Analysis of Thermal-developed Dry Non-silver Imaging Products and Technology (5)

3 Dye-type heat-sensitive recording material coloring materials:

Couplers are key compounds of dye-based thermal paper. Most of the characteristics such as color tone, color density, and image fastness are affected by the coupler. The coupler has the necessary characteristics 1 low water solubility. 2 The reaction with the developer is appropriate. 3 suitable melting point. 4 couplers and color bodies have high fastness. The color tone of the 5 color body is consistent with the standard. When the coupler has a high water solubility, it will dissolve in the micro-milling and mix with the developer liquid. In addition, the highly reactive coupler reacts with the filler contained in the paper to generate fog.

The dye-based thermal paper originally developed by NCR Company was a cyan heat-sensitive paper whose patent scope was limited to cyan color, and Japan has actively conducted research on black couplers. The black color thermal recording paper is applied to many fields such as facsimile, and other colors are limited by the image and two-color color. The following table is representative of the coupler, the most common type of black coupler.

The color developer is also a key compound and it is required that the property be based on the coupler other than the coupler property 5. Although there are many problems with bisphenol A (2,2-bis(4-hydroxyphenyl)propane), its raw material polyester can be mass-produced, the price is very cheap, and it has always been in the mainstream. The following table is representative of the coloring agent. (b) In the table, high sensitivity can be achieved without using a sensitizer, and it is a coloring agent in the era of painting. (c)-(f) are used for the chemical resistant heat-sensitive recording material to be described later.

4 dye-based thermal recording material other materials:

The color formation of the direct thermal recording material begins with the hot dissolving color forming material. Thermal print heads are often in contact with the dissolving color forming material, so smearing problems caused by the coloring material can occur from time to time, and most of the problems that arise with direct thermal recording systems are here.

To solve this problem, a high oil-absorbing pigment is first added to the color forming layer to stabilize the liquefied color forming material in the color forming layer without transferring to the thermal print head. The degree of oil absorption is expressed by the amount of linseed oil that can be absorbed by 100 g of pigment. Generally, about 30 to 100 g of linseed oil is used. Representative examples include light calcium carbonate, fired clay, and aluminum hydroxide. When calcium carbonate is used for heat-sensitive recording materials, many processes have been performed with improved oil absorption. Natural inorganic pigments mostly contain high-hardness materials such as feldspar and quartz, and wear hot print heads, so they are used less frequently. Secondly, when the material is attached to the thermal print head, it must be quickly detached, so it is necessary to add a release agent in the color-forming layer. In general, metal soap is used. In many materials, zinc stearate is the single most effective release agent.

2.2 Near infrared reading type thermal recording material

The above-mentioned couplers in the coupler table can absorb light of 400-700 nm. The label needs to record the optically read barcode (OCR) in a thermal manner. The general OCR light source is an LED or laser with an emission center of 800-1000 nm. This requires a thermal paper capable of absorbing infrared light in this area. The color body of the coupler shown in the figure below can absorb around 1000 nm, so it can be used for this purpose. Coupling-type thermal paper using a heavy metal coupling agent as a color forming material may also be used, but its disadvantage is that coloring is concentrated in an uncolored state and the contrast is small.

2.3 Chemical resistance thermal paper

The dye-based thermal recording paper causes a color-forming reaction by the contact of a solvent of a color-forming material. The color forming reaction is a reversible reaction and the reverse reaction under specific conditions will result in discoloration.

In order to prevent the coloration caused by the solvent contact, a protective layer is first provided on the color-forming layer. The protective layer uses polyvinyl alcohol with low solvent permeability. In addition, reactive polymers such as itaconic acid-modified polyvinyl alcohol are also used, and can be cross-linked after coating to provide water resistance. This method is only effective for solvents with lower boiling points, and the high boiling point solvent slowly penetrates into the protective layer, so the effect is not great. Second, the color forming material is dissolved by heat, and a material that is hardly soluble in a solvent is used. As in (g) in the coupler table and (e)-(f) in the developer table. These are extremely effective in suppressing the fog caused by the solvent. However, in principle, the sensitizer is hard to work, so high sensitivity is a difficult point.

There are two kinds of decolorization of color bodies: First, decolorization during preservation. This is caused by the reverse reaction of the color forming reaction of the dye-based thermal paper, and the recrystallization of the coupler and developer produced by the reverse reaction promotes discoloration. In order to prevent this phenomenon, it is necessary to select a color forming material that can form a solid color body, or add a third substance to suppress recrystallization. The addition of the polyvalent phenol compound in the following figure to the color forming layer can significantly suppress the discoloration of the color image. These compounds have the property of being difficult to recrystallize by itself when dissolved, and form a fused mixture with the color former and the color developer to lower the melting point. In addition, complex polyvalent phenol molecules and complex couplers inhibit the reverse reaction by forming complex color bodies. These substances are called decolorization preventing agents.

Second, the reaction between the color former and the color former is an ionic reaction. At this time, the solvent is adsorbed, and the color body structure is destroyed. The decolorization caused by the plasticizer is also the reason. Generally, the plasticizer has a high boiling point and the protective layer is difficult to prevent. Improvements in coloring materials must be made.

The chemical-resistant heat-sensitive recording material for labels or the like developed in accordance with the above-mentioned problems does not cause the decolorization of fog and color bodies under a wide range of conditions.

2.4 Development and Market Outlook of Ordinary Direct Thermal Paper

The thermal recording paper can be used for facsimile, electrocardiogram recording, analysis instrument recording, automatic ticket vending machine ticket, optional mall POS tag, etc.

Around 1930, thermal recording paper was used for electrocardiogram recording. At that time, it was a wax-type thermal paper (physical thermal method). In 1950, 3M developed a thermal fax recording paper (organic acid metal salt method), and in 1964, NCR Corporation. Thermal recording papers having a thermal dye as a coupler have been developed. In recent years, the thermal paper production technology has matured, completely solved the technical problems of poor thermal paper stability and short shelf life. The storage period can reach 5 to 10 years. After special processed thermal paper, the storage period can reach 15 to 20 years. The technical problem of printing on thermal paper has also been solved. In addition, the increase in the cost of the thermal print head has greatly reduced the use of thermal paper, resulting in a dramatic increase in commercial demand. According to statistics, in 2000, China's thermal paper consumption was about 15,000 tons, Japan's usage was about 90,000 tons, and North America's usage was about 150,000 tons. It is expected that in the next 3-5 years, China's demand will reach at least 30,000 tons, with an output value of nearly 1 billion yuan. The main manufacturers of thermal paper include Japan's Ten Paper, Magic Paper, Mitsubishi Paper, Dawang Paper, Prince, Bach Paper, Ricoh, Fuji, etc., as well as Hilton Davis, Ciba-Geigy and Appleton in other countries. , Labelon, Nashua, 3M, BASF, etc. The units engaged in research and production of thermal paper processing in China include Tianjin Paper Technology Research Institute, Shanghai Papermaking Research Institute, Weihai, Qingdao Paper Mill, Benxi Record Paper Mill, Guangdong Guanhao, and others.

Nowadays, various waste thermal papers have brought great pollution to nature (at present, Japan is the world leader in the recycling technology of waste thermal recording papers), and with the newly emerged printing directly on plain paper. Compared with inkjet fax, the image quality is poor and the storage period is short. Therefore, experts predict that this new type of fax machine will gradually replace the current thermal fax machine, and the demand for thermal paper on fax communication will gradually decrease. The use will be gradually transferred to an electronic cash register for use as a thermal label. At present, from a global perspective, thermal paper fax machines have been banned from selling in the European and American fax machines. In China, from the trend of recent years, the shrinkage of thermal paper machines is not obvious. Plain paper for inkjet and ribbon thermal transfer fax machines is inexpensive, but it does not sell much in the fax paper market. Difficult to keep pace with thermal fax paper (colorless dye type). The reason is that ink jet and ribbon thermal transfer fax machines require the use of ink cartridges and ribbons, while the thermal fax machines do not require any other cost than thermal paper, and each fax is cheaper to receive.