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The expert's eye

What is a thermochromic material?

Thermochromism (from the Greek thermos for temperature and chromos for color) is a physical mechanism present in a material composed of a pigment or a dye whose optical properties change according to the temperature. The material becomes reactive when exposed to heat or cold and its color changes.

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What are the main families of thermochromic materials?

There are today 3 great families: organic, hybrids and inorganics products.

 

Thermochromic liquid crystals

It is the class of organic derivatives that has the particularity of changing state (phase transition) involving a series of transitions with intermediate physico-chemical properties between the crystal and the liquid (liquid crystals also called mesomorphic = from the Greek "of intermediate form").

During this transformation, the orientation of the molecules is completely changed. The rise in temperature leads to an increase in thermal agitation and a growing disorder from a highly organized phase (crystals) to a totally disordered phase (liquid). In the crystalline state, the order is three-dimensional managed by long range interactions while in the liquid crystal state it is short range controlled by a few molecules. It is the orientation of these units that distinguishes the type of mesophase: nematic, smectic and cholesteric.

The main characteristic of these liquid crystals called "thermotropic" is to contain at least one aromatic entity and more or less branched linear chains, such as 4-n-pentylbenzenethio-4'-n-decyloxybenzoate, the discoidal molecule hexa-4-octyloxybenzoate of triphenylene, linear polymers, with side chains or combined.

Birefringence, elastic constants, viscosity or transition area are parameters of primary importance for choosing the right mesomorphic products. Depending on the molecular arrangement, different colors of thermochromism are available. It is generally a sequence that goes from black (or even red, orange) at high temperatures to blue (violet) spectral colors at low temperatures.

 

Microencapsulated thermochromic compounds

This class of materials allows to reach thermo-sensitive properties between -5 °C and 80 °C. They are microcapsules made of three components: a colorant (color former), a weak acid (color developer) and a solvent.

  1. The dye is a chemochromic organic molecule, which has the ability to change state (colored to colorless) with its chemical environment (pH of the medium). The most used derivatives are: spirolactones, fluorans, spiropyrans, or fulgides.
  2. The weak acid plays on the balance of the acid/basic form of the dye. It is a proton donor. This component is responsible for the reversible response of the thermochromic material, and is responsible for the color intensity of the final product. The standard color developer is bisphenol A.
  3. The solvent is the third element of the thermochromic microcapsule. It is generally a polar solvent such as an alcohol or an ester.

The presence of a microcapsule is an undeniable advantage for maintaining the chemical integrity and reversibility of the encapsulated liquid and protecting it from the environment. However, this class of pigments is extremely sensitive to shear forces and temperatures above 250°C.

Hybrid and inorganic thermochromic derivatives

thermochromic-materialAt the scale of a hybrid or inorganic pigment, the thermochromism can be obtained from various physico-chemical mechanisms evolving with the temperature like the thermal expansion, the change of coordination, the modification of the crystalline field, the chemical decomposition.

For some materials, the thermal expansion of chemical bonds leads to the separation of cations from anions. The result is a progressive evolution of properties with temperatures. A material initially white (absorbing at the UV-visible border) can thus become yellow by progressive displacement of its absorption front (moved towards the visible wavelengths).

For other compounds, thermochromism is associated with a modification of the coordination with a phase transition. This is notably the case for the compound NiMoO4, which changes from green to yellow when the temperature increases. The coordination polyhedron of molybdenum changes symmetry. It goes from an octahedral symmetry at low temperature to a tetrahedral one at high temperatures. Some copper derivatives also exhibit this phenomenon due to the thermal expansion of chemical bonds by the Jahn-Teller effect.

In some cases, the modification of the crystalline field causes an abrupt change of electronic configuration with the passage from a weak field to a strong field. This phenomenon called spin transition is notably encountered for coordination complexes containing one or more metal centers with a 3d4, 3d6 or 3d7 configuration.

Concerning the evolution of the chemical reactivity coupled to thermochromism, a typical example is the change of the oxidation state of Nickel. The change from Ni(OH)2 to NiO + H2O at 200°C results in a color change from green to dark grey. It is also possible to react cobalt oxide (black) and alumina (white) to form the compound CoAl2O4 (blue) at high temperature. Barium carbonate (white) can also be mixed with hematite (red) to form the compound BaFeO3 (dark gray) at high temperature.

 

Conclusion

Examples of thermochromic materials are numerous, as are the mechanisms. Each of these generations has advantages and limitations. We put at your disposal our 15 years of experience in the field of thermochromism to design and produce inks and paints with high added value, and to develop on request new pigments or dyes.