Overview of the principles behind thermal paper
Release time:
2021-11-12
Thermal paper, also known as thermal fax paper, thermal recording paper, and thermal copying paper, is called heat-sensitive copying paper in Taiwan. Thermal paper is a type of processed paper; its manufacturing principle involves coating the original paper with a layer of "heat-sensitive coating" (heat-sensitive color-changing layer). Although there are more than a dozen chemical substances used in this color-changing layer, it contains at least the following compounds: colorless dye (a wide variety of dyes are used, the most common being fluorescent compounds); common color developers include bisphenol and p-hydroxybenzoic acid; sensitizers contain benzenesulfonamide compounds; common fillers include calcium carbonate (particles); common adhesives such as polyvinyl acetate; stabilizers, such as dibenzoyl phthalate; lubricants, etc. Therefore, the process is difficult and technically demanding.
The Principle of Thermal Paper
When thermal paper is placed in an environment above 70℃, the heat-sensitive coating begins to change color. The reason for this color change must be explained from its components.
The heat-sensitive components in the thermal paper coating are mainly two types: one is a colorless dye or latent dye; the other is a color developer. This type of thermal paper is also known as a two-component chemical thermal recording paper.
Commonly used colorless dyes include: crystal violet lactone (CVL) of the triphenylmethane phthalide system, fluorescein system, colorless benzoyl methylene blue (BLMB), or spiro pyran system substances.
Commonly used dispersants include: polyvinyl alcohol L-3266, polyvinyl alcohol GL-05, polyvinyl alcohol KL-03 (Japan Synthetic Chemical Products).
Commonly used topcoat and bottomcoat auxiliaries include: gohsefimer Z-200, polyvinyl alcohol T-350, polyvinyl alcohol N-300.
Commonly used color developers include: p-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxybenzoic acid, or aromatic sulfones.
When thermal paper is heated, the colorless dye and the color developer react chemically to produce color. Therefore, when thermal paper is used to receive signals and print on a fax machine or directly printed with a thermal printer, the image is displayed. Due to the many types of colorless dyes used, the color of the displayed characters varies, including blue, purplish red, and black.
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