When printing on the surface of different substrates, it is necessary to consider the appropriate ink. Absorbent dry inks should be used when the porous surface with good absorbency such as paper is used as substrate, and volatile dry ink should be used when printing on non-absorbent plastic surface.
I. Introduction
To predict the future development of ink, it is necessary to understand the future of printing. In the future, the development direction of the printing industry is to save resources, save energy, reduce public hazards, and estimate the type of printed materials will increase with the economic growth. Print is not only a means of disseminating, copying, and transmitting information as politics, culture, art, and propaganda, but also widely used in packaging materials and building decoration materials. As for the printed matter for packaging, the society's hobbies and requirements are more and more varied. The pattern of building materials is also changing with each passing day, and the commodities are developing in the direction of beautiful decoration and enhanced competitiveness. Therefore, the printing industry has a major task and the outlook is optimistic. Whether the ink can be transferred and distributed well on the printing press, there will be no problem with the reproduction quality of the printed matter and the printing process (for example, flying ink, dot definition, density, permeability, luster, stack rubber, stacking plate, etc.). Both are related to the rheology of inks, and the rheological properties of inks have long been given widespread attention. Only with proper rheology of the ink, the ink can be smoothly transferred, transferred, distributed, and reached the plate until it is finally transferred to the surface of the substrate.
The printing speed of modern printing presses is extremely fast. In a few seconds, the ink is subjected to a large amount of shearing, stretching, squeezing, and crushing. Finally, the ink is transferred to the substrate (paper) and fixed and dried. The composition of the ink itself and the force of the ink in the printing process are very complex, resulting in a complex and varied rheology of the ink 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 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 moisten the substrate, refers to the substrate surface free energy is higher than the surface tension of the ink, which is the 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.
Second, the liquid in the wet and non-wetting state of the solid surface
1. Wet and non-wet conditions
When the liquid is in contact with a solid surface, the two molecules in the interface region are both attracted by the same molecules on the same side of the interface and attracted by molecules on the other side. The resultant force of these two attractive forces is called interfacial tension. If the solid is low surface energy, its attraction is lower than the attractive force of the liquid phase molecules, the liquid molecules in the interface zone, there is tension to the internal liquid shrinkage, this is the non-wetting state; if it is a high surface energy solid, it attracts When the force is higher than the attractive force of the liquid phase molecules, the liquid molecules in the interfacial zone have a pressure that is adsorbed on the solid, which is the wetting state.
For example, the surface tension of mercury is: 0.476 N/m, and the surface energy of polystyrene plastic plates is: 0.042 N/m, which is much less than the surface tension of mercury. If mercury is dropped onto a polystyrene plate, the mercury droplets will tend to It is spherical and non-wet. If the surface tension of only: 0.225N / m of ethanol drops onto the polystyrene board, it can be spread, wet state.
In general, inorganic solids (metals, oxides) have a 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 surfaces with low surface energy (such as plastics), the ink does not wet the substrate easily. At this time, some processing or modification of the substrate surface is required before normal printing can be performed.
2. Contact angle
If the solid has a high surface free energy and its attraction is higher than the attractive force of the liquid phase molecules, the liquid molecules in the interfacial zone have a pressure that is adsorbed to the solid, and the liquid has the ability to displace the gas on the solid surface when the liquid is in the solid state. The surface is in a wet state; at this time, the smaller the contact angle θ is smaller than 90°, the better the wetting effect is. 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 zone have a tension that shrinks to the inside of the liquid, indicating that the liquid does not wet the solid, then the contact angle θ is larger than At 90°, the greater the contact angle, the poorer the wetting effect.
In general, solid molecules have strong interactions, hardness, 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, the interaction between solid molecules Weak, soft, low melting point, little crystallization, organic matter (paraffin, polymer), the surface free energy is low, not easily wetted by general liquids. However, whether the ink can be firmly attached to the substrate depends on the attraction between the ink and the substrate.
Third, the attraction between ink and substrate
The adhesion between the ink and the substrate mainly includes the chemical bond force (ie the force between the atoms), the intermolecular force (hydrogen bonding force and Van der Waals force), the interface electrostatic attraction and the mechanical force.
Chemical bond force
Chemical bonding forces are the forces between atoms, including ionic bonds, covalent bonds, and metallic bonds.
1 Ionic bond force. The interaction between positive ions and negative ions.
2 covalent bond force. That is, the interaction between two atoms through a common electron pair (overlapping of two atom electron clouds). Most of the molecules of organic compounds are composed of covalent bonds. The covalent bond is a relatively strong chemical bond.
3 metal bond force. The attraction between metallic cations due to the free movement of electrons. Metal atoms are connected by metal bonds into metal crystals. (one)