Cell wall density

Let s consider the case of a wood with an air-dried density 7 = 0.45, corresponding to a cell wall density of 1.45, to a void volume fraction of 0.69. In Table 1, the physical quantities used for estimating the values of the parameters used in the model are shown. The volumes indicated are normalized for untreated wood, so that -I- =. Ef = 134... 

Measurements of the dry cell wall density based on microscopic observations generally give lower values (1.42 g/cm ) than those obtained using pycnometrically (1.47 g/cm ) with toluene as a displacement medium (29). This discrepancy is attributed to various uncontrollable factors such as cell-wall ruptures produced during preparation of the microtomed sections. 

The difference between the oven-dry cell wall density determined using silicone and after solvent exchange using an inert liquid like hexane must be due to the presence of residual sub-microscopic pores within the oven-dry cell wall, that are only penetrated when the cell wall is swollen. Accordingly, the inaccessible void volume per kg of cell wall material can be determined from the difference between the apparent volume of the cell wall material in silicone and the true volume as measured in the solvent-exchanged hexane. This is 0.0303 x 10 m per kg, or about 4% on a volume basis, i.e. (0.0303/0.6825) x 100% (Table 3.2). 

Table 3.2. Apparent cell wall density of sitka spmce in various fluids (Weatherwax and Tarkow, 1968).

Displacement fluid Sitka spmce physical state Apparent cell wall density (kgm ) Apparent volume of cell wall material (m kg- ) Loss in apparent volume of cell wall material, based on the volume of unswollen cell wall (m kg )... 

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 ... 

Table 6.2. Influence of microfibril angle on mechanical properties essential for standing trees (Reiterer et al., 1999 2001). The data has been normalized, i.e. values have been adjusted by multiplying by cell wall density (1500 kg m ) and dividing by the acmal density of the sample. Maximum property values (highlighted in grey) occur at different MFAs.

Many mechanical properties of wood, such as bending and cmshing strength and hardness, depend upon the density of wood denser woods are generally stronger (6). Wood density is determined largely by the relative thickness of the cell wall and by the proportions of thick-walled and thin-walled cells present. 

At small strains the cell walls at first bend, like little beams of modulus E, built in at both ends. Figure 25.10 shows how a hexagonal array of cells is distorted by this bending. The deflection can be calculated from simple beam theory. From this we obtain the stiffness of a unit cell, and thus the modulus E of the foam, in terms of the length I and thickness t of the cell walls. But these are directly related to the relative density p/ps= t/lY for open-cell foams, the commonest kind. Using this gives the foam modulus as... 

Fig. 26.2. The microstructure of wood. Woods ore foams of relative densities between 0.07 and 0.5, with cell walls which ore fibre-reinforced. The properties ore very anisotropic, partly because of the cell shape and partly because the cell-wall fibres ore aligned near the axial direction.

Wood, then, is a foamed fibrous composite. Both the foam cells and the cellulose fibres in the cell wall are aligned predominantly along the grain of the wood (i.e. parallel to the axis of the trunk). Not surprisingly, wood is mechanically very anisotropic the properties along the grain are quite different from those across it. But if all woods are made of the same stuff, why do the properties range so widely from one sort of wood to another The differences between woods are primarily due to the differences in their relative densities (see Table 26.1). This we now examine more closely. 

The moduli of wood can be understood in terms of the structure. When loaded along the grain, the cell walls are extended or compressed (Fig. 26.5a). The modulus E ,ll of the wood is that of the cell wall, E, scaled down by the fraction of the section occupied by cell wall. Doubling the density obviously doubles this section, and therefore doubles the modulus. It follows immediately that... 

Like the modulus, the tensile and compressive strengths depend mainly on the density (Fig. 26.6). The strength parallel to the grain varies linearly with density, for the same reason that the axial modulus does it measures the strength of the cell wall, scaled by the fraction of the section it occupies, giving... 

The size exclusion limit of cell wall fragments is a measure of the density of the pectin network. When wild type and invertase plants were compared no significant differences in the pore size emerged (Table 2). 

B) cell density and (C) mean cell wall thickness under different C02 pressures. Reprinted from [59], 2006, Elsevier Science. 

Different factors contribute to the mechanical properties of plant tissue cell turgor, which is one of the most important ones, cell bonding force through middle lamella, cell wall resistance to compression or tensile forces, density of cell packaging, which defines the free spaces with gas or liquid, and some factors, also common to other products, such as sample size and shape, temperature, and strain rate (Vincent, 1994). Depending on the sample properties (mainly turgor and resistance of middle lamella), two failure modes have been described (Pitt, 1992) cell debonding and cell rupture. 

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