Thermochromic materials replace thermocouples and thermometers


In many industrial sectors, electronic equipment provides unparalleled precision. Today, their use is widespread. However, using electronics is not always the best solution for industrial safety. The use of thermochromic coatings is replacing thermo-sensors and/or thermometers: this is an industrial reality.

In which cases does a thermochromic coating have an advantage over electronics?

When it is impossible to use electrical wiring

Thermocouple et thermomètre industriel

One reason for using a thermochromic coating as a replacement for a thermometer and/or thermo-sensor may be the need to map the temperature in an environment where it is impossible to insert any electrical wiring.

This is the case, for example, in aerospace and defense, where thermochromic coatings have the function of mapping the temperature gradient of an operating engine.
The space in a combustion chamber is so small that the use of a thermo-couple is absolutely unthinkable. A product such as thermochromic paint, on the other hand, can be deposited in the combustion chamber. After drying, the engine is started, then stopped and dismantled. The irreversible nature of thermochromic paint makes it possible to analyse the gradient(s) of colour change. This data provides access to the thermal map achieved by the combustion chamber during operation. Such information is essential for optimizing the design of future engines: limiting their fuel consumption, the release of certain gaseous pollutants and the fatigability of structures.

A thermochromic material is safe and can act completely autonomously


Other fields call on thermochromic materials for their ability to act without electricity, in total autonomy. This type of demand occurs in particular for the storage of sensitive materials for years in the field of industrial maintenance (energy, petrochemical, nuclear, etc.). In this case, electrical autonomy is a requirement.

Thermochromism is a thermodynamic process that acts as a passive solution. The colour change only occurs when the temperature threshold is crossed.

It is all about the initial programming of the material to present the right threshold for triggering the temperature switch. Like a living being, the colour change information is written into the DNA of the thermochromic material itself. It is therefore a robust phenomenon that contrasts with the electronic fragility of a thermometer and/or thermo-sensor: failures, hacking, maintenance issues etc.

To map an object as a whole…

In predictive maintenance and/or industrial safety, the need may be to access the thermal behaviour of a large surface, an object or products in their entirety. This is the case, for example, of an industrial chimney (an oven, a refrigerated truck, etc.) whose external temperature is to be monitored during operation. The use of a thermo-sensor and/or thermometer gives access to local information but not to the global temperature map.

On the other hand, the simple change of colour of a thermochromic coating positioned on the outside of this industrial chimney makes it possible to identify abnormal areas of heating. This is a simple way to monitor any shrinkage of the thermal insulation and to schedule maintenance. We are entering the world of the industry of the future, of predictive maintenance.

… or control a thermal evolution unitary at lower cost

Pigment thermochromique irreversible

The thermochromic solutions approach is also selected in many sectors where unit control is required. This is the case for example in the medical and/or food industry. Unlike the use of a thermo-sensor and/or thermometer, a reversible thermochromic product with a memory effect or an irreversible thermochromic product makes it possible to control the thermal evolution of an element on an individual scale, of each medicine, each package: to identify, for example, a break in the food cold chain, to attest to the conformity of the storage of a medicine or a vaccine.

Another advantage of the thermochromic approach is that it combines the individual scale with ease of implementation and a low-cost solution. This is the case for all indicators of well-being and comfort. For example, thermochromic tablets on a pizza box, a drink,… which indicate a good consumption temperature.

To meet traceability or anti-counterfeiting challenges

Finally, there are more subtle uses for traceability, to easily identify a break in the cold chain. A thermochromic indicator can, for example, be positioned at the level of a scan-code. It becomes heat-sensitive and can be modified by adding or removing a code in the event of an anomaly. This approach makes it possible to remove an item from a supply chain or sales chain without alerting (worrying) the consumer.

Finally, there are other uses for thermochromic materials to introduce security features for anti-counterfeiting purposes. Depending on the needs, this can be visible. For example, a coating on packaging that will change colour if heated with a hot source in an attempt to peel off a label and open a sealed document. In other cases, the heat reactive material is introduced and will be revealed during authentication by an authority to demonstrate the origin of a product.

An infinite range of possibilities

stylo-effacableThis overview would not be complete without mentioning the world of entertainment applications. One example is the erasable ink found in supermarkets: this is a thermochromic material with a memory effect. The friction of the eraser causes a local heating around 45-60°. The ink changes colour and becomes transparent. The opposite effect (reappearance of the writing) is obtained by cooling in the freezer. Thermochromic inks are also used to decorate nails, hair, fun objects such as coffee cups, mugs, etc.

In conclusion

The applications are numerous! Many industrial fields select thermochromic materials. This approach to colour modification (in case of anomaly/correct consumption temperature) has the advantage of being understandable by all, easy to use, and easy to interpret. For brands, it is often an element of differentiation. A simple way to modernize (re-brand) existing products. Bringing a touch of design, innovation, interactivity with the environment, safety and consideration of consumer well-being.

However, particular attention should be paid to the origin of these materials. Some families of leuco dyes, for example, available on the market may contain bisphenol-A.


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