Water within the Cell Wall

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 

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A concentration referred to as thus equals the actual concentration of all forms of C02 in component j divided by K COz. This convention allows us to discuss fluxes in a straightforward manner, because C02 then diffuses toward regions of lower regardless of the actual concentrations and partition coefficients involved. For instance, to discuss the diffusion of C02 across a cell wall, we need to consider the partitioning of C02 between the air in the cell wall pores and the various types of C02 in the adjacent water within the cell wall interstices. Hence is the actual concentration of C02 plus H2CC>3, HCO3-, and CO32- in the cell wall water divided by the concentration of C02 in air in equilibrium with the cell wall water. 

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. 

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... 

The actual adsorption process is not fully understood. In partieular there is a dearth of experimental detail on the aetual distribution of the adsorbed water within the cell wall at different moisture eontents. At low relative humidities the water molecules penetrate the non-erystalline regions of the cell wall in the proeess of... 

WHERE IS THE ADSORBED WATER WITHIN THE CELL WALL ... 

A more logical correlation is between the initial reaction rate and the volume of water within the cell wall accessible to molecules of larger... 

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. 

Shiraishi, N., Murata, M. and Yokota, T. (1972). Polymerization of vinyl monomer within the cell wall of wood. II. Polymerization of methyl methacrylate in the presence of wood, water and carbon tetrachloride. Mokuzai Gakkaishi, 18(6), 299-306. 

The take up of water or other liquids within the cell walls of wood involve the take up of a molecule at a time and its movement from one adsorption site to another (molecular jump phenomenon) under a concentration gradient. This is distinct from flow of bulk liquids into the coarse capillary structure under a capillary force or pressure gradient. 

Wood treated with polyethylene glycol has considerable decay resistance under non leaching conditions in spite of it s non toxicity (17). This is probably due to the fact that there is insufficient water present within the cell walls to support decay... 

Bulking Treatment with Water Insoluble Chemicals. The chief shortcomings of dimensional stabilization of wood with polyethylene glycol are that it can be leached from the wood and that the wood feels damp when held for prolonged periods of time at relative humidities of 80% and above. It thus appears desirable to deposit water insoluble materials within the cell walls of wood. This can be done by a replacement process with waxes (42). Water in green wood is replaced by Cellosolve (ethylene glycol monoethyl ether) by soaking the wood in this... 

A simpler approach for depositing water insoluble chemicals within the cell walls of wood is to impregnate the wood with solvent soluble resin forming chemicals containing a catalyst that penetrate the cell walls followed by evaporation of the solvent and then heating to polymerize the resin. This has been accomplished with the following water soluble resin forming systems phenol, resorcinol, melamine and urea-formaldehydes, phenol -furfural, furfuryl-aniline and furfuryl alcohol (44). 

The dimensional stability of Impreg made in the aforegoing way increases with an increase in the resin content of the veneer up to about 70% antishrink efficiency at a resin content of 30 to 35%. This ASE value is less than that obtainable with polyethylene glycol because of loss of water and subsequent contraction of the resin forming chemicals within the cell walls as polymerization occurs. 

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]. 

Wood-inorganic material composites (WIC) have been developed in Japan [54]. A double-diffusion treatment of wood by two aqueous solutions of inorganic chemicals leads to the formation of water-insoluble deposits within the cell walls and voids. Water-impregnated wood was introduced into solutions (I and II) successively at 50°C for a desired period of time. Saturated aqueous solutions were prepared from (I) barium chloride plus a small amount of boric acid and (II) ammonium phosphate plus a small amount of boric... 

It follows that the gain or loss of water or other liquids or vapors into or out of wood tissue can be influenced greatly by the nature, amount, and distribution of wood polysaccharides. As will be discussed in Chapter 3, these same sorption phenomena (i.e., adsorption and desorption), together with the architectural arrangement of wood cells in the tree, are responsible for particular patterns in wood swelling and shrinkage when wood is subjected to various environments. The arrangement of polysaccharide molecules within the cell wall itself, especially that of the cellulose, also has a profound effect on the physical and mechanical properties of individual cells and wood as a whole. 

Submieroseopie void spaces within the cell wall are only accessible to fluids like water that swell and penetrate the cell wall. Non-swelling fluids sueh as silicone therefore over-estimate the volume occupied by the cell wall material and so underestimate its density. 

BET need not concern us here. What is significant is that when the BET equation is applied to the adsorption of water by wood (Figure 3.3) the BET allows one to estimate both the surfaee area on which water is being adsorbed within the cell wall and the average number of layers of water adsorbed on these surfaees. The results are quite startling (Table 3.4). 

Wood only shrinks when water is lost from the cell walls and it shrinks by an amount that is proportional to the moisture lost below fibre saturation point. To a first approximation the volumetric shrinkage is proportional to the number of water molecules that are adsorbed within the cell wall, and that in turn is related to the number of accessible hydroxyls on the cellulose, hemicelluloses and lignin, and to the amount of cell wall material, i.e. the basic density of the wood (Figure 4.2). 

Consider the following example of a timber having an unextracted basic density of 530 kg m, comprising 500 kg m of wood and 30 kg m of hydrophilic (water-soluble) extractives deposited within the cell wall... 

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