TMC temperature indicating paints comprise a range of different coatings which are based on mostly inorganic colour changing pigments dispersed in solvent based solutions of acrylic and / or silicon based polymers. They are useful in determining heat patterns and showing temperature distribution by showing changes in colour.

They are widely used in industrial applications with single colour change paints typically used to show heat exposure events. Used as a warning coating on insulated vessels they can be used to indicate where thermal linings breakdown and surface temperatures rise. In the motor industry they can show when components have exceeded certain temperature limits.

Paints with multiple colour changes are commonly used in the aerospace industry for thermal mapping of jet engine components with paints suited for combustors and turbine work at temperatures in excess of 1000ºC.

The different paints are based on a range of different chemicals and many of these are based on heavy metals such as lead, cobalt and chromates. This means some are restricted to specialist scientific testing and research work. Safety Data Sheets provide identification of any hazardous components.

[Image: Gas Turbine Combustion Chamber]

Temperature exposure causes chemical changes such as oxidation, crystal structure changes with loss of associated molecules such as water or ammonia, depending on type. The colour changes and the temperature they occur at are mostly dependent on the length of heating time so some knowledge of the time and temperature relationship of the paints is required for a more accurate understanding of temperatures that may have been experienced. The colour changes are chemical reactions which are accelerated by increasing temperature so a paint that takes several hours to change at one temperature can show the same colour change much sooner if the temperature is increased.

When a colour change temperature is quoted for thermal paint it needs to be qualified by a statement on the length of time required at this temperature for these reasons.

By measuring the temperatures and time relationship it is possible to produce a calibration curve of paints. Below is a typical curve. Most paints show this same characteristic curve. Calibrations have been carried out over the years using various methods including immersion of coupons into specialist furnaces and resistance generating test plates (known a s butterflies) which generate thermal gradients.

Thermal paints are typically applied by spraying with a dry film of around 18 – 28u typically required. Adhesion to metal surfaces requires thorough preparation with abrasive cleaning, such as grit blasting, and grease removal with suitable solvents. In thermal mapping in the aerospace industry applying the thermal paint directly to metal is preferred, especially in high temperature applications.