Ink color and UV curing
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UV inks were developed as early as the 1940s, but they have only been developed in the last 20 years, and the industry has a closer understanding of UV inks. However, due to the high price of UV inks, large investment in drying equipment, and the use of processes more complicated than conventional inks, these have hindered their wider application.
The trend of modern printing in the direction of fast, multi-color one-time printing has also put forward new requirements for inks. Therefore, the advantages of UV inks are even more remarkable. For example, UV inks do not dry on the press, but can be dried quickly on prints, which meets the requirements of continuous printing by the printing house as well as the requirements of high-speed, multi-color presses.
Effect of pigment on curing
In order to meet the high-end printing needs, it is not enough to rely on the three primary color inks, and it is necessary to use the spot color inks. However, for the UV inks, some of the pigment particles will compete with the photoinitiators or photosensitizers for the incident UV radiation energy. Therefore, the latter absorbs the radiant energy, so that the ultraviolet light irradiated onto the surface of the UV ink cannot be directly transmitted through, and only after multiple reflections and light scattering bypasses the above-mentioned pigment particles to reach below the ink layer, slowing down the drying speed, Reduced drying efficiency.
There are thousands of pigment particles contained in the UV ink, so the UV light must have sufficient strength to reach the bottom of the ink layer. If the light intensity is insufficient, the bottom of the ink layer is not irradiated with ultraviolet light, the ink cannot be completely cured, and the outer layer of the ink layer is hard and soft inside, 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 inks is different from the curing of UV varnishes. 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 the pigment and the reactive binder may also cause a decrease in ink storage period or Problems such as changes in color.
Pigment color and curing
When certain yellow pigments are used in a photoinitiator system composed of acrylates and amines, the color of the ink changes greatly. The more difficult to cure pigments are black, white, phthalocyanine and yellow pigments. White and black exhibit two extreme phenomena, white completely reflects light, and black does not reflect light at all.
It is even thought that it may not be possible to pass UV light through a film layer having a certain thickness and being opaque to the light, but optimizing the curing conditions will enable good curing of the pigment-containing UV ink. Some problems are It can be solved or improved by selecting the photoinitiator and pigment according to the absorption characteristics of the pigment by UV light, and the effect of the curing pigment can also be reduced by adjusting the UV light source.
Different pigments have different characteristics of absorption, reflection and scattering of light, so the degree of hindrance to UV curing is also different. Most pigments have a certain transmissive area in the range of part of UV light and visible light, which is called "the pigment". The spectral window" at which the photoinitiator can sufficiently absorb the radiant energy of the UV light. Therefore, determining the position of the window is very important for selecting a suitable photoinitiator. That is, the photoinitiator should match the pigment in the ink system and be within the wavelength range in which the pigment absorbs less UV light. Therefore, care should be taken when using UV inks to carefully observe the relationship between the pigment and the absorption spectrum of the photoinitiator.
The curing speeds of UV inks of different colors are different, because the wavelengths of light waves reflected by pigment particles of different colors are different, and the closer the wavelength of reflection is to the wavelength of UV light, the slower the curing speed, the required UV light. The higher the energy; conversely, the faster the cure, the lower the UV light energy required. Therefore, the UV curing speed must be adjusted for different color inks to reduce the effect of pigment on ink curing.
The pigment of ordinary ink hardly affects the drying speed. The inks of different hue are composed of different pigments, the same base material and the same auxiliary agent, but the situation of UV ink is completely different, because the pigments of different colors have different wavelengths of light. With selective absorption and conditional reflection, the curing speed of different hue inks will not be the same. In addition, printing needs to mix different colors of ink, that is, color matching, because any color is the result of stimulation of mixed wavelength, so is the UV ink; also consider the tinting strength of the pigment, between the pigment and other components. The possibility of interaction, the absorption of UV light by the color, etc., if there is strong absorption, it will inevitably reduce the curing speed, and make the curing process more complicated, plus the difference in pigment ratio, the curing conditions are also It is even more difficult to master, and it is necessary to practice and find a suitable curing speed. The relationship between the transmittance of UV light to a pigment mineral oil suspension having a concentration of 10% and the wavelength curve of UV light. These curves vary with wavelength, but at a given wavelength, the magenta color usually has the highest transmittance, and the others are yellow, cyan, and black, which are experimental curves of UV light intensity and cure speed. The sorting exactly matches.
For UV inks containing a white or black pigment system, since white can totally reflect the angle of all visible light, higher energy is required for curing. On the contrary, although black can theoretically absorb all light, it 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 380 nm, and can also totally reflect light having a wavelength of 420 nm. Therefore, the wavelength of the reflected light ranges from 380 to 420 nm, which is light-induced. The choice of agent raises higher requirements. When selecting a photoinitiator, the absorption spectrum should be investigated so that it can match the emission spectrum of the UV lamp, and it can absorb the peak in the wavelength range where the selected pigment has weak UV absorption, so that the curing can be achieved with half the effort. effect. Black pigments absorb in the entire visible spectrum. Therefore, the black ink system is the most difficult to cure. It is more difficult for the pigments with better hiding power and the black layer to have a thickness of 20μm or more. Different grades of carbon Black has a great effect on curing. In addition, the use of special black pigments may also change this situation.
Now, UV ink pigments have achieved certain results in technology, such as the newly developed acetylphosphorus oxide photoinitiator, BASF and Tsinghua Unisplendour's TPO and Ciba refined BAPO type belong to this. A photoinitiator, whose ultraviolet absorption peak is longer than a conventional photoinitiator, and is suitable for a pigmented curing system. Of course, there are still many topics worth studying, which require the joint efforts of people in the industry.