The latest water-based UV curable screen printing

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New water-based UV curable silk printing ink (Part 2)

v. raw materials of UV curable water-based ink

1. UV curable resin

UV curable water-based ink is generally composed of base resin, photoinitiator, additive and water. The base resin is the main component of the ink system. The performance of the final curing film of the ink is mainly determined by the base resin. It is in the "4+2" modern industrial system that Jiangbei new area focuses on. Commonly used resins include unsaturated polyester, polyurethane acrylate, acrylate polyacrylate, polyester acrylate

unsaturated polyester used in UV curable water-based inks is prepared by esterification of diols or polyols with maleic anhydride. In order to make the resin hydrophilic, diols (such as polyethylene glycol) can be used to introduce hydrophilic structures, and photoactivity can be obtained by capping with acids or alcohols containing photoactive groups

polyurethane acrylate resin is the most studied system at present. The difference from general polyurethane acrylates is only how to introduce hydrophilic structure into the molecule. Similar to the above unsaturated polyester, the hydrophilic structure can generally be introduced by diol units. For example, polyethylene glycol static torsion tester can be used to introduce alcohol into non-ionic hydrophilic chain segments, which is composed of host, control and measurement system; Using dimethylol propionic acid (DMPA) to introduce carboxyl group, an ionic self emulsifying resin was obtained. The photoactive group can be introduced by the reaction of hydroxyl group of hydroxyethyl (meth) acrylate (HEA or HEMA) with polyurethane terminal isocyanate group (-nco), or by the esterification reaction of acrylic acid with hydroxyl group of hydroxyl terminated polyurethane

acrylated polyacrylic acid has the advantages of low price, easy preparation, plump ink film, good gloss and so on. Generally, hydrophilic carboxyl groups can be introduced by copolymerization of acrylic acid with various acrylates, and hydroxyl or epoxy groups can be introduced by copolymerization of hydroxyethyl (meth) acrylate or glycidyl (meth) acrylate, so as to further introduce oxy acrylate, so as to obtain photoactivity

2. Photoinitiators

in addition to the requirements of photoinitiators used in oil-based systems in terms of absorption properties, no yellowing, etc., photoinitiators used in water-based systems must also have 2.4 good anti foam performance and requirements of compatibility with aqueous environment and low volatility, so as to ensure that the required concentration can be maintained when the ink film is pre dried (generally at 60 ~ 80 ℃). At present, the most commonly used photoinitiator is darocur 2959. Because its molecule has one more hydroxyethoxy group than darocur1173, its solubility in water is relatively large (the solubility of the parent darocur1173 is only 0.1%), so it is easier to volatilize during pre drying

VI. wettability of Waterborne UV curing ink on the substrate

in order to ensure the wettability of Waterborne UV curing ink on the substrate material, on the one hand, the surface energy of the plastic surface should be increased through surface modification, on the other hand, the surface tension of the ink should be reduced as much as possible when designing the ink formula. For general UV curing inks, the surface energy is about 35 ~ 38 Mn/m, which is obviously too high for polyolefin materials with low surface energy and untreated surface, which will lead to printing failure. The surface energy of Waterborne UV curing ink is much larger than that of general UV curing ink. At this time, monomers with low surface tension should be added to the Waterborne UV curing ink, and the plastic surface should be modified to ensure the wettability of the ink on the plastic surface

VII. Surface treatment and electrostatic control of plastics

for some plastic substrates with large surface energy, ideal adhesion can be obtained by adjusting the formula of UV curing ink and improving the curing process conditions. However, some plastic products and plastic films (such as PE and PP plastics) have small surface polarity and low surface energy, so they will encounter the problem of poor ink adhesion during printing, and the printing effect and adhesion fastness are difficult to meet the requirements, affecting the printing quality. Only through surface treatment can we improve the above problems and improve the printing quality

there are many methods to treat the plastic surface:

▲ mechanical method: sandblasting and sanding

▲ physical method: flame, corona, high-energy radiation, etc

▲ chemical method: surface oxidation, grafting, replacement and crosslinking, etc

during surface treatment, appropriate methods should be selected according to different plastics and process conditions, such as polyolefin PE, PP and other non-polar plastics. Generally, flame and corona treatment methods should be used to improve the surface energy, and nylon can be treated with phosphoric acid. The following describes various surface treatment methods:

1. flame and corona (spark) treatment. Flame and corona treatment are two better prepress treatment processes, which can greatly improve the surface energy of plastic sheets and form a very thin oxide layer on the surface

attention should be paid to proper flame and corona treatment to avoid "burning" the surface and forming too thick oxide layer, which will cause the ink layer and oxide layer to fall off after printing. Generally, during flame treatment, heat the plastic to a temperature slightly lower than the thermal deformation temperature and keep it for a certain time. These two methods can temporarily improve the surface energy of plastic. After treatment, the printing operation must be completed within 20min, otherwise the treatment effect will decline quickly

2. Degreasing treatment. The surface of plastic products stained with oil or film remover will affect the adhesion of ink. It can be cleaned by alkaline aqueous solution, surfactant and solvent to achieve the purpose of surface degreasing and cleaning. If the surface is rough by sanding, it can also play the role of degreasing

most plastics with good adhesion to screen printing ink, such as polystyrene, can be degreased. When degreasing, solvents that will not dissolve plastic should be used, such as methanol, ethanol, isopropanol, etc. Polyamide plastic can also be wiped with acetone

3. Chemical surface treatment. Solvent vapor and chemicals are used to treat the plastic surface, so that the smooth plastic surface can be corroded into a controllable uneven surface. At the same time, hydrophilic functional groups or other groups can be oxidized and introduced, so that the amorphous area of the plastic surface can be dissolved to form a rough surface

solvent steam treatment. When polyethylene and polypropylene plastic parts are treated in the steam of hot solvents (such as toluene and trichloroethylene) for 10 ~ 20s, the fixation fastness of the ink can be significantly improved

chemical oxidation method. After 90g ammonium persulfate, 0.6g silver sulfate and 1000g distilled water are prepared into a solution, polypropylene and polyethylene plastic products are placed in it and treated at room temperature for more than 20min, or at 70 ℃ for 5min. This method is applicable to hard plastic products and soft plastics. Acid treatment. For PE and PP plastic products with complex shapes, acid etching method can be used for prepress treatment, and the mixed acid of chromic acid and sulfuric acid is used for treatment at 50 ℃ for 10min

4. ultraviolet or plasma irradiation method. The surface energy of polyethylene and polypropylene plastic can be significantly improved by scrubbing with trichloroethylene and then irradiating with ultraviolet light or plasma. The fixation fastness of silk screen printing ink on it can be increased by more than 5 times, which is suitable for soft plastic products and hard plastic products. This treatment method mainly uses high-energy rays to irradiate the plastic surface. After the oxidation reaction, polar groups are generated on the surface to improve the adhesion of the ink. If the speed of data collection is not high enough

5. Electrostatic control in plastic printing

it is best to install humidity and temperature control equipment in the printing workshop, which can not only effectively control static electricity, but also better ensure the printing quality

because plastic is a very good insulating material, static electricity will affect the printing quality of plastic during the printing process. The printing failure caused by static electricity mainly includes the following manifestations:

(1) the printing materials are pasted together and cannot be separated

(2) the printing materials cannot be stacked together or can hardly be stacked

(3) the printing material is pasted on the bottom of the silk plate

(4) the material cannot be fed normally on the full-automatic printing machine

(5) the operator was shocked

an important characteristic of plastic is that it has good insulation performance, so it is easy to carry static electricity and is not easy to eliminate, resulting in printing failure. Therefore, the problem of removing static electricity should be considered in plastic printing. At present, the commonly used method is to remove static electricity with static electricity remover

source: printing industry

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