Understanding thermal decomposition kinetics of flame-retardant thermoset polylactic acid

The Flynn–Wall–Ozawa method was applied to study the local activation energy of flame retardant thermoset PLA, and the results showed that with an increase of conversion of thermal degradation, the local activation energy was increased slowly. When the conversion of thermal degradation was under 15%, the activation energy of flame retardant thermoset PLA was lower than that of thermoset PLA, attributed to the low bond energy of P–C bond. When the conversion of thermal degradation exceeded 15%, the dehydration charcoal effect of phosphorous compound slowed down the process of thermal degradation, and the activation energy of flame retardant thermoset PLA was higher than that of thermoset PLA, indicating that the addition of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) enhances the thermal stability of thermoset PLA. The Coats–Redfern method and invariant kinetic parameters method were used to understand kinetics details about this process including the activation energy and apparent pre-exponential factor, and estimated contribution ratios for the 18 kinetic functions. The results showed that the addition of DOPO didn't impact the most important mechanism of thermal degradation but changed the contribution ratios of the 18 kinetic functions. According to the functional relationship between decomposition rate with temperature and quality conversion rate, three-dimension surface plots were made to understand the change regulation of decomposition rate. We found that the addition of DOPO reduced the decomposition rate of thermoset PLA, attributing to the dehydration charcoal effect of phosphorous compound which restrained the interesterification of PLA, and thus enhancing the thermal stability of thermoset PLA.