Physical

• Physical property


  A characteristic of matter that may be observed and measured without changing the chemical identity of a sample
• Physical properties of wood

  • Appearance
  • Wood moisture relations
  • Thermal conductivity
  • Electrical resistivity
  • Sound properties
  • Electromagnetic properties
• Appearance of wood


  Color, texture, grain direction, topography and other characteristics of wood that may be visible to the naked eye or be magnified
• Wood moisture relations


  Hygroscopicity of wood and dimensional changes in wood, including density and specific gravity
• Wood exhibits a low thermal conductivity (high heat-insulating capacity) compared with materials such as metals, marble, glass, and concrete
• Thermal conductivity is highest in the axial direction and increases with density and moisture content; thus, light, dry woods are better insulators
• Electrical resistivity


  A fundamental property of a material that quantifies how strongly it resists or conducts electric current
• Wood is a poor conductor of electricity specially when dry. Its electrical conductivity increases with the moisture content
• Wood conducts sound better in the longitudinal direction of the grain than perpendicular to it
• A dense wooden structure reflects sound and can easily be made into surfaces that channel sound reflections
• Electromagnetic radiation


  The waves (or their quanta, photons) of the electromagnetic field, propagating (radiating) through space, carrying electromagnetic radiant energy
• Wood as a non-conducting and porous material tends to absorb EM waves
• This is the same principle that governs the use of EM waves in the determination of the moisture content in wood and the presence of defects and deterioration
• Relationship between wood and moisture

  Has more significance in wood utilization than any single property of the material
• The amount of moisture in wood practically affects all of its physical properties
• Types of water in wood

  Bound water, free water, water of constitution
• Bound water


  Water adsorbed by the cell wall from the surrounding air, held by hydrogen bonding
• Free water


  Liquid water contained in the cell lumen and intercellular spaces
• Water of constitution


  Water present in a molecule that cannot be removed without disrupting the molecule
• Water in wood may also be in vapor form found in the lumen and intercellular spaces
• Sorption sites in wood

  Where water molecules are held to cellulose by hydrogen bonding
• The functional contributions of the different chemical compounds on the total water sorption sites are 47, 37 and 16% for cellulose, hemicellulose, and lignin, respectively
• Moisture content of wood

  The amount of water contained in wood expressed as a percentage of the a reference weight of the wood
• Moisture content expressed on oven-dry basis


  Percentage of the oven-dry weight of the wood
• Moisture content expressed on wet basis

  Percentage of the initial weight of the wood
• Moisture content can exceed 100% on oven-dry basis, but cannot exceed 100% on wet basis
• The moisture content of green wood varies considerably among species and the different locations across the diameter of the tree
• The moisture content of heartwood ranges from 33 to 98% while for sapwood is from 44 to 249%
• Wi


  Initial weight
• Wo


  Oven-dry weight
• %MCwb


  Moisture content on wet basis
• Derivation of formulas


  1. (1 - %MCwb/100) * Wi = Wo / (1 - %MCwb/100)

  2. %MCwb = (Wi - Wo)/Wi * 100

  3. %MCwb = (1 - Wo/Wi) * 100

  4. %MCwb/100 = 1 - Wo/Wi

  5. Wo/Wi = 1 - %MCwb/100
• This method is commonly used in the pulp and paper and wood energy industries where the primary concern is the weight of logs during transport
• Moisture content can never reach 100%
• Converting between moisture content on wet basis and oven-dry basis

  1. %MCob = (%MCwb/(100 - MCwb)) * 100

  2. %MCwb = (%MCob/(100 + MCwb)) * 100
• Practice deriving the two formulas
• Moisture content in green wood

  • Heartwood: 33 to 98%
  • Sapwood: 44 to 249%
• In softwoods, the average green moisture content tends to decrease as the tree grows older
• In softwoods, the sapwood contains higher %MC compared to the heartwood
• In hardwoods, the heartwood portion of some species contains more water compared to the sapwood portion