A PHYSICAL APPROACH IN THE UNDERSTANDING OF THE PHENOMENA ACCOMPANYING THE THERMAL-TREATMENT OF LIGNIN

In this work, a mechanical approach is associated with the thermal beh avior of lignin. Samples of lignin are submitted to a thermal treatmen t accomplished at different final temperatures of pyrolysis. The dimen sional variations which occurred during the degradation of lignin are quantified by the Jacobian of the transformation. The increase of the Jacobian in the interval 180 to 260 degrees C signifies the formation and the development of a plastic phase by crazing as is indicated by s canning electron microscope micrographs of lignin residues. The appari tion and the evolution of the plastic phase are not a new fact in frac ture mechanics, but the characterisation of this phase and the mechani cs of its formation and its fracture allow us to explain some aspects until now unknown in the thermal decomposition of lignin. The importan ce of this work relies on the analogy made between the thermal behavio ur of lignin and the mechanical behaviour of polymers. This analogy pe rmits us to make some interpretations and, for the first time, to expl ain the weak weight loss and the exothermic effect of the thermal degr adation of lignin. (C) 1997 Elsevier Science B.V.