KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Contents Cell wall structure and wood chemistry www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics • Cell wall ultra-structure and its variation • Chemical components and their variation Cell wall structure and wood chemistry www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Cell wall structures • Cell walls in plants are likened to a composite material with a: Cell wall ultra-structure and its variation – reinforcing framework; and – cementing matrix • The reinforcing framework is made up of cellulose microfibrils, arranged parallel to each other • These are surrounded by a cementing matrix made up of hemicellulose, lignin and extractives http://www.lbl.gov/Publications/YOS/Feb/ 19Dec10 0.0 title ! www.csaw.utas.edu.au Cell wall structure and wood chemistry KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Cell wall structure Secondary layers • The mature wood cell wall is made up of a primary wall and three secondary layers, S1, S2, and S3 • Each wall is made of microfibrils with a specific arrangement • The primary wall, often referred to as the P layer, is very thin and its microfibrils are arranged randomly • It is the most highly lignified among the cell layers • The middle lamella is the intercellular layer separating adjoining cells Cell wall layers, intercellular material, lumen, and microfibril angle (MFA) Source: http://www.nature.com/nmat/journal/v2/n12/fig_tab/ nmat1025_F1.html Cell wall structure and wood chemistry – There are no microfibrils in the middle lamella and it contains most of the lignin found in wood www.csaw.utas.edu.au www.csaw.utas.edu.au S1 layer: – very thin outermost layer of a few lamella (layers of microfibrils) – The microfibrils lie parallel to one another in distinct right-handed and left handled spirals inclined to the longitudinal axis at an angle of 50° to 70° It forms about 10% of the total wall thickness S2 layer: – about 85% of the total wall thickness – Its microfibrils are laid out parallel to each other in a spiral at an MFA of 10° to 30° The number of lamellas vary from 30-40 in thin walled earlywood cells, to at least 150 in latewood cells – The S2 layer constitutes most of the bulk of the cell material, and contributes the most to the wood's physical properties S3 layer: – makes up 1% of the total thickness of the wall and its microfibrils are orientated at 60° to 90° to the cell axis Cell wall structure and wood chemistry www.csaw.utas.edu.au 1! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Microfibril angle • This inclination of the microfibril is termed microfibril angle (MFA) • Fibres with steeper arrangement of fibrils have superior strength, smaller longitudinal shrinkage and greater tangential shrinkage • The MFA varies from over 35 degrees in juvenile and compression wood to 10 degrees or less in mature wood In juvenile and compression wood, the microfibrils in the S2 layer are oriented at a considerable angle to the longitudinal direction http://www.forestprod.org/cdromdemo/jr/pics/wd3.jpg Cell wall structure and wood chemistry www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Comparison of the fibre structure in mature wood, juvenile wood and compression wood From Jozsa and Middleton (1994) in http://www.forestresearch.gov.uk/fr/INFD-6FMCB3 Cell wall structure and wood chemistry www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Basic chemical components of wood Chemical components and their variation Element % by dry weight carbon oxygen hydrogen others ~ 49%, ~ 44%, ~ 6%, ~ 1% (Nitrogen, calcium, silicon, etc.) These elements are found in the cellulose microfibrils, and the cementing matrix of hemicellulose, lignin and extractives 0.0 title ! www.csaw.utas.edu.au Cell wall structure and wood chemistry KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Chemical components of the cell wall Cellulose • Cellulose, hemicellulose and lignin are the three principal chemical components of wood • While extractives are only a minor component, they play an important part in wood processing and utilisation Chemical Component Softwood Hardwood Cellulose 45 ± 2% 42 ± 2% Hemicellulose 30 ± 5% 27 ± 2% Lignin 20 ± 4% 28 ±3 % Extractives ± 3% ± 2% www.csaw.utas.edu.au • Cellulose is the principal component of the cell wall • Made up of glucose units that bond themselves to form long polymer chains, cellulose forms very stable, highly crystalline structures • Over parts of their length, these chains lie parallel to each other and regularly spaced, forming long crystalline microfibrils • It is not soluble in water nor is it easily digested by human and animals • Enzymes, such as cellulase, have been use to breakdown cellulose to glucose Chemical composition of softwoods and hardwoods Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry www.csaw.utas.edu.au 2! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Hemicelluloses Lignin • Lignin is an encrusting substance that is laid down in the final phase of cell differentiation • It constitutes about one-third the weight of wood and imparts rigidity to the cell wall • Lignin occurs throughout the cell wall, being deposited in between the microfibrils, but is concentrated in the middle lamella • It is highly aromatic and is difficult to remove from wood without degrading it • Hemicelluloses acts as the connecting agents that bond the cellulose microfibrils together • Also built of sugar units, hemicelluloses are relatively low molecular weight, less crystalline, are made of shorter chains, and are branched • While some hemicelluloses can be removed by hot water, those bonded to the microfibrils can be extracted using cold dilute alkalies • The hemicelluloses of softwoods and hardwoods are not the same Those in softwoods are more complex, in both the number and structure of the hemicelluloses http://www.lbl.gov/Publications/YOS/Feb/ 19Dec10 Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Extractives The roles of extractives in wood • Extractives refer collectively to the various low molecular weight substances which can be separated from cellulose, hemicellulose and lignin using neutral organic solvents • They include a number of radically different chemical substances – The most widely used chemical classification considers the similarity in chemical structures, i.e., terpenoids, flavonoids, stilbenes, and lignans • Extractives found in the cell lumen can occur as a layer lining the cell walls or as hardened substances that fill the lumen www.csaw.utas.edu.au Extractives influence • color, odour, and taste • moisture and permeability • fluorescence • durability • flammability • workability – Those found in the cell walls occur in interstices or minute capillaries between the macroscopic components, or as deposits on the pits of both sapwood and heartwood Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Influence of extractives Influence of extractives • In species with significant amount of extractives, they can have significant effects on physical, chemical and some mechanical properties of wood (Kuo and Arganbright 1980) • In some species where extractives constitute less than 1%, they can contribute disproportionately to the characteristics of wood (Imamura 1989) – For example, extractives in amounts of only 0.05% or less were found to be responsible for the resistance of pines and western red cedar to fungal degrade and termite attack (Hillis 1962) Cell wall structure and wood chemistry www.csaw.utas.edu.au www.csaw.utas.edu.au • Extractive influence wood strength both directly and indirectly – Phenolic extractives can provide cross-linking sites for the cell polymers, and as such, contribute to wood strength directly • Extractives, specifically those found in the cell wall, contribute to the dimensional stability of wood They have a bulking effect – They can reduce the shrinkage of wood by occupying sites otherwise occupied by water Cell wall structure and wood chemistry www.csaw.utas.edu.au 3! KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Influence of extractives Influence of extractives Extractive can influence: • Drying Results can be contradictory – Meyer and Barton (1971) believe that the accumulation of extractives in the cell lumens during drying causes collapse in wood – Choong (1989) found that the occlusion of lumens with extractives may prevent or minimise collapse • Wood permeability and absorption during chemical treatment – Some extractives block flow paths and increase the contact angle between aqueous solutions and the cell walls This requires the use of higher pressures during chemical treatment with preservatives • The higher decay resistance of heartwood compared to sapwood in some species results from the presence of extractives that are toxic to fungi and other wood attacking organisms • The equilibrium moisture content and the sorption characteristics of wood Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its origin and characteristics Influence of extractives Summary • Cell wall ultra-structure • Water repellent extractives affect the wettability of wood surfaces, the application and penetration of adhesives and thus the strength of woodadhesives bonds • Some phenolic extractives can react with adhesives to retard polymerization and the reduce the strength of wood-glue bonds Cell wall structure and wood chemistry www.csaw.utas.edu.au – Layers, microfibril angle • Chemical components – Carbon, oxygen, hydrogen and others – Cellulose, hemicellulose, lignin, extractive • We have only covered the broad concepts – There is significant detail and variability in each of these areas www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics Cell wall structure and wood chemistry www.csaw.utas.edu.au KDA503/335 Timber, its origin and characteristics References • • • • • • • • • Cave, I.D 1969 The longitudinal Young's modulus of Pinups radiata Wood Science and Technology: 3:40-48 _ 1976 Modelling the structure of the softwood cell wall for computation of mechanical properties Wood Science and Technology 10:10-28 _ and J.C.F Walker 1994 Stiffness of wood in fast-grown plantation softwoods: The influence of microfibril angle Forest Products Journal 44(5):43-48 Ilic, H 2001 Rapid prediction of wood stiffness from microfibril angle and density Forest Products Journal March, 2001 Hillis, W.E 1971 Distribution, properties and formation of some wood extractives Wood Sci Technol 5: 272–289 Kalnins MA, and WC Feist (1993) Increase in wettability of wood with weathering For Prod Journal: 43(2):55–57 Kuo, M.L & D.G Arganbright 1980 Cellular distribution of extractives in redwood and incensecedar Part I Radial variation in cell-wall extractive content Holzforschung 34: 17–22 Mellerowicz, E., P Immerzeel and T hayashi 2008 Xyloglucan: The Molecular Muscle of Trees Ann Bot (2008) 102 (5): 659-665 doi: 10.1093/aob/mcn170 Ritter; GJ 1925 Distribution of Lignin in Wood Microscopical Study of Changes in Wood Structure upon Subjection to Standard Methods of Isolating Cellulose and Lignin Industrial and Engineering Chemistry 17(11):194-197 Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry www.csaw.utas.edu.au 4! ... of both sapwood and heartwood Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry KDA503/335 Timber, its origin and characteristics KDA503/335 Timber,... Chemical composition of softwoods and hardwoods Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry www.csaw.utas.edu.au 2! KDA503/335 Timber, its origin... characteristics of wood Cell wall structure and wood chemistry www.csaw.utas.edu.au Cell wall structure and wood chemistry KDA503/335 Timber, its origin and characteristics KDA503/335 Timber, its