CHEM-E0120: An Introduction to Wood Properties and Wood Products Wood structure I: Basic features, structure and cell types Mark Hughes 18 th September 2017
Today Making trees: photosynthesis Tree types & characteristics Macroscopic structure of wood Structural features and their effect on properties Microstructure Cell types Wood structure
Where do wood products come from? Wood is from trees! Where do trees come from? Trees are composed of a range of organic compounds that are ultimately synthesized from atmospheric carbon dioxide and water using sunlight as an energy source. This process is known as photosynthesis Carbon dioxide + water + light energy carbohydrate + oxygen
Photosynthesis Photosynthesis is the process whereby light energy is converted into chemical energy that can then be used by the plant to fuel its metabolism The energy is stored in carbohydrates (molecules containing C, H and O) such as glucose that form the building blocks of plants and all organic matter Light energy is absorbed by proteins called, reaction centres, that contain the green pigment chlorophyll. These reaction centres are contained in organelles that are most abundant in leaves Light (photons) is used in reactions to produce NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate), which are subsequently converted in light independent reactions (the Calvin cycle) into carbohydrates, by incorporating atmospheric CO 2
Tree growth Growth occurs in the vascular cambium which in which living cells divide and differentiate outwards to form the bark and inwards to form the wood. The cambium is a lateral meristem responsible for the increase in girth The apical meristem is responsible for increasing the height Meristems are the tissues found in regions of the plant where growth can take place
Tree types & characteristics Basic characteristics of the tree: Gymnosperms (softwoods; conifers) Gymnosperms have "naked seeds", attached to the surface of modified leaves (cones) Bear needles, which are usually evergreen Angiosperms (hardwoods; broadleaves; deciduous) Angiosperms have seeds enclosed in fruit (apple, pear. acorn, etc.) Bear leaves (which are lost in the autumn in temperate climates)
Softwoods Spruce Pine
Hardwoods Birch Oak Maple
Tree structure The growth of a tree is a combination of genetics and environmental factors Genes dictate species etc. Ecological factors influence growth. These include, for example: Climatic factors (temperature, rain, snow) Soil Location (stand / clearing) External stresses (wind loading, frost)
Environmental factors Snow load Wind load
Environmental factors Forest stand Clearing
Environmental factors Forest Park
The tree and timber from it A tree consists of: Roots Trunk Branches including needles/leaves We are mainly interested in the trunk: Support Mechanical function Optimised for its own purposes not ours! This can present a conflict
Orientation Trunk is pseudo-cylindrical (slightly tapered) Three directions: Longitudinal Radial Tangential Properties differ significantly in different directions it is anisotropic In many situations wood can be regarded as an orthotropic material
The effect of orientation (Source: Society of Wood Science & Technology)
Wood: Nature s cellular composite material
Structural levels Gross structure of wood: Visible to the naked eye Heartwood/sapwood, growth rings, grain, knots Microstructure of wood Visible under a light microscope Different cell types, morphology of cells THE FIBRE (CELL) The cell wall Visible by electron microscopy (some features by optical microscopy) Chemical composition Spectroscopic & chemical techniques Macro- & microstructure Ultrastructure & chemistry Providing background to: Appearance Properties Behaviour
Macrostructure and microstructure
Gross structure of wood Bark, pith, heartwood, sapwood Growth rings Growth features (defects, usually in wood products): e.g. knots, grain angle
Heartwood/sapwood Heartwood usually darker in colour (extractives), generally more durable Also gums and resins Sapwood lighter in colour, often perishable (Source: Society of Wood Science and Technology)
Extractives in heartwood Darkening of timber heartwood is caused by extractives. Different compounds (that are extractable by organic solvents). They include: Lipids Terpenoids Phenolic compounds Extractives have an effect on: Colour Durability (e.g. pine heartwood much more durable than sapwood) Can also affect gluing etc.
Growth rings (or year rings)
(Annual) growth rings Width varies according to ring age and external conditions The width of a growth ring in Finland is an average of 1.5 to 2 mm, however: Variation is great: Pine 0.1...10 mm Spruce 0.5...12 mm Birch 0.5...10 mm Composed of earlywood and latewood (springwood/summerwood)
Earlywood & latewood ( springwood & summerwood ) Earlywood lighter in colour as it is less dense than the darker latewood
Earlywood/latewood proportions Wood is stronger the more latewood it contains (Strong relationship between density and strength of wood) Strength etc. qualities can be determined according to the relative share of latewood Pine 25% (variation 15...50%) Spruce 15% (variation 10...40%) latewood
Differences between earlywood and latewood The share of latewood depends on a) ecological factors & b) species As growth decelerates latewood percentage grows A warm autumn increases the latewood percentage A drought in the autumn results in a lower proportion of latewood The relative and absolute share of latewood is greatest at the base of the tree Density (pine): Earlywood 300...370 kg/m 3 Latewood 810...920 kg/m 3
Knots
Knots Live (or tight) knot Dead (or loose) knot (Source: Wilson & White, 1986)
Microstructure Wood composed of cellular tissue that has different functions Most cells (~90%) are aligned mainly parallel to the axis of the tree (the grain direction ). Most trees contain spiral grain, where the grain angle is not parallel with the tree s axis About 10% of cells are arranged perpendicular to the axis of the tree, radiating from the pith outwards. These are known as rays Spiral grain in wood
Transverse surface Structure Radial surface Rays Trunk is pseudocylindrical (tapered) Tangential surface (Source: Wilson & White, 1986)
Wood structure (Source: Society of Wood Science and Technology)
Transverse section
Pine (transverse section) (x150 magnification)
Birch (transverse section) (x150 magnification)
Longitudinal sections Tangential Radial
Microscopic structure of wood Cellular structure Wood fibres
The cell Features: Tube like structure Wall thickness depends on function Void space in the centre is called the lumen Structures known as pits connect cells Formed by cell division
Pits: Interconnected cells Single pits: Pit aperture Cells contain structures known a pits These are like pores, connecting adjacent cells and facilitate the flow of fluids between cells They contain a flap (the torus), suspended on a cobweb-like structure known as the margo. This acts as a valve controlling fluid flow and can become irreversibly closed. In this state it is said to be aspirated
Cell types Softwood: Hardwood: Tracheids (support and conduction) Aspect ratio ~100:1 Parenchyma (storage mainly in the rays) Tracheids Parenchyma Fibres (very thick walled cells) whose main function in mechanical support Vessels (or pores), specialised conductive tissue
Hardwood cross-section
Summary Hierarchical structure at macroscopic and microscopic levels Anatomy differs between softwood and hardwoods Cell types and functions differ between hardwoods and softwoods Many natural features (defects?) that affect wood s properties
Literature and further reading Society of Wood Science and Technology: http://www.swst.org/teach/set2/struct1.html Dinwoodie, J.M. (2001): Timber: Its Nature and Behaviour Wilson, K. and White, D.J.B. (1986): The Anatomy of Wood: Its Diversity and Variability