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

1047

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

11 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.

11.3.2.4.1 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

sulfite and alkaline cooking procedures, as well as the heterogeneous distribution

of hemicellulose across the different cell wall layers, suggest that there is a difference

in the arrangement of hemicelluloses in the fiber walls of PHK and acid sulfite

fibers which presumably influences their chemical properties. Moreover, during

the final phase of alkaline pulping dissolved hemicelluloses (e.g., xylan) are

taken back by the fibers from solution. The redeposition of xylan plays an important

role in the papermaking properties of the resulting pulp fibers, but could also

adversely affect the manufacture of cellulose acetate. Yllner and Enstrцm showed

that on heating pentosan-free cotton linters with birch wood chips in kraft cooking

liquor, considerable quantities of xylan were taken up from the solution

[65,66]. Quite recently it was shown that during kraft cooking of cotton linters at