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 im- aged 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 con- centric 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. Wood Composites. http://dx.doi.org/10.1016/B978-1-78242-454-3.00001-9 Copyright © 2015 Elsevier Ltd. All rights reserved. 4 Wood Composites 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 cel- lulose 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 transpi- ration. 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 mid- dle lamella (ML), consisting mainly of lignin (amorphous oxyphenyl propane units), connects adjacent cell walls. There are also pectins in the ML which are polysaccha- rides (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 second- ary 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, pri- mary (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.