Analysis and Research on Drying Characteristics of Several Flexographic Printing Inks
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In recent years, the field of flexographic printing has developed rapidly, and its development prospects are promising. It is one of the main printing methods in the field of packaging and printing in the future. The ink is a coloring material used in the printing process. Different types of inks have different drying mechanisms and environmental effects due to their different compositions and properties. This paper mainly discusses solvent-based inks used in flexographic printing. Different drying methods of water-based inks and UV inks and their advantages and disadvantages.
Ink is a mixture of colorants, binders, fillers and auxiliaries. The colorant provides color to the ink; the binder is a liquid component in the ink, and is a carrier of a solid powder such as a coloring material and a filler, which uniformly disperses the coloring material and the filler therein, so that the ink has appropriate rheological properties and drying properties. . At the same time, it is a film-forming substance. The pigment adheres to the surface of the substrate firmly by the dry film-forming property of the binder. The filler can adjust the consistency, viscosity, viscosity, fluidity and yield value of the ink in the ink. It is ink-repellent and has the effect of adjusting the ink concentration; the auxiliary agent is an auxiliary material added to improve certain properties of the ink, such as a plasticizer, a lubricant, an antifoaming agent, and the like. Among the several components of the ink, the binder determines the drying properties of the ink and the quality of the film. The following is mainly explained from the viewpoint of the binder.
1. Solvent type ink
The binder of the solvent type ink is mainly composed of a solvent plus a resin. The resin is mainly polyurethane, epoxy resin, acrylic resin, polyester, phenolic resin, EVA resin, PP resin, chlorinated PP resin, and the like. The solvent includes organic solvents such as alcohols, esters, benzenes, ketones, and ethers. The organic solvent of the solvent-based ink accounts for 30% to 50% of the total amount of the ink, and accounts for about 80% of the spray ink. The drying mechanism mainly relies on the solvent in the ink to volatilize in space to complete the drying process. The drying process is: after the ink is transferred to the substrate, the solvent in the binder is volatilized into the air, and the remaining binder (mainly resin) and The pigments together form a solid film that solidifies on the surface of the substrate.
Since the solvent-based ink is dried by coagulating the resin and drying it, it has the characteristics of fast fixing and rapid drying, and is suitable for high-speed printing. In addition, since the ink contains a large amount of organic solvent, the fluidity is good, the surface tension is low, and the adhesion is strong. It is suitable for the printing of substrates such as absorptive and non-absorbent. The printed surface has good leveling and gloss, bright feeling, and has excellent over-network properties, making it possible to print fine patterns and fine text.
The disadvantage of solvent-based inks is that a large amount of organic compounds (VOC) will be volatilized during the drying process. Long-term contact will cause the skin to crack and penetrate into the skin and blood. It will endanger the blood and hematopoietic function of the blood. The liver and kidney cause certain damage, and the long-term work of the worker in this environment causes great damage to the nervous system. Therefore, for products printed using solvent-based inks, the VOC content is checked. For example, in the national tobacco industry standard YC263-2008, there are mainly sixteen substances for volatile organic compounds: benzene, toluene, ethylbenzene, xylene, ethanol, isopropanol, n-butanol, acetone, methyl ethyl ketone, acetic acid Ester, isopropyl acetate, n-propyl acetate, n-butyl acetate, propylene glycol methyl ether, 4-methyl-2-pentanone, cyclohexanone, the required content is as follows:
2. Water-based ink
The water-based ink is composed of water, a resin, an amine compound and other organic solvents, and the resin is mainly a water-soluble acrylic resin. The drying mechanism and process of the water-based ink are the same as those of the solvent-based ink, except that the protagonist of the volatilization is changed from solvent to water, and the evaporation rate of the water directly affects the drying speed of the ink.
The advantage of water-based ink is that the solvent used is water or alcohol, which significantly reduces VOC emissions, can prevent air pollution, does not affect human health, is not flammable, is stable in ink, does not corrode printed substrates, prevents color relocation, Strong water resistance, etc. It not only reduces the risk of fire caused by static electricity and flammable solvents, but also reduces the residual toxicity of the printing surface. It is convenient to clean the printing equipment, so it is especially used in the food, beverage, pharmaceutical and other packaging industries, as well as direct contact with the human body. Products such as T-shirts and other cotton, elastic fabrics, insole printing industry also have a wide range of uses. It is recognized as an environmentally friendly material in the world and is the only ink currently approved by the American Food Association for all printing inks. Therefore, the use of water-based inks is increasing.
The indicator that the water-based ink needs to control during use is the pH value, and its normal pH range is 8.5 to 9.5. When the pH is higher than 9.5, the alkalinity is too strong, the viscosity of the ink is lowered, the drying speed is slow, and the water resistance is deteriorated. When the pH is lower than 8.5, the alkalinity is too weak, the viscosity is increased, and the ink is easily dried. The jammed plate and the anilox roll cause the plate to be dirty and create bubbles. In addition, the water-based ink has a lower surface gloss than the solvent-based ink, which limits its application in applications where glossiness is high. At the same time, the drying time of aqueous inks is also a problem that should be noted.
3. UV ink
The UV ink connecting material is a UV type connecting material, mainly composed of a photo-solid resin + a crosslinking agent + a photosensitizer, and the drying mechanism is to decompose the photosensitizer to form a radical by ultraviolet irradiation, and these radicals make the photo-solid resin and cross-linking The agent crosslinks and cures on the surface of the substrate. The drying process is as follows: after the ink is transferred to the substrate, the photosensitizer is excited by ultraviolet rays to form a radical, and the free radical causes the photocurable resin and the crosslinking agent to be cross-linked and copolymerized, thereby completing the drying process.
The advantage of UV ink is that there is no solvent volatilization during the printing process, 100% of the solid matter remains on the substrate, and a very thin ink layer can achieve good printing effect. Under the irradiation of UV light, the ink can be instantly dried, and can be immediately Stack up and carry out subsequent processing. In the printing process, it does not emit organic matter into the air, does not affect human health, is not flammable, and is suitable for packaging printed matter with high hygienic conditions such as food, beverage, and medicine.
Since the drying device of the UV ink generates a high temperature during operation, if the heat is not removed in time, the life of the UV lamp will be shortened, and at the same time, the substrate will be heated, if it is printed on a thermal plastic. Too much heat can cause overprinting problems, so a cooling system must be provided to reduce the output energy of the dryer. At the same time, the ventilation of the workshop should be done, because the UV ink reacts with oxygen to disperse ozone when it is solidified, which causes the UV ink to have an odor. Generally, a ventilation duct with better sealing performance can be used, both in the unit and above the delivery table. To install.
In summary, the three inks mainly used in flexographic printing have advantages and disadvantages in the use process due to the composition of the binder and the drying method. In the actual printing process, the types of inks should be selected according to the characteristics of the printed products.