Curing and drying mechanism and influencing factor

2022-09-19
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Curing and drying mechanism and influencing factors of water-based UV inks

the viscosity of prepolymer in ordinary UV inks is generally very high, which needs to be diluted with active diluent. However, the currently used diluent acrylate compounds have varying 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, 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 at present

1 curing and drying mechanism of aqueous UV inks

aqueous 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 two drying forms: UV curing and water-based ink pervaporation. Specifically, there are mainly two drying methods: pre volatilization drying of water-based system and UV curing

1 . Pre evaporation 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 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)

conversely, the reaction in the pre drying process is:

2 UV curing film-forming mechanism

the curing of UV curing aqueous materials refers to that under the irradiation of UV light, the photoinitiator absorbs the radiation energy of UV light and splits into free radicals, which triggers the polymerization, cross-linking and grafting reaction of prepolymer, and solidifies into 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 generally be 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 interact with unsaturated groups in oligomers to initiate chain or polymerization reactions; ④ The polymerization reaction continues, and the liquid components are transformed into solid polymers. The curing process is shown in Figure 1

2. There are many factors affecting the curing and drying of water-based UV ink. This paper only discusses the main influencing factors, which include the following aspects:

1 The pre drying of the aqueous system has a great impact on the light curing. When it is not dried or not completely dried, the curing speed 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 is, the better the curing property is. Therefore, if preheating is applied, the solidifying 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 free radical light curing, which requires that the resin molecules must have unsaturated groups. Under the irradiation of ultraviolet light, the unsaturated groups in the molecules are crosslinked with each other, 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 activity of various functional groups generally increases in the following order: vinyl ether allyl methacryloyl acryloyl. Therefore, it is generally interpreted in the industry that acryloyl and methacryloyl are mainly introduced, so that the resin has a faster curing speed

3. Effect of pigments on UV curing of Waterborne UV ink

as a non photosensitive component of Waterborne UV curing ink, pigments compete with initiators to absorb UV light, which greatly affects the curing characteristics of the system in the UV curing system. Because the pigment can absorb the photoelectric encoder, there will be a pulse signal output to receive part of the radiation energy, which will affect the absorption of light by the photoinitiator, and then affect the concentration of free radicals that can be generated, which will reduce the curing speed. Various pigments have different absorptivity (transmittance) for different wavelengths of light. The smaller the absorptivity of pigments, the greater the transmittance, and the faster the curing speed of the coating. Carbon black has high ultraviolet absorption capacity, and the curing is the slowest. The white pigment has strong reflectance, which also hinders the 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 dosage, the curing rate of ink film decreases 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. Due to the absorption rate of black to ultraviolet light, the manufacturing process produces the largest structure equivalent to the structure produced by FFF method, making the light transmittance of black ink the lowest, so the change of its dosage has no obvious effect on the curing rate of 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 curing of the surface layer of the ink film and the non curing of the bottom layer, which is easy to produce the phenomenon of "wrinkle"

4. The effect of photoinitiator on aqueous UV curing

the function of photoinitiator is to decompose and produce free radicals after absorbing UV light energy, so as to initiate unsaturated bond polymerization in the system, crosslink and cure into a whole. The performance of photoinitiator is the key to the successful polymerization and curing of aqueous 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 a certain solubility with the water-based UV curing system and low water vapor volatility, so that the photoinitiator can be dispersed, which is conducive to obtain 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. 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, through the right to use a variety of photoinitiators, and adjust the proportion of various photoinitiators to obtain an ink film with fast curing rate and excellent performance. The amount of composite photoinitiator in the system including lightweight automobile chassis, all aluminum body, aluminum alloy wheel hub, various aluminum alloy cast control arms, steering knuckles, axle seats, etc. in the steering system should be appropriate, which is too low to be conducive to the absorption and 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 increase its content, the curing speed will decline

influence of light source, irradiation distance and light curing time

UV light source radiates light in one band, and the energy distribution of each wavelength is different. Among them, the wavelength is nm, the energy distribution of nm light is better, and the effect is 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 consider both the UV absorption characteristics of the pigments contained in the system and the UV absorption characteristics of the initiator

in the drying and curing process of water-based UV ink, the irradiation distance and UV curing time will also affect the UV curing. The closer the irradiation distance is, the stronger the light is, the faster the photoinitiator generates free radicals, the more double bonds 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 light curing of Waterborne UV ink must also have a suitable light curing time. The time is too short, and the curing of the ink is incomplete. When a suitable light curing time is reached, the tensile strength of the cured film reaches the maximum. If the light curing time continues to increase, the tensile strength of the cured film decreases, and the cured resin turns yellow

3 conclusion

at present, water-based UV ink has been successfully developed and applied in some printing. Hu1000 series water-based UV inks of nazdar ink company and Hydra UV water-based UV inks of reseda company have entered the Chinese market. Beijing Yingli technology and Beijing Hongji Printing Co., Ltd. jointly developed Hongying brand water-based UV ink, and Zhongshan Zhongqiao coating adhesive Co., Ltd. also launched its own water-based UV oil series. With the increasing requirements of environmental protection in the printing industry, water-based UV ink will be a hot spot in the application and research of ink in the future

(from the print world)

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