The residual lignin is not only a concern for optical properties, but also governs

The processability of dissolving pulps. It is reported that viscose filterability (determined

By the clogging constant) gradually deteriorates when increasing the residual

lignin content from about 0.17 to 0.36% [34].

Inorganic Compounds

The presence of certain inorganic compounds such as silicates, Ca salts, and catalytically

Active transition metal ions (Fe, Mn, Co, etc.) clearly impairs the filterabil-

Pulp Properties and Applications

Ity and spinnability of a cellulose spinning dope (e.g., viscose or lyocell type of

Fibers). Moreover, pulp contamination with inorganic compounds leads to a gradual

clogging of the spinnerets, and this alters the uniformity of the fiber titer [13].

In particular, the cations Ca2+ and Fe2+, as well as silicates, are considered to be

Detrimental in this respect. Although Fe(II), and to a lesser extent Cu(II), promote

Light-induced yellowing, both cations are involved in detrimental degradation reactions

In the presence of hydrogen peroxide bleaching (Fenton-type reaction).

Thus, all necessary measures must be undertaken (acid wash, chelation stage,

Etc.) to remove catalytically active cations.

Surprisingly, most of the harmful ash components are not distributed homogeneously

In the pulp, but are present as particulates in certain cell fractions, particularly

in the parenchyma cells [35]. Therefore, the only promising way to reduce

The amount of harmful ash components is an efficient mechanical pulp treatment

Which applies combined pressure screening of unbleached pulp and centrifugal

Cleaning after bleaching. This treatment ensures the removal of extremely small

Debris such as sand, bark specks, and shives. Again, the best way to control inorganic

Compounds is to reduce them to the lowest level economically feasible.

Macromolecular Properties

Molar mass, molar mass distribution

Since celluloses from natural sources and after chemical treatment are always

Polydisperse, the determination of the average molecular weight (e.g., by viscosimetry)

Is insufficient to predict specific product properties. Additional information

Is provided by the measuring molecular weight distribution (MWD) of dissolving

Pulps.

Measurements of MWD reveal a multimodal distribution for pulps produced

According to acid sulfite cooking, while the PHK pulps show a rather uniform

MWD. Dissolving pulps, being representative of various applications including

Viscose, acetate and high-viscosity ether, are compared in Fig. 11.11.

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