Compounds acting as model biphenyl and phenylcoumarane structural units in the lignin molecule (dehydrodivanillin and dehydrodiisoeugenol) were investigated by thermal analysis. The results were compared with previously obtained data on the thermal degradation of model phenylpropane monomer units of lignin. It was found that the mechanism of thermal degradation of these models and the thermal stabilities of the bonds depend on the structure. The thermal stability of the models increases in the absence of reactive functional groups in the side chain.The phenylcoumarane unit is relatively stable thermally. The possibility of a free radical mechanism for the thermal degradation is discussed.The study of the mechanism of lignin thermolysis is of theoretical and practical significance. Prominence has been given to this question in recent years, especially in connection with the application of DTA to investigate the mechanism and kinetics of degradation.However, because of the complexity and irregularity of the structure of lignin the interpretation of the thermal curves of this polymeric substance involves great difficulties. The use of model compounds seems to be of importance in solving this problem. The general principles of the thermal degradation of models with structures and functional groups characteristic of lignin fragments, or the intermediates of lignin pyrolysis can be applied to the lignin macromolecule as a whole.In this paper the recent results [1 ] obtained with model phenylpropane monomer units of lignin with different side chain structures 1-(4-hydroxyphenyl)propanone-1 (I), 1-(4-hydroxy-3-methoxyphenyl)propanol-1 (I1), l-(4-hydroxy-3-methoxy-phenyl)propanol-3 (III) and 1-(4-hydroxy-3-methoxy-phenyl)propen-1--ol-3 (IV) are discussed. This information is compared with the data on the kinetics of the thermal degradation of the model biphenyl and phenylcoumarane structural units in lignin [dehydrodivanillin (V) and dehydrodiisoeugenol (VI)]. 43". Thermal Anal 2, 1970
The process of thermal degradation of lignins of hard wood (birch, aspen, oak), obtained under the wood treatment with 66 % sulphuric acid in the form of acid-insoluble and soluble fractions, has been investigated by thermal methods. Activation energy values of lignin pyrolysis have been determined at the stage of increasing rate of weight loss. It has been established that the difference in their thermal properties is conditioned by the structural changes in the process of isolation from wood. It has been suggested that acid-soluble lignins have a higher density than acid-insoluble ones. Sulphuric acid lignins of birch, in particular acid-soluble lignin, are assumed to possess a more homogeneous structure than sulphuric acid lignins of other species Lignin, which goes into acid solution during wood treatment with concentrated acids and is precipitated by water from the acid solution, has been little investigated. Some time ago even its carbohydrate character was under consideration. However, at present the aromatic nature of this lignin is fully proved, but there is very little information in the literature on the structure of acid-soluble lignins.As is known, the thermal properties of polymer materials are directly related to their stereochemistry and even slight changes of atomic arrangement in the basic and side chains of a macromolecule affect its thermal stability, mechanism and kinetics of degradation. We have used this thesis for establishing whether there are differences in the structures in acid-soluble lignins and lignins insoluble in sulphuric acid. A derivatographic technique was used in the present work, as this is one of the most sensitive methods for indicating structural changes in a substance, lignin in particular, as was shown in our recent investigation [1 ]. The kinetics of lignin degradation was studied at the definite stages of the process on the basis of DTA data.Various kinds of hard woods were used to yield lignin preparations, since they provide a considerable yield of acid-soluble lignin as compared with coniferous woods.
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