The presence of condensed lignin structures formed during kraft pulping process have been identified and characterized in previous studies. Resinol-type structures, namely pinoresinol and secoisolariciresinol, have been characterized in kraft lignin and residual kraft lignin samples. It has been suggested that the difficult removal of residual lignin from pulp is affected by these structures, which are also associated with fatty acids. Sulphur has been suggested to play a role in initiation of radical reactions involved, but our hypothesis is that similar reactions can also take place in novel processes and applications involving other possible radical initiators.<br/><br/>As new processes and applications for biomaterials are being developed, it is essential to understand the underlying mechanisms of existing technologies. To get new insight to the mechanisms involved in formation of condensed structures, kraft lignin was heated in temperatures used in many processes used for biomaterials, namely 160-170 °C. In order to understand the role of fatty acids in the reactions, heat treatments were performed using tall oil fatty acids or the organic solvent decane as the reaction media. The washed and dried products were practically insoluble in organic solvents, but they could be analyzed using FT-IR. To increase the solubility of the samples, each were acetylated in a mixture of pyridine and acetic anhydride at 80 °C. As a result, main part of the samples was soluble in d6-DMSO for structural analysis with NMR, and in THF for analysis of molar masses with GPC. The thermal properties of the products were also characterized with TGA and DSC. In addition, the role of fatty acids in the reaction was examined by analyzing lignin samples with py-GC/MS in the presence and absence of fatty acids.<br/><br/>The structural analysis by NMR confirmed the presence of condensed structures in both samples. In addition, fatty acids were present in sample heated with tall oil fatty acids as the solvent, according to both NMR and FT-IR. Thermal analyses suggested that the materials were fused together instead of behaving as separate materials. Analysis with py-GC/MS at 200 °C showed that the composition of lignin-based volatiles were affected by the presence of fatty acids in the sample. Therefore, it is highly possible that similar reactions occur in processing and applications of biomaterials containing lignin and fatty acid-type extractives.

Formation of condensed structures during mild thermal treatment of kraft lignin with fatty acids through radical reactions

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.