Hemicelluloses properties

Derivatives of hemicellulose components have properties similar to the ceUulosic equivalents but modified by the effects of thek lower molecular weight, more extensive branching, labile constituents, and more heterogeneous nature. Acetates, ethers, carboxymethylxylan (184), and xylan—poly(sodium acrylate) (185) have been prepared. 

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

Recently, the alkah-soluble hemicelluloses of hardwood dissolving pulps have been investigated [28]. Their composition and molecular properties depended on the pulp origin and steeping conditions. The MGX of the j8-fraction from press lye had a low uronic acid content (ratio of MeGlcA to Xyl is about 1 20). The molecular weight of the hemicellulose fractions varied between 5000 and lOOOOg/mol. 

The number of reports about hemicelluloses that have been covered by this review indicates the significantly increased importance of all types of hemicelluloses as plant constituents and isolated polymers during the last decade. Attention has been paid not only to known hemicelluloses but also to the primary structure, physicochemical, physical, and various functional properties of hemicelluloses isolated from hitherto uninvestigated plants. The efforts to exploit a variety of plant as potential sources of hemicelluloses were pointed out particularly for agricultural crops, wood wastes, as well as for by-products of pulp and rayon fiber technologies. Many studies were devoted to characterize seed-storage hemicelluloses from plants that have been traditionally applied in food and medicine of many underdeveloped countries to find substitutes for imported commercial food giuns. 

Wood is a composite material that is made, up basically of a mixture of three main constituents, cellulose, hemicellulose, and lignin (see Textbox 54), all of them biopolymers synthesized by the plants, which differ from one another in composition and structure (see Textbox 58). The physical properties of any type of wood are determined by the nature of the tree in which the wood grows, as well as on the environmental conditions in which the tree grows. Some of the properties, such as the density of wood from different types of trees, are extremely variable, as can be appreciated from the values listed in Table 71. No distinctions as to the nature of a wood, whether it is a hardwood or a softwood, for example, can be drawn from the value of its specific gravity. 

In chemical pulping, a significant part of the hemicelluloses is dissolved from the fibres into the pulping liquor. The rest remains in the fibre or is adsorbed into it, significantly affecting the properties of the cellulose fibres or paper produced... 

Xylan has the general properties of insolubility in water, solubility in alkaline solutions, ease of acid hydrolysis, high negative optical rotation, and non-reducing action toward Fehling s solution. It can be placed in three general polysaccharide classes (1) pentosan, (2) glycan, and (3) hemicellulose. It is classed as a pentosan because it is principally a polymer of a pentose. It is by far the most abundant pentosan. 

The primary structural component of paper is cellulose but non-structural polysaccharides (hemicelluloses) and sometimes lignin may also be present in paper. The physical and mechanical properties of a sheet are, however, in large measure due to the cellulosic fibres. 

Fourth lesson - combination of different compounds in unique macrostructure provides unique performance properties. Starch is used extensively in nature to store carbon and energy. Starch is readily digested and must be protected from degradation by a resistant coating, for example, a seed (e.g. com, wheat or rice) or a skin (e.g. potato). Woody materials such as trees, soft plants and grasses are composed of a complex combination of aliphatic and aromatic compounds (cellulose, hemicellulose and lignin). 

Many of the physical, chemical and biological properties of wood can be understood by referring to the polymeric chemical constituents. In many cases of wood modification, these polymeric components are modified to some extent. The three structural polymeric components of the wood cell wall are cellulose, hemicellulose and lignin. There are many excellent texts describing the structure and function of these components, and only a brief account is given here. 

The surface wettability of heat-treated wood decreases due to a reduction in the hydroxyl content of the modified wood (Pdtrissans etal., 2003). There is a reduction in the water-sorption capacity, which is related to a reduction in the number of primary sorption sites (OH groups) within the wood cell wall, largely as a result of the removal/degradation of the hemicellulosic component. As remarked upon earlier, hygroscopic properties are strongly influenced by the treatment method employed. Podgorski etal. (2000) heated... 

Complex pyrolysis chemistry takes place in the conversion system of any conventional solid-fuel combustion system. The pyrolytic properties of biomass are controlled by the chemical composition of its major components, namely cellulose, hemicellulose, and lignin. Pyrolysis of these biopolymers proceeds through a series of complex, concurrent and consecutive reactions and provides a variety of products which can be divided into char, volatile (non-condensible) organic compounds (VOC), condensible organic compounds (tar), and permanent gases (water vapour, nitrogen oxides, carbon dioxide). The pyrolysis products should finally be completely oxidised in the combustion system (Figure 14). Emission problems arise as a consequence of bad control over the combustion system. 

Other abundant carbohydrates, such as hemicelluloses and pectin, are usually highly branched and thus not very suitable for fiber and film production. Hemicelluloses and some pectins are also acetylated in the native state, which makes them more resistant to enzymatic hydrolysis (20,21) and changes their solubility properties (9-77,75). Branching does not, however, preclude their utilization in such potentially large markets as thickeners and adhesives. Xylans, for example, show such a strong adhesion to cellulose fibers that they are very difficult to remove completely by both acidic and alkaline pulping processes (22). 

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