KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Content Wood-water relationships The technical aspects of the relationship between wood and water www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics This lecture • Introduction to wood-water relationships • Water in wood • Drying and dimensional change • Hygroscopicity Next lecture • Measuring moisture content • Drying and utility • Moisture and production • Moisture and service performance Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood in the tree or in service contains water • Wood in the tree or in service contains water Introduction to wood-water relationships – Its moisture content is the weight of water in the piece as a percentage of dry wood fiber Free water in the cell Bound water in the cell wall Water, water in wood, dimensional change and hygroscopicity www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Air dry timber Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood, dimensional change & hygroscopicity • Wood changes in size as it loses or gaining moisture • Wood is hygroscopic: it loses or gains moisture with fluctuations in environmental humidity Bound water in the cell wall Air dry timber Wood-water relationships www.csaw.utas.edu.au Water in wood Wood in the tree or in service contains water www.csaw.utas.edu.au 1! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Water Water content of air • Water (H2O) is a common molecule – Held together by electron sharing, it has an unbalanced electrostatic charge – The angle of the molecular bonds is 1050 • In nature, it existing in gaseous, liquid and solid forms + - xnet.rrc.mb.ca/rcharney/the%20water%20molecule.htm Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics • Humidity is a general term for the presence of water vapour in air – Absolute humidity - The amount of moisture in air It is usually expressed as the weight of water vapour in a unit weight of dry air – For any given temperature and pressure, there is a specific limit to the amount of water vapour that air can hold At a given temperature, relative humidity is the amount of moisture in air as a percentage of the maximum moisture carrying capacity of the air – These and similar relationships can be displayed on psychometric charts Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Moisture content of wood • The moisture content of timber is expressed as the weight of water in the piece as a percentage of the weight of oven dry wood fiber & weight of waterin the wood # $$ !! x100 = MC % % dry weight of the wood " http://www.av8n.com/physics/axes.htm Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Water in wood in standing trees Water in timber in service environments MC~120% Wet southern forests Wood-water relationships MC~45% Eastern Arnhem land forest www.csaw.utas.edu.au MC: 12-14% Sheltered unconditioned space Wood-water relationships MC: 8-9% Air-conditioned space www.csaw.utas.edu.au 2! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Water in wood and water to wood • Free - The water within cavities or lumens, in the cells of wood • Bound - the water weakly chemically bound in the cell walls of wood O H Hydroxyl group on wood polymer Bound water molecules UBC 2009 Wood474 Avramidis Wood and Water From UBC 2009 Wood474 Avramidis Wood and Water Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Removing moisture • When milled, timber has a relatively high moisture content • It is dried to increase its utility Drying and dimensional change To dry timber, a moisture gradient has to be established in the piece Free water Fibre saturation point is when the Bound water cell walls are saturated with bound water but the cell cavities Air dry timber are free of water It occurs at approx~ 30% moisture content Timber shrinks as the bound water is removed www.csaw.utas.edu.au Wood-water relationships KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Moisture and shrinkage Drying mechanisms Dryings two processes • Water has to removed from the wood surface 80 60 loss of moisture with little change in size low shrinkage 70 moisture content www.csaw.utas.edu.au free moisture 50 40 Fibre saturation point range over which shrinkage commences 30 straight line portion of graph where shrinkage is proportional to moisture content 20 Service EMC zone bound moisture 10 example of service conditions and Equilibrium Moisture Content high shrinkage fibre saturation point – With the control of temperature, humidity and air flow • Water has to move from the woods interiors to the surface – Through the bulk flow of vapour or liquid, or diffusion from cell to cell Shrinkage Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au 3! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Timber dries at different rates Shrinkage and cell structure • Pines have a relatively porous cell structure that allow bulk flow: • As moisture is drawn out of the cell wall, the wood fiber shrinks – They can be dried very quickly – Turnaround time from saw to store can be as low as a week • Hardwoods have a non-porous cell structure The moisture must diffuse through the cell wall – Drying can be very slow – It must be done carefully Wood-water relationships This cell will shrink more than this one Shrinkage/swelling ~ specific gravity ~ cell wall thickness www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Shrinkage and grain direction Shrinkage rates longitudinal ~ 0.1 - 0.3%, radial ~ 2.5 - 6.5%, tangential ~ - 13% Species Radial% Tang.% Radiata pine 3.5 Hoop pine 2.5 3.5 Cypress 3.5 Softwood • As the thicker latewood cells run tangentially, shrinkage is greatest in this direction, less radially, least longitudinally • Rays and the irregular arrangement of cells generate constraint and additional variability www.csaw.utas.edu.au Spotted gum 4.5 Karri 4.5 10 Sydney blue gum Grey Ironbark 5.5 7.5 Mountain ash 6.5 13 Wood-water relationships KDA503/335 Timber, its origin and characteristics Shrinkage effects Other shrinkage effects • Tangential shrinkage is almost always highest; • This tends to shorten the growth rings as the timber dries, distorting cut sections www.csaw.utas.edu.au Longitudinal shrinkage Shrinkage from 25% to 12% MC KDA503/335 Timber, its origin and characteristics Wood-water relationships Tangential shrinkage Hardwood tan ge nti al Wood-water relationships Radial shrinkage www.csaw.utas.edu.au • Simple shrinkage is complicated by other drying effects including: – Collapse and reconditioning, – Surface and internal checking – Grain deviation along or through the board, Wood-water relationships www.csaw.utas.edu.au 4! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Wood, dimensional change & hygroscopicity • Wood is hygroscopic • It loses or gains moisture to be in equilibrium with variation in environmental humidity • As it does so, its dimensions change Hygroscopicity Bound water in the cell wall Air dry timber www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Moisture exchange is dynamic Equilibrium moisture content • Equilibrium moisture content (EMC) is: Wood Surrounding environment Dynamic exchange of heat and water Wood-water relationships – the moisture content where timber neither gains nor loses moisture from the surrounding atmosphere www.csaw.utas.edu.au FWPA, 2009, Timber Flooring • There is a direct relationship between relative humidity, temperature and woods EMC Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Rate of change of MC Unit shrinkage rates Species • The rate of MC change in timber with changes in the surrounding environment varies with species type, density and other characteristics Radial% Tang.% Radiata pine 0.19 0.27 Hoop pine 0.18 0.23 Spotted gum 0.32 0.38 Nitens 0.33 0.22 Tas blue gum mature 0.61 0.49 Tas blue gum plantation 0.46 0.35 Mountain ash 0.23 0.36 Softwood Hardwood Unit Shrinkage (shrinkage for 1% change in MC) Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au 5! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Calculations of steady state ingress of MC% Uneven moisture ingress Unit width x unit movement per 1% x MC% change = total unit movement Back sawn Radiata pine board of 85mm cover with a 3%MC change 85mm x 0.27% x = 0.6885mm of movement either + or – Back sawn Tas Oak board of 85mm cover with a 3%MC change 85mm x 0.36% x = 0.918 mm of movement either + or – Quarter sawn Tas Oak board using the same parameters 85mm x 0.23% x = 0.5865 mm of movement either + or – • Moisture ingress is often uneven • Uneven moisture movement from one face of a board will distort the piece To calculate the total movement in the width of a floor per MC%: Unit width x unit movement per1% x MC% change x number of units = total movement in the width of the floor Back sawn Radiata pine board of 85mm cover in a 6000mm wide floor 85mm x 0.27% x = 0.6885mm x (6000 / 85) = 48.59mm Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Uneven moisture ingress in a bamboo floor Wood-water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Summary • Wood in the tree or in service contains water – Its moisture content is the weight of water in the piece as a percentage of dry wood fiber • Wood changes in size as it loses or gaining moisture • Wood is hygroscopic: it loses or gains moisture with fluctuations in environmental humidity Wood-water relationships www.csaw.utas.edu.au Wood-water relationships www.csaw.utas.edu.au 6! ... oven dry wood fiber & weight of waterin the wood # $$ !! x100 = MC % % dry weight of the wood " http://www.av8n.com/physics/axes.htm Wood- water relationships www.csaw.utas.edu.au Wood- water relationships... of wood • Bound - the water weakly chemically bound in the cell walls of wood O H Hydroxyl group on wood polymer Bound water molecules UBC 2009 Wood4 74 Avramidis Wood and Water From UBC 2009 Wood4 74... Wood4 74 Avramidis Wood and Water Wood- water relationships www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Wood- water relationships www.csaw.utas.edu.au KDA503/335 Timber,