Application technology of ink in different printing materials
When printing on different substrate surfaces, it is necessary to consider the appropriate ink. When the porous surface with better absorption such as paper is used as the substrate, the permeation drying ink should be used; while for printing on the non-absorbent surface such as plastic, the volatile drying ink should be used.
I. Introduction
To predict the future development of ink, it is necessary to understand the future printing form. The future development direction of the printing industry is to save resources, save energy, reduce pollution, and estimate that the types of printed materials will increase with economic growth. Printed matter is not only used as a means of disseminating, transmitting and transmitting information in politics, culture, art, propaganda, etc., but also widely used in packaging materials and building decoration materials. For printed materials for packaging, society's hobbies and requirements are becoming more and more diverse. The pattern of construction materials is also changing with each passing day. Commodities are developing in the direction of beautiful decoration and enhancing competitiveness. Therefore, the printing industry has a major task and optimistic prospects. Whether the ink can be transferred and distributed on the printing machine well, will there be problems with the copy quality of the printed matter and the printing process (such as flying ink, dot definition, density, permeability, gloss, rubber stacking, stacking, etc.), these Both are related to the rheology of the ink, and the rheological properties of the ink have long been widely valued. Only if the ink has the proper rheology can the ink be smoothly transferred, transferred, distributed, and reach the printing plate until finally transferred to the surface of the substrate.
The printing speed of modern printing machines is extremely fast. In a few seconds, the ink will be greatly sheared, stretched, squeezed and broken, and finally transferred to the substrate (paper) to be fixed and dried. However, the composition of the ink itself and the force of the ink in the printing process are very complicated, resulting in a complex and diverse rheological state during the printing process. This brings great difficulties to the study of ink rheological properties. In addition, you should also consider whether the ink can wet the substrate, and whether there is a certain interaction between the ink and the substrate. Only if the ink can wet the substrate and there is mutual attraction with the substrate, the ink film can be firmly adsorbed on the surface of the substrate. The so-called ink can wet the substrate means that the surface free energy of the substrate is higher than the surface tension of the ink, which is a prerequisite for the ink to adhere to the substrate. The surface tension of the ink is mainly determined by the surface tension of the solvent in the ink.
2. Wetting and non-wetting state of liquid on solid surface
1. Wetting and non-wetting state
When the liquid and the solid surface are in contact, the two molecules in the interface area are attracted by the same kind of molecules on the same side of the interface and the different kinds of molecules on the other side. The combined force of these two attractive forces is called interfacial tension. If the solid is of low surface energy, its attractive force is lower than that of liquid phase molecules. The liquid molecules in the interface area have a tension that shrinks toward the inside of the liquid, which is the non-wetting state; if it is a high surface energy solid, its attraction If the force is higher than the attractive force of the liquid phase molecules, the liquid molecules in the interface area have a pressure of being adsorbed on the solid, which is the wet state.
For example, the surface tension of mercury is: 0.476N / m, and the surface energy of polystyrene plastic plate is: 0.042N / m, which is much smaller than the surface tension of mercury. If mercury is dropped on a polystyrene plate, the mercury drop will tend to It is spherical and non-wet. If ethanol with a surface tension of only 0.225N / m is dropped on the polystyrene board, it can be spread out and wet.
In general, inorganic solids (metals, oxides) have high surface free energy and are easily wetted by general liquids; organic solids (paraffins, polymers) have low surface free energy and are not easily wetted by liquids. When the surface tension of the ink is less than the surface energy of the substrate, the ink can wet the substrate, creating the necessary conditions for printing; conversely, printing on a surface with low surface energy (such as plastics), the ink is not easy to wet the substrate, At this time, it is necessary to perform some treatment or modification on the surface of the substrate to be able to print normally.
2. Contact angle
If the solid has a high surface free energy and its attractive force is higher than that of the liquid phase molecules, the liquid molecules in the interface area have a pressure to be adsorbed on the solid, and the liquid has the ability to replace gas on the solid surface. The surface is in a wetting state; at this time, the contact angle θ is less than 90 °, the smaller the contact angle, the better the wetting effect. If the solid has a low surface free energy and its attractive force is lower than the attractive force of the liquid phase molecules, the liquid molecules in the interface area have a tension that shrinks toward the inside of the liquid, indicating that the liquid is not wetted by the solid, then the contact angle θ is greater than 90 °, the greater the contact angle, the worse the wetting effect.
Generally speaking, solid molecules have strong interaction force, high hardness, high melting point, and good crystallization. When they are inorganic substances (metals, oxides), the solid surface has high free energy and is easily wetted by general liquids; on the contrary, solid molecules interact Weak, soft, low melting point, rarely crystallize, when it is organic matter (paraffin, polymer), the surface free energy is low, and it is not easy to be wetted by general liquid. But whether the ink can be firmly attached to the substrate depends on the attraction between the ink and the substrate.
3. The attraction between ink and substrate
The adhesion between the ink and the substrate mainly includes the chemical bonding force (that is, the force between atoms), the intermolecular force (hydrogen bonding force and van der Waals force), the interface electrostatic attraction, and the mechanical force.
Chemical bond
The chemical bonding force is the force between atoms, including ionic bond, covalent bond and metal bond.
â‘ Ionic bond strength. The interaction force between positive and negative ions.
â‘¡Covalent bond strength. That is, the force between two atoms connected by a common electron (the overlap of the two atom electron clouds). Most molecules of organic compounds are composed of covalent bonds. A covalent bond is a relatively strong chemical bond.
â‘¢Metal key force. The attraction between metal positive ions due to the free movement of electrons. Metal atoms are connected into metal crystals by metal bonding force.
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