Softwoods lignin

FIGURE 1.3 First step in lignin softwood polymerization. 

Witayakran, S., and Ragauskas, A.J. (2009) Modification of high-lignin softwood kraft pulp with laccase and amino acids. Enzyme Microb. Technol., 44 (3), 176-181. 

With adjustments in composition to compensate for the chemical features of each aromatic hydroxyl source, a wood binder formulated with hardwood lignin, softwood lignin, or phenol (67) will be deemed highly effective if it can be 1. formulated at lower cost, 2. applied with conventional equipment, 3, reacted under the same process conditions, and 4. an adhesive that is so strong that wood parts formed with it fail in the wood phase most of the time and not in the adhesive phase. 

The word lignin is derived from the Latin word lignum meaning wood. It is a main component of vascular plants. Indeed, lignin is second only to polysaccharides in natural abundance, contributing 24—33% and 19—28%, respectively, to dry wood weights of normal softwoods and temperate-2one hardwoods. 

Table 1. Types and Frequencies of Interunitary Linkages in Softwood and Hardwood Lignins (Number of Linkages per 100 Units)...

The polydispersity of softwood milled wood lignin, as measured by M /M = 2.5, is high compared with that of cellulose and its derivatives. 

Typical compositions for softwood and hardwood kraft blackhquors are shown in Table 10. Most commercial kraft lignins are sulfonated kraft lignins or lignin amines. A few nonsulfonated products are, however, available. 

The aromatic nature of lignin contrasts with the aliphatic stmcture of the carbohydrates and permits the selective use of electrophilic substitution reactions, eg, chlorination, sulfonation, or nitration. A portion of the phenoUc hydroxyl units, which are estimated to comprise 30 wt % of softwood lignin, are unsubstituted. In alkaline systems the ionized hydroxyl group is highly susceptible to oxidative reactions. 

Softwoods are generally more resistant to acids than are hardwoods because they have high lignin and low hemiceUulose contents. In general, heartwood is more resistant to acids than sapwood, probably because of heartwood s higher extractive content and slower movement of Hquid into the heartwood. For these reasons, the heartwood of certain conifers has been widely used in the chemical industry. 

The fuel properties of wood can be summarized by ultimate and proximate analyses and deterrnination of heating value. The analytical procedures are the same as those for coal, but with some modifications. Analytical results generally vary about as much within a species as they do between species, except that softwood species generally have a higher carbon content and higher heating values than hardwood species because of the presence of more lignin and resinous materials in softwood species (see Fuels from waste). 

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