Hardwood kraft pulp structural features affecting refinability

Two industrial bleached Eucalyptus globulus kraft pulps (pulps A and B), cooked from the same wood stock with slightly different alkali charges and temperatures, showed different refinability in the laboratory beating of pulp using a PFI mill. The easy to refine pulp (A) and the difficult to refine pulp (B) did not reveal significant differences in their biometric data and general chemical composition, but pulp A showed ca. 5%-10% higher mechanical strength and twice as high refinability compared to pulp B. These differences were ascribed to slightly higher intrinsic viscosity and higher xylan content of pulp A compared to pulp B (1010 vs. 860 cm(3) g(-1) and 20.1 vs. 17.8%, respectively). The xylan structure and its location in fiber cells were also different, which affected the pulp's refinability. The presence of high proportion of xylan in the bulk of fibers, rather than in their outer layers, seems to favor the pulp beating. An interpretation was also proposed, according to which the aggregation of cellulose fibrils during cooking, i.e. a partial coalescence of fibril crystallites, may also have a negative effect on pulp refinability.