The water-based ink is composed of water-based polymer resin and emulsion, organic pigments, solvents (mainly water) and related additives through physical and chemical processes. The water-based ink has no volatile organic solvents, is not flammable, does not damage the health of ink manufacturers and printing operators, and has no pollution to the atmosphere. As a new type of printing ink, water-based ink does not have some toxic and hazardous substances in the solvent-based ink remaining in the printed matter and pollution of the packaged goods, improving the overall environmental quality.
The water-based flexographic ink is named according to the classification method of the ink according to the plate type and combined with the drying mechanism of the ink.
2. Technical basis for the formulation of water-based flexographic ink
1. Neutralization and salt formation mechanism of waterborne binder
Water-based inks use water-based binders. Waterborne binders are basically divided into three categories: colloidal dispersions, emulsion polymers, and water-dilutable polymers. At present, the ink industry mainly uses dilution type connecting materials. Many polymers (water-soluble) were originally insoluble in water or only partially soluble in water. Only by adding an acid or alkali can they be dissolved in water due to ionization. Typical examples are polyacrylic acid and polyamide, which are anionic and cationic electrolytes, respectively. The pH value of the aqueous solution of these substances is closely related to its viscosity, flocculation effect, stability, and dispersibility. Therefore, the molecular chain of the water-soluble polymer contains a certain number of strong hydrophilic groups, such as carboxyl groups, hydroxyl groups, amine groups, etc., but most of these polar groups can only form an emulsion when mixed with water. The carboxylate can be dissolved in water, so the production of water-soluble binders often uses high-acid synthetic resins, which are then neutralized with amines to form salts.
2. Drying film formation mechanism of volatilization, penetration, curing reaction or all three
In general, the printing process and the corresponding drying method determine the ink formulation system used. In order to meet the technical requirements of flexographic printing and the characteristics of water-based inks, according to different printing substrates, the drying methods of water-based flexographic inks are volatilization, penetration, curing reaction or both.
(1) Volatile drying mechanism
For flexographic printing where the substrate is a non-absorbent substrate, the drying method is mainly volatile drying, that is, the resin / solvent ink system is used. because:
â‘ The flexo printing machine has a very fast speed, from 80m / min to 200m / min. Generally speaking, the interval between the first color printing and the second color printing is only a few seconds to a few tenths of a second. For non-absorbent substrates, of all drying types, only volatile drying can meet this requirement.
â‘¡Only the liquid in the ink film can be removed from the ink film as soon as possible, for example, the solvent with a low boiling point has the characteristics of instant volatilization, so that the ink can be dried quickly. Fast drying also requires the ink to be a low viscosity and thin liquid.
â‘¢ For offset printing ink and lead printing ink, the ink can be evenly coated on the printing plate through the adjustment between the ink rollers, while the flexographic ink is only filled in the anilox roller net by its own fluidity and adhesion In the eyes and the ink is transferred to the printing plate, only a lower viscosity, that is, a thinner liquid can impart such properties. In addition, the ink must fill the concave eye in a short time. If the viscosity is too high, it is difficult to fill the concave eye. At the same time, it is difficult for the scraper to scrape off the ink on the wall part smoothly. Therefore, flexographic ink is generally in the form of a fluid with a low viscosity, and a large amount of solvent or water can only be carried out by means of volatilization or penetration. Of course, the viscosity is too low, otherwise imprinting will cause the ink in the dots to be easily deformed, making the reproducibility of graphics and text worse.
The drying speed of the ink first depends on the evaporation rate of the solvent in the ink. The main factors affecting the volatilization of solvents in ink are as follows:
â‘ Different resins have different degrees of slowing down the solvent evaporation rate. The greater the solubility of the resin, the more difficult it is to remove the solvent and the lower the volatilization rate.
â‘¡ The greater the proportion of pigment in the ink, the lower the volatilization rate of the solvent; the smaller the radius of the pigment particles, the greater the specific surface area, the lower the volatilization rate of the solvent; different types of pigments have different releasability of the solvent.
(2) Osmotic absorption and drying
For the flexo printing of absorbent substrates, the drying method is mainly permeable absorption drying. Absorbent substrates such as paper are absorbent for ink. The binder in the ink is a liquid film-forming substance, and at the same time, it can bind the toner particles and make them stick to the surface of the paper. After the transfer material is transferred to the paper, it penetrates into the paper until the toner particles in the ink become sufficiently close, and the gap between the particles is continuously reduced. The capillary force between the pigment particles produced is equal to the absorption connection force in the paper. Absorb penetration process. The degree of penetration of the connecting material into the paper is proportional to the capillary tension in the paper, that is, the ink absorption of the paper depends on the degree of looseness inside, that is, the tightness and uniformity of the paper. In actual printing, the effects of printing pressure, printing time, and ink viscosity should also be considered. The Olsson formula basically summarizes the relationship of the above factors to the depth of the ink pressed into the paper:
d = (p × r2 × t / 2η) 1/2
d: the depth of ink pressed into the paper
p: printing pressure
r: paper capillary radius
η: ink viscosity
Since the printing pressure is much greater than the capillary attraction, the latter is ignored.
The penetration of ink into the absorbent substrate is important for the ink curing and drying process. The amount of penetration is too small or too shallow, the ink does not stick well, and it is not easy to dry. However, if the penetration amount is too large or too deep, it will cause through-printing problems and reduce the gloss of the ink.
In addition, for absorbent substrates, there is also a volatile drying mechanism. Therefore, for flexo printing of absorbent substrates, the drying method is a dual film-forming mechanism of volatilization and penetration.
In the formulation of inks with special requirements, some reactive groups are also introduced to further improve their film-forming properties.
3. Many factors affecting the formula design of water-based flexographic ink
1. Requirements for printability
The process of transferring ink from the ink tank to the printing plate to the substrate is the process of ink transmission and transfer separation. First of all, it is required that the ink can be stably transferred to the printing plate during the printing process. Secondly, it is hoped that the ink on the plate surface will always be transferred to the substrate effectively in a certain state. The quality of transmission and transfer is related to the performance and accuracy of the printing press, and at the same time requires the ink to have the appropriate printability. The rheology of the ink itself becomes an important factor governing the suitability of the ink. Rheology includes viscosity, yield value, thixotropy, fluidity, adhesion, and filament length.
The cohesion of the viscous ink must be suitable for the entire printing process.
(1) 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 low viscosity and is easy to separate, and the solvent easily escapes from the surface of the ink film. Practice has proved that the plastic viscosity of the ink has a significant effect on the transfer rate of the ink. Under the same speed of the same printing machine, the split state of the ink layer with lower viscosity is more favorable to the transfer of ink than the viscosity. As shown in Figure 1.
(2) Requirements for substrates: For relatively smooth coated paper, the transfer rate is higher when the ink supply is sufficient, so the viscosity of the ink is required to be slightly higher; for the paper with soft structure such as offset paper, the viscosity of the ink should be slightly lower.
Thixotropy is "shear thinning". The ink should have a certain thixotropy. After the ink is mechanically rotated on the ink roller during the printing process, the fluidity of the ink changes, the viscosity decreases, the fluidity increases, and its ductility also increases. Conducive to the smooth and uniform transfer of ink. After the ink is transferred to the substrate, it loses the external force, because its thixotropy quickly changes from thin to thick without overflowing to the surroundings, ensuring the accuracy of the imprint. In particular, the printing of dots and text lines can prevent the dots from expanding and thickening caused by the ink infiltration and spreading on the paper. The thixotropy of ink is related to the following factors:
(1) Properties of pigments The inks made of pigments with strong surface adsorption have a large thixotropy.
(2) The shape of the pigment particles When the pigment particles are needle-shaped, they are more thixotropic than the ink made of spherical particles.
(3) The dosage of pigments Generally, the greater the proportion of pigments in the ink, the stronger the particle interaction, and the greater the thixotropy of the ink.
(4) The wetting ability of the pigment particles and the binder is low, and the ink has a large thixotropy.
(5) Molecular weight of resin The greater the molecular weight of the resin in the binder, the greater the thixotropy.
Printing suitability requirements for thixotropy: The printing operation requires the ink to have appropriate thixotropy, but if the thixotropy of the ink is too large, the ink will cause the ink supply to be not smooth in the ink fountain, and even the phenomenon of ink supply interruption will affect the continuous The uniformity and accuracy of ink supply during printing. Different types of printing will have different thixotropy of ink. The general screen printing, text printing and line printing require the thixotropy of the ink to be slightly larger; the large-area field version is slightly smaller.
In the printing process, the force that makes the ink film split and transfer to the corresponding object surface (ink roller surface, printing plate surface or substrate surface) is the adhesion, that is, the ink and the ink roller, printing plate, substrate Connectivity between surfaces. Under the action of adhesion, the ink splits first and then transfers. This is the dynamic response of the ink to adhesion. The ability of the ink itself to prevent the separation of the ink film in this dynamic process, called the adhesion of the ink, is essentially a performance of the cohesion of the ink (the connection force between the ink molecules) under the action of adhesion. The effect of ink adhesion on printability is:
(1) When the adhesion of the ink is large, the separation of the ink is difficult, resulting in uneven spreading of the ink on the printing press. When the ink layer is separated between the paper and the blanket, if this resistance greatly exceeds the binding force of the paper, it will produce a plucking phenomenon or even a peeling phenomenon.
(2) In multi-color printing, the first color is printed quickly while the ink of the previous color is not dry. Generally, the adhesion of the first color ink is required to be greater, and the ink of the subsequent colors should be gradually reduced. Otherwise, there will be It may appear that the back is not printed, and the front ink layer is stuck away.
Ink adhesion and drying After the ink is attached to the substrate, it changes from a liquid jelly to a solid film and adheres to the substrate. This process of change is called ink drying. This process is completed in two stages: the ink changes from liquid to semi-solid, and can no longer flow and transfer. It is the initial drying stage of the ink, which is expressed by the initial dryness. In the semi-solid ink, the main part of the linking material undergoes physical or chemical reaction to completely dry and form a film, which is the complete drying stage of the ink, which is expressed by thoroughness. The initial drying stage and thorough drying stage of the ink are collectively referred to as the fixed drying of the ink. The drying speed of the ink is related to the drying method of the ink. The drying method of the ink depends on the components of the ink binder. Different printing methods, substrates and printing machinery have different requirements for the drying of ink. The effect of the fixing and drying performance of the ink on the printability has the following two points:
(1) For paper substrates, ink can flow into its depressions and voids to be fixed, which is attached by the mechanical "anchor effect"; for substrates with smooth and non-absorbent surfaces, the adhesion of ink depends on the ink The wetting effect on the substrate, that is, the affinity of the liquid on the solid surface, also depends on the interaction between the ink and the substrate molecule. However, due to the accumulation and adhesion of paper during high-speed printing, the intermolecular distance will be drastically reduced, resulting in a significant increase in the binding force between the ink and the substrate molecule, and the wet ink will be reflected on the back of the next sheet. Therefore, in general, on the premise of not affecting the adhesion of the ink to the substrate, the ink adhesion should be appropriately reduced, or the fixing and drying of the ink should be accelerated to avoid the "backside rubbing" of the printed product.
(2) The drying speed of the ink affects the adhesion of the ink. As shown in Figure 2, as the drying time increases, the adhesion of the ink gradually increases to the maximum, and then decreases to zero. If the ink dries quickly, the surface of the ink film will be smooth, smooth and dry and lose its adhesion, the second color ink will be difficult to attach, and the ink will have a "crystallization phenomenon". The ideal overprint should be carried out in the time from the initial drying to the thorough drying of the ink.
If the drying is too fast, the ink will skin and paste on the surface of the printing plate, causing the printing ink to accumulate, and the gloss is poor, and "ink spots" appear; if the drying is too slow, the phenomenon of "rubbing" may occur on the back of the printing.
2. Requirements for performance of packaging
Hiding power The area of ​​the original ink that can be printed per unit weight is usually called the hiding power of the ink, and the unit is m2 / g. The hiding power depends on the following factors:
(1) The printing area of ​​inks with a large specific gravity of ink is less than that of inks with a small specific gravity of the same weight.
(2) Ink transfer amount The transfer amount is affected by the hardness of the ink fountain roller and the plate material, the ink carrying capacity and wear of the anilox transfer roller ink hole, and the installation position of the doctor blade.
(3) The content of colorant in the ink determines the size of the ink's hiding power.
(4) The hiding power of ink on non-coated paper or other absorbent printing materials is worse than that on non-absorbent printing materials, because ink can penetrate into absorbent materials and reduce the brightness of colors. When printing paper, the water-based ink penetrates the paper body at a slower rate than the solvent ink, and the hiding power is higher than the solvent ink of the same color strength.
Chemical resistance, scratch resistance, heat sealability, grease resistance, freezing resistance, durability, etc.
The above discusses many factors that affect the performance of water-based flexographic inks. On this basis, it is possible to further determine the selection and preparation of its basic formula, linking material system and its raw materials, and it is scheduled to develop the general performance requirements and technical indicators of the varieties.
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