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] [polymer]s with varying compositions, occurring in [wood], [grass]es and other [plant]s. The stiff lignin polymers are built up from cross-linked chains of various [phenol]s, [4-coumaryl alcohol], [coniferyl alcohol], [sinapyl alcohol], in varying proportions. Like in [cellulose], the only [element]s involved are C ([carbon]), H ([hydrogen]) and O ([oxygen]), but unlike the cellulose [macromolecule]s, lignin contains a high proportion of aromatic compounds ([compound]s containing [benzene ring]s). 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 [digest]ed by [ruminant] animals, lignin is not digestible by animals, nor by [human]s. Stems and straws of grasses and other plants contain more lignin than the [leaf|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 [fiber]s. 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 [friction]al [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 [carpenter]s, 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 [solution]s (e.g. [lignosulfonate] from the sulphite process) represent valuable [raw material]s 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 [factory|factories] as [fuel].

Reference:

Encyclopedia of Chemical Technology (ed.: R. E. Kirk and D. F. Othmer)

Lignin (thing)