The Fibre Saturation Point

Water in wood ean exist as either absorbed (also called free water) in the cell lumens and intereellular spaees or as adsorbed (also called bound water) within the cell walls. When wood dries water first evaporates from the lumens and intercellular spaees. The fibre saturation point is defined as the moisture content at which all the absorbed water has been removed but at which the cell walls are still fully saturated. This oeeurs at a moisture eontent of 25 to 35%. In most instances it is adequate to presume the fibre saturation point to be 30% moisture content. 

The concept of the fibre saturation point cannot be over emphasized. Its importance lies in the fact that the manner in which water is held is different in the adsorbed and absorbed states. The fact that the absorbed water can be removed before stripping off the adsorbed water indicates that a distinction can be made between these two states of sorption. For example, when green wood is dried there is no appreciable change in its mechanical properties until the fibre saturation point is reached. Below this moisture content they increase almost linearly with any further decrease in moisture content. Furthermore, wood only shrinks when the adsorbed water is removed from the cell walls. 

Absorption refers to the take up of liquid by capillary condensation within a porous solid as a result of surface tension forces. It is accompanied by a limited reduetion of the vapour pressure over the liquid as a result of the concave liquid-vapour mensicus. The energy required to evaporate the liquid is thus only slightly greater than that required to evaporate from a flat surface. The sap filling the lumens of wood is absorbed water. This bulk water can persist only at very high humidities, evaporating from successively smaller capillaries as the humidity falls. Table 3.3 states that lumens with radii exceeding 1 pm will drain and be free of all absorbed water when the relative humidity falls below 99.9% and so forth. 

By eontrast adsorption is different. Adsorbed water within the cell wall remains even at very low vapour pressures indicating that the attractive force between the adsorbent (in this case wood) and the adsorbate (water) is much greater than the 

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Figure 9.2 shows that, during recycling of chemically delignified (Kraft) pulp fibres, irreversible pore closure within the cell wall takes place which leads to a reduction in their cell wall water content as measured by the fibre saturation point (see Chapter 5). The net effect of this is a loss in fibre flexibility which, in turn, leads to less effective inter-fibre bonding. 

Liquid water, sometimes referred as free liquid water, is present in the porous structure at moisture contents above the fibre saturation point, here assumed constant at all temperatures 0.2 kg water per kg dry wood. The rate of evaporation in Equation (3), C4,+b]ti is indirectly solved from Equation (2) for water vapour, j=HjO. In the presence of liquid water equilibrium v iour pressure is assumed in the gas phase described as 18] ... 

At moisture contents below the fibre saturation point (he vapour pressure sinks as [9] ... 

In earlier work a sensitivity analysis of simulations of dry wood shows an 8% reduction in time of pyrolysis at 1 mm reduction of the final charcoal radius [I], This was shown to be in the same range as the influence from an exothermic heat of pyrolysis (150 kJ/kg). In this paper the inclusion of an axial convective flow is shown to influence the time of pyrolysis in the same range, a reduction of 6% (evaluated from Figure 2a where the times of conversion of Case 1 and Case 3 are estimated to 750 and 708 seconds respectively). For wood with a moisture content above the fibre saturation point the axial liquid permeability is shown to dominate the influence on the conversion time. 

Electrical moisture meters provide a quick and reasonably accurate nondestructive alternative. The direct-current resistance of the timber is measured or either the alternating-current capacitance or power loss can be measured. Direct-current resistance moisture meters are more common a pair of needles, a fixed distance apart, is driven into the wood across or along the grain (depending on the manufacturer s instructions) and the electrical resistance measured. The procedure is reasonably accurate between the fibre saturation point (defined later) at 30% and about 6% moisture content (at which point the resistance becomes too great to measure with reasonable accuracy). In this moisture content range, the relationship between electrical resistance and moisture content is represented by a log-log plot. 

The adsorption-desorption isotherms shown in Figure 3.1 relate to an experiment at 32°C. A rise in temperature causes a decrease in the equilibrium moisture content of wood at all relative humidities (Figure 3.2). There is also a corresponding reduetion in the moisture content of the wood at the fibre saturation point. Both... 

Raising the temperature tends to displace the equilibrium towards the left, i.e. less adsorbed water and a lower equilibrium moisture content. In this particular example the fibre saturation point drops from around 31% at 25°C to 23% moisture content at 100°C (Figure 3.2). 

There is a difficulty with the definition of the fibre saturation point. Strictly it corresponds to the moisture eontent when the cell walls are fully saturated (which would occur where the relative humidity is 1.0) and all the lumens are free of water. However at high relative humidities, above about 0.98, the lumen tips and pits begin to fill by capillary condensation, eausing a sharp upward break in the sorption curve. In praetiee it is extremely hard to measure moisture content at these humidities and extrapolation from lower relative humidities is not particularly accurate. 

It is worth making a simple calculation. Assuming that the fibre saturation point eorresponds to 30% moisture content, then at the fibre saturation point there are 300 kg of water (density 1000 kg m ) for every 1000 kg of oven-dry cell wall (density 1500 kg m ). At fibre saturation point the cell wall is fully swollen and the proportion of the swollen wall that is occupied by water will be ... 

In order to measure the fibre saturation point Feist and Tarkow (1967) used a sufficiently large water soluble polymer to preclude it penetrating the cell wall. Stone and Scallan (1968) reversed this approach and used a series of much smaller... 

Finally, absorbed lumen water has been detected by NMR (contributing a minute 0.3% wt/wt) when the moisture content was only 17% (Araujo et al., 1992). This blurs the definition of the fibre saturation point in that small amounts of absorbed water are present at moisture contents well below the fibre saturation point. 

Figure 3.5. NMR spectrum of wood as a function of moisture content (Nanassy, 1974). The spectral intensity of the broad component is due to hydrogen nuclei in dry wood tissue (6% of oven-dry mass) and to a monolayer of strongly adsorbed water molecules. The intensity of this component increases somewhat with moisture content, at least to the fibre saturation point. This implies that new internal surfaces are being created as the moisture content increases. The narrow component corresponds to the more mobile multilayers of adsorbed water. The narrow component is truncated because the idea was to record the broad spectmm which required expanding the vertical scale, so that the peak of the narrow spectmm is well off scale. With quantitative NMR techniques the areas under the broad and narrow components of the spectmm provide a measure of the number of hydrogen atoms in these two states.

Some typieal moisture content values for green wood are noted in Table 3.1. These values are considerably greater than the fibre saturation point. Absorbed water at the surface will evaporate and the lumber will dry provided the surrounding atmosphere is not totally humid. Indeed the absorbed water in the lumens cannot remain there in equilibrium with the atmosphere unless the relative humidity of the air is in excess of 99% (Table 3.3). If the wood is left under cover - keeping the rain off - it will eventually dry to a moisture content that will vary according to the temperature and humidity of the air (Figure 3.2). This moisture content will be below the fibre saturation point so all the absorbed water and some of the adsorbed water will have evaporated. If an even lower moisture content is required it is necessary to use a kiln to lower the relative humidity and raise the temperature (Figure 3.2). 

Early in the drying process it is inevitable that the moisture eontent at the centre of the pieee will be above the fibre saturation point while the fibres at and near the surface will be well below the fibre saturation point. There will be a moisture gradient within the wood and the system will not be in equilibrium. In this situation the surface fibres will have started to shrink and the overall volume of the pieee will be reduced even though the average moisture eontent is above fibre saturation. This accounts for the shrinkage of the wood at mean moisture eontents a little above the fibre saturation point (Figure 4.2). 

Figure 4.2. External volumetric shrinkage of 7/8 in. (22 mm) boards of loblolly pine (Stamm, 1964). The dotted lines extrapolate to the shrinkage intersection point, which is an estimate of the fibre saturation point.

With a slow-drying impermeable species a fan speed as low as 1.5-2.0 m s may be sufficient as the moisture eontent at the surface drops quickly below the fibre saturation point - no mass flow - and there is no point in installing overly powerful fans just to strip off surface moisture for the first few minutes of a long kiln schedule (>14 days) thereafter the slow rate of transfer of moisture from the centres of the boards to their surfaces becomes more important than the rate of evaporation. 

Only when the supply of moisture from the interior is no longer able to sustain the maximum cooling effect due to evaporation does the surface begin to dry out, its moisture content drops below the fibre saturation point and the temperature at the... 

The moisture content of the heartwood is only a little higher than the fibre saturation point (Table 3.1), so most pits in heartwood are aspirated. These pits are also clogged to some extent with extractives. Heartwood is impermeable. 

When wood is green the diffusion eoeffieient of water vapour in the lumens is about 10 times greater than that of adsorbed water in the eell wall, but when the wood is dry, c. 5% moisture content, the diffusion eoeffieient of water vapour in the lumen is about 1000 times greater than that of water in the eell wall the difference is least when the moisture content of the eell wall is elose to the fibre saturation point and greatest when the moisture content approaehes oven-dry. 

Some success has been reported with hardwoods, but only for those species that are not particularly difficult to dry using conventional schedules. Overall drying time can be halved but at the expense of additional degrade such as honeycombing and collapse. Some recalcitrant timbers may be dried successfully but only if they have been pre-dried to the fibre saturation point. Other hardwoods cannot be high-temperature dried at all. 

Collapse precedes conventional shrinkage, only occurring within saturated cells, whereas normal shrinkage occurs below the fibre saturation point. Where present, severe eollapse is revealed by a series of corrugated depressions matching earlywood... 

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