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,

Provided that the ozone treatment is followed by an alkaline treatment.

Tab. 11.8 Comparison of ZP and PZ treatments on (E/O)

pretreated hardwood sulfite dissolving pulps [46].

Sequence Ozone charge

[kg odt–1]

Viscosity*)

[mL g–1]

Brightness Carbonyla

[lmol g–1]

Carboxyl

[lmol g–1]

Initital

[% ISO]

72 h, 105 °C

[% ISO]

P 640 87.2 81.8 16.8 34.4

PZ 2 575 93.2 87.9 22.9 33.7

4 460 94.3 88.8 29.9 36.6

6 390 94.4 88.4 38.4 42.7

ZP 2 529 94.3 91.0 22.3 31.3

4 463 95.4 92.8 21.0 35.0

6 390 95.4 92.5 24.2 40.9

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