11 Pulp Properties and Applications

acetic anhydride in acid solution is more sensitive to accessibility as compared to

alkaline processing due to limited swelling conditions. The results indicate that

drying at 105 °C is unfavorable over drying at room temperature with respect to

the homogeneity of the cellulose triacetate solution, as expressed by the increased

turbidity value (Tab. 11.15).

Tab. 11.15 Chemical and physical characterization of a neverdried

beech acid sulfite pulp in the course of different drying

conditions [86].

Sample treatment Triacetate solution quality

Yellowness coefficient Turbidity

Air-dried

105 °C 0.59 296

130 °C 0.59 367

160 °C 0.89 1030

The loss in pore volume by drying at 160 °C is clearly reflected in a significant

Increase in the yellowness coefficient, haze, and the amount of undissolved particles.

The severe decrease in reactivity has also been described by Gruber et al.,

using comparable conditions [87].

To summarize, water uptake is largely controlled by morphological features of

the cellulose fiber, consisting of a hierarchy of domains of gradually different

accessibility. By drying swollen fibers, some of these domains become inaccessible

(hornification). In agreement with many previous reports, chemical reactivity in

particular has been found to be sensitive to drying conditions.

11.3.2.7 Degradation of Dissolving Pulps

The behaviour of dissolving pulps within heterogeneous degradation reactions

provides insight into their supramolecular structures, functionalities, and changes

In mwd. A comprehensive description of all relevant cellulose degradation processes

Is reviewed in Ref. [4]. The different modes of cellulose degradation comprise

chemical, mechanical, thermal, and radiation degradation. In many conversion

processes of cellulose, the molecular weight must be adjusted by controlled

degradation procedures.

The degradation of cellulose plays an important role in the chemical processing

of dissolving pulps. The aim of controlled cellulose degradation is to adjust polymer

properties related to the molecular weight such as solution viscosity (ethers)

or strength properties of the final product (regenerated fibers). The most important

conversion processes of dissolving pulps, viscose and cellulose ethers, operate

1056

11.3 Dissolving Grade Pulp

under alkaline conditions. There, molar mass is adjusted by oxidative alkaline degradation,

also known as ageing of alkali cellulose. In recent years, a new route of

controlled degradation of the dissolving pulp prior alkalization by high-energy radiation

has been extensively investigated and technologically developed by Fischer

et al. [88].

Alkali cellulose with a typical composition of 34% cellulose and 16% NaOH

(~1.9 mol NaOH mol–1 cellulose) is rather rapidly degraded at only a slightly elevated

temperature (30–50 °C), initiated by the uptake of oxygen. The reaction rate

can be accelerated by the addition of transition metal ions, particularly Co or Mn

salts. The course of the chain scissions, calculated from the weighted molecular

weight (determined by GPC measurement [68]) of alkali celluloses prepared from

both hardwood sulfite and PHK dissolving pulps as a function of reaction time at