Wood structure I: Basic features, structure and cell types

Transcription

1 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

2 Today Making trees: photosynthesis Tree types & characteristics Macroscopic structure of wood Structural features and their effect on properties Microstructure Cell types Wood structure

3 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

4 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

5 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

6 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)

7 Softwoods Spruce Pine

8 Hardwoods Birch Oak Maple

9 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)

10 Environmental factors Snow load Wind load

11 Environmental factors Forest stand Clearing

12 Environmental factors Forest Park

13 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

14 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

15 The effect of orientation (Source: Society of Wood Science & Technology)

16 Wood: Nature s cellular composite material

17 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

18 Macrostructure and microstructure

19 Gross structure of wood Bark, pith, heartwood, sapwood Growth rings Growth features (defects, usually in wood products): e.g. knots, grain angle

20 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)

21 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.

22 Growth rings (or year rings)

23 (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 mm Spruce mm Birch mm Composed of earlywood and latewood (springwood/summerwood)

24 Earlywood & latewood ( springwood & summerwood ) Earlywood lighter in colour as it is less dense than the darker latewood

25 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 %) Spruce 15% (variation %) latewood

26 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 kg/m 3 Latewood kg/m 3

27 Knots

28 Knots Live (or tight) knot Dead (or loose) knot (Source: Wilson & White, 1986)

29 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

30 Transverse surface Structure Radial surface Rays Trunk is pseudocylindrical (tapered) Tangential surface (Source: Wilson & White, 1986)

31 Wood structure (Source: Society of Wood Science and Technology)

32 Transverse section

33 Pine (transverse section) (x150 magnification)

34 Birch (transverse section) (x150 magnification)

35 Longitudinal sections Tangential Radial

36 Microscopic structure of wood Cellular structure Wood fibres

37 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

38

39 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

40 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

41 Hardwood cross-section

42 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

43 Literature and further reading Society of Wood Science and Technology: 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