Curing and drying mechanism of the hottest aqueous

2022-07-23
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Curing and drying mechanism of water-based UV ink and its influencing factors

the viscosity of prepolymer in ordinary UV ink is generally very large, which needs to be diluted with active diluent. However, the currently used diluent acrylate compounds have different degrees of skin irritation and toxicity. At the same time, many reactive diluent monomers still have the problem of incomplete reaction in the process of UV radiation, and the residual monomers are permeable, which is easy to bring health and safety hazards, And affect the long-term stability of the cured film. Although water-based inks have low viscosity and very low organic volatile matter (VOC) that can be easily adjusted, which is the key to ensure the accuracy of spring testing, their drying time is generally long, most of them need to be heated, the heat resistance of the substrate is also limited, and drying and curing crosslinking may occur at the same time. Water based UV ink uses water and ethanol as diluents, which combines the characteristics of water-based ink and UV ink. It is a new research direction in the field of UV ink

I. curing and drying mechanism of water-based UV inks

water based UV inks are mainly prepared from prepolymers (water-based UV curable resins), photoinitiators, pigments, amines, water, cosolvent and other additives. Its drying and curing combines UV curing and water-based ink pervaporation. Specifically, there are two drying methods: pre volatilization drying and UV curing of water-based system

1 pre volatilization drying mechanism

pre drying is a necessary process before UV curing. Without pre drying, the final result of UV curing will be unsatisfactory. In the manufacturing process of water-based UV ink, water-based UV curable resin can only be dissolved in water by adding an alkali or acid to make it become carboxylate. The reaction of salt formation by adding ammonia water can be shown as:

r-cooh+nh3--r-coo+nh4 (water soluble)

2 mechanism of UV curing film formation

uv curing curing of water-based materials refers to that under the irradiation of ultraviolet light, the photoinitiator absorbs the radiation energy of ultraviolet light and splits into free radicals, The prepolymer is initiated to undergo polymerization, cross-linking and grafting reaction, and solidifies into a three-dimensional polymer in a very short time to obtain a hardened film. In essence, chemical drying is achieved by forming chemical bonds. The curing process can be generally divided into four stages: ① the interaction between light and initiator, which may include the absorption of light and the interaction between photoinitiators; ② Photoinitiator molecules rearrange to form free radical intermediates; ③ Free radicals react with unsaturated groups in oligomers to initiate chain or polymerization reactions; ④ The polymerization continues, and the liquid components are transformed into solid polymers

II. Factors affecting the curing and drying of water-based UV inks

there are many factors affecting the curing and drying of water-based UV inks. This paper only discusses the main influencing factors. These factors include the following aspects:

1 The pre drying of the aqueous system has a great impact on the light curing. If it is not dry or incomplete, the solid environmental protection plastic bag will be used as fertilizer after dissolving water! The rate of "Dongguan Legion" of new materials is slow, and the gelation rate does not increase significantly with the extension of exposure time. This is because, although water has a certain effect on inhibiting the polymerization of oxygen, it can only make the surface of the ink film solidify rapidly, only to achieve surface drying, but not to achieve solid drying. Because the system contains a large amount of water, when the system is cured at a certain temperature, with the rapid evaporation of water on the surface of the ink film, the surface of the ink film solidifies rapidly, and the water in the film is difficult to escape. A large amount of water remains in the ink film, preventing the further curing of the ink film and reducing the curing speed. In addition, the ambient temperature during UV irradiation has a great impact on the curing of UV ink. The higher the temperature, the better the curing property. Therefore, if preheating is applied, the curing property of the ink will be enhanced and the adhesion will be better

2. The influence of Waterborne UV curing resin on light curing

Waterborne UV curing resin needs to undergo free radical light curing and develop high value-added aviation logistics, which requires that the resin molecules must have unsaturated groups. Under the irradiation of ultraviolet light, the unsaturated groups in the molecules are cross-linked, and the liquid coating becomes a solid coating. Usually, the method of introducing acryloyl, methacryloyl, vinyl ether or allyl is adopted to make the synthetic resin have unsaturated groups, so that it can be cured under appropriate conditions. Propionate is often used because of its high reaction activity. For the free radical UV curing system, with the increase of the double bond content in the molecule, the crosslinking speed of the film will increase, and the curing speed will accelerate. Moreover, resins with different structures have different effects on the curing rate, and the reaction activities of various functional groups generally increase in the following order: vinyl ether allyl methacryloyl acryloyl. Therefore, acryloyl and methacryloyl are generally introduced to make the resin have a faster curing speed

3. Effect of pigments on UV curing of Waterborne UV curable inks as a non photosensitive component of Waterborne UV curable inks, pigments compete with initiators to absorb UV light, which greatly affects the curing characteristics of the UV curing system. Because the pigment can absorb part of the radiation energy, it will affect the absorption of light by the photoinitiator, and then affect the concentration of free radicals that can be generated. As a result, the curing speed will be reduced. Each color of pigment has different absorptivity (transmittance) to different wavelengths of light. The smaller the absorptivity of the pigment, the greater the transmittance, and the faster the curing speed of the coating. Carbon black has high ultraviolet absorption capacity and slowest curing. The strong reflection of white pigment also hinders curing. Generally, the absorption order of ultraviolet light is: black purple blue cyan stunning yellow red

different proportion and concentration of the same pigment have different effects on the curing speed of the ink film. With the increase of pigment content, the curing rate of ink film decreased in different procedures. The amount of yellow pigment has the greatest influence on the curing rate of ink film, followed by red pigment and green pigment. Because black has the highest absorption rate of ultraviolet light, making the transmittance of black ink the lowest, the change of its dosage has no obvious 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 on the surface layer absorbs a large amount of ultraviolet light, which reduces the transmittance of ultraviolet light and affects the curing of the deep layer of the ink film, resulting in the surface layer of the ink film curing but the bottom layer not curing, which is easy to produce "wrinkle" phenomenon

4. The effect of photoinitiator on aqueous UV curing

the function of photoinitiator is that after it absorbs UV light energy, it decomposes to produce free radicals, thus initiating unsaturated bond polymerization in the system, crosslinking and curing into a whole. The performance of photoinitiator is the key to the successful polymerization and curing of Waterborne UV curing system. According to the principle that different initiators have a maximum absorption wavelength, when selecting the UV curing initiator, the UV absorption wavelength region of the photoinitiator should be just in the non absorption region of the pigment (the pigment has a wavelength window suitable for the absorption of the initiator), that is, the maximum transmission wavelength region of the pigment should coincide with the absorption wavelength region of the free radical initiator. In addition, the absorption peak of photoinitiator should be as close as possible to the main wavelength emitted by the light source

the photoinitiator must have certain miscibility with the aqueous UV curing system and low water vapor volatility, so that the photoinitiator can be dispersed, which is conducive to the satisfactory curing effect. Otherwise, during the drying process, the photoinitiator will volatilize with the water vapor, reducing the efficiency of the initiator. Different photoinitiators have different absorption wavelengths. Their combined use can fully absorb ultraviolet rays of different wavelengths, improve the absorption of ultraviolet radiation, and greatly accelerate the curing rate of the ink film. Therefore, the ink film with fast curing rate and excellent performance can be obtained through the right to use a variety of photoinitiators and adjusting the ratio of various photoinitiators. The content of compound photoinitiator in the system should be appropriate, too low is not conducive to the absorption competition with pigments; Too much light cannot enter the coating smoothly. At the beginning, the curing speed of ink increases with the increase of compound photoinitiator, but when the compound photoinitiator dose increases to a certain value, and then increases its content, the curing speed will decrease

influence of light source, irradiation distance and curing time

uv light source radiates light in one band, and the energy distribution of each wavelength is different. Where the wavelength is nm, the energy distribution of nm light is better, and the effect is the best when the wavelength is about 360nm in the UV-A region. In order to achieve the best combination of the curing system, the choice of UV light source should not only consider the UV 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 ink, the irradiation distance and curing time will also affect the curing. The closer the irradiation distance is, the stronger the illumination is, the faster the photoinitiator generates free radicals, the more the number of double bonds that cause polymerization, and the cross of the resin is 4 The higher the connection degree of the steel ball rotation fatigue test, the faster the curing speed of the ink film, and vice versa. The UV curing of water-based UV ink must also have an appropriate UV curing time. The time is too short, and the curing of the ink is incomplete. When an appropriate UV curing time is reached, the tensile strength of the cured film reaches the maximum. If the UV curing time continues to increase, the tensile strength of the cured film will decrease, and the cured resin will turn yellow

3 conclusion

at present, water-based UV ink has been successfully developed and applied in some printing. The hu1000 series water-based UV inks of nazdar ink company and the Hydra UV water-based UV inks of reseda have entered the Chinese market. Beijing Yingli technology and Beijing Hongji Printing Co., Ltd. jointly developed Hongying brand water-based UV ink

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