THERMAL-DECOMPOSITION OF MILLED WOOD LIGNINS STUDIED BY THERMOGRAVIMETRY MASS-SPECTROMETRY

Thermogravimetry/mass spectrometry has been applied to characterize 16 milled wood lignins isolated from grasses, softwoods and hardwoods. T he samples were thermally decomposed in inert atmosphere using a heati ng rate of 20 degrees C min(-1). The effect of acetylation as well as ZnCl2 and NaCl as catalysts on the thermal decomposition has been stud ied. The weight loss and evolution profiles of the most abundant degra dation products were monitored as a function of temperature. Correlati on has been found between the intensity of several products and the co mpositional parameters of lignins determined by wet chemical methods. The modification of OH groups by acetylation changed the decomposition patterns, but the acetic acid produced by thermolysis had no catalyti c effect on the formation of monomers. The evolution profiles of H2O a nd CO2 from the original and acetylated lignins proved that free OH gr oups promote the scission of COOH groups. The additives ZnCl2 and NaCl have different influence on the product distribution. NaCl promotes d ehydration, demethoxylation and recombination of the primarily formed radicals, although the maximum of the evolution profiles is not shifte d significantly. However, water and formaldehyde formation shifts to 6 0-80 degrees C lower temperature in the presence of ZnCl2 indicating t he occurrence of ionic decomposition mechanisms. (C) 1997 Elsevier Sci ence B.V.