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, due to the high price of UV inks, large investment in drying equipment, and the use of more complicated processes than conventional inks, these have prevented their wider application.
The trend of modern printing in the direction of rapid, multi-color printing at one time also puts forward newer requirements for inks. Therefore, the advantages of UV inks are even more significant. For example, the UV ink does not dry on the printing machine, but can be dried quickly on the printed product, which not only meets the requirements of continuous printing in the printing plant, but also meets the requirements of high-speed, multi-color printing machines.
Effect of pigment on curing
In order to meet the needs of high-end printing, it is not enough to rely on three primary colors inks, spot color inks are needed, but for UV inks, some pigment particles will compete with photoinitiators or photosensitizers for incident UV radiation energy. As a result, the latter's absorption of radiant energy is affected, so that the ultraviolet light irradiated on the surface of the UV ink cannot be directly transmitted through, but can only pass through the multiple reflection and light scattering to bypass the upper pigment particles and reach the bottom of the ink layer, which slows the drying speed. Reduced drying efficiency.
Thousands of pigment particles are contained in UV ink, therefore, 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 with ultraviolet light, and the ink cannot be completely cured, causing the outer layer of the ink layer to be hard and soft, and the surface is wrinkled due to the shrinkage during polymerization, which affects the printing quality. At the same time, it should be noted that the curing of UV ink and UV varnish are different, as shown in Figure 1 and Figure 2. In addition to affecting the curing of UV inks, pigments also affect the viscosity, fluidity, wettability, leveling, etc. of the ink layer; and the interaction between pigments and reactive binders can also cause the ink storage period to decrease or Problems such as color changes.
Pigment color and curing
When certain yellow pigments are used in a photo-initiated system composed of acrylates and amines, the color of the ink will change greatly. Pigments that are more difficult to cure are black, white, phthalocyanine, and yellow pigments. White and black exhibit two extreme phenomena, white reflects light completely, and black does not reflect light at all.
It is even thought that it seems impossible to let UV light pass through a film with a certain thickness and is opaque to it, but to optimize the curing conditions will make good curing of UV inks containing pigments, some problems are It can be solved or improved by selecting photoinitiators and pigments according to the absorption characteristics of pigments to UV light, or by adjusting the UV light source to reduce the impact of curing pigments.
Different pigments have different characteristics of light absorption, reflection and scattering, so the degree of hindering UV curing is also different. Most pigments have a certain transmissive area in the range of part of UV light and visible light. "Spectral window", in which the photoinitiator can fully absorb the UV light radiation energy, therefore, determining the position of the window is very important to select the appropriate photoinitiator. In other words, the photoinitiator should match the pigment in the ink system, and in the wavelength range where the pigment absorbs UV light weakly. Therefore, when using UV inks, care should be taken to carefully observe the relationship between the absorption spectrum of the pigment and the photoinitiator.
The curing speed of UV inks of different colors is different because the wavelengths of light waves reflected by pigment particles of different colors are different. The closer the reflected wavelength is to the wavelength of UV light, the slower the curing speed. The UV light required The higher the energy; conversely, the faster the curing speed, the lower the UV light energy required. Therefore, the UV curing speed must be adjusted for inks of different colors in order to reduce the effect of pigments on the ink curing.
The pigments of ordinary inks hardly affect the drying speed. The inks of different hues are composed of different pigments, the same base and the same additives, but the situation of UV inks is quite different, because different colors of pigments have different spectral wavelengths of light With selective absorption and conditional reflection, the curing speed of inks of different hues will not be the same. In addition, different colors of inks need to be mixed during printing, that is, color matching. Since any color is the result of the stimulation of the mixed wavelength, the same is true of UV inks; and the coloring power of the pigment, between the pigment and other components The possibility of interaction, the absorption of UV light by color, etc., if there is a strong absorption, it will inevitably reduce the curing speed, and make the curing process more complicated, coupled with the difference in pigment ratio, the curing conditions are also It is even more difficult to master, and it is necessary to go through practice to find a suitable curing speed. Fig. 3 shows the relationship between the transmittance of UV light to a pigment mineral oil suspension with a concentration of 10% and the wavelength curve of UV light. These curves vary with different wavelengths, but at a given wavelength, the magenta color usually has the highest transmittance, and the others are yellow, cyan, and black in turn, which is related to the experimental curve of UV light intensity and curing speed. The ordering is fully consistent, as shown in Figure 4.
For UV inks containing a white or black pigment system, white requires a full reflection angle for all visible light, which requires higher energy for curing. On the contrary, black, although theoretically capable of absorbing all light, is due to pigment particles. The blocking effect of light also hinders the deep curing of the ink, so it also requires higher energy, so the curing speed is slower.
The white pigment commonly used in inks is titanium dioxide (TiO2), which effectively absorbs light at a wavelength of about 380nm and can also reflect light at a wavelength of 420nm. Therefore, the reflected light has a wavelength range of 380 ~ 420nm, which causes light The choice of agent puts forward higher requirements. When choosing the photoinitiator, the absorption spectrum should be investigated to make it not only match the emission spectrum of the UV lamp, but also can absorb the peak in the wavelength range where the selected pigment has weaker UV absorption, so as to achieve a multiplier curing effect. The black pigment will absorb in the entire visible spectrum range. Therefore, the black ink system is the most difficult to cure. It is more difficult to cure pigments with better hiding power and films with a black layer thickness of more than 20 μm. Different grades of carbon Black will have a great effect on curing. In addition, the use of special black pigments may also change this situation.
At present, the pigments of UV inks have achieved certain results in technology, such as the newly developed acetylphosphorus oxide photoinitiator, TPO of BASF and Tsinghua Unisplendour Chemical Co., Ltd. and BAPO of Ciba refined belong to this The photoinitiator has an ultraviolet absorption peak that is longer than that of conventional photoinitiators, and is suitable for curing systems with pigments. Of course, there are still many topics worth studying, which require the joint efforts of industry insiders.
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