Heartwood permeability

Permeability. Although wood is a porous material (60—70% void volume), its permeability (ie, flow of liquids under pressure) is extremely variable. This is due to the highly anisotropic shape and arrangement of the component cells and to the variable condition of the microscopic channels between cells. In the longitudinal direction, the permeability is 50 to 100 times greater than in the transverse direction (13). Sapwood is considerably more permeable than heartwood. In many instances, the permeability of the heartwood is practically zero. A rough comparison, however, may be made on the basis of heartwood permeability, as shown in Table 3. 

Table 3. Relative Permeability of the Heartwood of Some Common Species, Decreasing from Group 1 to Group 4...

Timber species also vary in the treatability of the heartwood - with preservatives in an impregnation plant, and are classified into four categories according to the depth of penetration which can be achieved permeable (complete penetration) moderately resistant (6-18 mm lateral penetration) resistant (3-6 mm lateral penetration) and extremely resistant (no appreciable lateral and very little end-grain penetration). 

Impermeable timbers have a good resistance to polluted atmospheres where acid fumes rapidly attack steel. Wood has given excellent service in the buildings of chemical works and railway stations. Permeable wood species and sapwood can suffer defibration problems caused by the sulphur dioxide of industrial atmospheres. Tile battens are particularly vulnerable. The heartwood of Douglas fir, pitch pine, larch, Scots pine/European redwood and many tropical hardwoods give good service in these conditions. 

Pre-extraction of Wood. Since the major factor causing a reduction in the permeability of wood during heartwood formation is occlusion of the pit membranes with extraneous material, one would anticipate that pre-extraction of the wood with a suitable solvent would be a method of increasing the permeability of wood. This contention has been verified by a number of studies (15, 16, 17, 18, 19, 20, 3, 21). However, it appears doubtful that such treatments would be commercially feasible since the solvents are expensive and excessive time is required for the additional step in the treating process. 

Attempts have been made to increase the permeability of heartwood but without success. Furthermore, steaming has not been effective on species other than the southern pines (27). 

The important thing to remember in most of these experiments is that the bacteria have been used to improve the permeability of sapwood. For species like Sitka spruce which has sapwood that is impermeable to creosote, this type of treatment may prove to be commercially feasible. However, in order to fully exploit this method for increasing permeability, it will be necessary to find bacteria which have the ability to degrade heartwood pit membranes. Indeed, this may be possible since Greaves (50) has shown that some bacteria can increase the permeability of heartwood. In addition to this, it would be desirable to accelerate the reaction as much as possible in order to make it compatible with a commercial operation. 

To date, it has not been possible to degrade heartwood pit membranes with isolated enzymes. Since co-factors are probably required (42), it appears that use of a whole organism rather than isolated enzymes may be required if the permeability of heartwood is to be increased. 

The polymer loading of wood depends not only upon the permeability of wood species, but also on the particular piece of wood being treated (14). Since the void volume is approximately the same for the sapwood and heartwood for each species, it would be expected that the polymer would fill them to the same extent. 

The moisture content of heartwood in softwood trees is reduced to a level much lower than that of normal sap-wood (2, 24), During the moisture reduction period, the membranes of bordered pits in sapwood fibers have a strong tendency to become aspirated. This situation, together with that of pit membrane incrustation, greatly reduces the natural permeability of heartwood tissue to liquids and gases. 

With impermeable woods - and heartwood - the supply of moisture from the interior eaimot keep paee with evaporation of water vapour from the surface, because mass flow of water is not possible and diffusion is a much slower process. Thus the surfaee moisture eontent quiekly falls below fibre saturation and the evaporative front starts reeeding into the wood. Figure 8.6 shows the parabolic moisture content profile for a slowly air-dried impermeable hardwood. Similarly for permeable softwoods that have been dried below the irreducible moisture content, Stamm (1964, 1967b) reported parabolic moisture profiles that are consistent with diffusion of both water vapour and bound water. 

Even with a permeable wood diffusion assumes increasing importance as the average moisture content approaches the irreducible moisture content indeed, in every part of the board where the moisture eontent approaehes this value drying is diffusion controlled. Permeable and impermeable timbers of similar densities should dry from fibre saturation at about the same rate. The behaviour of mixed heart/sapwood boards is eomplieated sinee, at first, there is both an evaporative interface near the sapwood surfaee and one in the interior at the zonal boundary between heart and sapwood. For a board with only a slither of heartwood along one face, mass flow can only move to the sapwood faee so in effeet the board appears to be twice the width than it aetually is. Pang et al. (1994) predieted that such a 50 mm thick board would dry from green to 6% moisture eontent using a 140°C/90°C schedule in 14 hours, compared to 10 hours for sapwood and 11 hours for heartwood. 

In heartwood, due to metabolite deposition, aspiration or closure of bordered pits, or tylose development, the permeability is often reduced by one or several orders of magnitude. 

Table 40.7 summarizes some values of directional permeability available in the literature for different species. Depending on the experimental apparatus and the protocol nsed by the authors, some data are missing in the papers. For example, it is not always easy to calculate the permeability ratios from permeability value, or vice versa. Choong et al. (1974), for example, have reported the permeability values for sapwood and heartwood for the longitudinal direction, but not for the transverse directions. Only the mean anisotropy ratio is available in this paper. Perre (1992) and Perre et al. (2002) have used an experimental procedure to determine the longitudinal permeability and the anisotropy ratio on the same sample. In these instances, they just obtained the ratios and decided not to calculate the transverse permeability accordingly. 

The transverse permeability and the anisotropy ratios are very variable. The heartwood part of logs is usually much less permeable than the sapwood part (Comstock, 1967). This is due to tyloses development and extractives deposition (tannins, gums, etc.) in hardwoods and due to the aspiration of bordered pits in softwoods. 

Note L, longitudinal R, radial T, tangential S, sapwood H, heartwood 1, permeability to liquid g, permeability to gas a, air-dried sample o, oven-dried sample. 

The strategy of simulating the differences between heart-wood and sapwood lies in only two sets of parameters the permeability and the initial moisture content (for these experiments, 180% for sapwood and 70% for heartwood). The values of permeability used to differentiate sapwood from heartwood (Table 40.8) are based on the considerations concerning pit aspiration. 

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