Surface characteristics of thermomechanical pulps; the influence of defibration temperature and refining

The surface properties of thermomechanical pulps (TMP) were characterized by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Finnish and French spruce samples representing two different defibration temperatures as well as two different stages of refining were studied. In addition, samples from an RTS process were studied. The XPS results showed that the surface content of non-cellulosic components was higher for Finnish spruce compared with the French pulp samples. Different wood fibre cell wall structures and transition lamellae within the cell walls with characteristic microfibril orientation were identified by AFM. Besides the morphological features, the homogeneity of the surfaces was studied with AFM with respect to adhesion and stiffness. The refining was concluded to result in a cleavage taking place either within the secondary cell wall or at primary/secondary cell wall interface. No randomly oriented microfibrillar network structures typical for the primary cell wall were observed. The RTS process resulted in most homogeneous samples whereas heterogeneous structures with even damaged microfibrils were observed for e.g. the stage-2 refined TMP.