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 Enstrom 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

150 °C for 2 h, more than 70% of a xylan, which was added to the cooking liquor

(isolated from a eucalyptus pulp), was retained on the cellulose surface at a residual

effective alkali concentrations of approximately 10 g L–1, and still more than

20% in the case of alkali concentrations up to 50 g L–1 [67]. Xylan redeposition has

Been demonstrated unequivocally by carbohydrate and GPC analysis (Fig. 11.24).

The xylan distributions across the fiber wall of TCF-bleached acid sulfite and

PHK dissolving pulps, obtained from both spruce and beech wood, were investigated

employing a new enzymatic peeling technique [69,70]. A general characterization

Of the pulp substrates subjected to enzymatic peeling is provided in

Tab. 11.11.

Dissolving Grade Pulp

3 4 5 6 7

0.0

0.3

0.6

0.9

1.2

Xylan added linters cooked without xylan linters cooked with xylan

dW/d(logM)

Log Molar Weight

Fig. 11.24 Molar mass distribution of cotton linters after

cooking for 2 h at 150 °C with a residual EA-concentration of

10 g L–1 in the absence and presence of xylan [67]. The xylan

Was isolated from a eucalyptus kraft pulp. GPC measurements

were made according to [68].

Tab. 11.11 Characterization of TCF-bleached dissolving pulps

Made from spruce and beech wood produced by acid sulfite and

prehydrolysis kraft (PHK) cooking procedures [70].

Pulp type Bleaching

Sequence

Bleached yield

[%]

Viscosity

[mL g–1]

Brightness

[% ISO]

Xylan

[%]

Glucomannan

[%]

Beech-S (E/O)-Z-P 39.7 573 91.4 3.3 0.6

Beech-PHK O-A-Z-P 38.1 465 91.1 6.4 0.4

Spruce-S (E/O)-Z-P 42.3 526 90.0 1.8 2.4

Spruce-PHK O-A-Z-P 38.5 439 83.4 2.3 1.3

The enzyme treatment causes a peeling effect which removes fiber material,

Starting from the fiber surface. An analysis of the removed material by capillary

Zone electrophoresis (CZE) revealed fundamental differences in the radial distributions

Of hemicelluloses, as shown in Fig. 11.25. These differences may account

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