IS UNBLEACHED SOFTWOOD KRAFT PULP STRONGEST AT 30-KAPPA-NUMBER

In several kraft softwood pulp mills, unbleached pulp samples were obtained across kappa number ranges from the high teens to the mid-30s. Pilot-plant reference pulps were produced, at similar kappa numbers, from corresponding samples of mill chips. Among these families of pulps, tear-tensile strength was highest within a kappa number range from the low 20s to the mid-30s; it declined outside this range. In each mill, the strength gap between corresponding pilot-made and mill-made pulps was essentially constant across a wide range of kappa numbers.