There are many factors that affect the curing and drying of Waterborne UV coatings when using UV curing machine. This paper only discusses the main factors. These factors include the following aspects:
1. Effect of pre drying of aqueous system on UV curing
The drying conditions before curing have a great impact on the curing speed. When it is not dry or incomplete, the curing speed is slow, and the gelation rate does not increase significantly with the extension of exposure time. This is because of over packaging. 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. As the system contains a large amount of water, the system is subject to standards and certification when curing 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 consolidation and proofing of the ink film and reducing the curing speed. In addition, the ambient temperature during UV irradiation has a great influence on the curing of UV coatings. The higher the temperature, the better the curing property. Therefore, if preheating is applied, the curing property of the coating will be enhanced and the adhesion will be better.
2. Effect of photoinitiator on Waterborne UV curing
The photoinitiator must have certain miscibility with the water-based 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 for tobacco packaging 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 by using a variety of photoinitiators and adjusting the ratio of various photoinitiators. The content of compound photoinitiator in the system should be developed properly, too low is not conducive to the absorption competition with pigments; Too much light cannot enter the coating smoothly. At the beginning, the curing rate of the coating increases with the increase of the compound photoinitiator, but when the compound photoinitiator dose increases to a certain value, and then increases its content, the curing rate will decrease.
3. Effect of Waterborne UV curing resin on UV curing
The water-based UV curable resin needs free radical light curable flexible packaging, 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 group certification, so that it can be cured under appropriate conditions. Acrylate 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. The reaction activity of various functional groups generally increases 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.
4. Effect of pigments on UV curing of Waterborne Coatings
As a non photosensitive component in Waterborne UV curing coatings, 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 maintenance of the photoinitiator for the light absorption equipment, and then affect the concentration of free radicals that can be generated, which will reduce the curing speed. 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. White pigment has strong reflective property, which also hinders curing. Generally speaking, the absorption order of ultraviolet light is: Black > purple > Blue > cyan > Green > 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 varying degrees. 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 faster than that of the plate, 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.
Post time: Jul-05-2022