Permeability, wood

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. 

Chemical attack Damage is superficial in the case of impermeable timbers, but is deep with permeable wood species. 

Windy areas need fences with some permeability. Wood-rail fences, with gaps between the rails as wide as the rails themselves, are less easily blown down than solid fences. A heavy-duty trellis with a close mesh allows wind permeability at the same time as affording more privacy. 

The book opens with a paper on the structure and composition of wood to define the material under discussion and then considers molds, permeability, wood preservation, thermal deterioration and fire retard-ance, dimensional stability, adhesion, reconstituted wood boards such as fiberboard and particleboard, plywood, laminated beams, wood finishes, wood-polymer composites, and wood softening and forming. A final paper treats the common theme of wastewater management. Only one of the papers presented at the meeting is not included in this volume, and its subject of conventional wood preservation methods is adequately treated in detail elsewhere (e.g., Nicholas, D. D., Ed Wood Deterioration and Its Prevention by Preservative Treatments, 2 vols., Syracuse University Press, 1973). 

In the above rather simplistic discussions on density, the arguments put forward implicitly assume that wood is a homogeneous, isotropic material. Of course, in reality wood has a non-uniform, anisotropic, vascular structure. A low permeability wood could be expected to delay the escape of volatile pyrolysis products for a longer time than more permeable woods, increasing the opportunities for carbon-depositing secondary reactions to occur. This suggests that permeability could well be a better... 

In summary the temperature of the wood surface is determined by the cooling effect of evaporation, rising from the initial wet-bulb temperature when the surface is wet to the dry-bulb temperature as the wood approaches the equilibrium moisture content. With permeable woods the surface temperature remains at the wet-bulb for a considerable proportion of the schedule. With impermeable woods the surface temperature soon begins to rise toward the dry-bulb temperature, as there is no mass flow of water from the interior to keep the surface moist and above fibre saturation. 

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. 

Textile fibers can be air-formed ditectiy into end use configuration by including a shaped condensing surface or, as in the production of pillows, an air-permeable collection package. Aerodynamic web formation is a suitable means for processing brittie fibers such as glasses and ceramics, and stiff fibers such as metaUics and wood. 

Transport. Wood is composed of a complex capillary network through which transport occurs by capillarity, pressure permeability, and diffusion. A detailed study of the effect of capillary stmcture on the three transport mechanisms is given in Stamm (13). 

Once applied, the eure rate will depend heavily on the moisture eontent and/or moisture vapor permeability of the substrates between whieh the adhesive is sandwiehed. For wood substrates, typieally full eure oecurs in a day with the faster systems. Misting of the substrates with water is sometimes conducted prior to adhesive applieation to further speed eure, or when the substrates are substantially moisture impermeable. 

Wood in chemical applications gives remarkably good service in the most severe conditions. Impermeable woods give the best results as any chemical degradation is confined to the surface, and the breakdown products are generally less harmful than metal corrosion products. Because of its permeability all sapwood must be removed. The low heat conductivity of wood is a property of considerable advantage for conserving heat and wood is commonly used for hot wells. 

Nail sickness Nail sickness is chemical decay associated with corroded metals in marine situations. Chemical degradation of wood by the products of metal corrosion is brought about by bad workmanship or maintenance, or unsuitable (permeable) timber species, all of which permit electrolyte and oxygen access which promotes corrosion. Chemical decay of wood by alkali occurs in cathodic areas (metal exposed oxygen present). Softening and embrittlement of wood occurs in anodic areas (metal embedded oxygen absent) caused by mineral acid from hydrolysis of soluble iron corrosion products. 

Trickling filter systems are classified under the aerobic attached growth systems (Figure 30.7). Crushed rock and stone, slag, wood, or synthetic media with higher permeability are used to fill the filter bed. The size of the media is in the range of 25-100 mm diameter. The depth of the filter... 

However, they do more than merely replace. They are lighter than metals, less brittle than glass, tougher than wood, less permeable than paper, more resistant than ceramics, and more decorative than cardboard. Plastics, and polystyrene as one of them, have secured their own firm place in the modern world and it is hard to imagine life without them. 

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