Ink drying problem

The process of changing the ink from a fluid state to a solid state is called ink drying. The time required to complete this process is called drying. 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 the ink drying has a great influence on the quality of the prints. When the ink is dried too fast, the ink will crust on the surface of the printing plate, causing scumming, causing the printing ink to accumulate, and the gloss to be poor. “Dark spots” appear; drying is too slow and the back of the print may be The phenomenon of sticking and transfer occurs, and it is also not conducive to the next process. The drying of ink is a complicated process involving many aspects such as physics, chemistry, fluid mechanics, and surface treatment. At the same time, it is affected by the printing materials, ink composition, printing conditions, printing methods, printing machinery, and printing environment. The influence of factors, so it is necessary to make a discussion here. The focus is on the application of physics and chemistry knowledge to better control the drying properties of inks in terms of ink composition, substrates, printing environment and printing conditions.

First, the ink drying method Ink drying methods are broadly divided into three types: osmotic drying type, volatile drying type and oxidized conjunctiva type. Generally speaking, the drying of the ink is based on a certain drying method, which is accomplished with other drying methods.

Permeation drying type refers to the penetration of the ink on the substrate, and the absorption of the binder in the ink by the substrate. In general, for paper, paper is a porous material intertwined with fibers and has a very small pore size. When the ink is transferred to the paper, due to the capillary action of the paper fiber voids, it begins to absorb the binder in the ink. As the amount of the binder decreases, the liquid fraction also decreases, the cohesive force between the pigment particles gradually increases, and the ink gradually loses fluid. The nature of the solid-state form. It can be seen that the process that the paper absorbs 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 fine gaps between the ink particles, constituting an infinite number of capillaries. As the ink penetrates into the paper, the capillary action of the ink pigment particle voids prevents penetration. When the capillary action between the fibers of the paper is balanced with the capillary action between the pigment particles, the binder in the ink stops penetrating into the paper.

Volatile drying type refers to solvent molecules with higher energy in the ink to overcome the mutual attraction between the molecules in the ink and escape from the ink surface to the air, leaving the resin and pigment to form a solid film layer and fixate on the surface of the substrate. process. This type of dry form is mainly found in solvent-based embossed liquid inks.

The oxidative conjunctiva drying type refers to the process of using dry vegetable oil as the binder ink, absorbing the oxygen in the air, and then oxidizing and polymerizing to form a network structure and dry on the substrate surface. Such dry forms can form high-gloss, rub-resistant, and strong ink coatings. For example, offset inks, iron inks, and screen-printing inks fall into this type of drying.

Second, from the substrate to see the ink drying performance of the substrate on the ink drying performance point of view, as mentioned earlier, should focus on the paper, because the paper drying performance of the infiltration drying ink has a great impact. For other drying inks, the substrate itself does not absorb ink or the absorption is negligible.

In general, paper with a rough surface and loosely structured paper is advantageous for the drying of the ink; at the same time, the affinity of the paper for the ink also has an effect on the penetration, and the paper is more affinity for the ink (the smaller the contact angle), The faster the ink permeates, the faster the ink will dry.

The pH value of the paper has a considerable effect on the drying properties of the ink. The acidic paper has the effect of hindering the drying of the ink. Neutral or alkaline paper is beneficial to the drying of the ink. It is generally believed that the inorganic acid present on the surface of the paper passes through a water or water vapor medium and reacts with the driers in the ink, thereby weakening the activity of the driers. The test data in Table 1 can enhance this understanding.

It is worth mentioning that the water content of the paper has a certain degree of influence on the penetration of ink and drying by oxidation. Assuming that the water content of the paper is high, the fiber or coating on the surface of the paper is in a loose state, and the gravity of the molecule is reduced. The pores are blocked by the water film, which not only reduces the absorption of the ink binder, but also reduces the ability to absorb oxygen, which obviously delays the drying speed of the ink.

Third, from the composition of the ink to see the drying properties of ink We know that ink is mainly composed of the following four parts: the connection material (or resin), solvent (or water), pigments and additives. Among them, the solvent plays a direct and obvious role in the drying performance of the ink, and different resins, pigments, and auxiliaries also have a certain influence on the drying performance.

For a single solvent, the lower the boiling point, the faster the evaporation rate. However, there are exceptions. For example, ethyl acetate and ethanol have similar boiling points, but ethyl acetate evaporates almost twice as fast as ethanol. The boiling point of water is not high, but the volatilization rate is very slow. Generally, medium, low, and high solvents are used in combination to achieve the dryness of the ink. The volatilization rate of the mixed solvent, in turn, is constrained by various solvent molecules and is not a simple arithmetic mean. Table 2 lists the boiling points and volatilization rates of various solvents for reference.

After the resin is dissolved in the organic solvent, the volatilization rate of the solvent is reduced, and different resins have different degrees of slowing down the volatilization rate of the solvent. The greater the solubility of the resin, the more difficult the solvent to escape from it, the lower the volatilization rate, the worse the resin's release to the solvent. In the preparation of volatile dry ink, the selected resin must have a certain release of the solvent, otherwise, the solvent will not be easy to cause back dirt. In the oxidized conjunctiva drying ink, using dry vegetable oil as a binder, the more double bonds in the unsaturated fatty acid molecule, ie, the higher the degree of unsaturation, the slower the drying speed. For water-based inks, the higher the general emulsion resin content, the faster the drying rate.

When a pigment is added to make the ink, the evaporation rate is greatly reduced. The smaller the pigment particles, the larger the surface area, the faster the drying, and vice versa. ... The effect of pigment particle size on solvent volatilization rates is listed.

In addition to the special driers, other auxiliaries have little effect on the drying performance of the ink due to the small amount and the different effects.

Fourth, the printing environment and printing conditions on the ink drying the printing environment temperature, drying the ink on the print is beneficial. This can be explained by the increase in temperature, the acceleration of the movement of the molecules of the substance, and the promotion of permeation, volatilization and oxidative polymerization. In the same way, the increase in printing and drying temperature also helps the ink to dry. At the same time, slowing down the printing speed allows sufficient drying. The effect of the humidity of the printing environment on the drying of the ink is also very significant. The higher the relative humidity, the slower the drying speed of the ink. For example, under normal conditions, when the relative humidity is 65%, the ink drying time is 6.7 hours. When the humidity is increased to 75%, the same ink will take 23.3 hours to dry, delaying the ink drying time by more than three times. .

V. Concluding remarks In summary, the problem of ink drying on the substrate is a very complex and complicated issue. To obtain a proper drying rate, ink manufacturers and printers must make joint and unremitting efforts.

(Gao's <Guangzhou> Coatings Co., Ltd. Greater China Technical Center Wu Jiangli)

Reprinted from: Chemical Technology Forum

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