Cell wall changes caused by refining in fibre of Eucalyptus grandis W. Hill ex Maiden bleached kraft pulp: a ultra- and nano-structural approach

As a consequence of refining on bleached kraft pulps of Eucalyptus grandis W. Hill ex Maiden, at 25°SR, 30°SR and 35°SR, substantial modifications of the fibre are observed. Such changes affect the quality of pulp and are related to specific cellulose microfibrils distribution pattern, nanofissures, micropores, cell wall fibrillation, the presence of dislocations and to the partial or less frequent entire erosion of the S1 layer of secondary wall fibre. The Schopper–Riegler degree (°SR) is a useful index to measure the value of the mechanical treatment (refining) to which the pulp has been subjected to. When refining at 25°SR, the S2 layer presents a loose cellulose microfibril pattern, whereas after refining at 30°SR, the S2 layer displays concentric and successive lamellae and visible nanofissures. After refining at 35°SR, bleached kraft pulps are heterogeneous, and the S2 layer of the twisted fibre exhibits an outer zone with radially oriented fractures between cellulose aggregates and an electron-dense inner zone with cellulose microfibrils parallel to the axis. Fibre changes in length, diameter and wall thickness were analyzed with a nonparametric ANOVA by ranges (Kruskal–Wallis H test) which varied significantly.