With the wide application of plastic packaging in daily life, the plastic film printing industry has been greatly developed. Plastic film printing is a comprehensive technology. There are many problems that easily arise in the printing process. Sticking is one of the most common problems. In the high-temperature, high-humidity rainy season, adhesions are particularly prone to occur. Because adhesion is not immediately noticeable (unintuitive), it often causes great losses.
By adhesion, it is meant that after printing is wound up, the ink layer on the print is adhered to another contact surface (usually the back side of the film), or the ink is attached to another contact surface. The specific performance is that it cannot be unwound smoothly, or the printing pattern may be damaged and the contact surface may be stained due to ink peeling. When the adhesion is light (the previous stage of adhesion), it appears as a peeling sound upon rewinding, or an abnormal gloss on the ink surface of the overlapping portion.
The cause of adhesion of plastic film gravure printing products is relatively complex, involving many factors such as ink, solvent, printing film, and process conditions. Adhesion may occur during printing, compounding, slitting, and other processing. This article focuses on the influencing factors and solutions for blocking failures in the printing process.
From the actual statistical situation, the phenomenon of adhesion is mainly caused by the stickiness of the ink, the temperature is too high, the residual solvent is too much, the pressure is too large and other factors. The following is a detailed analysis.
First, the binder resin in ink
The binder resin used in the ink differs in the anti-blocking properties of the ink. At present, the inks used for the gravure printing of plastic films are all solvent-type inks and consist of a binder resin, a pigment, and a solvent. As a binder resin, it is required to have a firm bond with the printing film, no adhesion after printing, certain flexibility, and a certain high temperature, and it is not affected by external factors in a certain range.
1. The softening point of the binder resin
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 state of micro-melting under a high temperature environment, so that the printed product will be blocked after winding up. Increasing the softening point of the binder resin clearly helps to prevent blocking, but if the softening point is too high, the flexural properties of the printing ink layer will be deteriorated and brittle cracking will occur.
Taking the polyamide ink used for printing PE and PP films as an example, the linking material of this ink is a polyamide resin, which is generally a linear polymer of low molecular weight and is composed of dimer acid and alkyl (or aryl) II. Polyamines obtained by polycondensation usually have a softening point of 100-110°C.
2. Resin release of solvents
We all know that in order to adapt to the needs of printing or consider the economy, we often need to add a mixed solvent in the printing ink. 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 poorer the release of the solvent from the solvent. Poor release can directly cause the problem of residual solvents 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, but also to consider 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 poorer the desolvation of the resin. From this point of view, the softening point of the binder resin should also be increased accordingly.
3. Grinding time and grinding temperature
In the processing of ink, if the grinding time is too long, the temperature generated by the grinding will be too high, which will lead to partial denaturation of the binder resin, and will also have a certain impact on the adhesion.
Second, residual solvent
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 in the printing ink layer must diffuse into 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 within the printing ink layer, thereby causing a problem of residual solvents.
When the residual solvent in the printing ink layer reaches a certain concentration, the printing ink layer remains in a slightly fused (wet) state, which directly leads to the problem of blocking. 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. Solvent evaporation capacity
The volatilization rate of a single solvent is determined by its physical parameters. The correlation between volatility and solvent parameters is:
E=KP25M/d25
Where E is the solvent volatilization rate, P25 is the saturated vapor pressure of the solvent at 25°C, d25 is the specific gravity of the solvent at 25°C, M is the molecular weight, and 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 volatilization of the solvent will change, the volatile components will first escape, and the less volatile components will remain. As a result, the composition of the solvent will change and it will not be like a single solvent. That is, at a constant temperature, it escapes with a single volatilization rate, and the volatilization rate of the solvent gradually slows down.
If the solvent is unsatisfactory in purity (eg, contains too much high-boiling components) or excessively slow-drying solvents are used, severe residual solvent problems can occur under normal conditions. Therefore, the rational design of the mixed solvent formulation is a very important technical issue. It should be used with caution when using substitutes.
In addition, the surface characteristics, specific surface area, and concentration of the pigment all have a certain influence on the volatilization of the solvent. For the same pigment, the solvent volatilization rate will decrease with increasing pigment concentration; for different pigments, generally, the pigment has a small specific gravity, the solvent has a low volatilization rate, the pigment particles are small, and the solvent volatilization rate is small. Also low.
2. Dry conditions
Drying conditions include drying air temperature, air volume (wind speed), drying device structure, and the like. Bad 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 enhance the drying conditions. However, it should be noted that, in the case of a thick printing ink layer, if it is dried too fast, the surface of the ink layer will be quickly congealed and the internal solvent will not be able to escape, which will increase the amount of residual solvent. In addition, the temperature is too high, it is easy to soften the ink film.
3. Printing speed
The speed of printing determines the length of drying time that can be obtained for printing products. Therefore, the printing speed should be improved only when the printing ink layer is first ensured to be sufficiently dry.
4. Drying medium (air) humidity
The humidity of the drying medium (air) has a great influence on the amount of residual solvent. One is that the moisture in the air, if it enters the ink, will cause the overall volatility of the solvent to deteriorate. The second is the large amount of moisture in the drying medium (air), which also inhibits the volatilization of the solvent.
The humidity of the environment doubles and the drying speed of the ink is generally slowed by nearly twice. It is also for this reason that adhesion failures are most likely to occur when plastic films are printed during the rainy season. Therefore, in a high-humidity environment, the printing speed should be reduced as much as possible, and the contact between ink and air should be reduced to ensure that the ink can be fully dried.
The relative humidity of a printing shop should generally not exceed 70%. Of course, it should not be too dry, otherwise it will easily cause static electricity problems.
5. Printing film
Films of different materials have different selectivity for solvent absorption. Aluminum foil, polyester, etc. are non-absorbent films with less residual solvent; polypropylene-based films tend to leave hydrocarbon-based solvents, while water-absorbent films (eg, nylon, cellophane) tend to retain alcohol-based solvents. In addition, additives added to certain films can also affect the volatilization of the solvent, thereby increasing the amount of residual solvent.
Third, the adhesion of the printing ink layer
The fastness of adhesion of printing ink layers is also closely related to adhesion. When the adhesion fastness 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 must be ensured. The causes of poor ink fastness are as follows:
(1) Misuse of ink or mixing of different types of ink.
(2) Plastic film corona treatment or excessive moisture absorption.
(3) The additive in the plastic film is precipitated, or the air is adsorbed by the dust on the film, thereby affecting the adhesion of the ink.
(4) Whitening and deterioration of ink.
(5) Bad drying.
Fourth, cooling conditions and storage environment
The printed matter has a high heat after passing through the drying oven. The film coming out of the drying system still has a certain amount of waste heat. If it is not cooled back, it will cause waste heat accumulation inside the rolled semi-finished product, and then increase due to ink softening. The possibility of adhesion. Therefore, the printed film must be cooled before rewinding.
Cooling rollers are now commonly used for cooling. Cooling roll temperature, running smoothness, printing line speed, etc. are the main factors affecting the cooling effect. If you forget to use tap water for cooling during operation, the cooling roller loses its cooling effect. After the plastic film is completely printed and the roll is closed, the heat will accumulate in the film roll and the temperature will rise to 50-60°C. There are many examples of this causing adhesion.
In addition, during storage and handling, too high temperatures or poor ventilation may cause blocking.
V. Others
(1) The winding tension is too large
Too much tension in the winding can directly increase the transfer tendency 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 reduced as much as possible, and the winding diameter should not be too large.
(2) The ink layer has very weak cohesive force (unfavorable cohesion)
When the cohesion of the ink film is very weak, separation of the ink film occurs under a small pressure, causing blocking.
(3) The membrane is under excessive pressure
Too much pressure on the film roll will increase the tendency of the ink layer to move toward the contact surface. Vertically storing the film roll can effectively reduce the pressure between the ink film and the contact layer.
(4) The affinity between printing surface and contact surface is too strong
When the affinity of 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 blocking.
Six, to solve the adhesion failure method and precautions
By analyzing the above factors that affect adhesion, we can summarize the solutions and precautions for plastic gravure adhesion failures:
(1) Control the use of slow-drying solvents and residual solvents.
(2) Try to improve the performance of the drying oven so that it can supply sufficient heat and air volume.
(3) When the chill roll is rotated smoothly and the rewinder is rewinded, the temperature rise caused by the slippage of the paper tube and the roll is noticed.
(4) When printing or recoiling, pay special attention to thin films such as PET and NY. During transport and handling, the printed film should be kept upright and not tipped over. Otherwise, the local pressure of the film roll will be too large and cause blocking.
(5) The printed pattern should not be concentrated in a specific area. For multi-color printing, it should be considered in the design of the pattern can not be too much overlap, if possible, you can use spot color ink printing to prevent the local ink layer is too thick.
In the case where the printed pattern on the plastic film is deviated to one side, the pressure on the side of the rolled-up film on which the printed pattern is printed must be relatively large, and blocking is very likely to occur. Therefore, especially for this type of printed matter, care should be taken not to make it too tight.
(6) Take corresponding measures to improve the adhesion of the ink layer.
(7) The ink containing metal powder in the plastic film gravure ink has weak ink layer cohesion and poor adhesion, and it is very easy to cause adhesion under very small pressure and must be fully noted.
(8) Cellophane, nylon or corona-treated films on both sides,