Lignin is the stuff that makes trees “woody.” Usually constituting from one-fifth to one-third of wood, lignin strongly influences its chemical and physical properties. A major use of wood is for the production of pulp for paper and paperboard products. The residual lignin in pulp fibers greatly affects paper properties and, therefore, the uses of these pulps.
Lignin is a rather unusual substance. Polymerized through radical coupling of propenylphenols, it forms several types of interunit bonds and a three-dimensional network may result. The lignin of gymnosperms (softwoods) is made up mostly of a single monomer type. As a result, the lignins of gymnosperms do not differ much from species to species. However, the lignins of angiosperms (hardwoods) do vary considerably among species because these lignins are derived from two monomer types which are often present in differing proportions. Furthermore, the ratio of monomer types may vary among different cell types and between cell regions within a species.
This review, intended mainly for those unfamiliar with the details of lignin chemistry, will provide an overview of the formation, structure and distribution of lignins in wood and of the distribution of lignin in pulp fibers.