The viscose demonstrates both a clear reduction in the particle volume and a shift

Of the average particle size to lower values (Fig. 11.10).

The results confirm that reinforced oxidation, for example by ozone, contributes

To an improved dispersibility of pulp resins, and this is an important prerequisite

For the effective separation of these impurities. The efficiency of resin removal

Can be further enhanced by additional post-cleaning operations.

Although a certain amount of pulp resin may be beneficial for subsequent processing

Steps, by improving accessibility to the cellulose substrate, practical experience

Has taught that the best way to control extractives is to take measures to keep

them at a low level [32]. When the resin content falls below a certain level, the

Homogeneity of subsequent reactions may be impaired because of a lowered surface

Activity. In this case, the addition of small amounts of synthetic surface active

Agents overcomes that deficiency.

Residual lignin, brightness

The residual lignin content in dissolving pulps is generally very low. The kappa

Number, which specifies the amount of oxidizable (by KMnO4) structures containing

Double bonds in the pulp, is typically between 0.2 and 0.5 units which translates

to a residual lignin content of about 0.05% [33]. The main reason for aiming

At a low kappa number is the high demand on optical properties. Residual lignin

Structures strongly contribute to yellowing of the cellulosic products. The highest

Demands on brightness and brightness stability are given for viscose, lyocell, and

Acetate pulps. Similar brightness levels are required for dissolving pulps converted

To cellulose ethers for application in foodstuff and pharmaceuticals. Residual lignin

Is, however, not the only factor determining the optical properties of cellulosic

Substances. Therefore, the relationship between pulp brightness and brightness

Of the final product is also dependent upon the processing conditions, especially

In the case of alkaline derivatization procedures (e.g., viscose, ethers). In industrial

Operations using constant conditions, pulp brightness is clearly reflected in

The brightness of the final product.

Brightness – and thus residual lignin – is not a concern for pulps used for technical-

grade cellulose ethers (major applications: textile, paper, drilling muds, ceramics,

etc.). Nevertheless, bleaching to brightness levels of about 70–75% ISO is

Necessary to improve pulp reactivity and prevent precipitation of lignin compounds

In subsequent processing steps.

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