Temperature Indicating Paints
TMC Temperature Indicating Paints comprise a range of different coatings based on inorganic color changing pigments that are 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 color.
They are widely used in industrial applications with single color change paints to show heat exposure events. Used as a warning coating on insulated vessels, they can be used to show where thermal linings breakdown and where surface temperatures rise. In the motor industry, they can show when components have exceeded certain temperature limits.
Paints with multiple color changes are commonly used in the aerospace industry for thermal mapping of jet engine components. They are combined with paints suited for combustors and turbine that work at temperatures more than 1000ºC.
The different paints are based on a range of different chemicals. Many of these are based on heavy metals such as lead, cobalt, and chromates. This indicates some are restricted to specialist scientific testing and research work. Safety Data Sheets provide identification of any hazardous components.
Temperature exposure causes chemical changes. For example, oxidation, the crystal structure changes with loss of associated molecules such as water or ammonia, depending on type. The color changes and temperature they occur at are mostly dependent on the length of heating time. 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 color changes are chemical reactions that are accelerated by increasing temperature so a paint that takes several hours to change at one temperature can show the same color change much sooner if the temperature is increased.
When a color 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 temperature and time relationship, it is possible to produce a calibration curve of paints. The graph 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. Adhesion to metal surfaces requires thorough preparation with abrasive cleaning, such as grit blasting, and grease removal with suitable solvents. During thermal mapping in the aerospace industry, applying thermal paint directly to metal is preferred, especially in high temperature applications.