Lignin
These polymers are also constituents of wood (about 25–30%) [18, 19]. It is uncertain what the molecular weights of the polymers are as the materials are quite complex in structure. The extraction processes of lignin result in considerable loss of molecular weights. The structures of lignin vary, depending upon the source. Generally, they are considered to be polymers of coniferyl alcohol:
An idealized picture of lignin that formed from coniferyl alcohol was published by Freudenberg [20]. It appears reasonable, however, that some lignin's might, perhaps, also form in different trees, from other compounds not coniferyl alcohol, but related to it and also found in woods. Also, it appears plausible that several of such compounds, including coniferyl alcohol, actually participate together in lignin formation, depending upon the species of wood. Figure 8.1 shows the chemical structures of some of these compounds. Many attempts were made to convert lignin to a useful material for coatings and adhesives. Only very limited success, however, has been achieved. A reaction product with formaldehyde can be used as a wood adhesive. In addition, lignin obtained from wood pulping by the sulfate process (as a sulfonate) has been utilized to a limited extent as an asphalt extender and as an oil-well drilling mud additive.
Fig. 8.1 Compounds that occur in various woods and might, perhaps, participate in formation of lignin
