36 Drying of Wood

Pure waterWater in woodVaporL vΔh sL v−L fFiber saturation pointIceLiquidBound waterMoisture content≈ −L v /2FIGURE 36.10 Differential heat of sorption versus the moisture content. (Adapted from Skaar, C., Wood–Water Relations,Springer, Berlin, 1988.)Dh s ¼d ln P vR P vsM vd1 (36:4)TDimensionLThis heat of sorption is slightly higher than 1000 kJkg 1 for oven-dry wood and decreases rapidly withmoisture content to reach zero, with a horizontalslope at the FSP (Figure 36.10).RT36.2.1 .5 Sh rinkageIn trees, the cell walls of wood are in a fully swollencondition. However, when sorbed water is removedfrom the cell wall, new hydrogen bonds form betweenthe hydroxyl groups of the molecular chains andreduce the distance between the chains. This variationaffects the dimension in a direction normal to themicrofibril direction. This fact explains why the longitudinalshrinkage is usually negligible. In the transversedirections, the dimensional variations plottedagainst bound water content are very close to astraight line (Figure 36.11). Shrinkage can then bedefined by the total shrinkage (variation of dimensionbetween the green state and the oven-dry condition)or in terms of a shrinkage coefficient (variation ofdimension divided by the variation in moisture content).For most species, the tangential shrinkage isabout twice the radial shrinkage. Several featurescan explain this observation: the cell arrangement oftissues, the difference between earlywood and latewood,and the presence of ray cells. The extrapolationof length values against moisture content to the originalunshrunken length yields the so-called shrinkageintersection point (SIP) (Figure 36.11). Normally, thismoisture content is a little higher than the FSP, butoften the two moisture content values are assumed tobe same.FSP SIPMoisture contentFIGURE 36.11 Dimensional variations of a typical woodsample vs. moisture content.The shrinkage values tend to increase with basicdensity, but values vary strongly between and withinspecies. Table 36.6 indicates the order of magnitudevalues encountered in some species.In the case of compression wood, the microfibrilangle in the S 2 layer is large, which results in a lowertransverse shrinkage and a significant longitudinalshrinkage. In spite of the very low value of themicrofibril angle in the G layer, tension wood ofhardwood has also an abnormally high longitudinalshrinkage.As logs are sawn in green state, shrinkage occursafter sawing. The size of each section is reduced byshrinkage and also the shape of the section changescaused by the tangential or radial anisotropy inshrinkage (Figure 36.12):a. Only the quartersawn sections without the pithkeep their rectangular shapeb. Flatsawn sections cup into a trough-like shapeß 2006 by Taylor & Francis Group, LLC.