UV-curable inks use ultraviolet light-wave sensitization to form and dry the ink. The mechanism of UV ink drying and film formation is to use different UV spectra to generate different energy to polymerize different prepolymers and different monomers into polymers. So the UV 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. Among them, the energy distribution of light having a wavelength of 300 to 310 nm and 360 to 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.
The composition of UV inks is similar to that of traditional inks, but there are also many differences. The following is a comparison of the composition of the two inks:
1. Pigments: Many types of pigments are suitable as UV ink colorants, but you still need to pay attention when choosing. 1, many of the pigments in the dark will promote the natural polymerization of the vehicle, this natural polymerization is not carried out immediately, but it takes a long time to occur, it will make the UV ink inventory time shortened. 2. Different colors absorb UV light waves with different speeds, which often affect the polymerization of UV inks. 3, The acrylate used in the vehicle of UV ink is not a good wetting agent, and it often makes the pigment grinding more difficult, affecting the smoothness and evenness of the color film in the future.
2. Prepolymer: Prepolymer is a kind of high molecular weight and high viscosity monomer. It is like the resin in traditional ink. It is the main chemical for film formation and also determines various properties of UV ink. The resin used in traditional inks is an already polymerized compound, which may be solid or liquid; and this prepolymer is an unpolymerized liquid compound that needs to be polymerized by the action of ultraviolet light waves. There are three commonly used prepolymers: epoxy acrylates, amine ester acrylates, and polyester acrylates.
Epoxy acrylate - is produced by the action of epoxy resin and acrylic acid. UV inks made with epoxy acrylates dry extremely quickly, have good gloss, strong chemical resistance, and low cost, and are very popular with UV ink manufacturers. 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.
Amine ester acrylates are produced by the action of isocyanate and hydroxyl in acrylic acid. 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.
Polyacetate Acrylate - is produced by the action of hydroxyl and acrylic acid in polyester. The polyester acrylate has a low molecular weight, a low viscosity, a low price, good wettability, and high flexibility. 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.
In general, UV-drying inks have greater wear resistance than traditional solvent-based or epoxy-based inks. Their abrasion resistance is 1300% of the wear resistance of traditional inks, and its drying speed is also higher than traditional inks. Has greatly improved, from the original 7min to 15s, saving 96% of the time. UV inks also have extremely high solvent resistance after drying. This solvent resistance results in part from the high crosslink density (macromolecule weight) of the crosslinked dry ink layer. The higher degree of cross-linking makes it impossible for the molecules to separate and dissolve from each other under the effect of the solvent. In addition, UV inks can also be used with some solvent-resistant chemicals.
Let's take a look at the situation after replacing epoxy inks with UV inks. First, do not mix the ink before use. Second, of course, it is time-saving because the epoxy-based ink needs to be dried for a long time under a certain high temperature condition, and the UV ink is almost instantaneously dried. This is accompanied by the additional advantage of reducing the damage of the ink layer caused by heating. UV inks also save a lot of dry space.
There are currently three main factors that hinder the widespread use of UV technology.
One is that UV inks are harmful to the human body. Therefore, the safety of UV-curable inks must be given sufficient attention. There are two main hazards: the harm caused by radiation to the human body and the harm caused by the ink to the human body.
The second is insufficient understanding of the tremendous achievements made in the field of UV ink chemistry. A research report shows that the use of UV inks can increase productivity and work efficiency, reduce the rate of ink scrap, and greatly save the cost of ink. The result proves that it is cost-effective to purchase new curing equipment.
Third, UV inks have disadvantages such as poor layer performance and creases.
The use of UV-curable inks and coatings began in the 1970s and currently holds a large market share in the printing industry, including applications in lithography, styrene printing, and letterpress printing. It is predicted that by 2005, the market for UV-curable inks will increase from US$1.1 billion in 2001 to US$1.5 billion. Of course, the extent of growth depends on the development of the branch market.
For the moment, web offsets account for the largest share of the market. They can use UV-curable inks in the production of direct mail, business forms, folding cartons and credit cards. Narrow flexo printing has been used in a large number of UV inks and coatings. In packaging printing, the application of UV inks and coatings is growing, both in offset printing and flexo printing.
Energy-curing products are entering every corner of the market and are of great economic importance. Electron beam EB curing market share of about 30 million to 50 million US dollars, accounting for almost 13% of the entire energy curing market. Compared with UV curing drying systems, EB equipment is expensive and covers an area that is relatively large, so the growth rate is relatively slow. Although the price of equipment has been reduced, the chemical materials used are still relatively expensive.