Application of Fluorescent Ink in Screen Printing (I)

Fluorescent ink is one of the widely used inks in modern security printing. It is suitable for packaging and printing of documents, securities, documents, and advanced cigarettes, wines, medicines and cosmetics. Fluorescent anti-counterfeiting technology has also been applied to certain banknotes in China and South Korea. The appearance of fluorescent ink has made a great contribution to the development of screen printing and process reform.

Overview

The reason why fluorescent inks can emit light is mainly because their main components are fluorescent pigments, and fluorescent pigments are functional light-emitting pigments. The substance stores the absorbed sunlight energy and then expresses it in the form of heat, or produces a photochemical reaction, or emits it in the form of visible light. The absorbed radiant energy is shorter than the emitted energy wavelength. The phenomenon of absorbing light of different colors and then emitting light of different wavelengths is called fluorescence; the phenomenon of emitting light in the dark environment after absorbing light is called phosphorescence. The difference between the two is that the phosphor absorbs natural light or artificial light and emits light in a dark environment after the light is removed; and the fluorescent light must absorb light in the presence of light before it can emit light. The fluorescent ink can also emit light in the presence of noctilucent or violet light, but does not emit light in the dark and without light. The fluorescent ink not only exhibits its own color, but also has the ability to absorb light and change the light intensity so that the color it exhibits has a stronger light intensity, but the hue (or color) does not change. Fluorescent pigments and macromolecular resin binders, solvents and additives are compounded and ground to obtain fluorescent inks, and can be used for printing methods such as screen printing, gravure printing, or offset printing.

First, the principle of fluorescence and light storage and the classification of fluorescent pigments

1. The relationship between light and material color

When a beam of light interacts with a partially transparent substance, reflection, absorption and transmission occur mainly.

· Reflection

A part of the incident light can usually have two reflections in the same place: If the surface is very smooth, then a specular reflection occurs. The incident angle is equal to the reflection angle; if the surface has a certain degree of roughness, some diffusely reflected light will be scattered from the surface. This diffusely reflected light comes from the closer interaction of the same object and can exhibit a stronger color than the specular reflection. As a result, for a high-gloss ink film, when looking at light, it seems to lose its color in the display of light. Some substances produce a sense of color through light reflection. It includes various pigments, metal powders, pearlescent pigments, and the like. If glass beads are added to the paint to control the reflection of light, retroreflective inks can be made. The liquid crystal display ink uses color changes due to external conditions such as temperature and electric field.

· Absorption

Non-reflecting or non-scattering light enters the object. Some of the absorbed light is converted into heat energy. Some light is scattered inside the object. Some substances make the absorbed light energy re-radiate in the visible light range. It is fluorescent light storage. This fluorescent process will have a color. Some light may have internal reflections, while the remaining light is transmitted.

· Transmission

When the light beam enters or leaves the transparent object, there is often some degree of deflection, which is controlled by the difference in refractive index between the two media, and also varies with the angle of incidence. When light passes through a substance, the velocity decreases, the number of which depends on the nature of the substance and the wavelength (that is, the color), which results in dispersion in the prism.

2. Classification of fluorescent pigments

The fluorescent pigments can generally be classified into inorganic fluorescent pigments and organic fluorescent pigments according to the molecular structure.

· Organic fluorescent pigments

Also known as eye-type fluorescent pigments. The fluorescent pigment is prepared by sufficiently dispersing a fluorescent dye (phosphor) in a colorless, transparent and insoluble resin, and the color is determined by a fluorescent dye molecule. When sunlight strikes, it emits a high-brightness visible light other than the normal color because its amount of reflected light increases, absorbing ultraviolet rays that cannot be seen by the naked eye in the sunlight, and converting the short-wave ultraviolet light into a visible long wave. The more ultraviolet light content, the stronger the fluorescence that it emits, and can obtain different color tone fluorescent pigments by appropriate proportioning or matching with appropriate non-fluorescent pigments. In the absence of ultraviolet radiation, the pigment is a general shade pigment that is not fluorescent.

· Inorganic fluorescent pigment

Also known as UV fluorescent pigments. The fluorescent pigment is made of metal (zinc, chromium) sulfides or rare earth oxides combined with trace active agents and calcined. Colorless or light white, but under the irradiation of ultraviolet light, depending on the type and content of the metal and the activator in the pigment, a spectrum of various colors appears.

Second, the difference between fluorescent ink and ordinary ink

Ordinary inks or dyes display colors by reflected light, and each ink color has a different wavelength and output energy. For example, the red light reflects red and absorbs other colors. Fluorescent red not only has partially reflective red, but also converts other colors (green, blue, and purple) into red wavelengths that emit as red light, so the color appears to radiate from the interior of the object. It is this luminescent property that makes the ink known as fluorescent ink. Printers use fluorescent inks for printing to obtain four times the brightness and visibility of ordinary inks. Fluorescent inks are very bright in dim light, such as dusk and dawn.

The reflection characteristics of ordinary ink color and fluorescent ink color are different, and the unit of measurement is Angstrom (1 Angstrom = 10 "G1"). Fluorescent ink converts short-wavelength light into long-wavelength light and superimposes to generate fluorescence, which forms a through conversion and superposition. This kind of fluorescent color has special brightness and visibility.Usually, the daylight fluorescent color indicates that the life time is 30 days directly under the sunlight, but now the life of the color in the room has been greatly improved.The color can be extended indoors to Indefinitely, it can be used to express vivid colors and jumping colors.At present, fluorescent inks have the advantages of fine background, fast drying, and ease of use for thinner ink layers.