Dissolve out of the cell wall during pulping (macropores). The amount of

water within the cell wall – the so-called fiber saturation point (FSP) – can be adequately

measured with a solute exclusion technique [78,79]. Based on this technique,

Bredereck et al. [80,81] developed an inverse size-exclusion chromatography

(ISEC) technique in which the macromolecules used are a series of dextran

Fractions and mono-, di-, tri-, and tetrasaccharides. (The diameters of the macromolecules

are calculated from their diffusion coefficients using the Einstein–

Stokes formula.) The uptake of water by a pulp sample can be simply determined

By measuring the so-called water retention value (WRV), which comprises the

weight gain (in %) of a dry sample after swelling in a large excess of water and

subsequent centrifugation under defined conditions [82]. Scallan showed the

WRV to be a good measure of the FSP up to values of 1.8 H2O g–1 o.d. pulp [83].

The values determined from WRV and ISEC measurements listed in Tab. 11.13

Demonstrate the important influence of the supramolecular structure of dissolving

Pulps on water swelling.

Tab. 11.13 Results of ISEC and WRV measurements obtained

from a selection of typical dissolving pulps. Vp = pore volume;

Dp = average pore diameter; Op = specific pore surface of waterswollen

Pulps. The values were calculated according to the

model published by Bredereck et al. [81].

Pulp type Vp

[mL g–1]

FSP

[mL g–1]

WRV

[%]

Dp

[nm]

Op

[m2 g–1]

HW-S 0.60 0.50 73 5.1 235

HW-PHK 0.65 0.55 71 5.5 240

Cotton linters 0.45 0.39 54 4.8 190

As expected, the water-accessible pore volume of cotton linters is clearly lower

Than that of the dissolving wood pulps. Only minor differences in integral pore

Volume (Vp), WRV and specific pore surface (Op) were seen between acid sulfite

And PHK dissolving pulps. A slight advantage of the PHK over the acid sulfite

Pulp with regard to total pore volume was apparent (Fig. 11.27), though closer

Examination revealed that the PHK pulp provided a larger number of small pores

with an average diameter of 1.5–2.0 nm.

Never-dried cellulose substrates showed a considerably higher accessibility for

Water than did either the dried or rewetted samples. When pulps are dried, an

Pulp Properties and Applications

1 10 100

0.0

0.2

0.4

0.6

HW-PHK HW-Sulfite Cotton Linters

V

p

[ml/g]

D

p

[nm]

Fig. 11.27 Pore volume versus pore diameter of a selection of

Dissolving pulps, as determined by the ISEC method.

Irreversible loss of fiber swelling occurs; indeed, Maloney and Paulapuro reported

That most macropores may be rendered inaccessible, while none of the micropores

was irreversibly collapsed after drying and rewetting [77]. The mechanism leading

to these irreversible changes is termed “hornification”. Moreover, drying and even

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