Thermochromic pigments

Thermochromism Heat (intrinsic and indirect) Thermochromic pigments, temperature indicators and thermometers, security and novelty printing... 

Whilst this definition is academically accurate for many inorganic and organic materials, the label thermochromic has also been applied to important technical areas that involve other external influences as well as heat in the observed colour change, e.g. thermochromic pigments. 

Composite thermochromic pigments consist of three components a pH sensitive dye, a proton donor, which acts as the colour developer, and a hydrophobic, nonvolatile co-solvent. To achieve the desired effect the components are mixed in specific ratios and usually encapsulated to protect the system in subsequent applications. A review of the patent literature on these compositions has been published. ... 

Figure 1.19 Schematic of a composite organic thermochromic pigment.

The main application area for these indirectly thermochromic, reversible systems is the manufacture of organic thermochromic pigments. 

Composite thermochromic pigments consist of three main components pH-sensitive dye, proton donor that acts as a colour developer, and hydrophobic, non-volatile cosolvent. To achieve the desired effect, these three components are mixed in a specific ratio and most often encapsulated, which preserves the system from external influences and significantly increases applicability. 

The application area for thermochromic pigments is vast indeed. In the context of indicators they may be used for visual density identification of atomic species caused... 

Problems associated with fastness properties of thermochromic composites remain to limit a wide range of apphcation possibUities, and at the forefront of the most important issues to be dealt with is poor fastness to UV irradiation, which excludes long-term exposure to daylight and direct sunlight, thus limiting the products for indoor use only. It was also found that temperature directly influences the properties of organic thermochromic pigments, and therefore they should be shielded from undesirable sources of heat. 

Oda, H., 2008a. PhotostabiUzation of organic thermochromic pigments action of benzotriazole type UV absorbers bearing an amphoteric counter-ion moiety on the light fastness of color formers. Dyes and Pigments 76, 270—276. 

---