Ink and dirt behavior in repulping after artificial aging of coldset offset-printed newspapers at different temperatures

Effects of the temperatures to which offset-printed papers are exposed during storage and transportation in summer and the length of exposure to these temperatures on ink release from the surface of paper for recycling have remained largely unclear. This work aims to determine how severely temperatures of 30°C and 45°C and various exposure times to these temperatures affect the release of ink from the surface of paper and its fibers when coldset offset-printed newspapers are repulped at conventional alkaline deinking conditions. Reference tests were carried out at 60°C, which is the standard temperature at which aging tests are conducted on papers for recycling. The results showed that after exposing newspapers to 30°C for one week, strong ink fragmentation, significantly impaired ink release from fibers, and decreased brightness of fibers were observed after the repulping process. As the temperature was increased from 30°C to 45°C and then to 60°C, and the thermally aged newspapers were repulped, the ink fragmentation behavior, the ink content bound to the fibers, and the number and area of dirt specks significantly increased. Problems in releasing ink from fibers and breaking down ink-fiber agglomerates became more pronounced as the aging time and the temperature increased. ADDRESSES OF THE AUTHORS: Kalle Kemppainen (kalle.kemppainen@oulu.fi) and Heikki Upola (heikki.upola@oulu.fi): University of Oulu, Faculty of Technology, Fibre and Particle Engineering, P.O. BOX 4300, FIN-90014 Oulu, Finland. Mika Körkkö (mika.korkko@haarla.com): Haarla Oy, Pyhäjärvenkatu 5 A, FIN-33200 Tampere, Finland. Corresponding author: Kalle Kemppainen