How to solve the crusting problem of packaging printing ink?
When the concentration of printing ink increases to a certain value, its surface will be covered by a layer of molecules, then even if the use of supplementary solvents or grease to reduce the concentration of ink, but has been crust (gel) of the surface is not possible to accommodate more molecules. After this failure, not only to bring trouble to the printing, but also cause a great waste of materials. According to estimates: the light waste of 1 ‰, the heavy waste of nearly 1%. This tangible consumption to the packaging and printing enterprises to increase the heavy economic burden. In order to prevent the occurrence of such phenomena, ink producers or printing operators usually use manual or place polyethylene tube stirring and add anti-crusting agent to save the method, the purpose is only to reduce the printing costs to a minimum.
First, the packaging printing ink crust after the measures taken
As we all know, when the printing process in the ink hopper due to surface drying and the formation of a layer of film, either place a stirring device, or simply pour the ink, otherwise, the film will be attached to the roller, the result will be the formation of uneven layout, resulting in the printing of messy knife marks of pollution. The reasons for this, the ink community tends to be.
(1) the ink dries too quickly.
(2) poor ink flow.
(3) the ink precipitation has dried.
But familiar with this phenomenon is caused by the volatility of the ink system, or evaporation, or oxidation drying gradient is large, while the printing industry that: the ink barrel or ink hopper structure is not good, resulting in non-flowing stagnant parts, the air leaking from the dryer dried the surface layer of the ink or hot blowing wind blown to the ink hopper caused by the ink. Although there are different opinions, but around the emergence of this phenomenon, printing operators generally use the following treatment methods.
(1) can not be used to dry too fast ink.
(2) add medium to slow drying solvent.
(3) replace the ink has deteriorated (i.e., from poor mobility to excellent mobility).
(4) Constant stirring in the ink hopper.
(5) Setting a closed top cover on the ink hopper.
(6) adjust the angle of the hot blowing air.
General printing experience and common sense, when the dry speed is too fast, or ink concentration is too light, or poor flow, or ink thixotropy, or ink static electricity, will take the printing machinery to speed up the approach, not only to prevent the ink dry early knot on the printing plate, but also to overcome the formation of packaging printing ink film crust, eliminating the graphics of the patchy failure.
Second, stable packaging printing ink graphics film quality
Can control the free radical polymerization of packaging printing ink film, drying or solvent volatilization, and the active role of oxidation, you can achieve the purpose of preventing crusting. But this control role must be based on the need, neither affect the drying rate of the printing graphics into the film, but also does not affect the performance and quality of the ink products, which is the minimum standard of ink manufacturing.
For a long time, packaging printing ink to add anti-crusting agent (i.e., blocking), has a history of more than fifty years, most of the past forty years using phenols, and in the ink is the choice of benzoic acid, than the earliest choice of high boiling point solvent cover both time and effort. Because the phenolic compound itself is a proton-giving body, can produce peroxide radical reaction in the oxidation reaction process, generating ROOH, while forming a stable antioxidant radicals, capture the active radicals, and the termination of the oxidation polymerization reaction – to prevent the printing ink crust.
Third, the scope of application of anti-crusting agent and precautions
Packaging printing ink oxidation, especially the rate of volatilization of solvents, modified resin oils, the use of pigments, fillers, drying agent dosage, the finished ink storage environment temperature, humidity and other factors have a certain impact on the ink crust (gel), so in the use of anti-crusting agent (gel agent), must take into account all the above factors, but also should focus on the addition of seven aspects of the basic situation: the amount of.
1, the amount added. Generally calculated by the ink material (because the main ingredient of the crust is the main raw material of the ink) or by the total amount of packaging printing ink, while also taking into account the impact of other components. Usually add the amount of 0.1 – 0.3% between can be. Because of the difference in temperature and humidity throughout the year, there is a certain difference in the speed of the crust. Therefore, 0.1% in winter, 0.3% in summer, 0.2% in spring and autumn. Then depending on the composition of the formula is scientific.
2, ink film dry rate. Whether the oxidation or evaporation rate at room temperature or the rate of evaporation, we have proved in practice: in the manufacture of alkyd resin offset printing ink, methyl ethyl ketone oxime dosage is generally 1%, although the dry speed is slightly longer, but still in the standard range of packaging printing regulations. This is because the additives and catalysts formed complexes, so that the catalysts temporarily lose activity caused by the reason, so that with the traditional use of phenolic compounds to capture the active radicals to terminate the polymerization reaction principle is different, because the oxime compounds have little effect on the dry rate, if the dosage is too much, the volatile time is extended, the complexes disintegrate too slowly, often affecting the drying rate of packaging printing ink into film.
3, yellowing. Anti-crusting agent once more than 0.3%, often lead to printing graphics ink film yellowing, especially in white ink, after a long period of storage will produce yellowing problems. Therefore, strict control of the amount added or changed to butyraldehyde oxime can avoid the occurrence of yellowing failure.
4. Color degree. Packaging printing ink in addition to prevent crusting agent, often directly affect the ink color degree, usually toner oil impact greater than the color ink, white or yellow ink impact greater than the dark color packaging printing ink. Generally this phenomenon is difficult to observe by visual inspection, and has nothing to do with the amount added. Although we have known the packaging printing graphics into the film after drying the application of ink film hue is not affected, but in the presence of certain pigments (such as amphoteric pigments) and drying agents, packaging printing color graphics ink film hue between the significant difference, the color mixing has produced a problem. When encountering this situation, the ink should be made in the continuous proofing, the first test of color difference, the purpose is to avoid after printing due to discoloration affect the quality of the appearance of the ink film.
5. Printing ink film state. Although the anti-crusting agent will not have any effect on the state of the printed ink film, but packaging printing ink in low temperature long-term storage, will precipitate crystallization, which leads to packaging printing ink after the film turbidity and affect the ink film gloss. Such as the above because the material and the ink system of linking materials, pigments, fillers and drying agent design ratio is not appropriate, it will appear larger particles. Therefore, the test should be conducted before printing.
6. gloss and weather resistance. After adding anti-crusting agent, printing ink gloss will not only be affected, but also the light retention. After the addition of additives, the weather resistance of the printing graphics ink film is improved and enhanced.
7. Use method. Anti-crusting agent is generally added when the ink transfer, and at room temperature while stirring. For volatile ink, in order to control the ink, color materials in the grinding dispersion and storage period, especially during the printing process crust, can be used to add a part of the ink when making, before printing and then add a part of the appropriate.