Color changing materials are items that change color according to changes in temperature and /or heat. A color changing material can be light when it is cold and dark when it is hot or lighten up in the sun and darken in the shade.
What causes certain materials to change colors?.
Substances called Liquid Crystals (LCs) are very temperature sensitive and change to many different colors. Liquid Crystals have a temperature range from -25 degrees to about +250 degrees Fahrenheit and -30 to about 120 degrees Celsius. Liquid Crystal sheets can be used over and over again making them reversible. When above and below their temperature range, they are black. Liquid crystals last for years when they are not exposed to ultraviolet light and high temperatures.
Another substance that changes color with temperature is Leuco Dyes (LDs). Leuco dyes are colored when cool and become clear as they are heated. In order for the LD to change color the temperature change only needs to be five degrees Fahrenheit or three degrees Celsius. They can be exposed to a temperature only as high as 450 degrees Celsius. Leuco Dyes, unlike Liquid Crystals, are not sensitive so they are used for items where the accuracy of the temperature is not too important. An example of a fabric with Leuco Dye is the Nerf football that changes colors when touched by people. Again, how long the dye will last is dependent upon its exposure to UV light (the sun), exceeding temperatures, and strong solvents. If it is used on textiles, i.e. color changing T-shirts, the dye will not reveal any signs of fading until after about fifteen or twenty washings, if they are not with bleach. Bleach, like any other chemical, can affect the duration of the dye on the shirt.
Thermochromic Materials.
Some thermochromics in the past were made with mercury and are considered poisonous. Today thermochromic chemicals include the Leuco Dyes and Liquid Crystals. They are thermochromic because their color varies with temperature changes.
