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Intensity (see Fig. 11.18: hw-phk high p-factor) clearly changes the supramolecular

structure and results in a shift of the transition curve to a lower

NaOH concentration. Above 11% NaOH, the curve of the highly purified PHK

pulp proceeds similar to that of the sulfite pulp.

Closer examination of the mercerization behavior reveals that even between different

sulfite dissolving pulps, differences in the course of the curve have been

reported [58]. Recently, it was shown that pulping and subsequent purification

1044

11.3 Dissolving Grade Pulp

0 2 4 6 8 10 12 14

0

20

40

60

80

100

Medium Viscosity, Low Purity Medium Viscosity, High Purity

Low Viscosity, Low Purity Low Viscosity, High Purity

Na-Cellulose I [%]

NaOH concentration [wt%]

Fig. 11.19 Lattice transition from cellulose I to Na-cellulose I

of HW-sulfite dissolving pulps subjected to different pulping

and purification pretreatments in dependence on steeping lye

concentration at room temperature [56].

procedures of hardwood sulfite dissolving pulps clearly exert an influence on the

shape of the transition curve, especially beyond an NaOH concentration of 10% at

room temperature.

The enhanced viscosity degradation during final phase of sulfite cooking, combined

with reinforced purification during hot caustic treatment, accelerates the

transition to Na-cellulose I in the range of NaOH concentration of 10–12%. This

type of activation towards alkali cellulose formation is also expressed in a better

Viscose filterability, thus indicating an improved reactivity.

During the course of a cold alkali extraction treatment for the manufacture of

high-purity wood dissolving pulps, a partial lattice transformation from cellulose I

to cellulose II usually occurs, depending on NaOH concentration in the aqueous

pulp suspension (see Fig. 8.4). For the manufacture of very high-purity pulps (R18

> 98%), NaOH concentrations up to 10–11% are required, thus leading to a significant

shift in the crystalline structure to cellulose II, as shown in Tab. 11.10 (HWPHK-

CCE). After drying, the reactivity of the mercerized dissolving pulps may

alter [59]. It is reported that dried, partly mercerized dissolving pulps cannot be

converted to cellulose acetate under the normal processing conditions, but the

reactivity during nitration is not affected [60,61].

However, different pulps respond differently to cold alkali extraction, and hence

behave differently towards acetylation. Thus, bond cleavage and solution of hemicelluloses

may lead to increased reactivity, while drying may lead to decreased

1045

11 Pulp Properties and Applications

reactivity. It was shown that the reactivity of a cotton linters pulp towards acetylation

decreases steadily with increasing the concentration during cold caustic

extraction (Fig. 11.20). The loss in reactivity is due to drying of the pulp after the

cold caustic refining, which induces the formation of hydrogen bonding. With rising

alkali concentration the hydrogen bonds become more dense, and this finally

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