First, the introduction of flexographic printing Second, the composition and type of UV flexo ink Third, UV flexographic ink performance and technical requirements IV. Advantages and Prospects of UV Flexo Printing Inks Ji'An ZhongXiang Natural Plants Co.,Ltd. , https://www.jxzxessentialoils.com
Flexo printing has become one of the most dynamic printing methods in recent years and has developed rapidly worldwide with its unique advantages. Flexo printing has the advantages of thick ink layer, consistent ink color, printing with non-toxic water-based ink, wide range of printable media, strong equipment processing capability, low production cost, high economic benefit, and low investment in production. Therefore, flexo printing has developed rapidly in such fields as flexible packaging printing, trademark printing, newspaper printing, and folding carton printing. Especially in recent years, the market share of flexographic printing in various countries has expanded a lot. According to authoritative data from the United States, the U.S. flexo printing market accounted for more than 70% of the market in the field of flexible packaging, Europe is about 40%, and Asia is only about 15%.
In the future, due to the characteristics of flexo printing, the market share of printing in various countries will further expand. Due to the appearance and continuous improvement of a series of technologies, the original factors that restrict the printing of flexographic printing are continuously disappearing. Flexographic printing quality can now reach almost the level of offset printing. The increasing automation of flexo presses, the introduction of digital workflows, the development and use of CTP technologies, etc., have all contributed to the further development of flexo printing.
The water-based ink used in flexo printing has its own characteristics, such as water-based ink does not affect human health, is not easy to burn, ink is stable, colorful, does not corrode media, simple operation, low price, ink adhesion after printing Good advantages. However, it also has its disadvantages, such as slower drying speed, poor color saturation and poor stability. It is not suitable for printing large areas on the ground, and it is prone to adverse effects such as precipitation and delamination. Since 1990, Germans have begun to study solvent-free flexographic printing inks and systems to replace water-based and solvent-based inks. They found that UV inks can be printed on almost any substrate, and the quality of the prints is higher than aqueous and solvent inks. UV inks have the advantages of small dot gain, good brightness, wear resistance, good chemical resistance, no pollution, good hiding power, and relatively low cost.
The curing of UV inks is done instantaneously under the irradiation of ultraviolet light, so the composition of the ink is also different from that of other inks. UV inks consist mainly of pigments, prepolymers, reactive diluents, photoinitiators and additives.
UV ink pigments are similar to other types of inks, and prepolymers are unique to energy-curable inks. The UV ink prepolymer is the main body of the ink binder and determines the physical and chemical properties of the ink after curing. Prepolymers require lighter colors, faster curing speeds, and better weatherability and chemical resistance. Commonly used prepolymers are acrylic resins such as epoxy acrylic resins, polyester acrylic resins, and urethane acrylic resins. The monomer, used in the ink, acts as a reactive diluent for the system on the one hand, and crosslinks with other monomers or unsaturated double bonds and linear structure polymers on the other hand to form a film-like structure of a network structure. Photoinitiators are unique to UV inks and are the key to UV inks. They determine the drying speed of inks. The photoinitiators in UV flexo printing inks are generally ketones, ethers, and sulfides. They mainly absorb ultraviolet rays from 300 nm to 450 nm, and can cause polymerization of prepolymers after being irradiated with ultraviolet rays, thereby making the ink film short. Curing in time. UV ink additives generally have stabilizers, UV light absorbers, polyethylene wax and so on.
At present, there are mainly two types of UV flexible inks, ie, radical UV flexo inks and cationic UV flexo inks. The nature of the comparison is shown in Table 1.
As can be seen from the above table, both free radical and cationic UV flexo inks have their own characteristics. However, radically-used UV flexo inks are currently mainly used. The application prospects of cationic UV flexo inks are very large, especially in the field of food packaging and printing, but they need to be further improved.
1. Requirements for UV flexo ink itself (1) Viscosity UV flexo printing ink viscosity is not determined only by the ink itself, due to the special nature of flexo printing, the viscosity of the ink is also related to anilox roller, squeegee, etc. related. In practice, our requirement for ink viscosity is that the ink can be smoothly transferred from the ink tank to the anilox roller and the printing plate, and then smoothly transferred to the substrate, so that the density and the dot of the printing product do not become obvious. The change.
If the viscosity of the ink is too large, the color will darken, the amount of ink will increase, the drying speed will be slower, and the amount of ink supplied by the anilox roller will be insufficient due to the decrease of ink supply by the ink circulating pump. If the viscosity is too low, the color will change and the dots will expand, resulting in a decrease in the quality of the printed product. Therefore, in flexo printing, the most important variable is the change in the viscosity of the ink, which is related to the color quality, drying effect, and final quality of the printed matter.
The viscosity of the ink is generally controlled between 0.2 pa.s and 1 pa.s. When it is more than 2 pa.s, a heating ink fountain should be installed to ensure that the ink adheres well to the anilox roller. In the inking device, the ink viscosity requirement is relatively low to facilitate the ink transfer. However, when the ink is transferred to the substrate, the viscosity is required to be higher to control the dot expansion.
(2) Coloring power Coloring power is the basic requirement for inks, and UV flexographic inks are not otherwise. Reasonable tinting power is achieved by the pigment ratio in the ink. Insufficient pigments reduce the coloring power; excessive pigments reduce the fluidity of the ink and reduce the adhesion. Therefore, the optimal pigment ratio means that the ink can be smoothly transferred to the substrate and meet the color requirements and the control requirements for dot gain.
(3) Dispersibility Dispersion refers to the dispersion and uniformity of the pigment in the ink. UV inks, like other inks, require that the pigments be well dispersed and uniform in the ink so that color consistency and uniformity can be guaranteed.
(4) After the odor UV ink is dried, it will have an odor similar to that emitted by organic compounds such as acrylic acid. Although it is not necessarily harmful to the human body, it is undesirable for soft packaging printing. We can use solvent recovery system, catalytic oxidation technology to solve.
(5) Curing speed In UV inks, the amount of photoinitiator and the intensity of UV light determine the curing speed. Too much or too little photoinitiator is detrimental to print quality. Therefore, according to the types of substrates and printing processes and other factors, consider the amount of photoinitiator and the intensity of ultraviolet light.
2. Requirements of UV flexographic inks for equipment
UV lamps are the key equipment for UV ink drying. When UV inks are used for flexo printing, UV lamps should be provided for each color group. The determination of the intensity of the UV lamp is determined comprehensively by factors such as the type of substrate and the color of the ink. In addition, UV lamps should also be replaced in a timely manner. The lifetime of general UV lamps is 1000 hours. Flexo-printed plates should also use UV-resistant inks with better plates, such as DuPont's UVP-67 plate. The rubber roller must also have good anti-UV properties. A rubber roller made of EPDM rubber or silicon rubber can be used.
3. UV flexo ink common fault
(1) Incomplete curing
UV inks also often suffer from drying problems such as surface stickiness and incomplete bottom drying. The main reason is due to the improper use of photoinitiators and the inappropriate intensity of UV lamps. Actually, the more photoinitiators, the better. Excessive photoinitiator will dry the ink film too much and make the ink film brittle. Too little will cause poor drying and so on. Therefore, the amount of photoinitiator should be used in accordance with the principle that the ink can be just dried, not too much or too little.
For the UV lamp, the energy radiated to the surface of the ink film should also follow a moderate principle. We can control the received energy of the ink film by controlling the power of a single lamp, the number of lamps, and the distance between the lamp tube and the ink film. . In addition, UV lamp life is generally 800 to 1000 hours, the lamp should be replaced in production. When using a mercury lamp, it takes a few minutes to warm up before it can officially start printing.
UV inks should be stored in a ventilated, dark environment, with a typical storage temperature of 16°C to 28°C. During the printing process, the light in the workshop, especially around the ink tank, should not be too strong to prevent the UV ink from thickening and solidification.
(2) The odor odor problem is a common problem of UV inks. In general, the odor produced in the printing process has the following aspects: raw materials in the ink, by-products from the drying process, inks that are not completely cross-linked and dry, coatings on paper, coatings and additives on films, UV lamps And corona treatment equipment and so on. In general, in the production of inks, small odor monomers should be used as much as possible to enhance ventilation and exhaust during work. It is also possible to optimize the composition of substrates to reduce odors. At the same time, efforts should be made to research and develop low-odor UV flexo inks. The new technology in cationic inks provides a new solution featuring high printing speed, good adhesion, excellent dot reproduction and color strength, and it has almost the same odor as ordinary inks.
(3) Dilution
Since the UV ink has a high solid content, it is necessary to dilute it in order to improve its printability. However, in the past, many companies used ethanol to dilute them. This of course can improve the fluidity of the ink and reduce the viscosity of the ink. However, excessive addition affects the light curing and causes failure. The ideal diluent should be an acrylic monomer that can participate in the reaction and form a film. It is an excellent diluent for UV inks.
Ultraviolet (UV) curing technology has been hailed as “21st Century Green Industry New Technology†because it fully complies with the 3E principles of future development, namely Energy-Saving (Energy Conservation), Ecology (Eco-Environmental Protection), and Efficient (Efficient) . With the continuous enhancement of environmental awareness and the rapid development of high and new technology, UV curing technology has also gained more and more applications in the field of printing inks. UV printing inks were invented in the 1960s, but flexographic UV inks began to be used in flexographic printing after the 1990s, and were mainly used for the printing of fine products. Compared with traditional inks, UV inks have the characteristics of energy saving, low pollution, cost-effectiveness, and good printability.
Flexographic UV inks also have the property of protecting the product. In the field of flexible packaging printing, the use of heat-curing causes heat-sensitive substrates to be thermally deformed or even damage the substrate. This is bad for printing. The UV ink can be cured at a low temperature, avoiding the damage of various heat-sensitive substrates caused by the high temperature during the heat curing. Therefore, the UV ink is most suitable for the printing of various heat-sensitive material substrates. However, since the light emitted from the UV lamp contains about 60% of the infrared light, it is necessary to use a cooling device to remove the heat generated by the infrared rays in the printing of the substrate of the heat-sensitive material.
The UV ink has good printability, the printing process does not change the physical properties, the dot gain is small, and the printing quality is excellent. UV inks are significantly superior to traditional inks in terms of glossiness, sharpness, color saturation, etc. UV inks are superior to conventional inks in terms of water resistance, alcohol resistance, abrasion resistance, and chemical resistance.
In addition to the above characteristics, UV ink has its own advantages, such as smaller viscosity than other inks. Generally, the viscosity of the flexographic UV ink ranges from 250 cP to 2000 cP/25°C (1 pa·s=1000 cp); the ink layer of the UV ink The thickness is generally 2 ~ 5μm; the curing speed is generally 60m/min ~ 150m/min, the fastest can exceed 500m/min.
The huge flexo market will promote the development and application of new UV inks. As the technology matures, the development of UV water-based ink technology will gradually mature. According to reports, Sericol has developed a new type of narrow-web UV flexo printing ink Uvivid 800 series, the entire ink series are lower viscosity water-based ink, featuring high color density, with unparalleled printing effect. Uvivid ink does not foam or cavitation in the ink fountain and ensures optimal ink delivery. In addition to its low viscosity characteristics, it also has perfect imaging resolution and excellent dot coverage.
In short, with people's growing awareness of environmental protection, people's requirements for environmentally friendly ink is also increasing. In view of the above, UV inks will be an ideal environmentally friendly ink, and its development space is very broad. With the development of UV inks, the application of UV inks in flexo printing has become a reality. With the development of flexo and UV technology