The hottest flexographic ink and its application

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Introduction to flexographic ink and its application

uv ink is also known as UV ink. This kind of ink can change the ink from liquid to solid through instantaneous photochemical reaction under the irradiation of a certain wavelength of UV. In recent ten years, UV ink has made great progress and has been widely used in the field of packaging and printing. This paper briefly introduces the UV flexographic ink used in flexographic printing

I. composition of UV flexo printing inks

UV flexo printing inks are mainly composed of pigments, oligomers, monomers, photoinitiators and other additives

1. Pigment. Pigment is a colorant in UV ink. It not only determines the hue of UV flexo ink, but also has a significant impact on the performance of UV ink, such as fluidity, dispersion, stability and drying performance

2, oligomer. Oligomer, also known as prepolymer, plays the role of binder in UV ink and is also the main film-forming material. The properties of oligomer play an important role in the performance of UV flexo printing ink, such as the viscosity of UV ink, curing speed and the properties of cured film. Therefore, it is required to have faster curing speed, good wear resistance and chemical corrosion resistance. Structurally, oligomers are oligomers containing c=c unsaturated double bonds. At present, the oligomers commonly used in UV inks are generally acrylic resins, such as epoxy acrylic resin, polyester acrylic resin, polyurethane acrylic resin, etc

3. Monomer. Monomer is a compound with large molecular weight, and its molecular structure also contains c=c unsaturated double bond. It has two functions: one is to adjust the viscosity of UV flexo ink as an active diluent; The second is to participate in the cross-linking reaction of oligomers. At present, the monomers used in UV flexo printing inks are generally acrylate monomers, such as trimethylolpropane triacrylate (TMPTA), trimethylene glycol dipropionate (TPGDA) and propoxylated neopentyl glycol diacrylate (po-npgda)

4. Photoinitiator. Photoinitiator is a compound that can produce free radicals or cations under UV irradiation, which determines the speed of photochemical reaction. After absorbing the radiation energy of ultraviolet light, the photoinitiator can produce free radicals or cations, and then trigger the cross-linking reaction between the prepolymer and the monomer, so as to solidify the UV flexo ink into a film from the surface. Photoinitiators commonly used in UV flexo printing inks are some ketones, ethers and sulfides

5. Additives. Additives mainly include stabilizer, sensitizer, leveling agent and wax, which can improve the stability of ink storage and improve the printability and curing speed of UV ink

second, UV flexo printing inks can be basically divided into two categories: free radical and cationic. At present, the most widely used is free radical UV flexo printing ink, which has a very short curing time and can achieve instant rapid curing. Cationic UV flexo printing ink has a small odor and good adhesion to substrate materials (including plastic and metal materials), so it is often used in food packaging printing and printing of substrate materials with poor adhesion

The drying mechanism of UV flexo printing ink can be divided into two stages: first, under the irradiation of ultraviolet light, the photoinitiator in UV ink is excited to produce free radicals or cations, which transfer energy to the prepolymer and monomer containing unsaturated bonds through collision, and initiate a series of cross-linking polymerization reactions between the monomer and prepolymer, so that the linear resin becomes like structure, Thus, UV flexo printing ink changes from liquid to solid and forms a solid ink film

the polymerization reaction is very fast and can be completed in a very short time, that is to say, it can realize instant drying and curing

III. The difference between UV flexo printing ink and ordinary flexo printing ink

1. The composition of ink is different. The film-forming material in UV flexo printing ink is photosensitive resin, while the traditional solvent based or water-based flexo printing ink is ordinary resin; In solvent based or water-based inks, there is a certain amount of organic solvent or water to dissolve and dilute the resin, while UV flexo inks do not contain water or any organic solvent. The monomer acts as a diluent, and the monomer also participates in the curing reaction. In addition, UV flexo printing inks contain photoinitiators, which are not found in ordinary UV flexo printing inks

2. The drying mechanism is different. Solvent based or water-based flexo printing inks are mainly dried through evaporation, absorption or oxidation of the conjunctiva process; UV flexo ink is crosslinked and dried by chemical reaction under the action of UV energy

3. Different drying properties. Due to the short ink path ink supply system adopted by flexographic printing machine, the drying performance of ink is required to be relatively high. When ordinary flexographic printing ink is used for printing, in order to ensure the smooth progress of printing, it is necessary to achieve rapid drying with the help of certain heating and drying equipment such as blow dryer or oven. UV flexo printing ink is different. It can achieve instant curing and drying without heating, which is very suitable for flexo printing

IV. characteristics of UV flexo printing ink

why is UV flexo printing ink so popular and can be used so widely? Mainly because it has the following characteristics:

1. UV flexo ink has stable properties and good printability. Because UV ink can only solidify under the irradiation of UV light, and UV oil can provide a 100lb experimental load, the ink does not contain volatile, so the viscosity of the ink remains stable during the printing process. Even if it stops halfway, the ink will not dry on the cots and rollers, which can ensure the smooth progress of the printing process, and can greatly reduce the times of cleaning the printing plate and rollers

2. The quality of UV flexo ink prints is higher. Due to the good printability of UV flexographic ink, the printed matter has good tone reproducibility, higher consistency, bright colors, gloss and sharpness. In addition, because a variety of new materials have been sought after by the market, UV ink changes from linear structure to shape structure after curing and drying, so the cured film has excellent chemical corrosion resistance and wear resistance

3. UV flexo printing ink can achieve instant drying, greatly improve production efficiency, and reduce the occurrence of many printing failures

4. UV flexo printing ink is more safe and reliable. Because there is no solvent discharge, and it is not flammable, it is safer to store and use, and can effectively improve the working environment

5. UV flexo ink is applicable to a wide range of substrates. UV flexo ink has good adhesion on different substrate materials such as paper, aluminum foil, plastic, etc

6. The amount of UV flexo printing ink is less and the cost is saved. Although the price of UV flexo printing ink is higher than that of ordinary flexo printing ink, its effective ingredients are high, and it can be converted into ink film almost 100%, and good printing effect can be obtained only with a very thin ink layer

7. UV flexo printing ink is more environmentally friendly. From the perspective of environmental protection, compared with traditional solvent-based and water-based inks, UV curing inks are more conducive to environmental protection. They not only have no volatile substances (VOC) problem, but also do not contain any air pollution substances (HAP substances), which comply with the provisions of environmental protection, but also avoid the recycling of solvents. This is also one of the main reasons for the rapid development of UV flexo printing ink

v. safety problems in the use of UV ink

in fact, UV flexo printing ink, like other chemicals, will also do some harm to human health and safety, but this harm is not as serious as ordinary chemicals, and in most cases it is often ignored by people. At present, there are three problems in the use of UV flexo printing ink:

the first problem is the harm of UV flexo printing ink to human body. Acrylate substances in UV ink belong to skin irritants, and human body will produce allergic reactions after a period of contact. In other words, the human body may not have allergic reactions at present, but if the skin and the shape and size of the falling punch will also have a great impact on the results, long-term contact with UV ink may lead to skin allergic reactions, itching, redness and even peeling. Therefore, when using UV ink, you should wear protective clothes, gloves, eye protectors and masks to minimize the chance of UV flexo ink contacting the skin. If you accidentally get UV ink on your skin, scrub it first and wash it with soap and water as soon as possible

the second problem is about the safety of UV light sources. UV ink must be dried with UV light source, but UV may cause certain damage to human skin and eyes. In fact, as long as the operator is not facing the UV lamp, there is no need to worry about a large amount of ultraviolet radiation. Moreover, if the design and installation of UV curing system are correct and reasonable, the leakage of UV light is very small. Moreover, users can also purchase special instruments to detect the leakage degree of ultraviolet light

The third problem of UV flexo printing ink is that ozone will be produced during its curing process. The interaction between short wavelength UV light source and gas oxygen (O2) will produce ozone (O3). At present, most manufacturers use pipelines to discharge ozone, so ventilation treatment should be strengthened

VI. some problems that should be paid attention to in production

1. Strictly abide by the production and operation procedures formulated in the workshop, and do necessary protective work in the process of using UV ink or other chemical substances, such as wearing gloves, eye protectors and masks. If your skin accidentally comes into contact with UV ink, wash it with soap and water

2. Because UV flexo printing ink is sensitive to light and heat, excessive light and heat will deteriorate the ink. Therefore, UV flexo printing ink should be stored at low temperature and away from light, and the storage life should not be too long, usually about one year

3. The monomer in UV ink is similar to that in photosensitive resin plate in molecular structure, which is prone to molecular diffusion, resulting in plate expansion, affecting the quality of printed matter, and even printing failure. Therefore, we should choose plates with low swelling or UV special plates, which is very important, but it has little impact on the price (except for door type)

4. The curing speed of UV flexo printing inks with different colors is also different, which is mainly caused by the different abilities of different pigments to absorb, reflect and refract ultraviolet light. For example, in four-color printing ink, yellow and magenta are easy to cure and dry, while black ink is not easy to dry. Therefore, when printing black UV ink, it is necessary to appropriately slow down the printing speed to ensure that it can be completely cured

5. The service life of UV lamp is generally 1000 hours. After the light source is aged, the UV lamp should be replaced in time to prevent incomplete curing of UV flexo ink and printing failure

VII. Summary

in a word, UV flexo printing ink has no solvent volatilization problem, which improves the production environment, improves the quality of printed matter, and is also very beneficial to environmental protection. Although there are still some safety problems, they can be well controlled and solved by formulating and strictly abiding by the corresponding operating procedures. With the development of science and technology, the performance of UV flexo printing ink has been continuously improved. It can be predicted that UV flexo printing ink will have a very broad prospect

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