Journal of Chemical Technology and Metallurgy, 54, 1, 2019 30 INVESTIGATION ON THE KINETICS OF COATED PAPER THERMAL AGING Dimitrina Todorova, Veska Lasheva, Greta Radeva, Radostina Cenkova ABSTRACT The paper aging is primarily attributed to the presence of carbonyl groups formed as a result of the oxidation of the hydroxyl groups at the second and third carbon atom in the elementary unit of the cellulose macromolecule and the presence of both aldehyde and carboxyl groups in the cellulose materials. Reduction of paper whiteness degree is mainly connected with the presence of lignin, hemicelluloses, the presence of metals and the sizing agent type. The paper as an elasto-plastic, capillary porous material quickly breaks down at an elevated temperature. The latter accelerates the chemical reactions that occur in paper in the course of aging. The depth of the paper changes depends not only on the temperature but also on the duration of the impact. For different paper types differing in composition the reaction rate constant is different. Its temperature dependence is described by the Arenius equation. The aim of this work is to carry out a kinetic analysis of the infuence of the temperature increase on the ageing of coated paper used for book covers. The accelerated thermal aging is conducted at three different temperatures – 90°C, 105°C and 120°C with duration of 6 h, 12 h, 18 h, 36 h and 48 h. In order to examine the kinetics of the process the level of whiteness is determined. The data obtained from the thermal aging of the paper are treated us- ing the laws of polychrome kinetics. Keywords: coated paper, kinetics, thermal aging, degree of brightness. Received 20 June 2018 Accepted 14 September 2018 Journal of Chemical Technology and Metallurgy, 54, 1, 2019, 30-34 University of Chemical Technology and Metallurgy 8 Kliment Ohridski, 1756 Sofa, Bulgaria E mail: todorova.dimitrina@uctm.edu INTRODUCTION A typical external sign of aging of paper refers to the decrease of its degree of brightness and the deterioration of the paper physical and mechanical properties. The paper aging is primarily attributed to the presence of carbonyl groups formed as a result of oxidation of the hydroxyl groups at the second and the third carbon atom of the elementary unit of the cellulose macromolecule and the presence of both aldehyde and carboxyl groups in the cellulose materials. The decrease of the paper whiteness degree is mainly connected with the presence of lignin, hemicelluloses, the presence of metals and the sizing agent type [1 - 3]. The investigation of the kinetics of accelerated thermal ageing of different paper types provides data on the progress of the ageing process which may facilitate the elucidation of its mechanism with advantage of the methods of treatment and storage. Ageing is a complex process which does not obey formal kinetic principles. Some authors describe the degradation reactions of the cellulose chains due to the ageing processes as reactions of a frst or a second order [4 - 7]. But the equations derived consider the processes discussed as taking place on a homogeneous surface. It is worth noting that the infuence of the physical structure of the surface has to be taken into account [8] in a real heterogeneous system. In addition, the correlations between the kinetics of cel- lulose degradation and the changes in a wide range of macroscopic properties such as brightness, Pc number or tensile strength [8, 9] have also to be taken into account. The aim of this work is to carry out a kinetic analysis of the temperature increase effect on the ageing of coated paper used for book covers. The analysis accounts the complex infuence of the heterogeneous nature of the system focusing to explain the mechanism of the process.