Structure-Function Relationships in the Phenolation of Lignins from Different Sources

In recent years, significant amounts of various lignins became commercially available. Their technical utilization for thermosets such as phenol formaldehyde resins is widely discussed as an added value. However, the comparably low number of reactive sites is still limiting utilization in higher proportions. To overcome this obstacle, lignins can be activated by phenolation prior to resin synthesis. In this study, the applicability and outcome of phenolation was studied for a set of organosolv lignins from hardwood, softwood, and annual plants and was compared to their counterparts from commercial processes, i.e., kraft, sulfite, soda, and hydrolysis lignin. Thus, structural properties of various raw lignins could be linked to the increase of reactive sites upon phenolation. Large differences were found and could mainly be attributed to the number of aliphatic hydroxyl groups in the raw lignins. Highest activation was achieved for hardwood organosolv and softwood sulfite lignins. With ion-exchanged sulfite lignin in H+ form the phenolation could even be performed autocatalyzed to a high extent. In contrast, soda grass and softwood kraft lignin showed weak potential for activation. Additionally, the influence of ash and sulfur content, and the changes in molecular weight were elucidated.