Heterogeneity of Wood Structure

Wood is a very porous material. The high porosity is essential for the transport of

water in the sapwood of a living tree, where water flows from the roots to the top

through the cell lumina and is further distributed through the pits. Wood is, however

not very permeable for gases and liquids, because flow into or through it

must occur through a coarse system of capillaries. The void system of wood is different

in softwoods and hardwoods, sapwood and heartwood, springwood and

summerwood and additionally is also dependent on the individual wood species.

Softwoods consist mainly of longitudinal fibers, tracheids, with a tubular structure

of an average length of approximately 3.5 mm and a diameter of 30–35 lm.

The tapered ends overlap longitudinally by about one-fourth of their length. Penetration

occurs through their lumina and pits, which are covered by a membrane.

There are two types of pits, namely the simple pit and the bordered pit, but all are

characterized by the presence of a pit cavity and a pit membrane. In the simple pit

the cavity is almost constant in width, whereas in the bordered pit the cavity narrows

more or less abruptly toward the cell lumen. The pit membrane, which consists

of primary wall and middle lamella, contains pores with dimensions in the

range below of approximately 4 nm [2]. Within the heartwood of softwoods, mass

transport is very limited because the pores in the pit membrane are often sealed

by lignification or resinification.

Hardwood fibers are made up of several cell types, differentiated according to

their special functions (see Chapter I-2). Mass transfer occurs predominantly

through the lumina of the vessel elements, which are connected vertically to form

long tubes. The channels thus formed contribute very efficiently to the water

transport. If the vessels are plugged by tyloses, which frequently occurs during

the development of heartwood, the penetration rate almost ceases (as is the case

4 Chemical Pulping Processes

in white oak; see Tab. 4.9). The fibers of hardwoods are interconnected by pit

pairs, but they are smaller and fewer in number as compared to the softwood tracheids.

They are less effective for liquid transport. Electron microscopic studies

have not provided any evidence of pores through the membranes of hardwoods,

thus indicating that hardwood fibers are ineffective for liquor flow [3]. Springwood

is more easily penetrated than summerwood due to its wider lumina and its thinner

and more fissured cell walls. Reaction wood and wood knots are very dense

and thus more difficult to penetrate.

The water content of the wood determines not only the mechanical properties

but also the efficiency of impregnation prior to chemical pulping. The moisture

content in wood, MCd, is defined as the water in wood expressed as a fraction of

the weight of oven-dry wood [see Eq. (21)]:

MCd _

mwc _ mdc

mdc _21_

where mdc equals the mass of dry chips, and mwc the mass of wet chips.

The moisture content can be related to the dry solid content, DS, expressed as a

weight fraction using Eqs. (22) and (23):

DS _

mdc

mwc _

1 MCd _22_

MCd _

1 _ DS

DS _23_

And thus themass of thewet chips,mWC, can be calculated according to equation (24):

mWC _

mdc

DS _24_

On occasion, the moisture content is based on the total weight of wet wood, MCw,

which then can be expressed as:

MCw _

mwc _ mdc

mwc _

MCd

MCd 1 _25_

The amount of moisture content, MCd, in freshly cut green wood can vary considerably

within species, and can range from about 30% to more than 200%. In softwoods,

the moisture content of sapwood is usually greater than in heartwood. In

hardwoods, the difference in moisture content between heartwood and sapwood

depends on the species. Variability of moisture content exists even within individual

pieces cut from the same tree. The average moisture contents of a selection of

hardwoods and softwoods are listed in Tab. 4.7.

At the cellular level, moisture can exist as free water or water vapor in the cell

lumens and cavities, and as chemically bound water within the cell walls. Earlywood

tracheid lumens can hold more water because they are much larger than

the latewood tracheid lumens. The cell walls are denser in the latewood tracheids

and so contain more of the bound water. The bound water is held between microfibrils

in the cell wall and is closely associated with the polysaccharides by means

4.2 Kraft Pulping Processes

Tab. 4.7 Average moisture content of green wood, by species according to [4].

Wood species Moisture content [MCd]

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