WATER IN WOOD Free Water is in cell lumens, and intercellular spaces. Energy level like water sitting in a glass, “easily” lost from wood. Bound Water is in the cell walls. H-bonded to the cell wall components Lumen Cell Wall Free Water Bound Water SEM Photo by T. Zimmermann.
BOUND WATER Affects size of the wood Affects strength of the wood [spreading of microfibrils in the cell wall, esp. S2 because it’s the largest layer] “Harder” to remove from wood than free water as H-bonded.
Bound Water in Wood Wood (cell wall) is hygroscopic Changes in relative humidity affect the amount water in wood. Increase RH, increase amount of bound water, increase the size of the wood Decrease RH, decrease amount of bound water, decrease the size of the wood. Moisture Content Percent (% M.C.) is used to quantify the amount of water in wood.
% Moisture Content % M.C. = Ratio of the weight of water in the wood to the oven-dry weight of the wood Wt. of HOH in Wood X 100% % M.C. = Oven-dry Wt. Of Wood Oven-dry wood = water-free wood, drying wood in oven set at 105o C (220o F) until wood reaches constant weight.
% Moisture Content X 100% % M.C. = Original Wt. --- Oven-dry Wt. Oven-dry Wt. Of Wood In wood industry use oven-dry wt as the reference point. Pulp and paper industry computes % M.C. on “green weight” basis, ratio of weight of water to total weight (wood + paper).
WATER IN WOOD All wood in normal applications will have water in it Amount of water is proportional to the relative humidity At a given relative humidity, wood reaches a equilibrium moisture content (EMC).
WATER IN WOOD RH EMC 25% 5% 50% 9% 75% 14% 25% 5% 50% 9% 75% 14% Precondition wood to % M.C. it’ll be at when in use Interior use, kiln-dry to < 10%, 7--8 % Structural lumber 15-19 %
Fiber Saturation Point When cell walls saturated with water and no water in lumen Around 30% M.C. With changes in %M.C. below fsp get changes in strength and size of wood.
Water in Tree Above fiber saturation point Varies from sapwood to heartwood Sapwood Heartwood Birch 70 % 75% Beech 70 % 55% Loblolly Pine 110 % 35% In softwoods sapwood moisture content usually higher than heartwood M.C., hardwoods not consistent. Remember that these single numbers ?, expect variability