UV ink composition and curing factors

Ultraviolet light-curing inks, also known as uv inks, are used more and more widely in the printing industry, occupying important positions in areas such as offset printing, letterpress printing, and screen printing.

With the continuous development of science and technology, radiation curing systems have been increasingly used in printing inks. In particular, UV curing systems have high curing speed, solvent-free volatilization, space saving, and are suitable for a variety of substrates. The film has good hand feeling and the like, and its application has been widely used, and the usage has increased rapidly.

In the printing and packaging industry, uv light-curable inks play an important role in improving production efficiency and quality. In the field of offset printing and flexo printing, the application of uv inks has grown from business forms, direct mail, folding cartons, and credit card market production. With the ever-accelerating printing speed, the web has been continuously increased, and the curing ability of uv ink has also been challenged more fiercely. Therefore, this paper summarizes the curing mechanism of uv ink and the common factors that affect the drying of uv ink.

1.uv ink and its composition

Ordinary inks are composed of pigments, binders and auxiliaries. Most of them are dried by physical or chemical means by volatilization of solvents; while uv inks are photopolymerizable prepolymers (mostly Saturated or unsaturated dibasic acid esters and acrylic resins. As an ink base material, radicals are generated from a photoinitiator under the irradiation of ultraviolet light to cause polymerization of a base material and a photoexchanging reaction, so that the ink is instantaneously cured. The main components of the uv ink include a photopolymerization-active prepolymer, a coagent, a photosensitive solvent, and a photopolymerization initiator.

2. Curing mechanism of ink

Under the action of ultraviolet light, a photopolymerization initiator absorbs a photon of a certain wavelength to be excited to an excited state to generate free radicals or ions:



These radicals or ions react with unsaturated bonds in monomers such as prepolymers to form monomer groups and start a chain reaction:




Because the prepolymer is a multi-functional structure, the molecular exchange is three-dimensional, so the binder material changes from a liquid to a solid state. This reaction speed is very fast, and is completed in less than 1 s, that is, when the sheet reaches the sheet feeding device.

3. The main factors that affect the uv ink curing

The curing effect of uv ink is mainly evaluated in two aspects: first, full curing; second, curing at the optimal speed. The choice of the best curing speed is: First, it is cured by a uv machine at a certain speed. If it is cured, the speed is increased until the ink layer coming out of the curing machine can not be cured immediately. If the speed at this time is V, then 0.8V is the optimum curing speed. The main factors affecting the ink curing adequacy and the best curing speed are as follows:

3.1 External conditions

3.1.1 Temperature at the time of irradiation

The higher the temperature, the better the curability. Therefore, in order to obtain a high curability and good ink curing effect, preheating is generally performed.

3.1.2 Curing environmental conditions

When printing, room temperature conditions should be as constant as possible, generally 18 to 25 degrees is appropriate; oxygen has the effect of hindering the polymerization of the active prepolymer, so the oxygen concentration of the photo-curing reaction occasion is between 3010-6~5010-6.

3.1.3 Light Curing Initiators

Adding an appropriate amount of light and curing initiator is an effective method to increase the curing effect. However, overdosing will hinder the curing, but it is usually less than 4%.

3.1.4 The radiation intensity of the light source

The uv light source radiation is a band of light, and the energy distribution of each wavelength is not the same, and the uv lamp's spectral sensitivity range and uv ink to match; generally require the uv lamp's wavelength range is between 180 ~ 420nm, sensitive The area is between 350 and 420 nm.

3.2 Internal conditions

3.2.1 The nature of the pigment

For color uv inks, the first step in the arrangement of the printing color sequence should consider the ability of various pigments to absorb uv light quantum. For uv photons, the transmittance of magenta is 50%~60%, yellow is 20%~30%. The cyan color is 8%~20%, and the black color is 12%. Therefore, the order of their UV radiation curing rates is magenta-yellow-cyan-black, among which the black color is the slowest, so the printing sequence is black-blue-yellow. - Magenta is most suitable.

3.2.2 ink ink layer thickness

The thinner the ink layer, the better the curing performance, and the general colored ink layer can be fully cured within a film thickness range of 10 to 12 nm.

3.2.3 surface properties of printing materials

The adhesion of the ink depends on the ability of the ink to wet the substrate and the forces between the ink molecules. The surface treatment of the substrate for UV ink adhesion is extremely important, usually the surface tension of the ink is less than the critical surface tension of the substrate, in order to make the ink adhere well to the surface of the substrate.

Usually, monomers with lower surface tension are used to formulate inks. For materials with low surface tension, such as PP, PE and PET, the surface chemical composition and crystallization conditions and surface morphology can be changed by corona discharge treatment on the surface of the substrate. , increase the surface energy; on the plastic film and other materials, can be gently wipe with a clean cloth of alcohol, to remove the plasticizer and impure chemical substances, dust, greasy floating on the surface of the substrate, thereby improving the ink adhesion strength. Usually the substrate surface tension reaches 0.5N/m or so can get satisfactory adhesion effect.

4 Conclusion

There are many series of uv inks, and the diversity of its composition constitutes the complexity of its influencing factors. A full understanding of uv inks is a prerequisite for improving the use of uv inks, improving productivity and work efficiency, and reducing the scrap rate. This can greatly save the higher cost of ink, and not like some printers think that the use of uv ink will increase their total expenses; at the same time, it can also make up for its deficiencies in practical applications according to its use principles and influencing factors. . For example, the thickness of the ink film can be controlled by warming up, and the color overprinting sequence can be reasonably arranged to speed up the drying speed. For the disadvantage that it may cause harm to the human body, the operator can put on protective clothing and gloves by wearing the work clothes. Glasses to avoid, you can also use aluminum and other materials to UV seal, to prevent the leakage of ultraviolet light and so on. By adopting such a series of corresponding measures, we can maximize our strengths, avoid their shortcomings, and make them better serve us.