Screen Printing Spot Color Inks (Part 2)

1. Neutralization salt formation mechanism of aqueous binders

Water-based inks use aqueous binders. Waterborne binders fall into three categories: colloidal dispersions, emulsion polymers, and water-reducible polymers. At present, the ink industry mainly uses dilute binders. Many macromolecules (water-soluble) are originally insoluble or only partially soluble in water, and only by the addition of an acid or base can they be dissolved in water by ionization. A typical example is polyacrylic acid and polyamide, respectively, anionic and cationic electrolytes. The pH of the aqueous solution of these substances is closely related to its viscosity, flocculation effect, stability, dispersion, and the like. Therefore, a certain number of strong hydrophilic groups such as carboxyl groups, hydroxyl groups, and amine groups are contained in the molecular chain of the water-soluble polymer, but most of these polar groups can only form an emulsion when mixed with water. Carboxylates are soluble in water, so the production of water-soluble binders tends to use high-acid synthetic resins and neutralization with amines.

2. Volatilization, infiltration, curing reaction or dry film formation mechanism

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, ie, the resin/solvent ink system is used. because:
1 The speed of the flexo press is very fast. There are 80m/min to 200m/min. Generally, the first color is printed to the second color, and the interval is only a few seconds to a few tenths of a second. For non-absorbent substrates, only volatile drying meets this requirement in all types of drying.
2 Only the liquid in the ink film is removed from the ink film as quickly as possible, for example, the solvent with a low boiling point has the characteristics of instantaneous volatilization, so that the ink can be quickly dried. Fast-drying also requires the ink to be a low viscosity and a thin liquid.
3 For offset printing inks and printing inks, the ink can be evenly coated on the printing plate by the ink roller reconciliation. The flexographic printing ink is filled in the anilox roller only by its own fluidity and adhesiveness. In the eyes, ink is transferred to the printing plate. Only the lower viscosity, that is, the thinner than the liquid, can impart such properties. In addition, the ink must fill the concave eye in a short period of time. If the viscosity is too large, it is difficult to fill in the concave eye. At the same time, it is difficult for the scraper to smoothly scrape off the ink of the net wall portion. Therefore, the flexographic ink is generally in the form of a relatively low viscosity fluid. A large amount of solvent or water can only be generated by volatilization or permeation. Of course, the viscosity is too low, or imprinting will cause the ink in the dots to be easily deformed, which will deteriorate 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 volatilization of the solvent in the ink mainly has the following influencing factors:

1 Different resins have different degrees of slowness in solvent evaporation. The greater the solubility of the resin, the more difficult the solvent to escape, the lower the volatilization rate.
(2) 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 larger the specific surface area, the lower the volatilization rate of the solvent; and the different types of pigments have different solvent removal 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 material 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 pulling force 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 organization of the paper. In actual printing, the influence of printing pressure, imprinting time, and ink viscosity should also be considered. The Olsson formula basically sums up the relationship of the above factors to the depth of the ink pressed into the paper: d = (p×r2×t/2η)1/2

d: ink pressed into paper depth
p: printing pressure
r: Paper capillary radius
η: ink viscosity
Since the printing pressure is much greater than the capillary attraction, the latter ignores it.

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.
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Third, many factors affecting the water-based flexographic ink formulation design

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, tackiness, length of ink, and the like.

The viscosity of the cohesive force of the ink must be suitable for the printing process.

(1) The higher the printing speed, the lower the viscosity of the ink. Fast printing requires fast transfer and quick drying, that is, the ink has small viscous force, is easy to separate, and the solvent easily escapes 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 condition of the same speed of the same printing machine, the ink layer with a small viscosity has a better splitting state than the ink with a high viscosity. As shown in Figure 1.
(2) Substrate requirements: 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 loose paper such as offset paper should be slightly lower.

Thixotropic 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 ink is related to the following factors:

(1) Properties of the pigment The ink made of a pigment having a strong surface adsorption property is highly thixotropy.
(2) Shape of pigment particles When the pigment particles are needle-like, the thixotropy of the ink made from the spherical particles is larger.
(3) The amount of pigment In general, the greater the proportion of the pigment in the ink, the stronger the interaction of the particles and the greater the thixotropy of the ink.
(4) Wetting ability of the pigment particles and the binder The wetting ability is low, and the thixotropy of the ink is large.
(5) 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.

Adhesion In the printing process, the force that splits the ink film and transfers it to the corresponding object surface (ink roller surface, plate surface, or substrate surface) is adhesion, that is, ink and ink roller, plate, substrate The connection force between the surfaces. Under the action of adhesion, the ink first splits and then transfers. This is the dynamic response of the ink to the adhesion. The ability of the ink itself to prevent the separation of the ink film is manifested in this dynamic process, and the adhesion of the ink is essentially a manifestation of the cohesion of the ink (the connection force between the ink molecules) under the action of the adhesion. The effect of ink adhesion on printability is:

(1) When the adhesiveness of the ink is large, the separation of the ink is difficult, resulting in uneven spreading of the ink on the printer. When the ink layer is separated from the paper or the blanket, if the resistance greatly exceeds the binding force of the paper, a pulling phenomenon or even a peeling phenomenon will occur.

(2) In multi-color printing, after the ink of the preceding color is not dried, the color of the ink is quickly printed. Generally, the adhesiveness of the first color ink is required to be larger, and the ink of the back color is to be gradually reduced in adhesiveness, otherwise, there is It may happen that the back of the ink cannot be printed, and stick the front ink layer away.

After the ink adheres and the dried ink adheres to the substrate, it changes from a liquid jelly to a solid film and adheres to the substrate. This change process is called ink drying. This process is completed in two stages: the ink changes from a liquid to a semi-solid state and cannot be re-transferred. It is the initial drying stage of the ink and is expressed in terms of initial dryness. The physical or chemical reaction of the main part of the binder in the semi-solid ink is completely dry to form a film, which is a thorough drying stage of the ink, and is represented by thoroughness. The initial dry stage and the dry stage of the ink are collectively referred to as the fixation and drying of the ink. The drying speed of the ink is related to the drying mode of the ink. The ink drying method depends on the composition of the ink binder. Different printing methods, substrates, and printing machines have different requirements for ink drying. The effect of the fixing and drying performance of the ink on printability has the following two points:

(1) For paper-based substrates, the ink can flow into the cavity and voids to be fixed, which is attached by the mechanical "anchoring effect"; for a substrate with a smooth surface and no absorption, the adhesion of the ink depends on the ink. The wetting of the substrate, the affinity of the liquid to the solid surface, also depends on the interaction between the ink and the substrate molecules. However, high-speed printing due to the accumulation and adhesion of the paper will make the distance between molecules drastically reduced, resulting in significant increase in the binding force between the ink and the substrate molecules, ink will not dry back to the back of the next sheet. Therefore, in general, under the premise of not affecting the adhesion between the ink and the substrate, the adhesion of the ink should be appropriately reduced, or the fixation and drying of the ink should be accelerated, so as to avoid the “backside smudging” 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 tackiness of the ink gradually increases to its maximum value and then drops to zero. If the ink dries quickly, the surface of the ink film will become smooth and dry and lose its tackiness. The second color ink will be difficult to adhere to, and the ink will produce "crystallization