engleski

INFRARED AND RAMAN SPECTRA OF THE THERMOCHROMIC OFFSET PRINT AFTER UV ILLUMINATION

Thermochromic offset prints are exposed to UV light in an aging chamber at different time intervals for 40 hours. A commercially available offset dye (Chameleon) with an activation temperature of 27°C blue color was applied. Measurements of CIELab values ​​showed significant changes already after 1 hour of accelerated aging. The recorded IR spectra are a reflection of the overall structure of the thermochromic ink. In the spectra of illuminated thermochromic print samples in the area of ​​stretching vibrations of OH groups, a broad band originating from the OH groups of cellulose from paper is observed and covers the possibly formed stretching bands of OH groups of oxidation products. The shoulder at 1715 cm –1 , whose intensity increases with irradiation time, can certainly be attributed to carbonyl groups in the formed oxidation products (most likely ketones) and indicates the decomposition of the polymer. For durable materials, such as most coatings and plastics, short-wave UV radiation is the primary cause of cleavage of the primary covalent bonds in the polymer chain. Encapsulation with melamine resin is a common method widely described in the literature. The other materials from which the thermochromic microcapsules are made also contain -OH groups, so photooxidation of the microcapsules can be assumed. Heterogeneity in the photooxidative degradation of polymers is a well-described and established phenomenon. Although it is possible to record the Raman spectrum of a thermochromic ink, unlike ATR spectra, the Raman spectra of thermochromic prints are dominated by paper bands. Therefore, it is not possible to derive information about the structural changes of the samples during UV illumination by means of Raman spectroscopy.

UV, Raman, OH groups, carbonyl groups.

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