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Leuco Dyes
Leuco dyes are a very popular class of dyes. Their
uniqueness lies in the fact that in Leuco Dyes, the molecules show the
capacity to acquire two distinct forms, and one of it is colourless. A
prominent example can be given is that of the spiro form of an oxazine,
that is typically a colorless Leuco Dyes. Another example of leuco dye
is the crystal violet lactone. This dye while in its lactone form is
typically colourless or perhaps slight yellowish, but in conditions of
low pH, as it is protonated, it shows the colour that is intense violet.
Other examples can include phenolphthalein and thymolphthalein.
Features of Leuco Dyes |
- As earlier said, one of the unique features of Leuco Dyes is that
the colour changes upon heating. That is from coloured (when cool)
to colourless. In fact it is the uniqueness of leuco dyes that they
are also reversible. They change colour back and forth with
fluctuating temperatures usually for Leuco Dyes to change colour it
takes about 5ºF (3ºC) temperature change.
- Leuco Dyes are available in 15 distinct colours and a broad range
of temperatures.
- Leuco Dye products can be found in wide variety of forms. This
includes slurry, powder, water-based ink, epoxy, master batch, etc.
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Functioning
of Leuco Dyes |
Leuco Dyes are different from other dyes in its
functioning. They require typically a combination of chemicals in a
system. For protection of this special system of materials a process
called Microencapsulation is put to good use. The process of
microencapsulation works like this, in the begining it extracts a small
droplet out of the Leuco Dye system and puts protective wall coating
around it. This process is effectively shown in the photomicrograph.
As the image shows, actually it is these Leuco microcapsules that has
in it the complete color changing thermochromic system. For example
Leuco Dyes microcapsules when added in inks makes the ink display the
necessary thermochromic properties.
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Application
of Leuco Dyes |
Leuco Dyes are used for a wide array of applications,
materials and products. Examples of application of Leuco Dyes include
security printing, novelty bath toys, swimming pool toys, battery
testers etc.
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Limitations
of Leuco Dyes |
It is generally seen that the life expectancy of
Leuco Dyes are shortened by:
- Very high temperature applications.
- Sunlight (Ultra Violet exposure).
- Contact with some solvents.
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