The composition and structure of the surface material of kraft fibres
can affect properties of the pulp such as the reactivity in, for examp
le, bleaching processes. In order to study the nature of the surface m
aterial, conditions for a gentle removal of fibre surface material fro
m unbleached kraft fibres have been evaluated. An unbleached softwood
kraft pulp with a lignin content of 6.2% (kappa number 32) was treated
mechanically using three different types of laboratory equipment: a P
FI-mill, a high intensity mixer, and a disintegrator. The number of re
volutions and the pulp concentration were varied and the removal of su
rface material was followed by chemical and microscopical analyses. Th
ese analyses revealed that there is lignin-rich material present on th
e fibre surface which can be removed by a mild mechanical treatment. T
he lignin content of the surface material abstracted was between 2.5 a
nd 4.5 times higher than that of the fibres. The primary fines (obtain
ed by fractionation of unbeaten pulp, 20 mu m), also originating from
the fibre surface, had a lignin content almost 5 times higher than tha
t recorded in the fibres. For analytical studies of the surface materi
al, a mechanical treatment of the kraft fibres in a disintegrator at a
pulp concentration of 4.5% and at 20.000-200.000 revolutions is an ef
ficient and suitable treatment for gradual removal of material from th
e fibre surface.