Formulation design of the most popular UV curable

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Formulation design of UV curable flexographic ink

flexographic printing is a branch of relief printing. Flexographic printing is similar to relief printing in printing principle, but different from traditional relief printing in the composition of printing plate, ink and stamping process. Figure 1 briefly introduces the principle of flexographic printing

image transmission in the printing process is completed by a printing plate obtained by photochemical reaction or a rubber stick engraved with patterns

the ink is in an ink storage tank, and the technical parameters of the sling tension tester are introduced as follows through the ink transfer roller: (anilox) a certain volume of ink is transported to the printing plate. Anilox is composed of chromium alloy, and its surface is coated with ceramic wear-resistant layer or ceramic wear-resistant layer by ion spraying. The amount of ink transferred is determined by the number of lines on the surface of the ink transfer roller. The surface of thick anilox has about 120 lines per inch, while the ink roller used for printing fine patterns can reach lines per inch

technical requirements of flexographic printing inks

high quality inks have many important parameters, and the most important characteristic is good pigment wettability. For systems with high viscosity such as offset printing inks, the flocculation of pigments will not pose a problem. But for flexo printing ink, because the viscosity is very low, the pigment is easy to flocculate or even settle, which means that the resin suitable for offset printing ink is not necessarily suitable for flexo printing ink

of course, pigment wetting is not the only factor to be considered. It is also very difficult to achieve good rheology, reactivity, adhesion and bacterial resistance at the same time

◆ relatively low viscosity: the viscosity of standard UV flexo ink at a shear rate of 2500s-1 is 0 5Pa. s. The printing speed is determined by the construction conditions (in the open air or in a closed room), and also determines the viscosity of the final ink. If the viscosity is too high, the ink will accumulate between the ink roller and the doctor blade, so the printed pattern will have some discontinuous ink accumulation stains. If the viscosity of the ink is very low, the problem will be smaller. However, due to the small amount of ink in flexographic printing itself, the transmission of ink will be affected

the requirement of ink reactivity will also affect the viscosity of ink, because most high-level functional groups have high product viscosity. In order to improve the reactive activity of the ink without affecting the viscosity, the amount of photoinitiator must be increased, but this will also affect the odor and the cost of the ink. Therefore, the formulation designer must find the best combination point of ink viscosity and other characteristics

uv flexo printing ink has higher viscosity than solvent type and water-based flexo printing ink, so compared with solvent type and water-based flexo printing ink, using UV flexo printing ink can achieve the printing effect with screw drive and rack drive, which is closer to offset printing and gravure printing. Sometimes the viscosity of UV flexo printing ink can reach 2.5pa s。

◆ pigment wettability and color strength: flexo printing inks require a very high pigment content, often 15% to 20%, which makes it possible that the thickest ink film during printing is not more than 1 micron. This is very important for the printing materials and the printing price per square meter. When the pore density on the surface of the ink transfer roller is very low, a very high pigment content will cause many small and discontinuous printing spots

the viscosity is closely related to the wetting of pigments; If the pigment is not wetted well, it will not reflect the characteristics of Newtonian fluid, and the ink will also have corresponding flow difficulties. In addition, because the pigment is not dispersed well, the color intensity will also weaken

◆ rheology: ink with good fluidity will have good gloss. Newtonian fluid has the characteristic that the viscosity does not change with the shear rate, which not only determines whether the ink transmission is good, but also is equally important for the transmission of ink on the ink roller. When the ink passes through the roller, the bubbles extruded will enter the remaining ink. In this way, too many bubbles will offset the Newtonian rheology of the ink and lead to printing failure

◆ reactivity: at present, the commonly used printing speed is 350m/min, so the requirements for ink curing speed are increased

◆ adhesion on various substrates: since UV flexo printing ink is mainly used to print various very soft labels and packages, the ink must have good adhesion on various soft substrates such as PP, PE, pet, PVC, and PA

◆ other requirements: the ink should have scratch resistance, chemical resistance and bacterial resistance. Ink manufacturers often face a difficult problem when developing new ink varieties, that is, how to maintain good adhesion after the film is eroded by bacteria. In addition, the requirements of regulations should also be considered, especially in the application of food packaging

selected substances and their characteristics

the usual manufacturing method of UV flexo printing ink is two-step process: grind the resin and a certain amount of pigment on a three roller machine. During the grinding process, some monomers are often added, and sometimes some dispersant additives are added to make the color paste. Next, dilute the color paste with a low viscosity mixture containing monomer, initiator and leveling agent

polyester resin or modified epoxy resin is often used as the grinding base material in the grinding process (see Table 1). At the same time, some particularly effective monomers (see Table 1) used in the grinding or dilution stage are given to ensure smooth delivery

experimental results

▲ influence of grinding resin: the ink grinding equipment has a great influence on the dispersion effect of pigments. The monomer and pigment can be mixed and ground by ball mill, or the mixture of resin and pigment can be ground by three roller mill

the grinding effect depends largely on the type and fineness grade of pigments. There is no chemical change during the grinding of pigment

magenta flexographic ink

it is sometimes difficult to judge the quality of ink simply by the relationship between viscosity and shear rate. However, through the thinning index (SI), it is easy to find which ink has the best printability

for red pigment 57:1, the best rheological behavior can be obtained by mixing and grinding with polyester acrylic resin such as ebecryltm 450, ebecryltm 657 or ebecryltm 870. Due to the high polarity of epoxy resin, the viscosity of ink at low shear rate will be too high

compared with polyester acrylate, epoxy acrylate based inks have a lower gloss on paper. But this is not to say that epoxy acrylate cannot be used in this kind of ink, and epoxy acrylate can improve the curing speed of ink, and the price is low and will not increase the cost. In this case, the wetting of the pigment can be ensured by adding other components

black flexographic ink

for black flexographic ink, in order to improve the reactivity of the ink, epoxy resin can be appropriately used in the formula to replace polyester resin. Using oxidized carbon black pigment to disperse in epoxy resin will have different effects. The epoxy acrylic resin often used is based on bisphenol A epoxy resin. Ebecryltm 3702 is a fatty acid modified epoxy resin. It is a kind of binder with good wettability of pigments

ebecryltm 600 is not good in terms of pigment wettability, which will cause poor ink fluidity, so that it cannot be pumped. In fact, the fluidity of the ink is not good. After being diluted by dpgda, the optical density and coloring power decrease. These phenomena all explain the flocculation of the pigment. Ebecryl provides good rheological properties for TM 3701. Although the ink using ebecryltm 3701 has good optical density and transparency, it has low gloss on various substrates

▲ influence of monomer and dispersant: if the viscosity reducing component in the system is not selected well, no matter how appropriate the pigment content is, it will not achieve good results. Viscosity reducing components include monomers used to adjust viscosity. Of course, monomers also play a role in coordinating other aspects of performance. In addition, photoinitiators and additives are also essential

by diluting the red pigment 4BY with the same pigment content through low viscosity acrylate

ebecryltm 40 and ebecryltm 160, it is found that all inks have thixotropy without dispersant, but the inks based on ebecryltm 160 have better printability, optical density, gloss and transparency

when the dispersing agent is added in the grinding stage of the pigment, the Newtonian fluid characteristics of the ink become obvious and the optical density becomes higher. The experiment shows that the effect of dispersing agent combined with ebecryltm 160 is much better than that with ebecryltm 40. The same is true in alkoxylated neopentyl glycol derivatives. Although additives can not solve the existing problems to the greatest extent as we hope, sometimes the opposite results will occur. However, the gloss of ebecryltm 160 ink on paper is relatively better

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