11.3.2.5 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

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11 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

rays were removed was greatly improved, owing to a lowering of the lignin content.

The cost:benefit ratio of pulp fractionation was poor, mainly due to the high

costs caused by the extensive pulp losses and the need for additional equipment.

Consequently, pulp fractionation was discontinued in the early 1980s. Quite

recently, pilot plant trials were conducted using a specially designed pressure

screen with drilled screening plates [75]. Even though the fractionator was operated

with a split ratio of 15% short fibers and 85% long fibers, the reactivity of the

latter fraction towards xanthation was only marginally improved. However, the

more homogeneous long-fiber fraction revealed a narrower MWD and improved

optical properties, which also could be translated to a higher brightness of viscose

fibers. Again, these pilot plant trials confirm that pulp fractionation simply does

not pay unless the removed short-fiber fraction could be recovered as a product

with a price comparable to that of the long-fiber fraction.

11.3.2.6 Pore Structure, Accessibility

The cell wall of a pulp fiber is a hydrogel, and consists of carbohydrates that can

take up water and thereby increase in volume. The interaction between polysaccharides

and water within the whole cell wall structure is an important prerequisite

for the subsequent conversion of cellulose to its derivatives. Otherwise, the

reaction would be limited to the available surface. The swelling behavior of a pulp

depends strongly on its supramolecular structure which finally determines the

pore volume and pore structure. It has been shown that there are three distinct

fractions of water within the cell wall: (a) bulk water located in large macropores;

(b) freezing bound water held in micropores within the amorphous region of the

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