Plant Fibre: Molecular Structure and Biomechanical Properties, of a Complex Living Material, Influencing Its Deconstruction towards a Biobased Composite

Abstract

Plant cell walls form an organic complex composite material that fulfils various functions. The hierarchical structure of this material is generated from the integration of its elementary components. This review provides an overview of wood as a composite material followed by its deconstruction into fibres that can then be incorporated into biobased composites. Firstly, the fibres are defined, and their various origins are discussed. Then, the organisation of cell walls and their components are described. The emphasis is on the molecular interactions of the cellulose microfibrils, lignin and hemicelluloses in planta. Hemicelluloses of diverse species and cell walls are described. Details of their organisation in the primary cell wall are provided, as understanding of the role of hemicellulose has recently evolved and is likely to affect our perception and future study of their secondary cell wall homologs. The importance of the presence of water on wood mechanical properties is also discussed. These sections provide the basis for understanding the molecular arrangements and interactions of the components and how they influence changes in fibre properties once isolated. A range of pulping processes can be used to individualise wood fibres, but these can cause damage to the fibres. Therefore, issues relating to fibre production are discussed along with the dispersion of wood fibres during extrusion. The final section explores various ways to improve fibres obtained from wood.

Keywords: biobased composites; biological material; cell wall; hemicellulose; molecular interactions; plant fibre; reinforced plastics.

Figure 1

Overview of wood; from a living tree to the molecular organisation and its deconstruction to produce a short fibre-reinforced polymer composite. WPC, wood polymer composite. This figure outlines the topics covered by this review: the organisation of the aerial part of a tree and its trunk; the arrangement of the fibres in the wood; the structure of the cell wall and its molecular organisation; the deconstruction of the wood, fibre individualisation and integration into a WPC. This scheme is not pertinent for monocotyledons.

Figure 2

Aspects of various types of fibre. (a) Softwood shive, r = ray cell; (b) softwood fibre (tracheid); (c) bast fibre; (d) bast fibre bundle; (e) hardwood fibre.

Figure 3

Origins of different types of fibres. The fibres extracted and used are represented in colour. Orange represents the primary growth and green the secondary growth. On the left-hand side, the gymnosperm (softwood) stem contains tracheids. The tracheids have the function of support and sap conduction. On the right-hand side, the angiosperm is divided into two categories. At the top is a hardwood with fibre and vessels. The fibres have mainly a support role, while the vessels are the principal sap conductive elements. The bottom part shows how stem and leaf are grouped in non-wood plants. Bast fibres are found in the stem, and monocotyledon fibres are found in the leaf (excluding bamboo). Both have a support role and are often found in bundles with other elements. The dotted circle represents a bundle of fibres that can be referred as ‘technical fibre’.

Figure 4

Average composition of stem/straw of lignocellulosic material: (a) hardwood; (b) softwood; (c) monocotyledon (wheat*/corn† straw). Values extracted from Demirbas [42].

Figure 5

Cell wall model for hardwood from the middle lamellae to the lumen. ML: mid-lamella; PW: primary wall; CML: compound mid lamella; SW: secondary cell wall; WL: warty layer. This scheme represents the cell wall corner between four cells after the secondary cell wall formation. It shows the arrangement and the distribution of the various components of the cell walls. The grey area is the CML separating the four cells. On the left-hand side, the angle of the cellulose microfibrils and the width of the various layers (S1, S2, S3 and CML) can be found. In the secondary cell wall, the hemicellulose is distributed alongside one or linking two cellulose microfibrils. Some microdomains of high density lignification can be found in the S2 layer. In the primary cell wall, the microfibrils are not oriented. The microfibrils are held together in xyloglucan hotspots, and some calcium-pectin complexes have a load-bearing function between different fibres. On the right-hand side is the percent distribution of the various constituents of the cell wall.ML: mid lamellae, PW: primary wall, CML: compound mid lamellae, SW: secondary cell wall, WL: warty layer.

Figure 6

Cellulose conformation. Conformation of: (a) cellulose Iα; (b) cellulose Iβ. The grey and white rectangles represent glucose molecules linked together by β-(1-4) linkages (< and >). Two glucose molecules are linked such that each glucose molecule is rotated 180° in relation to each other, represented by grey and white rectangles. These two 180° linked glucose molecules form a cellobiose unit, the repeat unit of cellulose.

Figure 7

Deposition of cellulose, hemicellulose and lignin in the cell wall. The rosette is a protein complex in the cell membrane producing the cellulose microfibrils. The rosette moves along the microtubules. Hemicelluloses are synthesised in the Golgi apparatus and transported towards the cell wall via vesicles. Hemicelluloses are adsorbed onto the cellulose or other hemicelluloses. Lignin monomers are transported through the cell membrane via an unknown mechanism (probably a membrane transporter). The monomers polymerise together; this process might be due to the actions of laccase and peroxidase enzymes or be a random phenomenon.

Figure 8

Percentage of hemicellulose and pectins in the primary and secondary cell wall of: (a) hardwood; (b) softwood; and (c) grasses. X represent the presence, but no quantitative data available [90,99,100]. CW: cell wall; GAX: glucuronoarabinoxylans; GGM: galactoglucomannans; GM: glucomannans; GX: glucuronoxylans; MLG: mixed linked glucan; XG: xyloglucan.

Figure 9

Xylan substitutions in hardwood and softwood. (a) Hardwood unevenly distributed substituents domain; every xylose has a rotation of 120° (three-fold screw). As the substitutions are uneven, the xylan cannot bind the cellulose chain made of glucose. (b) Hardwood evenly spaced substituents domain; every xylose has a rotation of 180° and all decorations face one side allowing the adsorption onto the cellulose. Unevenly and evenly distributed domains are probably on one xylan molecule of around 120 xylose residues. (c) Softwood xylan decoration; the glucuronic acids are every six xylose residues, and the arabinose are at plus two xylose residues from the glucuronic acid. This domain xylan molecule can be adsorbed onto the cellulose.

Figure 10

Properties of water in the cell wall. (a) A water layer impairs the separation of two polysaccharide elements through surface tension; (b) but promotes sliding when the polysaccharides are under shear pressure.

Figure 11

Adaptation of the ‘rip, slip and stick’ model of molecular translation in a lignocellulosic material. (a) On the initial hydrogen bonding conformation; (b) a force is applied causing the hydrogen bonded network to stretch; (c) then after the applied force has ceased, the hydrogen-bonding network reforms in the (new) most favourable position.

Figure 12

Stages of the pulping process: (a) chemical pulping; (b) mechanical pulping.