Comparative Evaluation of Additives in Softwood Fractionation: Impacts on Lignin Recovery and Pulp Quality

<jats:p> Lignocellulosic biomass fractionation with concurrent lignin stabilization via chemical modification has been shown to enhance delignification efficiency and enable the recovery of lignin with a low degree of condensation, thereby increasing the application potential of both carbohydrate and lignin fractions. In this study, five different chemicals—ethylene glycol (EG), glyoxylic acid (GlyoxA), phenol (Phen), thiourea (ThioU), and thiolactic acid (TLA)—were evaluated for their ability to modify lignin during acid hydrotropic fractionation (AHF) of softwood in aqueous <jats:italic>p</jats:italic> ‐toluenesulfonic acid. Their effects were compared to AHF without additives. Among the tested modifiers, Phen achieved the highest lignin removal. However, TLA led to the highest recovery lignin yield, the lightest‐colored fractions, and pulp sheets with superior tensile properties, highlighting the strong potential of thiol‐based nucleophiles in wood delignification. ThioU, another sulfur‐based nucleophile, also produced lightly colored fractions, but lignin redeposition on fibers negatively impacted pulp mechanical properties. GlyoxA moderately improved lignin removal and yield, while EG had negligible effect compared to plain AHF. Notably, well‐defined, spherical lignin nanoparticles were obtained from TLA‐ and ThioU‐modified lignins, although ThioU‐lignin also formed film‐like structures due to nonprecipitated lignin. Other lignin samples yielded irregularly shaped nanoparticles. </jats:p>