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11.3 Dissolving Grade Pulp

The data in Fig. 11.12 also indicate that the viscose fibers derived from sulfite

pulps generally exhibit slightly lower mechanical properties at a given DP <100

weight fraction as compared to those made from PHK pulps. This may be caused

by the higher carbonyl content of the sulfite pulps at a comparable purity level.

GPC measurements of the corresponding fibers indicate that the increase in the

amount of low molecular-weight carbohydrates during alkaline degradation (ageing)

Is more pronounced for sulfite than for phk pulps. Surprisingly, a clear correlation

between the carbonyl content of the pulps and the working strength of

the regular viscose fibers made thereof could be established, as illustrated in

Fig. 11.13.

0 5 10 15 20 25 30 35

350

400

450

500

550

* Tensile strength times

Elongation

conditioned state

Working Strength [cN/tex*%]*

Laboratory pulps HW-Sulfite

Commercial pulps Representative Selection

Carbonyl groups [μmol/g]

Fig. 11.13 Maximum strength properties of 1.3 dtex regular

Viscose fibers in the conditioned state related to the carbonyl

content of dissolving pulps (selection of Fig. 11.12). The content

of carbonyl groups was determined according to the

CCOA method [41].

The content of carbonyl groups reflects both the amount of reducing endgroups

(short-chain molecules) and the oxidized groups thus undergoing degradation

reactions under strongly alkaline conditions.

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,

1037

1038 11 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