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Wood and Water

FW1035 Lecture 8. Wood and Water. Bowyer – Chapter 8, pp.165-184. 1. Moisture content of wood 2. Green moisture content 3. Bound and free water – Fiber Saturation Point 4. Equilibrium moisture content 5. Outdoor vs. indoor RH 6. Dimensional change in wood with changing MC.

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Wood and Water

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  1. FW1035 Lecture 8 Wood and Water Bowyer – Chapter 8, pp.165-184 1. Moisture content of wood 2. Green moisture content 3. Bound and free water – Fiber Saturation Point 4. Equilibrium moisture content 5. Outdoor vs. indoor RH 6. Dimensional change in wood with changing MC

  2. Wood moisture content is calculated based on the DRY wood weight Calculating the moisture content of a wood sample: %MC = (wet weight of wood) – (oven-dry weight of wood) X 100 (oven-dry weight of wood) A small sample of wood is considered oven dry after 24 hours at 103°C The moisture content of wood can be >100%!! If you have equal weights of dry wood and water in a wet wood sample, the MC will be 100%.

  3. The moisture content of green wood(freshly cut and unseasoned) Average MC’s for hardwoods and softwoods: • Hardwoods typically have a %MC between 60 and 100% • Softwoods %MC depends on heartwood or sapwood • 30-100% in heartwood • 110-220% in sapwood • Some species may show seasonal dependence (birch and ash species increase sapwood MC ~30% at leaf emergence)

  4. Average Green Moisture Content of Some Common Wood Species

  5. Where does water get absorbed in the Cell Wall? Microfibril arrangement figures (2) Water sorption sites are between microfibrils.

  6. Fiber Saturation Point Oven Dry OH OH HO HO H2O OH OH HO H2O HO OH H2O H2O HO OH OH   H2O HO HO H2O OH OH HO HO H2O OH H2O OH H2O HO HO H2O OH H2O HO OH H2O H2O OH HO H2O H2O HO OH OH HO H2O H2O H2O HO OH HO H2O OH H2O H2O H2O HO OH Swelling is caused by water getting into the sorption sites within cell wall

  7. States of Water in Wood Free Water- Liquid phase water within lumens and cell wall cavities. Bound Water- Water within cell walls H-bonded to cellulose and hemicellulose. Cell Wall Lumen Free Water Bound Water

  8. Fiber Saturation Point (FSP) Moisture content where bound water content is maximized, but there is no free water present. FSP = ~30%, on average

  9. FSP Can Vary Widely With Species • Extractives play a large role in determining the fiber saturation point. • Hydrogen bonding sites on lignin, hemicellulose and cellulose may already be ‘occupied’ • Does this chart reflect heartwood or sapwood?

  10. Equilibrium Moisture Content (EMC) The moisture content eventually attained by a piece of wood upon extended exposure to air at a constant given relative humidity and temperature. H2O H2O

  11. Moisture content of wood-in-use • Prior to use, commercial wood products are typically dried to 7-19% MC • Wood is hygroscopic - it absorbs moisture vapor from the surrounding air • Moisture content of wood in use changes with humidity of surrounding air

  12. Wood MC Relative Humidity Hygroscopicity of Wood The moisture content of wood is roughly proportional to the Relative Humidity of the surrounding air. MC@0%RH = 0% MC@100%RH = FSP

  13. Relative Humidity • A measure of the moisture content in air relative to the amount of moisture the air can hold at a given temperature. • Relative humidity (RH) depends on temperature - cold air can hold less moisture than warm air • For a sample of moist air: • Warm it up – RH goes down • Cool it down – RH goes up

  14. Indoor RH Change in a Temperate Climate Indoor RH  Exterior RH Indoor air RH affected by: 1. Heating (lowers RH) 2. Air Conditioning (lowers RH) 3. Humidification (increases RH)

  15. Average January Temperature and Interior Wood EMC Insert RH/MC maps Source: R.B. Hoadley, Understanding Wood

  16. Average July Minimum RH and Interior Wood EMC Source: R.B. Hoadley, Understanding Wood

  17. Average Indoor Wood Moisture Content A simpler view Source: The Hardwood Council, www.hardwoodcouncil.com

  18. Indoor Wood MC Cycles During the Year, Northern U.S. S W S W S W S Seasonal Change Coatings affect the rate of MC change and MC extremes during the year

  19. Rate of wood MC change depends on: 1. Air circulation rate around wood 2. Coatings 3. Size of wood member 4. Difference between wood MC and potential EMC

  20. The top of this wood slice got wet, swelled, and distorted the overall shape. Dry Side Wet Side

  21. Dimensional Change with Changing Moisture Content 1. Wood shrinks and swells with decreasing or increasing MC. 2. BUT, only for ΔMC below the Fiber Saturation Point (~30%) 3. Dimensional change is asymmetric STangential > SRadial >> SLongitudinal SRadial≈ 0.5 to 0.66 STangential S = Maximum possible shrinkage in a given direction

  22. Wood Cell Wall Structure Revisited • Model based on softwood longitudinal tracheid cell wall • Distinct layers • Chemical composition and microfibril orientation differ with layer • Cellulose microfibrils and lignin bonded by hemicellulose • Primary cell wall composed primarily of pectin (complex carbohydrate related to hemicellulose) • Notice S-Z-S orientation of the microfibrils in the S1-S2-S3 layers S1 increases burst strength S2 provides compression strength S3 increases resistance to collapse

  23. The S2 Layer Dominates Cell Wall MC-Based Dimensional Change • S2 layer is by far the thickest in cell wall • The S2 microfibril angle is close to the wood/cell longitudinal direction • Water gets between the microfibrils and pushes them apart • Transverse swelling of the S2 layer is constrained by the S1 and S3 layers

  24. Swelling is caused by water expanding the sorption sites in the wood cell wall Fiber Saturation Point Oven Dry OH OH HO HO H2O OH OH HO H2O HO OH H2O H2O HO OH OH   H2O HO HO H2O OH OH HO HO H2O OH H2O OH H2O HO HO H2O OH H2O HO OH H2O H2O OH HO H2O H2O HO OH OH HO H2O H2O H2O HO OH HO H2O OH H2O H2O H2O HO OH

  25. Average Total Shrinkage (Green to Oven Dry) = S SR ST SL SVol SR ST SVol SL is small. Usually assumed to be 0.1% for “normal” wood. Normal ≠ juvenile or reaction wood

  26. The magnitude of SVol depends on density, extractive content and ST/SR ratio 1. Density Higher wood density gives larger S values because more wood cell wall is present – more sorption sites to shrink or expand. SG SVol White Ash 0.6 13.3 Pignut Hickory 0.75 17.9 E. White Pine 0.35 8.2 Loblolly Pine 0.51 12.3

  27. 2. Extractive Content Higher extractive content gives lower S values because the wood will have a lower FSP meaning less water will be lost or absorbed. SG SVol Black Ash 0.49 15.2 Sassafras 0.45 10.3 Balsam Fir 0.36 11.2 Western Red Cedar 0.33 6.8

  28. 3. ST/SR ratio • A large difference between ST and SR is often caused by rays restraining shrinking in the radial direction. • SG ST/SR SVol • Basswood 0.37 1.4 15.8 • Aspen 0.38 1.9 11.5 • High density wood with low extractive content tendsto shrink and swell more. The trend is complicated by the nature of the extractives present (some are more efficient at occupying H-bonding sites) and ray restraint.

  29. Longitudinal Shrinkage (SL) SL is normally small (0.1%), and usually ignored. 8 foot 2x4 lumber changing from FSP  0% MC (0.001) (96 inches) = 0.1 inch 8 foot 2x4 lumber changing from 19% MC (KD)  10% (interior EMC) - approximately 1/3 of total possible dimension change (1/3) (0.1 inch) = 0.03 inch However, reaction and juvenile wood may have SL may be up to 3% if SL = 3%, 8 foot 2x4 lumber, FSP  0% MC, Δl = 2.9 inches 8 foot 2x4 lumber, 19%  10% MC, Δl = 1.0 inch

  30. Mechanisms for Unequal Tangential and Radial Shrinkage/Swelling 1. Ray restraint of shrinking/swelling in radial direction. - Rays are more dimensionally stable in their length than surrounding wood - act as “reinforcing rods” to restrain movement in the radial direction. SG Ray Volume ST/SR SVol Basswood 0.37 5.3% 1.4 15.8 Aspen 0.38 9.6 1.9 11.5

  31. Furniture Design

  32. Door Design

  33. 2. Earlywood - Latewood Interaction Dense latewood shrinks and swells more, and is also stronger than earlywood. In tangential direction, latewood movement dominates, stretching the weaker earlywood. In radial direction, additive effect of earlywood and latewood movement. Therefore, tangential shrinking/swelling is large because it is controlled by the latewood, while radial shrinking/swelling is relatively small because it is intermediate between large latewood and small earlywood effects. SG Ray Volume ST/SR SVol Red elm 0.53 13.0 1.8 13.8 White Birch 0.55 ~15 1.4 16.2

  34. Higher ST/SR ratios tend to be found in: Species with higher ray volumes (Ray Restraint) Ring porous hardwoods (Earlywood – Latewood Interaction) Softwoods with a large proportion of latewood in annual ring (Earlywood – Latewood Interaction)

  35. Shape Distortion upon Drying Caused by Asymmetric ST & SR Shrinkage

  36. Amount of dimensional change with changing moisture content (Dimensional Stability) will depend on: 1. Wood species 2. Inherent sample variability - density - anatomy - sapwood or heartwood - extractive content of heartwood 3. Magnitude of ΔMC below FSP 4. Anatomical direction 5. Mechanical constraint 6. Presence of reaction or juvenile wood

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