The behavior of irreversible thermochromic inks in the paper recycling process.

Thermochromic printing inks are special chromogenic inks that change their color by exposure at a certain temperature, which is usually called the activation temperature (TA). The color change can be irreversible or reversible. The property of irreversible color change in relation to temperature exposure over time, allows the development of numerous indicators. As such, they can be used to monitor the storage and transportation of temperature-sensitive products, such as refrigerated and frozen foods, drugs, temperature-sensitive chemicals, or biological materials, etc. Since the ink formulation is an important factor in the deinking process, the aim of this research is to determine the recycling efficiency of irreversible thermochromic prints. Thermochromic inks differ from conventional printing inks in formulation and pigment size due to the presence of microcapsules that are much larger than conventional pigment particles. For this study, two irreversible thermochromic printing inks are printed by screen printing in full tone, on one printing substrate. To explain the behavior of these inks in the recycling process, the hydrophobicity of the sample surface was examined, as one significant factor in the flotation process using the contact angle of water. Recycling of prints was carried out in laboratory conditions, by chemical deinking flotation in alkaline conditions. Sheets were made for each sample before and after flotation. Optical properties of the recycled samples show that these inks are difficult to recycle. The deinking flotation method is not entirely the best method for recycling of irreversible thermochromic inks because slight differences in the optical properties of the laboratory paper samples before and after flotation are achieved. Future research should go in the direction of new techniques such as adsorption and enzymatic deinking. [ABSTRACT FROM AUTHOR]