Thermochromism is the ability of
substanceto change colordue to a change in temperature. A mood ringis an excellent example of this, but it has many other uses. Thermochromism is one of several types of chromism.
The two basic approaches are based on
liquid crystals and leuco dyes. Liquid crystals are used in precision applications, as their responses can be engineered to accurate temperatures, but their color range is limited by their principle of operation. Leuco dyes allow wider range of colors to be used, but their response temperatures are more difficult to set with accuracy.
Some liquid crystals are capable of displaying different colors at different temperatures. 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, through anisotropic chiral or twisted
nematicphase, to the high-temperature isotropicliquid phase. Only the nematic mesophase has thermochromic properties; this restricts the effective temperature range of the material.
The twisted nematic phase has the molecules oriented in layers with regularly changing orientation, which gives them periodic spacing. The light passing the crystal undergoes
Bragg diffractionon these layers, and the wavelength with the greatest constructive interferenceis reflected back, which is perceived as a spectral color. A change in the crystal temperature can result in a change of spacing between the layers and therefore in the reflected wavelength. The color of the thermochromic liquid crystal can therefore continuously range from non-reflective (black) through the spectral colors to black again, depending on the temperature. Typically, the high temperature state will reflect blue-violet, while the low-temperature state will reflect red-orange. Since blue is a shorter wavelength than red, this indicates that the distance of layer spacing is reduced by heating through the liquid-crystal state.
Some such materials are
cholesteryl nonanoateor cyanobiphenyls.
Liquid crystals used in dyes and inks often come microencapsulated, in the form of suspension.
Liquid crystals are used in applications where the color 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 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.
Thermochromic dyes are based on mixtures of leuco dyes with suitable other chemicals, displaying a color change (usually between the colorless leuco form and the colored form) in dependence on temperature. The dyes are rarely applied on materials directly; they are usually in the form of
microcapsules with the mixture sealed inside. An illustrative example is the Hypercolorfashion, where microcapsules with crystal violet lactone, weak acid, and a dissociable salt dissolved in dodecanolare applied to the fabric; when the solvent is solid, the dye exists in its lactone leuco form, while when the solvent melts, the salt dissociates, the pH inside the microcapsule lowers, the dye becomes protonated, its lactone ring opens, and its absorption spectrum shifts drastically, therefore it becomes deeply violet. In this case the apparent thermochromism is in fact halochromism.
The dyes most commonly used are
spirolactones, fluorans, spiropyrans, and fulgides. The weak acids include bisphenol A, parabens, 1,2,3-triazolederivates, and 4-hydroxycoumarinand act as proton donors, changing the dye molecule between its leuco form and its protonated colored form; stronger acids would make the change irreversible.
Leuco dyes have less accurate temperature response than liquid crystals, and are used in applications where accuracy is not required. They are suitable for general indicators of approximate temperature ("too cool", "too hot", "about OK"), or for various novelty items. They are usually used in combination with some other pigment, producing a color change between the color of the base pigment and the color of the pigment combined with the color of the non-leuco form of the leuco dye. Organic leuco dyes are available for temperature ranges between about -5 °C and 60 °C, in wide range of colors. The color change usually happens in a 3 °C interval.
Leuco dyes are used in applications where temperature response accuracy is not critical: eg. novelties, bath toys,
flying discs, and approximate temperature indicators for microwave-heated foods. Microencapsulation allows their use in wide range of materials and products. The size of the microcapsules typically ranges between 3-5 µm (over 10 times larger than regular pigment particles), which requires some adjustments to printing and manufacturing processes.
An interesting application of leuco dyes is in the
Duracellbattery state indicators. A layer of a leuco dye is applied on a resistive strip to indicate its heating, thus gauging the amount of current the battery is able to supply. The strip is triangular-shaped, changing its resistance along its length, therefore heating up a proportionally long segment with the amount of current flowing through it. The length of the segment above the threshold temperature for the leuco dye then becomes colored.
Exposure to ultraviolet radiation, solvents and high temperatures reduce the lifespan of leuco dyes. Temperatures above about 200-230 °C typically cause irreversible damage to leuco dyes; a time-limited exposure of some types to about 250 °C is allowed during manufacturing.
Thermochromic inks or dyes are temperature sensitive compounds, developed in the
1970s, that temporarily change colorwith exposure to heat. They come in two forms, liquid crystals and leuco dyes. Liquid crystals are used in mood rings. Leuco dyes are easier to work with and allow for a greater range of applications. These applications include: flat thermometers, battery testers, clothing, and the indicator on bottles of maple syrupthat change color when the syrupis warm. The most well-known line of clothingutilizing thermochromics was Hypercolor. The thermometers are often used on the exterior of aquariums, or to obtain a body temperaturevia the forehead.
Thermochromic paint is a relatively recent development in the area of color-changing
pigments. It involves the use of liquid crystalor leuco dyetechnology. After absorbing a certain amount of light or heat, the crystallic or molecular structure of the pigment reversibly changes in such a way that it absorbs and emits light at a different wavelength than at lower temperatures. Thermochromic paints are seen quite often as a coating on coffee mugs, whereby once hot coffee is poured into the mugs, the thermochromic paint absorbs the heat and becomes colored or transparent, therefore changing the appearance of the mug.
Thermochromic papers are used for
thermal printers. One example is the paper impregnated with the solid mixture of a fluorandye with octadecylphosphonic acid. This mixture is stable in solid phase; however, when the octadecylphosphonic acid is melted, the dye undergoes chemical reaction in the liquid phase, and assumes the protonated colored form. This state is then conserved when the matrix solidifies again, if the cooling process is fast enough. As the leuco form is more stable in lower temperatures and solid phase, the records on thermochromic papers slowly fade out over years; this may lead to interesting effects in combination with accounting records, receipts from a thermal printer, and a tax audit.
Another good example of this is the color indicators on batteries. The indicator turns green if the battery still possesses a charge. This works by passing the charge of the battery through a small
resistoron the battery, and causes the pigment to absorb heat. Once the paint has absorbed enough heat from the current of the battery, it changes from black to green (usually), thus indicating that the battery still has a fair amount of charge left in it.
A simple-to-make thermochromic compound is
zinc oxide, which is white at room temperature but when heated changes to yellow due to various types of crystallattice defects. On cooling the zinc oxide reverts to white. Also lead(II) oxidehas a similar color change on heating. These solids are technically semiconductors, and the color change is linked to their electronic properties. Copper mercury iodideundergoes a phase transitionat 55 °C, reversibly changing from a solid material at low temperature to a dark brown solid at high temperature. Other such material is mercury(II) iodide, a crystalline material which at 126 °C undergoes reversible phase transitionfrom red alpha phase to pale yellow beta phase. Yet another example is nickel sulfate, green at room temperature but becoming yellow at 155 °C. Vanadium dioxidehas been investigated for use as a "spectrally-selective" window coating to block infraredtransmission and reduce the loss of building interior heat through windows. This material behaves like a semiconductorat lower temperatures, allowing more transmission, and like a conductor at higher temperatures, providing much greater reflectivity. [http://www.solgel.com/articles/August00/thermo/Guzman.htm] [http://www.azom.com/details.asp?ArticleID=2587] The phase change between transparent semiconductive and reflective conductive phase occurs at 68 °C; doping the material with 1.9% of tungstenlowers the transition temperature to 29 °C.
Other thermochromic solid semiconductor materials investigated for commercial use are CadmiumxZinc1-xSulfurySelenium1-y (x=0.5...1, y=0.5...1), ZnxCdyMercury1-x-yOxygenaSbSecTellurium1-a-b-c (x=0...0.5, y=0.5...1, a=0...0.5, b=0.5...1, c=0...0.5), HgxCdyZn1-x-ySbSe1-b (x=0...1, y=0...1, b=0.5...1). [http://www.patentstorm.us/patents/5499597.html]
Some minerals are thermochromic as well; for example some
chromium-rich pyropes, normally reddish-purplish, become green when heated. [http://minerals.gps.caltech.edu/mineralogy/undergrad/garnet_2001/garnet.html]
* [http://www.thermochromic-polymers.com/ Fraunhofer IAP - Department Chromogenic Polymers]
* [http://www.chromazone.co.uk/ Thermographic Measurements Ltd]
* [http://www.thermochromicinks.com/ B&H Colour Change Ltd]
* [http://www.geminnov.fr/ GEM'INNOV thermochromic microcapsules]
* [http://www.mutr.co.uk/pdf_files/SMARTCOL.pdf Thermochromic pigments]
* [http://www.colorchange.com/ Color Change Corporation]
* [http://www.t-m-c.com/ Liquid Crystal Resources LLC]
* [http://www.alsacorp.com/products/xposurepaint/xposurepaint_prodinfo.htm ALSA Corporation Exotic Paint]
* [http://jchemed.chem.wisc.edu/HS/Journal/Issues/1999/Sep/clicSubscriber/V76N09/p1201.pdf Thermochromism in commercial products, PDF]
* [http://www.changemeclothing.com/ Color Changing t-shirts]
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