Thermal paper is a special coated paper, its appearance is similar to ordinary white paper. The surface layer of the thermal paper is smooth. It is made of ordinary paper as the paper base. A layer of thermosensitive chromogenic layer is coated on the surface of the ordinary paper. The chromogenic layer is made of adhesive, developer, leuco dye (or The composition is called leuco dye), which is not separated by microcapsules, and the chemical reaction is in a "latent" state. When the thermal paper encounters a heat-generating print head, the developer and the leuco dye where the print head is printed chemically react and change color, forming graphics and text.
When the thermal paper is placed in an environment above 70°C, the thermal coating begins to change color. The reason for its discoloration has to be discussed from its composition. There are two main heat-sensitive components in the thermal paper coating: one is a leuco dye or leuco dye; the other is a developer. This type of thermal paper is also called two-component chemical thermal recording paper.
Commonly used as leuco dyes are mainly: crystal violet lactone (CVL) of tritylphthalide system, fluoran system, colorless benzoyl methylene blue (BLMB) or spiropyran system and other substances. Commonly used as color reagents are mainly: para-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aromatic sulfones.
After the thermal paper is heated, the leuco dye reacts with the developer to produce color, so when the thermal paper is used to receive the signal on the fax machine or print directly with the thermal printer, the graphics and text will be displayed. Since there are many kinds of leuco dyes, the colors of the written writing are different, such as blue, fuchsia, black, etc.