Application of anti counterfeiting ink technology in printing and packaging

Anti Counterfeiting Ink is one of the most important technical branches of anti counterfeiting technology, which involves many disciplines, such as optics, chemistry, electromagnetism, computer technology, spectroscopy technology, printing technology, etc., and belongs to an interdisciplinary subject. Among them, chemical anti counterfeiting materials and technologies are the basic technologies of anti counterfeiting ink, which occupy a very important position in research and application. Ink anti counterfeiting technology is a very practical technology in the fields of bill anti counterfeiting, securities and trademark packaging, which can not only meet the requirements of public first-line anti counterfeiting identification, but also meet the requirements of second-line and third-line senior experts and management departments for identification and supervision.

1.the concept and types of anti counterfeiting ink
Anti counterfeiting ink refers to the ink with anti counterfeiting function, that is, a special ink made by adding anti counterfeiting materials with special properties to the ink binder and processing through a special process. It is also composed of three parts: pigment, binder and ink additive. Anti counterfeiting ink relies on the special functions of pigments and binders in ink to play an anti counterfeiting role.
Anti counterfeiting ink can be divided into relief printing, gravure printing, screen printing, offset printing and water-based flexographic Printing Ink according to printing forms. Anti counterfeiting ink can be divided into paper ink, iron printing ink, newsprint ink and Plastic Ink according to different substrates.

According to its anti counterfeiting function, anti counterfeiting ink mainly includes the following types:

(1) heat-sensitive color-changing anti counterfeiting ink, the principle of which is that the pigment adopts substances whose color changes with temperature;

(2) photochromic anti counterfeiting ink, the principle of which is to add photochromic or photoactive compounds to the ink;

(3)humidity-sensitive color-changing anti counterfeiting ink, the principle of which is to add substances whose color changes with humidity to the pigment;

(4)Pressure-sensitive color-changing anti counterfeiting ink, the principle of which is to add pressure-changing compounds or microcapsules into the ink;

(5)UV fluorescent anti counterfeiting ink, the principle of which is to add visible fluorescent compounds excited by ultraviolet light into the ink;

(6)The principle of infrared fluorescent anti counterfeiting ink is to add visible fluorescent compounds excited by infrared rays into the ink.

(7)Anti alteration and anti counterfeiting ink, in which substances with color-developing chemical reaction to chemical substances used for alteration are added.

(8)Visual color-changing anti counterfeiting ink, the pigment adopts multi-layer interference optical broken film.

(9)Magnetic anti counterfeiting ink, its pigment uses magnetic substances, such as iron oxide and cobalt doped in iron oxide. The characteristic of these anti counterfeiting ink is to observe the color changes of ink samples by implementing different external conditions, mainly in the form of light, heat and spectrum detection, so as to achieve the purpose of anti-counterfeiting.

2.the category of anti counterfeiting ink

2.1.Ultraviolet fluorescent anti counterfeiting ink
A special ink that can emit visible light (wavelength 400 ~ 800 μm) under the irradiation of ultraviolet light (wavelength 200 ~ 400 μ m). According to the wavelength of the excitation light source, it can be divided into short-wave ultraviolet light excitation fluorescent anti counterfeiting ink and long-wave ultraviolet light excitation fluorescent anti counterfeiting ink. Generally, it refers to the visible violet anti counterfeiting printing ink excited by short-wave ultraviolet light with an excitation wavelength of 254μm, and the visible fluorescent anti counterfeiting printing ink excited by long-wave ultraviolet light with an excitation wavelength of 365 μ m..
According to the color of printing ink, it can be divided into colored fluorescent ink and colorless fluorescent ink. The former is to add fluorescent compounds into colored ink, while the latter is to add colorless ink. The printed pictures and texts of the former are visible to the naked eye, while the latter is invisible.

The composition of ultraviolet fluorescent ink is mainly fluorescent powder, and there are three kinds of commonly used fluorescent materials according to their composition. (1)Organic fluorescent materials are unsaturated molecules with large conjugates, which are mostly excited by sunlight, and their fluorescence stability is poor and easy to be oxidized and decomposed; (2) Inorganic fluorescent materials synthesized at high temperature have good radiation resistance and high stability, but they are difficult to disperse in oily media, so the synthesis is difficult and the water resistance is poor; (3)Chemically synthesized rare earth organic complexes are fluorescent materials, which are the main materials of anti counterfeiting ink. The commonly used organic rare earth complexes are simple in preparation, easy to disperse, dissolve and refine in oily medium, colorless in visible light and show strong fluorescence effect under ultraviolet excitation.

In recent years, China's ultraviolet fluorescent ink has developed rapidly, and colored (visible) ultraviolet fluorescent ink and colorless (invisible) ultraviolet fluorescent ink have been successfully developed. These fluorescent inks can emit red fluorescence, blue fluorescence, yellow fluorescence and green fluorescence respectively under specific ultraviolet irradiation.

The tax stamp on the invoice is printed with colored ultraviolet fluorescent ink. During the day, the tax stamp on the invoice is red like the seal of the general unit. When the invoice is illuminated with purple light in a dark place, the tax stamp is bright red, while the seal of the general unit is black.

2.2.Heat-sensitive anti counterfeiting ink
Heat-sensitive anti counterfeiting ink is an ink that can change color under the action of temperature. Usually, it is divided into reversible and irreversible thermochromic anti counterfeiting inks, which usually refer to the thermochromic temperature of 34℃ ~ 100℃. According to the different temperature required for color change, it can be divided into hand temperature color change anti counterfeiting ink and high temperature color change anti counterfeiting ink. Hand-temperature color-changing anti counterfeiting ink refers to ink that can change color at 34℃ ~ 36℃; High-temperature color-changing anti counterfeiting ink refers to ink that can change color at 40℃ ~ 100℃ or even higher. To truly meet the requirements for making heat-induced anti counterfeiting ink, the following conditions must be met: high color-changing sensitivity and bright color change in a short time; The discoloration temperature is appropriate, and the discoloration temperature should be controlled at 40℃ ~ 100℃; Good stability, sensitive and effective in different climatic conditions.

Mechanism of reversible discoloration of organic compounds: the reversible discoloration of organic compounds is due to the change of molecular structure with the change of temperature, that is, the movement between electron donor and electron acceptor; Two spatial configurations or two crystal structures are mutually variable; Molecules are heated to open the ring or produce free radicals, which causes the color change of ink.

(2)Irreversible thermochromic compound. Irreversible color-changing compounds refer to compounds whose colors cannot be recovered when heated and cooled. For example, the dichloride of cobalt chloride penta-ammonia complex changes from pink at room temperature to purple at 120℃, sky blue at 170℃ and black above 230℃. When it is cooled to room temperature, it still maintains the color corresponding to the highest temperature. The ink made of thermochromic compounds as pigments is called thermosensitive anti counterfeiting ink, which can also be called thermochromic ink. Thermochromic ink has been widely used in China. It is rapid, accurate and simple to test the temperature with thermochromic ink printed matter, without any special auxiliary instruments and special training for operators.

Heat-sensitive anti counterfeiting ink is easy to detect and suitable for public identification, and is widely used in the packaging of famous brand products, especially cigarette packaging. The disadvantage is that offset printing and relief printing have poor color rendering effect, while screen printing and gravure printing have good effect.

2.3. Photosensitive color-changing anti counterfeiting ink
Under the sunlight, the ink changes from colorless to colored, but it can be restored to colorless anti counterfeiting ink after the light source is removed. On the surface, this kind of ink is discolored by sunlight, but in essence it is discolored by ultraviolet light. At present, several kinds of sunlight-excited color-changing anti counterfeiting inks successfully developed in China are characterized by color-changing effect in sunlight, which can change from colorless to purple, blue, yellow and other colors, and can also be designed to change from colored to colorless.
Photochromic ink has the advantages of convenient identification and good anti counterfeiting performance, and has been widely used. The disadvantage is that offset printing and embossing are poor.

2.4. Magnetic anti counterfeiting ink
Magnetic anti counterfeiting ink is an anti counterfeiting printing ink made of magnetic powder materials (iron oxide and cobalt doped in iron oxide, etc.) as a functional component. It is the most commonly used anti counterfeiting ink. Its outstanding features are dark appearance and simple detection instrument, and it is mostly used for anti counterfeiting of bills and certificates. There are two kinds of magnetic ink: characteristic magnetic ink means that the magnetic powder in the ink contains a specific magnetic signal, while non-characteristic magnetic ink means that the magnetic powder in the ink does not contain a specific magnetic signal, but only has magnetism.
In addition to the letter recognition method of magnetic ink used by banks and postal services, there is also a reader, also known as an optical scanner, which is a device that uses optical principles to recognize letter symbols. Another main purpose is to print magnetic tape strips, shopping cards, meal tickets, magnetic cards, IC cards and visiting cards on credit cards. Magnetic ink used in anti-counterfeiting packaging is mainly printed with magnetic ink at a certain position of the packaging and detected by special means, so as to achieve the purpose of anti counterfeiting. The printing methods are generally flat printing and embossed printing.

2.5. Visual color-changing anti counterfeiting ink
Visual color-changing anti counterfeiting ink is a kind of high-tech security anti counterfeiting ink, which can be printed by traditional engraving intaglio method without complicated or additional testing equipment to distinguish authenticity. It is suitable for banknotes, trademarks, securities, packaging and printing. Visual color-changing anti counterfeiting ink has good concealment and is difficult to decipher. It is a safe ink developed for the rapid development of color reproduction technology, such as laser scanning color separator and color copier.
The principle of visual color-changing anti counterfeiting ink is that when the viewing angle changes by 60 degrees, two completely different colors will be produced. The first color is seen when the viewing angle is 90 degrees. When the viewing angle is twisted by 60 degrees to 30 degrees, the first color is offset and the second color is enhanced.

2.6. Pressure-sensitive anti counterfeiting ink
Pressure-sensitive anti-counterfeiting ink refers to ink that can appear color under the action of pressure or friction. It is made by adding special chemical reagents or color-changing substances to ink. When the colored or invisible pictures and texts printed with this ink are rubbed and pressed by hard objects or tools, chemical pressure discoloration or microcapsule rupture and dye leakage occur, resulting in colors (red, blue, black, green, purple, yellow, etc.). You can choose the displayed color and design the steganography according to the user's requirements.
2.7. Anti-tampering and anti counterfeiting ink
Anti-alteration and anti counterfeiting ink refers to the ink with color-developing chemical reaction to the chemical substances used for alteration. Print the shading of invoices and all valuable securities with anti-alteration ink. When encountering correction liquid , these shading will disappear or change color, so as to find the traces of alteration.
Anti-counterfeiting principle of anti-tampering ink: (1) Pigments that can fade with correction fluid, such as some basic dyes, are printed with the shading of the amount column of securities. (2) using other chemicals that can change color, such as biphenol, to make anti-alteration ink. Checks printed with this ink, such as capital columns, will change color or show the words "invalid" once they encounter correction fluid.

3.Problems needing attention in printing of anti counterfeiting ink
3.1. Many anti counterfeiting inks are colorless inks, that is, they are colorless or white on printed matter. Therefore, before printing such inks, the ink roller, ink tank and ink knife must be cleaned to avoid imprinting of printed matter and affect the anti counterfeiting effect.
3.2. Many anti counterfeiting inks have poor alcohol resistance, such as fluorescent and heat-sensitive inks. Therefore, when using such inks, it is best not to use alcohol dampening.

3.3. Limited by the characteristics of raw materials, many anti counterfeiting inks are worse than ordinary inks in offset printing. Therefore, it is necessary to work harder in printing and strengthen product inspection to prevent ink color differences from affecting product quality and anti-counterfeiting effect.

3.4.Because the direct irradiation of 254nm ultraviolet light will harm people's skin and eyes, therefore, when short-wave fluorescence is used, the detection lamps with protective devices must be used for product detection to avoid affecting health.