Impairs the accessibility of the acetylation agent. When subjecting a low-grade dissolving

pulp with a residual pentosan content of 7.8% to cold alkali extraction, the

reactivity towards acetylation changes only marginally. This may be explained by

two counteracting effects. On the one hand, the removal of higher amounts of low

molecular-weight hemicelluloses, especially when using 10% NaOH, results in

the formation of a more homogeneous pulp with increased accessibility to the

reagent. On the other hand, a denser network of hydrogen bonds ensures that

pores are closed and accessibility becomes more difficult. In contrast, reactivity of

the kraft pulp increases continuously with increasing the NaOH concentration

which may be accounted for by removal of the hemicelluloses. In the case of

unpurified kraft pulp, the effect of improving reactivity due to pulp homogenization

prevails up to a NaOH concentration of about 15%. However, by exceeding

this NaOH concentration a slight decrease in reactivity is observed due to a

decreasing accessibility.

0 5 10 15 20

0.0

1

2

3

Cotton Linters Bagasse-PH-Soda Bagasse-Kraft

DS-Acetyl (after 4 h)

NaOH concentration [%]

Fig. 11.20 Effect of NaOH concentration during

cold caustic extraction on the reactivity

towards acetylation of cotton linters, bagasse-

PH-Soda and bagasse-kraft pulps [62].

Initial pulp purities, measured in terms of

alpha-cellulose content: 99.1%, 91.9%, and

75.8% for cotton linters, bagasse-PH-Soda and

bagasse-kraft pulps, respectively.

1046

11.3 Dissolving Grade Pulp

11.3.2.4 Cell Wall Structure

The architecture of both native (cellulose I) and regenerated fibers (cellulose II) is

constituted by micro- and macrofibrils. The latter are the structural elements of a

single cellulose fiber. The cross-section of cotton linters fibers amounts to

17–27 lm, while that of spruce sulfite pulp fibers is 21–40 lm [63]. The cellulose I

fibers are a buildup of single cell-wall layers that differ in their fibril texture, and

this contributes to the fiber properties.

The fibrillar morphology of pulps is highly dependent upon the pulping process

and pulping conditions. It is assumed that the active sulfite cooking chemicals,

comprising hydrogen sulfite and hydrated sulfur dioxide, penetrate through the

pits into the middle lamella where the pulping reaction starts from the primary

wall across the cell wall. As a consequence, the primary wall is sometimes completely

removed after acid sulfite pulping. Pulping under alkaline conditions (the

kraft process), however, enables rather uniform pulping reactions across the cell

wall layers due to the high swelling properties of white liquor. The morphology of

cellulose samples is evaluated mainly using electron microscopy (scanning and

transmission electron). A very good resolution of the fibrillar structure is obtained

by the transmission electron microscopy (TEM) surface replica technique

described by Purz et al. [64] and by Fink et al. [53]. A characteristic micrograph of

eucalyptus sulfite dissolving pulp is shown in Fig. 11.21a, with the primary wall

having been largely removed. The morphological architecture of the sulfite pulp is

clearly dominated by the S1 layer (Fig. 11.21a).