Here is some background and scientific information about the way in which Thermochromism works. Thermochromism is the ability of a substance to change colour due to temperature change. A mood ring for example is an example of materials that change colour to the rise and fall of the temperature around it.
The two main products that posses this unique quality to change colour are liquid crystals and leuco dyes. Liquid crystals are used in precision application, as their reaction can be engineered to accurate temperatures, however their colour range is limited and would not be hugely beneficial when applied to clothing. Leuco dyes however, allow a wider range of colours to be applied to certain fabrics.
Some liquid crystals are capable of emitting different colours at different temperature levels.
This change is dependent on selective reflection of certain wavelengths by the crystallic structure of the material, as it changes between the low temperature crystallic phase.
When heated the compounds that form the base colour of the crystal open up and create new compounds that reflect a different colour range which is visable.
Liquid crystals are used in applications where the colour change has to be accurately defined. They find applications in thermometers for room, refrigerator, aquarium, and medical use, and in indicators of level of propane in tanks. Liquid crystals are usually in a state of suspension usually in some sort of liquid or solution.
Liquid crystals are difficult to work with and require specialized printing equipment. The material itself is also typically more expensive than alternative technologies. High temperatures, ultraviolet radiation, some chemicals and or solvents have a negative impact on their lifespan. We as a group propose to use leuco dyes as they are more user friendly especially when applied to clothing.
Thermochromic dyes are based on mixtures of leuco dyes with suitable other chemicals, displaying a colour change usually between the colourless leuco compound and the coloured form. The dyes are rarely applied directly and are usually in the form of microcapsules with the mixture sealed inside.
Hypercolour is the most prominent example of a clothing company exploiting the rare properties of the chemicals. Leuco dyes have less accurate temperature response than liquid crystals, and are used in applications where accuracy is not required. The dyes are usually used in combination with some sort of pigment , producing a colour change between the colour base pigment and the colour of the pigment combined with the colour of the non leuco form of the leuco dye.
An interesting application of leuco dyes is in the Duracell battery state indicators that inform the user of the current battery levels. The use of leuco dyes is broad and different companies have utilized their qualities to best suite their product roles.
Exposure to ultraviolet radiation, solvents and very high temperatures reduce the lifespan of the compounds as their molecular structure is broken or deformed. Temperatures above 200-230 degrees C typically cause irreversible damage. Leuco dyes may be exposed to high temperatures of around 250 degrees C for limited duratiions during the manufacturing process.
Here are some images of mood rings as well as Hypercolour T shirts that change colour due to temperature change.