The general aim of the pulping process is to delignify the wood matrix by chemically degrading and/or sulfonating the lignin to water-soluble fragments to liberate the cellulose. The origin of the lignin (wood species), the delignification type, and the recovery process from pulping liquors have remarkable influence on the structure of this biopolymer in both pulp and dissolved lignin [1, 10, 13-18]. The different structural characteristics of lignin have influence on its performance toward further processing [19, 20] and determines its suitability for different proposes.

This section deals with the most important lignin types available from the pulp and paper industry. There will be reference to lignin from emerging processes of lignocellulosic biomass conversion to ethanol and saccharide-derived chemicals. The major differences between lignins are derived from distinct delignification processes. In general, the characteristics of the lignins obtained are dependent of
the balance between the two main groups of reactions having an opposite effect: (i) cleavage reactions and introduction/liberation of hydrophilic groups, leading to dissolution of lignin fragments and (ii) condensation reactions which increase the molecular weight of lignin.