Modifications in the bulk and the surface of unbleached lignocellulosic fibers induced by a heat treatment without water removal: effects on fibre relaxation of PFI-beaten kraft fibers

The effect of heating lab- and mill cooked refined and unrefined unbleached kraft pulp in a rotating lab reactor at different temperatures, pH and consistencies was evaluated based on sheet and fiber properties. The observed significant decrease in tensile strength with increasing temperature was attributed to the drop in fiber bendability and to the drastic increase in fiber curl. The increased fiber curl at increased temperature (labelled as “reversed latency”), was attributed to two potential mechanisms: (a) the removal of hemicelluloses from the fiber bulk and the re-aggregation of the fibrillar cell wall structure and (b) the local damage of the fiber surface and the presence of the fully softened hemicelluloses–lignin-matrix which caused the fiber to bend and kink.