Enhancing the Inherent Flame Retardancy of Polylactic Acid by Anchoring Phytic Acid-Lysine Using Epoxidized Tannic Acid

Inherent flammability is a major limitation of polylactic acid (PLA) in industrial applications. Several flame retardants (FRs) have been explored to address this, but many undergo complicated synthesis routes, often accompanied by toxic chemical use. Moreover, these FR systems negatively affect the biodegradability of PLA. In this study, epoxidized tannic acid (ETA) and a phytic acid-lysine (PALys) biobased FR were used. The former has two functions: an anchor and a carbon source, while the latter functions as a synergistic phosphorus-nitrogen (P-N) FR. The anchoring process of PALys to the PLA end groups using ETA was accomplished through melt blending. The heat and shear allow the epoxy groups in ETA to react to OH groups in PALys and PLA. The addition of 1 wt % ETA and 5 wt % PALys improved PLA thermal stability by reducing the weight loss rate from 40.69 to 2.99 wt %/min. Moreover, microscale combustion calorimeter (MCC), limiting oxygen index (LOI), UL-94, and cone calorimeter tests were performed to evaluate the efficacy of the FR system. PLA/1ETA/5PALys achieved a high LOI value of 34% vol, a UL-94 V-0 rating, and 50% reduction in the flame out time. Char formation was also observed during the combustion tests. Additionally, the FR mechanism was evaluated using scanning electron microscopy-energy-dispersive spectroscopy and thermogravimetry-mass spectrometry. The latter established the gas-phase action by the emission of noncombustible gases, such as ammonia, CO₂, and H₂O, from the decomposition of lysine and ETA, which reduces the combustible gas concentration. Furthermore, phosphorus from PALys was able to dehydrate ETA and PLA to catalyze the formation of stable aromatic and phosphorus-containing char.