Formulation design of the hottest water-based flex

2022-08-16
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Formulation design of water-based flexographic ink

I. concept of water-based flexographic ink

water based ink is composed of water-based polymer resin and lotion, organic pigments, solvents (mainly water) and related additives through physical and chemical processes. Water based inks are non-volatile organic solvents, non flammable, will not damage the health of ink manufacturers and printing operators, and will not pollute the atmospheric environment. As a new type of printing ink, water-based ink has no residue of some toxic and harmful substances in solvent based ink in printed matter and pollution to packaged goods, and improves the overall environmental quality

water based flexographic inks are named according to the classification method of inks according to the type of printing plate and combined with the drying mechanism of inks

II. Technical basis of water-based flexographic ink formulation

1 Neutralization and salt formation mechanism of water-based binders

water based inks use water-based binders. Water based binders are basically divided into three categories: colloidal dispersion, lotion polymer and water diluted polymer. At present, the ink industry mainly e uses diluted connecting materials. Many polymers (water-soluble) are not soluble in water or only partially soluble in water. Only by adding an acid or base 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, dispersion, etc. Therefore, the molecular chain of water-soluble polymers contains a certain number of strong hydrophilic groups, such as carboxyl, hydroxyl, amino, etc., but most of these polar groups can only form emulsion when mixed with water. The carboxylate can be dissolved in water, so the manufacture of water-soluble binder often uses synthetic resin in high acid state, and then neutralizes it with amine to form salt

2. Evaporation, penetration, curing reaction or the drying film-forming mechanism of the three

generally speaking, the printing process and the corresponding drying method determine the ink formulation system used. In order to meet the technological requirements of flexographic printing and the characteristics of water-based inks, according to different printing substrates, the drying methods of water-based flexographic inks include volatilization, infiltration, curing reaction or the emergence of aircraft carriers with both of them, which once again provides China with its military status

(1) volatile drying mechanism

for flexographic printing with non absorbent substrate, the drying method is mainly volatile drying, that is, using resin/solvent ink system. Because:

① flexographic printing machines are fast, ranging from 80m/min to more than 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, in all types of drying, only volatile drying can meet this requirement

② only when the liquid in the ink film is removed from the ink film as soon as possible, for example, the solvent with low boiling point has the characteristics of instant volatilization, can the ink be dried quickly. Quick drying also requires the ink to be a low viscosity and thin liquid

③ for offset printing ink and lead printing ink, the current situation of the automotive industry is very good. Through the coordination between ink rollers, the ink is evenly coated on the printing plate, while flexo printing ink only depends on its own fluidity and adhesion to fill the eyes of the roller and transfer the ink to the printing plate. Only low viscosity, that is, thinner liquid, can give such properties. And the ink must be filled in the concave hole in a very short time. If the viscosity is too large, it is difficult to fill the concave hole. At the same time, it is difficult for the scraper to smoothly scrape the ink from the wall. Therefore, flexographic ink is generally in the form of fluid with low viscosity, and a large amount of solvent or water can only be carried out by volatilization or penetration. Of course, too low viscosity is not enough, otherwise imprinting will cause the ink in the dot to deform easily, making the reproducibility of graphics and texts worse

the drying speed of the ink depends first on the evaporation speed of the solvent in the ink. The main influencing factors of solvent volatilization in ink are as follows:

① different resins slow down the speed of solvent volatilization differently. The greater the solubility of the resin, the more difficult it is to remove the solvent and the lower the volatilization rate

② the larger the proportion of pigment in ink, the lower the volatilization rate of solvent; The smaller the radius of pigment particles, the larger the specific surface area, and the lower the evaporation rate of solvent; Different kinds of pigments have different extractability to solvents

(2) osmotic absorption drying

for flexo printing of absorbent substrates, the drying method is mainly osmotic absorption drying. Absorbent substrates such as paper e ink are absorbent. The binder in the ink is a liquid film-forming material, which can bind the pigment particles and make them adhere to the paper surface at the same time. After the binder is transferred to the paper, it penetrates into the paper until the pigment particles in the ink become close enough, and the gap between the particles continues to shrink. The absorption and penetration process is completed when the capillary tension between the pigment particles is equal to the absorption and bonding force in the paper. The penetration of the connecting material into the paper is directly proportional to the capillary tension in the paper, that is, the ink absorption of the paper depends on the internal looseness, that is, the tightness and tissue uniformity of the paper. In actual printing, the influence of printing pressure, embossing time and ink viscosity should also be considered. Olsson formula basically summarizes the relationship between the above factors and the depth of ink pressed into the paper:

d = (P × r2 × t/2 η) 1/2

d: depth of ink pressed into paper

p: printing pressure

r: capillary radius of paper

η: Ink viscosity

because the printing pressure is much greater than capillary gravity, the latter is ignored

the penetration and absorption of ink to absorbent substrate is very important for the curing and drying process of ink. The penetration is too small or too shallow, the ink is not firmly adhered, and it is not easy to dry. However, if the penetration is too large or too deep, it will cause penetration problems and reduce the gloss of the ink

in addition, for absorbent substrates, there is also a evaporation drying mechanism. Therefore, for the flexo printing of absorbent substrate, the drying method is the dual film-forming mechanism of volatilization and penetration

in the preparation of some ink with special requirements, some reactive groups are also introduced to further improve its film-forming performance$ Page break $

III. many factors affecting the formulation design of water-based flexographic ink

1 Requirements for printability

the process of ink from ink tank to printing plate to substrate is the process of ink transmission, transfer and separation. First of all, it is required that the ink can always be stably transmitted to the printing plate during the printing process. Secondly, it is hoped that the ink on the plate will always be effectively transferred to the substrate in a certain state. The quality of transmission and transfer is related to the performance and accuracy of the printing machine, and at the same time, the ink is required to have the corresponding printability. The rheology of the ink itself has become an important factor controlling the suitability of the ink. Rheology includes viscosity, yield value, thixotropy, fluidity, adhesion, ink length, etc

viscosity the cohesion of ink must meet the requirements of the whole printing process

(1) the higher the printing speed, the smaller the viscosity of the ink is required. Fast printing requires fast transfer and fast drying, that is, the ink viscosity is small, easy to separate, and the solvent is easy to escape from the surface of the ink film. Practice has proved that the plastic viscosity of the ink has a significant impact on the transfer rate of the ink. Under the same conditions of the same printing machine speed, the split state of the ink layer with low viscosity is more beneficial to the transfer of the ink than that with 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 high, so the viscosity of the ink is required to be slightly higher; For offset paper and other paper with soft structure, the ink viscosity should be slightly lower

thixotropy thixotropy is "shear thinning". The ink should have certain thixotropy. After the ink is mechanically rotated on the ink roller during the printing process, the fluidity of the ink is E-shaped, the viscosity is reduced, the fluidity is enhanced, and its ductility is also increased, which is conducive to the smooth and uniform transfer of the ink. After the ink is transferred to the substrate, it loses the effect of external force. Because of its thixotropy, it quickly thickens and does not flow around, ensuring the accuracy of imprinting. In particular, dot and text line printing can avoid dot enlargement and line thickening caused by ink infiltration and spreading on the paper. The thixotropy of inks is related to the following factors:

(1) properties of pigments. Inks made of pigments with strong surface adsorption have large thixotropy

(2) shape of pigment particles when pigment particles are needle shaped, the thixotropy of ink made of spherical particles is greater

(3) the amount of pigment generally, the larger the proportion of pigment in the ink, the stronger the particle interaction, and the greater the thixotropy of the ink

(4) the wetting ability of pigment particles and binder is low, and the thixotropy of ink is large

(5) molecular weight of resin the greater the molecular weight of the resin in the binder, the greater the thixotropy

requirements of printing adaptability on thixotropy: printing operation requires ink to have appropriate thixotropy, but if the thixothermal aging denaturation of ink is too large, ink supply in the ink bucket will not be smooth, and even ink supply interruption will occur, affecting the uniformity and accuracy of ink supply during continuous printing. Different types of printing have different thixotropy of ink. Generally, the thixotropy of ink is slightly larger for the printing of line version, text version and line version; The large-area field version is slightly smaller

adhesion in the printing process, the force that causes the ink film to split and transfer to the corresponding object surface (ink roller surface, plate surface or substrate surface) is adhesion, that is, the connecting force between the ink and the ink roller, plate and substrate surface. Under the action of adhesion, ink first splits and then transfers, which is the dynamic response of ink to adhesion. The ability of ink itself to prevent the separation of ink film in this dynamic process is called the adhesion of ink, which is essentially a manifestation of the cohesion of ink (the connecting force between ink molecules) under the action of adhesion. The influence of ink adhesion on printability. 3 let the technology introduce the machine performance to you on the spot:

(1) when the ink adhesion is large, it is difficult to separate the ink, resulting in uneven extension of the ink on the printing machine. When the ink layer is separated between paper or blanket, if this resistance greatly exceeds the binding force of the paper, there will be galling or even peeling

(2) in multi-color printing, when the ink of the front color is not dry, the second color is printed quickly. Generally, the adhesion of the first color ink is required to be larger, and the adhesion of each color of the rear ink should be gradually reduced. Otherwise, it is possible that the rear ink will not be printed, and the front ink layer will be stuck away

ink adhesion and drying after the ink is attached to the substrate, it changes from a liquid colloid to a solid film, which is bonded to the substrate. This change process is called ink drying. This process is completed in two stages: the ink changes from liquid to semi-solid, which can no longer flow and transfer. It is the initial drying stage of the ink, which is expressed by initial dryness. The main part of the binder in semi-solid ink undergoes physical or chemical reaction and completely dries and solidifies into a film. It is the complete drying stage of the ink, which is expressed by thorough dryness. The initial drying stage and thorough drying stage of ink are collectively referred to as the fixed drying of ink. The drying speed of ink is related to the drying method of ink. The drying method of ink depends on the composition of ink binder. Different printing methods, substrates and printing machines have different requirements for ink drying

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