Matching UV ink to UV light source
The promotion and use of UV inks has greatly solved the problem of high-quality printing of high-speed printing of thermal printing paper and other special printing materials. However, due to the different properties of the materials themselves, poor drying is still observed when using UV inks in production. why? This is because the matching of the UV ink with the UV source is ignored.
First, UV ink
1. Composition The UV ink is an ink that undergoes cross-linking polymerization under the irradiation of UV light and instantly solidifies into a film. It is mainly composed of a photopolymerizable prepolymer, a photosensitive monomer, a photopolymerization initiator, an organic pigment, and an additive. Among them, the photopolymerization initiator is the most important component of the entire UV ink and is the beginning of photopolymerization. Commonly used photopolymerization initiators include aromatic ketones and acetoin ethers.
2. Features
(1) Instant drying at low temperature; (2) Good gloss and high adhesion; (3) No solvent and good stability; (4) High printability to meet the needs of various substrates; (5) Energy saving and improvement Efficiency; (6) Environmental pollution is small.
3. Curing mechanism Under the irradiation of UV light, the photopolymerization initiator in the UV ink absorbs photons of a certain wavelength and is excited to an excited state to form a radical or an ion. Then, by the transfer of the intermolecular energy, the polymer such as the polymerizable prepolymer and the photosensitive monomer is brought into an excited state to generate a charge transfer complex. These complexes are continuously crosslinked and polymerized to form a film. Its photochemical reaction history can be expressed as: wherein D is a photopolymerization initiator, and A is a photopolymerizable prepolymer and a photosensitive monomer. In the curing process of UV inks, the excitation of the photopolymerization initiator to the excited state is a critical step that determines the efficiency of the entire photochemical reaction. It means that the photopolymerization initiator changes the electron distribution in the molecule under the irradiation of UV light, and the molecule is excited to be an excited state, generating a radical. As we all know, due to the different state of molecular motion, the molecule contains a variety of energies, of which electron energy is the main body, and its energy distribution is not continuous, but a step-like energy level distribution. The change in the state of molecular motion is that the molecule absorbs a certain wavelength of radiation, from low energy level to high energy level, forming an excited state. According to the law of photochemical equivalence, the light absorption of a substance is carried out in units of photons, and the molecule can only absorb one photon at the same instant; and only when the energy of the photon is exactly equal to the difference between the two energy levels of the molecule, the molecule can be absorbed by the molecule. That is to say, the material molecules are selectively absorbed by light. Different UV inks, in which the photopolymerization initiators are different, have different photons to be absorbed, so each UV ink is only sensitive to light of a particular wavelength in the spectrum of the UV source. Moreover, photochemical reactions are related to the number of effective photons. The more effective photons, the greater the probability that the photopolymerization initiator will absorb the photons, and the faster the photochemical reaction speed. Suppose that the total energy of a UV light source radiates light is E, where the energy of light with a frequency of υ is Eυ, and the ratio of Eυ in E is k, then: Eυ=kE in the formula E2-E1=hυ, hυ It is the energy of a photon, so the number of photons with a frequency of υ is N=Eυ/hυ=kE/hυ Visible: N is proportional to k and E. That is, the number of photons is determined by the total energy of the radiation light of the UV light source and the distribution of the light energy of each wavelength. It can be seen from the above that the UV ink selectively absorbs photons of UV light, and its drying is affected by the total energy of the UV light source and the light energy distribution of different wavelengths.
Second, UV light source
1. Constructing a UV light source is a device that emits UV light in a UV curing system. It is usually composed of components such as a light box, a lamp, a mirror, a power supply, a controller, and a cooling device. According to the material contained in the lamp, it can be classified into metal halide lamps, high-pressure mercury lamps and ozone-free lamps. Its performance parameters are: arc length, characteristic spectral power, operating voltage, operating current and average life.
2. Spectral characteristics
Although the UV light source emits mainly UV light, it is not a single wavelength of light, but a light in a band. Different UV sources have different ranges of emitted light, and the distribution of spectral energy in the band is also different. Take the metal halide lamp M08-L41 imported from Japan as an example, its spectral distribution is shown in Figure 1. As can be seen from Fig. 1, the UV light source radiates light in one wavelength band, and the energy distribution of the light of each wavelength is different. Among them, the energy distribution of light having a wavelength of 300 to 310 nm and 360 to 390 nm is relatively high. If the photopolymerization initiator in the UV ink used is selected to absorb light of the partial wavelength, its drying effect will be better. This effect is especially noticeable in large ink quantities. We encountered such a problem in the production of black mark (the black mark requires a large amount of ink): the same UV light source, the same amount of ink, the same working conditions, the use of Japanese imported SPA concentrated UV black ink when the imprint is not dry When the domestic Hanghua UV161 black ink is used, the drying is good. why? I think this is a problem that UV inks match UV light sources.
Third, the matching of UV ink and UV light source
The matching of the UV ink to the UV source is such that the photon that is selectively absorbed by the photopolymerization initiator in the UV ink used is the portion of the UV source spectrum that has the highest energy distribution. For a printing company, it is unrealistic to analyze the spectral characteristics of the UV light source and the photosensitivity of the UV ink on its own, which is unrealistic, which deviates from the production purpose of the enterprise. However, companies should not ignore this problem. The technical department should obtain technical information about the spectral characteristics of the UV source from its equipment supplier, and then select the ink curve from the ink supplier to match the UV ink of the UV source spectrum. In this way, the matching between the two can be better solved, and the preconditions for considering other factors affecting drying are also provided.
Solving the matching between UV ink and UV light source is beneficial to speed up the drying speed of ink, is conducive to improving labor productivity, is conducive to improving the utilization rate of energy, and is beneficial to reducing the production cost of enterprises.
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