Tab. 11.12 Characterization of TCF-bleached beech PHK pulps

prepared according to the Visbatch® process comprising four

different levels of P-factor [72].

Pulp type Bleaching

Sequence

Bleached yield

[%]

Viscosity

[mL g–1]

Brightness

[% ISO]

Xylan

[%]

Glucomannan

[%]

O-A-Z-P 44.4 466 90.7 15.6 0.6

O-A-Z-P 38.1 465 91.1 6.4 0.4

O-A-Z-P 34.7 488 91.2 3.9 0.3

O-A-Z-P 30.4 470 89.0 2.3 0.2

The decrease in surface xylan concentrationmay be explained by both the enhanced

Removal of xylan from the outermost surface layers through reinforced prehydrolysis

Conditions, and the diminished reprecipitation of xylan from solution during the

Final phase of cooking due to a significant reduction in xylan concentration.

Fiber Morphology

Hardwoods contain a wide variety of cell types of extremely heterogeneous morphology,

And this is reflected in the distribution of fiber dimensions in the fully

Bleached pulp (e.g., fiber length, coarseness, etc.). This complex cell morphology

Pulp Properties and Applications

Of predominantly hardwood dissolving pulps (e.g., beech, birch, aspen) might

Cause problems in subsequent conversion processes due to an inhomogeneous

Course of reaction. Moreover, some of the cell types such as the parenchyma cells

Are highly enriched in noncellulosic compounds, the removal of which during

Pulping and bleaching operations remains insufficient. This material does not

React completely with reagents to form homogeneous solutions and products

(e.g., viscose, acetate). Gruber et al. reported that even the short fiber fraction of

Spruce yields lower degrees of substitution during the course of heterogeneous

nitration and carbanilation than do the long fiber fractions [73]. Moreover, their

Derivatives are less soluble and contain a large proportion of gels. The lower reactivity

Of the short fiber fraction in sulfite spruce dissolving pulp is due to its more

Rapid swelling, which impedes diffusion of the reagents.

Therefore, much effort has been undertaken since the early days of dissolving

Pulp production to fractionate the pulp fiber into more homogeneous fractions in

Order to overcome the problems in subsequent processing steps. Several attempts

Were pursued to construct suitable devices for selective separation of the short

fiber fraction (wood ray, “0-fibers”, etc.). By using special drum filters with a

0.3-mm mesh screen [74] or so-called “side hills”, the total pulp losses were

usually kept at 4–6%, while the extent of resin removal was reported to be between

50 and 90%. Moreover, bleachability of the screened pulps from which the wood

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