Wood Composites

Martin P Ansell

• Publisher: Woodhead Publishing
• Year: 2015
• ISBN:
  9781782424772
• Topic: Technologie et ingénierie
• Subtopic: Science des matériaux

Recent progress in enhancing and refining the performance and properties of wood composites by chemical and thermal modification and the application of smart multi-functional coatings have made them a particular area of interest for researchers. Wood Composites comprehensively reviews the whole field of wood composites, with particular focus on their materials, applications and engineering and scientific advances, including solutions inspired biomimetrically by the structure of wood and wood composites.

Part One covers the materials used for wood composites and examines wood microstructure, and wood processing and adhesives for wood composites. Part Two explores the many applications of wood composites, for example plywood, fibreboard, chipboard, glulam, cross-laminated timber, I-beams and wood-polymer composites. The final part investigates advances in wood composites and looks at the preservation and modification of wood composites, environmental impacts and legislative obligations, nano-coatings and plasma treatment, biomimetic composite materials, the integration of wood composites with other materials and carbonized and mineralized wood composites.

• Comprehensively reviews the entire field of wood composites in a single volume
• Examines recent progress in enhancing and refining the performance and properties of wood composites by chemical and thermal modification and the application of smart multi-functional coatings
• Explores the range of wood composites, including both new and traditional products

Part One

Materials for wood composites

1

Wood microstructure – A cellular composite

M.P. Ansell

Abstract

Wood possesses a cellular, three-dimensional microstructure and is described as a natural composite material with orthotropic elastic properties. The mechanical properties of the wood cell wall, comprised of primary and secondary cell wall layers, are dictated by the orientation of stiff cellulose microfibrils in a matrix of hemicellulose and lignin. The hemicelluloses are closely associated with the cellulose microfibrils and these carbohydrate polymers are encrusted with non-carbohydrate lignin, affording protection from moisture and microbial organisms. The orientation of microfibrils has a direct influence on elastic properties of the wood cell wall and varies as a function of position in the tree and within annual rings. Numerous models of the wood cell wall have explored this relationship. Bound water content in the cell wall also affects mechanical properties together with swelling and shrinkage. The chapter concludes by presenting expanded forms of Hooke’s law for both wood (3D) and balanced veneers (2D), which are directly related to the wood composite microstructure.

Keywords

Wood microstructure

Microfibril angle

Chemical composition

Modelling the wood cell wall

Elastic properties of wood.

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.

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

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.

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 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).

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.

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.

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.

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 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.

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 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 reopen. 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 strengthenin...