Wood Microstructure Ł a Cellular Composite

Wood microstructure – A cellular composite 1

M.P. Ansell

1.1 Introduction

The constituent materials for wood composites are by definition derived from trees and manufactured from a variety of wood products which include logs, sawn timber, strands, chips, fibre or nano-cellulose. It is therefore appropriate to begin a chapter on wood microstructure with an image of trees (Figure 1.1) before exploring the macro-, micro- and nano-scale features of wood. Timber (or lumber) refers generally to wood, harvested from trees, which has been converted into sawn wood at the sawmill and may be used for the manufacture of wood composites such as glue-laminated timber (glulam) and cross-laminated timber. Alternatively the bark from the tree may be removed and logs steamed to allow rotary peeling into veneer for the manufacture of plywood (Chapter 4) and laminated veneer lumber (LVL) (Chapter 6). A further option is to de-bark and process the log into strands, chips or fibre for the manufacture of oriented strandboard, chipboard (Chapter 6) and medium density fibreboard (MDF) (Chapter 5), respectively. The reinforcement for state-of-the-art wood composites may be comprised of nano-cellulose (Abdul Khalil et al., 2012; Lee et al., 2014) derived from the acid digestion of wood in order to exploit the very high-elastic modulus of cellulose. It is therefore essential to understand wood microstructure, as it has a fundamental influence on the properties of wood composites. A cross-section through a Douglas fir trunk is shown in Figure 1.2. The pith at the centre of the tree is surrounded by heartwood, which in turn is surrounded by the sapwood (collectively known as secondary xylem) and finally the vascular cambium where new wood is formed by cell division at the interface with the bark. The bark (secondary phloem and cork) expands as the tree lays down new cells at the phloem to xylem interface. The heartwood is a zone of inactive cellular tissue that has ceased to conduct water and it is often darker than the sapwood. Heartwood, sapwood and ray parenchyma cells (referred to hereafter as ray cells) are clearly seen in the cross-section of oak imaged in Figure 1.3. Starchy material is stored in the ray cells within the sapwood zone (Barnett and Jeronimidis, 2003). The stem (trunk) of temperate and coniferous trees consists of concentric annual rings, the first of which is located at the central pith and the age of the tree is indicated by the number of annual rings present. Within each annual ring the light-coloured concentric rings (Figure 1.2) are the earlywood, formed early in the growing season and the darker, denser concentric rings are the latewood. Tropical trees have no discernible annual rings as there is little seasonal variation in climate.

Figure 1.1 Avenue of mature weeping silver limes (Tilia tomentosa Petiolaris) at the University of Bath.

Figure 1.2 Cross-section of coniferous Douglas fir. Image supplied by Henri D. Grissino-Mayer, Dept. Geography, University of Tennessee, http://web.utk.edu/~grissino.

The principal directions and planes associated with the orthotropic structure of wood are labelled in Figure 1.4a, and a sketch diagram of a wood wedge (Figure 1.4b) illustrates the heartwood and sapwood, annual rings and the position of radial cells. Defects in timber include knots and splits and other features include resin canals (Dinwoodie, 2000) frequently observed in sawn softwood. Wood microstructure – A cellular composite 5

Figure 1.3 Stained cross-section of deciduous English oak (Quercus robur) with heartwood (darker inner zone), sapwood (lighter outer zone) and ray parenchyma (radial cells) clearly defined.

L Growth L TS direction R T Cambium Resin Sapwood canals (phloem) Heartwood TS R L Bark

RLS Pith RL

T R Radial cells Woody xylem Latewood Earlywood TLS Annual ring (a) (b) Figure 1.4 Sketch diagrams of (a) tree stem and (b) wood wedge (softwood). L = longitudinal, R = radial, T = tangential, TS = transverse section, RLS = radial–longitudinal section and TLS = tangential–longitudinal section.

1.2 Cellular microstructure

1.2.1 Softwoods and hardwoods Softwoods are made up of two types of cells, tracheids and rays (see Figures 1.5 and 15.1). The two major functions of tracheids involve supporting the mass of the tree and transporting water and mineral salts from the roots up the stem (Barnett and Bonham, 2004). Around 90% of cells in softwoods are tracheids, which are aligned parallel with 6 Wood Composites

L L

T R T R

25KV 1215 100 . 0U BATHU 25KV 1216 100 . 0U BATHU Figure 1.5 SEM images of Scots pine (Pinus sylvestris) softwood. (a) 3D sections with a complete annual ring within the cross-section and (b) tracheid width is ~30 μm and bordered pit openings are visible on the radial–longitudinal section. the trunk (longitudinally) and hence allow the vertical transportation of fluids whilst also acting as the primary structural elements. In contrast, ray cells are located in the radial–longitudinal (RL) plane and ensure radial movement of water and minerals between the tracheids (Dinwoodie, 2000) as well as storing starchy material. Rays are the sole means of translocating products of photosynthesis from the inner bark into the tree as well as storage. The pit openings imaged in Figure 1.5b allow the movement of moisture from tracheid to tracheid. Many of these openings are termed bordered pits and contain cellulose membranes which act as valves and control the passage of moisture in response to internal pressure (Choat et al., 2008). Bordered pits, whilst allowing bi-directional flow, act as stop valves when there are sudden differences in pressure which are caused by breaks and embolisms in the water column. Once closed, bordered pits do not re- open. The tracheids in softwoods range in length from 2 to 4 mm so the presence of pits is essential to allow water to pass from cell to cell as it passes from the ground to the leaves in the process of transpiration. Due to the thin walls and large lumen of the cellular material, earlywood is less dense than the latewood and is responsible for conducting water up the stem (Desch and Dinwoodie, 1996). The latewood is produced later in the season and due to its thicker walls and smaller lumens, it is responsible for supporting and strengthening the tree. The earlywood-to-latewood ratio within an annual ring is known to vary from year to year depending on the climate and growing conditions, in turn affecting the mechanical properties of the wood. In evolutionary terms, hardwoods are much younger than softwoods and their cellular structure is more complex (Figure 1.6). The longitudinal cellular elements include fibres and tracheids together with much larger vessels. In oak (Figure 1.6a), the vessels develop in the earlywood in concentric rings and the structure is termed ring porous. In species such as beech, the vessels are randomly distributed and the structure is termed diffuse porous. In hardwoods, vessel size in often a function of water potential. Large vessels soon form embolisms as water potential falls during the growing season, hence the need for formation of smaller vessels. Where water Wood microstructure – A cellular composite 7

L L R T T R

25KV 1209 100 . 0U BATHU 25KV 1213 100 . 0U BATHU Figure 1.6 SEM images of English oak (Quercus robur) hardwood. (a) 3D sections with a complete annual ring within the cross-section including large ring porous vessels and (b) cross-section, illustrating variation in the diameter of longitudinal cells, and tangential longitudinal section, containing a high proportion of medullary ray cells. potential is low, all trees have small diameter conducting elements. The radial cells occupy a much larger volume in hardwoods, sometimes as high as 50% (Figure 1.6b), but they perform the same role as in softwoods.

1.2.2 Structure of the wood cell wall

Softwood tracheids are approximately 30 μm wide and contain a number of distinct features including thin cell walls, pits and a distinct transition between earlywood and latewood. Some cells include internal helical thickening within the cell cavity which prevents the collapse of light weight thin cell walls under high suction during transpiration. All tracheids contain both a primary and secondary walls, with the secondary wall being split into three separate layers: S1, S2 and S3 (Figure 1.7). The thicknesses of the layers are typically 0.1–0.3 μm for the S1 layer, 1–5 μm for the S2 layer and 0.1 μm for both the S3 and primary layers (Mark, 1967). The middle lamella (ML), consisting mainly of lignin (amorphous oxyphenyl propane units), connects adjacent cell walls. There are also pectins in the ML which are polysaccharides (Whiting and Goring, 1982). A cross-section through Douglas fir earlywood is imaged in Figure 1.8. The section was cut with an ultra-microtome using a diamond knife and examined under the light microscope. The ML, primary wall and secondary wall can just be distinguished, but the magnification is not high enough to see the layers within the secondary wall. Three bordered pits are in the field of view and radial cells are present in the top left-hand corner of the image. When viewed in the transmission electron microscope (TEM), it is possible to distinguish the ML, primary (P) and secondary (S) cell wall layers (Figure 1.9a). An image of a longitudinal section (Figure 1.9b) reveals two S2 layers in a double cell wall, but it is not possible to identify the cellulose content which acts as a ‘fibre’ reinforcement in a ‘matrix’ of hemicelluloses and lignin. Cellulose is nature’s stiffest molecule with a value of ~138 GPa computed by Nishino et al. (1995). Microfibrils, comprising clumps of well-aligned cellulose with 8 Wood Composites

T P + ML R

S1+S2+S3

Approx. 30 µm

S3 Secondary S 2 wall S1

P Primary wall ML Middle lamella

Figure 1.7 Structure of softwood tracheids showing primary and secondary cell walls and orientation of cellulose microfibrils (black lines).

Figure 1.8 Ultrathin cross-section through Douglas fir earlywood (EW) examined by light microscopy. Wood microstructure – A cellular composite 9

S2 layer S3

S1 P

ML S2 S2 layer

1 µm 2.5 µm

Figure 1.9 (a) Cross-section through the junction between three EW tracheids in unstained Douglas fir. The cell wall layers are identified (scale bar = 1 μm). (b) Longitudinal section through double cell wall (scale bar = 0.5 μm) (Lacrosse, 2010). a typical diameter of 5–60 nm (Donaldson, 2007) and length of 1–3 μm (Abe and Yano, 2009), determine the mechanical properties of the wood (Plomion et al., 2001). The mechanical strength and stiffness of wood is strongly influenced by the thick S2 layer where the cellulose microfibrils, represented by the black lines in Figure 1.7, are disposed in a right-hand spiral (Z-helix) around the tracheid axis at an angle varying between 5° and 30° (Donaldson and Xu, 2005) within a matrix of hemicellulose, lignin and pectins. Within the S2 layer the angle of the spiral to the cell axis is termed the microfibril angle (MFA), and low MFA values result in high elastic modulus (Cave, 1968; Cave and Walker, 1994). The S1 and S3 layers have much thinner walls but provide a cru- cial role in strengthening the cell against lateral deformation and providing horizontal stiffness to the wood (Booker and Sell, 1998). As a result, the wood cell wall is com- monly modelled as a helically reinforced multi-layer composite (Mark, 1967). MFA measurements in the S2 layer have illustrated a decrease in MFA from the pith of the tree to the bark. Studies from the mid-1900s reported variability in MFA in conifers from one annual ring to another (Phillips, 1941) at different heights of the tree (Pillow et al., 1953) and within growth rings (Donaldson, 1992). Helical thickening may be present on the inside of the lumen, for example, in Douglas fir tracheids (Butterfield and Meylan, 1980), which complicate the measurement of MFA (Wang et al., 2001). The ability to measure the MFA within the S2 layer allows correlations to be made between MFA and the mechanical properties of the wood, in particular, the modulus of rupture and modulus of elasticity (MOE). Unfortunately, due to the nano-scale of microfibrils, they cannot be observed directly with a light microscope (Donaldson and Frankland, 2004). A wide range of techniques have been developed to measure MFA 10 Wood Composites

(Butterfield, 1998; Donaldson, 2008) and the most common methods include polar- ised light microscopy, iodine precipitation under a light microscope, X-ray diffraction and infrared spectroscopy. The results obtained from these techniques often differ within a given wood sample and most techniques do not provide enough magnification to obtain precise values of MFA (Ansell and Mwaikambo, 2009). Over the last 80 years, the iodine staining method has been adapted and improved, initially by Bailey and Vestal (1937) and in the last 30 years by Senft and Bendtsen (1984) and Donaldson and Frankland (2004). X-ray diffraction was a very popular technique towards the end of the twentieth century because it involved little prepara- tion time compared to previous methods. Its main disadvantages are the equipment cost and the overlapping reflections from conflicting planes (Cave and Robinson, 1998). The most advanced MFA measurement technique to date using X-ray diffraction is the SilviScan-3 system (CSIROpedia, 2014), a rapid non-destructive wood analysis system combining optical imaging, X-ray densitometry and X-ray diffractometry. In addition to MFA measurement, it provides a thorough overview of the properties of the wood sample, measuring properties such as density and both tracheid and growth ring dimensions. Bawcombe (2012) measured the MFA in Douglas fir grown in the south-west of England as a function of cambial age, sampling height and location using the SilviScan-3 system. He found a strong correlation between MFA and MOE in juvenile and mature wood, whereas flexural strength was more closely associated with density. In Figure 1.10, the density of a radial section from a Douglas fir stem is plotted as a function of distance from the pith with measurements taken at 25 μm intervals. There are approximately 34 annual rings within the section and across each annual ring, the density increases markedly from the earlywood to the latewood as expected.

1200

1000

) 800 −3 m

600

400 Density (kg

200

0 0255075 100 125 150 Distance from pith (mm) Figure 1.10 Typical density profile of Douglas fir as a function of distance from the pith (0–155 mm) from X-ray densitometry measurements in the SilviScan-3 (Bawcombe, 2012). Wood microstructure – A cellular composite 11

35 Mean early wood microfibril angle 1.3 m Mean early wood microfibril angle 8 m

) 25 (° e

gl 20 an

br il 15 crofi

Mi 10

0 246810121416182022242628303234363840 Cambial age (years) Figure 1.11 Mean earlywood microfibril angle as a function of cambial age at stem heights of 1.3 m (upper characteristic) and 8 m (lower characteristic) in Douglas fir from X-ray diffractometry measurements in the SilviScan-3 (Bawcombe, 2012).

The SilviScan-3 system is capable of measuring mean MFA as a function of cambial age (Figure 1.11). At 8 m in height from the ground the chracteristic is shorter because fewer annual rings have been laid down. At both 1.3 m and 8 m, the MFA decreases initially away from the pith and eventually reaches a fairly constant value (20° at 1.3 m) after about 10 years. The latewood (Figure 1.12) exhibits a similar

35 Mean late wood microfibril angle 1.3 m Mean late wood microfibril angle 8 m

) 25 (° e

gl 20 an

15 rofibr il

Mic 10

0 246810 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 Cambial age (years) Figure 1.12 Mean latewood microfibril angle as a function of cambial age at stem heights of 1.3 m (upper characteristic) and 8 m (lower characteristic) in Douglas fir from X-ray diffractometry measurements in the SilviScan-3 (Bawcombe, 2012). 12 Wood Composites

35 ) 30

15 Microfibril angle (° 10

0 0255075100 125150 Distance from pith (mm) Figure 1.13 Microfibril angle profile of Douglas fir as a function of distance from the pith (0–155 mm) from X-ray diffractometry measurements in the SilviScan-3 (Bawcombe, 2012). trend but at 1.3 m the mean MFA levels out at approximately 10°. Hence the denser latewood portion of each annual ring has a smaller MFA than the much less dense earlywood portion and therefore the latewood is expected to possess a much higher elastic modulus. A more precise MFA profile with measurements made at 200 μm intervals is plotted in Figure 1.13 as a function of distance from the pith at 8 m in stem height for approximately 32 annual rings. The SilviScan-3 system is able to measure the range of MFAs across an annual ring and beyond the first few annual rings, the MFA settles down to a range of from approximately 15° in the earlywood to close to 0° in the latewood. Feedstock materials for wood composite products include solid wood, veneer, strands, chips and fibres. Material derived from a single tree will possess a wide range of densities and MFAs, which will depend on location between the pith and bark, and which will determine mechanical stiffness and strength. Hence the randomisation of particles in products such as chipboard and MDF or the cross lamination of veneer products such as plywood and LVL will act to even out physical properties and reduce the variability encountered in solid wood.

1.2.3 Chemistry of the constituents of the wood cell wall Dinwoodie (2000) summarises the chemical composition of softwoods and hardwoods. The cellulose content (by mass) of softwoods (42 ± 2%) and hardwoods (45 ± 2%) represents the crystalline, macromolecular, microfibrillar component of the primary and secondary wood cell walls. The hemicellulose content (by mass) of softwoods (27 ± 2%) and hardwoods (30 ± 5%) is lower in molecular weight and is a semi-crystalline matrix component of the wood cell walls. The amorphous lignin content (by mass) of softwoods (28 ± 3%) and hardwoods (20 ± 4%) is the second major Wood microstructure – A cellular composite 13

H OH CH OH H OH 2 CH2OH O O O HO H H H OH OH H H H OH H H H OH H H H H H OH H H O O O O

CH2OH H OH CH2OH H OH n Figure 1.14 Cellulose molecule with repeat unit (in brackets) containing an oxygen linkage between two d-glucopyranose ring units. constituent of the matrix which surrounds the microfibrils. Dinwoodie states that the residual chemical components are soluble extractives, 3 ± 2% by mass in softwoods and 5 ± 4% by mass in hardwoods, which do not contribute to the multi-layer composite structure of the wood cell wall. Cellulose is an unbranched linear polysaccharide (natural carbohydrate) with a molecular repeat unit (Figure 1.14) containing two d-glucopyranose ring units (mono- saccharides) connected by β-1→4 glycosidic oxygen linkages (Marchessault and Sundararajan, 1983) allowing the molecular chains to bend and rotate. There are approximately 10,000 glucopyranose units in wood cellulose chains (Sjoström, 1981). During the formation of woody plant cell walls, living cells differentiate into tra- cheary cells (e.g. tracheids) with secondary thickening (Ansell and Mwaikambo, 2009). The living plant primary cell wall (Cosgrove, 1997) is lined with a plasma membrane and the plant cell contains microtubules which are aligned in the same orientation as cellulose in the primary cell wall (Ledbetter and Porter, 1963). Biosynthesis of woody cellulose is initiated by synthase enzyme complexes bound into the cell membrane and these synthases can be imaged as 25–30 nm diameter rosettes (Lerouxel et al., 2006). Vincent (1999) and Doblin et al. (2002) describe the process by which enzyme rosettes progress around the cell membrane depositing trails of cellulose microfibrils of the order of 5 nm in diameter containing around 40 cellulose molecules which aggregate into microfibrils with a diameter of about 20 nm. It is proposed that microfibrils are enclosed in a sheath of hemicellulose allowing them to align in the same manner as nematic liquid crystals (Neville, 1993). The organisation of microtubules is likely to determine microfibril length and microfibril orientation may be influenced by stimuli from the external environment (Wasteneys, 2004). An alternative model for microfibril deposition is postulated by Emons and Mulder (2000) who envisage microfibrils being spun from the primary cell wall guided and aligned by cortical microtubules in a mechanism analagous to liquid crystal formation. Hence axial, transverse, crossed, helical and random tex- tures may be laid down controlled by the synthase enzyme complexes in the cell membrane. Kondo et al. (2002) mimicked the natural deposition of plant cellulose by the biodirected epitaxial nanodeposition of polymers on oriented macromolecular templates. Biosynthesis and crystallisation of cellulose occurs in two forms, triclinic I and α monoclinic I (Kataoka and Kondo, 1996; Nishino, 2004). It is thought that the I β α phase in the secondary cell walls is strain-induced whereas when cell formation ceases 14 Wood Composites

(020)

Occluded Water

3 nm

(110) 86.5º 3 nm 7 nm 7 nm Figure 1.15 Section through a microfibril comprising four elementary fibrils of laminated crystallised cellulose I. Modified from Frey-Wyssling (1954). the stable strain-free I phase is favoured (Nishiyama et al., 2002). O’Sullivan (1997) β reviews the literature on the structure and orientation of plant cellulose. X-ray diffraction studies by Frey-Wyssling (1953, 1954), Tsekos et al. (1993) and others confirm that cellulose chains are disposed parallel to the (020) plane but the microfibril surface and the surface of the plasma membrane are parallel to the (110) lattice plane of the cellulose (O’Sullivan, 1997). Frey-Wyssling’s classic model of the fine structure of elementary fibrils with cross-sections of 3 nm by 7 nm surrounded by randomly oriented paracrystalline cellulose molecules is reproduced in slightly modified form in Figure 1.15. A microfibril is envisaged comprised of four elementary fibrils with overall dimensions of about 9 nm by 20 nm. Microfibrils in algal cellulose are reported to be flat ribbons of width 10–35 nm with thickness to width in the range 1:7 to 1:3 (Balashov and Preston, 1955). Estimates of the theoretical elastic modulus of the cellulose I molecule along the chain axis vary – for example, 167.5 GPa (Tashiro and Kobayashi, 1991) and 250 GPa (Vincent, 1999). However, experimental estimates are in the range 130–140 GPa (Eichhorn and Davies, 2006; Nishino et al., 1995). A typical softwood is highly porous due to the high volume fraction of cell lumens and has a density of the order of 500 kg m−3 and elastic modulus of the order of 10 GPa. However the predicted elastic modulus of the wood cell wall, which is a composite of cellulose microfibrils in a matrix of principally lignin and hemicellulose, is much higher (see Section 1.2.4) and the nano-cellulose extracted from wood fibre has the potential to be a highly effective reinforcement for polymer composites. Hemicellulose is the second most abundant polysaccharide in nature (Saha, 2003) next to cellulose. The main chain is comprised of d-xylopyranose units (Sun et al., 2001) which are joined by β-1,4 linkages to form either a linear or branched structure – for example, xylans, Figure 1.16. Timell (1967) reviews the chemistry of wood hemicelluloses and describes their role in the wood cell wall. In the secondary cell walls, hemicellulose is seen as a Wood microstructure – A cellular composite 15

−OOC O H3CO HO H3C O OH O O HO O O HO O O O O O O OH n O O OH

H3CO O OH

HO Figure 1.16 Example of xylan structure. The main chain is comprised of d-xylopyranose units joined by β-1,4 linkages. Author: Yikrazuul.

‘matrix’ phase in association with the the microfibrillar cellulose ‘reinforcement’ and lignin encrusts both the cellulose and the hemicellulose. Timell examines the chemical structure of softwoods and hardwoods and evidence for the distribution of wood polysaccharides in the cell wall is based on evidence from emerging chromatographic techniques. He speculates that hemicelluloses, which always occur in conjunction with lignin, may act as an interface between cellulose and the encrusting lignin. Kacurakova et al. (2000) identify hemicelluloses in plant cell walls including xyloglucans, xylans, glucomannans and galactoglucomannans using Fourier Transform Infra-Red spectroscopy (FT-IR). The IR absorption bands allow the structure and composition of these polysaccharides to be identified. Lignin in the wood cell wall protects the cellulose and hemicellulose polysaccharides from attack by enzymes and microbial organisms. It has a 3D polymeric structure comprising oxyphenol propane units free radical polymerised from cinnamyl alcohols. Lignin monomers may react with lignin polymers to either lengthen the polymer chain or form cross-links. Cellulose and hemicellulose are carbohydrates unlike lignin and the latter performs the key role of repelling water in plant cell walls (Thielemans et al., 2002). In the production of wood pulp for papermaking wood chips are treated with a solution of sodium hydroxide and sodium sulphide at high temperature and pressure to break down lignin and hemicelluloses, leaving largely cellulose from which paper is made. Fromm et al. (2003) determined the distribution of cell wall lignin in spruce and beech using TEM and backscattered scanning electron microscopy (SEM). The highest concentration of lignin was found in the ML and cell corners, and the lignin was closely associated with the orientation of the cellulose microfibrils in the wood cell walls. An overall picture of the structure of the wood cell wall as visualised by Fratzl and Weinkamer (2007) is presented in Figure 1.17. Their drawing of microfibrils in the S2 layer surrounded by a matrix of hemicellulose and lignin is related to a drawing of a softwood tracheid sourced from Fengel and Wegener (1989). 16 Wood Composites

~ 20 µm

S2 Hemicelluloses Ln reinforced with lignin

S1 L2 L1

Cellulose Fibrils

3 nm Figure 1.17 Softwood tracheid with cell wall layers (Fengel and Wegener, 1989) and structure of cell wall (Fratzl and Weinkamer, 2007).

1.2.4 Models for the composite structure of the wood cell wall Observations of the composite structure of the wood cell wall have led to the analysis of its elastic properties and the stress versus strain response of hollow cells and cell aggregates. The literature is extensive and some key papers are reviewed below which reflect progress in this field since the publication of early work, for example, Phillips (1941). Richard E. Mark’s famous text on the cell wall mechanics of tracheids (Mark, 1967) described methods for the analysis of stresses in cell walls in relation to the distribution of microfibrils in the primary and secondary cell walls. He performed tests on micro-tensile specimens of Juniperus virginiana L. which were 8–10 tracheids wide and approximately one or slightly more tracheid thick in the necked portion of the specimen. The RL specimens were comprised of earlywood tracheids. Failure loads were variable, but failure was initiated in the S1 layer by shear followed by rupture of the outer region of the secondary S1 and S2 cell walls by chain scission of primary bonds and/or cleavage of secondary bonds. Mark evaluated five parameters in order to perform a stress analysis, namely the net cross-section of cell walls, the proportion of primary (P) and secondary (S) cell walls in these sections, the proportion of cellulose and lignin/hemicellulose in these sections, the elastic properties of these components and the filament winding angles of microfibrils. He then used orthotropic elasticity theory to predict the mechanical response of cells walls to applied stress on the basis of assigning microfibril filament winding angles to the P, S1, S2 and S3 concentric cell wall layers. He calculated that maximum stresses developed in the cellulosic framework of the tangential wall of the S2 layer. However, Mark emphasises that the cross-laminated microfibrils in the S1 layer performed a vital role in providing a high shear modulus of rigidity. Wood microstructure – A cellular composite 17

Cave (1968) evaluated elastic stiffness constants for the wood cell wall by treating it as a two-phase fibre composite where MFA was represented as a gaussian distribution or series of gaussian distributions. He used these elastic constants to determine the longitudinal Young’s modulus of earlywood cells in Pinus radiata and compared theoretical values with experimental results with good agreement. Bodig and Jayne (1982) discuss the molecular organisation of microfibrils in a two-phase fibre/matrix model of the wood cell wall resulting in an equation for the relationship between stress and strain for the cell wall subjected to plane strain. Hollow cylinder models for single wood cells complicate the analysis because shear coupling of cell wall layers, with different thicknesses and microfibril orientations, occurs. A model is required with asymmetric, anisotropic cell wall layers. However as a first approximation the consideration of the S2 layer alone is usually sufficient because of the high proportion of the cell wall occupied by this layer (Fengel, 1969). Harrington et al. (1998) determined elastic constants for secondary cell wall layers (S1, S2, S3) and the compound ML (P + M) of a softwood using a sequential nano- structural homogenisation procedure. Nine elastic constants for each wall layer were determined including the elastic moduli (Et, Er, El), Poisson’s ratios (νrt, νlt, νlr) and shear moduli (Grl, Glt, Gtr) at a moisture content of 12% where t = tangential, r = radial, l = longitudinal. Good agreement was found with measured data taken from the literature (Astley et al., 1998). Bergander and Salmén (2002) modelled the symmetric double wood cell wall comprising the S3, S2, S1, P, M, P, S1, S2 and S3 layers. Each layer was separated into two separate layers of polysaccharides (cellulose microfibrils in a matrix of hemicellulose) and lignin. Elastic constants for the lignin, hemicellulose and cellulose were selected at a moisture content of 12% and different properties were modelled for cellulose, hemicellulose and lignin. The cellulose and matrix components were coupled using Halpin–Tsai equations and properties in the longitudinal and radial orientations were computed as a function of MFA – for example, Figure 1.18. The elastic constants of the cellulose dominated the elastic properties of the double cell wall in the longitudinal direction. In contrast, the transverse cell wall modulus was dominated by the properties of the hemicellulose and lignin and the S1 layer plays a key role in determining transverse properties. Burgert (2006) reviewed the micromechanical design of wood cells by considering the role of cellulose, hemicelluloses and lignin in the cell wall layers. Micromechanical testing of individual cells in tension was reported and the role of microindentation in determining the properties of cell wall materials with high resolution. Microtensile tests were combined with X-ray diffraction to follow changes in MFA under load and with Raman spectroscopy to follow stretching of cellulose microfibrils. The paper con- cludes by stating the value of micromechanical tests in understanding the structure– fiunction relationship of plants. A thorough review of mathematical modelling of the cellular mechanics of plants is provided by Bruce (2003), who also calculated the mechanical properties of cell walls from measurements on plant tissue (Hepworth and Bruce, 2000). 18 Wood Composites

105 )

104 Model 1: Lignin A, hemicellulose A Model 1: Lignin A, hemicellulose A Model 1: Lignin A, hemicellulose C Model 2: Lignin A, hemicellulose C Experimental data Picea abies [3]

Longitudinal modulus (M Pa Experimental data Pinus radiata [21]

010 20 30 40 50 Fibril angle of the S2 layer (°) Figure 1.18 Longitudinal modulus versus S2 microfibril angle for the wood cell wall showing the relative insensitivity to changes in the elastic moduli of hemicellulose and lignin (Bergander and Salmén, 2002).

The structure of the wood cell wall was physically modelled by Gordon and Jeronimidis (1980), who simulated the helical disposition of microfibrils in the secondary cell wall by winding resin-impregnated glass or carbon fibres into helically reinforced tubes at several winding angles between 10° and 35°. These reinforced ‘fibres’ demonstrated tension buckling as observed in single wood cells and were able to absorb maximum amount of energy at a winding angle of 15°. Arrays of these model fibres bonded together into notched beams possessed a high work of fracture of up to 5.105 J m−2 at a winding angle of 15°.

1.3 Moisture and the composite wood cell wall

Trees have to function as living organisms so their structure and development is inextricably associated with water. As a result, sawn wood must be dried to a moisture content that is in equilibrium with its end-use environment and which eliminates problems associated with bio-degradation. The moisture content of wood has a profound effect on mechanical properties of the wood cell wall and hence the macroscopic properties of the wood. The swelling and shrinkage of wood is described by Siau (1984) in his classic monograph on transport processes in wood. Moisture in wood can be classed as bound water in the wood cell wall and unbound water in the cell lumens. As wood containing unbound water is dried in an oven or kiln the fibre saturation point is reached at between 28% and 32% moisture content, based on dry weight, below which moisture is removed from the cell wall until the wood is oven dry. Water is thought to penetrate amorphous regions of the cell and bond Wood microstructure – A cellular composite 19 with free hydroxyl groups by hydrogen bonding (Yamamoto et al., 2002). Changes in moisture content result in much greater expansion of wood transverse to the cell axis compared to expansion along the cell axis. The classic Barber and Meylan (1964) model considered the cellulose content (microfibrils) of the wood cell wall to be stiff and unreactive whereas the remaining substances (lignin–hemicellulose matrix) were water-reactive and compliant. Cave (1978a,b) modelled the multi-layer cell wall in terms of these two phases in order to quantify swelling and shrinkage resulting from changes in bound water. He assessed the swelling and shrinkage characteristics, individually, of cellulose, lignin and hemicellulose and concluded that changes in stiffnesses of these components corresponded to change in dimensions and Young’s modulus of wood as a function of MFA. Cave’s model enabled deformation from internal moisture changes and deformation from external stresses to be unified in one model. Yamamoto et al. (2002) reviewed progress in refining cell wall models since the work of Cave. They considered tree growth stress, anisotropy of shrinkage and swelling in wood and the relationship between moisture content and the elastic modulus along the grain using a two-phase model. Qing and Mishnaevsky (2009) also presented a computational model which characterised the hydro- elastic and shrinkage properties of softwood latewood including the influence of moisture, density and microstructure on stiffness properties. They conclude that MFA influences longitudinal properties of wood but has negligible influence on transverse properties. With respect to moisture changes, shrinkage coefficients in the longitudinal direction are approximately one-tenth of those in the transverse direction.

1.4 Orthotropic properties of wood

1.4.1 Elastic properties of wood As indicated in Figures 1.4–1.7, three major axes – longitudinal (L), radial (R) and tangential (T) – describe the coordinate axes of wood. The designation of the axes makes sense if the whole tree trunk (Figure 1.4) or a small incremental cube of wood (Figure 1.7) is considered, with structural features defined by the axes. The whole tree is considered to be orthotropic in that the longitudinal direction is a single axis of anisotropy with optimum properties determined principally by the microfibrils in the S2 cell wall. All radial directions have lateral symmetry and mechanical properties are inferior to those in the longitudinal direction. CES EduPack software allows longitudinal (l) and ‘transverse’ (t) moduli to be plotted as a function of density (Figure 1.19) but the precise orientation of ‘transverse’ properties is not specified. There is a clear, approximately linear relationship between modulus and density for both the longitudinal and ‘transverse’ orientations. The oval fields represent the range of values for Young’s modulus and density at 12% moisture content. 20 Wood Composites

Beech (Fagus sylvatica) (l) Greenheart (l) Douglas fir (Pseudotsuga menziesii (coastal)) (l) Ash (Fraxinus excelsior) (l) 20 Fir (Abies alba) (l) Pine (Pinus sylvestris) (l) Keruing (l) Lignumvitae (l) Lignumvitae (t)

Balsa (l) (ld) 10 Ebony (l) Ash (Fraxinus excelsior) (l) Chestnut (l) Hemlock (l) (ld) Iroko (l) Greenheart (t) 5 Ebony (t) s (GPa)

lu Cedar (Thuja occidentalis) (l) Oak (Quercus robur) (t) Keruing (t) du Beech (Fagus sylvatica) (t)

mo 2

Douglas fir (Pseudotsuga menziesii (coastal)) (t) Ash (Fraxinus excelsior) (t)

un g’s Iroko (t) 1

Yo Chestnut (t) Pine (Pinus sylvestris) (t) Balsa (t) (ld) 0.5 Cedar (Thuja occidentalis) (t) Spruce (Picea sitchensis) (t)

0.2 300 400500 600700 800900 1000 110012001300 Density (kg m−3) Figure 1.19 Young’s modulus (GPa) versus density (kg m−3) for selected softwood and hardwood species in longitudinal (l) and transverse (t) directions.

Bodig and Jayne (1982) presented the matrix relationship between strain and stress linked by elastic compliances in terms of the orthogonal L, R and T axes for a small cube of wood stressed in three dimensions as follows:

é 1 --nn ù RL TLL 000 ê EEE ú ê L R T ú ég L ù és L ù ê ú ê--nn1 ú ê ú ê LR TR 000ú êg ú EEE ês ú ê R ú ê LRT ú ê R ú ê ú ê --nn 1 ú ê ú ê LT RT 000ú êg T ú ês T ú ê EEL RTE ú ê ú = · ê ú ê ú ê 1 ú ê ú êg RT ú ê 000 00ú êss RT ú ê ú ê GRT ú ê ú ê ú êg ú 1 ês ú ê LT ú ê 0000 0 ú ê LT ú ê ú ê ú GLLT ê ú êg ú ê ú ês ú ë LR û 1 ë LR û ê 00000 ú ê ú (1.1) ë GLR û

The terms in the left-hand side 1 × 6 column matrix are the strains (γ) which results from the application of the right-hand side 1 × 6 column matrix of stresses

(σ) linked by a 6 × 6 matrix of compliances. γL, γR and γT are the strains normal to Wood microstructure – A cellular composite 21

the cube face in the L, R and T directions, respectively. γRT, γLT and γLR are the shear strains which act on the cube face, for example, γRT acts on the R face (perpendicu- lar to the R axis) in the T direction. The stresses use the same subscripted notation as the strains. The moduli of elasticity EL, ER and ET correspond to the L, R and T axes, respectively, and the shear moduli GRT, GLT and GLR link the shear strains to the corresponding shear stresses. There are six Poisson’s ratios required for wood. Due to the symmetry of the compliance matrix of orthotropic wood, the following relationships are expected:

nnLR RL nnLT TL nnRT TR ===,, (1.2) EEL R EEL T EER T

In practice, these relationships hold quite well but values of νRL and νTL are relatively small so errors may be expected in computing elastic parameters from Equation (1.2). Many factors influence the elastic properties of wood species including density and moisture content, but the following approximate relationships between elastic moduli are proposed by Bodig and Jayne (1982) as follows:

EELR::ET » 20 :.16:1

GGLR:: LT GRT » 10 :.94:1 (1.3) EGLL::R » 14 1

Values for the elastic parameters of wood species are listed by Hearmon (1948), Bodig and Jayne (1982), Dinwoodie (2000) and Forest Products Laboratory (2010). The reader should note that the subscript notation used by these authors differs, but the notation of Bodig and Jayne has been used here. The anisotropy of wood is a function of the cellular structure of wood and the disposition of cellulose in the cell wall. The incentive to produce flat panel products for furniture and construction applications led to the development of plywood, the earliest examples of which were discovered in ancient Egypt – for example, a Third Dynasty coffin in the step pyramid at Saqqara (Lucas and Harris, 1962). If a single layer of peeled veneer is flattened the plane of the veneer is longitudinal–tangential with the radial direction out of plane. For a symmetric bonded double layer of veneer under biaxial stress, the 2D relationship between applied in-plane stresses and resulting in- plane strains is simplified from Equation (1.1) as follows:

é 1 -n TL ùù ê 0 ú ég L ù EE és L ù ê ú ê L T ú ê ú ê ú ê ú -n LT 1 ê ú g = ê 0 ú· s ê T ú EE ê T ú ê ú ê LT ú ê ú ê ú ê 1 ú ê ú ëg LT û ës LT û ê 00 ú (1.4) ëê GLT ûú 22 Wood Composites

A balanced cross-lamination of veneers will modify the compliance terms in the 3 × 3 matrix but ensures that in-plane elastic properties are more isotropic.

1.4.2 Strength of wood The strength of wood depends on the loading mode in tension, compression, shear or bending. Data within CES EduPack software enables the longitudinal (l) and ‘transverse’ (t) tensile strength of the same species selected in Figure 1.19 to be plotted as a function of density (Figure 1.20). There is an approximately linear relationship between tensile strength and density for both the longitudinal and ‘transverse’ orientations.

1.4.3 Advantages of wood composites Sawn wood is an orthotropic material with planar orientation defined by longitudinal, radial and tangential axes. Elastic properties are controlled by cellular microstructure and the nanostructure of the cell walls such that properties are maximised in the longitudinal direction. For solid wood, the 3D relationship between measured strain and applied stress depends on a matrix of elastic compliances. In 2D wood composite products such as wood veneer and plywood, a simpler relationship is required with fewer elastic constants as described above. The strength of wood is also greatest along the grain and fracture modes are usually related to planes of weakness, which may be eliminated by the structure of wood composites. Shrinkage of solid wood is also

Ash (Fraxinus excelsior) (l) Greenheart (l) Douglas fir (Pseudotsuga menziesii (coastal)) (l) Pine (Pinus sylvestris) (l) Fir (Abies alba) (l)

100 Spruce (Picea sitchensis) (l) Keruing (l) Lignumvitae (l) Ebony (l) Balsa (l) (ld) Chestnut (l) Beech (Fagus sylvatica) (l) Hemlock (l) (ld) Oak (Quercus robur) (l) (MPa) Cedar (Thuja occidentalis) (l)

th Iroko (l)

Greenheart (t) reng

st Ash (Fraxinus excelsior) (t) Keruing (t) Lignumvitae (t) 10 ile Fir (Abies alba) (t) Oak (Quercus robur) (t) Pine (Pinus sylvestris) (t)

Tens Oak (Quercus robur) (t) Chestnut (t) Hemlock (t) (ld)

Iroko (t) Ebony (t) Cedar (Thuja occidentalis) (t) Beech (Fagus sylvatica) (t) Balsa (t) (ld) Spruce (Picea sitchensis) (t)

Douglas fir (Pseudotsuga menziesii (coastal)) (t) 1

300 400 500 600 700 800 900 1000 1100 12001300 Density (kg m−3) Figure 1.20 Tensile strength (MPa) versus density (kg m−3) for selected softwood and hardwood species in longitudinal (l) and transverse (t) directions. Wood microstructure – A cellular composite 23 anisotropic and is a function of dimensional changes in the wood cell wall. There are therefore advantages in manufacturing wood composite products such as plywood and panel products based on fibres, chips or strands which are more isotropic and more dimensionally stable.

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