INFLUENCE OF SAWING ORIENTATION ON MOISTURE MOVEMENT THROUGH SOFTWOOD BOARDS

The microstructure of sapwood in a softwood, such as radiata pine, is relatively simple. The bulk of the structure is made up of fibrous elements called tracheids that are aligned axially in the tree. Other cells known as parenchyma cells and ray tracheids, along with open structures called resin canals, make up the remainder of the sapwood tissue. These are the cells that form the xylem in the tree and are found in sawn sapwood boards.

In temperate climates, the dimensions of the axial tracheids change depending on which part of the growing season they form in. Tracheids formed early in the growing season have large diameters and thin cell walls and are relatively short in length. These make up the earlywood that has a creamy white appearance. Tracheids formed later in the season have smaller diameters and thicker cell walls, but are longer. These make up the latewood bands that are a light brown colour. The contrast between the earlywood and latewood gives the appearance of the annual growth rings. Each growth ring represents the seasonal accumulation of woody material.

The lumens of the axial tracheids in both the earlywood and latewood are connected by structures called bordered pits. These bordered pits are situated mainly on the radial faces of the tracheids and facilitate longitudinal and tangential movement of moisture in the xylem. A few of these pits connect to ray tracheids in the ray tissue, which allows radial moisture movement to occur. These pits are however smaller and less permeable than the pits between axial tracheids.

The size and permeability of the pits between axial tracheids also differ between earlywood and latewood. Earlywood pits are larger, more numerous and more permeable than latewood pits. The torus and supporting strands of bordered pits in earlywood are more flexible than in latewood, which means that they are more likely to aspirate. Many latewood pits remain unaspirated at all times.

The permeability of softwood to moisture movement cannot easily be characterised as it varies with density, direction of flow, and moisture content. Some of the changes are reversible and others are not. The variation with density is based on the anatomical differences between the less dense earlywood and the denser latewood. The larger-diameter lumens and more frequent, open-structured bordered pits of the earlywood mean that the green permeability to liquid water is higher compared with the latewood. The green permeability is greatest in the axial direction, about 2000-times more permeable than that in the tangential and radial directions, which are similar. The radial permeability of latewood is 10 times smaller than that of earlywood.

When wood is dried, there are processes that occur that change the permeability. Firstly, the bordered pits in the earlywood aspirate, as do some of the pits in the latewood. This reduces the permeability, to gas and liquid, in both types of wood, but the effect is more significant in the earlywood because a greater proportion of the pits aspirate. The radial permeability is also modified when the ray tissue dries below fibre-saturation point and the ray parenchyma collapse, leaving a more open structure. This increases the permeability of the wood in the radial direction.

The driving force for strain development in boards during drying is the moisture-induced shrinkage that occurs in the wood as it dries below FSP and begins losing bound moisture from the cell walls. In the absence of any restraint this would produce a free shrinkage strain. In a drying board, however, the shrinkage is seldom unrestrained as there are differences in moisture content throughout the board and therefore different levels of moisture-induced shrinkage. Variations in density will also result in variations in moisture-induced shrinkage.

Whenever adjacent zones of wood in a drying board experience different levels of moisture-induced strain then they are restrained by each other and a drying stress develops in response. The development of stress leads to stress-related responses in the form of instantaneous strain, mechano-sorptive strain and creep strain. These stress-induced strains act to offset, or relieve, the moisture-induced shrinkage strains. Instantaneous strain is a strain produced immediately on loading of a stress and can be elastic or plastic depending on the magnitude of the stress and the temperature of the wood. Mechano-sorptive strain develops due to the interaction between moisture loss and stress during drying. This is a difficult strain to analyse and is not considered in this study. Creep strain is a delayed time dependant strain that results in permanent deformation of the wood to relieve stress. At high temperatures, the rate of creep strain is accelerated due to thermal softening so it can counteract other strain development during drying, particularly under high temperature conditions.

When boards of rectangular cross-section are cut from a log the orientation of the annual growth rings to the long face (the drying face) of the board varies. Where the growth rings are roughly parallel to the long face then the boards are called flat-sawn and where they are perpendicular the boards are called quarter-sawn. In most boards the growth ring orientation is somewhere between the two extremes. The sawing patterns used in softwood processing, in New Zealand, usually result in more flat-sawn than quarter-sawn boards.

Sawing orientation influences the rate of drying because of inherent differences in permeability. Pang & Haslett (2002) have shown that quarter-sawn boards dry more slowly than flat-sawn.

In a drying kiln, the boards are stacked such that the short faces are butted together and the long faces are exposed to the hot air. This means that most of the moisture evaporates from the long faces so that drying is essentially one-dimensional. For quarter-sawn boards this means drying is in the tangential direction of the wood, whereas for flat-sawn boards drying is in the radial direction of the wood.

The objective of these experiments was to measure moisture and strain profiles in both flat-sawn and quarter-sawn boards at different stages in drying at high temperatures. These profiles illustrate the different mechanisms of drying for the two extremes of sawing orientation.

The wood used for these experiments were rough-sawn, green sapwood boards, 50mm x 100mm, obtained from a local sawmill in Christchurch from Pinus radiata timber originating from North Canterbury. The boards were taken immediately from the grading table, wrapped in plastic and stored in a refrigerated room at approximately 4oC until required.

A week before the tests, the boards were cut into 50×100×800mm test boards, which were painted on the ends and sides to prevent moisture loss from these surfaces during drying. The test boards were then re-wrapped in plastic and refrigerated until the day of the experiment. Twenty flat-sawn and twenty quarter-sawn test boards were prepared in this manner. These boards were dried in the same tunnel as the moisture profile samples under the same conditions.

RESULTS

Below fibre saturation, the value of S has no physical meaning, and was not calculated. To calculate S, the moisture content at fibre saturation was taken from the temperature-dependent equation given by Chen et al. (1997), with the wood temperature assumed to have reached the quasi-steady-state temperature at the boiling point by the time the first sample board had been withdrawn after 100 minutes. This assumption yielded a value of 0.22 for X_FSP.

The initial green moisture content of each board (X_bg) can be determined from the green weight of the board (W_bg) and the oven-dry weight (W_bo).

However, the oven-dry weight of the board is unknown, but can be determined from the dried weight of the board (W_bd) and that of the sample (W_sd), knowing the oven-dry weight of the sample (W_so) from equation (5):

It follows therefore that:

The unsliced long dimension of each partially dried slice was determined from the measurements made on the sample at 5mm intervals prior to slicing. The long dimension measurements were interpolated, between the 5mm intervals, to get a measurement for each individual slice (l_i) from the thickness measurements for each slice. A measure of the instantaneous strain was then calculated using equation (7).

where l_s is the long dimension of the slice after cutting and is the average of the l_i values for the 20 slices.

The residual shrinkage was also calculated for each slice using the following equation:

where l_od is the oven-dried length of the slice.

Fig 3 shows the moisture saturation profiles for flat-sawn timber and Fig 4 those for quarter-sawn timber. Each curve represents a separate board with a unique density profile. In Fig 3, the positions of peaks in the density profiles, representing latewood bands, are indicated by the enlarged symbols. This feature is not included in profiles for quarter-sawn boards as the density profile is effectively uniform, and thus moisture content is also plotted directly rather than moisture saturation.

The strain profiles determined in the second set of experiments are shown in Figure 5 for flat-sawn boards and Figure 6 for quarter-sawn boards. On most of the plots moisture content and percentage saturation are shown. However, where only moisture content is plotted, it means the board was dried below fibre saturation point and percentage saturation is no longer relevant.

There is a clear difference in the shape of the moisture profiles for the two sawing orientations illustrated in Figs 3 and 4, respectively. Flat-sawn boards exhibit minima in moisture saturation at the latewood bands (Fig 3), whereas quarter-sawn boards show a relatively uniform moisture content profile in the wet core of the board (Fig 4). There are also differences in the moisture profiles close to the surface. With flat-sawn wood, the moisture level at the surface falls below fibre saturation point, early in drying, while there is a small change in the moisture saturation of the core. On the other hand, with quarter-sawn timber, the entire core moisture content falls uniformly with only a small gradient at the surface.

This difference in behaviour is likely to be a reflection of the orientation of the growth rings with respect to the drying surface. In flat-sawn wood the growth rings are in series with respect to moisture movement. The moisture movement seems to be impeded by the less permeable latewood bands in the core, resulting in higher saturation at the board centre and lower saturation in subsequent growth rings nearer the surface, with a sharp drop in saturation over the outermost growth ring. Moisture evaporation probably occurs over this growth ring for much of the drying process, with sap moving as liquid from the core to it, until the bordered pits have sufficiently aspirated to render the remaining liquid immobile.

In quarter-sawn wood, the growth rings run parallel to the direction of the moisture movement. The measured moisture content at any position represents some kind of average over a number of rings. Since the mean permeability is essentially uniform normal to the surface, the moisture gradient is smoother than in flat-sawn wood, with a less pronounced dry surface zone.

For flat-sawn wood it appears that the evaporative zone withdraws into the wood while there is still free moisture in the core. The position of the evaporative zone is also quite distinct and is associated with the position of the latewood bands. With quarter-sawn wood the evaporative zone stays near the surface, though it is not as easy to place due to the gentler slope of the moisture profile. This slope might be caused by existence of non-uniform moisture profiles orthogonal to the direction of moisture movement. Such non-uniform profiles may result from drying variation caused by differences in wood density.

The growth rings have a threefold effect on the development of drying strains. Firstly, the rings influence the moisture-loss rate and the moisture profile, which determine the development of mechanosorptive and shrinkage-related strains, respectively. Secondly, the earlywood and latewood zones shrink differently because of differences in cell wall properties. Thirdly, the structure of successive wood zones, with the latewood bands acting as stiffening rings, influences the transmission of stresses through the board. It is thus expected that the strain behaviour will depend on the grain orientation with respect to the direction of moisture movement.

One difference between the strain profiles for the two grain orientations is that the magnitude of strains measured in quarter-sawn boards is roughly half that of flat-sawn boards. This is because for the quarter-sawn boards the strain is measured in the radial direction of the wood and that for flat-sawn in the tangential direction. For radiata pine the radial shrinkage is roughly half of the tangential shrinkage.

For flat-sawn boards, the strain profiles for 94% average moisture content (Fig 5a) shows that most of the wood within 5mm of the surface is in tension, whereas the layer of wood right at the surface and in the core is in compression. The compression in the core is expected as a reaction to the wood nearer the surface shrinking as it dries below fibre saturation point. The surface compression seen here has not been reported before. We believe that early in the drying process the wood right at the surface dries below fibre saturation point and thus shrinks, while the surface zone (the outermost growth ring) is still mostly above fibre saturation point and therefore does not shrink. This difference in shrinkage causes significant tensile stress in the surface layer. This tensile stress can be higher than the elastic limit and causes some non-elastic deformation. With further drying, the wood in the surface zone dries below the fibre saturation point and attempts to shrink; therefore, the compressive stress in this zone is reversed. However, due to the non-elastic elongation, the deformation in the thin surface layer cannot be recovered and a compressive stress develops. Such stress changes are consistent with model predictions (Pang, 2001). Overall the strain profiles follow the anticipated pattern, with a strain reversal during drying, so that towards the end of the drying process the core is in tension, balanced by the surface zones in compression.

The instantaneous strain profile illustrates how drying stresses can be transmitted through the board. Fig 5a shows a step change in the instantaneous strain at the second latewood band from the surface, while there is very little variation in strain over the earlywood bands. This suggests that an entire growth ring acts as a single unit when reacting to strain development.

The strain profiles for the quarter-sawn boards are similar to those for flat-sawn boards in the early part of drying (Fig 6a), though the profile of compression in the core is different. Later in the drying (Fig 6b), the strain profiles change, with increasing tension near the surface and a decrease in compression in the core. There are two aspects of these profiles that are unusual. Firstly, there seems to be shrinkage occurring above fibre saturation point, as indicated by a tensile strain in wood with a positive saturation. Secondly, the level of compressive strain increases further into the core. The first of these observations can be explained by the existence of non-uniform moisture profile parallel to the direction of strain. This means that for each slice taken for strain measurement contains wood above and below fibre saturation point, so while the overall saturation is above zero some parts of the slice are shrinking. The second observation can be explained by the stiffening effect of the latewood bands constraining the board to shrink uniformly. The higher compressive strain in the core is likely to be due to other mitigating effects, such as shrinkage strain and mechanosorptive strain, being lower than they are closer to the surface, due to the higher saturation.

The strain resulting from an uneven moisture profile in parallel to the drying surface could also be involved in the formation of collapse as the wood shrinks more in the tangential direction, the direction of collapse, than in the radial direction. Collapse in the form of surface “washboarding” is a feature of drying quarter-sawn timber (Deyev and Keey, 2001).

The quarter-sawn strain profile towards the end of drying (Fig 6c) is unusual, with the wood entirely in tension. The other half of the sample (25-50mm) that was not sliced may have affected this profile, if it did not dry as quickly and therefore has not shrunk as much.

The difference in physical structure and permeability between earlywood and latewood tracheids causes the drying behaviour of sapwood boards of softwood timber to depend on the way the boards are cut from the log. In flat-sawn timber, the latewood bands are in series with the direction of moisture movement: in quarter-sawn timber, the latewood bands run parallel to the movement of moisture. Thus the latewood bands in flat-sawn timber tend to form barriers to moisture movement, resulting in stepwise changes in the moisture profile and a sharp drop over the outermost growth ring. By contrast, the moisture profile is more uniform when quarter-sawn boards are dried, with a less marked moisture gradient near the surface. These differences in moisture profile also influence the strain development. In our tests, the strain behaviour for flat-sawn boards conforms to the expected behaviour. On the other hand the strain behaviour of the quarter-sawn boards is unusual, with highest compression in the core and apparent shrinkage above fibre saturation point.

Chen, G.; Keey, R.B and Walker, J.C.F. 1997. The drying stress and check development on high-temperature kiln-seasoning of sapwood Pinus radiata boards. Part I Moisture movement and strain model. Holz als Roh- Werkst., vol.55, 351-360.

Deyev, A and Keey, R.B. 2001. Observation of the “washboard effect” on the surface of quarter-sawn Pinus radiata boards under kiln-drying conditions. New Zealand Journal of Forestry, vol.46(2), 26-33.

Keey, R.B, Langrish, T.A.G and Walker, J.C.F. 2000. Kiln drying of lumber. Springer Series in Wood Science, Springer Verlag, New York.

McCurdy, M.C and Keey, R.B. 1999. Moisture saturation profiles in Pinus radiata during high temperature drying. IPENZ Transactions: Civil Section, V26/N1CIV, 29-33.

Pang, S. 2001. Modeling of stresses and deformation of radiata pine lumber during drying. Proceedings of 7^th International IUFRO Wood Drying Conference, Tsukuba, Japan: 238-245.

Pang, S and Haslett, A.N. 2002. Effects of sawing pattern on drying rate and residual drying stresses of Pinus radiata lumber. Paper submitted to Maderas: Ciencia Y Tecnologia.