Ink color and UV curing

UV inks were developed as far back as the 1940s, but they have not been developed until the last 20 years. The industry has also become more aware of UV inks. However, due to the higher price of UV inks, the large investment in drying equipment, and the use of a more complex process than conventional inks, these have hindered their wider application.

The trend of modern printing in the direction of rapid, multi-color one-time printing has also made new requirements for inks. Therefore, the advantages of UV inks are even more pronounced. For example, UV inks are not dried on the printing press, but can be quickly dried on printed matter. This not only meets the requirements for continuous printing in the printing factory, but also meets the requirements for high-speed, multi-color printing presses.

Effect of pigments on curing

In order to meet high-end printing needs, it is not enough to rely on three-color inks. Spot inks are required. However, for UV inks, some pigment particles “grab” incident UV radiation energy with photoinitiators or photosensitizers. Thus affecting the latter's absorption of radiant energy, so that the ultraviolet rays irradiating the surface of the UV ink can not be directly transmitted through, but only through multiple reflections and light scattering to bypass the above pigment particles to reach below the ink layer, slowing down the drying speed, Reduces drying efficiency.

Thousands of pigment particles are contained in the UV ink. Therefore, the 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 is not irradiated with ultraviolet light, and the ink cannot be completely cured, resulting in hard internal softness of the ink layer, and the surface is wrinkled due to volume shrinkage during polymerization, which affects the printing quality. At the same time, it should be noted that there is a difference between UV ink curing and UV light oil curing. 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 decline or Problems such as color change.

Pigment color and curing

When using certain yellow pigments in photoinitiating systems 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 show two extremes. White completely reflects light, and black does not reflect light at all.

It is even believed that it may not be possible to allow UV light to pass through a layer that has a certain thickness and is opaque to this light. However, optimizing the curing conditions will make it possible to cure UV inks containing pigments with some problems. It can be solved or improved by selecting the photoinitiator and the pigment according to the absorption characteristics of the pigment to UV light, and can also reduce the influence of the cured pigment by adjusting the UV light source.

Different pigments have different characteristics of light absorption, reflection and scattering, so the degree of hindrance to UV curing is also different. Most pigments have a certain transmissive area in a part of the UV light and visible light range, which is called the pigment. "Spectral window", in which the photoinitiator can fully absorb the UV light radiant energy. Therefore, determining the position of this window is very important for the selection of a suitable photoinitiator. That is, 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 inks, care should be taken to observe the relationship between the absorption spectrum of the pigment and the photoinitiator.

The curing speeds of UV inks of different colors are not the same, because the wavelengths of light reflected by pigment particles of different colors are different, and the closer the reflected wavelength is to the wavelength of UV light, the slower the curing speed is and the required UV light is. The higher the energy, the faster the curing speed and the lower the required UV light energy. Therefore, the UV curing speed must be adjusted for different colors of ink in order to reduce the impact of the pigment on the ink curing.

Normal ink pigments have almost no effect on the drying speed. Different hue inks are composed of different pigments, the same binder and the same additives, but the UV inks are quite different because of the different color of the pigments on the light of different spectral wavelengths. With selective absorption and conditional reflection, the curing speed of the different hue inks will not be the same. In addition, printing needs to be mixed with different colors of ink, that is, color, because any color is the feeling of the stimulation of the mixed wavelength, UV ink is also the case; but also consider the coloring power of the pigment, pigment and other components The possibility of interaction, absorption of UV light, and other issues, if there is a strong absorption, will inevitably reduce the curing speed, and make the curing process more complex, together with the difference in the proportion of pigment, the curing conditions will also It is more difficult to master and must be practiced to find a suitable curing speed. The transmittance of the UV light to the pigment mineral oil suspension having a concentration of 10% is related to the wavelength curve of the UV light. These curves vary with wavelength, but at a given wavelength, usually the magenta color has the highest transmittance, and the other is yellow, cyan, and black, which is the experimental curve of UV light intensity and curing speed. Sort exactly matches.

For UV inks containing a white or black pigment system, because white light is totally reflective of all visible light, it requires a higher energy for curing. On the contrary, although black light 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. In addition, it can totally reflect light having a wavelength of 420 nm. Therefore, the wavelength of reflected light is in the range of 380 to 420 nm. The choice of photoinitiator puts forward higher requirements. When selecting a photoinitiator, the absorption spectrum should be examined so that it can both match the emission spectrum of the UV lamp and enable it to absorb the peak in the wavelength range where the selected pigment is weaker for UV absorption, so as to achieve a more efficient cure. effect. The black pigments absorb in the entire visible spectrum. Therefore, the black ink system is hardest to cure. It is even harder for the pigments with good hiding power and the ink layer thickness of 20 μm or more to be cured. Black will have 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 technical achievements, such as the newly developed acetyl phosphorus photoinitiator, BASF and TPU of Tsinghua Unisplendour Chemical Co., Ltd., and BAPO of Ciba Specialty Chemicals belong to this group. Photo-initiators have a UV-absorbing peak that is longer than conventional photoinitiators and are suitable for pigmented curing systems. Of course, there are still many issues worthy of study and require the joint efforts of people in the industry.

Author: Liu Yongqing

Reprinted from: Printing Technology