Temperature Indicating Coatings and Materials Information
Temperature indicating coatings and materials include labels, paints, crayons, or other markers that change color or shape when exposed to a certain temperature. These products are either non-reversible and are designed for one-time use or are reversible and can change back and forth to provide an indication of the present temperature.
Temperature indicating paints and materials incorporate substances that exhibit thermochromism, a property for a material to undergo a change in color in response to a change in temperature. Most thermochromic substances are micro-encapsulated. This increases the stability and life expectancy of the product. The three most common types of thermochromic substances used in temperature indicating paints and materials are leuco dyes, thermochromic liquid crystals, and chemical indicators.
Leuco dyes contain molecules that exhibit two forms, one of which is colorless. They undergo a reversible color change that occurs over a temperature differential of approximately 5ºF (3ºC). Available in several different forms-- slurry, powder, water-based ink, epoxy, or master batch-- leuco dyes are engineered to change color over a broad range of temperatures and are available in several different colors.
Thermochromic Liquid Crystals (LCs)
Thermochromic liquid crystals, similar to liquid crystal displays, are color changing crystals that undergo a reversible thermally induced color change. LCs are microencapsulated and coated on a black backing. As they approach their temperature rating they pass through the colors of the spectrum in sequence from tan to green to blue before turning back to black. They are calibrated to display green at a rated temperature. LCs can possess acute thermal sensitivity, detecting temperature changes as small as 0.2ºF, although they can be costly when compared to other temperature indicating paints and materials.
Chemical Indicators (CIs)
Chemical indicators are almost always irreversible temperature indicators. These one-time use products undergo a phase change when they reach their rated temperature. Typically this involves a sharply defined melting point that triggers an interaction between a chemical compound and a substrate to produce a permanent color change. CIs are primarily used for temperature monitoring and quality control. Supplied as an adhesive strip or label they denote whether temperature falls below or exceeds a rated threshold.
Extreme temperatures, UV light, and strong solvents can rapidly deteriorate life expectancy.
Application | Industry
Temperature indicating paints and materials are used to monitor wear and friction that can arise when machinery parts are overheated, to monitor the operation of engines and valves for warranty protection, measuring surface temperatures on printed circuit boards (PCB), optimization of temperatures in processing applications, and quality control of equipment used in food preparation. The following list of industries makes use of temperature indicating inks and materials for these purposes:
Food and beverage
Maintenance / MRO
Medical / Healthcare
Defense / Military
OEM / Industrial
Process / Firing equipment
Semiconductor / Wafer
Temperature indicating paints and materials include the following types of products.
Coating: Coatings are thin films deposited upon materials to add or enhance desired properties. Temperature indicating coatings are used to gauge temperatures on equipment for petrochemical applications, critical electronic components, and circuit breakers. They provide color indication of surface temperature and may also possess required features such as conductivity or corrosion resistance.
Paint: Paint is a pigmented liquid or powder used to protect and/or beautify substrates. Color changing paint can be used to monitor surface temperatures of equipment or machinery. Monitoring of surface temperatures alerts operators of potential hazards or required maintenance.
Ink: Industrial inks are liquids, powders, or transfer tapes that are printed, sprayed, rolled, or transferred onto substrates or media to create markings, patterns, print or graphics. Color changing ink contains both a conductive ink and a leuco dye or thermochromic liquid crystals. They are commonly used to display battery life or to indicate that food and beverage products have reached a desired temperature.
Crayon/Pencil: Temperature indicating crayons or pencils make chalky marks that become glossy, glazed over or shiny at a specific temperature. Some pencil or crayon marks may change color with temperature. They are typically sold as sets to accommodate a specific temperature range. They are used for applications such as preheating, welding and fabrication, annealing, soldering, molding, thermal processing, and other machine or equipment operations where precise surface temperature indication is desired.
Film/Label: Film and label products are a thin sheet of plastic, tape, or label for adhering to a surface. They can be secured to machinery or equipment and register temperature based on chemical reactions or liquid crystal indicators. A temperature indicating label may consist of a scale to show the exact temperature, or may simply produce a change in color that indicates a temperature threshold has been breached.
Marker/Pen: Markers and pens incorporate color changing ink, paint or marking materials. Like paints and coating they may be used to monitor surface temperatures and alert operators of potential hazards or required maintenance.
Marking material: Marking materials include inks, laser marking materials, coatings, tapes, and supplies used to identify industrial parts, products, materials and packaging.
Pyrometric cone, disc or ring: Pyrometric cones, discs or rings change shape when a specific temperature in a furnace, kiln, or oven is reached. Witness cones are another name for pyrometric products because these products record or witness the same thermal conditions as the parts they accompany through an industrial heating or firing process.
The indicating temperature is the temperature or temperature range at which the coating or material provides an indication of the temperature through a reversible or irreversible color change, shape change, or appearance change.
Thermal sensitivity is a measurement of precision describing the temperature range over which color or shape change takes place.
Adhesive coatings are used to bond the product to a surface.
Chemically resistant coatings or labels resist acids, alkalis, oils, and general chemicals.
Heat resistant products resist damage from heat, or are formulated for use in high-temperature environments.
Irreversible temperature indicating materials or coatings only change color, shape, or appearance once after the first exposure to the indicating temperature of the material. These single-use color changing products are used for quality control and preventive maintenance of electrical systems, product packaging, medical applications, or to monitor overheating of HVAC components.
Reversible temperature indicating materials or coatings change color, shape, or appearance multiple times after the exposure to the indicating temperature of the material. The color change is reversible after the product cools down. A reversible temperature indicator may incorporate a scale to monitor the actual temperature.
ASTM F1416 – Standard guide covering information on the selection of commercially available time-temperature indicators (TTIs) for noninvasive external package use on perishable products, such as food and pharmaceuticals.