Hemicelluloses), and the change of the molecular distribution to a narrow,

Monomodal type of distribution with a minimum amount of low molecularweight

Carbohydrates. The extent of purification should thus be adjusted to the

Need of the dissolving process, and pulp grades of varying purity level are available.

It is a well-known fact that the mechanical properties of the viscose fibers

Correlate quite well with the amount of short-chain molecules. As early as 1941,

Hermans stated that the chain-length distribution in the dissolving pulp is a crucial

property in the production of rayon fibers [1]. In addition, by using sulfite and

Prehydrolysis-kraft (PHK) pulps of different purity levels, Avela et al. were able to

Demonstrate that all strength characteristics are significantly reduced with an

increase in the low molecular-weight fraction [2]. The short-chain molecules represent

The weakest part in the fiber; this means that, the shorter the molecules, the

Lower will be the number of molecules linking the crystalline regions. In a recent

Study, a correlation between the strength properties of rayon fibers and the

Amount of low molecular-weight fraction (expressed as the DP50-fraction) was

Established, using a set of dissolving pulps prepared by different organosolv processes

[3].

In general, caustic extraction steps are conducted to remove short-chain carbohydrates

From wood pulp that resisted the pulping process, in order to obtain

Favorable product characteristics such as improved material properties (e.g.,

Increased fiber strength), higher brightness and brightness stability. These alkaline

purification procedures can be carried out in two different ways – as either

cold or hot caustic extractions. While the cold process, which is conducted at 20–

Handbook of Pulp. Edited by Herbert Sixta

Copyright © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-30999-3

©2006 WILEY-VCHVerlag GmbH&Co.

Handbook of Pulp

Edited by Herbert Sixta

40 °C and high sodium hydroxide concentration (1.2–3.0 mol L–1), involves mainly

Physical changes, the hot purification process, operated in the range between

70 °C and 130 °C and low sodium hydroxide concentration (0.1–0.4 mol L–1),

Induces multiple carbohydrate degradation reactions. In addition to cleavage of

The terminal glycosyl groups, one by one via b-alkoxy elimination (peeling reaction)

Until the reducing end group is converted into a corresponding aldonic acid

(alkali-resistant metasaccharinic acid end group), a series of fragmentations to

Mainly short-chain organic acids (mainly C2 and C3 hydroxy acids) occurs at elevated

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