Insight into the evaluation of colour changes of leuco dye based thermochromic systems as a function of temperature

• Leuco dye based three-component thermochromic composites have been studied.
• Investigation of wide range of component concentration ratios.
• Thorough evaluation of dynamic colour change in temperature dependence.
• Introducing cumulative colour difference as a most appropriate parameter to evaluate.

Evaluation of colour change as a function of temperature was studied on twenty eight samples of bulk ternary thermochromic composites using varying concentrations of crystal violet lactone as a colour former, bisphenol A as a developer and tetradecanol as a co-solvent. A number of parameters describing the colour change – reflectance, trichromatic values X and Y, visual colour densities DX and DY, and several parameters in CIELAB space, were obtained as a function of temperature. From these functions, the colour contrast, temperature sensitive interval, rate of colour change and width of hysteresis loop as the characteristics of thermochromic composites were evaluated and compared. The results show that cumulative colour difference ΔEC, representing the length of the path of colour change in CIELAB space, might be the best parameter in such an evaluation. Taking into account both the rate of colour change and the width of hysteresis loop the best performing thermochromic system could be selected.

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