The viscosity of the prepolymer in ordinary UV inks is generally very large, and it needs to be diluted with active diluents. The currently used diluent acrylate compounds have varying degrees of skin irritation and toxicity. At the same time, many reactive diluent monomers are in ultraviolet In the process of light radiation, there is also the problem of incomplete reaction. The residual monomers are permeable, which can easily cause hidden dangers in health and safety, and affect the long-term performance stability of the cured film. Although the water-based ink has a very easy to adjust low viscosity and very low organic volatile matter (VOC), but its drying time is generally longer, most of them need to be heated, the heat resistance of the substrate is also limited, drying and curing crosslinking may be simultaneously Deficiencies occur. Water-based UV inks use water and ethanol as diluents, combining the characteristics of water-based inks and UV inks, which is a new research direction in the field of UV inks. 1 Curing and drying mechanism of water-based UV ink Water-based UV inks are mainly prepared from prepolymers (water-based photocurable resins), photoinitiators, pigments, amines, water, co-solvents and other additives. Its drying and curing combine two types of drying: UV light curing and water-based ink pervaporation. Specifically, there are two main drying methods: pre-volume drying of the water-based system and ultraviolet light curing. 1. Pre-volume drying mechanism Pre-drying is a process that must be carried out before photo-curing. Without pre-drying, the final result of photo-curing will be unsatisfactory. In the manufacturing process of water-based UV ink, water-based photocurable resin can be dissolved in water by adding an alkali or acid to make it into a carboxylate salt, and the reaction of making salt by adding ammonia water can be shown as: R-COOH + NH → R-COO + NH (water-soluble) In turn, the reaction during the pre-drying process is: R-COO + NH → R-COOH (water insoluble) + NH ↑ 2. Photocuring mechanism The curing of UV-curable water-based materials means that under the irradiation of ultraviolet light, the photoinitiator absorbs the radiation energy of ultraviolet light and then splits into free radicals, which initiates the polymerization and crosslinking of the prepolymer and cures in a short time. The formation of a three-dimensional network polymer polymer to obtain a hardened film is essentially chemical drying by forming chemical bonds. The curing process can generally be divided into four stages: ①interaction between light and photoinitiator, which may include absorption of light and interaction between photoinitiators; ② photoinitiator molecules rearrange to form free radicals Intermediate; ③ The free radical interacts with the unsaturated group in the oligomer to initiate a chain or polymerization reaction; ④ The polymerization reaction continues and the liquid component is converted into a solid polymer. 2 Factors affecting the curing and drying of water-based UV ink 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 have the following aspects: 1. Effect of pre-drying of water-based system on light curing The drying condition before curing has a great influence on the curing speed. When it is not dried or is not completely dried, the curing speed is slow, and the gelation rate does not increase significantly with the extension of the exposure time. This is because, although water has a certain effect on inhibiting the polymerization inhibition of oxygen, it can only cure the surface of the ink film quickly and only achieve surface drying, but not solid drying. Because the system contains a lot 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 quickly solidifies, and the water in the film layer is difficult to escape, and a large amount of water remains in the ink film. This prevents further curing of the ink film and reduces the curing speed. In addition, the ambient temperature during UV irradiation has a great influence on the curing of UV inks. The higher the temperature, the better the curability. Therefore, if pre-heating is applied, the curability of the ink will be enhanced and the adhesion will be better. 2. Effect of waterborne UV curing resin on light curing Radical photocuring of water-based UV-curable resins requires that the resin molecules must bear unsaturated groups. Under the irradiation of ultraviolet light, the unsaturated groups in the molecules cross-link with each other, changing from a liquid coating to a solid coating Floor. The method of introducing acryloyl, methacryloyl, vinyl ether or allyl is usually adopted to make the synthetic resin have unsaturated groups, so that it can be cured under suitable conditions. Acrylates are often reactive due to their high reactivity used. For the free radical ultraviolet curing system, as the content of double bonds in the molecule increases, the cross-linking speed of the coating film will increase, and the curing speed will be accelerated. Moreover, resins with different structures have different effects on curing speed. 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 the main method, so that the resin has a faster curing speed. [next] 3. Effect of Pigment on the Photocuring of Waterborne UV Ink As a non-photosensitive component in water-based UV-curable inks, pigments and initiators compete to absorb UV light, which greatly affects the curing characteristics of the system in the UV-curing system. Since the pigment can absorb a part of the radiant energy, this will affect the absorption of light by the photoinitiator, and in turn affect the concentration of free radicals that can be generated, and as a result, the curing speed will be reduced. Each color pigment has different absorption rate (light transmittance) for light of different wavelengths. The smaller the absorption rate of the pigment, the greater the light transmittance, and the faster the curing speed of the coating. Carbon black has a high ultraviolet absorption capacity, and it cures the slowest. The white pigment is highly reflective, which also hinders curing. In general, the order of absorption of ultraviolet light is: black> purple> blue> cyan> green> yellow> red. The proportion of the same pigment is different, and the effect on the curing speed of the ink film is different. With the increase of the amount of pigment, the curing rate of the ink film has decreased to varying degrees. Among them, the amount of yellow pigment has the greatest influence on the curing rate of the ink film, followed by red pigment and green pigment. Because the absorption rate of ultraviolet light by black is the largest and the transmittance of black ink is 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, reducing the transmittance of ultraviolet light, affecting the curing of the deep ink film, resulting in the curing of the surface layer of the ink film and the bottom layer Without curing, it is easy to produce "wrinkle skin" phenomenon. 4. Effect of photoinitiator on waterborne UV light curing The role of the photoinitiator is to decompose and generate free radicals after it absorbs ultraviolet light energy, thereby initiating the polymerization of unsaturated bonds in the system, and crosslinking and curing into a whole. The performance of the photoinitiator is the key to the smooth polymerization and curing of the waterborne photocuring system. According to the principle that different initiators have a maximum absorption wavelength, the wavelength range of the ultraviolet light absorption of the photoinitiator should be exactly in the non-absorptive region or the micro-absorptive region of the pigment when selecting the photocuring initiator The wavelength window that the initiator absorbs), that is, the wavelength region of 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 main wavelength emitted by the light source. The photoinitiator must have certain compatibility with the aqueous photocuring system and low water vapor volatility, so that the photoinitiator can be dispersed, which is conducive to obtaining a 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, and their combined use can fully absorb ultraviolet rays of different wavelengths, improve the absorption of ultraviolet radiation, and thus greatly accelerate the curing rate of the ink film. Therefore, it is possible to use a variety of photoinitiators and adjust the ratio of various photoinitiators to obtain an ink film with a fast curing rate and excellent performance. The content of composite photoinitiator in the system should be appropriate, too low is not conducive to competition with the absorption of pigment; too high light can not enter the coating smoothly. The curing speed of the ink increases with the increase of the composite photoinitiator at the beginning, but when the dosage of the composite photoinitiator increases to a certain value, and then increase its content, the curing speed will decrease instead. 5. Effects of UV light source, irradiation distance and light curing time The UV light source radiates light in a wavelength band, and the energy distribution of the light of each wavelength is different. Among them, the energy distribution of light with a wavelength of 300 nm to 310 nm and 360 nm to 390 nm is better, and the best effect is achieved when the wavelength is about 360 nm in the UV-A region. 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 ink, the irradiation distance and light curing time will also affect the light curing. The closer the irradiation distance, the stronger the light, the faster the photoinitiator generates free radicals, the more double bonds that cause polymerization, the higher the degree of crosslinking of the resin, and the faster the curing speed of the ink film, and vice versa The slower. The photo-curing of water-based UV ink must also have a suitable photo-curing time. The time is too short and the curing of the ink is not complete. When a suitable photo-curing time is reached, the tensile strength of the cured film reaches the maximum, and the photo-curing time continues to increase Instead, the tensile strength of the cured film decreases, and the cured resin exhibits yellowing. 3 Conclusion At present, the water-based UV ink has been successfully developed and applied in some printing. The HU1000 series water-based UV inks of American Nazdar Ink Company and the Hydra-UV water-based UV inks of Liseda Company have all entered the Chinese market. Beijing Yingli Technology and Beijing Hongji Printing have jointly developed the Hongying brand water-based UV ink. Zhongshan Zhongqiao Coating Adhesive Co., Ltd. has also launched its own water-based UV ink series. With the increasing demands of the printing industry for environmental protection, water-based UV inks will be a hot spot for future ink application and research.
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