On the drying problem of ink and its influence on

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On the drying of ink and its impact on printing quality

the change process of ink from fluid state to solid state is called ink drying, and the time required to complete this process is called drying time. The drying time is short, the ink drying speed is fast, the drying time is long, and the ink drying speed is slow

the speed of ink drying has a great impact on the quality of prints. If it dries too fast, the ink will crust and paste on the surface of the printing plate, causing the printing ink to accumulate, with poor gloss and ink spots; If the drying is too slow, the back of the printed matter may be sticky, dirty and transferred, which is not conducive to the next process. The drying of ink is a relatively complex process, involving many aspects of knowledge such as physics, chemistry, hydrodynamics and surface treatment. At the same time, it is affected by many factors such as substrate, ink composition, printing conditions, printing methods, printing machinery and printing environment. Therefore, it is necessary to make a discussion here. The key point is to apply the knowledge of physics and chemistry. There are eight ways to protect the durometer from ink composition, substrate and printing: better control the drying performance of ink in terms of environment and printing conditions

I. ink drying methods

ink drying methods are generally divided into three types: osmotic drying, volatile drying and oxidized conjunctival type. Generally speaking, the drying of ink is based on a certain drying method, accompanied by other drying forms

penetration drying type refers to that the ink has penetration effect on the substrate, which also has absorption effect on the connecting material in the ink. Generally speaking, paper is a porous material intertwined with fibers, and the pore size is very small. When the ink is transferred to the paper, it begins to absorb the binder in the ink due to the capillary effect of the paper fiber gap. With the reduction of binder, the liquid content also decreases, and the cohesion between pigment particles gradually increases. The ink gradually loses the nature of fluid and presents a solid form. It can be seen that the process of the paper absorbing the binder in the ink (that is, the penetration process of the ink into the paper) is the drying process of the ink on the paper. On the other hand, there are also tiny gaps between ink particles, forming countless capillaries. When the ink penetrates into the paper, the capillary effect of the ink pigment particle gap means that the carton fully meets the requirements and prevents penetration. When the capillary action between the paper fibers and the capillary action between the pigment particles are balanced, the binder in the ink stops penetrating into the paper

volatile dry type refers to the process that solvent molecules with large energy in the ink overcome the mutual attraction between molecules in the ink, escape from the ink surface to the air, and make the remaining resin and pigment form a solid film and fix on the substrate surface. This kind of drying form mainly exists in solvent based concave convex liquid ink

the dry type of oxidized conjunctiva refers to the process in which the ink with dry vegetable oil as the connecting material absorbs the oxygen in the air, undergoes oxidative polymerization, forms a like structure, and dries and solidifies on the surface of the substrate. This kind of drying form can form high gloss, friction resistant and firm ink film, such as offset printing ink, iron printing ink and silk printing ink

second, from the substrate to see the drying performance of ink

in terms of the impact of substrate on the drying performance of ink, as mentioned above, we should focus on paper, because paper has a great impact on the drying performance of osmotic drying ink. For other drying inks, the substrate itself does not absorb the ink or the absorption is very small, which can be ignored

generally speaking, paper with rough surface and loose structure is beneficial to the drying of ink; At the same time, the affinity of paper to ink also affects the penetration. The more affinity of paper to ink (the smaller the contact angle), the faster the penetration of ink. Obviously, the faster the drying of ink at this time

the pH size of paper also has a considerable impact on the dryness of ink. Acidic paper hinders the drying of ink, while neutral or alkaline paper is conducive to the drying of ink. It is generally believed that the inorganic acid on the surface of the paper reacts with the drier in the ink through the medium of water or steam, thereby weakening the activity of the drier. The test data in Table 1 can strengthen the understanding of this aspect

it is worth mentioning that the water content of the paper has a certain impact on the penetration and oxidative drying of the ink. Assuming that the water content of the paper is very high, the fibers or coatings on the surface of the paper are in a loose state, the attraction between molecules is reduced, and the pores are blocked by the water film, which not only reduces the absorption of the ink binder, but also reduces the absorption of oxygen, obviously delaying the drying speed of the ink

third, from the composition of ink to see the drying performance of ink

we know that ink is mainly composed of the following four parts: connecting material (or resin), solvent (or water), pigment and additives. Among them, solvents play a direct and obvious role in the drying performance of ink, and different resins, pigments and additives also have a certain impact on the drying performance

for a single solvent, the lower the boiling point, the faster the volatilization rate. But there are exceptions, such as ethyl acetate and ethanol have similar boiling points, but the volatilization rate of ethyl acetate is almost twice that of ethanol. The boiling point of water is not high, but the evaporation speed is very slow. Generally, medium, low and high solvents are used together to meet the dryness requirements of the ink. The volatilization rate of mixed solvents is restricted by various solvent molecules, which is not a simple arithmetic mean. Table 2 lists the boiling points and volatilization rates of different solvents for reference

after the resin is dissolved in organic solvents, the volatilization rate of the solvent is reduced, and different resins slow down the volatilization rate of the solvent differently. The greater the solubility of the resin, the harder the solvent is to be removed from it, and the lower the volatilization rate, that is, the worse the release of the resin to the solvent. When preparing volatile dry ink, the selected resin should have a certain release of the solvent, otherwise, poor solvent removal is easy to cause the back to rub dirty. The scratches on the surface of the oxidized conjunctiva drying sheath are often caused by the twists and turns of the cable core. In the type ink, dry vegetable oil is used as the connecting material. The more double bonds in the molecules of unsaturated fatty acids, that is, the higher the degree of unsaturation, the slower the drying speed. For water-based inks, generally, the higher the content of lotion resin, the faster the drying speed

when pigment is added to make ink, the volatilization speed is greatly reduced. The smaller the pigment particle, the larger the surface area, the faster the drying, and vice versa. And the larger the proportion of pigment in ink, the lower the volatilization speed of solvent and the slower the drying. At the same time, different kinds of pigments have different solvent stripping properties. For example, black and blue pigments have poor solvent stripping properties and low volatilization speed. Table 3 lists the influence of pigment particle size on solvent volatilization rate

in addition to special driers, other additives have little effect on the drying performance of ink due to their small amount and different functions

IV. The Influence of printing environment and printing conditions on ink drying

the increase of printing environment temperature is beneficial to the drying of ink on printing pieces. This can be explained by the fact that the temperature rises and the movement of material molecules accelerates, thus promoting the infiltration, volatilization and oxidative polymerization. Similarly, the increase of printing drying temperature is also conducive to ink drying. At the same time, slowing down the printing speed can make the drying sufficient. The humidity of the printing environment also has a significant impact on the ink drying. The higher the relative humidity is, the slower the ink drying speed is. For example, under normal conditions, the ink drying time is 6.7 hours when the air relative humidity is 65%. When the humidity increases to 75%, the same ink will take 23.3 hours to dry, delaying the ink drying time by more than three times

To sum up, the drying problem of ink on the substrate is a wide-ranging and relatively complex problem. To obtain an appropriate drying rate, ink manufacturers and printing manufacturers need to make joint unremitting efforts

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