Leuco Dyes

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 used.

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.

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• 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) temperature change.
• 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 temperatures.

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 thermochromic properties.

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.

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.