WPS 264 Water in wood & EMC

Hygroscopic property of wood

• Wood exhanges water vapour with environment until equil reached

• If air drier, wood wil desorb water. If air more humid, wood will adsorb water.

• Phenomenon is called sorption

Bound & free water

• Free (capillary) water; liquid water within lumens & cell wall cavities.

• Bound water; water within cell walls hydrogen bonded to cellulose & hemicelluloses.

  • Bound to hydroxyl groups of cellulose, hemicelluloses and also lignin but less.

  • Limited by number of soption sites

Water in cell wall

Water molecules adsorbed to hydroxyl groups

• At high MCs up to 10 H₂O molecules can be attracted to each possible sorption site.

• OH groups of cellulose are crosslinked to each other in crystalline areas, which decrease sorption sites.

Removal of water

• Free water removed first, then water starts to leave cell wall

• All “dry” wood still contains water in cell wall

Physical properties change drastically if bound water also removed

FSP

• MC at which cell wall is saturated & voids are empty.

• ~30% MC

• FSP decreases with increasing temp; ~0.1% for each 1C.

• Higher extractives means lower FSP.

• FSP decreases with increasing density.

• Increasing temp decreases FSP.

• Hydrogen bonding sites on lignin, hemicelluloses & celluloses may already be ‘occupied’.

• Most physical properties (shrinkage, mechanical & electrical properties) change below FSP. FSP can be experimentally determined by measuring this property as function of MC.

Equilibrium moisture content

• MC of wood eventually attained by piece of wood upon extended exposure to air at a constant relative humidity & temp.

• Only possible in confined space eg. kiln

• Species vary slightly in EMC (about 5%), but increases with relative humidity.

• Low EMC attributed to high content of extractives

• Temp influences EMC to small degree. EMC lower at higher temps.

• Temps > 100C has permanent effect; wood becomes less hygroscopic

• Most composite wood products have lower EMC

EMC of wood products

• EMC generally lower

• Plywood & laminated wood closer to raw wood

• Particle/fibreboard are considerably different

  • densification

  • addition of resin

  • coatings

  • fillers

  • → means less hygroscopic

Desorption

• Loss of moisture called desorption

• Begins by evaporation of surface water

• Free water from cavities drawn to surface by capillary forces & evaporates from surface

• Bound water diffuses to surface & evaporates

Adsorption

• In greenwood hydroxyl groups of cellulose in cell wall are saturated with water molecules

• When dried, hydroxyl groups move closer & weak cellulose bonds are formed

• Fewer adsorption sites than originally if humidity increases again

Sorption hysteresis

• At constant temp → isotherm sorption curve

• Desorption curve up to 3% higher than adsorption curve

• If humidity changes frequently → middle curve

• Initial desorption curve always higher than adsorption or further desorption curves

• After first drying, hygroscopicity of wood is permanently reduced due to crosslinked of hydroxyl groups during first desorption curve

Hysteresis and species

• Vary with species

• General form of sorption curve always the same, vertical position changes

• Caused by extractives occupying binding sites.