WPS 264 Shrinkage & swelling

FSP and S&S

• Dimensional changes only below FSP

• Swelling caused by water sorption site in cell wall

S&S with face

• Dimensional changes are anisotropic (differ along length)

• Axial is smallest

• Radial is larger

• Tangential largest

S&S affected by

• Moisture

• Extractive content

• Anatomical structure

• Density

• Chemical composition

• Mechanical stress

Moisture influence

• Amount of moisture gained or loss from 0-FSP determines amount of deformation

• Nearly linear in all growth directions

• Can reach beyond FSP if internal stress have developed in too large samples;

  • large sample might have non-uniform MC

  • use small, standard size samples to determine MC influence

Density influence (& AC)

• Magnitude of S&S increases with density

• Larger amount of cell substance, thicker cell walls

• Contain more moisture in cell walls

• Density affects anisotropy of S&S

• Anisotropy coefficient: ratio between tangential/radial shrinkage

• AC increases with increasing density (difference in shrinkages smaller in denser woods)

Mass of water on wood

• m_water [kg/m³] = r * V * u

Influence of structure

• Latewood S&S more because of higher density (up to 3.5x)

• Tangential shrinkage changes more because rings are tangentially arranged, especially for woods with pronounced density differences

Influence of extractives

• Large content of extractives (in cell walls) reduces S&S

• Removing extractives increase S&S

Influence of mechanical stress

• Permanent deformation of wood cells changes S&S characteristics

• Large compression → more shrinkage

• Large tension → less shrinkage

• Cell dimensions permanently reduced

Chemical composition

• Lignin is main influencial component

• Higher lignin → less S&S

• Lignin content reduced in high density wood

• HW shrink more than SW because lower lignin content

Anisotropy & cell-wall structure

• Mainly attributed to cell-wall structure

• Secondary wall consists of 3 layers with different orientation of microfibrils

• In S2 microfibrils nearly parallel to cell axis

• In S1 & S3 microfibrils nearly transverse to cell axis

• Swelling in S2 is in proportion to number of microfibrils

• S1 & S3 restrain swelling, because different orientation

• Axial orientation of microfibrils in S2 results in small axial shrinkage. If they were perfectly longitudinal, no axial shrinkage could be observed

• Any deviation results in axial shrinkage & swelling

Anisotrophy & rays, early-/latewood, middle lamella

• Rays radially orientated

• Microfibrils of the parenchyma cells parallel to cells

• Restrain radial S&S

• Removal of rays sometimes increases radial shrinkage (HWs)

• Difference in early-/latewood results in greater tangential shrinkage, especially in trees with pronounced density differences (ring-porous)

• Some diffuse porous woods have higher tangential shrinkage than radial

• Middle lamella has high lignin content in radial cell walls → higher shrinkage in radial direction after lignin removal

S&S coefficients

• All given

• Calculated with wet dimension, dry dimension & green dimension. Answer is in %

• Volumetric change is sum of individual directional S&S coefficient, but can be estimated with only tangential & radial coefficients.

Shrinkage & dimensional change equations

• All variables are percentages converted to decimals.