Design principle of optical color-changing anti-counterfeiting ink
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With the large number of high-security documents such as checks, banknotes, passports, and credit cards being forged, the industry has been looking for new technologies in the optical field that can successfully protect files from being copied. Optically variable inks have this feature that successfully protects high security documents from being falsified. Today, in terms of passports or credit cards, more than 50 countries in the world use anti-counterfeiting technology with color corners as a means of counterfeiting to achieve anti-counterfeiting purposes.
First, what is optical color changing ink
The optically variable ink is a printing ink containing a high-precision interference filter in a micro structure and displaying two different colors by different angles .
The color-shifting properties characteristic of optical color-changing inks have been chosen by more than 50 countries for currency security, which exhibits different color variations at viewing angles of 900 and 300, thus ensuring that colors are not copied.
This high performance interference filter for use on optically variable inks is produced by precision vacuum deposition on roller coating equipment. This material must be able to achieve its maximum optical properties to achieve the desired anti-counterfeiting effect, mica or through the wet quartz raw material has this property.
The optical properties of pigment particles make it possible to produce strong color transition properties (colors produced by physical phenomena). Pigment particles are only about 1 μm thick and 20 μm wide. The principles of reflection, absorption, incidence and interference are generally shown in.
It can be seen that we only use physical phenomena to analyze the ink when it is accurate to each layer of the material. Optically variable inks are based on interference color filters fabricated in vacuum memories to exhibit this color transition characteristic of the ink, but the choice of such color transition materials is confusing. In general, synthetic metal media performs much better than full media because of its high opacity and high cost performance.
Second, the feasibility of the design
Let's first distinguish between the following three options:
Pigment design (interference filter);
Ink design (chemical method);
Document design (graphics and tables);
1. Pigment design
This vacuum storage method provides a lot of room for the possibility of pigment design. It is achieved by changing the thickness of the materials used in each layer and the order between the different layers. This design shows that the ink is changed in color angle by superposition of metal, medium, metal, medium and metal. In this way, the ink can get the highest chroma, saturation, hue and brilliant colors. To give a simple example, Figure 3 below shows the design of a five-layer pigment, each layer having its own specific task to achieve.
1.1 reflector. It may consist of highly reflective materials such as aluminum, nickel, chrome, iron, silver or gold, which act like a mirror, giving the reflector a uniform color surface. We can choose the material that really meets the requirements through cost-effective, high reflectivity and chemical resistance. Obviously, this macroscopic characteristic of the material will affect the final conductivity, magnetic properties, etc. of the pigment particles, as well as the ink holding capacity.
1.2 spaced devices. The spacer material is roughly classified into an inorganic material or an organic material, and in order to exert the maximum performance of the material, the material must have the lowest optical standard. It has been experimentally proved that a material having a refractive index of <1.6 is most suitable, such as silica, magnesium dioxide or plexiglass.
The separation distance and optical thickness between the absorbing layer and the reflective layer determine the color characteristics at a certain angle, which may be any color from a certain wavelength region between the UV light and the nearest infrared ray. We must undoubtedly consider the characteristics of the visible band outside of electromagnetic radiation, which allows the "user" to be applied to high-security documents (during classification and processing).
An example of the above case is the suppression at the near infrared angle. This concept allows us to design a pigment that is visible at 90° near-infrared and not visible at other angles (Figure 4). This unique feature can be easily distinguished by inspection equipment.
The absorbing material and its thickness determine the primary properties of the color at an angle of 90°, such as chemical resistance and natural visible properties such as interaction with the ink, contact, separation, and deposition on fluid inks. To date, chromium, nickel, iron and their oxides or alloys have been successfully used in the design of such pigments. Today, optical color-changing inks have become the latest industry standard with their powerful color transition characteristics and magnetic properties.
2. Ink design
High security documents Traditional printing processes use offset, gravure and letterpress printing techniques. Interference Pigment Particles Unless we explicitly indicate the use of offset printing, our first consideration is to apply the ink system to the gravure printing process.
Gravure or stencil printing is characterized by the transfer of ink from the engraved plate to a depth of 70-150 μm under high printing pressure. The printing speed of the banknote is about 2 m/s. The remaining ink is removed from the engraving plate using a plastic roller or paper, and the thickness of the transfer ink layer is about 30-50 μm, which can be instantaneously cured by hot air, or at the beginning. Oxidative drying is carried out by the mutual squeezing action of the paper within 24 hours.
The ideal optical color-changing pigment is an artificial pigment containing a curved product. In order to enable it to be transferred to a substrate at one time (gravure printing), we need a low viscosity ink medium. Conversely, in the process of intaglio, a bonding system of high viscosity and strong hydrophilic properties is sought.
These two characteristics are difficult to fuse with natural optical color-changing pigments, but high-performance gravure optically variable inks can be obtained if the correct polymer surfactants, diluents and catalysts are selected, and the inks are now available It is used by companies in more than 50 countries. For example, the United States, the Czech Republic, Russia, Italy, and the like.
The gravure-printed optically variable ink and banknote design have been well integrated, it does not require additional printing routes or pressure, and is highly cost effective. The intaglio optical color-changing ink is matched with the color of the optical color-changing ink by printing on the offset substrate, and then the effect of sharply increasing color is seen at a certain angle. In this way, the color transition image can rely on a change in the viewing angle to cause a visual information to complete the "appearance" or "disappear" transition .
Screen printing refers to printing techniques that transfer low viscosity inks through a set of screens at a lower printing pressure by a defined series of doctor blades. This technique is simple and feasible, but has significant limitations in resolution, speed, drying and safety of the ink. Recently, many European banknote printing has chosen screen printing technology because it allows images to be transferred by very small pigments such as micro-interference filters used in optical color-changing inks. The low viscosity allows parallel alignment of each pigment particle during deposition and when it reaches the substrate, and the original foil can be reused. Our next development goal is to obtain a screen optical color-changing ink that meets all of the following requirements and has oxidizing properties:
1 Compatible with all high security printed substrates in the industry.
2 Very good support for overprinting and gravure overprinting of offset printing.
3 perfect display color transition characteristics and the compatibility of the interference tablets defined above.
Today, this type of screen optical color-changing ink has been used industrially for many years, and the circulation of hundreds of millions of banknotes has also proven the viability of this concept. An important disadvantage of the printing pressure mentioned above is speed. Therefore, this printing structure requires a new development from our perspective, that is, UV-curable optical color-changing ink.
This interference pigment with special characteristics excludes a free exciter-based ink that has been explicitly described. Also known to the experts is that the concept of light initiation uses all the systems needed for UV curing to stimulate penetration. A practical approach is to use a cationic system that can polymerize all of the cations that contain UV excitation to the surface of the ink film. This so-called secondary hardening effect has a strong dependence on temperature. Such ink systems typically exhibit the advantages and disadvantages of the following two rules and characteristics during the shelf life.
advantage:
There is no limit to the stability of printing; high reactivity; high resistance; low volatility.
Disadvantages:
two systems; limited in positioning; can not overprint.
Another option is the water-based screen printing ink. To date, its limitation is that it is not allowed to apply such systems to industry. The poor performance is mainly due to the lack of stability in storage, which can only be achieved by the interaction between groups of polymers and pigment particles. This phenomenon leads to a large increase in the viscosity of the ink, and the performance of the color stored for more than two months is lost, so its chemical compatibility still needs to be studied, and we are sure that these problems will be solved in the near future.
An important future option is to enhance the safety of optically variable inks by combining optically variable pigments with mechanically readable materials. A fluorescent, phosphorescent, or infrared absorbing pigment is allowed to be added to the optically variable pigment of the optical color changing ink. All of these materials have no color by themselves and therefore do not affect the color transition properties of optically variable inks.
3. Document design
High security documents have undergone a dramatic change in their aesthetic and image design due to the addition of new optical security features. Its most important optical safety feature is the fusion of optical color-changing inks and printing on printing equipment, which is easily accomplished by gravure printing.
This opens up a new dimension for the three-color design of traditional banknotes. Optically variable inks are only part of the design and can be used to print optically variable inks anywhere in the document. In particular, when the document is viewed obliquely, the contrast with the changed color can be enhanced by selecting the color of the offset base in the optically variable print.
Gravure printing provides additional security for the design of optically variable prints that are produced by gravure printing of optically variable inks and are either significant or proven.
The size of the print directly affects the perception and credibility of this feature. It has been experimentally proven that when selecting an image environment, the surface required to provide optical security features is based on the background image. The application of screen printing technology provides a good theoretical study of the positioning of pigment flakes on the ink film, and improves the color appearance and color leap.
The solvent-based ink oxidative drying system provides a preferred choice for the substrate and ensures overprinting on the intaglio in any environment.
The use of UV curing systems in Austrian currency and franc circulation has been considered to significantly improve some of the limitations of performance image resolution.
The ideal application and design should include the color on the optically variable ink, which is pre-printed by offset printing at the same printing pressure, and a solvent-based optically variable color ink can also be applied to screen printing. Overprinting on the intaglio plate enhances the safety and color appearance of the ink. Swiss banknotes or Swiss Union Bank notes clearly show this concept.
Further development of optically variable features can be accomplished by printing two very close optically variable inks. For example, green - magenta / green - blue or gold - green / magenta - green. When the colors of the two optically variable inks match, the same color can be displayed at an angle, and an appropriate image can be used to display visual information in a dynamic manner (logo, named numbers, Seal).
A similar solid surface can be obtained by embossing an optically variable ink on screen printing by relying on a gravure press, a set of plates or a sheet conveyor to carry a negative image. For example, bank checks from Credit Suisse and UBS are implemented through this principle.
Third, the conclusion
Optical color-changing inks are inks based on high-performance interference filters, which are manufactured on roller coating equipment and are now widely used in industry. Its high cost performance, excellent durability, design and application freedom make it possible to achieve this through the aforementioned high security presses.
Printed products are tightly controlled by the materials and inks provided, and in combination with high-security presses, optically variable inks end up with excellent optical security features.