Direct thermal dyes are members of a class of compounds referred to in
the imaging industry as color formers or leuco dyes. The oldest members of
that class have simple triarylmethane structures, and have been employed for
years in various dyeing applications. More complex triarylmethane compounds,
such as phthalides and fluorans, are now used in various imaging systems to
produce color. Color is derived from all of these compounds via the same
mechanism, on a molecular level. That is, an event of activation produces a
highly resonating cationic system whose interaction with incident light
produces reflected light of a specific color. The activation event in the
case of a direct thermal system is the creation of a melt on the paper
involving dye and an acidic developer.
The three major performance parameters in a thermal system are background
color, image density, and image stability. The three major dye physical
parameters affecting thermal performance are chemical constituency, purity,
and particle size. Those dyes having the best combination of characteristics
which can also be manufactured economically dominate the marketplace.
Manufacturing high performance dyes for the thermal market involves
multi-step, convergent reaction sequences performed on large scale.
Intermediates must be manufactured at the right time, and at the right quality
to be useful.