Colorimetric properties of reversible thermochromic printing inks

The colorimetric properties of three, leuco dye-based thermochromic inks at 31 C activation temperature were studied. The inks had similar pigment particle size distribution and comparably thick microcapsule shells with similar stability against oxygen plasma etching. The colour of the inks was dependent on temperature as well as the thermal history of the sample, which gives rise to colour hysteresis. The area of the observed hysteresis loop in CIELAB colour space determines how different colours appear on the sample when decolourisation and colourisation processes are compared. The largest colour difference achieved for the same sample that arose because of different thermal history and temperature were determined. Four characteristic temperatures were ascribed to the two chemical reactions causing colour hysteresis. The stability of the decolourised state at cooling <activation temperature was confirmed as >10 h. The reversibility of the thermochromic effect diminished approximately linearly with the highest heating temperature. (C) 2010 Elsevier Ltd. All rights reserved.