An evaluation of thermochromic prints based on microencapsulated liquid crystals using variable temperature colour measurement

A variable temperature colour measurement technique has been developed to characterise the temperature-dependent colour change, known as ‘colour play’, observed with thermochromic prints based on microencapsulated liquid crystals. The effect is most pronounced over a black background. The liquid crystals exhibiting the thermochromic effect adopt a chiral nematic phase with an underlying smectic A phase. A variety of methods for presenting and interpreting the data obtained, as the colour of the print passes through the spectrum, is explored. The effect of temperature on the wavelength of reflected light, a*b<ast; values, lightness, chroma and hue angle have been studied. The ‘blue phase’, exhibited transiently as the liquid crystal passes out of the chiral nematic phase to form an isotropic liquid, has been detected as an increase in lightness and a hue shift. Prints on nylon/lycra are found to have acceptable wash fastness, but show some sensitivity to light.