Lignin hemicellulose matrix

In ARP pretreatment of hybrid poplar, deUgnification is increased with the treatment severity, but xylan removal occurs only to a certain extent. Increase of temperature from 175 to 195 °C in ARP does not affect xylan/lignin removal in hybrid poplar, yet the glucan digestibility of treated solid is increased. This is caused by an increase of cellulase accessibility to cellulose created by breakage of certain bonds and reconfiguration of the components in hemicellulose-lignin matrix. Crystallinity index of hybrid poplar increases after the ARP treatment. It is primarily caused by the removal of amorphous components,... 

The main components of wood are cellulose, hemicelluloses, lignin, proteins, and small molecules including several inorganic compounds. The pyrolysis products of each individuai type of macromolecular component of wood were described in Part 2 of this book. Some differences are seen in the pyrolysis of pure macromolecular components as compared to the pyrolysis results of the same component from the wood matrix. This is seen mainly for cellulose and hemicelluloses. However, the pyrolysis components are well recognizable, and it is common to use wood pyrolysates to trace a certain group of compounds to celiulose, hemicellulose, lignin, small molecules existent in wood, etc. 

Within the various layers of wood cell walls, the hemicelluloses, lignin, extraneous and mineral constituents are concentrated in the spaces between microfibrils or elementary fibrils. The hemicelluloses and lignin form a matrix surrounding the cellulose. Within a given microfibril, lignin and the hemicelluloses may penetrate the spaces between cellulose molecules in the amorphous regions. 

The authors stated (8) that the middle lamella and the matrix of the cell wall is effected by chemical changes such as oxidation and hydrolysis to a higher extent than the cellulose microfibrils. Ultrastructural changes would then be expected to occur in areas rich in hemicelluloses, lignin, or pectic substances. Later, Borgin et al. (10) found that the lignin in the ancient wood samples had been oxidized. A decrease in lignin content was evident when the samples were analyzed by a hydrochloric acid procedure. 

Although the internal cell wall structures of diflFerent woods are not well understood, a variety of models have been proposed (4). These models vary in detail, but all have the cellulose fibrils surrounded by hemicellulose, which in turn is embedded within a lignin matrix. Hemicellulose apparently occurs as an interface between lignin and cellulose, but is covalently bonded only to lignin (2). Apparently cellulose and hemicellulose form before lignin is added in a space-filling manner (4). 

Lignin is a very complex molecule that consists of different types of phenolic monomers such as p-coumaryl, coniferyl and synapyl alcohols. It fills the spaces in the cell wall between the cellulose and the hemicelluloses matrix and is tightly bound to them via hydrogen and covalent bonds. This improves the plant s rigidity and compactness as well as its resistance against microbial attack (by restricting enzymatic access to the cellulose and hemicellulose). Lignin is amorphous with no fixed structure and therefore can only be represented by a hypothetical formula. 

Wang et al. [54] studied the effect of WG on the degumming process of jute fiber in order to improve the fiber properties. It was found that the WG concentration, sodium hydroxide concentration, and treatment time were the three most important parameters for the degumming process. The authors concluded that the degumming process was an effective method for ranoving hemicellulose, lignin, pectin, and certain other noncellulose materials. WG or alkali treatments depend on several variables such as the concentration of the alkaline solution, temperature, and the duration of the treatment. These variables directly affect the adhesion between the fiber and the matrix, and consequently, they also affect the mechanical and thermal properties of the fiber-reinforced composites. 

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. 

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. 

Fig. 26.3. The molecular structure of a cell wall. It is a fibre-reinforced composite (cellulose fibres in o matrix of hemicellulose and lignin).

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