INVESTIGATION OF THE IMPACT OF CHEMICAL AGENTS AND SUBSTRATES ON THE STABILITY OF THERMOCHROMIC PRINTING INKS

This research aims to analyse the chemical stability of thermochromic (TC) prints to determine which substances affect the TC effect and which parameters contribute to their better stability. Two different types of paper were chosen: synthetic and bulky, each distinguished by its unique chemical composition and absorbency characteristics. This study employed two TC inks based on leuco dyes—a 45°C activation temperature offset TC ink and a 31°C activation temperature TC screen printing ink. The offset TC printing ink dries through oxidative polymerisation, whereas the screen printing TC ink dries using UV radiation. Following printing, the samples underwent a chemical stability test in accordance with the ISO 2836 standard by employing water, ethanol, oil, and alkali. From the obtained results, we can conclude that the printing substrate significantly affects the stability of the TC ink. Samples printed on bulky paper were found to be less stable compared to those printed on synthetic paper. The paper's structure enables bulky paper to absorb more ink and chemicals, thereby accelerating the degradation of the TC ink. The greatest impact on the TC ink was shown by samples exposed to alkali printed with offset TC ink, while a slightly lesser impact was observed on samples printed with screen printing TC ink and exposed to oil and ethanol. UV TC inks require special drying conditions and drying units, while TC inks that dry by oxidative polymerization require an adequate air source and fewer conditions, making them cheaper. All these parameters ultimately define the cost of the product itself and the profitability of the entire printing process. The results of this study show that it is necessary to consider the type and purpose of the product itself and accordingly choose the appropriate TC ink and substrate for printing.)