Thermal paper is paper that is impregnated with a chemical that changes color when exposed to heat. It is used in thermal printers and particularly in cheap, lightweight devices such as adding machines , cash registers , and credit card terminals .
The paper is impregnated with a solid-state mixture of a dye and a suitable matrix; a combination of a fluoran leuco dye and an octadecylphosphonic acid is an example. When the matrix is heated above its melting point, the dye reacts with the acid, shifts to its colored form, and the changed form is then conserved in metastable state when the matrix solidifies back quickly enough.
Most direct thermal papers require a protective topcoating to:
- Reduce fading of the thermal image caused by exposure to UV light, water, oils, grease, lard, fats, plasticizers, and similar causes
- Provide improved printhead wear
- Reduce or eliminate residue from the thermal coating on the thermal printheads
- Provide better anchorage of flexographic printing inks applied to the thermal paper
- Focus the heat from the thermal printhead on the active coating.
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