Discussion on the drying performance of ink and its influence 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. Short drying time, fast ink drying speed, long drying time and slow ink drying speed.

The speed of ink drying has a great influence on the quality of printed matter. If it is dried too fast, the ink will crust and paste on the surface of the printing plate, which will make the printed matter ink accumulate, have poor luster and appear "ink spots"; If drying is too slow, the back of printed matter may be sticky and transfer, which is not conducive to the next process. The drying of ink is a complicated process, involving many aspects of knowledge such as physics, chemistry, fluid mechanics and surface treatment. At the same time, it is influenced by many factors such as substrate, ink composition, printing conditions, printing methods, printing machinery and printing environment, so it is necessary to discuss it here. The key point is to apply the knowledge of physics and chemistry to better control the drying performance of ink from the aspects of ink composition, substrate, printing environment and printing conditions.

1.Drying method of ink
There are three drying methods of ink: osmotic drying, volatile drying and oxidized conjunctiva drying. Generally speaking, the drying of ink is mainly based on a certain drying method, accompanied by other drying forms.
Permeable drying type means that ink has a penetrating effect on the substrate, and the substrate also has an absorbing effect on the connecting material in the ink. Generally speaking, paper is a porous substance interwoven with fibers, and its 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 action of the fiber gap in the paper. With the decrease of binder, the liquid content also decreases, and the cohesion between pigment particles gradually increases, and the ink gradually loses its fluid properties and presents a solid form. It can be seen that the process of paper absorbing the binder in ink (that is, the infiltration process of ink into paper) is the drying process of ink on paper. On the other hand, there are tiny gaps between ink particles, which constitute countless capillaries. When the ink penetrates into the paper, the capillary action of the gap between the ink pigment particles prevents the penetration. When the capillary action between paper fibers and pigment particles are balanced, the penetration of the binder in the ink into the paper stops.
Volatile drying refers to the process that solvent molecules with great energy in ink overcome the mutual attraction between molecules in ink and escape from the ink surface to the air, so that the remaining resin and pigment form a solid film and are fixed on the substrate surface. This kind of drying form mainly exists in solvent-based gravure liquid ink.

The dry type of oxidized conjunctiva refers to the process that the ink with dry vegetable oil as the binder absorbs oxygen in the air, then undergoes oxidative polymerization to form a network structure and is dry-fixed on the surface of the substrate. This kind of drying form can form high gloss, friction resistance and firm ink film, such as Offset Printing Ink, iron Printing Ink and Screen Printing Ink.

2.Drying performance of ink from substrate
As far as the influence of substrate on the drying performance of ink is concerned, as mentioned above, we should focus on paper, because paper has a great influence on the drying performance of osmotic drying ink. And other drying methods of ink, the substrate itself does not absorb ink or the absorption amount is very small, which can be ignored.

Generally speaking, paper with rough surface and loose structure is beneficial to ink drying; 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, and obviously the faster the drying of ink at this time.

The PH of paper also has a great influence on the drying of ink. Acidic paper can hinder the drying of ink, while neutral or alkaline paper is beneficial to the drying of ink. It is generally believed that the inorganic acid existing on the paper surface reacts with the drier in the ink through the medium of water or water vapor, thus weakening the activity of the drier.

It is worth mentioning that the water content of paper has a certain influence on the penetration and oxidation drying of ink. Assuming that the water content of paper is high, the fibers or coatings on the surface of paper are in a loose state, the attraction between molecules is reduced, and the pores are blocked by water films, which not only reduces the absorption of ink binders, but also reduces the absorption of oxygen, which obviously slows down the drying speed of ink.
3.Drying performance of ink from its composition
We know that ink mainly consists of the following four parts: binder (or resin), solvent (or water), pigment and additives. Among them, solvents play a direct and obvious role in the drying performance of ink, while different resins, pigments and additives also have certain effects on the drying performance.
As far as a single solvent is concerned, the lower the boiling point, the faster the volatilization speed. But there are exceptions, such as the boiling points of ethyl acetate and ethanol are similar, but the evaporation rate of ethyl acetate is almost twice that of ethanol. The boiling point of water is not high, but the volatilization speed is very slow. Generally, medium, low and high solvents are used together to meet the drying requirements of ink. The evaporation rate of mixed solvents is restricted by various solvent molecules, which is not a simple arithmetic average. After the resin is dissolved in the organic solvent, the volatilization speed of the solvent is reduced, and different resins have different degrees of slowing down the volatilization speed of the solvent. The greater the solubility of the resin, the more difficult it is for the solvent to escape from it, and the lower the volatilization speed, that is, the worse the release of the resin to the solvent. When preparing volatile drying ink, the selected resin should have a certain release to the solvent, otherwise, poor solvent extraction will easily lead to dirty back. However, in the oxidized conjunctival drying ink, 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 Ink, the higher the content of emulsion resin, the faster the drying speed.

When pigment is added to make ink, the volatilization speed is greatly reduced. The smaller the pigment particles, the larger the surface area, and the faster the drying, and vice versa. And the greater 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, such as black and blue, which have poor solvent stripping properties and low volatilization speed. In addition to the special drier, other auxiliaries have little effect on the drying performance of ink because of their small amount and different functions.

4.Influence of printing environment and printing conditions on ink drying
The increase of printing environment temperature is beneficial to the drying of ink on printed parts. This can be explained by the increase of temperature and the acceleration of molecular movement of substances, thus promoting penetration, volatilization and oxidative polymerization. Similarly, the increase of printing drying temperature is also beneficial to ink drying. At the same time, slowing down the printing speed can make the drying sufficient. The humidity of printing environment also has a significant influence on ink drying. The higher the relative humidity, the slower the ink drying speed. For example, under normal conditions, when the air 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, which delays the ink drying time by more than three times.