It is a special fine paper that is coated with a material
formulated to change color when exposed to heat. It is used in
thermal printers, 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
dye and a suitable matrix; a combination of a fluoran leuco dye for
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. The reactant acid in thermal paper
is often bisphenol A (BPA).
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 often generate enough heat
from friction to produce a mark. Multicolor thermal paper first
became available in 1993 with the introduction of the Fuji
Thermo-Autochrome (TA) system.[1] This was followed in 2007 by
Polaroid's development of the Zink ("zero-ink") system.[2] Both of
these methods rely on multi-layer coatings with three separate
colorizing layers, with different methods used for independent
activation of each layer.
