Lignin matrix material

Wood can be considered as a biological composite of hollow tubes of cellulose fibers held together by a lignin matrix gluing material. Liquids are transported up and down the trees through the tubular plumbing system. About 90 percent of the wood tissue of softwood trees consists of fiber tracheid cells for liquid conduction and support. The hardwood trees evolved after the softwoods and have specialized water conduction cells called vessels. 

As all pits develop in softwoods and hardwoods, a specialized pit membrane remains within the pit complex (Figure 19, D and E). This membrane is initially constructed from the compound middle lamella in all cases, but in its fully difierentiated state the membrane can differ considerably between various cell types, between softwoods and hardwoods, and to some extent even between different species (3). In hardwoods, pit membranes are observed to be thin and generally nonporous partitions of microfibrils, matrix materials, and lignin (Figure 20). Movement of liquids through the pit complex to an adjacent cell must therefore occur largely by diffusion rather than by free liquid translocation. Fortunately, hardwoods have an effective alternate mechanism for liquid movement, at least in the vertical direction, and that mechanism is the vessel system. 

Figure 2.8. (a) Hemicelluloses with long persistence lengths are relatively inflexible and can only snake between microfibrils, (b) Schematic assembly of polydiverse xylans and lignins during secondary wall formation of a hardwood, (c) Microfibrils cluster in arrays surrounded by wider regions of matrix material (schematic from spruce wood). 

Figure 4.62 One-electron oxidation of feruloyl groups. The radicals can couple at all possible spin sites, although (3-04 is the most frequent, g Lignin, the matrix material of plant cells, is an oxidative polymer of three related alcohols, i-coumaryl, coniferyl and sinapyl alcohol, formed by coupling of radicals at all possible spin sites. The feruloyl side-chains of pectin and hemicellulose can participate in this polymer formation, generating covalent links between polysaccharides and lignin.

Wood is a composite material made up of cellulose fibres embedded in a relatively hydrophobic lignin matrix (Fig 3.10). For a complete breakdown of wood the lignin matrix has to be removed ore at least made hydrophilic. 

Composites can occur naturally or may be synthetically produced. For example, wood is a naturally occurring, water-plasticized composite consisting of oriented cellulose fibers in a continuous, cross-linked matrix of lignin. With synthetically produced composites, the matrix may, for example, be a metal (an example of which is steel-reinforced concrete). Here, the discussion will be limited to composites with synthetic polymers as the matrix material. 

The concept of composite materials is ancient to combine different materials to produce a new material with performance and efficiency imattainable by the individual constituents. An example is adding straw to mud for building stronger mud walls. Some more recent examples, but before engineered materials became prominent, are steel rods in concrete, cement and asphalt mixed with sand, fiberglass in resin, etc. In nature, examples abound a palm leaf, cellulose fibers in a lignin matrix (wood), collagen fibers in an apatite matrix (bone), etc. 

Wood as a form of natural plant fiber is a composite material in which the cellulose fibers as reinforcing elements are embedded in the lignin matrix. It is used as fuel or as a construction material, for packaging, artworks, and paper. Lignins are aromatic amorphous oligomers of di- and trisubstituted phenyl propane units obtained from the wood as by-products of the pulp and paper industries by solvent extraction. Lignin can be used after fractionation as fillers or as antioxidants and can be modified by esterification [105,106],... 

Hemicelluloses are important cell wall components being associated with cellulose microfibrils and the lignin matrix. In hardwoods they constitute almost one third of the total organic material present with 0-acetyl-(4-0-methylgIucurono)xylan as the major component. 

The use of fibres and fabrics as additives to reinforce matrix materials in structures that are often referred to as composites goes back into prehistory, as in the use of straw to reinforce clay bricks. As usual, nature developed such structures first. Examples are wood (cellulosic fibres in a lignin matrix) and bone (collagen fibres in an inorganic matrix). A composite need not be based on fibres - it is a material or product formed by intimate combination of two or more distinct physical phases, so the sh ls of crustaceans (calcium carbonate in a chitin matrix) are composite structures. However, the word composite now commonly brings to mind structures consisting of fibres embedded in a matrix of some other material, whether plastic, ceramic or metal (i.e. fibrous composites), and even tends to be used particularly for structures in which the fibres are laid out in organized fashion before the matrix material is consolidated around them. 

Wood is a hard, fibrous tissue found in many plants. It is an organic natural material and can be treated as a natural composite of cellulosic fibers embedded in the lignin matrix. It is a heterogeneous, hygroscopic material composed of cells and cell walls. It contains 40-50% cellulose, 15-25% hemicellulose, and 15-30% Hgnin. 

Wood is composed of three major polymeric materials cellulose, hemicelluloses, and lignin. Cellulose is the common name used for the glucan present in wood, which constitutes about 42 percent of wood s dry weight. Cellulose is the primary component of the walls of cells making up wood fibers and is the main structural material of wood and other plants. Paper, paperboard, and other wood fiber products thus also are composed mostly of cellulose. The chemical structure of the cellulose macromolecule is shown in Fig. 7.4. In the plant the degree of polymerization (DP) of cellulose is approximately 14,000. Closely associated with cellulose in the wood structure and paper products are other polysaccharides called hemicelluloses, which often have been labeled as the matrix material... 

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