Thermal paper is a special coated paper with a similar appearance to ordinary white paper. The surface of the thermal paper is smooth. It uses plain paper as a paper base and is coated with a layer of thermal coloring layer. On one side of the paper surface, the chromonic layer is composed of an adhesive, a color developer, a leuco dye, which is not separated by microcapsules, and the chemical reaction is in a “latent” state. When the thermal paper is encountered, in the case of a hot print head, the developer and the leuco dye at the place where the print head is printed are chemically reacted and discolored to form a graphic image.
When the thermal paper is placed in an environment above 70°C, the heat-sensitive coating begins to change color. The reason for its discoloration is also to talk about its ingredients.
There are two main types of heat sensitive materials in thermal paper coatings: one is a leuco dye; the other is a color developer. This type of thermal paper is also referred to as a two-component chemical type thermal recording paper.
Commonly used as leuco dyes are crystal violet lactone (CVL), fluoran system, colorless benzoylmethylene blue (BLMB) or spiropyran system of triphenylmethyl phenyl hydrazine system. Commonly used as a color developer are para-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aromatic sulfone.
When the thermal paper is heated, the leuco dye reacts with the developer to produce a color. Therefore, when the thermal paper is used to receive the signal printing on the fax machine or directly printed by the thermal printer, the graphic is displayed. Since many varieties are used as leuco dyes, the colors of the written characters are different, and there are blue, magenta, black, and the like.
Crystal violet lactone (CVL) is one of the most widely used and best-performing substances in triarylmethane ketone thermal and pressure sensitive dyes. CVL is an important functional dye for the production of pressure sensitive materials. CVL is a kind of heat sensitive and pressure sensitive dye with fast color, high chroma and good oil solubility. It is widely used in commercial subpoenas, terminal subpoenas and computer records for computer output. Studying the synthesis method of CVL has important practical significance for promoting the production of functional dyes. A general dye is a colored conjugated system which has the property of dyeing fibers or coloring a resin, and the conjugated system of a heat-sensitive, pressure-sensitive dye is interrupted, itself is colorless, but it has a function with a color developer, and the performance of hair color. Under acidic conditions, lactone ring crack formation quinoid π nonferrous group, which emits a brightly high concentration of blue-violet, with reflected light absorption at 609.8 nm.
CVL is used in dark blue press-sensitive and heat sensitive dyes for press-sensitive action paper and carbonless copying paper. The package is double plastic lined paper drum, net weight 20 Kg.
The synthesis of p-dimethylaminobenzaldehyde was carried out by vilsmeier reaction using N,N-dimethylaniline as raw material. The effects of reaction temperature, reaction time and ratio of raw materials on product yield were discussed. When the ratio of N,N-dimethylaniline and phosphorus oxychloride is 1:1.3 (mol/mol), the yield is 87.8%. The p-dimethylaminobenzaldehyde, N,N-dimethylaniline and m-dimethylaminobenzoic acid are used as raw materials, and the crystal violet lactone is obtained by condensation and oxidation, and the yield is 41.5%.
The equimolar ratio of N,N-dimethylaniline and p-dimethylaminobenzaldehyde was reacted at 80°C for 0.5h in the presence of hydrochloric acid and urea, and then added with m-dimethylaminobenzoic acid at 90°C for 5h. The reaction mixture was neutralized and suction filtered to give colorless crystal violet lactone (MVL). Then, in the presence of a transition metal and an organic nitrogen compound, the colorless crystal violet lactone is converted into crystal violet lactone (CVL) by oxidation in air at 70 to 80°C in an alkaline solution, and the obtained product has a melting point of 177-179°C, the yield is about 90%. In the air oxidation step, suitable transition metals are manganese and iron with a pH of 11.