Thermochromic ink–paper interactions and their role in biodegradation of UV curable prints

In this study, biodegradability aspects of UV thermochromic leuco dye print on three different paper materials (synthetic, recycled, and bulky) were studied using the soil burial test under anaerobic conditions. Biodegradation of UV curable thermochromic prints were evaluated for changes by visual examination, microbial growth assay, weight loss measurements, Fourier transform infrared spectroscopy, scanning electron microscopy and colorimetric measurements. Results showed a better absorption of ink into the bulky paper structure, followed by recycled paper. Synthetic paper is not absorbent. The ink binder altogether with classic pigment and microcapsules penetrates into the structure of bulky paper. Recycled paper absorbs ink binder mostly with classic pigment. Better adsorption of binder into the bulky paper structure results in thinner layer of ink binder on the paper surface. On non-absorbent synthetic paper, microcapsules are covered with thicker layer of ink binder. In the case of bulky paper, the highest rate of biodegradation was observed, resulting in a higher number of bacteria, higher weight loss, higher changes in colour (destruction of almost all microcapsules) and almost complete reduction in thermochromic effect of the prints. Results show that microcapsules, which penetrate into the structure of bulky paper, are promoting the rate of bulky paper biodegradation. The opposite behaviour was noticed for the penetration of classic pigment into the structure of recycled paper, which resulted in remarkable reduction of recycled paper biodegradation rate. The thicker layer of ink binder (containing classic pigment) on the surface of microcapsules on recycled paper resulted in slower rate of microcapsules degradation and smaller colour change of the print. The thickest layer of ink binder and classic pigment on synthetic paper surface causes the slowest rate of biodegradation of print.