viscosity
Viscosity refers to the measure of the ability of molecules in a fluid to slow down or reduce the mutual motion of molecules due to the interaction. In inkjet ink, fineness is an important parameter to measure its performance: if the viscosity is too low, the ejected ink droplets will be easily broken; if the viscosity is too high, it will cause the ink to break due to dragging and appear drawing phenomenon tw. The viscosity of the ink also has a great influence on the jetting speed of the ink: if the viscosity is too low, it will drip directly from the nozzle, which will affect the jetting effect; if the viscosity is too high, the jetting speed of the ink will be reduced.
There are many factors that affect ink viscosity, such as resins, pigments and additives in ink components will affect the viscosity of ink: the influence of resin on ink viscosity mainly lies in its structure; while pigment is based on its composition in ink components. The content in the ink affects the viscosity of the ink in direct proportion.
dispersion
The dispersion effect of the pigment in the water-based ink directly determines the dispersion effect of the ink, which is mainly affected by the properties and interactions of the components in the system. In the development process of the ink, the dispersion of the pigment is divided into the wetting of the pigment, the pulverization of the pigment and the stabilization of the pigment.
Wetting is the prerequisite for the dispersion of pigments, mainly by using surfactants or resins to replace the fluid on the surface of pigment particles, thereby improving the mutual solubility of the connecting resin and pigments; crushing refers to the use of mechanical force to condense the aggregated pigments according to the process requirements. The particles are crushed to a suitable particle size to form a suspension dispersion; stabilization means that the ground suspension dispersion can be kept in a stable state during storage without aggregation or precipitation by adding other additives.
The dispersion state and dispersion stability of the pigment in the ink will affect the performance of the ink. If the pigment particles in the inkjet ink are coagulated or precipitated, the nozzles will be blocked and normal printing cannot be achieved. At the same time, the drying, film-forming and color rendering properties of the ink are all closely related to the dispersibility of the pigment. Therefore, the pigments used in ink development must have high requirements in terms of dispersibility and dispersion stability.
Surface Tension
The surface tension of inkjet ink has a great influence on the formation of ink droplets on the surface of the substrate and the quality of the printed product during the printing process. For example, whether there is liquid overflow at the nozzle, the drawing length of the droplet when it breaks, the attachment degree and stability of the droplet, the speed and running curve of the droplet ejection all have a great influence on the surface tension of the ink.
When the surface tension of the ink is too small, the ink is not easy to produce tiny ink droplets, and there will be stringing phenomenon when it breaks, which will cause the ink droplets to fly unstable during printing. to the quality of printed products; when the surface tension of the ink is too large, the wettability of the nozzle will become poor, and the surrounding ink will gather together to make the ink drop deflect during operation, and it will also make the ink drop on the surface of the substrate. The wetting, penetrating and drying properties of the ink are affected. In general, inks with lower surface energy than the substrate are selected for printing.
pH value
For the preparation of water-based ink, alkali-soluble resin is often used as the binder, which makes the prepared ink alkaline, and its pH value is different from that of solvent-based ink. How the change of pH value of water-based ink affects the performance of ink has always been a difficult problem in the field of water-based ink application research.
drying performance
The drying performance of the ink is one of the important indicators to measure the quality and performance of the ink, and the good drying performance of the ink is one of the necessary conditions for obtaining high-quality printed matter. When the ink is dry, it includes initial drying and thorough drying: the ink on the surface of the substrate changes from a liquid state to a semi-solid that no longer flows, which is the initial drying stage; the ink’s thorough drying stage refers to the occurrence of the binder in the semi-solid ink component. React until dry to form an ink film.
Adhesion performance
Ink adhesion refers to the ability of the dried ink film to bond to each other through physical and chemical interactions on the surface of the substrate. The influencing factors mainly include the mechanical binding force during the printing process, the adsorption capacity of the two sides in contact, the interaction capacity of chemical bonds and the mutual diffusion effect.
The adhesion of ink is an important index to measure the performance of ink; the ink with good adhesion can make the ink absorb well on the surface of the substrate after printing and realize a good printing process.
There are mainly two types of adhesion mechanisms for the ink film: one is mechanical adhesion, and the other is chemical adhesion. Among them, the surface properties of the substrate and the ink film strength after ink drying determine the size of the mechanical adhesion; and the attraction between the molecules at the contact surface of the two affects the change of the chemical adhesion. We usually think that the adhesion of ink is mainly affected by the chemical adhesion of the ink film, such as molecular electrostatic force, hydrogen bond force, chemical affinity and van der Waals force.
The adhesion of the ink will also be affected by the molecular structure of the compound in the film-forming resin (some polar groups -OH, -COOH) and the binding force between the polar groups on the surface of the substrate, so W should be selected when preparing the ink. A film-forming resin with good wettability, so that the film-forming compound can fully contact the polar groups on the surface of the substrate during printing to generate an interaction force to enhance the adhesion of the ink.
