Thermal paper is a special fine paper that is coated with a chemical that changes color when exposed to heat. It is used in thermal printers and particularly in inexpensive or lightweight devices such as adding machines, cash registers, and credit card terminals.
The surface of the paper is coated with a solid-state mixture of a dye and a suitable matrix; a combination of a fluoranleuco dye and octadecylphosphonic acid as 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 a metastable state when the matrix solidifies back quickly enough.
Usually, the coating will turn black when heated, but coatings that turn blue or red are sometimes used. While an open heat source, such as a flame, can discolor the paper, a fingernail swiped quickly across the paper will also generate enough heat from friction to produce a mark.
Most direct thermal papers require a protective top-coating to:
• reduce fading of the thermal image caused by exposure to UV light, water, oils, grease, lard, fats, plasticizers, and similar causes
• provide improved print head wear
• reduce or eliminate residue from the thermal coating on the thermal print heads
• provide better anchorage of flexographic printing inks applied to the thermal paper
• focus the heat from the thermal print head on the active coating.
Links: Thermal Paper from Wikipedia