LEUCO DYE APPLICATIONS

Leuco Dyes (LD's) generally do not make accurate thermometers (like liquid crystal TC's), and are better used for general temperature range or novelty applications.  They are very effective at too cool, OK, too hot type indication.  By combining LD's with regular permanent color pigments, a change from one color when cool to another color when warm is possible.  For example, a blue LD combined with a yellow regular pigment would change from green when cool (blue + yellow) to yellow when warm (clear + yellow).  Please note that it is always necessary to go from a darker color to a lighter color.  LD's can be designed to change color over temperatures ranging from as low as 23ºF (-5ºC) up to about 140ºF (60ºC) and are available in a wide range of colors.

Some examples of LD applications are bath toys, Frisbees, swimming pool toys/products, and hot wheels cars.  LD's can be used in a wide array of materials and products. 

There are some limitations with LD's.  Their life expectancy could be shortened by very high temperature applications, sunlight (UV exposure), and contact with solvents.  The highest temperature that can be applied to LD's without damaging it is approximately 400ºF to 450ºF.  LD's should not be kept in direct sunlight for extended periods of time or brought into contact with solvents.  Testing will help determine which solvents prove harmful.

Cool	Hot

Did you know the Duracell^® battery tester is actually a thermochromic device?

It works by visually displaying the heat generated as current passes through a variable resistance printed conductive ink. There are three primary functional layers, a black to clear LD layer on top of a permanent graphic yellow layer on top of a conductive ink.

When the battery is tested, current passes through the conductive ink and heats a portion of the LD ink changing it from black to clear. If the battery is weak, the current is low and only the high resistance area gets hot. The LD over the high resistance area turns clear, exposing a small portion of the yellow bar and indicates "replace". When the battery is fresh, the current is high and both the high resistance and low resistance areas get hot. This exposes the full yellow bar indicating "good". The LD ink is designed to change color above typical ambient conditions (over 100° F) so that false indications are rare but not so high the battery is significantly drained during testing.