- Thermo Electric
Thermochromic Textiles convert thermal radiation to produce a chromatic effect, therefore altering the perceived colour to the human eye. The transition between displayed colours, in the instance of thermochromic, is reversible because it is entirely dependant upon temperature to provoke the colour change process. Other colour changing materials are available but the stimulus can vary from light, photochromic, to electrical current, electrochromic. The way this interaction occurs and is implemented into textile products varies because of the material; there are two types of material which enable this reaction;
• Liquid Crystals
• Leuco Dyes
Objects are perceived as a colour because of the wave length within the visible light section of the Electromagnetic Spectrum the object does not absorb. If a material can change the way it interacts with rays within the light spectrum it will alter its perceived colour.
Liquid crystals are used where precision is a requirement as they can be engineered to react at a programmable temperate rate; applications of such include thermometers and LCD screens, and more novel products such as mood rings. Because of their liquid state, to be used in textile applications they are microencapsulated in an aqueous solution. To ensure maximum visual impact the microencapsulated product is often screen printed so it remains on the product surface. The temperature range at which they display different colours is engineered to meet the application, above and below the temperature range they will display black. The colour range they display follows that of the electromagnet spectrum within the visible light range, from red through to violet. The range between the red and the blue start is defined as the bandwidth. The temperature to evoke the colour change in the bandwidth can be as little as 1°C or much larger such as 20°C.
A black background maximises visual impact, as black absorbs the light rays, ensuring the desired colour is the only one bounced back. Liquid Crystals work through a change in their molecular structure which alters the range of visible light they are able to reflect back, transitioning from black (no Reflection), through the visible light spectrum and then back to black.
Liquid Crystals in textile applications are sensitive to soiling and contact with foreign matter. This is influenced by the material used as the microcapsule shell wall, as this degenerates the capsules burst resulting in a loss of the effective ingredients.
Leuco dyes can use a wider range of colours although are limited to between two to three different colours in each application and with reduced accuracy in terms of temperature range. They are a dye so must be used over the secondary colour wishing to be displayed, for example a red Leudo dye over a yellow back ground, will change from red to transparent, thus displaying the yellow background, and perhaps a orange colour as an in between state. They are generally more robust and less expensive,
The dye pigments and the required solvent, which can be alcohol based such as ethanol, are microencapsulated. In a cool state the solvent will remain solid and the colour will be displayed due to electron interaction. When the temperature rises and the solvent becomes a liquid this electron interaction is disrupted and therefore no visible colour is displayed. Therefore Leuco dyes differ to that of liquid changing crystals as molecular structure does not change as a result of temperature, the solvent they are in reacts to the temperature and as a result of this mixes with the dye to provoke not a colour change, but a colour absence.
Video explaining how colour works: