Water-based UV ink curing mechanism and its influencing factors

Prepolymers in general UV inks are generally very viscous and need to be diluted with reactive diluents. The currently used diluents, acrylates, have varying degrees of skin irritation and toxicity, while many reactive diluents are used in UV light. In the process of radiation, there is still a problem of incomplete reaction. Permeability of residual monomers is apt to bring about hidden dangers of health and safety, and affect the long-term stability of the cured film. Although water-based inks have easily adjustable low viscosity and very low organic volatiles (VOC), their drying time is generally longer, most of them need to be heated, the heat resistance of the substrate is also limited, and the drying and curing cross-linking may be simultaneous. There are other deficiencies. Water-based UV inks use water and ethanol as diluents, combining the characteristics of water-based inks and UV inks. It is a new research direction in the field of UV inks.

First, the curing drying mechanism of water-based UV ink

Water-based UV inks are primarily formulated from prepolymers (water-based photocurable resins), photoinitiators, pigments, amines, water, cosolvents, and other additives. The drying curing combines two kinds of drying modes of UV light curing and water-based ink pervaporation. Specifically, there are two main drying methods: pre-volatilization drying and ultraviolet light curing of the aqueous system.

1 Pre-evaporation drying mechanism

Pre-drying is a necessary step before photocuring. Without pre-drying, the final result of photocuring is not ideal. In the manufacture of water-based UV inks, water-based photocurable resins can be dissolved in water by adding a base or acid to make them carboxylate salts, where the reaction to form salts by adding ammonia water can be shown as:

R-COOH+NH3--R-COO+NH4 (water-soluble)

2 photocuring mechanism

The curing of UV-cured water-based materials means that under the irradiation of ultraviolet light, the photoinitiator absorbs the radiant energy of the ultraviolet light and then splits into free radicals, causing polymerization of the prepolymer, cross-linking of the grafts, and curing in a short time. Into a three-dimensional network of high-molecular polymer, to obtain a hardened film, the essence is to achieve chemical drying through the formation of chemical bonds. The curing process can generally be divided into four stages: 1 interaction between light and initiator, which may include the absorption of light and the interaction between photoinitiators; 2 photoinitiator molecules rearrange, form the middle of the free radicals (3) Free radicals react with unsaturated groups in the oligomers to initiate chain or polymerization reactions; (4) Polymerization continues, and liquid components are converted into solid polymers.

Second, the factors that affect the curing of water-based UV ink drying

There are many factors that affect the curing and drying of water-based UV inks. This article only discusses the main influencing factors. These factors are as follows:

1. The pre-drying of aqueous system has a great influence on photocuring. When it is not dry or incomplete, the curing speed is slow, and with the extension of exposure time, the gelation rate has no obvious increase. This is because, although water has a certain effect on inhibiting the inhibition of oxygen, this only allows the ink film surface to rapidly solidify, and it only reaches the surface dryness, and it cannot reach the solid state. Because the system contains a large amount of water, when the system is cured at a certain temperature, the surface of the ink film rapidly solidifies as the moisture on the surface of the ink film rapidly evaporates, the water in the film layer is difficult to escape, and a large amount of water remains in the ink film. The ink film is prevented from further curing and the curing speed is reduced. In addition, the ambient temperature at the time of UV irradiation has a great influence on the curing of the UV ink. The higher the temperature, the better the curability. Therefore, if the preheating is performed, the curability of the ink will be enhanced and the adhesion will be better.

2. Effect of water-based UV curing resin on light curing

Water-based UV-curable resins undergo radical photo-curing. This requires that the resin molecules must have unsaturated groups. Under ultraviolet irradiation, the unsaturated groups in the molecules crosslink with each other, and the liquid coating becomes a solid coating. Floor. The method of introducing an acryloyl group, a methacryloyl group, a vinyl ether, or an allyl group is generally adopted so that the synthetic resin has an unsaturated group, so that it can be cured under suitable conditions. It is often used. For radical-type UV curing systems, as the content of double bonds in the molecule increases, the cross-linking speed of the coating film increases and the curing speed increases. Also, resins of different structures have different effects on the curing speed, and the reactivity of various functional groups generally increases in the following order: vinyl ether <allyl <methacryloyl <acryloyl. Therefore, the introduction of acryloyl and methacryloyl groups is generally the main, so that the resin has a faster curing speed.

3. The effect of pigments on the photocuring of aqueous UV inks

As a non-photosensitive component in water-based UV-curable inks, pigments compete with initiators for absorbing UV light, which largely affects the curing characteristics of systems in UV curing systems. Because the pigment can absorb a portion of the radiant energy, this will affect the absorption of light by the photoinitiator, which in turn affects the concentration of free radicals that can be generated, and as a result, the curing speed is reduced. Each color pigment has different absorption rate (light transmittance) for different wavelengths of light, the smaller the absorption rate of the pigment, the greater the transmittance, and the faster the curing speed of the coating. Carbon black has a higher UV absorption capacity and cures the slowest, and the white pigment's high light reflectivity prevents it from curing. The order of UV absorption is generally: black> purple> blue> cyan> stunning> yellow> red.

The same concentration ratio of the different pigments has different effects on the curing rate of the ink film. With the increase of the amount of pigment, the curing rate of the ink film has a different program. The amount of yellow pigment has the greatest influence on the curing rate of the ink film, followed by red pigment and green pigment. Since black has the highest absorptivity for ultraviolet light, the black ink has the lowest light transmittance, so the change in the amount thereof has no significant effect on the curing rate of the ink film. When the amount of pigment is too large, the curing rate of the surface layer of the ink film is fast, but the pigment of the surface layer absorbs a large amount of ultraviolet light, reduces the light transmittance of the ultraviolet light, and affects the solidification of the deep ink film, resulting in the surface curing of the ink film and the bottom layer Not cured, easy to produce "wrinkle skin" phenomenon.

4. Effect of photoinitiators on aqueous UV light curing

The role of the photoinitiator is that after it absorbs the ultraviolet light energy, it generates free radicals through decomposition, thereby initiating the polymerization of unsaturated bonds in the system and curing the cross-linking into a whole. The performance of the photoinitiator is the key to the successful polymerization of the waterborne photocuring system. According to the principle that different initiators have a maximum absorption wavelength, the photo-curing initiator should be selected such that the UV-absorbing wavelength region of the photo-initiator is just in the non-absorption region of the pigment (the pigment has a wavelength suitable for the absorption of the initiator. The window, ie, the wavelength region of the maximum transmission of the pigment should coincide with the absorption wavelength region of the radical initiator. In addition, the absorption peak of the photoinitiator should be as close as possible to the dominant wavelength emitted by the light source.

The photoinitiator must have a certain compatibility with the aqueous photocuring system and low water vapor volatility so that the photoinitiator can be diffused, which is favorable for obtaining a satisfactory curing effect. Otherwise, during the drying process, the photoinitiator will volatilize with the water vapor, reducing the initiator's efficiency. Different photoinitiators have different absorption wavelengths, and their combined use can fully absorb ultraviolet light of different wavelengths and increase the absorption of ultraviolet radiation, thereby greatly accelerating the curing rate of the ink film. Therefore, a variety of photoinitiators can be used with the right to use, and the ratio of various photoinitiators can be adjusted to obtain ink films with fast curing speed and excellent performance. The amount of complex photoinitiator in the system should be appropriate, too low is not conducive to competition with the absorption of the pigment; high light can not enter the coating smoothly. The curing speed of the ink begins to increase with the increase of the complex photoinitiator, but when the complex photoinitiator dose increases to a certain value, the content of the photoinitiator increases, and the curing speed will decrease.

5. Effect of UV light source, irradiation distance and light curing time

The UV light source radiates light in one wavelength band, and the energy distribution of each wavelength light is not the same. Among them, the energy distribution of light with a wavelength of 300-310 nm and 360-390 nm is good, and the wavelength of about 360 nm in the UV-A region is the best. In order to achieve the best combination of curing system, the choice of UV light source should not only consider the UV light absorption characteristics of the pigments contained in the system, but also take into account the UV absorption characteristics of the initiator.

In the drying and curing process of water-based UV inks, the irradiation distance and the photo-curing time also affect the photo-curing. The closer the irradiation distance and the stronger the light, the faster the photoinitiator generates free radicals. The more the number of double bonds that are polymerized, the higher the cross-linking degree of the resin, and the faster the curing speed of the ink film. The slower. Water-based UV inks must also have a suitable photocuring time for photocuring. The time is too short and the ink is not cured. When a suitable photocuring time is reached, the tensile strength of the cured film is maximized and the photocuring time continues to increase. The tensile strength of the cured film decreased instead, and the cured resin appeared yellowing.

3 Conclusion

At present, water-based UV inks have been successfully developed and have been used in some printing applications. The United States Nazdar Ink Co., Ltd.'s HU1000 series of water-based UV inks, Lizda's Hydra-UV water-based UV inks have entered the Chinese market. Beijing Yingli Technology and Beijing Hongji Printing jointly developed Hongying water-based UV ink,