There are many printing properties of gravure inks, and these printing properties directly affect the printing quality of gravure inks, mainly including adhesion, color density, surface tension, viscosity, dryness, etc., which are briefly introduced below.
Surface Tension
The most basic characteristic of the liquid surface is the tendency to shrink, which manifests itself in the phenomenon that the small liquid drop takes a spherical shape, such as the water drop and the small mercury drop on the lotus leaf, and the liquid film shrinks automatically. This is the result of surface tension and surface free energy. Surface tension is one of the inherent properties of gravure printing inks. In the preparation and printing of gravure printing inks, ink leveling, wetting and adhesion to substrates are all related to surface tension. The wettability of the ink can be adjusted by using the principle of surface wetting. If the surface tension of the gravure printing ink is high, a solvent with a small surface tension can be added for adjustment; otherwise, a solvent with a high surface tension can be added.
viscosity
In a flowing liquid, if the flow velocity of each layer of the fluid is different due to some external factors, there will be an action force and a reaction force between the liquid layers with different flow velocities on the two contact surfaces. This pair of forces is called The internal friction of the liquid. Generally liquids have this property, which is called the viscosity of the liquid, and the physical quantity to measure the viscosity of the fluid is called the viscosity. Viscosity is a property that prevents fluid flow. It is a measure of the ability of fluid molecules to interact to hinder the relative motion between molecules, that is, the resistance of fluid flow. The viscosity of printing ink is one of the main parameters of ink physical properties.
There are many methods of viscosity testing and measurement, such as rotational viscosity, Ford cup and Zion cup, etc. The measuring cup used in this article is the Zion cup. The original ink viscosity of gravure ink is generally tested by Zion cup, and the diluted viscosity is generally tested by Zion cup.
The binder of volatile drying ink is mainly composed of resin and solvent. After the ink is transferred to the surface of the substrate, the organic solvent is released from the shackles of the resin, and relies on its own volatilization ability to form a film from the ink and enter the gas phase. At this time, only resin and other components are left in the surface film layer of the substrate, and the resin is originally solid. During the ink manufacturing process, the solvent first soaks into the gaps of the resin molecules, and then expands the gaps until the resin molecules are completely isolated. From the outside, This process is the swelling and dissolution of the resin. When the solvent leaves the resin, the resin molecules will approach again until physical crosslinking occurs. At this point, the resin loses its fluidity and becomes solid, completing the volatilization process.
Adhesion
Adhesion is the mutual attraction that occurs where two different substances come into contact. This mutual attraction is a manifestation of the molecular force between the molecules of the two substances. It only shows up when the molecules of the two substances are in close proximity. From a microscopic point of view, the surface of a solid is always "rough", so it is difficult to show the effect of adhesion when two solids are in contact. Liquids and solids can be in close contact, and it is easy to show the effect of adhesion between them. The phenomenon that the liquid wets the solid is the result of the adhesion force.
Adhesion has a major impact on print quality. In the process of plastic film printing, the poor adhesion of ink will lead to a series of printing quality problems. For example, the ink film will be peeled off when the film is rewound, causing the backside to be smeared or color biting during the overprinting process. These problems will seriously affect the quality of printed matter, resulting in waste. Therefore, enhancing the ink adhesion has a very important relationship with the quality of the printed matter. The adhesion generated between the ink and the substrate mainly includes the chemical bond force, that is, the force between atoms, the interface electrostatic attraction, the hydrogen bond force between molecules, van der Waals force and mechanical force.
(1) Chemical bond force. Chemical bonds are the forces between atoms, including ionic bonds and covalent bonds. These forces have little to do with the adhesion of the ink to the substrate. The adhesion between the ink and the substrate is mainly the intermolecular force, and the formation of chemical bonds between the ink and the substrate is a minority.
(2) Intermolecular forces. The intermolecular forces mainly include hydrogen bonding forces and van der Waals forces.
①Hydrogen bonding force. Hydrogen bonding is a form of intermolecular interaction between van der Waals forces and chemical bonds. The bond energy of the hydrogen bond is much greater than that of the van der Waals force. If a sufficient number of hydrogen bonds can be formed at the interface between the ink and the substrate, it is undoubtedly more beneficial than the van der Waals force. There are many compounds that can form hydrogen bonds, and they all contain groups composed of atoms with small atomic radius and high electronegativity and hydrogen atoms, such as water, alcohol, cellulose, polyamide, polyacrylocyanide and other high molecular substances can produce hydrogen bonding. The properties of hydrogen bonds depend on the atomic radius and electronegativity of the bonding atoms. The smaller the atomic radius, the greater the electronegativity and the greater the bond energy.
② Van der Waals force. Van der Waals forces are also intermolecular forces, including dispersion forces, induction forces and orientation forces. There is only dispersion force between two polar molecules. The force between a polar molecule and a non-polar molecule is not only the induction force, but also the dispersion force. The three non-polar molecules include dispersion force, induction force and orientation force. planted.
(3) Interface electrostatic attraction. When the ink is in close contact with the metal substrate, because the metal loses electrons, the non-metal is easy to get electrons, so the electrons can be transferred from the metal to the non-metal, causing a potential difference between the two interfaces, resulting in electrostatic attraction.
(4) Mechanical force. Under the action of printing pressure, the ink fills the gaps or concaves on the surface of the substrate, and after curing, an engaging force is generated in the interface area, which does not work in plastic printing.
color density
For gravure printing, when the pigment in the ink is fixed, the color density of the ink can be used to measure the amount of ink transferred to the substrate during the printing process. Therefore, the transfer characteristics of the ink can be measured by measuring the color density value. The higher the color density of the ink, it means that the amount of ink transferred to the printed matter is more, and the performance of ink transfer and transfer is better. In the printing process, the ink is transferred to the bearing surface through transmission and transportation. Always keep the stable, uniform and appropriate amount of ink transmission and transfer, so that the printed matter can obtain a more uniform ink layer, which is an important guarantee for obtaining high-quality printed matter. There are many effects on ink transmission and transferability. In terms of ink composition, when other components of the ink are fixed, the type of resin has a great influence on the ink. This is because the rheological properties of the resin and the ink have a great relationship, and the rheological properties of the ink during printing are closely related to the transmission and transfer performance of the ink. The resin affects the rheological properties of the ink and thus indirectly affects the color density of the ink.
Overprint rate
In multi-color printing, the ink of the latter color is attached to the ink of the previous color, which is called the transfer of ink. The order of ink overprint and the amount of ink overprint directly affect the reproducibility of printed matter. The overprint rate is also called the ink receiving force of the ink. It refers to the comparison of the performance of one ink when it is printed on the surface of another ink. It is an important physical quantity to measure the degree of ink overprint and can also reflect the reproduction effect of ink overprint color. , the higher the value of the overprint rate, the better the overprint effect.
Chroma and Gamut
Ink color can be expressed by chroma, which is a physical quantity describing color.
The color gamut of the ink is the display range of the ink color, which determines whether the ink can better reproduce the color of the printed matter. Generally, the color of printed matter is combined by several monochrome overprints, so the color gamut of the ink is determined by the chroma of the monochrome ink. Sometimes, ink factories or printing factories can also use existing inks to prepare spot colors according to customer requirements instead of overprinting to achieve color reproduction. The hue and color density of the ink directly determine the color gamut of the ink. The pigments in the ink need to have high color brightness, tinting strength, light resistance and chemical stability, and at the same time, the pigment powder is required to be well dispersed in the resin, so that the ink will have high color concentration and good transparency , Bright hue. At the same time, the cost of the ink should also be considered when designing the formula, because the red, yellow, and orange pigments with good light resistance and chemical stability are very expensive.
