Application and development prospect of water-base

  • Detail

The application and development prospect of water-based ink in flexible packaging printing

with the progress of the times and the continuous enhancement of environmental protection awareness, at the same time, the "green revolution" triggered by all walks of life around the world is also constantly impacting packaging printing. People realize that the inhibition of ink solvent and odor, one of the main materials of packaging printing, is one of the urgent problems to be solved at present. Taking flexographic ink printing, which is widely used at present, as an example, flexographic printing is the fastest-growing printing method so far. The reason is that flexographic printing has extensive adaptability and economy of printing. More importantly, flexographic printing uses alcohol soluble ink, UV ink and water-based ink, which has good printing adaptability and excellent environmental protection performance, and is in line with the development trend of modern printing. At present, flexo printing is the most popular in the United States. The development trend of flexo printing in Japan, Brazil and Western Europe is also very good, and its development process in China is also obvious to all. The development of flexographic ink has gone through various periods from aniline ink, solvent ink to alcohol soluble ink, water-based ink and UV ink, especially the use of water-based ink and UV ink has played an important role in the title of "green printing" on the crown of flexographic printing. This paper focuses on the composition, application and development prospect of water-based ink. 1. Composition of water-based ink the main difference between water-based ink and solvent ink is that the solvent used in water-based ink is not organic solvent but water, and its binder is composed of resin and water. The main components of water-based ink are colorant, water-based binder, solvent and various additives, as shown in Figure 1. Generally, the following proportions can be referred to: colorant: 12~40% resin: 20~28% water + alcohol: 33~50% alkali: 4~6% additives: 3~4% colorant: mixed water-based adhesive of pigment, dye or filler: resin or auxiliary resin oil binder ink solvent: ethanol or mixed solvent based on alcohol auxiliary agent: diluent, stabilizer, defoamer, dispersant, lubricant, etc. Figure 1 The basic composition of water-based ink

1.1 colorant colorant usually uses pigments with strong alkali resistance, which requires good dispersion in the aqueous system. The colorant of water-based ink is essentially the same as that of solvent ink, with a few exceptions, which can be selected according to specific circumstances. At present, in order to meet the decorative needs of packaging materials, pigments with bright colors and strong coloring power are mostly used. Sometimes in order to improve the color strength and viscosity of ink, dyes can also be used. However, no matter what kind of pigment is used, it is best to use it after being treated with anionic and non-ionic surfactants. The reason is that the surface tension and polarity of water in water-based ink are relatively large, which makes it difficult to disperse the colorant. If untreated, the uneven distribution of the colorant will be detrimental to printing. 1.2 binder water-based ink binder is an important factor affecting the quality of water-based ink. Compared with ordinary inks, water-soluble resins are often used in water-based ink binders. These polymer resins contain hydrophilic groups, such as -nh2, -oh, -cooh, etc. There are many kinds of them, which can be selected according to different occasions and uses. At present, the research and development of ink by domestic ink companies mostly focus on the research of connecting materials, and the following types are usually available for reference: (1) water soluble connecting materials: this kind of connecting materials include polyvinyl alcohol, hydroxyethyl cellulose and polyvinyl pyrrolidone. This kind of connecting material can be permanently dissolved by water, so the use range of inks prepared with it will be limited to a certain extent, and they can only be used in occasions that do not contact water. (2) Alkali soluble binder: this kind of water-based ink binder can be dissolved by water during printing, and become insoluble after printing drying. This kind of connecting material is usually an alkaline solution of acid resin, in which an appropriate amount of ammonium hydroxide is added, and the two form a soluble resin salt after chemical action. In the process of ink drying, ammonia volatilizes and turns the ink into a substance insoluble in water. The performance of this kind of ink mainly depends on the type of acid resin used. Traditionally, we usually use rosin modified maleic acid resin as the connecting material. Because of its wide range of raw materials, short supply cycle and reasonable price, it has been widely used in a certain period of time. However, the ink made of this kind of connecting material has poor wear resistance, water resistance and viscosity stability, so it forces printers to turn to the development of new connecting materials. At present, acrylic resin is widely used as connecting material at home and abroad. It has significant advantages in gloss, weather resistance, water resistance, chemical resistance and pollution resistance, so that it has excellent performance whether in direct dispersion, dissolution or synthesis of polymer lotion. There are three forms of acrylic resin: alkali soluble, non-ionic polymer and acid soluble polymer. In water-based inks, the alkali soluble type is mainly used (because the non-ionic type is hydrophilic and has poor water resistance after film formation; the acid soluble type has no suitable volatile acid and has smell). In the preparation of water-based inks, ammonia or amines or their mixed forms are usually used to make the resin become water-soluble ammonium salts. After printing on the substrate, with the volatilization of ammonia, the ink film will have a certain gloss, water resistance and friction resistance after drying. Acrylic resin has two forms in the binder of water-based ink: one is that acrylic acid-base soluble resin forms resin solution through ammoniation, commonly known as resin oil. It is the main body of binder, used to disperse pigments and prepare water-based inks. The properties and solid content of the resin directly affect the viscosity, pigment dispersion and ink transfer performance of the water-based ink; The other is lotion, which is formed by polymerization of acrylic acid, methacrylic acid or other monomers (such as styrene) through lotion. It also has a certain degree of polymerization and acid value, and these properties directly depend on the control of conditions in lotion polymerization. Because the conditions of lotion polymerization are harsh and difficult to control, the performance and stability of lotion become more important. The application of lotion in ink has a significant impact on improving the gloss, water resistance and dryness of ink. Especially for high-grade water-based ink, lotion is essential. Lotion can be divided into film-forming lotion or non film-forming lotion according to different properties. According to different substrates, the proportion of these two lotion in water-based ink is also different. (3) Diffusion binder: this binder is a fine resin particle suspended in water, which is usually called latex. This kind of binder usually contains propylene, ethylene or styrene butadiene polymer. Latex has more significant advantages than the resin dissolved by measuring the stress change on the sample: first, latex can have high density and low viscosity, which can produce high-quality thin coatings; Second, latex can also contain quite large polymer molecules, which are generally harder than small molecules, wear-resistant, heat-resistant, and good adhesion. Although macromolecules usually cause the viscosity of the solution to be too high, latex does not have this problem. This is the unique performance of latex ink. The biggest problem of latex ink is that it is difficult to print and clean, because once the latex condenses, it will become insoluble, so latex is usually used as coating ink. In addition, latex can also be mixed with alkali soluble binder, which can not only retain some unique properties of latex, but also have the printability of alkali soluble binder. It should be pointed out that sometimes, in order to achieve a certain purpose, several kinds of connecting materials can be mixed. At this time, we should take into account the impact on other properties of water-based ink. 1.3 add a small amount of alcohol, ethanol, isopropanol or n-propanol to the pure water of solvent. The addition of alcohol helps to improve the stability of the ink, accelerate the drying speed, reduce the surface tension, and isopropyl alcohol also plays a role in defoaming. 1.4 auxiliary agents auxiliary agents mainly include diluents, stabilizers, defoaming agents up to 0.001 ⑴ 000mm/min, diluents, quick drying agents, slow drying agents, antifoam agents and other additives. The stabilizer can reduce the viscosity of water-based ink and adjust the pH value of water-based ink, so that the pH value of water-based ink is controlled at 8.5~9 5, improve the stability of water-based ink. Defoamer can break or inhibit the foam that has been produced in water-based ink, while antifoam can prevent or inhibit the formation of foam. Sometimes in order to enhance the scratch resistance of ink, wax additives can be added, usually polyethylene wax. In addition, there are some other additives, which can be selected according to specific needs. The reference values are shown in Table 1. In order to make the resin soluble in water, the alkaline substances in water-based inks mostly use ammonia or organic bases. Ammonia water is cheap. I believe that water-based ink can form a film with good water resistance after drying, but the smell is large, so some organic bases are usually used together with ammonia water

table 1 Dosage of various auxiliary agents for water-based inks and their effects dosage of auxiliary agents effect defoamer 1%~2% eliminate bubbles quick drying agent 0~3% speed up the drying of water-based inks slow drying agent 0~10% delay the drying of water-based inks thickener about 1% increase the viscosity of water-based inks diluent 3%~4% reduce the viscosity of water-based inks wax about 1% increase the abrasion resistance of ink film diluent appropriate amount reduce the color depth stabilizer appropriate amount maintain the pH value of water-based inks 8.5~9.5

in China, Due to the late start of flexographic printing, people are used to the control of solvent based inks, and the control of water-based inks either uses inertial thinking or does not know where to start. As a result, a lot of human, material and financial resources are often wasted and heavy losses are incurred. Therefore, it is particularly important to correctly understand the technical control indicators of water-based ink. Viscosity control: viscosity is a measure of preventing fluid flow. It is the ability of the interaction between fluid molecules to hinder the relative movement of its molecules, that is, the resistance of fluid flow. Viscosity directly affects the transfer of ink and the quality of flexo prints. The viscosity is not only related to the viscosity and density of the resin in the binder, but also related to the type and particle size of the pigment. Low viscosity and fast ink transfer will cause light color, dot deformation, uneven ink transfer and other disadvantages; The viscosity is high, the ink transfer is slow, the ink color is uneven, and the "color" is sometimes not deep, which is easy to cause dirty versions and pasted versions. Low viscosity can be adjusted by mixing new ink; High viscosity can be diluted with water or water and ethanol, or adjusted with ink stabilizer. In addition, the temperature also has a great impact on the viscosity of the ink. Usually, the viscosity decreases when the temperature increases, and vice versa. Therefore, in order to maintain the consistency of the density of the printed matter during the printing process, the temperature of the printing workshop should be kept constant. PH value control: the connecting material in ink is mainly alkali soluble resin, and the pH value is controlled in the range of 8.5~9.5. The printing performance of ink is the best, and the printing quality is the most stable. As ammonia volatilizes constantly during the printing process, operators will add new ink and various additives to the ink from time to time, so the pH value is changing at any time. When the pH value is higher than 9.5, the alkalinity is too strong, the viscosity of the ink will decrease, the drying speed will slow down, and the water resistance will become poor; When the pH value is lower than 8.5, the alkalinity is too weak, the viscosity of ink will increase, the drying speed will become faster, and it is easy to block the printing plate and the roller, causing dirt on the plate and bubbles. It is suggested that the pH value and viscosity should be controlled together in actual control, especially in chromatic printing, which should be paid more attention to. Usually, in addition to the viscosity cup, a simple pH indicator should be prepared for detection at any time. When the pH value is low, a pH stabilizer or a small amount of alkaline substances can be added. When the pH value is high, a solvent or diluent can be added for dilution. Thickness control: the so-called oil

Copyright © 2011 JIN SHI