Reeb, James E.

Western Dry Kiln Association

Western Dry Kiln Association. Meeting (49th : 1998 : Reno, Nev.)

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WHY DOES MY WOOD SHRINK LIKE THIS?Jim ReebOregon State UniversityCorvallis, OregonWood shrinks approximately 8% along the tangential surface, 5% along theradial surface, and for "normal" wood, only about 0.1% along the longitudinalsurface. Therefore, the very best behaving wood will change shape, and do thisdifferently in different directions, while losing or gaining moisture.The story is: wood will shrink when placed in an environment of lowermoisture content and swell when placed in an environment of higher moisturecontent. We can see the results but we cannot see why this occurs because ithappens at the microscopic level.Three main components of wood are: Lignin, cellulose and hemicellulose.Lignin makes wood fibers stiff and strong and is the "cement" between individualfibers. It deters insect and fungal attack and functions as a water barrier.Chemically, it is a complex, 3-dimensional substance. Cellulose is the majorchemical component of wood fibers. It is composed of long chains (10,000 + ) ofglucose sugar molecules. Cellulose is a straight-chain polymer, that is, the glucosemolecules are strung end to end to make, at the molecular level, a large molecule.Hemicelluloses "coat" the cellulose chains and help the lignin bind to the cellulose.Hemicellulose are branched-chain polymers made up of sugars, one of which isglucose. Hemicellulose is made up of hundreds of sugar molecules rather than tensof thousands for cellulose.Basic Structure of the Cell WallMicrofibrils can be considered the building blocks of wood. Microfibrils area core of crystalline cellulose encased in a shell of short-chain hemicelluloses thatare partly linked to the cellulose core. Wood cells are made up of microfibrils. Theway microfibrils are laid down in the cell wall depends on where in the cell wall theyare located.The Primary Cell Wall and Compound Middle LamellaPlease refer to FIGURE 1 for the following section. The primary cell wall isvery thin. It is made up of microfibrils that are dispersed in a loose, irregularinterwoven pattern. The microfibrils are laid down at an angle of orientation ofabout 85 degrees with the cell axis. The angle of orientation becomes successivelysmaller from the inner to the outer parts of the primary cell wall as the maturing cellenlarges.Intercellular material, extending between the cell walls of adjacent woodcells, is known as the true middle lamella. It is made up of an isotropic, plastic-likesubstance that is mostly pectin. The plastic nature allows slippage between cellsas they are enlarging. Since the primary cell wall is very thin, it is hard todistinguish where the primary cell wall starts and the true middle lamella ends.WDKA	 8	 May 1998Camped middle lawilaFIGURE 1. Layering structure of a wood cell.Source: Dry Kiln Operator's ManualCompound middle lamella refers to the true middle lamella and the primary cell wallsof two adjacent wood cells, that is, it fills the space between two wood cells.The Secondary Cell WallThe secondary cell wall is much thicker than the primary cell wall. It isdivided into layers, S1, S2 and S3, and contains a high percentage of cellulose.Because of the density, volume and structure of the secondary cell wall, it isresponsible for the major source of strength for the woody cell. More importantlyfor this paper, the secondary cell wall plays a major roll in the shrink/swell behaviorof wood. This is especially true of the S2 layer because it is much thicker than theS1 and S3 layer.The S1 layer and the S3 layer of the secondary cell wall in softwoods isabout 4 to 6 lamellae thick. A lamellae is a cluster of microfibrils. The microfibrilsof the S1 layer are laid down at an average angle of 50 to 70 degrees measuredfrom the cell axis. The microfibrils of the S3 layer are laid down at an average angleof 60 to 90 degrees measured from the cell axis, about the same as the S1 layer.The S2, or central, layer of the secondary cell wall consists of a densenetwork of lamellae that are aligned nearly parallel to each other The S2 layer insoftwoods can be up to 40 lamellae thick in thinner walled springwood, and over150 lamellae thick in summerwood. The microfibrils of the S2 layer, for normalmature wood, are laid down at an average angle of 10 to 30 degrees from the cellaxis (close to being parallel to the axis). It is the microfibril angle of the S2 layerswithin individual wood cells that drives the shrink/swell behavior in a larger pieceof wood.WDKA	 9	 May 1998Shrinking and SwellingAs wood loses moisture below the fiber saturation point, it loses boundwater and it shrinks. This bound water is held in the cell wall structure and becausethe S2 layer is the thickest, it holds most of the bound water. Shrinking occurs asbound water escapes from between cellulose and hemicellulose molecules, betweenmicrofibrils. As water leaves, the microfibrils move closer together. Swelling is thereverse of this process.Wood shrinks across the grain, both tangentially and radially, but not muchalong the grain. Since the microfibrils are aligned almost parallel to the long axis ofthe cell, we can envision these microfibrils coming closer together as the waterleaves the cell but without much change lengthwise in their relationship with thecell axis. As an example, place two pencils parallel and even on a piece of paper.Mark the top and bottom with a mark. Now bring them together. They get closerbut there isn't any change as far as up and down is cancerned. But as microfibrilscome closer together, the cell gets narrower. Because the microfibrils are notexactly parallel to the cell axis there is some longitudinal shrinkage but it is usuallyonly 0.1% to 0.2%.Juvenile Wood and Compression WoodJuvenile wood is the first wood produced in a stem. It extends out from thecenter of a stem from 5 to 25 annual growth rings depending on species. We areinterested in the S2 layer of these juvenile wood cells. Juvenile wood has a muchhigher microfibril angle in the S2 layer than mature wood cells, those produced laterin the stem. This means less shrinkage across the grain as the microfibrils comecloser together but up to ten times more shrinkage in the longitudinal direction.Grab your two pencils. Place them parallel to each other but at an angle of about45 degrees to the long axis of the paper. Mark the top pencil at its top and markthe bottom pencil at its bottom. Now, move the pencils together and remark thetop and bottom. Its obvious that these have shrunk much more longitudinally thanthose at only a slight angle to the long axis.Let's use an example to see how this affects drying your lumber. A 20 ft.two-by-four, will shrink longitudinally about a quarter of an inch (mature woodshrinks longitudinally about 0.1% so .001 X 240 inches = 0.24 inches, actuallyonly about half of this since you will dry it to about 15% MC and not 0%). Yournext 20 ft. two-by-four came from near the center of the log and is about halfjuvenile wood and half mature wood. Half of your two-by-four wants to shrink 0.24inches along its length but the other half wants to shrink up to 2.4 inches along itslength. In this case the piece of lumber will either bow or crook depending onwhere the two zones of wood are located.Compression wood forms in softwoods. It forms on the underside of leaningtrees and the underside of branches. Fiber length is shorter than in normal maturewood and, as with juvenile wood, has a much higher microfibril angle in the S2 layerthan normal mature wood. Although behaving similarly to juvenile wood whenlosing moisture, compression wood has been reported to shrink up to seventy timesmore in the longitudinal direction than normal mature wood. If your 20 ft. two-by-WDKA	 10	 May 1998four contains half compression wood, that half may want to shrink as much as 16inches in the longitudinal direction. Severe bow or crook can be expected.We can definitely see the results of shrinking and swelling of wood eventhough we cannot see how this occurs since it is occurring at the molecular level.ReferencesDry Kiln Operator's Manual. 1991. Edited by W. T . Simpson. Ag. Handbook 188,USDA For. Serv., For. Prod. Lab., Madison, WI. 274 pp.Haygreen, J. G. and J. L. Bowyer. 1996. Forest Products and Wood Science: AnIntroduction. 3rd Ed., Iowa State University Press, Ames, Iowa. 484 pp.Panshin, A. J. and C. de Zeeuw. 1980. Textbook of Wood Technology. 4 th Ed.,McGraw-Hill, Inc., New York. 722 pp.WDKA	 11	 May 1998

Why does my wood shrink like this?

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Why does my wood shrink like this?

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