Lignin hygroscopicity

Since many lignin preparations are hygroscopic, it is imperative to dry the lignin samples completely such as at 50 °C over phosphorous pentoxide (P205) (<20mmHg) for 48 h. The dried samples are kept in a desiccator over P205. 

Effect of Wood Species and Extractives. The sorption isotherms of all woods are generally similar in shape. However, there may be considerable variations among them with respect to the absolute values of hygroscopicity. This variation may be because of differences in the proportion of the primary wood constituents, such as cellulose, hemicellulose, and lignin in different woods or more importantly, because of differences in the kind and quantity of ex-... 

Some chemical reactions of the wood components involve functional groups that do not form part of the polymer chain and may have only a slight effect on some wood properties and may enhance some. For example, esterification or etherification of free hydroxyl groups in carbohydrates or lignin may reduce hygroscopicity, increase dimensional stability, and actually increase wood strength by reducing the equilibrium moisture content. 

Cellulose, the hemicelluloses, and lignin sorb moisture to different extents (Figure 1). The hemicelluloses are more hygroscopic than cellulose, which is more hygroscopic than lignin (5). This means that lignin (noncrystalline and probably totally accessible), the hemicelluloses (all noncrystalline and nearly totally accessible), the noncrystalline portion of cellulose, and the surfaces of the cellulose crystallites are responsible for moisture uptake by the wood cell wall. 

The plant cell walls of bast fibers consist mainly of cellulose, hemicellulose, and lignin. The uptake of water by a hygroscopic material as cellulose or hemicellulose is a hydration process involving accessible hydroxyl groups, or in this case -CH2OH groups of the host material. Water molecules absorbed by dry cellulose form a true cellulose hydrate, and the reaction is exothermic, which provides the driving force (Pizzi 1987). 

Woody biomass, either gymnosperms (softwoods) or angiosperms (hardwoods) is inherently anisotropic and hygroscopic it is porous material, with the porosity caused by the hollow fibers that make up the woody material. Any number of wood technology texts (see, for example, Haygreen and Bowyer [13]) describe the physical structure of woody biomass. The porosity or void volume of wood exists as macropores—tracheids, rays, resin canals and related structures useful in moving moisture and nutrients within the tree. Chemically, woody biomass is comprised of cellulose, the hemicelluloses, one or another type of lignin, and extractives such as pinoresinol, catechin, pinosylvin, and related compounds. 

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. 

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