Kinetic analysis of the thermal decomposition of cellulose: The main step of mass loss

Abstract

Modulated thermogravimetry (MTG) has confirmed the conclusion of Shafizadeh and Bradbury [F. Shafizadeh, A.G.W. Bradbury, J. Appl. Polym. Sci. 23 (1979) 1431–1442] that the thermal decomposition of cellulose over the wide range of mass loss is essentially the same in both air and nitrogen. At the beginning of the decomposition oxygen interacts only with surface cellulose, resulting in only ≅3% of mass loss. The two-phase model of cellulose is proposed to explain all observable phenomena related to both the pyrolysis (inert atmosphere) and the oxidative decomposition (air or oxygen). According to the model, the decomposition occurs through a migration of chain ends from the phase of polymer cellulose into the phase of products (char, gases and high-boiling tar). The interface between two phases is the region of location of chain ends; therefore unzipping of cellulose is preferable in comparison to the random scission of macromolecules. That is why the yield of levoglucosan can reach 60%. It evolves as a result of the unzipping by the transglycosylation with activation energy of about 200 kJ/mol. Oxygen interacts with products of the decomposition but does not penetrate into the matrix of polymer cellulose. The oxidation is the fast reaction subsequent to the depolymerization; therefore, the main step of the degradation is independent of the oxidative reactions.