Preparation and mechanism study of a high efficiency bio-based flame retardant for simultaneously enhancing flame retardancy, toughness and crystallization rate of poly (lactic acid)

Polylactic acid (PLA) is regarded as one of the most promising sustainable bioplastics in response to the global energy crisis and white pollution problems. However, the inherent high flammability , poor crystallinity and brittleness of PLA composites severely limit their application in the emerging fields of electronics and rail transit. Although the current phosphorus-nitrogen flame retardants exhibit excellent flame-retardant efficiency in PLA, they are often synthesized mainly through traditional petrochemical-based compounds and their function is confined to improving the flame-retardant performance of PLA. Herein, a novel multi-functional bio-based additive chitosan methyl phosphonate (CMP) was synthesized through the self-assembly of marine organism-derived chitosan (CS) and methylphosphonic acid. The incorporation of CMP simultaneously enhanced the fire safety, toughness and crystallization rate of PLA composites. The introduction of only 4 wt% CMP gave the PLA the UL-94 V-0 rating and high LOI of 30.6% due to its excellent gas-phase effect. It meant that the final PLA bio-based composites fully met the stringent flame-retardant requirements in the emerging fields. Moreover, the crystallization rate of PLA/CMP composites was significantly enhanced and the crystallinity of PLA/CMP4 was improved from 9.08% of pure PLA to 18.66% with the increase of 105%. Because of the uniform dispersion of CMP in PLA matrix and the good interface between them, PLA/CMP4 maintained almost the same strength as pure PLA and its impact strength was slightly improved compared to that of pure PLA. This research provided a novel strategy for the preparation of a high-performance and multi-functional PLA bio-based composites, which was of great significance to expand the practical application value of PLA bio-based composites in the current environment of advocating recyclable development. • A multifunctional bio-based flame retardant (CMP) was designed and synthesized. • Only 4 wt% CMP enabled PLA to obtain the desirable UL-94 V-0 rating. • CMP exerted efficaciously the inhibitory effect in gas phase. • CMP significantly improved the crystallization rate and toughness of PLA bio-based composites.