Assessing the combined benefits of clay and nanofibrillated cellulose in layered TMP-based sheets

A new concept for both furnish composition and z-directional furnish arrangement involving the interaction between specific thermo-mechanical pulp fractions (TMP), nanofibrillated cellulose (NFC) and clay in oriented layered laboratory sheets is presented. Used separately, NFC improves the strength properties of paper while fillers enhance the optical properties. Synergy effects of clay–NFC interactions are assessed. The study comprises a structural assessment, including laser profilometry, scanning electron microscopy (SEM) and field-emission (FE)-SEM analyses. In addition, optical and strength properties are assessed. It is demonstrated that a potential reduction of strength properties caused by filler addition may be counteracted by appropriate NFC addition to specific layers in the z-direction. Based on an estimation of an overall quality index considering five variables, it is concluded that the best sheet construction is obtained when placing the fillers in surface layers with the TMP accept fraction and the NFC in the centre of the sheets together with the refined TMP reject fraction.