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A leuco dye is a dye whose molecules can acquire two forms, one of
which is colorless. Leuco dyes are a key component of some thermochromic
dyes and thermal printer papers, photochromic materials, and of the Flexplay
DVD discs with limited play time, where leuco form of methylene blue is
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
colorless or perhaps slight yellowish, but in conditions of low pH, as it is
protonated, it shows the color that is intense violet. Other examples can
include phenolphthalein and thymolphthalein, which undergo reversible color
change between colored and colorless form at a specific electrode potential.
Features of Leuco Dyes
- Leuco Dyes change color when get heated and when cooled, they again
change the color. This is the uniqueness of Leuco dyes that they are
reversible. They change color back and forth with fluctuating
temperatures. For Leuco Dyes to change color it takes about 5ºF (3ºC)
- Leuco Dye products can be found in wide variety of forms. This
includes slurry, powder, water-based ink, epoxy, master batch, etc.
- Leuco Dyes are available in 15 distinct colors and a broad range of
Functioning of Leuco Dyes
Leuco Dyes require a special system i.e.
combination of chemicals; their functioning is different from other dyes.
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 beginning 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 photo-micrograph.
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
Application of Leuco Dyes
Leuco Dyes are used for a widely used in various
applications, materials and products. Examples: security printing, novelty
bath toys, swimming pool toys, battery testers etc.
Limitations of Leuco Dyes
It is generally seen that the life expectancy of
Leuco Dyes are shortened by:
- Contact with some solvents.
- Sunlight (Ultra Violet exposure).
- Very high temperature applications.