In microcrystalline areas as the main reason for hornification [85]. The effect of

thermal treatment on the physical and chemical properties was examined by

using a never-dried beech acid sulfite pulp [86]. When the temperature exceeds

105 °C, thermal degradation proceeds in parallel with hornification, as shown in

Tab. 11.14.

The content of carboxyl groups decreased while the content of carbonyl groups

Increased, particularly at temperatures above 105 °c. The increase in carbonyl

groups is caused by the cellulose degradation, thus creating new reducing endgroups

(see Tab. 11.14). The course of brightness reveals the effect of heat-induced

yellowing caused by the formation of chromophores, with a probable participation

of carbonyl groups and residual noncellulosic compounds such as hemicellulose,

resins, and certain metal ions. The effect of pore collapse along with the decrease

In pore volume is clearly illustrated in Fig. 11.28.

1054

11.3 Dissolving Grade Pulp

Tab. 11.14 Chemical and physical characterization of a neverdried

beech acid sulfite pulp during the course of different

drying conditions [86]. Thermal treatment was carried out

for 12 h.

Sample treatment Brightness

[% ISO]

Viscosity

[mL g–1]

WRV

[%]

Vp

[mL g–1]

COOH

[lmol g–1]

CO

[lmol g–1]

Never-dried 91.2 581 91.2 0.92 24.5

Freezed-dried 0.86

Air-dried 91.2 581 91.2 0.69 30.9 25.6

105 °C 90.1 546 90.1 0.60 30.5 27.5

130 °C 88.9 489 88.9 0.56 28.2 29.7

160 °C 79.1 350 79.1 0.43 27.6 37.3

1 10 100

0.0

0.2

0.4

0.6

0.8

1.0

never-dried freeze-dried air-dried

dried at 105 єC dried at 130 єC dried at 160 єC

dried twice at 160 єC, intermediate re-wetting

V

P

[ml/g]

D

P

[nm]

Fig. 11.28 Pore volume versus pore diameter (ISEC method)

of a beech acid sulfite pulp subjected to different drying

procedures [86].

As expected, freeze-drying is the only drying procedure which largely preserves

the pore and void system of a never-dried pulp fiber. The loss in pore volume is

negligible compared to all other drying procedures (see Fig. 11.28 and Tab. 11.14).

When discussing hornification, the question always arises to what extent pulp

reactivity is affected by this irreversible loss of pore volume. The reaction with

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11 Pulp Properties and Applications

acetic anhydride in acid solution is more sensitive to accessibility as compared to

alkaline processing due to limited swelling conditions. The results indicate that

drying at 105 °C is unfavorable over drying at room temperature with respect to

the homogeneity of the cellulose triacetate solution, as expressed by the increased

turbidity value (Tab. 11.15).

Tab. 11.15 Chemical and physical characterization of a neverdried

beech acid sulfite pulp in the course of different drying

conditions [86].

Sample treatment Triacetate solution quality

Yellowness coefficient Turbidity

Air-dried

105 °C 0.59 296

130 °C 0.59 367

160 °C 0.89 1030

The loss in pore volume by drying at 160 °C is clearly reflected in a significant

Increase in the yellowness coefficient, haze, and the amount of undissolved particles.

The severe decrease in reactivity has also been described by Gruber et al.,

using comparable conditions [87].

To summarize, water uptake is largely controlled by morphological features of

the cellulose fiber, consisting of a hierarchy of domains of gradually different

accessibility. By drying swollen fibers, some of these domains become inaccessible

(hornification). In agreement with many previous reports, chemical reactivity in

particular has been found to be sensitive to drying conditions.