What is Lignin?
Lignin or lignen is a complex chemical compound most commonly derived from wood and an integral part of the secondary cell walls of plants. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood. It is one of the most abundant organic polymers on Earth, superseded only by cellulose, employing 30% of non-fossil organic carbon and constituting from a quarter to a third of the dry mass of wood. As a biopolymer, lignin is unusual because of its heterogeneity and lack of a defined primary structure.
Complex Lignin Structure
Lignin is formed by removal of water from sugars to create aromatic structures. These reactions are not reversible. There are many possible monomers of lignin, and the types and proportions depend on the source in nature. Some typical monomers are shown in the sketch:
The OH groups (either the alcoholic OH's on the chains or the phenolic OH's on the aromatic rings) can react with each other or with the aldehyde or ketone groups. When an OH reacts with another, an ether linkage is formed. As we have seen, an OH reacts with an aldehyde to form a hemiacetal. The reactions of OH groups with ketones forms ketals. An early stage in the condensation of various monomers to form lignin is shown in the next sketch:
There are several groups shown in red that can react further. Some will simply extend the polymer while others would establish cross linking. The monomer that is shaded in orange has three of its functional groups linked to other monomers, so it is starting a branch or cross link. The large lignin molecules fill three dimensions and are heavily cross linked. Sometimes lignin is isolated as a brown powder, but more often it is a gummy mixture of lignins with a wide range of molecular weights.
The Nature of Lignin
In nature it is very resistant to degradation, being held together with strong chemical bonds; it also appears to have a lot of internal H bonds. It is bonded in complex and various ways to carbohydrates (hemicelluloses) in wood. This picture of usefulness and stability presents quite a contrast to the familiar lignin in ground wood paper, which is so unstable and so troublesome in books and records of value. The contrast can be explained by the radical effect of pulping and bleaching on the lignin as it is separated from the fibers.
Lignin is actually not one compound but many. All are complex, amorphous, three-dimensional polymers that have in common a phenyl propane structure, that is, a benzene ring with a tail of three carbons. In their natural unprocessed form, they are so complex that none of them has ever been completely described, and they have molecular weights that my reach 15,000 or more.
Lignins are not acids, though most of them contain certain carboxylic acids, and wood gives off acids as it deteriorates, as do paper and board that contain lignin. This deteriorates cellulose and other sensitive materials nearby, as well as the cellulose fibers within the lignin-containing paper itself. This is why permanent paper standards in the past have always specified that no ground wood or unbleached fiber should be used to make the paper. Recently, though, it has been recognized that calcium carbonate filler would effectively cancel this destructive effect of lignin for an indefinite period of time, and the current draft of the ANSI Z39.48 standard permits as much as 7.5% lignin.