Lattice planes is widened from the original 0.61 nm to more than 1.2 nm due

to incorporation of the sodium hydrate ion [20]. At a NaOH concentration between

160 and 190 g L–1 the lattice transformation to Na-cellulose I is completed.

This structure gives rise to a better reactivity with chemical reactants due to the

Better accessibility of the hydroxyl groups on C6 and C2 (e.g., CS2 in the case of the

Viscose process). It is well known that the transition curve from cellulose I to Na-

Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

Cellulose I depends also on the supramolecular structure of the dissolving pulp.

Sulfite pulps generally require a lower lye concentration to achieve full lattice conversion

than do PHK pulps [21]. The somewhat higher mercerization resistance

May be due to the less degraded primary cell wall of the latter, restricting swelling

by NaOH [20]. The changes in supramolecular structure upon alkali treatment of

Two dissolving pulps, beech acid sulfite and eucalyptus PHK pulps, have been

Investigated using solid-state CP-MAS 13C-NMR spectroscopy (Fig. 8.4).

0 100 200 300 400

Eucalyptus-PHK: Cellulose I Na-cellulose I Na-cellulose II

Beech-Sulfite: Cellulose I

Proportion [%]

NaOH concentration [g/l]

Fig. 8.4 Lattice transition from cellulose I to Na-cellulose I

And Na-cellulose II of beech sulfite and eucalyptus-PHK pulps

Depending on NaOH concentration. Data were recorded

Using solid-state CP-MAS 13C-NMR spectroscopy (according

to [22]).

Over the range of NaOH concentration from about 160 g L–1 to 270 g L–1, the

Structure of Na-cellulose I prevails, while beyond this concentration level a further

Lattice conversion to Na-cellulose II arises. The NMR-spectrum of this lattice type

indicates cleavage of the intramolecular hydrogen bond between O-3-H and O-5′,

and thus the coordination of an additional Na+ ion to O-3 [23]. A series of comprehensive

Reports provides further information on the changes in supramolecular

Structure that occur during the treatment of cellulose with aqueous solutions

[20,22,24–26].

Pulp Purification

8.3

Cold Caustic Extraction

The extent of purification, measured in terms of R18 and R10 values and residual

Hemicellulose content (xylan in case of hardwood pulp), depends primarily on the

NaOH concentration and the temperature (see Section 8.2). Additionally, the reaction

Time, the position of the cold caustic extraction (CCE) within the sequence,

And the presence of dissolved hemicelluloses may have an influence on the efficiency

Of purification. In industrial CCE treatment, emphasis is placed on efficient

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