexibility of the printing ink layer will deteriorate, and it will become prone to brittle cracking. Take polyamide inks, which are commonly used for printing on PE and PP films. The binder is a polyamide resin, which is a low-molecular-weight linear polymer obtained by polycondensation of a dimer acid and an alkyl (or aryl) diamine. The softening point is usually 100–110°C.
When printing, a mixed solvent is often added to the ink to meet printing needs and consider costs. Here is an interesting phenomenon: 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 molecule. However, the better the solubility of the solvent in the resin, the worse the resin’s release of the solvent, which can lead to residual solvent problems. From this perspective, when formulating a mixed solvent, the amount of solvent must be controlled within a reasonable range, which must satisfy the printability of the ink and also take into account the overall solubility of the solvent. For the same type of binder resin, the lower the resin softening point, the easier it is to dissolve, and the worse the desolventability. Therefore, from this perspective, it is necessary to appropriately increase the softening point of the binder resin.
In addition, if the grinding time is too long when processing the ink, the high temperature generated by the grinding may partially denature the binder resin, which will also affect the adhesion problem. This is like a precise mechanical structure, where any small problem may affect the overall operation.
2. The solvents in gravure inks do not completely evaporate after printing
When the printing ink is transferred to the plastic film, the solvents on the surface of the printed ink layer will evaporate first, while the solvents inside will have to diffuse and penetrate to the surface of the ink film before they can continue to evaporate and dry. When the drying of the printed ink layer is almost complete, the surface of the ink film has cured, which restricts the diffusion and evaporation of the solvents inside, and the problem of residual solvents arises.
In plastic film printing, solvent evaporation is the way the ink dries, and the rate of solvent evaporation has a crucial impact on the drying of the ink layer and the printing quality. A slow evaporation rate gives good reproducibility of the plate and produces a beautiful color in the printed matter, but it also makes the printed matter stick together easily. Conversely, a fast evaporation rate can cause the printed matter to appear whitish. This is a delicate balancing act, and choosing the right evaporation rate is the key to success in plastic film gravure printing. In continuous color printing, if the solvent has not completely evaporated while the film is running between the two printing stations, it will stick to the rollers when printed at the next station.
After the film is printed, if the organic solvents in the ink do not completely evaporate in the drying system, and there is residual heat after the film is wound, this creates conditions for the residual solvents to continue evaporating, which can lead to sticking. This is especially the case during the hot and humid season, and the problem is more serious when the film is wound too tightly or under pressure after printing. When the residual solvent in the printed ink layer reaches a certain concentration, the printed ink layer will remain in a slightly melted (wetted) state, which directly causes sticking. Therefore, the residual solvent content must be strictly controlled. In addition to the solvent release of the binder resin, the residual solvent content is also affected by the following factors.
1. Solvent volatility
The evaporation rate of a single solvent is determined by its physical parameters, while the solvents in the printing ink film are mixed solvents, and each solvent has a different evaporation rate. This is like a complex chemical cocktail, with the highly volatile components escaping first, while the less volatile ones remain behind, causing changes in the solvent composition. Unlike a single solvent, which evaporates at a constant rate at a constant temperature, the solvent evaporation rate will gradually slow down. If the solvent purity is not up to standard (e.g. containing too many high boiling point components) or if slow-drying solvents are used excessively, serious residual solvent problems will arise under normal conditions. Therefore, the rational design of mixed solvent formulations is an extremely important technical task, and even greater care must be taken when using substitutes.
In addition, the surface characteristics, specific surface area and concentration of the pigment also have an impact on solvent evaporation. For the same pigment, the solvent evaporation rate decreases with increasing pigment concentration; for different pigments, in general, the solvent evaporation rate is low for pigments with low density and small particles.
2. Drying conditions
Drying conditions include the drying air temperature, air volume (air speed), and the structure of the drying device. Poor drying will increase the amount of residual solvent. Increasing the air temperature and air volume (air speed) can enhance the drying conditions. However, it should be noted that if the printed ink layer is thick, drying too quickly will cause the ink layer surface to quickly form a film, preventing the internal solvent from escaping. This is like accelerating a car. If not controlled well, it may cause the brakes to fail.
3. Printing speed
The printing speed determines the drying time of the printed matter. Only when the printing ink layer is fully dried can the printing speed be increased. This is like a running competition, you must first ensure that your physical strength can support the whole process, and then you can consider speeding up.
4. Humidity of the drying medium (air)
The humidity of the drying medium (air) has a significant impact on the amount of residual solvent. On the one hand, the moisture in the air entering the ink will worsen the overall volatility of the solvent; on the other hand, the presence of a large amount of moisture in the drying medium will inhibit solvent volatilization. During the rainy season, when the ambient humidity doubles, the drying speed of the ink generally slows down by nearly two times. This is why printing on plastic film is most prone to adhesion problems at this time. Therefore, in high humidity environments, the printing speed should be slowed down to reduce the contact between the ink and the air and ensure that the ink is fully dried. At the same time, the relative humidity in the printing workshop should generally not exceed 70%, but it should not be too dry either, otherwise static electricity problems are likely to occur.
5. Substrate film
Films of different materials have different selective absorption tendencies for solvents. Non-absorbent films such as aluminum foil and polyester generally have less solvent residue; polypropylene films tend to retain hydrocarbon solvents, while absorbent films (such as nylon and cellophane) tend to retain alcohol solvents. In addition, the additives added to some films can also affect solvent evaporation, thereby increasing the amount of residual solvent.
Countermeasures: The rate at which solvents in ink evaporate depends not only on the boiling point, vapour pressure and latent heat of evaporation of the solvent, but also on the temperature, humidity, air volume, solute and ink layer thickness of the operating environment. Therefore, the rate at which solvents in ink evaporate must be adjusted at any time according to changes in conditions. If the ink layer dries too quickly at normal temperatures, a solvent with a slower evaporation rate can be added; if not, a solvent with a faster evaporation rate can be added. It can also be diluted with a mixed solvent of xylene, ethanol and isopropanol (each of the three solvents is mixed in a ratio of 1/3, and the mixed solvent is more effective than a single solvent). If you feel that it is drying too quickly, you can replace some of the ethanol with butanol (butanol can increase the gloss of the ink). However, be careful with the amount added so as not to affect the drying of the printed matter.
The solvents used for gravure printing on plastic film are mainly alcohols, supplemented by benzenes. The alcohols are mostly ethanol and isopropanol, and the benzenes are mostly toluene and xylene. Due to the needs of nitrocellulose, esters such as ethyl acetate and butyl acetate are also added. In short, the organic solvents in the ink must be allowed to evaporate completely before winding. Reduce the speed appropriately and increase the air volume. After the film is printed, place it loosely in a wire-woven basket so that the ink layer can continue to dry in the air and cure before being slit and made into bags. After the film is printed and made into bags, wrap it well and place it upright in a cardboard box to reduce the pressure between the film surfaces.
3. Adhesion fastness of the printed ink layer
The adhesion of the printed ink layer is closely related to adhesion. When the adhesion of the printed ink layer is poor, under pressure the printed ink film can easily transfer to another film in contact with it, causing adhesion. Therefore, ensuring good ink adhesion is crucial. The causes of poor ink adhesion are as follows:
1. Whether the wrong ink has been used or different types of ink have been mixed. This is like using the wrong seasoning when cooking, which can completely change the taste of the dish.
2. Poor corona treatment of the plastic film or excessive moisture absorption. Corona treatment is like giving the film surface a “facelift”, and poor treatment can affect ink adhesion.
3. The additives in the plastic film are precipitated, or air is adsorbed onto the film by dust, which affects the adhesion of the ink.
4. The ink whitens and deteriorates.
5. Poor drying.
Other major problems
1. Poor ink properties of plastic gravure inks. Some plastic gravure inks have a binder with a low melting point, which is prone to adhesion when the operating environment is hot and humid. Countermeasures: First, where conditions permit, install an air conditioner in the operating workshop to control the room temperature between 18°C and 20°C and the relative humidity below 65%. Second, replace the ink with a good ink property.
2. Poor suitability of thin plastic film for printing packaging. Reasons: First, plastic film processed using resins not intended for packaging is used; second, there is insufficient opening agent in the resin. Countermeasures: Replace the plastic film.
3. The influence of static electricity. Reason: The static electricity generated by the plastic film causes the film to adhere to each other. Countermeasures: Add an antistatic agent to the plastic.
4. Cooling conditions and storage environment. Printed products pass through a drying oven with a lot of heat, and the film coming out of the drying system also has residual heat. If it is not cooled back, the residual heat inside the rolled semi-finished product will accumulate, the ink will soften, and the possibility of adhesion will increase. Therefore, printed film must be cooled before winding, and cooling rollers are now commonly used for cooling. The temperature of the cooling roller, its smooth operation, the printing line speed, etc. are the main factors affecting the cooling effect. If you forget to use tap water to cool it during operation, the cooling roller will lose its cooling effect. After the film is printed and rolled up, heat will accumulate in the film roll, and the temperature can rise to 50-60°C. This type of adhesion is not uncommon. In addition, high temperatures or poor ventilation during storage and handling may also cause adhesion.
5. Excessive winding tension. Excessive winding tension will increase the tendency of the printed ink layer to transfer to the contact surface. Therefore, while ensuring that the roll is wound neatly, the winding tension should be minimized as much as possible, and the winding diameter should not be too large.
6. The ink layer has very weak cohesion (extremely poor cohesion). When the cohesion of the ink film is weak, the ink film will separate under very little pressure, causing adhesion.
7. The pressure on the film roll is too high. Excessive pressure on the film roll will increase the tendency of the ink layer to transfer to the contact surface. Storing the film roll vertically can effectively reduce the pressure between the ink film and the contact surface.
8. The affinity between the printing surface and the contact surface is too strong. At this time, part (or all) of the ink layer can easily transfer to the contact surface under pressure, causing adhesion.
4. Solutions to adhesion faults and precautions
Through the analysis of various factors affecting adhesion, we can summarize the solutions and precautions for adhesion faults in plastic gravure printing.
1. Adding additives to the resin can effectively prevent internal sticking and has a good effect. For example, oleic acid amide (chemical structure C17H33CONH2, iodine value ≤ 86g iodine / 100g, acid value < 0.8mgKOH/, melting point 72 – 76°C, white or yellow waxy) can be used. It is a slip agent. It is added to the PE resin and mixed with the screw at a temperature of about 50°C. It melts and disperses evenly in the plastic. After high-temperature extrusion, some oleic acid amide oozes out to the surface, forming an extremely thin wax film. This prevents the film layers from coming into direct contact and forming an affinity, thus preventing sticking.
The operating method is as follows: First, crush the oleic acid amide, as the incoming material may not be uniform, and not crushing it will cause the additive to concentrate locally. After crushing, sift through a 50-mesh screen to remove coarse particles, add to the LDPE pellets, and mix at a ratio of 100 parts resin to 0.1 part oleic acid amide. After simple stirring, add to the hopper and directly blow mould, with no other changes to the process. This has a significant effect, not only preventing the inner layers of the film from sticking together, but also forming a uniform, extremely thin layer of wax on the film, which greatly improves the finish and transparency of the film. After gravure printing, the brightness of the ink is significantly improved, and there is no effect on the ink receptivity. However, when the dosage exceeds 0.3%, the surface tension of the film is difficult to control and the ink receptivity decreases. 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 printed film to above 38×10 – 5N/cm². When the content in the formula is 0.2%, it will be easier to disassemble and clean the screw and die head. At the same time, its lubricity has a protective effect on the screw, barrel and die head. Oleamide also has a certain antistatic effect in plastics. Adding oleamide when blowing colored films can improve color dispersion, prevent the accumulation of color masterbatch, and has no adverse effect on the color. It also has no effect on processes such as heat sealing bags and slitting and cutting.
2. Control the use of slow-drying solvents and residual solvents. Maximize the performance of the drying oven to ensure that it can provide sufficient heat and air volume. Ensure that the cooling roller rotates smoothly. When rewinding, pay attention to the abnormal temperature rise caused by the sliding of the paper tube and roller. Pay special attention to thin films such as PET and NY during printing or rewinding inspection. During transportation and handling, keep the printed film rolled up upright to avoid it falling over and prevent localised excessive pressure on the film roll causing it to stick. The printed pattern should not be concentrated in a specific area. For multi-color printing, the design of the pattern should take into account the fact that there should not be too much color overlap. If possible, spot color ink can be used to prevent the ink layer from becoming too thick in some areas.
3. If the printed pattern on the plastic film is biased to one side, the side with the printed pattern will be under greater pressure during winding, which can easily cause adhesion. Therefore, for this type of printed product, special attention should be paid to not winding too tightly, while taking measures to improve the adhesion of the ink layer. In gravure printing on plastic films, the ink layer containing metal powder has weak cohesion and poor adhesion, and it is easy to cause adhesion under very little pressure.
4. Cellophane, nylon or films with corona treatment on both sides have strong affinity with the ink layer on the back after winding, and the risk of adhesion is high. Therefore, appropriate countermeasures must be taken before printing. If only one side is printed, the corona treatment device should be adjusted to treat only one side. Reduce the printing speed and increase the temperature of the drying oven. After printing, the film must be fully cooled by a cooling roller before being wound. For polyolefin films that are prone to stretching, be especially careful not to wind them too tightly.
5. Store printed products in a cool place. During storage, keep them ventilated and dry. Do not store them for too long. Avoid storing printed products that have been wound up in direct sunlight or near heat sources. In hot climates, take measures to ventilate and cool the warehouse.

Gravure printing on plastic film is a complex and comprehensive technology. There are numerous problems during the printing process, and one of the most common problems is adhesion, which is particularly likely to occur during the hot, humid rainy season. Because adhesion is non-intuitive, it often causes huge losses by the time it is discovered. It is like a bomb hidden in the dark, ready to detonate at any time and threaten the quality of the printed matter. Therefore, we must gain an in-depth understanding of the root causes of the adhesion problem, take effective preventive and remedial measures, and ensure print quality.

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