Organic thermochromic pigments

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. 

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. 

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. 

Typically the chromophores used are pH sensitive, i.e. acidochromic or ionochromic (see section 1.4), and the medium is one whose pH varies with temperature. A variety of pH sensitive chromophores are known that are readily synthesised and can be modified to provide a wide range of shades. Application of these pH sensitive dyes in pressure and thermally sensitive papers for digital printing, e.g. fax papers, where the whole object is for the process to be irreversible, will be covered under ionochromism. The main area of application for reversible indirect thermochromic materials is as thermochromic organic pigments, as will be described below (section 1.3.4.1). 

Colorants Coloration of plastics, dyes, organic and inorganic pigments Benzidene (diarylide) (yellow), nickel azo (yellow), benzimidazole, copper phthalocyanine (blue-green) and isoindolinone (yellow-orange and reds), heavy metal oxides and sulfides, heavy metals and titanium dioxide, dihydroindolizine (DHI) and thermochromic antimony... 

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