Analysis on the Influencing Factors and Development Trend of UV Curing Ink
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Printing inks have been expected to develop low-contamination, high-speed, and efficient curing methods. UV curing technology has many significant advantages over traditional methods. Such as fast curing speed, no volatile organic solvents, saving space, good printing quality, etc. Especially no pollution, it is a green technology. Therefore, it is the development direction of printing ink. This technology is also widely used in coatings, adhesives and the like.
First, the composition of UV curing ink
The UV curable ink consists of a photoactive oligomer (Oligomer), a photoinitiator (PI), a reactive monomer (reactive diluent), a colorant (pigment), and other necessary auxiliaries. The use of light energy to initiate cross-linking produces a physical chemical reaction to achieve the purpose of drying. Compared with traditional inks, UV light-solid inks have great differences in their use.
Pigment: Gives the ink the proper color and color strength.
Oligomer: not only ensures good wettability of the pigment in the initial state, but also gives it good fluidity, printing properties and film forming properties.
Reactive Diluent: Adjusts the viscosity, adhesion and cure speed of the ink.
Photoinitiator: Select a photoinitiator with an effective curing reaction in combination with the application conditions.
Additives: Many types of additives are used in ink formulations to regulate different properties of the ink, such as atomization, morphology, humidity and defoaming.
Second, the mechanism of UV curing ink
Immediately after the UV curable ink is sprayed onto the substrate, monomer 1 and oligomer 2 are polymerized to 3 at the instant to cause the liquid ink droplets to solidify on the printed material. The UV light does not have enough energy to initiate the polymerization process, and the curing adds a photoinitiator to the ink formulation. These compounds form free radicals 4, which are necessary to initiate the polymerization process.
After UV curing ink is irradiated by ultraviolet light, it can be divided into four stages from liquid to solid:
1 an interaction between light and a photoinitiator, which may include absorption of light or interaction between a photosensitizer and a photoinitiator;
2 photoinitiator molecular chemical rearrangement to form a free radical intermediate;
3 radicals interact with the unsaturated groups in the prepolymer and the monomer to initiate chain polymerization; the polymerization continues, and the liquid component is converted into a solid polymer;
4 The free-chain chain polymerization continues to change the liquid component into a solid polymer.
Third, the important factors affecting UV curing ink
1. Photoinitiator
Photoinitiators are a key component in the initiation and completion of UV curing processes. They absorb UV energy from the surface that is concentrated on the printing surface. UV can split the photoinitiator into active substances, and then start a chemical reaction called polymerization. This polymerization converts the liquid ink into a solid film. There are two types of photoinitiators commonly used in screen printing inks, free radical photoinitiators and cationic photoinitiators.
The UV light does not have enough energy to react with the active molecules in the coating to form free radicals. Therefore, a free radical photoinitiator is added to the ink. When photoinitiator molecules are exposed to a particular wavelength of UV light, they absorb UV light and generate free radicals that cause the crosslinking process. Free radical photoinitiators are the most commonly used and are used in over 90% of UV screen printing inks.
UV inks formulated with cationic photoinitiators typically contain different monomers and oligomers than those formulated with free radical photoinitiators. But what really distinguishes these inks is the way the photoinitiator reacts. The cationic photoinitiator is typically an aromatic sulfide salt which forms an acid catalyst upon exposure to UV light. Unlike radical chemicals, the cation reaction continues after UV light is removed. The cationic photoinitiator is present in approximately 5% to 8% of the UV ink used by screen printers.
Free radical initiators have a fast curing speed. The performance is easy to adjust, but there are problems such as severe shrinkage, oxygen inhibition, and poor adhesion.
The cationic initiator has the advantages of small volume shrinkage, strong adhesion, abrasion resistance, high hardness, no oxygen inhibition, and post-cure. However, it also has the disadvantages of slow curing speed, low variety of oligomers and reactive diluents, high price, and difficult to adjust the properties of cured products, thus limiting its practical application.
However, free radical and cationic hybrid initiators can fully exploit the advantages of both. There are two types of free radical and cationic hybrid initiators: one is a hybrid system composed of an unsaturated acrylate and an epoxy resin; the other is a hybrid system composed of an unsaturated acrylate and a vinyl ether compound.
2. Reactive diluent
UV-curing coatings are commonly used as reactive diluents, which are an extremely important part of UV-curing formulations because they not only adjust viscosity, but also membrane materials, which have a significant impact on the final properties of the coating.
The reactive diluent contains at least one double bond in which the radical is copolymerized with the prepolymer. Reactive diluents are used in combination with unsaturated oligomers in most cases. Reactive diluents are monofunctional or polyfunctional acrylates used to adjust the viscosity and functionality of the coating. It can adjust the flexibility, hardness, toughness and adhesion of the film. Since low molecular weight monomers are highly volatile and highly toxic, esters of acrylic acid and polyol are used.
Reactive diluents are important, in addition to their effects on viscosity and film formers, for the following reasons:
2.1 They and the photoinitiator system are the main factors determining the curing speed of the system.
2.2 They bind high molecular weight prepolymer molecules together and rely on their functionality, which typically contributes significantly to accelerated full cure. Like the bifunctional reactive diluent, it is easy to form a crosslinked network.
2.3 They enhance or improve the performance of the cured coating. When the amount of prepolymer is the same, different diluents produce different properties and different curing speeds.
3. Pigments
The ink is a colored system, and it is necessary to add a pigment during preparation. In the UV-curable ink, since the added pigment absorbs ultraviolet rays, the absorption amount of the initiator to ultraviolet light is reduced, causing the pigment to "shield" the initiator, thereby reducing the efficiency of initiation. . In order to solve this contradiction, the ultraviolet spectrum of the pigment must be analyzed and compared with the ultraviolet absorption of the initiator to find the "spectral window". Therefore, understanding the spectral window of a pigment is important for selecting a suitable photoinitiator. In principle, photoinitiators which have a very good absorption in the spectral window of the pigment should be chosen. This is important to improve the UV curing speed of colored ink systems.
Since the absorption of ultraviolet light by the pigment affects the photoinitiation efficiency of the photoinitiator, the pigment content has a great influence on the curing speed of the UV ink.
Note: pigment layering 0.92um
Fourth, the advantages of UV curing ink and the need to improve the place
Advantages of UV-curable ink: high printing speed; wide range of printing materials; consistent color; environmentally friendly; low odor;
What needs to be improved: Currently, pigment wettability, cure speed and adhesion are an aspect that needs improvement.
In lithographic inks, it is necessary to achieve better ink balance, brightness and fluidity, and improve pigment wettability; for flexo inks, because of the increase of pigment filler in the ink, the improvement of pigment wettability is especially important. In order to prevent flocculation in production, the primary and corresponding problem remains wettability. In both ink systems, fluidity is improved as the wettability becomes better.
Another aspect of pigment wetting is manifested in production costs. Grinding is a costly process and must be controlled at low temperatures to prevent gelation. In order to increase the yield, it has been considered to replace the conventional three-roll mill with a ball mill in some continuous operations. The better the wettability, the better the abrasiveness and dispersibility.
Curing speed In the initial application, the curing of the ink is effective. At present, the requirements for it are getting higher and higher, the new printing process requires a built-in lamp with good performance, and the price of the photoinitiator used in the ink system is also limited. For example, the lithographic printing ink can withstand the speed of 400m/min or l5000 impressions/h on the paper sheet, and the flexo printing process also has higher requirements, such as the speed required for label printing has exceeded 150m/min.
Adhesiveness Faces the development of performance improvements in UV-curable inks. In research areas such as pigment wetting, cure speed and adhesion, some valuable new products that help meet higher standards are being introduced.
Fifth, the development trend of UV curing ink
The properties of UV inks were examined and their characteristics were very obvious. If the printed product is quick-drying, easy to operate continuously, the production process has low energy consumption, does not pollute the environment, and the print brightness, color strength and durability are good. It can be used for varnish and hot-press foil. The instantaneous drying property of UV ink makes it It is used for web printing as well as for the permissible transient process.
In fact, in some areas, especially from the environmental and performance aspects, UV inks have an advantage. UV inks now dominate the printing industry, such as label printing, packaging printing and carton printing.
With the latest advances in raw materials for ink production, future UV-curing products will take us to the end. In the early days, basic UV inks were sold along with various additives to improve functionality. Printing practitioners are forced to play the role of a chemist to achieve proper ink performance. Today, ink suppliers are incorporating more technology into their inks and developing new products based on customer needs. Raw material suppliers are now expected to develop products based on the requirements of ink formulators, unlike at the beginning, when ink suppliers could only use a limited number of ink components.
The competition among ink suppliers has forced them to seek new and creative ways to improve the performance of inks, as well as to find new applications to benefit from UV screen printing. In general, due to the development of new The UV ink and UV screen printing market is growing at an alarming rate, and this trend will continue as raw material suppliers and ink manufacturers work together to develop products and develop new performance standards.