All about thermochromic materials
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 (colors) change according to the temperature. The thermochromic materials (pigments, dyes, inks, paints...) become 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 compounds.
Thermochromic liquid crystals
It is the class of thermochromic organic products 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 (short/long range interactions) 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 of the dyes, 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) at low temperatures.
Microencapsulated thermochromic pigments
This class of compounds allows to reach thermochromic 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.
- The dye is a 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.
- 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.
- 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 thermochromic pigments (dyes) is extremely sensitive to shear forces and temperatures above 250°C.
Hybrid and inorganic thermochromic compounds
At 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 thermochromic materials, the thermal expansion of chemical bonds leads to the separation of cations from anions. The result is a progressive evolution of colors with temperature. A thermochromic product 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 products, thermochromism is associated with a modification of the coordination with a phase transition. This is notably the case for the pigment 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 powder derivatives also exhibit a color transition 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 (from low spin state to high spin state). 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 color) and alumina (white color) to form the pigment CoAl2O4 (blue color) at high temperature. Barium carbonate powder (white color) can also be mixed with hematite powder (red color) to form the BaFeO3 pigment (dark gray color) at high temperature.
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.