WPS 264 Structural influence on density

Effect of structure

• Wood composed of mostly dead cells

• Consists of walls and cavities

• Density of cell wall is ~1.5g/cm³

• Density of wood depends on ratio of cell wall & voids

• Most woods density is 0.3-0.9g/cm³. Void volumes vary from 95% in light woods to 10% in heavy woods

Chemical composition

• Amount of cellulose, hemicellulose & lignin

• Densities:

  • Cellulose; ~1.5g/cm³

  • Hemicellulose; similar to cellulose, difficult to determine

  • Lignin; ~1.3g/cm³

• Wood density influenced by composition of components

• Variation of component amount:

  • Cellulose: 40-45%

  • Lignin: 17-35%

  • Large difference also in compression/tension wood

Percentage of void volume

• C = [(1 - r₀) / r_w] * 100

• At ovendry density: 100 - 66.7 * r₀

• given

Species difference reason

• Comes from anatomical difference;

  • various cell types (tracheids, parenchyma, vessels)

  • distribution ratio of these cells

  • cell wall thickness

  • volume cavities

• These are difficult to determine, therefore relationship of density & structure is determined via:

  • width of growth rings

  • proportion of latewood (denser)

Latewood proportion

• Latewood denser (thicker cell wall, smaller lumen)

• Density of wood increases with latewood proportion

• Diffuse-porous HWs make relationship unclear

Porous types (recap)

• Ring porous: abrupt difference in vessel sizes

• Semi-ring porous: gradual difference in vessel sizes

• Diffuse porous: no difference in vessel sizes

Growth ring width

• Ring-porous HWs: density increases with ring width because early-/latewood are equally wider

• Density of latewood can be lower than earlywood if large amount of vessels present eg. oak, chestnut

• Diffuse porous HWs: no clear relationship

• SWs: density decreases with increasing ring width because wider rings relates to more earlywood

Tree age

• SW denser when old

• HW less dense when old

Extractives

• Compounds of various chemical composition:

  • fats, gums, resins, sugars, oils, tannins, alkaloids

• Proportion of extractives: 1-20%

• Higher extractive amount means higher density

• Cause of higher density in heartwood compared to sapwood

• Removal of extractives causes decrease in density

Compression wood

• Tracheids are ~30% shorter

• 10% less cellulose & more lignin & hemicellulose

• 10-40% higher density but not more strength

• (In SW)