Editor's note: Gravure printing is an important printing method for packaging printing, especially in the printing of flexible packaging plastic film. As the absorptivity of plastic film is not as good as that of paper, there may be a phenomenon that the ink adhesion is not strong. This paper analyzes the reasons for this phenomenon and provides some solutions for your reference.
The method for judging the adhesion fastness of ink printing is generally determined by using glass paper pressure sensitive adhesive tape. The printing fastness varies with the ink type. Careful reference should be made to the specific solution. The main reasons are:
First, misuse ink type or mixed with different inks.
There are many types of inks, suitable for different substrates. If you use the wrong ones, there will be loose adhesion. For example, the gravure ink of cellophane is nitrocellulose, and the ink of printing paper is rosin. If it is used wrongly, it will produce The phenomenon of poor adhesion makes the ink also play a decisive role in the problem of the fastness of the printing ink layer. For different substrates, the corresponding ink should be used. For cellophane, nitro-Silicon resin ink can be used. For polypropylene film, chlorinated polypropylene resin ink can be used. For polyethylene film, polyamide resin can be used. Ink and so on. In actual production, in order to adjust the viscosity and volatilization rate of the ink, a certain amount of solvent is often added. However, when using solvents, mixed solvents are often used. Several problems should be noted:
(1) When adding the solvent, the solvent with lower surface tension should be used as far as possible in order to reduce the surface tension of the entire ink system, thereby improving the wettability of the printing film;
(2) When the solid content of the work is too high, the wetting effect will be relatively low, thereby affecting the ink fastness. On the contrary, if too much solvent is added, the effective components that the ink adheres to the film will be relatively reduced. Fastness decreased.
In order to solve the contradiction between the fastness of the ink layer and the volatilization rate of the ink, a resin (rubber) or other suitable compatible adhesive resin or rubber corresponding to the ink may be added to the diluent or the ink. This can also improve the fastness of the ink layer to a certain extent. At the same time, the ink is left for a long time, or mixed with impurities, which can cause deterioration of the connecting material. Therefore, fresh ink should be used as much as possible or the changed ink should be replaced in time.
Second, the ink swollen, or solid material precipitation caused by weak adhesion, to prevent the ink from swell, you can not use excessively quick-drying solvents, should be combined with the right amount of slow-drying solvents, to minimize the opportunity for ink and air contact, The moisture in the air invades the ink, increasing the viscosity of the ink and allowing the resin to precipitate and gel.
In the printing, there is a drop in gloss, a decrease in the concentration, a poor transfer, and a failure in plugging the plate. Therefore, the ink tank should be sealed and the air blower should not leak to the printing roller. When the printing pattern is extremely low, the ink consumption is particularly low. Use new ink as much as possible, for example: When the ink is a polyamide resin ink, usually toluene:isopropanol:ethyl ester 5:3:2 or methylbenzen:xylene:isopropanol 20%:50%:30% , Can effectively prevent the ink from swelling, usually when the room temperature is higher than 25 °C printing, the chance of ink swelling is higher, the best indoor temperature is 21 °C - 23 °C printing, relative humidity 65%, are conducive to printing , reduce the probability of poor adhesion, and if the chlorinated polypropylene resin ink using toluene: xylene 80%: 20%, or toluene: ethyl ester: butanone 70%: 20%: 10% are conducive to increase its viscosity It also effectively prevents the bloating factor.
In the printing process, if the supply solvent evaporates before the ink is dried, the condition is good, the heat provided by the drying is insufficient, the distance from the entrance of the drying tunnel, or the printing speed is too slow, so that the material is partially printed and goes into the drying process. If the time of the road is dragged on too long, it will be prone to "whitening" failures, and will also affect the problem of poor adhesion. ("Blank" first, the ink is cold, the water vapor in the air becomes a drop and mixes with the ink, causing the gelatinization to become turbid and precipitated. The "whitening" product has significantly reduced gloss, transparency, and adhesion.) It is to increase the printing speed and make the printing roller close to the baking mouth. If the above conditions cannot be achieved, a slow-drying solvent can be used to make the drying time slow. However, when a large amount of slow-drying solvents are used, problems such as residual odor and adhesion of the printed materials are apt to occur, but they cannot be added too much. Therefore, in order to improve the adhesion of the ink, the actual operating conditions and printing effects should be adjusted?
Third, the lack of heating, also caused the adhesive failure, when gravure printing, the drying machine to provide enough heat, insufficient heat, viscosity is poor, because the gravure ink in the binder is resin, to achieve a high enough temperature only viscosity. However, overheating causes the film to wrinkle or shrink, which also brings disadvantages to cooling. Can easily cause the accumulation of residual heat, so that the temperature after winding is too high, resulting in adhesion.
Fourth, poor substrate surface conditions cause poor adhesion.
1, such as polyethylene, polypropylene, nylon, polyester and other plastic films, the surface is very smooth and there is no affinity for the ink, the ink is not easy to attach to the top, so it needs to be treated to change its surface condition and chemical structure, increase the surface roughness and Polarity between molecules can make the ink bond firmly. The surface treatment of plastic film is flame method, chemical reagent oxidation method and electric discharge treatment method, and the use of hydrogen on the molecular chain of polyolefin is easily oxidized.
The non-polar molecules were changed to polar molecules, and the materials treated with spark discharge were measured by infrared spectroscopy. The presence of hydroxyl groups and carboxyl groups was found. In addition, the impact of the electron beam will also create microscopic pores on the surface of the film, which will become rough and facilitate the penetration of the ink: the fastness of the treated film will be greatly increased, the effect of the EDM treatment, and the voltage level used by the EDM, resulting in new The amount of eco-oxygen is related to the number of treatments: the conditions are suitable to increase the surface tension of non-polar polyethylene and polypropylene from the original 29-30 dyne/cm to 40-45 dyne/cm, regardless of the After the surface treatment, the surface tension of the plastic is more than 38 dyne/cm to meet the printing fastness. Check the level of surface tension, you can use standard surface tension liquid. When preparing standard surface tension fluids, the precision of the balance is required to be high and the purity of the reagents is high. For ease of observation, less than 0.03% of paint can be added to the tension fluid as a coloring agent.
During the measurement, the room temperature was 25°C. The degreased cotton was dipped in a little test solution and brushed on the treated surface. If the liquid was evenly distributed in the place where it was made, it did not shrink into a drop of beads, indicating that the surface of the test was The surface tension reaches the surface tension of the standard solution used. If within 5–10 seconds, the applied liquid shrinks into beads, and the test surface is wetted discontinuously and evenly, the surface bacteria on the test surface are lower than the value indicated by the standard solution, if less than 38. Dyne / cm, the printing is not strong, the need for EDM again, has reached more than 38 dyne / cm Caixing. There are 38 Dyne/cm standard Dynein pens in the country. It is very convenient to use. If the line is even and no shrinkage, it can meet the requirements and the expected fastness can be achieved by printing.
2. Hazards caused by additives Most of the raw materials used for printing contain various additives. For example, polypropylene contains antistatic agents, moisture-proof film contains slip agents, and plasticizers. Once these additives float on the surface of the film, the adhesion of the ink to the substrate is impaired, the printing fastness is greatly reduced, and the printing is not due to additives. The following method can be used to judge: Use solvents (alcohol, Acetone, ethyl acetate, etc.) The surface of the film is rubbed and cleaned. After drying, the same ink and the same conditions are used to print. If the treated surface has a better print fastness than the original, it is an additive problem. Films are particularly prone to this situation in the winter. When the presence of the additive is confirmed, this thin film cannot be printed anymore. It is necessary to exchange another brand of film, or change the ink type to ensure the product's normal print fastness.
3, surface moisture absorption nylon, ordinary glass paper, vinyl resin and other raw materials due to easy moisture absorption, the surface of the water molecular film, the ink and the substrate are separated, so that the printing fastness is greatly reduced, therefore, the storage conditions of these materials And printing conditions, should be as dry as possible, do not let it damp, and ultimately improve the adhesion of ink.