The rewritable technology in ThermoRewrite is based on the technology for traditional thermal recording paper in which fields Mitsubishi Paper Mills Ltd. has long history. The brilliant image in ThermoRewrite is generated using Leuco dye. Like other traditional thermal recording paper, ThermoRewrite consist of similar developer as well as Leuco dye. In the ThermoRewrite, however, specially designed developer has responsible for its rewritability. The typical color/decolor reversible reaction of Leuco dye is as follow.
Rewritable
process of Thermorewrite
The control of cooling process following
the application of heat to ThermoRewrite dominates the selection between
printing and erasing. When the mixture of Leuco dye and developer is heated
up to their melting point, typically these are around 170 Celsius, melting
mixture state is created. From this melting mixture state it follows two
way depending on cooling rate of material. One is quick cooling process
which results in colored state that responsible for printed image. Second
is slow cooling process which results in decolored state. In practice
the application of heat with thermal head usually realize quick cooling
rate resulting in colored state (printing process). On the other hand,
the application of so called static heat results in slow cooling rate.
The static heat can usually be realized with hot iron stamp, heat roller,
and so on. In this case we can estimate the "static erase temperature".
For ThermoRewrite it is around 120 Celsius.
Microscopic
view of rewritable process
Next figure shows more detail description
of rewritable process of ThermoRewrite. In the melting mixture state at
high temperature, dye and developer are mixed together. If this melting
mixture state is cooled down quickly, this situation can almost be kept
during the cooling process, finally resulting in some kind of amorphous
glass. Then in this amorphous state, dye and developer can react each
other forming colored state of Leuco dye. On the other hand, if melting
mixture state is cooled down slowly, the crystallization of developer
come to be dominant process. Finally, Leuco dye and developer can no longer
react each other since most of dye molecules and developers are separated.
This is why slow cooling process with static heat leads decolored state
(erasing process). Once the colored state is formed, however, the material
does not need to be heated up to the melting point of dye and developer
to reach the decolored state. There exist a region of temperature below
the melting point (decolorable region), in which the colored state shifts
to the decolored state. Typical temperature representing this region is
about 120 Celsius. In fact most of printer for our rewritable material
have an erase device which provide static heat of about 120 to 140 Celsius.
