When thermal paper is placed above 70 degrees Celsius, the thermal coating begins to change color. The cause of its discoloration should also start with its composition.

components in thermal paper coatings: one is colorless dye or cryptochrome dye; The other is a color developer. This type of thermal paper is also known as two-component chemical thermal recording paper.

Commonly used as colorless dyes are mainly substances such as crystalline violet lactone (CVL) of triphenylmethanylphthalide system, fluorane system, colorless benzoylmethylene blue (BLMB) or spiropyran system.

Commonly used dispersants are mainly: polyvinyl alcohol L-3266, polyvinyl alcohol GL-05, polyvinyl alcohol KL-03 (produced in Japanese synthetic chemistry).

Commonly used top coating and bottom coating additives are: gohsefimer Z-200, polyvinyl alcohol T-350, polyvinyl alcohol N-300.

Commonly used as color developers: para-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aramid sulfone. 

After the thermal paper is heated, the colorless dye and the color developer chemically react to produce color, so when the thermal paper is used to receive a signal on the fax machine or directly printed with a thermal printer, the picture and text are displayed. Since there are many varieties used as colorless dyes, the color of the handwriting is different, including blue, purple red, black, etc.