Kinetics of Thermal Decomposition of Polylactide Previously Subjected to Thermal, Bio and Photo Degradation

摘要

Polymer degradation is a rather complex process influenced by several external parameters (such as temperature, water, UV, microorganisms...) that affects the intrinsic characteristics of the materials. In the case of polylactide (PLA), different degradation mechanisms have been reported for different degradation types and a characterization of the thermal decomposition of degraded PLA would provide additional information about the macroscopic effects of these processes. Thus, the influence of prior thermal, biological and photo degradation on the kinetics of the thermal decomposition of PLA was assessed by Thermogravimetry (TG/DTG) under non-isothermal and Argon environment conditions. The thermograms (TG) and their first-order derivative curves (DTG) of all samples evidenced one solely decomposition process. Kissinger, Friedman and Flynn- Wall-Ozawa isoconversional methods were used to estimate the apparent activation energies (E_a) of the thermal decomposition of PLA subjected to different degradation processes. The activation energy average (Ea_(iso)) from these three methods was analyzed as a function of the degradation time. The data were analyzed following different models proposed in the literature. All the degradation processes promoted changes in the Ea_(iso) with increasing degradation time especially in samples previously exposed to thermal degradation. Moreover, some differences were evident in the thermal decomposition kinetic function of PLA due to the different degradation types, rather than to a mere molar mass decay. This fact indicates that the thermal decomposition mechanism depends on the degradation state of the material (former degradation, initial molar mass, chemical composition...), and explains that different models have been proposed in the literature for the thermal decomposition of PLA. Polylactide subjected to bio and photo degradation undergo autocatalytic thermal decomposition mechanisms, while thermally degraded samples exhibit a more complex mechanism. Acknowledgments: MINECO for ENE2011-28735-C02-01 and GVA for APOSTD/2013/036 -APOSTD/2013/054.