Abstract The basic concepts and film forming principles of UV-curable inks (UV inks) are reviewed. Comparing the advantages of UV inks with traditional inks, the factors affecting the UV ink curing performance, the methods of use and precautions, and the UV inks are introduced. The market made predictions.
Keywords Ink; UV Curing; Screen Printing
First, basic concepts and principles
UV-curable inks (UV inks) use ultra-violet light-wave sensitization to form and dry the ink. The mechanism of UV ink drying and film formation is a chemical change. Using different ultraviolet spectrums, different energies are generated to polymerize different prepolymers and different monomers into polymers. So the UV ink film has good mechanical and chemical properties.
The types of UV inks include UV frosting, UV ice cream, UV foam, UV wrinkles, UV embossing, UV refraction, UV gemstones, UV light fixing inks, UV varnish, and other specialty packaging inks. On the surface of the substrate with metallic mirror gloss, the UV ink is printed on the screen by the screen printing process. After being treated with ultraviolet light, the device produces a unique visual effect, which is elegant, dignified and luxurious. It is mainly used in high grades. , Exquisite chic tobacco, wine, cosmetics, health products, food, medicine, packaging and printing.
The UV ink is selectively absorbed by the UV light. Its drying is affected by the total energy of the UV light source and the distribution of the light energy at different wavelengths. Under the UV light irradiation, the photopolymerization initiator in the UV ink absorbs a certain wavelength. Photons, excited to the excited state, form free radicals or ions. Then, a polymer such as a polymerizable prepolymer and a photosensitive monomer is brought into an excited state by energy transfer between molecules, and a charge transfer complex is produced. These complexes continuously crosslink and polymerize to form a film.
The UV light source is a device that emits UV light in a UV curing system. Usually consists of light boxes, lamps, mirrors, power supplies, controllers, and cooling devices. According to the different materials filled in the lamp, they can be divided into metal halide lamps, high-pressure mercury lamps, and odorless lamps. Its performance parameters mainly include: arc year, characteristic spectrum, power, operating voltage, operating current and average life.
Although the UV light source emits the main UV light, it is not a single wavelength light but a light in one wavelength band. Different UV light sources emit light in different wavelength bands, and the distribution of spectral energy in the band is also different. The UV light source radiates light in one wavelength band, and the energy distribution of each wavelength light is not the same. The energy distribution of light with a wavelength of 300-310 nm and 360-390 nm is preferable. The UV light source has a wide range of wavelengths, and the effect is best when the wavelength is about 360 nm in the UV-A region.
Second, UV ink and traditional ink composition comparison
The composition of UV inks is similar to that of traditional inks. The difference is that the film formation of UV ink is a chemical action, from monomer to poly cooperation, while the film formation of traditional ink is a physical function, the resin is already a polymer, and the solvent dissolves the solid polymer into a liquid polymer, helping The ink is applied to the substrate, and then the solvent is volatilized or absorbed, allowing the liquid polymer to return to its original solid state. The following is a comparison of the composition of the two inks:
UV Ink Traditional Ink Pigment Pigment Prepolymer Resin Monomer Solvent Additive Additive
1. Pigments Many types of pigments are suitable as UV ink colorants, but they need to be carefully selected. First of all, many pigments promote the natural polymerization of the vehicle in the dark. This natural polymerization does not take place immediately, but takes a long time to occur, thus reducing the UV ink inventory time. In addition, there are differences in the speed of absorption of UV light by different colors, which often affects the polymerization of UV inks. In other words, the speed of printing is affected by the speed of drying, thereby reducing the mechanical and chemical properties of the color film. Furthermore, the acrylate used in the vehicle of the UV ink is not a good wetting agent and tends to make it difficult to grind the pigment, affecting the smoothness and evenness of the color film in the future. To successfully deploy a good UV ink, an ink chemist must not only have experience with pigments, but also have some knowledge of physics and chemistry.
2. Prepolymer prepolymer is a kind of monomer with high molecular weight and high viscosity. It is like the resin in traditional ink and is the main chemical for film formation. It also determines various properties of UV ink, such as mechanical and chemical properties. , physical and so on. The resin used in traditional ink is a compound that has been polymerized, and may be a solid or a liquid; and this prepolymer is a non-polymerized liquid compound that needs to be polymerized by the action of ultraviolet light waves. There are three main categories of prepolymers commonly used: epoxy acrylates, urethane acrylates, and polyester acrylates.
a. Epoxy acrylate epoxy resins and acrylics produce epoxy acrylates. UV inks made with epoxy acrylates dry extremely quickly, have good gloss and chemical resistance, and at the same time, are inexpensive and are very popular with UV ink manufacturers. If used to formulate UV varnish, coated on paper or cardboard, the paper or board can have good chemical and mechanical properties.
After the epoxy acrylate is polymerized by UV light waves, it often exhibits the unique properties of the acrylic resin, and also shows the strong chemical resistance of the epoxy resin. Its greatest feature is poor wettability of pigments and high viscosity. If used alone as a film-forming agent for UV inks, it tends to make the leveling and diffusion of color inks worse.
b. The ester acrylate acts as isocyanate and hydroxyl in acrylic acid to produce the amine ester acrylate. The amine ester acrylate is polymerized by ultraviolet light and has the characteristics of polyurethane resin. The color film has high lightness, high flexibility, strong elasticity, and good adhesion. UV inks composed of amine ester acrylates are particularly suitable for printing various plastics and foils.
c. Poly Acrylic Acid Acrylates Polyester acrylates are obtained when the hydroxyl and acrylic acids in the polyester react. Polyester acrylates have a low molecular weight and therefore have a low viscosity. Its price is very cheap, wet and flexible. Commonly used as a carrier for grinding pigments. It can sometimes be used as a thinner for UV inks to adjust the viscosity of epoxy acrylates and urethane acrylates. It has strong adhesion to non-porous substrates and enamel sheets, aluminum sheets and plastic sheets, and is often used as a printing UV ink for such articles.
Its biggest drawback is its poor chemical resistance. Many alkaline chemicals can erode it. Because the molecular weight is low, the polymerization time is longer, in other words, the drying is slower.
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