Composite 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.  

Switching of colour over a few degrees change in temperature Variable switching temperature by choice of co-solvent Wide choice of colours from yellow to red, blue, green and black 

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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. 

Figure 1.19 Schematic of a composite organic thermochromic pigment.

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. 

The pigment is coloured in the solid form of the material because in this state the colour former interacts with the developer, possibly via an ion-pair complex. Melting of the composite interferes with this interaction, leading to a negative thermochromic effect and a loss of colour. This is shown schematically in Figure 1.19. Possible mechanisms for the colonr change have been examined. ... 

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