On Drying of Ink and Its Effect on Printing Quality (I)

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 ink drying has a great influence on the quality of printed materials. Drying is too fast. The ink will crust and paste on the surface of the printing plate, causing the printing ink to accumulate, resulting in poor gloss and “ink spots”; 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 to better control ink drying performance in terms of ink composition, substrates, printing environment, and printing conditions.

First, the ink drying method

Ink drying methods are generally 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

With respect to the influence of the substrate on the ink drying performance, as described above, the paper should be emphasized because the paper has a great influence on the drying performance of the infiltration drying ink. 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.

It is worth mentioning that the water content of the paper has a certain degree of influence on the penetration of ink and the oxidation drying. Assuming that the moisture content of the paper is high, the fiber or coating on the surface of the paper is in a loose state, and the intermolecular force of gravity 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.