A KINETIC-STUDY OF THE THERMAL-DEGRADATION OF CHITOSAN AND A MERCAPTAN DERIVATIVE OF CHITOSAN

The thermal degradation behaviour of chitosan and N-(2-hydroxy-3-mercaptopropyl)-chitosan (HMPC) has been studied by means of dynamic thermogravimetric analysis (TGA) in the range 30-600-degrees-C. Dynamic experiments showed that thermal decomposition of both products proceeds in four steps. The apparent activation energy values for each degradation step were obtained following the procedure of Broido. The first decomposition stage of both products was also studied by isothermal TGA and it was found that the apparent activation energy varies with degree of conversion, which is a manifestation of the complexity of this decomposition stage for both polysaccharides. In the pyrolysis-mass spectra of chitosan and HMPC the lower stability of the latter is apparent, since it exhibits peaks at m/z 34, 47 and 76, not observed for chitosan, due to evolution of H2S and CH3S and C2H4OS fragments, respectively. The IR spectra of residual chitosan chars from isothermal pyrolysis gave an indication of degradation of the ring together with dehydration over the whole temperature interval studied. In the IR spectra of HMPC pyrolysis residues no appreciable degradation of the glycopyranosic ring was observed up to 200-degrees-C; at higher temperatures, the behaviour is similar to that of chitosan.