Shrinkage, wood volumetric

If it is hypothesized that the cell cavity remains constant in size as wood changes moisture content it can be shown (JO) that the volumetric shrinkage Shf of a wood of swollen volume specific gravity Gf can be predicted, based on a modification of Equation 8, as in Equation 10. 

Volumetric shrinkage data on other woods have also indicated that the ratio Shf/Gg tends to approximate the fiber-saturation moisture content Mf. For example, a mean ratio was found for Shf/Gg of 27 for 170 Australian woods (33). Data on tropical woods suggest somewhat lower values for this same ratio. The mean value for 140 Indian woods was approximately 20, considerably lower than the values for U.S. woods. This may indicate that tropical woods are less hygroscopic than temperate-zone woods, possibly because of their higher mean extractive contents. 

It is important to appreciate that water molecules do not penetrate a porous cell wall. Rather the dry cell wall expands as water is adsorbed within the non-crystalline regions of the wall and the accompanying volumetric swelling of the cell wall roughly corresponds to the volume of water adsorbed. Conversely when green wood is dried the volumetric shrinkage of the cell wall corresponds to the volume of adsorbed water that is removed. 

Wood only shrinks when water is lost from the cell walls and it shrinks by an amount that is proportional to the moisture lost below fibre saturation point. To a first approximation the volumetric shrinkage is proportional to the number of water molecules that are adsorbed within the cell wall, and that in turn is related to the number of accessible hydroxyls on the cellulose, hemicelluloses and lignin, and to the amount of cell wall material, i.e. the basic density of the wood (Figure 4.2). 

The axial, radial and tangential shrinkages, which together account for the volumetric shrinkage, are directed by features of wood structure that resist shrinkage, e.g. the quantity of ray tissue in the radial direction, or by features of the... 

If it had been assumed that the wood was extractive-free then the estimated oven-dry volumetric shrinkage would be ... 

This wood has 30 kg of extractives in a cubic metre of green wood and, assuming a density of 1400 kg m for these extractives (Tarkow and Krueger, 1961), they will occupy 0.021 m of the swollen cell wall per m of swollen wood. If the extractives were not present, the extractive-free basic density would be 500 kg m and the wood would shrink by 15%, i.e. the cell wall would shrink by 0.15 m per m of swollen wood. However the water soluble extractives occupy 0.021 m of the cell wall for every m of swollen wood, so the wood and the cell wall can only shrink by 0.150-0.021 m, i.e. 0.129 m per m of swollen wood rather than by 0.150 m per m which would be expected of an extractive-free wood having a basic density of 500 kg m. The oven-dry volumetric shrinkage will be only 12.9% rather than 15.0%. 

Figure 2 is a plot for volumetric shrinkage of waterlogged oak (i). It shows that at the same maximum moisture content level (indicating equal values of conventional density and thus no mass loss), the waterlogged wood nevertheless would have more than twice the shrinkage of recent wood. Although there may have been no net mass loss, some wood substance could... 

Figure 2. Volumetric shrinkage of archaeological oak wood as a function of maximum moisture content (Adapted from ref 1.)...

The wood-to-bath volume ratio was about 1 5. The dehydration phase in acetone necessitated four successive baths for 3.5 months. The acetone-resin exchange was then performed by four baths lasting 4 months. The irradiation dose absorbed by the items was 45 kGy, with dose rates ranging between 0.5 and 1 kGy/h. Although the treatment result was satisfactory for most of the artifacts, the very thin plates warped during polymerization, a phenomenon caused by volumetric shrinkage of the cured resin (Figures 10 and 11). 

Wood density is regarded as the easiest and most reliable measure of wood quality. Data from the drying of 106 North American hardwoods show that the volumetric shrinkage of wood increases with its basic density (Stamm, 1964). There... 

Wood density is regarded as the easiest and most reliable measure of wood quality. Data from the drying of 106 North American hardwoods show that the volumetric shrinkage of wood increases with its basic density (Stamm, 1964). There is also a correlation between the permeability and the basic density for the sapwood of a softwood (Nijdam and Keey, 2000). Presorting a sapwood load into high-density and low-density groups can reduce the moisture variability of the final recombined kiln-dried boards. 

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