1. The measures taken after packaging printing inks are skinned. Everyone knows that when the ink in the ink fountain forms a layered film due to the drying of the surface layer in the printing process, either a stirring device is placed or the ink is simply dumped, otherwise, The film will adhere to the roller, which will result in unevenness on the plate surface, resulting in contamination of the printed product with chaotic knife marks. For this reason, the ink industry tends to:
(1) The ink dries too fast;
(2) Poor ink flowability;
(3) The precipitation of ink has dried. However, it is not known that this phenomenon is caused by the gradient of volatilization, evaporation, or oxidative drying of the ink system, and the printing industry believes that the structure of the ink tank or ink fountain of the ink is not good, resulting in a non-flowing stagnation part. The leaked air dries the surface of the ink or the hot air blows the ink into the ink fountain.
Although there are different opinions, the processing methods generally adopted by printing operators around this phenomenon are:
(1) Do not use ink that dries too quickly;
(2) Add medium and slow drying solvents;
(3) Replace the deteriorated ink (that is, from poor fluidity to excellent fluidity);
(4) Keep stirring in the ink fountain;
(5) Set a closed top cover on the ink fountain;
(6) Adjust the hot blowing angle. People who have common knowledge in printing experience, when the drying speed is too fast, or the ink concentration is too light, or the flow is not good, or the ink thixotropy is large, or the ink is static, it will take the method of speeding up the printing machine speed, which prevents the ink It is dried on the printing plate early, and the crust formed by the film of the packaging printing ink is overcome, and the marking failure of the picture and text is eliminated.
2. Stabilizing the quality of the film and text of the packaging printing ink can control the radical polymerization, drying or solvent evaporation of the packaging printing ink film, and the active effect of oxidation, which can achieve the purpose of preventing skinning. However, this control function must be based on the need, which does not affect the drying rate of the printed graphic film, nor the performance and quality of the ink product, which is the minimum standard for ink manufacturing.
In order to achieve this goal, the author recommends the following three methods:
Method 1: Add antioxidant and retarder. In the printing ink system, the role of adding antioxidants or retarders is to prevent the generation of free radicals in the process of film formation of packaging and printing graphics, so as to achieve the interruption of ink film oxidation and volatile polymerization. Once the solvent evaporates in a very short period of time, the oxidation process of the oil will also be reduced, and the free radicals generated by its ink will continue to polymerize into a film (such as Tianjin Lisheng Chemical Factory product series).
Method 2: Add oxygen barrier. A liquid paraffin called oxygen barrier is added to the packaging printing ink, and the generated vapor is filled between the ink tank, tank, and box to form an oxygen barrier (film), which effectively prevents the vapor pressure. And it is very easy to evaporate the solvent in the ink system.
Method 3: Add complexing agent. Use oxime compounds (methyl ethyl ketone oxime, butyraldehyde oxime, cyclo ethyl ketone oxime vs hydroquinone, etc.) to control the activity of the ink system drier, which can delay or prevent skinning. For a long time, packaging printing inks have been added with anti-caking agents (that is, polymerization inhibitors) for more than 50 years. Most of the past 40 years have used phenols. In ink, benzoic acid is used, which is higher than the earliest choice. The boiling solvent cover saves time and effort. Because the phenolic compound itself is a proton donor, it can generate peroxide radical reaction during the oxidation reaction to form ROOH, and at the same time form a stable antioxidant free radical, capture the active radical, and terminate the oxidative polymerization reaction- Prevents the skinning of printing ink. With the development of science and technology, in recent years, people have used oxime compounds as ink anti-skinning agents, but found that butyraldehyde and methyl ethyl ketoxime have the best effect to prevent ink skinning. Some ink manufacturers and packaging and printing operators believe that the anti-gel effect of oxime compounds has exceeded that of traditional phenol compounds. In the 1980s, China developed the methyl ethyl ketoxime anti-skin agent for the first time. Following the advent of butyraldehyde oxime, China's application and effects have reached the world's advanced level.
Third, the application scope and precautions of anti-caking agent, especially the oxidation of packaging printing ink, the volatilization rate of solvent, the oil of modified resin, the pigment, filler, the amount of drier used, the storage environment temperature and humidity of the finished ink Various factors have a certain influence on the crust (gel) of the ink. Therefore, when using the anti-crust agent (gel), the above factors must be taken into consideration. At the same time, the amount of addition should be paid attention to. Basic situation:
1. Amount added. Generally calculated according to ink materials (because the main component of the skin is the main raw material of ink) or calculated based on the total amount of packaging printing ink, at the same time, it must also be affected by other components. Usually the amount of addition is between 0.1% and 0.3%. Because of the difference in temperature and humidity throughout the year, there is a certain difference in the speed of the skin. Therefore, it is 0.1% in winter, 0.3% in summer, and 0.2% in spring and autumn. It depends on the composition of the formula to be scientific.
2. Drying rate of ink film. Whether it is the oxidation or volatilization rate or the evaporation rate at room temperature, we have proven in practice: in offset inks made by alkyd resin, the amount of methyl ethyl ketoxime is generally 1%. Although the drying speed is slightly longer, it is still in packaging printing Within the specified standard range. This is because the complex formed by the auxiliary agent and the drier causes the drier to temporarily lose its activity. Therefore, it is different from the traditional principle of using phenolic compounds to trap living radicals to terminate the polymerization reaction. It has little effect on the drying rate. If the amount is too much, the volatilization time will be prolonged and the complex disintegration will be too slow, which will often affect the film drying rate of packaging printing ink.
3. Yellowing. Once the anti-skin agent exceeds 0.3%, it will often cause yellowing of the printed graphic ink film, especially when used in white ink, it will cause yellowing problem after long-term storage. Therefore, strictly controlling the amount of addition or changing to butyraldehyde oxime can avoid the occurrence of yellowing failure.
4. Color and luster. Packaging and printing inks often have a direct effect on the color of the inks after the addition of anti-skin agents. Usually, the adjustment ink affects more than the color inks, and the white or yellow inks affect the packaging and printing inks more than dark colors. Generally, this phenomenon is difficult to observe by visual inspection and has nothing to do with the amount of addition. Although we already know that the packaging and printing graphic film drying has no effect on the hue of the ink film application, but in the presence of certain pigments (such as amphoteric pigments) and driers, the packaging and printing color graphic ink film There will be a significant difference between the tones, which has troubled toning. In this case, the color difference test should be done first during the continuous proofing of the ink making. The purpose is to avoid affecting the appearance quality of the ink film due to color change after printing.
5. The state of printing ink film. Although the anti-skin agent will not have any effect on the state of the printing ink film, the packaging printing ink will precipitate crystals during long-term storage at low temperature, which will cause the turbidity of the packaging printing ink after film formation and affect the gloss of the ink film. If the material and the binder, pigment, filler and drier in the ink system are not properly designed and matched, larger particles will appear. Therefore, it should be tested before printing.
6. Gloss and weather resistance. After adding the anti-skin agent, the gloss of the printing ink will not only be affected, but also the gloss retention. After adding additives, the weather resistance of the printed graphic ink film has been improved and improved.
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