When thermal paper is placed in an environment above 70℃, the thermal coating begins to change color. The reason for this color change lies in its composition.
The thermal components in the thermal paper coating are mainly of two types: one is a colorless dye, also known as a leuco dye; the other is a color developer. This type of thermal paper is also called a two-component chemical thermal recording paper.
Commonly used colorless dyes include: crystal violet lactone (CVL) of the triphenylmethane phthalide system, fluorane systems, colorless benzoylmethylene blue (BLMB), or spiropyran systems, etc.
Commonly used dispersants include: polyvinyl alcohol L-3266, polyvinyl alcohol GL-05, and polyvinyl alcohol KL-03 (produced by Nippon Synthetic Chemicals).
Commonly used top and bottom coating additives include: gohsefimer Z-200, polyvinyl alcohol T-350, and polyvinyl alcohol N-300.
Commonly used color developers include: para-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid, and aromatic sulfones.
When thermal paper is heated, the colorless dye reacts chemically with the color developer to produce color. Therefore, when using thermal paper to receive signals on a fax machine or directly print with a thermal printer, the text and images are displayed. Because there are many types of colorless dyes used, the displayed text colors vary, including blue, magenta, and black.