Temperature sensitive materials that permanently change its color at a characteristic temperature were analysed. Four different samples were tested: thermal paper, a commercially available printing ink and printing ink, which is under development (two different versions).
The influence of static and dynamic heating on the color of the sample were analysed. The static heating was obtained in a laboratory oven with adjustable temperature, while for the dynamic one a special device with the adjustable heating rate and temperature range was used.
Visible reflection spectra were measured and used to determine the corresponding color values. These color values were applied to calculate the differences of color appearing in the sample in dependence on the heating temperature.
The analysed samples are different in the following details:
a) the temperature range where the color changes,
b) the size of the color difference between the initial and completely colored state,
c) depending of the color change at given temperature on the thermal history,
d) different coloration in the case of static and dynamic heating of the sample.
In general, the color of the analysed samples is independent on their thermal history. All of the analysed samples have similar dependence of the color on the heating temperature and have a stable color in the fully coloured states. Interestingly, the analysed thermal paper shows the entire coloration only after the corresponding sample has cooled down, which was not observed on any other sample.