Content, indicating the fragmentation of microfibrils. Clearly, temperatures

beyond 140 °C do not contribute to further purification due to alkaline hydrolysis.

0 2 4 50 100 150

80.0

R18 R10

R values [%]

Temperature [.C]

Fig. 8.19 Development of R18 and R10 contents as a function

Of temperature of a high-viscosity spruce sulfite pulp during

HCE [4]: HCE-conditions: 120 kg NaOH odt–1, 4 h.

Xylan versus R18 Contents

Prolonged acid sulfite cooking causes both the removal of hemicelluloses (e.g.,

Xylan) and the degradation of cellulose, resulting in a low-viscosity pulp. HCE

Treatment of low-viscosity sulfite pulps allows reduction to a very low xylan content,

While the R18 content remains rather close to that of pulps with a higher

Initial viscosity at a comparable yield level (Fig. 8.20). These data conclude that the

R18 content of medium- to high-viscosity pulps partly contains alkaline-stable

Hemicelluloses. On the other hand, part of the degraded cellulose is not included

In the R18 fraction of the low-viscosity pulp.

Hot Caustic Extraction 957

81 84 87 90 93

Xylan content [%]

Medium Viscosity: R18 Low Viscosity: R18

R18 content [%]

Purification yield [%]

Xylan Xylan

Fig. 8.20 R18 and xylan contents related to purification yield

during (E/O)-treatment of hardwood sulfite dissolving pulp

(HW-S) [33]. Low-viscosity pulp: viscosity 490 mL g–1, kappa

number 6.2, xylan content 4.5%; Medium-viscosity pulp: viscosity

730 mL g–1, kappa number 6.2, xylan content 6.5%.

1 2 3 4 5 6

Unbleached (E/O)-treatment:

Medium viscosity Low viscosity Medium viscosity Low viscosity

R18 content [%]

Xylan content [%]

Fig. 8.21 R18 versus xylan content during (E/O)-treatment of

hardwood sulfite dissolving pulp (HW-S) [33]. Low-viscosity

pulp: viscosity 490 mL g–1, kappa number 6.2, xylan content

4.5%; Medium-viscosity pulp: viscosity 730 mL g–1, kappa

number 6.2, xylan content 6.5%.

Pulp Purification

The distinct difference in the residual xylan contents of low- and medium-viscosity

Hardwood sulfite dissolving pulps at a given R18 content is clearly shown in

Fig. 8.21. The xylan content of the medium-viscosity pulp is approximately 1%

higher than that of the low-viscosity pulp when compared at a level of 95% R18

(2.8% versus 1.8% xylan).

Purification versus Viscosity

The removal of short-chain carbohydrates through HCE treatment results in a

Slight increase in viscosity because stepwise degradation (peeling) has only a small

Effect on the molecular weight of long-chain cellulose. The effect of HCE on viscosity

Has been expressed as a negative change of chain scissions for both spruce

And beech dissolving pulps to consider different levels of initial viscosity.

The data in Fig. 8.22 reveal a clear relationship between the degree of purification

And viscosity increase, reflecting the removal of short-chain material. The

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