Reaction mechanism and application of light interference type color ink

Optical interference type color shift ink is a kind of modern anti-counterfeit ink with high technological content and relatively complex manufacturing technology. It is called optical variable ink, also known as optical color change ink. It was introduced in the 1970s. Light interference type color ink prints have a dynamic color change effect, and different colors can be observed at different angles. Especially when observed at around 0° and 60°, two distinct colors will appear. This discoloration effect comes from the light interference pigment in the ink.

The light interference type discoloration ink was first studied by Canada in 1975, and successfully developed a light interference multi-layer composite film capable of selectively absorbing and reflecting sunlight. In 1981, the United States began to develop thin-film interference filters. In 1984, it developed a method of transferring light-transmitting films to other surfaces, and applied for patents on the production of optically-varying thin films whose coating color changes with viewing angle. Since then, Swiss SICPA Corporation has purchased the personnel, equipment and all inventions of the American Coatings Laboratory. In June 1987, it recommended this product to the 7th International Conference on Banknotes Anti-counterfeiting.

The light interference type color-changing ink has complex manufacturing processes and high prices. The ink has a FOB price of up to 3,000 US dollars per kilogram, and the sales channels are relatively strictly controlled and have strong anti-counterfeiting capabilities. The manufacturing principle and process of the ink are complicated and confidential, and only a brief introduction is made here.

Reaction mechanism of light interference type color ink

1. Photochromism mechanism

Certain substances in a certain wavelength of light irradiation, the chemical structure can change, the absorption spectrum of visible light changes, resulting in color changes; and then under another wavelength of light irradiation or heat, restore or not restore the original s color. This phenomenon of reversible or irreversible coloration and achromatization of matter is called "photochromism."

The reversible photochromic process of matter can be divided into two steps: coloration and achromatism. The so-called coloring means that the substance undergoes color change under certain wavelengths of light; achromatism means that the discolored material is heated or irradiated with another wavelength of light to restore the original color. This process of material photochromism can be simply expressed as AB. For most photochromic substances, this is a reversible photochemical reaction and the process can be represented by AB. The substance first absorbs photons, and it transitions from the ground state to the excited state, and then a photochromic reaction takes place. From substance A to substance B, it is a color forming process. The achromatization process has two ways: one is that the substance B undergoes a thermal reaction by endothermic change from the ground state directly into the substance A; the other is through the absorption of light energy from the ground state transition to the excited state, a photochromic reaction occurs, and then back To substance A, as shown in Figure 1.

2. Reaction mechanism of light interference type color ink

It is known that color is the sensitometric response of the eye and brain to the visible spectrum (wavelength range 400-700 nm). When white light (natural light) shines on an object, we only feel the reflected light (reflection color) on the surface of the object, and the rest of the wavelength is absorbed by the object. Absorption and reflection of light waves by objects are determined by the intrinsic characteristics of the objects and vary widely. However, no matter what the angle of observation or how the light source changes, the color of the object remains the same. In contrast, "interference color" does not belong to the characteristics of the object itself and it is due to light interference. If the soap bubble itself is colorless and transparent, but when its thickness meets the conditions of light interference, it will present a rainbow-like color. The "interference color" changes color with the incident angle of the light and the observation angle, which is an effect that "reflection color" cannot achieve.

Object coloration comes from two aspects. First, the light is irradiated on the surface of the object, and the object selectively absorbs part of the visible light waves to make the object appear color. Secondly, the light is incident on the inside of the transparent object. After being reflected and refracted, the light wave interferes and becomes color. The interference film for light-variable ink is designed based on the latter principle and belongs to a technical product that is simple in principle and difficult to manufacture. Its color principle is shown in Figure 2. A beam of light from the light source is incident on the inside of the transparent film or on the surface of the thin plate, and then part of the light waves are refracted. The refracted light enters the film, and reflection and refraction are generated on the lower surface. Both the reflected light and the transmitted light come from the same light wave, and they satisfy the coherent light conditions, and interference phenomena can be produced. For natural light (multicolor light) we can filter out one of the wavelengths of light by interference, even if its reflected light intensity is zero. To get this result, the following conditions must be met:

N1=(n0n2)1/2 (1)
N1h=λ/4,3λ/4.....(2l+1)λ/4 (2)

Where h is the thickness of the medium, ie, the optical thickness of the monolayer film is an odd multiple of the incident light λ/4; l is the number of layers of the low refractive index film. The refractive index n1 of the film material should be between n0 and n2, but in fact these two conditions are difficult to satisfy because it is difficult to obtain such materials.

The use of multilayer high-refractive dielectric films can solve the above problems.

From equation (1), it can be seen that to increase the refractive index n1 of the film material, it is necessary to increase the refractive index n2 of the base material. Multi-layer coating is to first plate a layer of high-refractive film on the substrate, called H film, and then plate a layer of low-refractive-index film called L film to obtain a multilayer composite film.

Similarly, the wavelength of light reflected by each film is an odd multiple of the incident light wave, and the difference between nH and nL should be large, such as nH (H) film with InS (n = 2.4), nL (L) film can be MgF2 (n = 1.38). In the same batch, if the L and H films are arranged in the opposite order, a multi-layered retroreflective film with different anti-reflection effect can be produced. These two kinds of composite films can produce wonderful effects with the change of viewing angle. If the substrate is previously covered with a layer of polymeric material, the material is at least soluble in a solvent such as acrylic resin (soluble in acetone). On this material, a multi-layer composite film that meets the requirements is obtained under strict control by vacuum deposition. The film-coated multi-layer composite film is placed in a pre-selected solvent, and the multi-layer composite film is peeled off one by one to form fine fragments. The ratio of the upper and lower surface area of ​​this debris to its side area is at least 3:1. This fragment was then vacuum dried to produce a photochromic material. This multi-layered film fragment selectively absorbs a portion of the light waves and reflects the remaining light waves to exhibit color. The photochromic ink can be produced by adding these multi-layered film fragments to specially designed binders and adding transparent dyes and other fillers.

Application of light interference type color ink

The optically variable ink itself is a reflective ink with pearlescent and metallic effects. In products printed with light-variable ink, the ink color patches present a pair of inter-changeable colors, such as: magenta-blue, green-blue, cyan-green, etc. If the ink blot is tilted around 45°, the pattern can be made from one Hue shifts to another hue. Because only the ink film on the print is thicker, significant color drift may occur, so its print characteristics cannot be imitated by other inks and printing methods.

The dynamic color-changing effects of printed products of light-variable inks cannot be duplicated with high-definition scanners, color copiers, and other equipment. The design and production techniques are highly specialized, the production process is complicated, and the investment is huge. It is difficult to imitate. Therefore, it is applied to currency security by more than 70 countries. Since China's 1999 edition of RMB 100 used optical ink printing technology, the anti-counterfeiting technology has not been copied on counterfeit money and has become an important mark for identifying serious fake RMB. In addition, it is also widely used in the security of various types of securities, famous trademarks, certificates and documents, and product packaging.

Source: "Printing Technology"