A physical approach in the understanding of the phenomena accompanying the thermal treatment of lignin

Abstract In this work, a mechanical approach is associated with the thermal behavior of lignin. Samples of lignin are submitted to a thermal treatment accomplished at different final temperatures of pyrolysis. The dimensional 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°C signifies the formation and the development of a plastic phase by crazing as is indicated by scanning electron microscope micrographs of lignin residues. The apparition and the evolution of the plastic phase are not a new fact in fracture mechanics, but the characteristation of this phase and the mechanics of its formation and its fracture allow us to explain some aspects until now unknown in the thermal decomposition of ligain. The importance of this work relies on the analogy made between the thermal behaviour of lignin and the mechanical behaviour of polymers. This analogy permits us to make some interpretations and, for the first time, to explain the weak weight loss and the exothermic effect of the thermal degradation of lignin.