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

Wood Structure and Properties

Sapwood and Heartwood

Sapwood

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

Heartwood

• 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

Item	Comment
Availability of raw material	Limited - by-product of oil
Extraction process	Possible pollution problems
Energy consumed	High - due to chemical processes
Health and safety	Fire hazard
Waste disposal	Many 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.