An eco-sustainable, lactic acid-based phosphaphenanthren-containing flame-retardant plasticizer: Synthesis, properties, and mechanism

Biobased multifunctional additives are favored by industry due to their significant application advantages in the modification of poly(lactic acid) (PLA). In this study, a novel phosphaphenanthren-containing flame-retardant plasticizer (APGOL) with flame-retardant and plasticizing function derived from l-lactic acid was synthesized and characterized. Moreover, the plasticizing and flame-retardant effects of the APGOL in PLA were investigated in depth. The phosphaphenanthren of the APGOL structure endowed PLA with the enhanced flame retardancy, such that the free radical can be captured in the gaseous phase, and the carbon residue layer can be formed in the condensed phase. The addition of the APGOL led to the rise of the LOI values from 18.5% to 32.0%, whereas it led to the decline of the peak heat release rate (pHRR) and the total heat release (THR) of the blends by 20.01% and 31.18%, respectively. Notably, the APGOL-plasticized PLA samples generally reached the UL-94 V-0 grade. Besides, the APGOL led to a significant increase of the activation energy of PLA, such that the APGOL was confirmed to exhibit "good" thermostability. Furthermore, the APGOL significantly accelerated the crystallization rate of PLA. As indicated by the analysis result of the mechanical data, the tensile modulus and elongation at the break of the PLA blend (the APGOL only) reached 860.1 MPa and 330.6%, respectively. In comparison with the ATBC, the APGOL-plasticized PLA blends had higher tensile strength and tensile modulus. It is noteworthy that the APGOL can maintain the long service life and stable performance of the plasticized PLA products for its excellent anti-volatility and anti-migration stability. This study can provide a reference for the development of a flame-retardant plasticizer from renewable resources.