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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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This explains the different pattern of hemicelluloses removal as compared | | | Content is almost negligible. Therefore, the main objective of the alkali |