A thermochromic coating (from the Greek thermos for temperature and chromos for color) is, by definition, made of a pigment/colorant whose optical properties change according to temperature. The thermochromic material becomes reactive when exposed to a heat source.
What are the main families of thermochromic materials?
There are today 3 great families of materials: the organic thermochromic materials (with the example of the liquid crystals or `leucodyes’), the thermochromic hybrids and the thermochromic inorganic materials.
The thermochromic liquid crystals
It is about the class of organic materials which present the characteristic to change state (transition of phase) with the temperature by making intervene a series of transition with physico-chemical properties intermediate between the crystal and the liquid (also known as mesomorphs = of Greek ” of intermediate form “).
During this transformation, the orientation of the molecules is completely upset. The rise in temperature leads to an increase in thermal agitation and a growing disorder from a highly organized phase (crystal) 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, transition temperature are parameters of primary importance to choose the right thermotropic phase. Depending on the molecular arrangement, different colors of thermochromic liquid crystals are available. It is generally a sequence that goes from black (see red, orange) at high temperatures to blue (violet) spectral colors at low temperatures.
Micro-encapsulated thermochromic materials
This class of materials allows to reach thermochromic properties between -5 °C and 80 °C. They are microcapsules made of three components: a dye (color former), a weak acid (color developer) and a solvent.
- The dye is a chemochromic organic molecule, which has the ability to change state (colored to colorless) depending on 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 gives the reversible function to 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 component of the thermochromic microcapsule. It is generally a polar solvent like an alcohol or an ester.
The presence of a microcapsule is an undeniable advantage to preserve the chemical integrity and the reversibility of the encapsulated liquid and to protect it from the environment. However, this class of pigments is extremely sensitive to shear forces.
Hybrid and inorganic thermochromic materials
At the scale of a hybrid or inorganic pigment, the thermochromic behavior 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. This results in a progressive evolution of properties with temperature. 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 change in coordination. 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 temperature. 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 black. 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 (black) at high temperature.
Examples of thermochromic materials are numerous, as are the mechanisms. Each of these generations has its advantages and limitations. We put our 15 years of experience in the field of thermochromic materials at your disposal to design and produce inks and paints with high added value.