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described by Purz et al. [64] and by Fink et al. [53]. A characteristic micrograph of
Eucalyptus sulfite dissolving pulp is shown in Fig. 11.21a, with the primary wall
Having been largely removed. The morphological architecture of the sulfite pulp is
Clearly dominated by the S1 layer (Fig. 11.21a).
Identification of the cell wall layers is possible by the preferred orientation of
The exposed cellulose microfibrils with respect to the cell axis. The latter is indicated
By arrows in the micrographs.
On the other hand, residues of the primary wall can be detected for the PHK
viscose pulp (low P-factor) (Fig. 11.21b, top). Apparently, the type of pulp – as well
as process conditions during pulp manufacture – exert an influence on the exposure
S1
Fig. 11.21 Transmission electron microscopic surface replica of
(a) eucalypt sulfite dissolving pulp (b) eucalyptus PHK pulp
(c) eucalyptus PHK pulp with high P-factor [56].
S1
PW
S1
Pulp Properties and Applications
Of cell wall layers. Clearly, mild prehydrolysis combined with alkaline cooking is
Less detrimental to cellulose microfibrils than acid sulfite cooking, thus leaving
The primary cell wall morphology largely unaffected. However, PHK cooking,
Using intensified prehydrolysis conditions to produce pulps of very low residual
Hemicellulose content (e.g., xylan), further removes the primary cell wall layers, as
Shown in Fig. 11.21c. Therefore, the microfibrillar structure of this PHK pulp
(high P-factor) is almost comparable to that of the sulfite pulps.
The different behavior of the three dissolving pulps with regard to lattice transition
As a function of NaOH concentration can be explained by their different morphological
Structure. It is interesting to note that the supramolecular structure
May be influenced by the cell wall structure of the dissolving pulps. Also, the
Markedly higher reactivity of sulfite and highly purified PHK pulps in ageing of
Alkali cellulose and xanthation may well be caused by a weakening of the outer
Layers of the wood fiber cell wall due to the higher hydrolytic action during final
Phase sulfite cooking and intensified prehydrolysis.
Distribution of Hemicelluloses across the Fiber Wall
In the manufacture of dissolving pulps, the objective is to remove as much hemicelluloses
As possible without causing too-severe cellulose deterioration. An
Important topic with regard to further process development is how the remaining
Hemicelluloses are located within the fiber wall. The different chemistry of acid
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Reactivity towards acetylation changes only marginally. This may be explained by | | | Sulfite and alkaline cooking procedures, as well as the heterogeneous distribution |