During alkalization of a beech sulfite dissolving

pulp (93.4%R18, 4.0%xylan) at three

different temperatures: (a) 20 °C; (b) 50 °C; (c)

80 °C [12]. Caustic treatment: 5%consistency,

30 min reaction time, NaOH concentrations:

20, 40, 60, 80, 100, 140, 180, 280, and 340 g L–1.

Reactions between Pulp Constituents and Aqueous Sodium Hydroxide Solution

Dissolved hemicelluloses was analyzed by anion-exchange chromatography (AEC)

With pulsed amperometric detection (PAD) after separation into beta- and

gamma-cellulose fractions [17]. It is noted that the proportion of beta-cellulose

decreases with increasing temperature, particularly above 50 °C. While the absolute

amount of gamma-cellulose remains fairly constant at 20–50 °C throughout

The whole range of NaOH concentrations investigated, the increase in the total

amount of dissolved hemicelluloses at 80 °C is mainly attributed to an increase in

the gamma-cellulose fraction (see Fig. 8.2). The fact that up to 90% of the gammacellulose

Fraction consists of degraded carbohydrates (equal to non-neutral sugars)

Clearly indicates that the removal of hemicelluloses through alkaline treatment at

C is mainly governed by chemical degradation reactions (e.g. peeling reaction).

As stated previously, the extent of chemical degradation reactions decreases with

Decreasing temperatures. Accordingly, the amount of degraded carbohydrates

Decreases at lower temperatures. The beta-cellulose fraction originating from any

Alkaline treatment consists almost exclusively of neutral sugars (except for uronic

Acid side chains and oxidized end groups). As anticipated, the maximum yield of

Beta-cellulose corresponds with the maximum solubility of the hemicelluloses

(sum of beta- and gamma-celluloses) at an alkaline treatment at 20 °C and 50 °C,

And with the lowest xylan content in the pulp residue at any temperature investigated.

Parallel to its highest yield, the beta-cellulose consists of the highest glucan

content (74 wt.%, 59 wt.% and 47 wt.% based on beta-cellulose for 20 °C, 50 °C,

and 80 °C, respectively). It can be assumed that the glucan fraction derives from

Degraded cellulose and comprises the highest molecular weight within the betacellulose.

Surprisingly, the treatment at 80 °C also produces a beta-cellulose fraction

Enriched with degraded cellulose at the same conditions where a complete

Removal of alkali soluble xylan occurs. This indicates that at a lower lye concentration

The cellulose structure is opened by inter- and intramicellar swelling, even at

High temperatures. Apart from degraded cellulose, the predominant hemicellulose

Fraction in beech sulfite dissolving pulps is made up of xylan, while the glucomannan

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