engleski

Evaluation of light stability of liquid crystal inks

Thermochromic (TC) inks change their colour by exposure to different temperature. The two most important types apply liquid crystals (TLCs) or leuco dye-based composites. The composites with leuco dyes change colour reversibly by selective absorption of the incident light. Contrary, TLCs do not absorb visible light, but reflect it within a narrow spectral region, which produces practically monochromatic colours such as in rainbow (iridescent colours), frequently named as "colour play". The TC inks are used in security printing, brand protection, smart packaging, medical applications, marketing and novelty printing. TC printing inks have low stability against high temperature and light. The leuco dye - based inks are proved to have poor stability of pigment capsules, polymer envelopes and binder against light and high temperatures, but the stability of TLC inks has not yet been evaluated. Therefore, the TLC inks were printed on paper - based substrates and exposed to daylight for varying time periods. The changes to the dynamic colour due to exposure to daylight were measured, using Ocean Optics spectrophotometer with 50 mm integrating sphere operating in (8°:di) measuring geometry, within 400-700 nm spectral region. The results show poor stability of dynamic colour against daylight, thus the thermochromic effect in TLC inks disappears in a short time.

thermochromic printing inks; liquid crystals; leuco dyes; light stability

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