Nature’s binder material in wood
Lignin is a hard natural polymer, into which flexible cellulose fibers are embedded, thus forming a rigid, reinforced natural composite, wood. From a chemical point of view lignin is very far from a sharply defined substance. Its composition varies appreciably within the same plant and from one species to another.
An aromatic polymer
Lignin is rather a broad term for a number of complex aromatic polymers with varying compositions, occurring in wood, grasses and other plants. The stiff lignin polymers are built up from cross-linked chains of various phenols, 4-coumaryl alcohol, coniferyl alcohol, sinapyl alcohol, in varying proportions. Like in cellulose, the only elements involved are C (carbon), H (hydrogen) and O (oxygen), but unlike the cellulose macromolecules, lignin contains a high proportion of aromatic compounds (compounds containing benzene rings). A circumstance that complicates the chemical picture even more is that lignin and cellulose also occur in a combined form, as “lignocellulose”.
No nutritional value
In contrast to cellulose, which can be digested by ruminant animals, lignin is not digestible by animals, nor by humans. Stems and straws of grasses and other plants contain more lignin than the leaves, which reduces their nutritional value to domestic animals. Research indicates that certain white rot fungi are capable of selectively bio-degrading the lignin in agricultural wastes like straw. Such biological treatment would make the cellulose content of these waste products available to farm animals as low-cost feed.
A thermoplastic polymer
Paper consists mainly of cellulose fibers. The purpose of the pulp and paper industry is consequently to remove the 15-25% lignin contained in wood, thus freeing the desired cellulose fibers. This is accomplished by chemical means (the acidic “sulphite” method, and the alkaline “kraft”- or “sulphate” method) or mechanically, by grinding wood slabs against wet rotating grindstones. The mechanical method (used mainly for low-quality paper like newsprint) depends on the fact that lignin is a thermoplastic polymer, i.e. it softens at elevated temperatures. The frictional heat generated by the grinding softens the lignin and the cellulose fibers are then easily brushed loose.
The thermoplasticity of lignin is also utilized by carpenters, when they are bending pieces of wood (e.g. when making frames for boat hulls) into new shapes. The wood is first heated with steam or boiling water. The thermoplastic lignin turns soft and the wood can be bent into the desired shape. When the wood is cooled, then the lignin hardens again and the bent shape becomes permanent.
Burnt as fuel
Chemical pulping produces huge quantities of chemically modified, dissolved lignin. In theory such modified lignin solutions (e.g. lignosulfonate from the sulphite process) represent valuable raw materials for the chemical industry, as they contain a rich mixture of various aromatic compounds. In reality the demand for lignosulfonate as a chemical raw material is marginal – most of the dissolved lignin produced by the pulp and paper industry is simply burnt by the factories as fuel.
Encyclopedia of Chemical Technology (ed.: R. E. Kirk and D. F. Othmer)