Pulp Quality

The kraft cooking process and subsequent oxygen delignification stage must be

regarded as combined processes from the point of view of yield and pulp strength.

The balance between cooking and oxygen delignification determines the quality

profile of the pulp. According to today’s knowledge, an unbleached kappa number

between 25 and 30 (preferably around 27) ensures the lowest productions costs,

taking into consideration both wood yield and chemical consumption combined

with the unchanged pulp quality of a softwood kraft pulp [37,135]. In agreement

with the results reported by Iribarne and Schroeder [12], strength properties are

not affected as long as a certain threshold viscosity is not exceeded [135]. If the

intrinsic viscosity of an oxygen-delignified softwood kraft pulp is kept above a level

of about 870 mL g–1, the relationship between tear index and tensile index seems

not to be affected. As an example, the data in Fig. 7.58 show that the zero span

tensile index of a loblolly pine kraft pulp increases proportionally to the intrinsic

viscosity, and reaches a plateau value above an intrinsic viscosity of about 750–

800 mL g–1. These values are somewhat lower than those reported before or elsewhere,

most likely because of the relatively low initial viscosity of the pulps used

in this study [136].

It is however important to point out that the unbleached kappa number must

not fall significantly below a level of 25.

0 200 400 600 800 1000

Unbleached Pinus taeda L. kraft pulps

Oxygen-delignified kraft pulps

Zero span tensile index [Nm/g]

Viscosity [ml/g]

Fig. 7.58 Relationship between intrinsic viscosity and zerospan

tensile index of unbleached Pinus taeda L. kraft pulps

and oxygen-bleached kraft pulps produced thereof according

to Iribarne and Schroeder (after recalculation) [12].

7.3 Oxygen Delignification 733

Unbleached kraft pulps – and also oxygen-delignified kraft pulps – show pronounced

swelling properties in an aqueous environment. The swelling properties

of the pulp fibers are affected by the acid groups, predominantly carboxyl groups.

These groups also determine the ion-exchange capacity of cellulose materials and

they contribute to the bounding of fibers. Furthermore, cations adsorbed by the

carboxyl groups play an important role in the discoloration mechanism of pulp

and products made thereof (e.g., paper, cellulose fibers, films). Carboxyl groups

originate from the hemicelluloses, the lignin fraction and, to a much smaller

extent, also from fatty and resin acids. The carboxyl groups in the hemicelluloses

are included in the 4- O -methyl-a-d-glucuronic acid side chains of the xylan fraction

which are, however, degraded to hexenuronic acids during alkaline cooking.

Hexenuronic acid and 4- O -methyl glucuronic acids are almost not degraded during

oxygen delignification. The observed decrease in carboxyl groups during oxygen

delignification of kraft pulp is attributed predominantly to the removal of lignin

[137]. The oxygen delignification of sulfite pulps, however, also contributes to

a reduction of carboxyl groups due to the dissolution of hemicelluloses.

7.4

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