4.2b: Timber

4.2b
Timber
Timber is a major building material that is renewable and uses the Sun's energy to renew itself in a continuous cycle.

Characteristics of Natural Timber
There are two types of natural timber, called hardwood and softwood. These names do not refer tot he properties of the weed necessarily as some softwoods can be relatively hard and some hardwoods can possess the mechanical properties of a softwood

     Structure of Natural Timber

Wood Structure of Hardwood and Softwood

• Natural timber is a natural composite material compromising celulose-fibres in a lignin matrix

     Moisture content of natural timber

• When living trees are cut down the timber is of very little use in engineering, product design and so on. This is due to the moisture of the wood. Most of this moisture needs to be removed and a equilibrium moisture content achieved

     Seasoning

• is the commercial drying of timber which reduces the moisture content of the wood, making the timber highly usable. Methods include kiln seasoning, air-drying, and kiln-drying

     Timber defects

• Warping is the distortion in wood caused by uneven drying, which results in the material bending or twisting
• Bowing is the warping along the length of the face of the wood 
• Twisting is the warping where the two ends of a material do not lie on the same place

     Hardwood

• hardwoods come from deciduous (meaning that they lose their leaves in the wintertime) trees. 
• the trees can take up to a hundred years to go (slow growth), making it more dense and more expensive
• not all hardwoods are hard, such as balsa which is used for model planes
• Hardwoods are sold by the cubic metre and sawn depending on what the customer want
• Deciduous trees grow in the temperate, subtropical and tropical regions of the world
• Types include:

  

  Mahogany wood

   

  

  Types of hardwood

     Softwood

• generally come from coniferous trees which are evergreen
• can often be harder than hardwood
• Softwoods grow faster than hardwoods and so are cheaper and are easier to work with as their are softer than hardwoods
• Coniferous trees grow tall and straight, providing for long planks of wood
• more environmentally friendly to use softwood rather than hardwood because it takes 3 times faster to grow, is more sustainable
• grown in temperate regions
• Types include:

  

  Types of softwood

Characteristics of man-made timbers
Manufactured boards are timber sheets which are produced by gluing wood layers or wood fibres together. Manufactured boards often made use of waste wood materials. Manufactured boards have been developed mainly for industrial production as they can be made in very large sheets of consistent quality. Boards are available in many thicknesses but commonly 3, 6, 9, 12, 15, 18, 20, and 25 mm.


      Man-made timbers

• come in sheet material
• made from particles of wood, mixed with a resin/glue to make the sheets (made from the by-products of timber)
• not recyclable afterwards, possible to reuse
• the dust produced from cutting the dust is not particularly good for you (you should be wearing a mask)
• Tensile strength depends on the man made timber, plywood has high tensile strength, particle board and MDF are very low
• could be used for flooring, boarding up a window, a filler for a table top for example
• some man-made timbers are resistant to damp environments, depending on the type
• Longevity - depending on type
• Aesthetic properties  - appears like real wood
• is a sufficient alternative to real timber as it is becoming scarce and more expensive
• Types include:
  
• MDF - Smooth, even surface. Easily machined  and painted or stained. Also available in water and fire resistant forms. Used mainly for furniture and interior panelling due to its easy machining qualities. Often veneered or painted
• ADVANTAGES: cheaper at cost and inexpensive, saves trees as it is recycling process, easy to color, no knots or kinks, easy to cut and drill without damaging
• DISADVANTAGES: comparatively weaker than wood, furniture doesn't last long, can crack or split under extreme stress, absorbs water quicker than wood, doesn't take nails and screws easily, contains VOC that can cause irritation to lungs and eyes, dust produced during the manufacture of MDF is very harmful to breathing.
• Plywood - A very strong board which is constructed of layers of veneer or piles which are glued at 90 degrees to each other. Laminated material meaning that it is made out of many layers (odd numbers) - grains are put at opposite from one another:
• ADVANTAGES: the alteration of the grains done inside plywood (called cross graining) makes the material have strength, gives it stability, has a reduced possibility of bending, it is lighter than hard wood, has an ecological importance
• DISADVANTAGES: weaker than wood, not durable like wood, not always easy to sculpture, long-term can get attacked my insects, surface of plywood (if not painted well) can be peeled off
• Chipboard/Particle board - made from chips of wood glued together. Usually veneered or covered in plastic laminate (for aesthetic or practical purposes)
• ADVANTAGES: cost effective,  is sold at low prices, provides smooth and flat surface and is aesthetically attractive, does not dent or distort easily, can be easily transported and handled, minimal maintenance
• DISADVANTAGES: low strength, in presence of moisture particle board expands and warps, discoloration can occur, cannot support heavy loads, can be toxic (resin can break down over time)

Treating and finishing timbers

Timber is seasoned as a part of its preparation for commercial use. This process reduces the moisture content so that it becomes workable. The remaining moisture, albeit small, means that the wood never really stabilises and continues to swell and shrink, with humidity and temperature variations. Timber treatments and finishes are used to protect, enhance and improve the mechanical properties.

1. Timber treatments

Timber treatments are an additive preservative to improve the timber's resistance to attack and improve its durability.

(Examples of wood preservatives)

2. Timber finishes

Timber finishes are applied to the surface of the timber and is usually carried out to achieve one or both of the following reasons :

• Aesthetics - to improve the materials' natural beauty 
• Function - to protect it from environmental impact, heat, and moisture 
• Examples of timber finishes are varnish/estapol, finishing oils, and wood wax.

Recovery and disposal of timber

Timber is one of the most renewable and 'earth friendly' materials available which makes the principle and practical aspect of recovery and recycling quite straightforward.

• Recovery and recycling - turning waste timber into usable products. 
• Uses for recycled waste wood include traditional feedstock for the panel board industry, which still accounts for the majority of recycled wood. Other uses include animal beddings, equestrian and landscaping surfaces, play areas and filter beds.