2 Factors affecting the curing and drying of aqueous UV inks
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. Effect of pre-drying of aqueous system on photocuring
The curing condition before curing has a great influence on the curing speed. When it is not dry or incomplete, the curing speed is slow, and with the prolonging of the exposure time, the gelation rate does not increase significantly. 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 Waterborne UV Curing Resin on Photocuring
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, and the acrylate is often due to its high reactivity and 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. Furthermore, the 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. Effect of Pigments on Photocuring of Waterborne UV Ink
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 high UV absorption capacity and is the slowest to cure. White pigments have a high degree of light reflectivity, which also hinders curing. In general, the absorption order of ultraviolet light is: black> purple> blue> cyan> green> 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 decreased to varying degrees. 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. Photoinitiator Effect on Waterborne UV 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 or micro-absorption region of the pigment (the pigment has a suitable The wavelength window of absorption by the initiator, ie the wavelength region of the maximum transmission of the pigment, should coincide with the wavelength region of absorption 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 in combination, 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. The energy distribution of light with a wavelength of 300-310 nm and 360-390 nm is preferable, and the wavelength of about 360 nm in the UV-A region is the best. In order to achieve the best combination of curing systems, the choice of UV light source must take into account the UV light absorption characteristics of the pigments contained in the system, while taking 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 inks. Zhongshan Zhongqiao Paint Adhesive Co., Ltd. has also launched its own water-based UV ink series. With the increasing demand for environmental protection in the printing industry, water-based UV inks will be a hot topic in ink application and research in the future.
Source: "Printing World" Author: Pingpei Yong
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. Effect of pre-drying of aqueous system on photocuring
The curing condition before curing has a great influence on the curing speed. When it is not dry or incomplete, the curing speed is slow, and with the prolonging of the exposure time, the gelation rate does not increase significantly. 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 Waterborne UV Curing Resin on Photocuring
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, and the acrylate is often due to its high reactivity and 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. Furthermore, the 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. Effect of Pigments on Photocuring of Waterborne UV Ink
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 high UV absorption capacity and is the slowest to cure. White pigments have a high degree of light reflectivity, which also hinders curing. In general, the absorption order of ultraviolet light is: black> purple> blue> cyan> green> 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 decreased to varying degrees. 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. Photoinitiator Effect on Waterborne UV 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 or micro-absorption region of the pigment (the pigment has a suitable The wavelength window of absorption by the initiator, ie the wavelength region of the maximum transmission of the pigment, should coincide with the wavelength region of absorption 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 in combination, 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. The energy distribution of light with a wavelength of 300-310 nm and 360-390 nm is preferable, and the wavelength of about 360 nm in the UV-A region is the best. In order to achieve the best combination of curing systems, the choice of UV light source must take into account the UV light absorption characteristics of the pigments contained in the system, while taking 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 inks. Zhongshan Zhongqiao Paint Adhesive Co., Ltd. has also launched its own water-based UV ink series. With the increasing demand for environmental protection in the printing industry, water-based UV inks will be a hot topic in ink application and research in the future.
Source: "Printing World" Author: Pingpei Yong
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