Fluorescent ink printing process

Fluorescent ink is divided into colorless and colored fluorescent ink. It is one of the widely used inks in modern packaging anti-counterfeiting printing. It is suitable for packaging and printing of official documents, securities, documents and high-grade cigarettes, wine, drugs, cosmetics and other brand-name goods.

Fluorescent anti-counterfeiting materials are an effective method for counterfeiting banknotes and securities. If inexpensive ultraviolet light pens are used together, they can exert greater effects. The study of fluorescent materials should be carried out in terms of stability, compatibility, red shift of absorption spectrum, etc.; it can also be carried out by changing genes, under ultraviolet irradiation, fluorescence discoloration and other more complex directions to increase confidentiality. Fluorescent anti-counterfeiting technology has also been applied to certain banknotes in China and South Korea.

The main components of fluorescent inks are fluorescent pigments, and fluorescent pigments are functional light-emitting pigments. The difference between these pigments and common pigments is that when irradiated with light (including ultraviolet light), they can absorb certain forms of energy and do not convert into heat energy. Instead, photons are excited, and the absorbed energy is released in the form of low-energy visible light, thereby generating fluorescent phenomena of different hue. Different shades of light combine to form an unusually bright color, and when the light ceases to irradiate, the luminescence disappears, and this is called a fluorescent pigment. Fluorescent inks can be prepared by blending fluorescent pigments with polymeric resin binders, solvents, and additives. The ink or ink prepared by adding 0.5% of the fluorescent substance and auxiliaries can be used and can be fixed on the banknotes and securities by printing methods such as screen printing plate, gravure plate, or offset printing plate or adsorbed by fibers.

(a) Classification of fluorescent pigments

1 Organic fluorescent pigment

This fluorescent pigment is prepared by sufficiently dispersing a fluorescent dye (phosphor) in a transparent, brittle resin (carrier), the color of which is determined by the fluorescent dye molecule. Among them, the daylight type fluorescent pigment emits a high-brightness visible light other than ordinary colors when it is irradiated with sunlight. This is due to the increase in the amount of reflected light, which absorbs ultraviolet rays that cannot be seen by the naked eye and turns the short-wave ultraviolet light into the naked eye. Long wave. The more ultraviolet light content, the stronger the fluorescence that it emits, and through appropriate blending (note the fluorescence quenching) or with a suitable amount of non-fluorescent pigments, a variety of different hue fluorescent pigments can be obtained. In the absence of ultraviolet radiation, the pigment is a general shade pigment that is not fluorescent.

2 inorganic fluorescent pigments

Also known as UV fluorescent pigments. This fluorescent pigment is made of metal (zinc, chromium) sulfides or rare earth oxides combined with trace active agents and calcined. Colorless or light white, it shows various colors of visible light (400nm-800nm) under the irradiation of ultraviolet light (200nm-400nm) depending on the type and content of the metal and the activator in the pigment. According to the different wavelength of excitation light source, it can be divided into short-wave ultraviolet excitation fluorescent pigment (excitation wavelength of 254nm) and long-wave ultraviolet excitation pigment (excitation wavelength of 365nm).

The chemical formula of these pigments is: nZnS(1-n)?CdS: where n is 0.15-1.0 and A(active agent) is copper, silver or manganese, and the amount thereof is about 0.003%-?1.0%.

The difference in characteristics between them is mainly the color of the fluorescence, the brightness, and the color of the pigment itself. If copper is used as an activator, the color can range from dark red to green, which varies with n (from 0.5 to 1). With silver as the activator, the color can range from dark red to dark blue, which varies with n (from 0.15 to 1.0). This red is brighter than the copper-activated red. When manganese is used as an activator, the color of fluorescence is between yellow and orange, and they are not related to n. When the value of n is high, the color of the pigment is still lighter than that activated with copper and silver, and it has yellow fluorescence.

The pigment itself is colorless or light white under sunlight, but it has a wide range of flash colors under the excitation of ultraviolet light.

Method for producing inorganic fluorescent pigment (UV light excitation and coloration)

The method for producing such pigments is as follows:

1 Dissolve zinc oxide or cadmium oxide in dilute sulfuric acid;

2 Purify the solution by sedimentation and then filter;

3 precipitation of zinc sulfide or cadmium sulfide with hydrogen sulfide, filtration;

4 Place the filter cake in the original tank and rinse with water;

5 pressure filter, and rinse it, then dry;

6 mixed with flux salt and activator and dried;

7 placed in the cocoon, calcined;

8 Mix with water, filter, wash;

9 Paste again, surface treatment, drying, sifting.

This is some basic production process. The particle size can be controlled by the type and amount of flux salts (such as sodium chloride, barium chloride, magnesium chloride, etc.), the temperature, and the calcination time (1 to 2 hours) at 1300 to 2000°C. The surface treatment is usually coated with silicate or phosphate substances, which contributes to the stability and quality of the pigment.

Inorganic fluorescent pigment technical indicators:

Light color appearance

Density 4.1-4.6kg/m2

Oil absorption 13~35g/100g

Particle size <10m

Excellent chemical resistance

(b) Fluorescent ink

1, the choice of resin

The fluorescent pigment prepared by the above method is called a fluorescent resin pigment, which is different from fluorescent pigments such as fluorescent yellow, fluorescent orange, etc., which are commercially available. Fluorescent resin pigments have high requirements for the choice of resin. Must have the following characteristics:

(1) has a high softening point;

(2) Good resistance to various chemical properties;

(3) Transparent color, no toxicity;

(4) Easy to synthesize and crush.

As a synthetic resin for a fluorescent dye carrier, polymethacrylates, polyvinyl chloride, alkyd resins, uric acid resins, styrene resins, and copolymers thereof are generally used. As a fluorescent dye, thiobright yellow pigment FF, fluorine is used. Golden light green, bright yellow 6G, Rhodamine B and so on.

(III) Characteristics of Printing Process

Fluorescent inks are suitable for printing on paper and vinyl films. There are two types of paper-based fluorescent inks: oxidative drying and evaporation drying. The fluorescent film for the ethylene film is an evaporation-drying type. Daylight fluorescent pigments typically have a particle size of 3.5 microns for the coatings industry, medium particle sizes for gravure inks and fabric inks, and ultra-fine particle sizes for offset, print, and flexography. In letterpress inks, the smallest particle size can reach 0.25-0.5 microns.

1 ink fluorescence color choice

The fluorescent color of the fluorescent ink is red, yellow and green inks. The wavelengths of the transmitted and red inks of the red and yellow inks have a large difference between the wavelengths of the fluorescent light and the ultraviolet fluorescent light, so the resulting fluorescent color is obvious. When the green ink is printed on a light background or a white bottom, the ink layer can reach a certain thickness. Better fluorescence effect. Therefore, when there is no special color requirement, red and yellow fluorescent inks should be used.

For fluorescent inks, gravure printing, flexographic printing, and screen printing should be used. Daylight-type fluorescent inks should use gravure printing and screen printing due to the poor color rendering effect of embossing and printing. The rheological properties of most fluorescent inks exhibit plastic flow and low viscosity. These characteristics determine the printing process is different from the printing process of common inks.

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