Endgroups from the total amount of carbonyls

Assuming that the amount of reducing endgroups

Relates to the MWn determined from

GPC measurement [45].

3 4 5 6 7

0.0

0.3

0.6

0.9

O

-charge: 6 kg/odt: PZ ZP

dW/d(log MM)

Log Molecular Weight

Fig. 11.16 Differential MWDs of beech sulfite

Dissolving pulps prepared by TCF bleaching

With Z-stage prior and after P-stage according

to (E/O)ZP and (E/O)PZ, applying identical

conditions in each stage [46]. Molecular weight

(MWn in brackets) after (E/O)ZP: 205 (38)

kDa, after (E/O)PZ: 290 (45) kDa; for more

Detailed characterization, see Tab. 11.8.

Dissolving Grade Pulp

15 20 25 30 35 40 45

0.0

HW-Sulfite Dissolving Pulp:

(E/O)-Z-P (E/O)-P-Z

Color Reversion, 100*ΔR

Oo

/R

Oo

Carbonyl groups [μmol/g]

Fig. 11.17 Color reversion, DR/R after 72 h

treatment at 105 °C, as a function of the overall

Carbonyl group content of beech sulfite dissolving

Pulps prepared by TCF bleaching with

Z-stage prior and after P-stage according to

(E/O)ZP and (E/O)PZ, applying a series of different

ozone charges: (E/O)ZP: 2, 4, 6 kg odt–1;

(E/O)PZ: 2, 4, 6, 6.2 kg odt–1 [46].

The results indicate clearly that carbonyl groups promote both cellulose degradation

reactions and color reversion [47]. Even though delignification and bleaching

Is more selective when ozonation represents the final stage according to an

(E/O)-P-Z-sequence, yellowing can only be reduced when the final bleaching

Sequence is reversed and the peroxide step follows an ozone stage. The important

Role of carbonyl groups with respect to stability of polysaccharides is comprehensively

reviewed by Gratzl [47].

Upon oxidative bleaching treatments, carbonyl groups are partly oxidized to carboxylic

Acids (see Tab. 11.8). Higher concentrations of carboxyl groups have shown

Not to adversely affect viscose fiber processing. However, the thermal stability of

The pulp is negatively influenced with an increasing amount of carboxyl groups

[48]. Moreover, carboxyl groups – particularly from the uronic acids present in the

pulp – are considered to promote heat-induced yellowing of kraft hardwood and

softwood pulps [49]. Acetate-grade pulps, which are used for plastic molding,

should therefore have a carboxyl group content which is as low as possible [42].

Supramolecular Structure

The supramolecular structure of cellulose is adequately characterized by the

Widely accepted two-phase model (fringe fibrillar model) representing low-ordered

(amorphous) and highly ordered (crystalline) regions. The ratio of crystalline-toamorphous

domains – the so-called “degree of crystallinity” (order) – is predomi-

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