Abstract: In this paper, the most frequently occurring phenomenon of "stick-back" in plastic gravure printing is explained in detail in all aspects, including the manufacturing and use of ink. Also from the ink production point of view, some suggestions and methods are given.
Keywords: gravure, plastics, inks, adhesives, additives
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. There are two phenomena of adhesion: First, the plastic film printing back surface adhesion, usually called "back sticking", is the imprint of the film printed after drying, after printing the roll or bag, the back of the stick in the storage period Dirty phenomenon. 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. In severe cases, the parts cannot be torn between the prints, and the ink layer of the image is shifted to cause the product to be scrapped. The second is that the surface of the plastic tubular film sticks to each other and the mouth of the bag is difficult to open. The user refuses to use it. Both of the above situations are more likely to occur in summer. The high temperature of the operation shop and the relative humidity are one of the main causes of adhesion. But sometimes it is related to the too little content of the slip agent in the PE resin itself, and it is also closely related to a series of factors such as air volume and speed.
After the printing of plastic film or transparent paper, the imprints appear dry, but after the printing of the reel or bag storage, there is a phenomenon that the back surface becomes dirty. In severe cases, the printed parts cannot be separated, and the ink layer of the image is shifted. , causing the product to be scrapped. This phenomenon is more likely to occur in the hot season, but the high temperature is not the only cause of back tack. It is also closely related to a series of factors such as the plastic, ink, solvent volatilization rate, operating environment, air volume, and illumination.
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.
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 have a softening point of usually 100 to 110°C.
Under normal circumstances, in order to meet the needs of printing or considering economical, often mixed solvents are added to the ink at the time of 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, and the better the solubility of the solvent to the resin, the worse the release of the resin 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.
In addition, when the ink is processed, if the polishing time is too long, the temperature generated by the polishing will be too high, which will cause partial degeneration of the resin of the binder, and will also have a certain impact on the adhesion.
Second, the solvent in the gravure ink is not 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 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.
Plastic film printing is the drying of ink by solvent evaporation. The volatilization rate is an important factor that determines the drying of the ink layer and affects the printing quality. The slower evaporation of the solvent, the better the reproducibility of the printing plate, the more beautiful the print color, but easy to stick; Therefore, the choice of a suitable solvent for evaporation speed is the key to the gravure printing of plastic films. In continuous color printing, the solvent must be completely evaporated after the film is in operation between the two printing stations. Otherwise, the rollers will stick when printing at the next station. After the film is printed, the organic solvent in the ink is not thoroughly evaporated in the drying system. After the film is wound, there is still residual heat, and the solvent remaining in the ink layer brings about the condition of continuing to volatilize, and the adhesion is formed. In the hot and humid season, after printing, the film is wound up tight or under pressure.
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, the volatility of the solvent
The volatilization 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 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;
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 stay. This will cause the composition of the solvent to 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, under normal circumstances, 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, 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, the humidity of the drying medium (air)
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, some additives added to the film can also affect the volatilization of the solvent. This increases the amount of residual solvent.
Countermeasures, the volatilization rate of the solvent in the ink, not only depends on the boiling point of the solvent, vapor pressure, vapor latent heat, but also depends on the operating environment of temperature, humidity, wind base, solute and ink layer thickness and other conditions. Therefore, the volatilization rate of the solvent in the ink should be adjusted at any time according to the change of conditions. At normal temperatures, if the ink layer dries too quickly, a slow solvent should be added; otherwise, a fast solvent. 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 alcohol (butanol has the effect of increasing the luster of ink). However, it must be noted that the amount of addition should be appropriate so as not to affect the drying of the prints.
Solvents for gravure printing of plastic films are mainly based on alcohols, together with benzenes, alcohols, ethanol and isopropanol, and benzene and toluene. Due to the requirement of nitrocellulose, esters such as ethyl acetate and butyl acetate are also added. In short, the organic solvent in the ink is sought to completely evaporate before being wound up. Slow down the speed and increase the air volume. After the film is printed, it is loosely placed in a wire woven basket so that the ink layer can continue to be ventilated and dried. After solidification, the film is cut and bagged. After the film is printed and bagged, the film is wrapped and placed in the carton to reduce the film surface. The pressure between.