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

thermal treatment has a negligible effect on the amount of nonfreezing bound

water. On drying, the water molecules are partially removed and new hydrogen

bonds are formed directly between cellulose molecules [84]. It is likely that, in

addition to hydrogen bonds, van der Waals bonds are also involved in hornification,

though the exact nature of irreversible bonding in pulp fibers has not yet

been established. Newman and Hemmingson have postulated a co-crystallization

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.

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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 .C dried at 130 .C dried at 160 .C

dried twice at 160 .C, 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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