Facile Synthesis of Phosphoramide Salts as Flame Retardants for Poly(Lactic Acid) with Enhanced Crystallization and Retained Tensile Properties

Abstract

The improvement of both the inherent combustibility and crystallization characteristics of PLA has long posed a significant challenge. In this study, this issue was effectively addressed through the facile synthesis of a series of phosphoramide salts containing phosphoryl groups, termed PPA-X (where X represents the corresponding diamine), using phenylphosphoric acid instead of phenylphosphonic dichloride. This approach avoided the typical problems associated with traditional flame retardants containing phosphoryl groups, including highly vigorous reactions, the need for acid-binding agents, and the generation of corrosive hydrogen chloride gas. A 3wt% loading of PPA-P-phenylenediamine (PPA-PPD) enabled PLA to self-extinguish within 1 second, achieve V-0 rating in vertical burning tests, and attain a limiting oxygen index (LOI) of 27.1%. Moreover, the crystallization half-time (t1/2) at 115 °C for PLA was significantly reduced from 22.9 minutes to 12.5 minutes, while its crystallinity increased from 13.8% to 41.2%. Importantly, the tensile strength and elongation at break of PLA were well-preserved. Overall, this study presents a simple and low-toxicity strategy for developing highly effective flame retardants and nucleating agents for PLA, with promising potential for industrial applications.

Keywords: Poly(lactic acid), phosphoramide salt, flame retardant, crystallization rate.