Properties of Wood Flashcards
How is the structure represented?
Bundle of tubular cells (fibres) glued together by lignin.
Which direction are cells aligned?
Usually longitudinal.
What are Trackeids?
- Longitudinal aligned cells
- Softwood - 3 to 5mm long (aspect ratio (l/d) = 100)
- Hardwood - 1mm long
- Used for mechanical support and water/sap transfer.
Cell make up
- Thin primary walls (forms first)
- Three layered secondary walls (forms second), provides most strength
S1 (outside layer of three layer secondary wall)
- 15 degrees off horizontal
- Thin
- Almost perpendicular to cell axis
- Acting mainly perpendicular to grain
S2 (middle layer of the three)
- Thick walls
- Microfibrils parallel to axis
- Providing strength in longitudinal direction
S3 (inner layer of the three)
- similar to that of S1
What is Cellulose?
= 50% of wood weight (about)
- Building block (glucose = sugar)
- With growth linear cellulose arrange into ordered strands, fibrils.
What is Lignin?
= 23 to 33% of softwood weight
= 16 to 25% of hardwood weight
- mostly intercellular
- intractable, insoluable, materials (chemically)
- bounded to cellulose
- glue holding tube together
- longitudinal shear limited to strength of lignin
What is Hemicellulose?
- Polymeric units made from sugar
- Different to cellulose (several sugars tied up in structure)
= 20 to 30% hardwood
= 15 to 20% softwood - main sugar units xylose (hardwood) and mounose (softwood)
What are Extractives?
= 5 to 30% of wood substance
- polyphenolics, colouring matters, essential oils, fats, resins, waxes, gums, starches, ad simple intermediates
- removed with water, alcohol, acetone and benzene
- some are toxic to resin, natural durability
What is Ash?
= 0.1 to 3% of wood material
- Calcium, potassium, phosphate and silica
What is the chemical composition of wood?
- 50% Carbon
- 44% Oxygen
- 6% Hydrogen
- 0.1% Nitrogen
- 0.1 to 3% Ash
Specific gravity factors
- around 1.5
- float initially on water (voids of air), when saturated will sink (takes years)
- seasoned wood less than 2/3 weight water
- denser the wood the stronger it is
Compression, is wood strong or weak under?
Stronger longitudinally
Traverse strong at 1/3 of og thickness
Tension, is wood strong of weak under?
Strong parallel to grain
Weak perpendicular to grain
No adjustment to allowable stress if average equilibrium moisture content is less than what?
15%
Causes of deterioration in wood?`
- Fire
- Decay
- Termites
- Chemicals
- UV
- Moisture fluctuations
- Insects
- Mechanic abrasions
Fungus on wood. What it does and how.
- Feeds on wood
- Spreads via spores
- Secrete enzyme that depolymerizes cellulose, resulting in rot.
Fungus growth.
- Food (cell structure, cell content)
- Needs temperature of 5 to 40 degrees
- Moisture (fibre saturation, below 20% MC fungi is inhibited)
- Oxygen
Fungus damage.
- Stains (sapstains) and decay damage.
Prevent Fungus
- Minimise moisture
- Exclude air
What termites do
- Feed off wood
- rest in soil (moist, warm)
- if moisture in wood is low they build shelter tubes for direct contact between soil and wood
Protect from termites how?
- Block access to wood from soil
- Preservatives
