The principle of thermal paper

Thermal paper is also called thermal fax paper, thermal recording paper, thermal copy paper, and thermal copy paper in Taiwan. Thermal paper is a kind of processed paper, and its manufacturing principle is to coat a layer of “heat-sensitive paint” (heat-sensitive color change layer) on high-quality base paper. Although there are more than a dozen chemicals used in this color change layer, there are at least the following compounds: leuco dyes, the dyes are of a wide variety, the most commonly used are fluorescent compounds; the color developing agent accounts for less than 20%, and the commonly used are double Phenol and p-hydroxybenzoic acid; sensitizers account for less than 10%, which contains benzenesulfonic acid amide compounds; fillers account for less than 50%, commonly used calcium carbonate (fine particles); adhesives account for less than 10%, Such as polyvinyl acetate; stabilizers, such as dibenzoyl terephthalate; lubricants, etc. Therefore, the process is difficult and the technical requirements are high.

The principle of thermal paper:
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 color 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, leuco benzoyl methylene blue (BLMB) or spiropyran system and other substances.
The commonly used dispersants are mainly: polyvinyl alcohol L-3266, polyvinyl alcohol GL-05, and polyvinyl alcohol KL-03 (produced by Japan Synthetic Chemicals).
The commonly used top coat and bottom coat additives are mainly: gohsefimer Z-200, polyvinyl alcohol T-350, and polyvinyl alcohol N-300.
“Para-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aromatic sulfones are mainly used as color developing agents.

When 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 color of the written handwriting is different, such as blue, purple, black and so on.