Analysis of the reasons for poor adhesion of the h

Analysis of the reasons for the poor adhesion of plastic gravure printing

gravure printing is an important means of packaging printing, especially in the printing of flexible packaging plastic film. As the absorption of plastic film is not as good as that of paper, ink may not stick firmly. This paper analyzes the reasons for this phenomenon and provides some solutions for your reference

generally, the method to judge the printing adhesion fastness of ink is to use cellophane pressure-sensitive adhesive tape. The printing fastness varies with the ink model. You should carefully refer to the instructions to make specific solutions. The main reasons are:

first, misuse the ink model or mix different inks

there are many types of inks, which are suitable for different substrates. If they are used incorrectly, the adhesion will be weak. For example, the intaglio ink of cellophane is nitrocellulose, and the ink of printing paper is rosin. If they are used incorrectly, the adhesion will be weak. Therefore, the ink also plays a decisive role in the fastness of printing ink layer. For different substrates, corresponding inks should be used. For cellophane, nitro cellophane resin ink can be used. For polypropylene film, chlorinated polypropylene resin ink can be used. For polyethylene film, different fixtures should be designed according to different experimental forces and the shape and size of samples. Polyamide resin ink should be used, etc. In actual production, a certain amount of solvent is often added to adjust the viscosity and volatilization speed of ink. However, mixed solvents are often used when adding solvents, and several problems should be paid attention to:

(1) when adding solvents, solvents with low surface tension should be selected as far as possible, in order to reduce the surface tension of the whole ink system, so as to improve the wettability of the printing film

(2) when the working solid content is too high, the wetting effect will be correspondingly low, which will affect the fastness of printing ink. On the contrary, if too much solvent is added, it will relatively reduce the effective components that the ink adheres to the film, which will also cause the fastness to decline

in order to solve the contradiction between the fastness of the ink layer and the evaporation speed of the ink, the resin (rubber) corresponding to the ink or other appropriate compatible bonding resin or rubber can 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, if the ink is placed for a long time or mixed with impurities, it will lead to the deterioration of the connecting material: therefore, try to use fresh ink or replace the deteriorated ink in time

second, the ink swells, or there is solid precipitation, which leads to poor adhesion.

to prevent the ink from swelling, you can't use too much fast drying solvent, and you should use it together with an appropriate amount of slow drying solvent. You should try to reduce the chance of the ink contacting the air. Because the water in the air invades the ink, the ink viscosity increases, resin precipitation, and gel

there are failures such as gloss reduction, concentration reduction, poor transfer, blocking plates, etc. in printing, so the ink tank should be sealed, and the wind of the blower should not be leaked to the printing roller. When the printing pattern is particularly small, due to the consumption of ink is particularly small, new ink should be used as much as possible. For example, when the ink is polyamide resin ink, toluene: isopropanol: ethyl ester 5:3:2 or a: xylene: isopropanol 20%: 50%: 30%, It can effectively prevent the ink from swelling. Generally, when the indoor temperature is higher than 25 ℃, the probability of ink swelling is higher. It is best to print at the indoor temperature of 21 ℃ -23 ℃ and the relative humidity of 65%, which is conducive to printing and reduce the probability of poor adhesion. For example, the chlorinated polypropylene resin ink using toluene: xylene 80%: 20%, or toluene: ethyl ester: butanone 70%: 20%: 10% is conducive to improving its viscosity and effectively preventing swelling factors

in the printing process, before the ink is dried, if the heat supplied by the evaporation of the solvent is sufficient, the situation is good. The heat provided by drying is insufficient, and it is far from the entrance of the drying channel, or the printing speed is too slow, so that the time from the printed part of the material to entering the drying channel is too long, which is prone to "whitening" failure and will also affect the problem of weak bonding. (first, when the ink is cold, the water vapor in the air becomes water droplets and mixes into the ink, causing gelation to become turbid and precipitated. The gloss, transparency and adhesion of "albino" products are significantly reduced.) The solution is to increase the printing speed and make the printing roller close to the drying channel. If the above conditions cannot be achieved, slow drying solvent can be used to slow the drying time. However, when slow drying solvents are widely used, problems such as residual odor and adhesion of printing materials are easy to occur, but they should not be added too much. Therefore, in order to improve the adhesion of ink, it should be adjusted according to the actual operating conditions and printing effect

III. insufficient heating

also causes the failure of weak bonding. During gravure printing, the heat provided by the dryer should be sufficient. If the heat is insufficient, the viscosity is poor, because the connecting material in gravure ink is resin, which has viscosity only when it reaches a sufficiently high temperature. However, heated self methyl methacrylate structural adhesive has been put on the market for nearly two years, which will wrinkle or shrink the film, which is also detrimental to cooling. It is easy to cause the accumulation of residual heat, so that the temperature after winding is too high and adhesion occurs

IV. poor adhesion caused by poor surface condition of substrate

1. Plastic films such as polyethylene, polypropylene, nylon, polyester, etc. have a very smooth surface and have no affinity for the ink, so the ink is not easy to attach to it, so it needs to be treated to change its surface condition and chemical structure, increase the surface roughness and polarity between molecules, so as to make the ink bond firmly. The surface treatment of plastic film includes flame method, chemical reagent oxidation method and electric spark treatment method. It is easy to be oxidized by using the activity of hydrogen on polyolefin molecular chain

the existence of hydroxyl and carboxyl groups can be found by changing non-polar molecules into polar molecules and measuring the materials treated by electric spark with infrared spectroscopy. In addition, the impact of the electron beam will also produce micropores on the film surface, making it rough and easy for the ink to penetrate: the printing fastness of the treated film is greatly increased. The effect of spark treatment can be achieved when the oil level reaches 1.5 of the oil window after being electrified until the oil level is seen. It is related to the high and low voltage used by the spark, the amount of new state oxygen generated, and the number of treatments: the surface tension of non-polar polyethylene and polypropylene can be achieved under appropriate conditions, From the original dyne/cm to dyne/cm, no matter which kind of plastic, after surface treatment, the surface tension is greater than 38 dyne/cm, which can meet the printing fastness. To check the surface tension, a standard surface tension liquid can be used. When preparing standard surface tension liquid, it is required to have high accuracy of balance and high purity of reagent. For the convenience of observation, paint less than 0.03% can be added to the tension liquid as the chromogenic agent

when measuring, the room temperature is 25 ℃, the degreased cotton is dipped with a little test solution, and it is painted on the treated surface. If the liquid can be evenly distributed in the prepared place and does not shrink into a drop of beads, it indicates that the surface tension of the tested surface has reached the surface tension of the standard solution used. If the applied liquid shrinks into beads within 5-10 seconds, and the test surface is wetted discontinuously and evenly, it indicates that the surface strain of the test surface is lower than the value indicated by the standard liquid. If it is lower than 38 dyne/cm, the printing is not firm, and it needs to be treated by electric spark again until it reaches more than 38 dyne/cm. There are 38 dyne/cm standard liquid dyne pens in China, which are very convenient to use. If the lines are drawn evenly and do not shrink, it means that the requirements are met, and the printing can achieve the expected fastness

2. Hazards caused by additives

most of the raw materials used for printing contain various additives, such as antistatic agents in polypropylene, smoothing agents and plasticizers in moisture-proof films. Once these additives float on the surface of the film, the adhesion of the ink to the substrate will be damaged, which greatly reduces the printing fastness. Whether the poor printing is caused by additives can be judged by the following methods: wipe and clean the film surface with solvents (alcohol, acetone or ethyl acetate, etc.), and then use the same ink and the same conditions to print after drying, but for crystalline polymers, If the printing fastness of the treated surface is better than the original, it indicates that it is a problem of additives, which is particularly easy to happen in winter. When the existence of additives is confirmed, this film cannot be used for printing. The film of another brand must be replaced, or the ink model must be changed to ensure the normal printing fastness of the product

3. Surface moisture absorption

nylon, ordinary cellophane, vinyl resin and other raw materials are easy to absorb moisture, and there is a water molecular film on the surface, which separates the ink from the substrate, which greatly reduces the printing fastness. Therefore, the storage conditions and printing conditions of these materials should be kept dry as far as possible, do not let it be affected by moisture, and finally improve the adhesion of the ink