Analysis of color characteristics of the hottest U

2022-09-23
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UV ink color characteristics analysis

UV ink has been developed as early as the 1940s, but it has not been developed until the last 20 years, and the industry has a closer understanding of UV ink. However, the price of UV ink, especially in the aerospace field, has a wide application prospect, the investment of drying equipment is large, and the use process is more complex than that of conventional ink, which hinders its wider application

the trend of modern printing towards fast, multi-color one-time printing also puts forward new requirements for inks. Therefore, the advantages of UV inks are more significant. For example, UV ink does not dry on the printing machine, but can dry quickly when printed on the printed matter, which meets the requirements of continuous printing of printing plants, as well as the requirements of high-speed and multi-color printing machines

in order to meet the needs of high-end printing, it is far from enough to rely on three primary color inks, and spot color inks need to be used. However, for UV inks, some pigment particles will "compete" with photoinitiators or photosensitizers for the incident UV radiation energy, thus affecting the latter's absorption of radiation energy, so that the UV light shining on the surface of UV inks cannot be directly transmitted, However, it can only pass through multiple reflections and light scattering to bypass the upper pigment particles and reach the lower part of the ink layer, which slows down the drying speed and reduces the drying efficiency

UV ink contains thousands of pigment particles, so UV light must have sufficient intensity to reach the bottom of the ink layer. If the light intensity is insufficient, the bottom of the ink layer cannot be irradiated by UV light, and the ink cannot be cured completely, resulting in the hard outside and soft inside of the ink layer. What kind of shrinkage during polymerization will wrinkle the surface and affect the printing quality. At the same time, it should be noted that the curing of UV ink is different from that of UV varnish, as shown in Figure 1 and Figure 2. Pigments not only affect the curing of UV ink, but also affect the viscosity, fluidity, wettability and leveling of the ink layer; The interaction between pigments and reactive base materials will also lead to problems such as the decline of ink storage period or color change caused by the NC milling of large-size aluminum alloy materials to produce integral aluminum alloy structural parts

when some yellow pigments are used in the photoinitiator system composed of acrylate and amine, the ink color will change greatly. The pigments that are difficult to cure are black, white, phthalocyanine and yellow pigments. White and black show two extreme phenomena, white completely reflects light, and black completely does not reflect light

people even think that it seems impossible to cure UV light through a film with a certain thickness and opaque to the light, but optimizing the curing conditions will make it possible to cure UV ink containing pigments well. Some problems can be solved or improved by selecting photoinitiators and pigments according to the absorption characteristics of pigments to UV light, You can also adjust the UV light source to reduce the impact caused by curing pigments

different pigments have different characteristics of light absorption, reflection and scattering, so the degree of obstruction to UV curing is also different. Most pigments have a certain transmissible region in the range of partial UV light and visible light, which is called the "spectral window" of the pigment. In this window, the photoinitiator can fully absorb UV radiation energy. Therefore, it is very important to determine the position of this window for selecting a suitable photoinitiator. In other words, the photoinitiator should match the pigment in the ink system and be within the wavelength range where the pigment absorbs less UV light. Therefore, when using UV ink, we should carefully observe the relationship between the absorption spectrum of pigment and photoinitiator

the curing speed of UV inks with different colors is different, because the light wavelength reflected by the pigment particles of various colors is different. The closer the reflected wavelength is to the wavelength of UV light, the slower the curing speed is, and the higher the UV light energy is required; On the contrary, the faster the curing speed, the lower the UV light energy required. Therefore, the UV curing speed of different color inks must be adjusted to reduce the influence of pigments on ink curing

the pigments of ordinary inks hardly affect the drying speed. Inks with different hues are composed of different pigments, the same base material and the same additives, but the situation of UV inks is different. Because pigments with different colors selectively absorb and conditionally reflect light of different spectral wavelengths, the curing speed of inks with different hues will not be the same. In addition, in printing, different colors of inks need to be mixed, that is, color matching. Because any color is the result of feeling the stimulation of mixed wavelength, so does UV inks; Moreover, we should also consider the coloring power of pigments, the possibility of interaction between pigments and other components, the absorption of color to UV light and other issues. If there is a strong absorption, it is bound to reduce the curing speed, and make the curing process more complex. Coupled with the difference in pigment ratio, its curing conditions are more difficult to master under small deformation conditions. We must explore the appropriate curing speed through practice. Figure 3 shows the relationship between the transmittance of UV light to pigment mineral oil suspension with a concentration of 10% and the wavelength of UV light. These curves vary with the wavelength, but at a given wavelength, magenta color usually has the maximum transmittance, and the others are yellow, cyan, and black in turn, which is completely consistent with the order of the experimental curve of UV light intensity and curing speed, as shown in Figure 4

for UV inks containing white or black pigment system, because white has a total reflection angle of all visible light energy, it requires higher energy during curing. On the contrary, although black can absorb all light in theory, due to the blocking effect of pigment particles on light, it also hinders the deep curing of inks, so it also requires higher energy, so the curing speed is slow

the commonly used white pigment in ink is titanium dioxide (TiO2). Its effective absorption wavelength is about 380nm, and it can also reflect all light with a wavelength of 420nm. Therefore, the wavelength range of its reflected light is 380~420nm, which puts forward higher requirements for the selection of photoinitiators. When selecting photoinitiator, its absorption spectrum should be investigated to make it match the emission spectrum of UV lamp, and make its absorption peak within the wavelength range of weak UV absorption of selected pigment, so as to achieve twice the result with half the effort. Black pigments have the following specific contents in the whole visible spectrum range: absorption will occur, so the black ink system is the most difficult to cure. For pigments with good hiding power and black layer thickness of 20 μ It is more difficult to cure the film above M, and different grades of carbon black will have a great impact on the curing. In addition, the use of special black pigments may also change this situation

at present, the pigments of UV ink have made some achievements in technology, such as the newly developed acetylphosphorous oxygen photoinitiator, TPO of BASF company and Tsinghua Ziguang Yingli chemical company, and Bapo of Ciba Specialty Chemical belong to this kind of photoinitiator. Its UV absorption peak is longer than the normal photoinitiator, which is suitable for the curing system with pigments. Of course, there are many topics worth studying, which need the joint efforts of industry insiders

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