503 335w6 wood water 1

Wood-water relationships 1 The technical aspects of the relationship between wood and water www.csaw.utas.edu.au Wood-water relationships 1 Content This lecture • Introduction to

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Wood-water relationships 1

The technical aspects of the

relationship between wood and

water

www.csaw.utas.edu.au

Content

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

KDA503/335 Timber, its origin and characteristics

Introduction to wood-water

relationships

Water, water in wood, dimensional

change and hygroscopicity

Wood in the tree or in service contains water

•  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

Free water in the cell

Air dry timber

Bound water in the

cell wall

Wood, dimensional change & hygroscopicity

•  Wood is hygroscopic: it loses or gains moisture

with fluctuations in environmental humidity

Air dry timber

•  Wood changes in size as it loses or gaining

moisture

cell wall

Water in wood

Wood in the tree or in service contains water

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Water

•  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

Water content of air

•  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

http://www.av8n.com/physics/axes.htm

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

%

100 MC x

wood the of weight dry

wood the in water of weight

=

!!

"

#

$$

%

&

Water in wood in standing trees

Water in timber in service environments

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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

From UBC 2009 Wood474 Avramidis Wood and Water

Hydroxyl group on

UBC 2009 Wood474 Avramidis Wood and Water

Drying and dimensional change

Removing moisture

•  When milled, timber has a relatively high moisture content

•  It is dried to increase its utility

Free water

Air dry timber Bound water

To dry timber, a moisture gradient

has to be established in the piece

Fibre saturation point is when the

cell walls are saturated with bound water but the cell cavities are free of water It occurs at approx~ 30% moisture content

Timber shrinks as the bound water is removed

Moisture and shrinkage

0

10

20

30

40

50

60

70

80

loss of moisture with little change in size

range over which shrinkage commences

straight line portion of graph where shrinkage is proportional to moisture content

example of service conditions and

Equilibrium Moisture Content

Shrinkage

free moisture

fibre saturation point

bound moisture

Fibre saturation point

Service EMC zone

Drying mechanisms

Dryings two processes

•  Water has to removed from the wood surface –  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

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Timber dries at different rates

•  Pines have a relatively porous cell

structure that allow bulk flow:

– 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

Shrinkage and cell structure

•  As moisture is drawn out of the cell wall, the wood fiber shrinks

This cell will shrink more than this one

Shrinkage/swelling ~ specific gravity ~ cell wall thickness

Shrinkage and grain direction

•  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

tangential

Shrinkage rates longitudinal ~ 0.1 - 0.3%, radial ~ 2.5 - 6.5%, tangential ~ 4 - 13%

Tangential shrinkage

Radial shrinkage

Longitudinal shrinkage

Softwood

Hardwood

Shrinkage from 25% to 12% MC

Shrinkage effects

•  Tangential shrinkage is almost always highest;

•  This tends to shorten the growth rings as the

timber dries, distorting cut sections

Other shrinkage effects

•  Simple shrinkage is complicated by other drying effects including:

–  Collapse and reconditioning, –  Surface and internal checking

–  Grain deviation along or through the board,

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Hygroscopicity

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

Air dry timber

cell wall

Moisture exchange is dynamic

environment

Dynamic exchange of heat and water

Equilibrium moisture content

•  Equilibrium moisture content (EMC) is:

–  the moisture content where timber neither gains nor loses moisture from the surrounding atmosphere

•  There is a direct relationship between relative humidity, temperature and woods EMC

FWPA, 2009, Timber Flooring

Rate of change of MC

•  The rate of MC change

in timber with changes

in the surrounding

environment varies

with species type,

density and other

characteristics

Unit shrinkage rates

Softwood

Hardwood

Unit Shrinkage (shrinkage for 1% change in MC)

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Calculations of steady state ingress of MC%

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 3 = 0.6885mm of movement either + or –

Back sawn Tas Oak board of 85mm cover with a 3%MC change

85mm x 0.36% x 3 = 0.918 mm of movement either + or –

Quarter sawn Tas Oak board using the same parameters

85mm x 0.23% x 3 = 0.5865 mm of movement either + or –

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 3 = 0.6885mm

x (6000 / 85) = 48.59mm

Uneven moisture ingress

•  Moisture ingress is often uneven

•  Uneven moisture movement from one face of a board will distort the piece

Uneven moisture ingress in a bamboo floor

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

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