Imaging materials for transfer thermal imaging technology
1) Thermal transfer imaging materials
Generally, the thermal transfer imaging material is composed of a heat-resistant smooth layer of a support and a dye transfer layer.
The support of the ribbon must have the characteristics of good heat resistance, high strength and low price. From these points of view, PET film is the best material, and its thickness is usually between 3.0 and 9.0 μm, but from the perspective of improving thermal conductivity, resolving power, and reducing body weight and weight, try to use thinner material. As the support of the recording medium, a paper base is usually used.
The main function of the heat-resistant smooth layer is to reduce friction, so that the thermal head moves back and forth on its surface without hindrance and is not damaged. Therefore, it must have the characteristics of heat resistance, smoothness, no wear on the thermal head, no charge, and no adverse effects on the dye transfer layer. Its main components are synthetic resin with high heat resistance and various lubricating substances. Thermosetting cross-linked polymers can be used as needed to improve physical strength and heat resistance.
Due to the different transfer mechanism, the molten dye transfer layer is mainly composed of inorganic or organic pigments and binder resin. During the heat transfer, all the heated parts are transferred. As a binder, it is mainly a waxy substance and a small amount of synthetic resin with a low softening point. According to different applications, in order to improve the heat resistance, friction resistance and chemical resistance of the image, the amount of wax can be reduced, and various synthetic resins are used more.
The thermal transfer imaging material has a single-layer structure and a double-layer structure. The single-layer structure is shown in Fig. 4 and is composed of pigment, filler, wax or resin, and a support. Among them, the role of fillers such as Al2O3 particles is to increase the heat transfer speed of the thermosensitive coating, so that the surrounding binder and wax melt as soon as possible and accelerate the transfer of the binder and the coloring agent.
The double-layer structure shown in Figure 5 is composed of a wax layer, a dye layer, and a support. The dye diffuses into the molten wax coating under the action of a heat source, and the wax coating is transferred to the receiving medium to form a blot. By changing the heat energy of the heat source, the number of dye molecules entering the wax coating can be controlled, that is, the density of the final formed image, and there is a good linear relationship between the image density and the pulse width of the printed signal.
2) Sublimation imaging materials
The image effect of the sublimation transfer is not only related to the printer, but more importantly, it has a direct relationship with the imaging material. That is, the dye donor and dye acceptor that form the image are the key to the sublimation imaging.
â‘ Dye donor
The basic structure of the dye donor is shown in Fig. 6, which usually includes a very thin base, which is positive on the base. There are coatings on both sides. The front side of the base is coated with a dye layer, which includes printing dyes and polymeric adhesives. There is an auxiliary layer or bottom layer between the dye layer and the base. The back side of the base is coated with a sliding layer. The purpose is to provide a lubricated surface to prevent the thermal print head from getting scratched.
As the support base of the dye donor, not only the flatness is required to be good, but also it can adapt to the sudden change of temperature, because the dye should be sublimated and transferred during printing. According to the requirements of the recorded data, the sudden change of temperature is very frequent. Supports that can meet these conditions are: polyesters, generally with a thickness of 2 μm to 30 μm.
The sliding layer is coated on the back of the film base to protect it and prevent the print head from adhering. The sliding layer is usually required to be as thin and uniform as possible to avoid affecting the degree of heat transfer; it has no effect on the thermal head, do not contaminate the thermal head; The layer has an adverse effect and usually consists of a cross-linked resin and a lubricating material.
The dye layer can contain a single color dye or a continuous repeating area containing different color dyes, such as cyan, magenta, yellow, and black.When the dye donor contains two or more main color dyes, the color can be obtained image. Generally speaking, the dye layer contains sublimation dyes, adhesives, etc., among them, the general requirements for sublimation dyes are high sublimation and good storage stability at room temperature; good heat resistance, no heat is generated under the heating condition of the heating head Decomposition; good color reproducibility; high molecular absorption coefficient; good light resistance, moisture resistance, chemical resistance; high solubility in water-binding resins or high particle dispersion.
â‘¡Dye acceptor
The basic structure of the dye acceptor is shown in Fig. 6, which is to coat the bottom layer first on the supporting film, and coat the dye receiving layer on the top. Generally, the support is required to have heat resistance, homogeneity, smooth surface and certain flexibility. The thickness is generally 100-200 μm. The base can be transparent film or various plastic films, such as polyolefin.
The dye-receiving layer is intended to receive the dye transferred by the dye donor heated. There are generally absorbable resins in the dye receiving layer, such as polyester. In order to improve the clarity of the transferred image, improve the whiteness of the receiving layer, improve the retransfer of the protected transferred image, and give the receiving surface rewriting stability, accept White pigment can also be added to the layer, this pigment is generally titanium oxide. In order to improve the light resistance of the transferred image, UV absorbers, light stabilizers, antioxidants, etc. can also be added. In order to improve the release performance, a release agent can also be added to the dye-receiving layer. As the release agent, solid wax and the like are generally used.
There can also be one or more intermediate layers between the film base and the receiving layer. The intermediate layer plays different roles with different materials, which can play a buffering role, loosen the dye anti-diffusion effect, and can also meet two or more functions. It can also be used as an adhesion layer. For example, the dye anti-diffusion layer is to prevent the dye from diffusing to the film base. The adhesive used for these layers can be water-soluble or oil-soluble, but it is usually water-soluble, especially gelatin. [next]
3) Structural characteristics of thermal transfer imaging and thermal sublimation imaging materials
The sublimation imaging material is that the sublimation dye in the heated part is transferred in the form of a single molecule. The film-forming resin and other components as the dispersed phase do not transfer with the dye molecule; while the thermal transfer imaging material is that all components in the heated part are transferred. Therefore, the coating structure and production process of sublimation imaging materials are much more complicated than thermal transfer imaging materials. Sublimation thermal transfer imaging materials are usually designed as multi-coating structures. Among them, at least two coatings contain sublimation dyes, that is, at least one dye supply layer? Hereinafter referred to as the lower layer? And a dye diffusion layer? The following is referred to as the upper layer, and the ability of the lower layer to release dye is greater than the upper layer. That is to say, if the lower layer and the upper layer are separately coated on the same support, and they are respectively combined with the same image receiving paper and thermally transferred by a thermal head or laser heating with the same energy supply, it is transferred to the image receiving paper The optical density of the dye on the upper layer should be greater than that of the upper layer, so as to ensure that the density of the dye obtained in each printing is uniform and does not decrease in multiple thermal transfer printing.
4 Features of Dye Sublimation Printing
1) With high-quality image and photo output functions
Since the density of each color of the dye-sublimation printer is controlled by the temperature of the print head, and each printed color point can exhibit a 256-level density change, the pigment is applied to the paper in the gas phase through the sublimation process. The combination of primary colors can form a continuous gradation. Furthermore, because the color sublimation printer does not have the problem of ink droplet diffusion, its actual resolution has reached a very ideal state. The effect of inkjet printing. Therefore, as far as the printing effect is concerned, the images printed by sublimation can be as delicate and smooth as spray, and the color fidelity and reproducibility of the printed images are superior to inkjet printers and color laser printers.
2) Long-term storage without fading
When using a sublimation printer to output an image, the printer will put a protective film on the image. This function can be described as a unique function of the sublimation printer. After the image is coated, the photo not only has waterproof and anti-oxidation characteristics, but is saved It is much longer than the photos output by traditional inkjet printers. It has the characteristics of long-term preservation and no fading, and its overall color feel will be more bright and vivid.
3) Slow printing speed
Because the dye-sublimation printer is cyclically printed with three primary colors, each time a color is printed, the paper must go back and forth in the printing channel, so it is necessary to walk the paper three times to complete a printing task. Therefore, the efficiency of dye-sublimation printers is much slower than that of inkjets. The speed of traditional color inkjets is almost three times faster than that of dye-sublimation printers, and dye-sublimation printers cannot increase the printing speed by reducing the printing resolution. This makes the sublimation printer unsuitable for applications that often require continuous printing, which prevents it from achieving greater development in the commercial field.
4) Not suitable for text printing
Because the dye sublimation printer mixes solid pigments of four colors including black when printing text, the purity of its black is very low, and it cannot be compared with inkjet at all, so it is not suitable for printing as text. the use of.
5) The printing format is narrow
At present, most ordinary sublimation printers only have an output capacity of 4 × 6 inches, which is just the size of ordinary photos. Compared with the inkjet A4 or even A3 format, the sublimation printer still has a big gap, but a little larger The format of the sublimation printer is not only expensive, but also expensive in consumables, which is not suitable for mass users.
6) The use environment is very demanding
Dye-sublimation printers are very sensitive to dust. If dust enters the print head or ribbon, the print quality is likely to have a serious impact. For example, a long white thin line is formed, causing the entire photo to be scrapped. In addition, the dye sublimation printer has high requirements for working temperature. If it works continuously for a long time, it may affect the color accuracy due to poor heat dissipation.
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