ALKALINE DELIGNIFICATION OF HARDWOODS IN A FLOW-THROUGH REACTOR WORKING AT A LOW RESIDENCE TIME .6. CHARACTERIZATION OF KRAFT POPLAR LIGNINS BY C-13 NMR

Kraft Poplar lignins extracted with a flow-through reactor are characterized by C-13 NMR. Two series. corresponding respectively to strong and weak alkali, are analysed and discuted with regard to the main steps of alkali delignification, during kraft cooking. In general, quantitative analysis shows that the majority of structural changes occur at the beginning of the cook. Later, the solubilization of the lignin corresponds to fragments that are more and more modified due to the progressive degradation of the macromolecular structure of lignin to end up with samples presenting a great anisotropy of structure. When the alkalinity is decreased, the structural changes evolve more slowly The DEPT NMR sequence analysis shows the presence of methylene carbons in the lignin structures which indicates some degradation reactions during the flow-through cooking.