Thermal paper is also called thermal fax paper, thermal recording paper and thermal copy paper, which is called thermal copy paper in Taiwan Province Province. Thermal paper is a kind of processed paper, and its manufacturing principle is to coat a layer of "thermal paint" (thermochromic layer) on high-quality base paper. Although there are more than a dozen chemicals used in this color-changing layer, there are at least the following compounds: leuco dyes are various, and fluorescent compounds are the most commonly used; The chromogenic agent accounts for less than 20%, and bisphenol and p-hydroxybenzoic acid are commonly used; Sensitizer accounts for less than 10%, which contains benzenesulfonamide compounds; The filler accounts for less than 50%, and calcium carbonate (granules) is commonly used; The binder accounts for less than 10%, such as polyvinyl acetate; Stabilizers such as dibenzoyl terephthalate; Lubricant, etc. Therefore, the process is difficult and the technical requirements are high.
The principle of making thermal paper is to coat a common paper base with granular powder, which is composed of colorless dye phenol or other acidic substances, and the middle is separated by a film. Under the heating condition, the film melts and the powder is mixed to produce a color reaction. The color handwriting on thermal paper is unstable and easy to fade, which makes it difficult to identify the contents of the document.
When the heat-sensitive paper is placed above 70℃, the heat-sensitive coating begins to change color. The reason for its discoloration should also be discussed from the composition.
There are two kinds of heat-sensitive components in the coating of heat-sensitive paper: one is leuco dye or leuco dye; The other is a color developer. This kind of thermal paper is also called two-component chemical thermal paper.
Commonly used as leuco dyes are crystal violet lactone (CVL), fluorane system, leuco benzoyl methylene blue (BLMB) or spiropyran system of triphenylmethane phthalein system.
Commonly used color developers are: p-hydroxybenzoic acid and its esters (PHBB, PHB), salicylic acid, 2,4-dihydroxy benzoic acid or aromatic sulfone.
When heat-sensitive paper is heated, leuco dye reacts with developer to produce color, so when heat-sensitive paper is used to receive signals and print on fax machine or directly on heat-sensitive printer, images and texts will be displayed. Because there are many kinds of leuco dyes, the colors of handwriting are different, such as blue, purple and black.
Common lottery tickets, supermarket receipts, hospital registration receipts, etc. All use heat-sensitive paper.