WPS 264 S&S defects

S&S degradation

• Reduction or increase in dimensions or varying S/S as result of MC or density distribution leads to:

  • opening/tightening of joints

  • change of cross-sectional shape

  • warping

  • casehardening

  • honeycombing

  • collapse

  • loosened or raised grain

Dimensional changes

• Appear in all wood

• Magnitude influenced by tension/compression wood & knots

Joints

• Doors can’t close in winter or is too loose if shrank

Change of cross-sectional shape

• Results from different radial & tangential shrinkage

• Depends on where on log wood was cut

Warping

• Results from different r&t S&S

• Lumber boards longer in tangential direction are most likely to warp

• Warping also caused by MC differences

Checking (splitting of fibres)

• Unequal dimension change due to MC gradient in wood

• Appear at ends or on surface where moisture is lost quicker

Loose knots

• Unequal shrinkage between knot & wood might lead to loosening of knot

Honeycombing

• Internal checking of wood occuring in core at high MC & too high temp in kiln drying

Collapse

• Severe distortion of cells caused by eg. high kiln temps

Loose / raised grain

• Raised grain: rough lumber surface caused by latewood being raised above earlywood

• Loose grain: partial separation of growth rings, loose portions protrude

Control of shrinkage

• Mechanical modification

• Water repelling coating

• Bulking treatment

• Reduction of hygroscopicity

• All expensive or experimental

Mechanical modification

• Avoid anisotrophy eg. plywood

• Adjacent layers with grain direction at right angles

• Decrease in shrinkage in direction of length & width

• Magnitude of shrinkage close to longitudinal value of solid wood

Water repelling coating

• Exterior coating protects atmospheric variation through retardation of moisture exchange

• S&S also retarded but not prevented. EMC not affected.

• Interior coating eg. alkydic resin (10-15%) + paraffin wax (0.5-1%)

• Wood immersed in solution, which changes hygroscopic character of wood.

• Protective coatings wear off with time

Bulking treatment

• Wood kept swollen by depositing chemicals (salt, sugar, ect.) in cell walls

• Reduces strength

• At high humidities salt, sugar, ect. appear on surface & deplete

• Monomeric methacrylates, vinyl compounds or styrene with a swelling agent can be used the same way

• They polymerise in cell wall & reduce shrinkage up to 90%

• Mechanical properties, weather & insect resistance improved

• Expensive

Reduction hygroscopicity

• Reduced by thermal treatment; breakdown of hemicelluloses (oak 160C & 6atm reduces shrinkage by 70%)

• Replacement of hydroxyl groups with acetyl groups reduces shrinkage up to 75% (expensive)

• Over-treatment results in loss of stability

• Crosslinking of hydroxyl groups with formaldehyde vapours reduces shrinkage by ~90%; strength reduced due to hydrolysis of cellulose & hemicelluloses by acidic catalysts

Drying

• Best method if done carefully

• Dry according to specified relative humidity rather than specified MC

• Outdoor wood especially subject to environmental factors

• Wood shrinks in dry, hot summer & dwells in wet winter

• Dry to EMC of area, not lower