Third, volatilization, penetration, curing reaction or a combination of three dry film formation mechanism Source: 21st Century Fine Chemicals Network
In general, the printing process and the corresponding drying method determine the ink formulation used. In order to meet the technical requirements of the flexographic printing and the characteristics of the water-based ink itself, according to different printing substrates, the drying mode of the water-based flexographic printing ink has volatilization, infiltration, curing reaction or both.
(1) Volatile drying mechanism
For the flexographic printing of substrates with non-absorbent substrates, the drying method is mainly based on volatilization and drying, that is, a resin/solvent ink system. Because, first of all, the speed of the flexo press is very fast, from 80m/min to 200m/min. Generally, the first color is printed to the second color, and the interval is only a few seconds to several tenths of seconds. For non-absorbent substrates, only volatile drying meets this requirement in all types of drying. Secondly, only the liquid in the ink film can be removed from the ink film as quickly as possible, for example, the volatile nature of the solvent with a low boiling point can cause the ink to rapidly dry. Fast drying also requires the ink to be a low viscosity and thin liquid.
In addition, the flexo ink is filled in the anilox roller mesh by its own fluidity and adhesiveness and is transferred onto the printing plate. Only the lower viscosity, that is, the thinner liquid has such a property. In addition, the ink must fill the mesh in a short period of time. If the viscosity is too large, it is difficult to fill in the mesh, and it is difficult for the scraper to smoothly scrape the ink on the wall portion. Therefore, the flexographic ink is generally in the form of a low-viscosity fluid, and a large amount of solvent or water is only generated by volatilization or permeation. Of course, the viscosity is too low, or imprinting will cause the ink in the dot to be easily deformed, which will degrade the reproducibility of the graphic and text.
The drying speed of the ink first depends on the volatilization rate of the solvent in the ink. The following factors affect the volatilization of the solvent in the ink: Different resins have different degrees of slowing down the volatilization rate of the solvent. The greater the solubility of the resin, the more difficult the solvent to escape, the lower the volatilization rate; the greater the proportion of the pigment in the ink, the lower the volatilization rate of the solvent; the smaller the radius of the pigment particles, the larger the specific surface area, the volatilization rate of the solvent The lower; different types of pigments have different solvent release properties.
(2) Osmotic absorption drying
For flexo printing of absorbent substrates, the drying method is mainly osmotic absorption drying. Absorbent substrates such as paper absorb ink. The binder in the ink is a liquid film-forming substance that binds the pigment particles and adheres to the surface of the paper. After the binder is transferred to the paper, it penetrates into the interior of the paper until the pigment particles in the ink become sufficiently tight and the gap between the particles is continuously reduced. The resulting capillary pull between the pigment particles is equal to the absorption binding force in the paper. Absorb infiltration process. The degree of penetration of the binder into the interior of the paper is proportional to the pull of the capillary in the paper, that is, the ink absorption of the paper depends on the degree of looseness inside the paper, that is, the tightness and the organization of the paper. In actual printing, the influence of printing pressure, imprinting time, and ink viscosity should also be considered.
The penetration of ink into the absorbent substrate is important for the ink curing and drying process. The amount of penetration is too little or too light, the ink is not sticky, and it is not easy to dry. However, if the amount of penetration is too large or too deep, it will cause problems with printing, and it will also reduce the gloss of the ink. In addition, for the absorbent substrate, there is also a volatile drying mechanism. Therefore, for the flexo printing of absorbent substrates, the drying method is a dual film-forming mechanism of volatilization and infiltration. In some particularly demanding ink formulations, some reactive groups are also introduced to further improve their film forming properties.
Fourth, should pay attention to technical control indicators
1. Viscosity
Viscosity is a property that impedes the flow of fluids and is a measure of the ability of fluid molecules to interact with each other and hinder their ability to move relative to each other, that is, the resistance to fluid flow. Viscosity is the most important control indicator in ink applications because it directly affects the ink transfer transfer performance and flexographic print quality. Viscosity is related to the viscosity and density of the resin in the binder, but also related to the type and particle size of the pigment. In the printing, the low viscosity of the ink transfer is fast, resulting in the disadvantages of light color, large dots, high light dot distortion, uneven ink transfer, etc.; high viscosity, slow ink transfer, will affect the transfer performance of the anilox roller, resulting in ink color Inconsistent colors, sometimes dark prints, and dirty, dirty version, and other ills.
Low viscosity can be adjusted by mixing with new ink; high viscosity can be diluted with water, water and ethanol (50% each), and can also be adjusted with ink stabilizer. In addition, in the printing process, the temperature has a greater influence on the viscosity of the ink, usually the viscosity increases when the temperature rises, and vice versa, the viscosity increases. Therefore, in the printing process, in order to maintain the consistency of the print density, the temperature of the printing shop should be kept constant.
2. PH value
Since the binder used in the ink is mainly an alkali-soluble acidic resin, the control of the pH value is very important. Normally, the pH should be controlled between 8.5 and 9.5. At this time, the printing performance of the ink is the best and the quality of the print is the most stable. As amines are constantly evaporating during the printing process, operators also need to add new inks and various additives from time to time, so the pH of the ink is changing at any time. When the PH value of the ink is higher than 9.5, the alkali is too strong, the viscosity of the ink will decrease, resulting in slower drying speed and poorer water resistance; when the PH value is lower than 8.5, the alkali is too weak, and the viscosity of the ink will increase. High, resulting in faster drying speed, easy to jam the printing plate and anilox roller, causing the plate surface to be dirty, and it will produce bubbles.
Since the influence of PH value and viscosity are directly related, it is recommended that the two be linked together in actual printing. Especially in case of color printing, this problem should be paid more attention. Usually, in addition to the viscosity cup, a simple pH indicator should also be prepared for testing at any time. When the PH value of the ink is relatively low, a pH stabilizer or a small amount of an alkaline substance may be added. When the pH is high, a solvent or diluent may be added for dilution.
3. Particle thickness
The so-called ink thickness refers to the degree of dispersion of solid materials such as pigments and fillers mixed in the linked department. The thickness of the ink is also an important quality indicator because it is not only related to the application properties of the ink (including tinting power, hue, stability, etc.), but also related to the rheological properties and economic benefits of the ink. Ink is too thick to cause defects such as heaping ink, stencil, etc., and the coloring power and hue of the ink will also be deteriorated. If the ink is too thin, the liquidity will be large, and it will flow after printing. This flow will cause the dot expansion, affecting Print quality. In addition, the thickness of the ink also affects its dispersibility. When the dispersibility of the ink is not satisfactory, its printing performance is also not satisfactory, and the print surface is not smooth, smooth and soft, and it will produce a "block"-like feeling, which greatly reduces the printing effect.
In the actual printing, inks with different thicknesses and fineness can be selected according to specific requirements. In the case of prints with relatively fine mesh and high precision, the fineness of the ink must be relatively high; when the print is not very precise in terms of print or field prints, the thickness of the ink may be appropriately reduced.
4. Dryness
Dryness of ink is a very important technical indicator. In addition to the performance of the ink itself, the drying speed of the ink is related to the printing speed, the drying capacity of the drying equipment, and the performance of the substrate. This article only considers the ink's own factors. The principle for selecting the ink drying property is usually: the ink should be able to be fully dried before the next printing and before rewinding or stacking under the existing conditions of the printing machine, but cannot be dried on the printing plate.
Ink uses water as the solvent and soluble acidic resin as the linking material. It is mostly used for the printing of corrugated cartons and paper bags. Therefore, the drying form is mainly osmotic drying and a small amount of volatile drying. Since water is used as a solvent, its volatilization rate is lower than other solvents, and thus the ink drying is slower than the solvent ink. Therefore, a small amount of alcohol is generally added to the solvent to increase the drying speed of the ink. In addition, the selection of different types of resins under solvent-specific conditions also affects the drying properties of the ink. Because different resins have different curing speeds on the substrate and different solvent release characteristics, the ink drying performance is also different.
In practical applications, the drying property of ink is also related to its viscosity and PH value. The ink is dry but may be too viscous, and can be solved by further dilution; the ink dries quickly, it may be PH value below 8.5, and stabilizers may be added for adjustment. In addition, a quick-drying agent or a slow-drying agent may be added to the ink according to the actual printing speed, and the drying speed of the ink may be appropriately changed to obtain a stable printing effect. The amount of quick-drying agent or slow-drying agent is generally 1% to 2% of the total amount of ink.
Fifth, many factors that affect the formula design of water-based flexographic ink
1. Printability requirements
The process of transferring the ink from the ink tank to the printing plate to the printing material is the ink transfer and transfer separation process. First, it is required that the ink be stably transported to the printing plate at all times in the printing process, and secondly, it is desired that the ink on the plate is always transferred to the printing material in a certain state. The quality of the transfer and transfer is related to the performance and precision of the press, and at the same time the ink is required to have the appropriate printability. The rheology of the ink itself has become an important factor in controlling ink suitability. Rheology includes viscosity, yield value, thixotropy, fluidity, adhesion, length of ink, and the like.
Viscosity, which is the cohesion of the ink, must be suitable for the printing process. The higher the printing speed, the lower the viscosity of the ink. Fast printing requires fast transfer and fast drying, that is, the ink has small viscous force, is easy to separate, and the solvent easily overflows from the surface of the ink film. Practice has proved that the ink plastic viscosity has a significant effect on the transfer rate of the ink. Under the same speed of the same printing machine, the ink layer with the lower viscosity has better splitting state than the ink with larger viscosity. From the substrate point of view, for relatively smooth coated paper, when the ink supply is sufficient, the transfer rate is higher, so the viscosity of the ink is required to be slightly higher; the viscosity of the ink used for the loose-textured paper such as offset paper should be slightly lower.
Thixotropy, thixotropy is "shear thinning." The ink should have a certain thixotropy. After the ink is mechanically rotated by the ink roller during the printing process, the fluidity of the ink changes, the viscosity decreases, the fluidity increases, and the ductility increases. It facilitates the smooth and even transfer of ink. After the ink is transferred to the substrate, it loses its external force, because its thixotropy quickly thickens from the thin and does not flow to the surroundings, ensuring the accuracy of the imprinting. Especially the printing of dot and text line printing can avoid the dot expansion and line thickening caused by the ink infiltrating and spreading on the paper. The thixotropy of the ink is related to the following factors: the nature of the pigment, the high thixotropy of the ink made from pigments with strong surface adsorption, and the shape of the pigment particles. When the pigment particles are needles, the thixotropy of the ink made from the spherical particles is greater; The amount of pigment, the greater the proportion of pigment in the ink, the stronger the particle interaction, the greater the thixotropy of the ink; the wetting ability of the pigment particles and the binder, the low wetting ability, the large thixotropy of the ink; the molecular weight of the resin, The greater the molecular weight of the resin in the binder, the greater the thixotropy.
The suitability of printing for thixotropy: The printing job requires the ink to have appropriate thixotropy, but if the thixotropy of the ink is too large, the ink in the ink fountain will cause the ink supply to be not smooth, and even the phenomenon of ink supply interruption will occur, which will affect the continuous Uniformity and accuracy of ink supply during printing. Different types of printing will have different thixotropy on the ink. The printing of general cable, text, and line printing requires a slightly greater thixotropy of the ink; a large field version is slightly smaller.