Surface chemistry and morphology of different mechanical pulps determined by ESCA and AFM

Abstract

The surface chemistry and morphology of spruce (Picea abies) mechanical pulps have been investigated by Electron spectroscopy for chemical analysis (ESCA) and Atomic force microscopy (AFM). As determined by ESCA, the content of lignin was slightly higher on the pulp surface than in the whole pulp. The surface coverage by lignin remained approximately the same after peroxide and dithionite bleaching, whereas on the ozone treated pressure groundwood (PGW) pulp the amount of lignin was strongly reduced. The AFM studies showed that the surfaces of mechanical pulp fibres are very heterogeneous, i.e. different cell wall layers are exposed along the fibre surface. It was found that the surfaces of unbleached PGW and thermomechanical pulp (TMP) fibres consist of both granular and fibrillar structures, whereas the surfaces of unbleached chemithermomechanical pulp (CTMP) fibres are almost fully covered by granular material. This granular layer, which is interpreted to originate from the middle lamella and the primary wall, was removed from the CTMP fibre surface after peroxide bleaching, exposing the underlying structure. However, the surface coverage by lignin determined by ESCA was not reduced. This suggests that peroxide bleaching of sulphonated CTMP pulp in some way modifies the lignin and pectin rich surface and that lignin occurs also in a non-granular form on the surface of mechanical pulps. Peroxide or dithionite bleaching had no effect on the morphology of PGW and TMP fibres. The highest amounts of extractives were found on unwashed and unbleached TMP pulp. It turned out that only part of the extractives was removed from the surface by alkaline treatment and by peroxide bleaching.