Thermogravimetric analysis of caffeic and rosmarinic acid containing chitosan complexes

Thermal behaviour and stability of chitosan (CH) and phenolic acids complexes could be of high importance in regard both the practical applications and understanding the complex formation. Thermal degradation of insoluble complexes of CH and caffeic (CACH) or rosmarinic acid (RACH) was investigated in air and nitrogen atmosphere. Thermal decomposition of CACH proceeded in two stages, first of which was attributed to decarboxylation of adsorbed CA in the temperature interval from 150 °C to 200 °C, and second one was related to decomposition of CH backbone at temperatures higher than 200 °C. Thermal degradation of RACH proceeded in one stage at temperatures higher than 200 °C, in the broad temperature range and was related to decomposition of formed complex. Values of apparent activation energy (E_a) related to the second stage of thermal decomposition of CACH and decomposition of RACH at conversion values (α) from 0.1 to 0.8 were determined by using Flynn-Wall-Ozawa method. For CACH the dependence between E_a and α was very similar to that of CH and showed that products of CA thermal degradation formed in the first stage didn't influence the thermal degradation of CACH in the second stage. Meanwhile, for RACH, initially, values of E_a increased with an increase of conversion degree, at certain value of α reached the maximum which depended on the RA to CH molar ratio, and then decreased.