http://www.auf.asn.au/scratchbuilder/timber.html
4.3.1 - Describe the structure of natural timber
Natural timber is a natural composite material comprising cellulose fibres in a lignin matrix. The tensile strength of timber is greater along the grain (fibre) than across the grain (matrix).
4.3.2 - Outline that timber can be classified according to the conditions needed for tree growth
Consider temperate and tropical conditions. A general knowledge of the geographical distribution of world timber resources is required.
Softwoods originate from coniferous trees (also known as evergreens, or gymnosperms). Rather than losing their leaves all at one time, they tend to shed their leaves (which resemble and are called "needles") throughout the year and bear cones. Hardwoods, on the other hand, come from deciduous trees which lose their leaves throughout the year and produce seeds covered with some kind of covering (sometimes a fruit or nut).
The classification of timber is based on the species it comes from, rather than its relative hardness or density. Hardwoods include exotic deciduous species (such as ash, mahogany, oak, walnut and balsa) and Australian native evergreen species (such as jarrah, silky oak, brushbox, iron bark and spotted gum). Softwoods come from coniferous species, such as true cedar, fir, pine and spruce.
4.3.3 - Outline that conifer trees are referred to as softwoods and that these grow only in temperate regions
Characteristics of softwood trees
Cellulose content - 42% +/- 2%
Lignin content - 28% +/- 3%
Extractives content - 3% +/- 2%
Fibre length - 2-6 mm
Coarseness - 15-35 mg/100 mm
- Cellulose - consists of long, straight chains of glucose molecules. It forms the skeleton of the plant wall. These fibres are long, strong and translucent.
- Lignin - is a three dimensional phenolic polymer network. This "glue from hell" holds the cellulose fibres together and makes them rigid. Chemical pulping and bleaching processes selectively remove the lignin without significantly degrading the cellulose fibres.
- Extractives - account for 3(+/-2)% of softwoods. These materials include plant hormones, resin and fatty acids along with other substances that help the tree grow and resist disease and pests. These substances are highly toxic to aquatic life and account for much of the acute toxicity of pulp mill effluent.
4.3.4 - Outline that deciduous trees are referred to as hardwoods and that these grow in both temperate and tropical regions
Characteristics of hardwood trees
Cellulose content - 45% +/- 2%
Lignin content - 20% +/- 4%
Extractives content - 5% +/- 3%
Fibre length - 0.6-1.5 mm
Coarseness - 5-10 mg/100m
More seriously
| Hardwood | Softwood | |
|---|---|---|
| Density: | higher density thereby harder | Lower density thereby most varieties are softer than hardwood. |
| Found in regions: | Trees supplying hardwood are found throughout the world from the Boreal and Taiga forests of the North to the tropics and down into the far South, excluding antartica. | Found in the northern hemisphere. |
| Definition: | Comes from deciduous trees that drop their leaves every year. | Trees that are conifer and have needles, and normally do not lose needles. |
| Properties: | Broad leaves; enclosed nuts; higher density. | Less dense; less durable; high calorific values. |
| Type: | Mostly deciduous. | Evergreen |
| Cost | Expensive. | Less expensive. |
| Examples of trees: | Aspen, Poplar, Birch, Elm, Maple, etc. | Pine, spruce, cedar, fir, larch, douglas-fir, etc. |
| Applications: | Used for furniture but less frequently than softwood. | Widely used as woodware for building and furniture. |
Resource: http://www.diffen.com/difference/Hardwood_vs_Softwood
4.3.5 - Discuss the issues relating to the consideration of timber as a renewable source
4.3.6 - List two examples of composite timbers
Particle Board
Also known as chipboard, is an engineered wood product made from wood particles, such as wood chips, sawmill shavings, or even saw dust. This type of composite timber is generally cheaper, denser and more uniform than conventional wood and plywood. It is usually used as a substitution for them when appearance and strength are less important than cost.
Plywood Board
Plywood is an engineered board made from thin sheets of wood, called plies or wood veneers. Softwood plywood is usually made either of Douglas fir or spruce, pine, and fir (collectively known as Spruce-pine-fir), and is typically used for construction and industrial purposes. Hardwood plywood is used for some demanding end use.
4.3.7 - Characteristics of particle board, laminated woods, pine wood and mahogany.
4.3.8 - Criteria for the selection of timber for different structural and aesthetic design contexts.
4.3.9 - Reasons for treating or finishing wood.
4.3.10 - Three differences in the selection of timbers for flooring if it were made of a hardwood, a softwood or a composite material.
