Wood hygroscopicity

Adsorption. Wood is highly hygroscopic. The amount of moisture adsorbed depends mainly on the relative humidity and temperature (Fig. 1). Exceptions occur with species with high extractive contents (eg, redwood, cedar, and teak). The equihbtium moisture contents of such woods ate generally... 

In green wood, the cell walls are saturated, whereas some cell cavities are completely filled and others may be completely empty. Moisture ia the cell walls is called bound, hygroscopic, or adsorbed water. Moisture ia the cell cavities is called free or capillary water. The distiaction is made because, under ordinary conditions, the removal of the free water has Htde or no effect on many wood properties. On the other hand, the removal of the cell wall water has a pronounced effect. 

The strength of a bond increases with the wood density in the region of approx. 0.7 to 0.8 g/cm Above this density, a decrease of the bond strength occurs. Performance and properties of wood-based panels are strongly influenced by the properties of the used wood. The anisotropy as well as the heterogeneity, the variability of various properties and the hygroscopicity have to be taken into account. Also the orientation of the wood fibers bonding solid wood has to be considered. 

Zinc in contact with wood Zinc is not generally affected by contact with seasoned wood, but oak and, more particularly, western red cedar can prove corrosive, and waters from these timbers should not drain onto zinc surfaces. Exudations from knots in unseasoned soft woods can also affect zinc while the timber is drying out. Care should be exercised when using zinc or galvanised steel in contact with preservative or fire-retardant-treated timber. Solvent-based preservatives are normally not corrosive to zinc but water-based preservatives, such as salt formulated copper-chrome-arsenic (CCA), can accelerate the rate of corrosion of zinc under moist conditions. Such preservatives are formulated from copper sulphate and sodium dichromate and when the copper chromium and arsenic are absorbed into the timber sodium sulphate remains free and under moist conditions provides an electrolyte for corrosion of the zinc. Flame retardants are frequently based on halogens which are hygroscopic and can be aggressive to zinc (see also Section 18.10). 

Wood is a hygroscopic material, due to the fact that the cell wall polymers contain hydroxyl groups. In an environment containing moisture, dry wood will absorb moisture until it is in equilibrium with the surrounding atmosphere. Similarly, saturated wood, when placed in an atmosphere of lower relative humidity (RH), will lose moisture until equilibrium is attained. If the wood is placed in an environment where the RH is stable, it will attain a constant moisture content (MC), known as the equilibrium moisture content (EMC). At this point, the flux of water molecules into the cell wall is exactly balanced by the outward flux into the atmosphere. 

Martins and Banks (1991) modified pine and wood samples with phenyl or butyl isocyanate and investigated the water vapour sorption properties of the modified wood. Modification resulted in a reduction in EMC at a given RH up to about 35 % WPG with butyl isocyanate, although reaction with phenyl isocyanate above 25 % WPG resulted in an increase in sorption due to cell wall damage caused by the reaction. The butyl derivative was more effective at reducing hygroscopicity over all the WPGs studied. 

The surface wettability of heat-treated wood decreases due to a reduction in the hydroxyl content of the modified wood (Pdtrissans etal., 2003). There is a reduction in the water-sorption capacity, which is related to a reduction in the number of primary sorption sites (OH groups) within the wood cell wall, largely as a result of the removal/degradation of the hemicellulosic component. As remarked upon earlier, hygroscopic properties are strongly influenced by the treatment method employed. Podgorski etal. (2000) heated... 

The material occupies space within the ceU wall that would otherwise be available to water molecules, which leads to a reduction in the hygroscopicity of the wood. 

Keith, C.T. and Chang, C.I. (1978). Properties of heat-darkened wood. I. Hygroscopic properties. Report, Eastern Forest Products Laboratory, Canada, No. OPX213E. 

Kumar, S., Dev, I. and Singh, S.P. (1991). Hygroscopicity and dimensional stabihty of wood acetylated with thioacetic acid and acetyl chloride. Journal of the Timber Development Association of India, 37(1), 25-32. 

Yano, H. and Minato, K. (1992). Improvement of the acoustic and hygroscopic properties of wood by a chemical treatment and application to the violin parts. Journal of the Acoustical Society of America, 92(3), 1222-1227. 

Yasuda, R., Minato, K. and Yano, H. (1993). Use of trioxane for improvement of hygroscopic and acoustic properties of wood for musical instruments. Wood Science and Technology, 27(2), 151-160. 

Air-dried wood, with twenty par cent, of hygroscopic moisture, Kiln-dried ten ... 

Kiln-dried wood, without hygroscopic moisture,. 

H 0. white, hygroscopic, crystalline powder, decomposes to nietaphns-phate on healing, soluble in water and acids, insoluble in alcohol. Uses Hireprooting wood, and as a stabilizer for plastics. 

Wood that has not been treated will absorb moisture from the surrounding air until its moisture content reaches an equilibrium condition. The hygroscopicity of wood treated with inorganic fire-retardant chemicals is usually greater than that of the untreated wood and is dependent on size and species of wood, temperature, relative humidity, and type and amount of chemicals used (8,60). 

Zinc chloride will add considerably to the equilibrium moisture content of wood in the range of 30 to 80 percent relative humidity (8). Ammonium sulfate will add at relative humidities exceeding 65 percent, and borax and boric acid will attract water at lower humidities. Phosphate salts affect hygroscopicity mostly when relative humidity exceeds 80 percent (58). 

Continuing research must yield information on the treatment-related properties of fire-retardant-treated wood and methods for their improvement. The properties of the conventional salt treatments which need improvement especially are hygroscopicity, strength properties, gluing, and finishing. 

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