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Lecture 14

FOR400Y1 Lecture Notes - Lecture 14: Lignin, Ochroma, Cell Wall


Department
Forestry
Course Code
FOR400Y1
Professor
Ning Yan
Lecture
14

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FOR400Y - Lecture 14: Wood is Good
- Wood is simple at the molecular level
- 50% carbon by weight
- 45% oxygen by weight
- 6% hydrogen by weight
- Carbon content is especially important
- If you keep your wood products, you’ve sequestered carbon for a very long time
- Wood is highly variable
- Between species
- Within a species
- Influenced by growing conditions
- Within a tree
- Gross structures like heartwood & sapwood & bark & juvenile wood,
tension wood, compression wood
- Within a cell
- Material that possesses really unique properties/characteristics
- Extremely high strength to weight ratio compared to other materials
- Density ranges from extremely hard and dense (lignite) to balsa
- Float or sink
- Good insulator/poor conductor of heat or electricity
- Because filled with air
- Can’t have steel hydropoles, so have to use wood
- Contain a variety of “unique organic chemicals”
- Softwoods are much lighter than hardwoods
- Softwoods wood float
Wood is highly variable
- Structure within a cell
- Fibre diameter is 25-35 microns
- Most of the cell is air
- The density of the cell wall is 1.5g/cm3
- This is really dense but because of all the air in the lumen, makes the wood quite
right
Variable composition of cell wall
- Cell wall is composed of 3 layers
- Layers are made up of microfibrils
- Orientation of the microfibrils is specific for each layer
Model of plant cell wall
- Cell wall structure
- Microfibrils are bundles of cellulose molecules (approx. 36 cellulose chains)
- Crystalline mycelles and amorphous regions

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- Macrofibrils consist of numerous cellulose microfibrils dispersed in hemicellulose and
lignin
- Most of the microfibrils are aligned longitudinally in cell wall
- Gives tree strength to withstand wind forces, snow
- Lignin is a polyphenolic compound that glues together all the microfibrils
- Cellulose is all sugar
- Fungi/microbes like to eat sugar (which is 60-80% of the wood)
- The rest is is lignin which microbes don’t like, so this protects wood from
microbes
- Lignin is also hydrophobic and water-resistant so that the water can move around
inside the tree without being absorbed by the cellulose
Variable composition of wood structure
- Softwoods only have tracheids or fibre cells
Anisotrophic behaviour
- Means it has different properties in different directions
- All the fibrils align in one direction
- If load wood on longitudinal axis, wood very strong, if we load it on the other axis, it’s
really weak
- Consider liquid flow: If you have to cross the cell walls, you have different properties than
if you were following the hole
The Bad about wood
- Oak and balsa have very poor relative strength compared to other materials
- If we load wood in tensile direction (pulling it), it’s really weak
- If we load wood in compression (pushing down), it’s as strong as concrete
Wood in Compression
- Used as beams and columns
- Compression columns
The good and bad
- How do we use pieces of wood to construct houses?
- Have to be aware of what wood is good for
- Wood is very flexy, so need joists in decks/floors
Why is Wood Good?
- Highly variable
- From a material perspective, possesses really unique properties/characteristics for
variety of applications
- Good building material
- Don’t even need expensive tools
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