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Functional Groups (Carbonyl, Carboxyl)
Cellulose, particularly from wood pulp, contains small amounts of oxidized
Groups. The carbonyl groups originate from the reducing endgroups and oxidized
Groups along the cellulose chain. Carboxyl groups are present in the residual
Hemicelluloses (predominantly as uronic acids), but are also introduced by pulping,
Bleaching, and other oxidation processes. Oxidized positions in cellulose,
Pulp Properties and Applications
Such as keto or aldehyde groups, are sources of polymer instability, where subsequent
cleavage will primarily occur, especially under alkaline conditions [42]. The
Carbonyl groups are a main reason for strength loss and decreased performance
Parameters in textiles, paper and other cellulosic materials, and they govern ageing
kinetics and promote thermal and light-induced yellowing processes [43]. Discoloration,
Or color reversion, is a serious deterioration of dissolving pulp quality. The heatinduced
Brightness loss is particularly attributed to low molecular-weight carbohydrates
(hemicelluloses), uronic acids, and carbonyl groups [44]. During pulping and
Bleaching operations, and particularly when strong oxidizing agents (e.g., ozone) are
Used, carbonyl groups are introduced into the pulp, thus enhancing color reversion.
In a comprehensive study, the effect of placing ozonation before (Z-P) and after
(P-Z) standard peroxide bleaching for three different ozone charges (2, 4, and 6 kg
O3 odt–1, respectively) on the effect of yellowing and the introduction of carbonyl
groups was investigated by using an (E/O) pretreated beech sulfite dissolving pulp
[46]. With increasing ozone dosage from 2 to 6 kg odt–1, the overall carbonyl content
increased from 22.9 to 38.4 lmol g–1 in the case of a PZ-sequence, whereas
Reversing the sequence to ZP the carbonyl content changed only marginally, from
22.3 to 24.2 lmol g–1. The courses of viscosity and carboxyl group contents were
Quite comparable for both sequences, as shown in Tab. 11.8.
However, the optical properties are considerably better with peroxide (P) as a
Final bleaching stage, which clearly relates to both the low overall carbonyl group
Content and the specific profile of the additionally introduced carbonyl groups
Along the cellulose chain. The data in Fig. 11.14 reveal that the pattern of keto
Groups generated along the cellulose chain is not influenced by the ozone charge,
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Results depicted in Fig. 11.7 clearly confirm the relationship between the maximum | | | Provided that the ozone treatment is followed by an alkaline treatment. |