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The Composition of Thermal Paper

6 January 2014

Thermal paper is a special paper that is used in conjunction with thermal printers to primarily produce receipts for ATM’s, cash registers and other points of sale (POS). The reason that this paper is used so commonly with POS systems is that no ink is involved in the process, which makes the printers small and more manageable than traditional printers that require either toner or ink cartridges to be regularly replaced.

To give you a brief overview of what happens when an item is printed through a thermal printer, the special paper is subjected to heat in specific configurations that turn the paper black and so show the writing/imagery that has been programmed to be shown. The reason that the paper turns black and even other colours when exposed to heat is that it has been impregnated with specific chemical combinations that are designed to react this way.

The base chemical composition of thermal paper consists of:

1. Leuco Dyes - Different types of these dyes can be used to create different colours once the substance is subjected to both an acid and heat. These dyes are usually triaryl methane phthalides.
2. Developers - These are the acids that provide the colour when combined with leuco dyes and thermal heat. The most common of these organic acids are phenols such as Bisphenol A (BPA) and Bisphenol S (BPS).
3. Sensitisers - In order to mix the developer and leuco dye properly when melting, a sensitiser is added. These sensitisers also serve to optimise the colourisation temperature of the leuco dyes.
4. Stabilisers - When stored in hot or humid conditions, the dyes in thermal-sensitive paper tend to revert to their crystalline, colourless forms. To stabilise the paper and prevent this from happening as easily, chemicals similar to the developers (such as complex phenols that are multifunctional) are added to the paper to keep the paper from being compromised.