Ink thixotropy in production
Phenomenon 1: In the printing production, the commonly used series of magenta, transparent yellow and other inks have higher viscosity. After the printing machine, after repeated manual stirring and ink roller transfer, the solidification state is rapidly reduced, the ink becomes thinner and the viscosity decreases. , resulting in blurred layout printing, unclear dot and so on.
Phenomenon 2: Some inks with large thixotropy often turn thick after being stopped in the middle. Seriously, even ink can not be transferred, resulting in inefficient production.
The principle of ink thixotropy
Most inks are thixotropic because it is the ink that has such properties that it can be transferred well and homogenized.
The unique internal structure of the ink also determines its thixotropy. The ink is mainly composed of a pigment, a binder, a filler and a small amount of additives. In a static state, the pigment is dispersed in the binder to form a dispersion system, the surface of the pigment is charged, and the binder contains a certain amount of polar medium. Under the action of surfactants, a regular arrangement is formed near the interface to achieve a balance between gravity and repulsion, forming a so-called "diffusion double layer", and internal components produce a stable "shelf" network structure. It appears gelatinous. Under the external force such as agitation and friction, the interaction between particles and particles, particles and medium, medium and medium causes the "shelf" structure to be disassembled, so the fluid becomes thinner, in a sol state, and the viscosity decreases; external force action time The longer the particle and the electric layer are destroyed, the more serious the viscosity is. When the external force is removed, the double layer of the particle returns to its original state, which shows that the viscosity is restored and the ink becomes sticky.
Factors affecting ink thixotropy
After understanding the principle of ink thixotropic, we know that the structure of the ink composition is the main cause of thixotropy.
(1) The adsorption of the pigment surface is strong. The stronger the surface adsorption, the greater the thixotropy of the ink. For the same type of carbon black, inks made without ozone treatment have much greater thixotropy than those treated.
(2) The size and shape of the pigment. In general, the larger the particle, the greater the thixotropy, and vice versa; the needle-like and plate-like thixotropy is greater than the spherical shape.
(3) Pigment solid content. Under normal circumstances, considering the relationship between production cost and solid content and color concentration, we found that when the solid content reaches a certain range, the color density no longer changes, and as the solid content increases, the pigment molecules attract each other and occur. Flocculation causes the thixotropy to increase.
(4) The specific gravity of the resin in the binder. The greater the specific gravity, the greater the thixotropy.
(5) Excipient components in the binder (such as the use of aluminum soap). Aluminum soap is the main component of the gel, and it is also the main substance of the ink with thixotropic properties. Its dosage directly affects whether the dots can be full after printing, and whether the level can be clear.
Of course, the effects of the thixotropy of the ink include not only the above points, but also the quality of the pigment, such as density, wettability, and the like.
Effect of ink thixotropy on print quality
In printing, ink thixotropic has a greater impact on print quality. The thixotropy of the ink is relatively large, and the viscosity of the ink is large before the thixotropic occurs. It is very viscous. After being printed on the ink fountain of the printing machine, it is difficult to be uniformly conveyed, and the ink does not appear in serious, resulting in the so-called "blocking ink" phenomenon; Due to the large thixotropy of the ink, the ink is blocked and the ink is not smooth, resulting in unstable ink supply, lighter and lighter ink, and unclear local dots, which seriously affect the quality and effect of the printed matter. For this reason, the operator often stirs the ink fountain to detect the viscosity and the like. When the ink thixotropy is too small, the viscosity of the ink can not be lowered after the machine is put on the machine, and the ink transfer is not smooth, which also affects the quality.
To this end, some printing machines are equipped with a stirrer in the ink fountain to fully agitate the ink, improve fluidity, reduce viscosity, and prevent ink blocking and paste. Appropriate thixotropy can improve printability and enhance the dotness and stiffness of the printed image. Therefore, we should understand the product requirements and select the appropriate thixotropic ink during production and use.
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