First, water-based flexographic ink concept
Water-based inks are formed by mixing physical and chemical processes of aqueous polymer resins and emulsions, organic pigments, solvents (mainly water) and related auxiliary agents. Water-based inks have no volatile organic solvents, are non-flammable, do not harm the health of ink manufacturers and printing operators, and have no pollution to the atmosphere. As a new type of printing ink, water-based ink does not contain certain poisonous and harmful substances in solvent-based ink residues in the printed matter and contamination of packaged goods, improving the overall environmental quality.
Water-based flexographic inks are based on the classification of the ink by the printing plate type and are combined with the drying mechanism of the ink.
Second, the technical basis of water-based flexographic ink formulations
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, that is, a resin/solvent ink system. 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 itself is filled with the fluidity and adhesiveness of the anilox roller. 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 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.
Water-based inks are formed by mixing physical and chemical processes of aqueous polymer resins and emulsions, organic pigments, solvents (mainly water) and related auxiliary agents. Water-based inks have no volatile organic solvents, are non-flammable, do not harm the health of ink manufacturers and printing operators, and have no pollution to the atmosphere. As a new type of printing ink, water-based ink does not contain certain poisonous and harmful substances in solvent-based ink residues in the printed matter and contamination of packaged goods, improving the overall environmental quality.
Water-based flexographic inks are based on the classification of the ink by the printing plate type and are combined with the drying mechanism of the ink.
Second, the technical basis of water-based flexographic ink formulations
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, that is, a resin/solvent ink system. 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 itself is filled with the fluidity and adhesiveness of the anilox roller. 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 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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