Heat Resistance

Few pigments degrade at temperatures normally associated with coatings. However, at higher temperatures, pigments become more soluble and shading can occur. Thus, for organic pigments, heat stability is closely related to solvent resistance.
Pigments that prove to be satisfactory at a certain stoving temperature may be totally inadequate in an application requiring 10°C more.

Chemical stability is also likely to be critical at elevated temperatures. This is typically the case in powder coating systems. Another key area is coil coatings, as metal complex pigments may react with stabilizers at elevated temperatures, causing major shifts in shade.

Modifications can also occur in the crystal structure of pigments when subjected to elevated temperatures. Pigments with a highly crystalline structure are usually more heat resistant than polymorphic pigments, where the different crystal modifications may respond differently to heat. Typically, inorganic pigments have enhanced heat stability, though an exception is yellow iron oxide, which loses water from the crystal at high temperatures.

Heat stability is system dependant and this must be reflected in any test. All tests assess color at various temperature intervals and evaluate the color difference between the sample in question and a standard that has been processed at the minimum temperature.