Thermal paper is also known as thermal fax paper, thermal recording paper, thermal copy paper, in Taiwan is called thermal copy paper. Thermal paper is a kind of processing paper, its manufacturing principle is to coat a layer of high-quality base paper "thermal paint" (thermochromic layer). Although there are more than ten kinds of chemicals used in the chromotropic layer, there are at least the following compounds: colorless dyes, the most commonly used are fluorescent compounds; color reagents account for less than 20%, commonly used are bisphenol, p-hydroxybenzoic acid; sensitizers account for less than 10%, including benzenesulfonamide compounds; Fillers account for less than 50%, commonly used calcium carbonate (particles); adhesive about 10%, such as polyvinyl acetate; stabilizers, such as phthalic acid diphenyl; lubricants. Therefore, the technology is difficult and the technical requirements are high.

Principle of thermal paper

When the heat sensitive paper is placed above 70 C, the thermal sensitive coating begins to change color. The reason for its discoloration depends on its composition.

There are two kinds of heat-sensitive components in the coatings of heat-sensitive paper: one is a colorless dye or a leuco dye; the other is a chromogenic agent. This kind of thermal paper is also known as two group differential thermal recording paper.

The main colorless dyes are crystal violet lactone (CVL), fluorone system, colorless benzoyl methylene blue (BLMB) or spiropyran system.

The commonly used dispersants are: polyvinyl alcohol L-3266, polyvinyl alcohol GL-05, polyvinyl alcohol KL-03 (synthetic chemical product of Japan).

Gohsefimer Z-200, polyvinyl alcohol T-350 and polyvinyl alcohol N-300 are commonly used as top and bottom coat additives.

The main coloring agents are p-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid or aromatic sulfone.

When the thermosensitive paper is heated, the colorless dye reacts with the developer to produce color. When the thermosensitive paper receives the signal from the fax machine or prints directly with the thermosensitive printer, the picture and text are displayed. There are many kinds of colorless dyes, so the color of the handwriting is different, such as blue, purple, black and so on.