Wood Structure and Properties: Sapwood, Heartwood, and Polymer Applications

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Wood Structure and Properties

Sapwood and Heartwood


  • Width varies with species, growth rate, and tree age.
  • Typically represents 20-50% of the total radius (except in very young trees).


  • Formed from former sapwood cells.
  • Acidity increases, extractives form, and color changes.
  • Resistance to fungal and insect attack increases.
  • Many timbers develop gums and resins in the heartwood.

Functions of Cells in a Growing Tree

  • Primary function: Conduction of water and dissolved minerals from roots to leaves.
  • Secondary function: Mechanical support of the tree.
  • Tertiary function: Food storage for spring growth.

Characteristic Differences between Softwoods and Hardwoods

  • Hardwoods: Water-conducting elements distributed throughout the annual ring.
  • Softwoods: Water-conducting and strengthening elements segregated in springwood and summerwood, respectively.

Polymer Applications in the Construction Industry

Environmental Impact of Polymers

Availability of raw materialLimited - by-product of oil
Extraction processPossible pollution problems
Energy consumedHigh - due to chemical processes
Health and safetyFire hazard
Waste disposalMany do not break down and require recycling

Importance of Carbon in Polymer Production

  • Lightest element with four electrons in the outer shell - good for bonding.
  • Forms covalent bonds by sharing electrons with other atoms to achieve an octet.
  • Bonding patterns can result in crystals, amorphous arrangements, or long chains (fibers).

Visco-Elastic Behavior and Polymer Types

Visco-Elastic Behavior

  • Mechanical properties lie between Hookean (stress ∝ strain) and Newtonian (stress ∝ rate of strain).
  • Stress is a function of strain and time.

Thermoplastic Polymers

  • Chains of molecules are separate and can slide over each other.
  • Long chain molecules held together by weak Van der Waals forces.
  • Heating weakens intermolecular forces, softening the material into a viscous melt.
  • Cooling allows re-solidification, and the process can be repeated.

Thermosetting Polymers

  • Chains become cross-linked, producing a solid material that cannot be softened or flow.
  • Formed in a two-stage chemical process:
  1. Long chain polymerized molecules produced.
  2. Cross-linking takes place.
Rigid structure with mechanical properties affected by heat. Cannot be softened by heating and must be molded prior to polymerization. Comprise large three-dimensional molecules with higher strength and stiffness than thermoplastics. Increased rigidity leads to increased brittleness, requiring toughening with fibers for impact resistance.

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