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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
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.
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
C 90.1 546 90.1 0.60 30.5 27.5
C 88.9 489 88.9 0.56 28.2 29.7
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
Pulp Properties and Applications
Acetic anhydride in acid solution is more sensitive to accessibility as compared to
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Dissolve out of the cell wall during pulping (macropores). The amount of | | | Alkaline processing due to limited swelling conditions. The results indicate that |