Kneading Laser Inked Wastepaper: A Laboratory Study

Three laboratory devices; a Lampen mill, a PFI mill and a Disk Shearer, were used to study the kneading of laser inked pulp at 30% consistency. The aim was to determine the relative effects of three types of kneading action at different specific energy levels on the brightness, tensile strength and freeness values of kneaded pulp. Laser printed sheets of a known dot size and printed area were used as the wastepaper source. The rate of ink speck break down varied with each kneading device, and with ink speck size, pulp temperature and pulp load per unit kneading area. PFI mill kneading reduced brightness four times faster than Lampen mill kneading. Disc Shearer kneading matched PFI mill kneading at high disc speeds (40rpm) and plate pressures (294 kPa), and Lampen mill kneading at lower disc speeds (10rpm) and plate pressures (84 kPa). Large ink specks broke down faster than smaller ink specks, and pulp at 80°C broke down ink specks slower than pulp at 20°C. Freeness reduction and tensile strength increase was minimal with Lampen mill kneading but significant with PFI mill kneading. Disc Shear kneading, on the other hand, increased freeness and decreased tensile strength due to fiber curl. Results show that a critical kneading pressure and shear rate exist where permanent fiber property change starts to occur. Operating close to the critical point is advantageous from an energy and process capacity point of view.