Elements of Formula Design for Water-based Flexographic Printing Inks (2)

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.

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 ink adhesion capacity should be appropriately reduced, or the fixation and drying of the ink should be accelerated in order to avoid the “backside smearing” 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 be smooth and dry and lose its tackiness. The second color ink will be difficult to adhere and the ink will have a "crystallizing phenomenon". The ideal overprint should be done in the ink dry to dry time.

If the drying is too fast, the ink will be crusted on the surface of the plate, and the plate will be pasted, causing the printing ink to accumulate, resulting in poor gloss and "ink spots". If the drying is too slow, the back of the printed product may suffer from "scratching" phenomenon.

2. Requirements for the use of packaging materials

The area where the original ink per unit weight of hiding power can be printed is generally 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 ​​the ink with a large specific gravity of ink is less than the ink with a small specific gravity.

(2) The transfer amount of the ink is affected by the hardness of the ink fountain roller and plate material, the ink carrying capacity and wear of the ink transfer hole of the anilox roller, the installation position of the doctor blade, and the like.

(3) The content of the colorant in the ink determines the hiding power of the ink.

(4) The hiding power of ink on uncoated paper or other absorbent printing materials is worse than that on non-absorbing printing materials because the penetration of ink into the absorbent material reduces the lightness of the colors. When printing paper, the water-based ink seeps into the paper faster than the solvent ink, and the hiding power is higher than solvent inks of the same color strength.

Chemical resistance, scratch resistance, heat-resistant sealability, grease resistance, freezing resistance, durability, and the like.

The above discusses many factors that affect the performance of water-based flexographic inks. On this basis, the selection and preparation of its basic formulas, binder systems and raw materials can be further determined, and the general performance requirements and technical specifications of the varieties to be developed are scheduled.