Fiber saturation point

Fiber saturation point is the bound moisture content of ceUular materials such as wood. 

Fiber-saturation point is the moisture content of celhilar materials (e.g., wood) at which the cell walls are completely saturated while the cavities are liquid-free. It may be defined as the equihbrium moisture content as the humidity of the surrounding atmosphere approaches saturation. 

Capillary Flow Moisture which is held in the interstices of solids, as liquid on the surface, or as free moisture in cell cavities, moves by gravity and capiUarity, provided that passageways for continuous flow are present. In diying, liquid flow resulting from capiUarity appUes to liquids not held in solution and to aU moisture above the fiber-saturation point, as in textiles, paper, and leather, and to all moisture above the equiUbrium moisture content at atmospheric saturations, as in fine powders and granular solids, such as paint pigments, minerals, clays, soU, and sand. 

FIGURE 68 Wooden bowl. A wooden bowl made of boxwood, first century c.e., from Qumran, Israel the wood used to make the bowl seems to have been imported from Turkey. The excellent preservation of the bowl is due to extremely hot and dry environmental conditions in the region. Three conditions are necessary for wood to decay (1) a favorable temperature (0-32°C), (2) moisture in excess of the fiber saturation point (above 25-30%) and (3) an adequate supply of oxygen. If any one of these is eliminated wood remains well preserved for long periods of time. 

It is very difficult to restrict the oxygen from microorganisms so control measures based on this approach would probably be fruitless. Wood below the fiber saturation point does not decay. Therefore, by restricting the amount of water in the wood cell wall below the fiber saturation point, the microorganisms will not thrive. 

Specific Heat. The specific heat of textiles, particularly wool, has been the subject of recent investigations. For moisture contents above the fiber saturation point of wool, reduced, supercontracted, and chemically modified wool fabrics exhibited endothermic peaks at -30 to 0°C that resulted from the heat of fusion of absorbed water. In that temperature range, a significant increase in the specific heat of the wool fabrics was also... 

The fiber saturation point (FSP) of cotton is the total amount of water present within the cell wall expressed as a ratio of water to solid content. It is equivalent to the water of imbibition of the fiber, also called its water retention value. The FSP has been measured using solute exclusion, centrifugation, porous plate, and hydrostatic tension techniques. It occurs at RVP greater than 0.997 and from the review of the papers, it has been concluded that the studies have yielded a value for FSP in the range of 0.43 to 0.52 g/g [303]. 

Changes in the moisture content of the wood cell wall have a major effect on the mechanical properties of wood [5]. At moisture contents from oven-dry (OD) to the fiber saturation point (FSP), water accumulates in the wood cell wall (bound water). Above the FSP, water accumulates in the wood cell cavity (free water) and there is no tangible strength effect associated with a change in free water content. However, at moisture contents between OD and the FSP, water does affect strength. Increased amounts of bound water interfere with and reduce hydrogen bonding between the polymers of the cell... 

Table 4 Fiber Saturation Point of Control and Acetylated Aspen Flakes...

Weight percent gain Fiber saturation point... 

Wood is subject to water iafiltration by both Hquid and vapor. As the moisture content iacreases, the wood wiU sweU imtU it reaches its maximum dimension at its fiber-saturation point (about 30% moisture). Variation in the bound water content between zero and 30% wfll aUow the wood to shrink and sweU. Rapid dimensional changes resulting from changes in the level of bound water cause the wood to crack and spHt. These cracks wfll then aUow moisture to absorb easily and quicldy into the wood. At moisture content levels above the fiber-saturation point, moisture wfll be present as free water, which in turn promotes the rate of wood decay. 

At equihbrium with relative humidity below 100%, the moisture in wood is present primarily in the cell wads. The moisture content at which the cell walls would be saturated and the cell cavities empty is called the fiber saturation point. Actually, such distribution is impossible. Beginning at 90% relative humidity, some condensation may occur in small capillaries. The determination of the fiber saturation point is based on the fact that certain properties of wood (eg, strength and volume) change uniformly at first with increasing moisture content and then become independent of the moisture content (Fig. 2). The equihbrium moisture content (usually determined by extrapolation), at which the property becomes constant at 25 to 30% moisture, is represented by the fiber saturation point. 

As a consequence of its hydrophilicity, wood tissue will seek to maintain, through either gain or loss of moisture, an equilibrium moisture content with the surrounding atmosphere. If the wood takes on water, the cell walls proceed to swell until the cell walls become water-saturated. The latter moisture content is called the wood s fiber saturation point. In contrast, loss of wood water (below the fiber saturation point), due to diffusion and evaporation, results in wood shrinkage. 

The water content of the wood cell wall has a strong influence on the wood s mechanical properties, and a higher moisture content, at least below the fiber saturation point, and normally is inversely related to most strength properties see Chapter 5). This situation is easily reconciled if one considers that the takeup of water below the... 

At the cellular level, the true density of dry cell wall substance (i.e., within the cell wall) has been determined to be about 1.5 g/ cm, varying to some extent with the method of measurement and species (2). There are voids within the dry wood cell wall, but the void volume here (i.e., micropores) is reported to be only about 2-4%. However, this figure would be expected to increase as wood moisture content is increased to the fiber saturation point (28). 

The fiber-saturation point is defined as the moisture content at which the cell cavities are empty of liquid water but the cell walls are still saturated with bound water (6). The fiber-saturation point is designated as mf (fraction of dry mass) or Mf (percent of dry mass). 

This result is as expected based on thermodynamic considerations and is discussed later in this chapter. The apparent fiber-saturation point Mf, which is obtained by extrapolating the sorption isotherm to 100% relative humidity, decreases approximately 0.1%/ C rise in temperature (22),... 

Stamm and Loughborough (32) first reported that this relationship has been reported (32) to be approximately valid for woods of the continental United States. The mean value of the ratio Shf/G was 27 for 107 hardwood species and 26 for 52 softwood species of the United States. These ratios should be equivalent to the fiber-saturation point Mf if the green volume specific gravity Gg is taken to be... 

Mechanical Properties. Many of the important mechanical properties of wood increase exponentially as the moisture content decreases below the fiber-saturation point (28) (see Chapter 5). This relationship can be expressed as... 

Figure 20. Calculated curves of specific gravity G and of density p (gj cmr) of wood vs. moisture content which assume a constant cell cavity volume and a 30% fiber-saturation point. (Adupted from Ref 45.)...

When wood below the fiber-saturation point interacts with water, heat is evolved, and there are changes in the free energy and entropy of the sorbed water. Furthermore, the wood exerts swelling forces that can be measured. These effects can be treated by classical thermodynamic methods although moisture sorption by wood is not a perfectly reversible process because sorption hysteresis is involved, as was pointed out in the section on Moisture Sorption Isotherms (p. 136). 

Hawley (64) first demonstrated the complex nature of moisture flow through wood above the fiber-saturation point resulting from capillary forces associated with air bubbles and pores of variable radii interconnecting cells. Using Comstock s (65) simplified structural model for softwoods, Spolek and Plumb (66), however, were able to predict the capillary pressures in southern yellow pine as a function of percent of water saturation of the cell cavities. Such a quantitative analysis would be more difficult to implement in the case of woods other than southern yellow pine because their structures and permeabilities are more variable in most cases. However, computer modeling techniques are developing to the point where more general models may become feasible. 

Wood strength is related to the amount of water in the wood fiber cell wall (7-JO). At moisture contents from oven-dry (OD) to the fiber-saturation point, water accumulates in the wood cell wall... 

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