Thermal paper is a special coated processed paper whose appearance is similar to ordinary white paper. The surface layer of thermal paper is smooth. It is made of ordinary paper as the paper base, and a layer of heat-sensitive color-developing layer is coated on the surface of the ordinary paper. The color-developing layer is made of adhesive, color developer, colorless dye (or It is called leuco dye) and is not separated by microcapsules, and the chemical reaction is in a "latent" state. When the thermal paper encounters the heat-generating print head, the color developer and the leuco dye in the place where the print head prints will undergo a chemical reaction to change color and form a graphic.

When the thermal paper is placed in an environment above 70°C, the thermal coating begins to change color. The reason for its discoloration also starts 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 color developer. This type of thermal paper is also called two-component chemical type thermal recording paper.

Commonly used as leuco dyes are: crystal violet lactone (CVL) of trityl phthalide system, fluorane system, colorless benzoyl methylene blue (BLMB) or spiropyran system and other substances. Commonly used as color reagents are: p-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aromatic sulfone and other substances.

When the thermal paper is heated, the colorless dye and the developer undergo a chemical reaction to produce color, so when the thermal paper is used to receive a signal on a fax machine or print directly with a thermal printer, the graphics and text will be displayed. Since there are many kinds of leuco dyes, the colors of the handwriting that appear are different, such as blue, purple, black and so on.

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