Summary
In order to characterize the stabilities of dimensions and shape of wood and the effect of an improvement of sorption behavior, up to now, only the maximum values of swelling in the tangential and radial directions were mostly determined. An analysis of the sorption and swelling of wood under conditions which exist during the use in practice shows that instead of the maximum values of swelling rather two other characteristics physically defined are required for a distinct determination of the sorption and swelling behavior of wood in the region of relative humidities between ψ=35 and 85%:
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(1)
the ratio of swelling q=Δα/Δu. It indicates the percentage change dα of lateral dimensions of the wood for each du=1% change of moisture content, and this in the region of the moisture contents of u 1≈7% up to u 2=20% which is important for practice, and
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(2)
the coefficient of swelling h=Δα/Δψ. It indicates the percentage change of lateral dimensions of the wood for each dψ=1% change of the relative humidity. paraBecause the swelling behavior is highly influenced by the hygroscopicity of the wood the two characteristics of swelling include a third one of the sorption behavior:
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(3)
the ratio of sorption s=h/q which can be used as a direct and numerical measure of the hygroscopicity of wood and of improved wood products, i.e. wood-plastic-combinations. paraFor the determination of the three characteristics q, h and s it is recommended to condition the wood specimens at two different relative humidities of about ϕ1≈35% and ϕ2≈85% until equilibrium state. It is advantageous to dry and condition the test pieces previously in a standard climate at a relative humidity of ϕc≈65%. For, by this way, the first shrinkage βc of the green wood, which is very important for the drying of wood, can be determined. paraAs a result of this study it is furthermore recommended that for a sufficient judgment of the sorption and swelling behavior of a wood species, the descriptions showing the properties of wood species ought to offer the three physical characteristics in the tangential as well as radial direction of the wood: q (ratio of swelling), h (coefficient of swelling), and βc (shrinkage from green to air-dry condition).
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Noack, D., Schwab, E. & Bartz, A. Characteristics for a judgment of the sorption and swelling behavior of wood. Wood Science and Technology 7, 218–236 (1973). https://doi.org/10.1007/BF00355552
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DOI: https://doi.org/10.1007/BF00355552